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The charging process of the Li–CO2 battery was also studied systematically (Fig. 5). The Li–CO2 battery with no illumination exhibits a high charge voltage of about 4.17 V, whereas the charge voltage sharply drops to 3.28 V after illumination (Fig. 5a). Impressively, the photo-assisted Li–CO2 battery with the SiC/RGO cathode outperforms the majority of Li–CO2 batteries (Table S1†). Furthermore, the fast response of the charge voltage to the on–off irradiation (Fig. S10†) demonstrates the superior kinetics under illumination, which is partially responsible for the decrease of charge overpotential. As presented in Fig. 5b, LSV curves of the batteries obtained under illumination also show a lower onset voltage and higher current density compared with that in the absence of light, in accordance with the galvanostatic charging profiles. Such a light response phenomenon presented in the charging process further supports the proposed working mechanism for the photo-assisted Li–CO2 batteries (Fig. 1c): the photoexcited holes play a key role in the decomposition of the discharge products due to their high oxidation capability. Thus, reduction of the charge voltage could be achieved by providing extra energy from light irradiation to compensate the required input energy from the external circuit. Also, the kinetics of the battery could be promoted under illumination in virtue of the quick response of the photo-excitation process compared to that of the electric field, which is also beneficial in reducing the charging overpotential. The morphology evolution of the discharge products upon charging was further investigated via SEM analysis. After the recharge process under illumination, the nanosheet-like discharge product disappears completely (Fig. 5d), whereas some membranous discharge product remains on the surface of the cathode charged in the absence of light (Fig. 5e). Analogously, XRD patterns of recharged cathodes show that the typical diffraction peaks of Li2CO3 completely disappear after photo-assisted charging, while these peaks still exist after being recharged without illumination (Fig. 5c and S7†), consistent with the results obtained from FTIR spectra (Fig. 5f and S8†). Overall, the light-assisted Li–CO2 battery possesses better reversibility in comparison with the conventional Li–CO2 battery, owing to the facile CO2ER process promoted by the photogenerated holes.

What's the cathode?

SiC/RGO

Then Li–O2 batteries were fabricated with PFOSF treated Li anodes. A carbon nanotube film was used as the air-cathode and 1 M LiTFSI in TEGDME as the electrolyte. The fabricated LOBs were charged and discharged repeatedly. SEM and XRD results (Fig. S12†) show that the cathode side reaction is reversible. As shown in Fig. 5, the bare Li based cell can only cycle for 43 cycles with the discharge voltage suddenly dropped to 2.1 V at a current density of 300 mA g−1 with the capacity limited at 1000 mA h g−1. To find out the reason, the cell was disassembled after cycling for 10 cycles, the bare Li foil became black and many cracks and pulverizations were observed under SEM (Fig. S13a and c†). In contrast, LOBs with the PFOSF based Li anode exhibited largely improved cycling performance which could be enhanced to 185 cycles without using any cathode catalyst (Fig. 5). The Li surface was flat and kept its pristine silvery color after 10 cycles with about 40 μm thickness of the protective film covered on the Li surface (Fig. S13b and d†). LOBs with different SEI thicknesses were assembled and tested. As shown in Fig. S14,† the cycling performance was improved. The cycle life was elongated to 104, 92, 140, 185 and 167 cycles after PFOSF treatment for 10, 20, 30, 60 and 180 min, respectively, so the optimized PFOSF treatment time is 60 min. The EIS of the LOBs further explained this phenomenon. In Fig. S15,† the ohmic resistance (Rs) and charge transfer resistance (Rc) gradually increased within 30 cycles for LOBs with the pristine Li anode, which was induced by the pulverization of the Li anode and accumulation of side products due to the severe side reactions of Li metal with the O2-rich electrolyte. In contrast, for PFOSF-Li based LOBs, the values of Rohm and Rctr almost remained stable, which were far smaller than those of bare LOBs, indicating the well protecting effect of the LiF coating on the Li surface in suppressing the side reactions of Li metal with the electrolyte.

What's the cathode?

air-cathode

Then Li–O2 batteries were fabricated with PFOSF treated Li anodes. A carbon nanotube film was used as the air-cathode and 1 M LiTFSI in TEGDME as the electrolyte. The fabricated LOBs were charged and discharged repeatedly. SEM and XRD results (Fig. S12†) show that the cathode side reaction is reversible. As shown in Fig. 5, the bare Li based cell can only cycle for 43 cycles with the discharge voltage suddenly dropped to 2.1 V at a current density of 300 mA g−1 with the capacity limited at 1000 mA h g−1. To find out the reason, the cell was disassembled after cycling for 10 cycles, the bare Li foil became black and many cracks and pulverizations were observed under SEM (Fig. S13a and c†). In contrast, LOBs with the PFOSF based Li anode exhibited largely improved cycling performance which could be enhanced to 185 cycles without using any cathode catalyst (Fig. 5). The Li surface was flat and kept its pristine silvery color after 10 cycles with about 40 μm thickness of the protective film covered on the Li surface (Fig. S13b and d†). LOBs with different SEI thicknesses were assembled and tested. As shown in Fig. S14,† the cycling performance was improved. The cycle life was elongated to 104, 92, 140, 185 and 167 cycles after PFOSF treatment for 10, 20, 30, 60 and 180 min, respectively, so the optimized PFOSF treatment time is 60 min. The EIS of the LOBs further explained this phenomenon. In Fig. S15,† the ohmic resistance (Rs) and charge transfer resistance (Rc) gradually increased within 30 cycles for LOBs with the pristine Li anode, which was induced by the pulverization of the Li anode and accumulation of side products due to the severe side reactions of Li metal with the O2-rich electrolyte. In contrast, for PFOSF-Li based LOBs, the values of Rohm and Rctr almost remained stable, which were far smaller than those of bare LOBs, indicating the well protecting effect of the LiF coating on the Li surface in suppressing the side reactions of Li metal with the electrolyte.

What's the anode?

Li

Then Li–O2 batteries were fabricated with PFOSF treated Li anodes. A carbon nanotube film was used as the air-cathode and 1 M LiTFSI in TEGDME as the electrolyte. The fabricated LOBs were charged and discharged repeatedly. SEM and XRD results (Fig. S12†) show that the cathode side reaction is reversible. As shown in Fig. 5, the bare Li based cell can only cycle for 43 cycles with the discharge voltage suddenly dropped to 2.1 V at a current density of 300 mA g−1 with the capacity limited at 1000 mA h g−1. To find out the reason, the cell was disassembled after cycling for 10 cycles, the bare Li foil became black and many cracks and pulverizations were observed under SEM (Fig. S13a and c†). In contrast, LOBs with the PFOSF based Li anode exhibited largely improved cycling performance which could be enhanced to 185 cycles without using any cathode catalyst (Fig. 5). The Li surface was flat and kept its pristine silvery color after 10 cycles with about 40 μm thickness of the protective film covered on the Li surface (Fig. S13b and d†). LOBs with different SEI thicknesses were assembled and tested. As shown in Fig. S14,† the cycling performance was improved. The cycle life was elongated to 104, 92, 140, 185 and 167 cycles after PFOSF treatment for 10, 20, 30, 60 and 180 min, respectively, so the optimized PFOSF treatment time is 60 min. The EIS of the LOBs further explained this phenomenon. In Fig. S15,† the ohmic resistance (Rs) and charge transfer resistance (Rc) gradually increased within 30 cycles for LOBs with the pristine Li anode, which was induced by the pulverization of the Li anode and accumulation of side products due to the severe side reactions of Li metal with the O2-rich electrolyte. In contrast, for PFOSF-Li based LOBs, the values of Rohm and Rctr almost remained stable, which were far smaller than those of bare LOBs, indicating the well protecting effect of the LiF coating on the Li surface in suppressing the side reactions of Li metal with the electrolyte.

What's the cathode?

Vanadium sulfides, such as VS2, have been often used as sulfur host materials for lithium–sulfur batteries (LSBs), however, their high-symmetry and layered crystalline structure often lead to a poor rate-capability and a limited cycling stability of the resultant LSBs. Thus, in this work, a type of distorted NiAs-type structured V2S3 phase was designed and attempted to use it as a sulfur host for LSBs. The results showed that the prepared V2S3-nanocrystal decorated carbon nanofiber (CNF@V2S3) electrode films are freestanding, highly conductive and flexible. And the resultant CNF@V2S3/S cathodes show a high specific capacity (1169 mA h g−1 at 0.1C), an excellent rate capability (retain 78.9% at 2.0C), an ultra-low delay rate per cycle of 0.0071%, and a low self-discharge rate of 3.65% per month. A series of analyses indicate that these high electrochemical performances are mainly due to the high polarity, high conductivity and high catalytic activity of V2S3 nanocrystals, as well as the improved diffusivities of Li ions. This research could provide some new insight into the design of sulfur host materials for high-performance LSBs.

What's the cathode?

CNF@V2S3/S

Along with the high PCE of the cSiPV module described above, much attention should be paid to photoelectric-charge/galvanostatic-discharge capability (i.e., electrochemical redox kinetics) of the bQSSB to develop highly efficient cSiPV–bQSSB. To this end, we chose LiCoO2 (LCO) and Li4Ti5O12 (LTO) as cathode and anode active materials in the bQSSB, respectively, owing to their fast rate performance and structural stability. Other electrode active materials with high capacities and wide electrochemical voltages can be used to develop advanced bQSSBs, which will be an interesting topic in future studies. In addition, to secure well-interconnected electronic networks in the battery electrodes that are affected by dispersibility of conductive additives (herein, carbon black (CB) powders), we modified the surface of CB powders in the electrodes. The CB powders were grafted with diethylenetriamine (DETA) to promote their inter-particle repulsion. The DETA grafting process is described in the Experimental section. The beneficial effects of the DETA-modified CB (denoted as DETA CB) powders on the formation of the electronic networks are conceptually illustrated in Fig. 2A. The X-ray photoelectron spectroscopy (XPS) spectra (Fig. 2B) showed characteristic N 1s peaks around 400 eV (assigned to amine groups), verifying the successful grafting of DETA on the CB powders. Additionally, the zeta potential of the DETA CB powders was +32 mV (vs. + 0.5 mV of the pristine CB powders), revealing the enhanced electrostatic repulsion between the powders (Fig. 2C).

What's the cathode?

LiCoO2 (LCO)

Along with the high PCE of the cSiPV module described above, much attention should be paid to photoelectric-charge/galvanostatic-discharge capability (i.e., electrochemical redox kinetics) of the bQSSB to develop highly efficient cSiPV–bQSSB. To this end, we chose LiCoO2 (LCO) and Li4Ti5O12 (LTO) as cathode and anode active materials in the bQSSB, respectively, owing to their fast rate performance and structural stability. Other electrode active materials with high capacities and wide electrochemical voltages can be used to develop advanced bQSSBs, which will be an interesting topic in future studies. In addition, to secure well-interconnected electronic networks in the battery electrodes that are affected by dispersibility of conductive additives (herein, carbon black (CB) powders), we modified the surface of CB powders in the electrodes. The CB powders were grafted with diethylenetriamine (DETA) to promote their inter-particle repulsion. The DETA grafting process is described in the Experimental section. The beneficial effects of the DETA-modified CB (denoted as DETA CB) powders on the formation of the electronic networks are conceptually illustrated in Fig. 2A. The X-ray photoelectron spectroscopy (XPS) spectra (Fig. 2B) showed characteristic N 1s peaks around 400 eV (assigned to amine groups), verifying the successful grafting of DETA on the CB powders. Additionally, the zeta potential of the DETA CB powders was +32 mV (vs. + 0.5 mV of the pristine CB powders), revealing the enhanced electrostatic repulsion between the powders (Fig. 2C).

What's the anode?

Li4Ti5O12 (LTO)

Low-cost and high-safety aqueous Zn ion batteries have been considered as promising alternatives to Li-ion batteries, provided that a stable Zn metal anode could be developed. The dendrite growth and the low Coulombic efficiency (CE) are the primary two issues afflicting the design of advanced Zn metal anode. Inspired by the complexing agent in the electroplating industry, acetonitrile (AN) is proposed as an electrolyte additive to guide the smooth growth of Zn. The enhanced intermolecular interactions between Zn2+ and the mixed H2O/AN solvents lead to the supersaturating of adatoms on the current collector, as revealed by the complementary theoretical and experimental studies. Consequently, homogeneous nucleation and smooth growth of Zn is enabled for achieving exceptional stability up to 1000 cycles with an excellent CE of 99.64% on average. Application-wise, the incorporation of complexing agent in the electrolyte is fully compatible with the cathode while maintains the non-flammable nature for safe operation. The solvation chemistry regulation strategy provides a promising route to stabilize Zn metal anodes.

What's the cathode?

Low-cost and high-safety aqueous Zn ion batteries have been considered as promising alternatives to Li-ion batteries, provided that a stable Zn metal anode could be developed. The dendrite growth and the low Coulombic efficiency (CE) are the primary two issues afflicting the design of advanced Zn metal anode. Inspired by the complexing agent in the electroplating industry, acetonitrile (AN) is proposed as an electrolyte additive to guide the smooth growth of Zn. The enhanced intermolecular interactions between Zn2+ and the mixed H2O/AN solvents lead to the supersaturating of adatoms on the current collector, as revealed by the complementary theoretical and experimental studies. Consequently, homogeneous nucleation and smooth growth of Zn is enabled for achieving exceptional stability up to 1000 cycles with an excellent CE of 99.64% on average. Application-wise, the incorporation of complexing agent in the electrolyte is fully compatible with the cathode while maintains the non-flammable nature for safe operation. The solvation chemistry regulation strategy provides a promising route to stabilize Zn metal anodes.

What's the anode?

Zn metal

Low-cost and high-safety aqueous Zn ion batteries have been considered as promising alternatives to Li-ion batteries, provided that a stable Zn metal anode could be developed. The dendrite growth and the low Coulombic efficiency (CE) are the primary two issues afflicting the design of advanced Zn metal anode. Inspired by the complexing agent in the electroplating industry, acetonitrile (AN) is proposed as an electrolyte additive to guide the smooth growth of Zn. The enhanced intermolecular interactions between Zn2+ and the mixed H2O/AN solvents lead to the supersaturating of adatoms on the current collector, as revealed by the complementary theoretical and experimental studies. Consequently, homogeneous nucleation and smooth growth of Zn is enabled for achieving exceptional stability up to 1000 cycles with an excellent CE of 99.64% on average. Application-wise, the incorporation of complexing agent in the electrolyte is fully compatible with the cathode while maintains the non-flammable nature for safe operation. The solvation chemistry regulation strategy provides a promising route to stabilize Zn metal anodes.

What's the electrolyte?

Low-cost and high-safety aqueous Zn ion batteries have been considered as promising alternatives to Li-ion batteries, provided that a stable Zn metal anode could be developed. The dendrite growth and the low Coulombic efficiency (CE) are the primary two issues afflicting the design of advanced Zn metal anode. Inspired by the complexing agent in the electroplating industry, acetonitrile (AN) is proposed as an electrolyte additive to guide the smooth growth of Zn. The enhanced intermolecular interactions between Zn2+ and the mixed H2O/AN solvents lead to the supersaturating of adatoms on the current collector, as revealed by the complementary theoretical and experimental studies. Consequently, homogeneous nucleation and smooth growth of Zn is enabled for achieving exceptional stability up to 1000 cycles with an excellent CE of 99.64% on average. Application-wise, the incorporation of complexing agent in the electrolyte is fully compatible with the cathode while maintains the non-flammable nature for safe operation. The solvation chemistry regulation strategy provides a promising route to stabilize Zn metal anodes.

What's the anode?

Zn metal

Novel portable power sources featuring high flexibility, built-in sustainability and enhanced safety have attracted ever-increasing attention in the field of wearable electronics. Herein, a novel flexible self-charging sodium-ion full battery was feasibly fabricated by sandwiching a BaTiO3-P(VDF-HFP)-NaClO4 piezoelectric gel-electrolyte film between an advanced Na3V2(PO4)3@C cathode and hard carbon anode. Besides the considerable flexibility and electrochemical storage performance, the as-designed device also delivers sound self-charging capability via various stress patterns, regardless of whether under static compression, repeated bending or continuous palm patting. Serially connected self-charging devices are able to drive several electronic devices with a good working state. Specifically, a unique theory of electromagnetic fields was successfully introduced to deduce the direct self-charging mechanism, where no rectifier was applied and the battery was charged by the built-in piezoelectric component. This work presents an innovative approach to achieve a new sustainable, safe and flexible sodium-ion battery for self-powered wearable electronics.

What's the cathode?

Na3V2(PO4)3@C

Novel portable power sources featuring high flexibility, built-in sustainability and enhanced safety have attracted ever-increasing attention in the field of wearable electronics. Herein, a novel flexible self-charging sodium-ion full battery was feasibly fabricated by sandwiching a BaTiO3-P(VDF-HFP)-NaClO4 piezoelectric gel-electrolyte film between an advanced Na3V2(PO4)3@C cathode and hard carbon anode. Besides the considerable flexibility and electrochemical storage performance, the as-designed device also delivers sound self-charging capability via various stress patterns, regardless of whether under static compression, repeated bending or continuous palm patting. Serially connected self-charging devices are able to drive several electronic devices with a good working state. Specifically, a unique theory of electromagnetic fields was successfully introduced to deduce the direct self-charging mechanism, where no rectifier was applied and the battery was charged by the built-in piezoelectric component. This work presents an innovative approach to achieve a new sustainable, safe and flexible sodium-ion battery for self-powered wearable electronics.

What's the anode?

hard carbon

Novel portable power sources featuring high flexibility, built-in sustainability and enhanced safety have attracted ever-increasing attention in the field of wearable electronics. Herein, a novel flexible self-charging sodium-ion full battery was feasibly fabricated by sandwiching a BaTiO3-P(VDF-HFP)-NaClO4 piezoelectric gel-electrolyte film between an advanced Na3V2(PO4)3@C cathode and hard carbon anode. Besides the considerable flexibility and electrochemical storage performance, the as-designed device also delivers sound self-charging capability via various stress patterns, regardless of whether under static compression, repeated bending or continuous palm patting. Serially connected self-charging devices are able to drive several electronic devices with a good working state. Specifically, a unique theory of electromagnetic fields was successfully introduced to deduce the direct self-charging mechanism, where no rectifier was applied and the battery was charged by the built-in piezoelectric component. This work presents an innovative approach to achieve a new sustainable, safe and flexible sodium-ion battery for self-powered wearable electronics.

What's the electrolyte?

BaTiO3-P(VDF-HFP)-NaClO4

We have successfully synthesized a self-supported CoFe@NCNT/CFC electrode for mechanically flexible ZABs, through a facile strategy. The optimized CoFe@NCNT/CFC cathode shows excellent bifunctional electrocatalytic activities with a half-wave potential of 0.873 V for the ORR and E10 = 1.506 V for the OER. The flexible all-solid-state ZABs with the CoFe@NCNT/CFC cathode exhibit a large open-circuit voltage of 1.426 V and a large power density of 37.7 mW cm−2 as well as robust stability. More importantly, ZABs assembled with the self-supported CoFe@NCNT/CFC cathode can still work even under extreme bending conditions. Our strategy opens a new way for flexible self-supported catalysts for high-performance portable and rechargeable energy storage devices.

What's the cathode?

CoFe@NCNT/CFC

We have successfully synthesized a self-supported CoFe@NCNT/CFC electrode for mechanically flexible ZABs, through a facile strategy. The optimized CoFe@NCNT/CFC cathode shows excellent bifunctional electrocatalytic activities with a half-wave potential of 0.873 V for the ORR and E10 = 1.506 V for the OER. The flexible all-solid-state ZABs with the CoFe@NCNT/CFC cathode exhibit a large open-circuit voltage of 1.426 V and a large power density of 37.7 mW cm−2 as well as robust stability. More importantly, ZABs assembled with the self-supported CoFe@NCNT/CFC cathode can still work even under extreme bending conditions. Our strategy opens a new way for flexible self-supported catalysts for high-performance portable and rechargeable energy storage devices.

What's the cathode?

CoFe@NCNT/CFC

Likewise, the cycling performances of bare and coated LMO cathodes were compared using a half-cell (Li|LMO). The cycling performance of the Li|LMO half-cell was examined within the voltage range of 3.3to 4.5 V at 55 °C. The charge and discharge voltage profiles of the bare LMO and surface modified LMO samples (PLMO-10 min, LMO-10 min, and LMO-30 min) are presented in Fig. S3a.† The initial discharge capacity of bare LMO, PLMO-10 min, LMO-10 min, and LMO-30 min are 128.01, 125.42, 127.52, and 130.144 mA h gLMO−1 at 0.3C and 55 °C, respectively. Fig. S3b† displays the cycling performances of all samples measured with the bare LMO and PVDF@LGLZNO fibrous film coated LMO half-cells at 0.3C and 55 °C. The discharge capacity of bare LMO sharply fades to 11 mA h gLMO−1 (9% capacity retention) after 100 cycles, while those of PLMO-10 min, LMO-10 min, and LMO-30 min were 15, 38, and 110 mA h gLMO−1 after 100 cycles at 55 °C and 0.3C, corresponding to capacity retentions of 12.20%, 30.14%, and 83.29%, respectively. Also, the cycling performance of the cell with LMO-50 min at 55 °C and 0.3C is depicted in Fig. S3c.† The above results indicate that LMO-30 min demonstrates better cycling performance at 55 °C compared to other cells, while the bare and PLMO coated cathodes have poor resistance to elevated temperature. The corresponding cells degrade faster than they did at 25 °C as seen in Fig. 2b. The effect of coating on capacity retention at 25 °C and 55 °C is apparent. The optimized PVDF@LGLZNO fibrous film coated on the surface of the LMO electrode acts as a better protective film that could substantially decrease manganese dissolution at high temperatures and minimize the adverse effects caused by electrolyte decomposition.

What's the cathode?

coated LMO

Likewise, the cycling performances of bare and coated LMO cathodes were compared using a half-cell (Li|LMO). The cycling performance of the Li|LMO half-cell was examined within the voltage range of 3.3to 4.5 V at 55 °C. The charge and discharge voltage profiles of the bare LMO and surface modified LMO samples (PLMO-10 min, LMO-10 min, and LMO-30 min) are presented in Fig. S3a.† The initial discharge capacity of bare LMO, PLMO-10 min, LMO-10 min, and LMO-30 min are 128.01, 125.42, 127.52, and 130.144 mA h gLMO−1 at 0.3C and 55 °C, respectively. Fig. S3b† displays the cycling performances of all samples measured with the bare LMO and PVDF@LGLZNO fibrous film coated LMO half-cells at 0.3C and 55 °C. The discharge capacity of bare LMO sharply fades to 11 mA h gLMO−1 (9% capacity retention) after 100 cycles, while those of PLMO-10 min, LMO-10 min, and LMO-30 min were 15, 38, and 110 mA h gLMO−1 after 100 cycles at 55 °C and 0.3C, corresponding to capacity retentions of 12.20%, 30.14%, and 83.29%, respectively. Also, the cycling performance of the cell with LMO-50 min at 55 °C and 0.3C is depicted in Fig. S3c.† The above results indicate that LMO-30 min demonstrates better cycling performance at 55 °C compared to other cells, while the bare and PLMO coated cathodes have poor resistance to elevated temperature. The corresponding cells degrade faster than they did at 25 °C as seen in Fig. 2b. The effect of coating on capacity retention at 25 °C and 55 °C is apparent. The optimized PVDF@LGLZNO fibrous film coated on the surface of the LMO electrode acts as a better protective film that could substantially decrease manganese dissolution at high temperatures and minimize the adverse effects caused by electrolyte decomposition.

What's the electrolyte?

LMO

Likewise, the cycling performances of bare and coated LMO cathodes were compared using a half-cell (Li|LMO). The cycling performance of the Li|LMO half-cell was examined within the voltage range of 3.3to 4.5 V at 55 °C. The charge and discharge voltage profiles of the bare LMO and surface modified LMO samples (PLMO-10 min, LMO-10 min, and LMO-30 min) are presented in Fig. S3a.† The initial discharge capacity of bare LMO, PLMO-10 min, LMO-10 min, and LMO-30 min are 128.01, 125.42, 127.52, and 130.144 mA h gLMO−1 at 0.3C and 55 °C, respectively. Fig. S3b† displays the cycling performances of all samples measured with the bare LMO and PVDF@LGLZNO fibrous film coated LMO half-cells at 0.3C and 55 °C. The discharge capacity of bare LMO sharply fades to 11 mA h gLMO−1 (9% capacity retention) after 100 cycles, while those of PLMO-10 min, LMO-10 min, and LMO-30 min were 15, 38, and 110 mA h gLMO−1 after 100 cycles at 55 °C and 0.3C, corresponding to capacity retentions of 12.20%, 30.14%, and 83.29%, respectively. Also, the cycling performance of the cell with LMO-50 min at 55 °C and 0.3C is depicted in Fig. S3c.† The above results indicate that LMO-30 min demonstrates better cycling performance at 55 °C compared to other cells, while the bare and PLMO coated cathodes have poor resistance to elevated temperature. The corresponding cells degrade faster than they did at 25 °C as seen in Fig. 2b. The effect of coating on capacity retention at 25 °C and 55 °C is apparent. The optimized PVDF@LGLZNO fibrous film coated on the surface of the LMO electrode acts as a better protective film that could substantially decrease manganese dissolution at high temperatures and minimize the adverse effects caused by electrolyte decomposition.

What's the cathode?

PLMO coated

All the electrochemical measurements were carried out on a CHI 660E electrochemical workstation with a standard three-electrode system in 0.5 M aqueous urea solution, and 1 M KOH solution with or without 0.5 M urea at room temperature. A mercury oxide electrode (Hg/HgO), platinum electrode (Pt), and the prepared materials were used as the reference electrode, the counter electrode, and the working electrode, respectively. The mass loading of the catalysts on the nickel foam is 1.5 mg cm−2. And the amount of commercial Pt/C and IrO2 on NF is the same as the loading of MoP@NiCo-LDH/NF-20. The two-electrode electrolyser uses a 007-2H exchangeable membrane electrolytic cell (50 mL) with Nafion 117 as the diaphragm. The synthesized catalysts were used as the cathode and anode electrodes with a distance of 5 cm between the two electrodes. During the test, the anode was connected to the working electrode and the cathode was connected to the counter and reference electrodes. Current densities were calculated based on the geometric area. All potential values were measured on an Hg/HgO electrode and converted to reversible hydrogen potential (RHE) according to the following equation: E (V vs. RHE) = E (V vs. Hg/HgO) + 0.059pH + 0.098 V.

What's the cathode?

All the electrochemical measurements were carried out on a CHI 660E electrochemical workstation with a standard three-electrode system in 0.5 M aqueous urea solution, and 1 M KOH solution with or without 0.5 M urea at room temperature. A mercury oxide electrode (Hg/HgO), platinum electrode (Pt), and the prepared materials were used as the reference electrode, the counter electrode, and the working electrode, respectively. The mass loading of the catalysts on the nickel foam is 1.5 mg cm−2. And the amount of commercial Pt/C and IrO2 on NF is the same as the loading of MoP@NiCo-LDH/NF-20. The two-electrode electrolyser uses a 007-2H exchangeable membrane electrolytic cell (50 mL) with Nafion 117 as the diaphragm. The synthesized catalysts were used as the cathode and anode electrodes with a distance of 5 cm between the two electrodes. During the test, the anode was connected to the working electrode and the cathode was connected to the counter and reference electrodes. Current densities were calculated based on the geometric area. All potential values were measured on an Hg/HgO electrode and converted to reversible hydrogen potential (RHE) according to the following equation: E (V vs. RHE) = E (V vs. Hg/HgO) + 0.059pH + 0.098 V.

What's the anode?

To investigate the electrochemical Na ion storage mechanism on the Na[Ni0.5Mn0.5]O2 and Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathodes upon Na ion extraction/insertion, cyclic voltammetry was performed (Fig. S7†). Both cathodes underwent a series of phase transitions (O3hex. → O′3mon. → P3hex. → P′3mon. → → ). The intensity of the redox peaks of the Na[Ni0.5Mn0.5]O2 cathode, especially during the hexagonal O3′–hexagonal O3′′ phase transition, became gradually polarized and reduced in height with cycling, indicating high capacity fading continuously with structural degradation. In comparison, the redox peaks of the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode hardly changed, which is consistent with the excellent Na intercalation stability of the cathode. In addition, ex situ X-ray absorption near edge structure (XANES) analysis was conducted (Fig. 3a and S8†) to examine the oxidation state of transition metals during the charge–discharge process. For both cathodes in the as-prepared state, Ni and Mn were divalent (2+) and tetravalent (4+), respectively. The Ni K-edge absorption spectrum clearly shifted toward the higher energy region after charging at 4.3 V, indicating that a change in the oxidation state of nickel from the divalent state to the tetravalent state occurred due to electrochemical oxidation in the Na cell. In comparison, although a shape change of the white line was observed, which is associated with a local geometry change due to the redox reaction of the surrounding electrochemically active metals, no significant edge shift was observed at the Mn K-edge, suggesting that manganese ions are electrochemically inactive in the tetravalent state, which is consistent with other previous reports. On discharge, the average oxidation state of Ni returned to its original value. Furthermore, this reaction mechanism was also confirmed through prediction of the net magnetic moments on Mn and Ni ions of Nax[Ni0.5Mn0.5]O2 based on first-principles calculations. As presented in Fig. S9,† in the case of O3-Na1[Ni0.5Mn0.5]O2, the integrated spin moments of Ni and Mn atoms were approximately 0 and +3, respectively, which indicates the existence of Ni2+ and Mn4+ ions in O3-Na1[Ni0.5Mn0.5]O2. During 1 mol Na+ extraction from the structure, the integrated spin moment of the Mn atoms remained unaltered and that of the Ni atom gradually increased from 0 to +2, which indicates the Ni2+/Ni4+ redox reaction of O3-Na1[Ni0.5Mn0.5]O2 during Na ion extraction/insertion. This prediction is consistent with the experimental results based on the XANES analysis. These results suggest that 0.01 mol of Ca2+ ions was successfully incorporated into the Na layer rather than a transition metal layer without interrupting the charge-transfer reaction.

What's the cathode?

Na[Ni0.5Mn0.5]O2

To investigate the electrochemical Na ion storage mechanism on the Na[Ni0.5Mn0.5]O2 and Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathodes upon Na ion extraction/insertion, cyclic voltammetry was performed (Fig. S7†). Both cathodes underwent a series of phase transitions (O3hex. → O′3mon. → P3hex. → P′3mon. → → ). The intensity of the redox peaks of the Na[Ni0.5Mn0.5]O2 cathode, especially during the hexagonal O3′–hexagonal O3′′ phase transition, became gradually polarized and reduced in height with cycling, indicating high capacity fading continuously with structural degradation. In comparison, the redox peaks of the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode hardly changed, which is consistent with the excellent Na intercalation stability of the cathode. In addition, ex situ X-ray absorption near edge structure (XANES) analysis was conducted (Fig. 3a and S8†) to examine the oxidation state of transition metals during the charge–discharge process. For both cathodes in the as-prepared state, Ni and Mn were divalent (2+) and tetravalent (4+), respectively. The Ni K-edge absorption spectrum clearly shifted toward the higher energy region after charging at 4.3 V, indicating that a change in the oxidation state of nickel from the divalent state to the tetravalent state occurred due to electrochemical oxidation in the Na cell. In comparison, although a shape change of the white line was observed, which is associated with a local geometry change due to the redox reaction of the surrounding electrochemically active metals, no significant edge shift was observed at the Mn K-edge, suggesting that manganese ions are electrochemically inactive in the tetravalent state, which is consistent with other previous reports. On discharge, the average oxidation state of Ni returned to its original value. Furthermore, this reaction mechanism was also confirmed through prediction of the net magnetic moments on Mn and Ni ions of Nax[Ni0.5Mn0.5]O2 based on first-principles calculations. As presented in Fig. S9,† in the case of O3-Na1[Ni0.5Mn0.5]O2, the integrated spin moments of Ni and Mn atoms were approximately 0 and +3, respectively, which indicates the existence of Ni2+ and Mn4+ ions in O3-Na1[Ni0.5Mn0.5]O2. During 1 mol Na+ extraction from the structure, the integrated spin moment of the Mn atoms remained unaltered and that of the Ni atom gradually increased from 0 to +2, which indicates the Ni2+/Ni4+ redox reaction of O3-Na1[Ni0.5Mn0.5]O2 during Na ion extraction/insertion. This prediction is consistent with the experimental results based on the XANES analysis. These results suggest that 0.01 mol of Ca2+ ions was successfully incorporated into the Na layer rather than a transition metal layer without interrupting the charge-transfer reaction.

What's the cathode?

Na0.98Ca0.01[Ni0.5Mn0.5]O2

As the key component in HZBs, cathodes work as the electroactive materials in rechargeable Zn-ion batteries and the catalyst in Zn–air batteries. They link both electrochemical reactions at the same time and have attracted great interest. Lee et al. initially prepared NiO/Ni(OH)2 nanoflakes on carbon paper as cathodes for HZBs, which opens the door for hybrid zinc systems. M. Ni's group and Zhi's group prepared Co3O4 nanosheets on carbon cloth to fabricate a Zn–Co3O4/air hybrid battery. Wang et al. constructed NiCo2S4 nanotubes on an N-doped carbon network derived from filter paper and fabricated a Zn–NiCo2S4/air hybrid battery with high capacity. Li et al. used NiCo2O4 nanorod@carbon coated nickel foam as the cathode to build a hybrid battery which exhibited good stability. Tan et al. prepared a Zn–Ag/air hybrid battery and achieved good reversibility and stability. Very recently, Zhi's group reported Ni3S2 nanosheets on Ni foam as a high-performance HZB cathode. Although rapid developments have been made for HZBs, defects such as slow oxygen reduction/evolution reactions (ORR/OER) and inferior energy storage properties still limit their practical applications. Therefore, it is still a crucial issue to explore novel cathode candidates for HZBs to realize both high efficiency and high energy storage properties at the same time.

What's the cathode?

NiCo2O4 nanorod@carbon coated nickel foam

As the key component in HZBs, cathodes work as the electroactive materials in rechargeable Zn-ion batteries and the catalyst in Zn–air batteries. They link both electrochemical reactions at the same time and have attracted great interest. Lee et al. initially prepared NiO/Ni(OH)2 nanoflakes on carbon paper as cathodes for HZBs, which opens the door for hybrid zinc systems. M. Ni's group and Zhi's group prepared Co3O4 nanosheets on carbon cloth to fabricate a Zn–Co3O4/air hybrid battery. Wang et al. constructed NiCo2S4 nanotubes on an N-doped carbon network derived from filter paper and fabricated a Zn–NiCo2S4/air hybrid battery with high capacity. Li et al. used NiCo2O4 nanorod@carbon coated nickel foam as the cathode to build a hybrid battery which exhibited good stability. Tan et al. prepared a Zn–Ag/air hybrid battery and achieved good reversibility and stability. Very recently, Zhi's group reported Ni3S2 nanosheets on Ni foam as a high-performance HZB cathode. Although rapid developments have been made for HZBs, defects such as slow oxygen reduction/evolution reactions (ORR/OER) and inferior energy storage properties still limit their practical applications. Therefore, it is still a crucial issue to explore novel cathode candidates for HZBs to realize both high efficiency and high energy storage properties at the same time.

What's the cathode?

Ni3S2 nanosheets on Ni foam

In summary, we have reported a facile strategy for preparing Pt3Ni1/NixFe LDHs as a binder-free catalytic electrode for HSABs. The HSAB with the Pt3Ni1/NixFe LDHs cathode delivered a higher open circle potential of 2.98 V and a lower ΔV of 0.50 V, together with remarkable cycling stability and excellent rechargeability with an average high round-trip efficiency of 79.9% during 350 cycles, which outperforms the HSABs assembled with the commercial catalyst composed of 20% Pt/C and RuO2. The remarkable charge–discharge performance and long-term cycling stability are mainly ascribed to the following: (1) the abundant Ni2+ vacancies alter the surface electronic structure of NiFe LDHs, which promotes electron/ion transfer kinetics and induces the strong interactions with PtNi nanoalloys. (2) The uniformly dispersed ultrafine PtNi nanoparticles on Pt3Ni1/NixFe LDHs enhance the electronic conductivity and provide abundant catalytic active sites, thus greatly boosting the ORR and OER in an alkaline medium. (3) The 3D hierarchically porous architectures of Pt3Ni1/NixFe LDHs offer high integrity and durability, high accessibility for electroactive sites, and enhance the interfacial kinetics that facilitates media transfer, as well as prevent catalytic by-product aggregation. (4) The binder-free design is conducive to exposing more active sites, leading to good catalytic performance. Thus, we believe that the discovery of the bifunctional Pt3Ni1/NixFe LDHs electrocatalyst sheds new light on the rational design of high-performance binder-free catalytic electrodes for large-scale application in energy conversion and storage.

What's the cathode?

Pt3Ni1/NixFe LDHs

Lithium manganese oxide (LMO) is one of the most promising cathode materials for lithium-ion batteries. However, the dissolution of manganese and its deposition on the anode surface cause poor cycling stability. To alleviate these issues, a film composed of polyvinylidene difluoride (PVDF) and Li5.6Ga0.26La2.9Zr1.87Nb0.05O12 type garnet (PVDF@LGLZNO) is coated directly on the LMO electrode and it functions as a promising artificial cathode–electrolyte interphase (CEI). The film thickness is optimized taking into account the electrospinning–processing time. To realize a cell with good capacity retention, excellent rate capability and resilience under harsher conditions (e.g. elevated temperature or high rates), the coated LMO cathode is coupled with a new anode which consists of sulfurized carbon derived from polyacrylonitrile (S-C(PAN)). The electrode (LMO-30 min) coated with the PVDF@LGLZNO composite material shows outstanding cycling stability and rate capability, as well as capacity retention when compared to the bare electrode both at room temperature (25 °C) and elevated temperature (55 °C). The PVDF@LGLZNO fibrous film coating suppresses the dissolution of manganese both at high C-rates and 55 °C, as supported by XPS, whereas PVDF coated and bare LMO cathodes are not able to prevent further deterioration of themselves. The film significantly minimizes undesirable side reactions at the cathode–electrolyte interface and reduces charge transfer resistance. The new cell with PVDF@LGLZNO (LMO-30 min) modified cathode and S-C(PAN) anode delivers capacity retention of 77% after 1000 cycles at 1C, corresponding to an average capacity decay of 0.023% per cycle.

What's the cathode?

Lithium manganese oxide (LMO)

Lithium manganese oxide (LMO) is one of the most promising cathode materials for lithium-ion batteries. However, the dissolution of manganese and its deposition on the anode surface cause poor cycling stability. To alleviate these issues, a film composed of polyvinylidene difluoride (PVDF) and Li5.6Ga0.26La2.9Zr1.87Nb0.05O12 type garnet (PVDF@LGLZNO) is coated directly on the LMO electrode and it functions as a promising artificial cathode–electrolyte interphase (CEI). The film thickness is optimized taking into account the electrospinning–processing time. To realize a cell with good capacity retention, excellent rate capability and resilience under harsher conditions (e.g. elevated temperature or high rates), the coated LMO cathode is coupled with a new anode which consists of sulfurized carbon derived from polyacrylonitrile (S-C(PAN)). The electrode (LMO-30 min) coated with the PVDF@LGLZNO composite material shows outstanding cycling stability and rate capability, as well as capacity retention when compared to the bare electrode both at room temperature (25 °C) and elevated temperature (55 °C). The PVDF@LGLZNO fibrous film coating suppresses the dissolution of manganese both at high C-rates and 55 °C, as supported by XPS, whereas PVDF coated and bare LMO cathodes are not able to prevent further deterioration of themselves. The film significantly minimizes undesirable side reactions at the cathode–electrolyte interface and reduces charge transfer resistance. The new cell with PVDF@LGLZNO (LMO-30 min) modified cathode and S-C(PAN) anode delivers capacity retention of 77% after 1000 cycles at 1C, corresponding to an average capacity decay of 0.023% per cycle.

What's the anode?

S-C(PAN)

In view of the bifunctional catalytic performance of MoP@NiCo-LDH/NF-20 towards both UOR and HER, the material was used as both anode and cathode to form a two-electrode electrolyser (MoP@NiCo-LDH/NF-20‖MoP@NiCo-LDH/NF-20). Fig. 7a shows the LSV curves of electrolysis cell voltages for both water and water with urea. At 100 mA cm−2, urea–water electrolysis (UOR & HER) requires much less driving voltage (1.405 V) than pure water electrolysis (OER & HER) (1.697 V). The inset of Fig. 7a illustrates a histogram of the driving voltages required at 20, 40, 60, and 100 mA cm−2. It can be seen intuitively that the driving voltage required by urea auxiliary electrolysis is smaller than that required by pure water electrolysis no matter whether the current density is low or high. Fig. 7b shows the polarization curves of the MoP/NF‖MoP/NF, NiCo-LDH/NF‖NiCo-LDH/NF, MoP@NiCo-LDH/NF-20‖MoP@NiCo-LDH/NF-20, and Pt/C/NF‖IrO2/NF electrolysis cells in 1 M KOH with 0.5 M urea. At 100 mA cm−2, the cell voltages of MoP/NF‖MoP/NF, NiCo-LDH/NF‖NiCo-LDH/NF, MoP@NiCo-LDH/NF-20‖MoP@NiCo-LDH/NF-20, and Pt/C/NF‖IrO2/NF are 1.494, 1.579, 1.405, and 1.708 V, respectively (Table S1†). The results show that at the same current density, the driving cell voltage of MoP@NiCo-LDH/NF-20‖MoP@NiCo-LDH/NF-20 is the lowest among all four cells. The significant bifunctional catalytic activity of MoP@NiCo-LDH/NF-20 exceeds those of reported non-precious metal catalysts (Table S2†).

What's the cathode?

MoP@NiCo-LDH/NF-20

In view of the bifunctional catalytic performance of MoP@NiCo-LDH/NF-20 towards both UOR and HER, the material was used as both anode and cathode to form a two-electrode electrolyser (MoP@NiCo-LDH/NF-20‖MoP@NiCo-LDH/NF-20). Fig. 7a shows the LSV curves of electrolysis cell voltages for both water and water with urea. At 100 mA cm−2, urea–water electrolysis (UOR & HER) requires much less driving voltage (1.405 V) than pure water electrolysis (OER & HER) (1.697 V). The inset of Fig. 7a illustrates a histogram of the driving voltages required at 20, 40, 60, and 100 mA cm−2. It can be seen intuitively that the driving voltage required by urea auxiliary electrolysis is smaller than that required by pure water electrolysis no matter whether the current density is low or high. Fig. 7b shows the polarization curves of the MoP/NF‖MoP/NF, NiCo-LDH/NF‖NiCo-LDH/NF, MoP@NiCo-LDH/NF-20‖MoP@NiCo-LDH/NF-20, and Pt/C/NF‖IrO2/NF electrolysis cells in 1 M KOH with 0.5 M urea. At 100 mA cm−2, the cell voltages of MoP/NF‖MoP/NF, NiCo-LDH/NF‖NiCo-LDH/NF, MoP@NiCo-LDH/NF-20‖MoP@NiCo-LDH/NF-20, and Pt/C/NF‖IrO2/NF are 1.494, 1.579, 1.405, and 1.708 V, respectively (Table S1†). The results show that at the same current density, the driving cell voltage of MoP@NiCo-LDH/NF-20‖MoP@NiCo-LDH/NF-20 is the lowest among all four cells. The significant bifunctional catalytic activity of MoP@NiCo-LDH/NF-20 exceeds those of reported non-precious metal catalysts (Table S2†).

What's the anode?

MoP@NiCo-LDH/NF-20

Ni-rich layered cathode materials are at the forefront to be deployed in high energy density Li-ion batteries for the automotive market. However, the intrinsic poor structural and interfacial stability during overcharging could trigger violent thermal failure, which severely limits their wide application. To protect the Ni-rich cathode from overcharging, we firstly report a redox-active cation, thioether-substituted diaminocyclopropenium, as an electrolyte additive to limit the cell voltage within the safe value during overcharging. The organic cation demonstrates a record-breaking electrochemical reversibility at ∼4.55 V versus Li+/Li and solubility (0.5 M) in carbonate-based electrolyte. The protection capability of the additive was explored in two cell chemistries: a LiNi0.8Co0.15Al0.05O2/graphite cell and a LiNi0.8Co0.15Al0.05O2/silicon–graphene cell with areal capacities of ∼2.2 mA h cm−2 and ∼3 mA h cm−2, respectively. With 0.2 M addition, the LiNi0.8Co0.15Al0.05O2/graphite cell survived 54 cycles at 0.2C with 100% overcharge. Moreover, the cell can carry an utmost 4.4 mA cm−2 (2C) with 100% overcharge and a maximum capacity of 7540% SOC at 0.2C.

What's the cathode?

Ni-rich layered

Ni-rich layered cathode materials are at the forefront to be deployed in high energy density Li-ion batteries for the automotive market. However, the intrinsic poor structural and interfacial stability during overcharging could trigger violent thermal failure, which severely limits their wide application. To protect the Ni-rich cathode from overcharging, we firstly report a redox-active cation, thioether-substituted diaminocyclopropenium, as an electrolyte additive to limit the cell voltage within the safe value during overcharging. The organic cation demonstrates a record-breaking electrochemical reversibility at ∼4.55 V versus Li+/Li and solubility (0.5 M) in carbonate-based electrolyte. The protection capability of the additive was explored in two cell chemistries: a LiNi0.8Co0.15Al0.05O2/graphite cell and a LiNi0.8Co0.15Al0.05O2/silicon–graphene cell with areal capacities of ∼2.2 mA h cm−2 and ∼3 mA h cm−2, respectively. With 0.2 M addition, the LiNi0.8Co0.15Al0.05O2/graphite cell survived 54 cycles at 0.2C with 100% overcharge. Moreover, the cell can carry an utmost 4.4 mA cm−2 (2C) with 100% overcharge and a maximum capacity of 7540% SOC at 0.2C.

What's the cathode?

Ni-rich

The non-renewability of fossil energy and growing environmental pollution have spurred the development of sustainable energy technologies, such as fuel cells and metal–air batteries. Of these, zinc–air batteries represent a promising energy technology for next-generation portable electronics, due to their good rechargeability and high theoretical density (1370 W h kg−1), where the device performance is largely dictated by the reversible catalysts at the cathode for oxygen reduction (ORR) and oxygen evolution reactions (OER). Platinum-based nanoparticles have been the catalysts of choice for ORR, whereas Ir and Ru-based nanoparticles for OER. Yet their high costs and low natural abundance have hindered the practical applications of the technology. Thus, development of low-cost and high-performance electrocatalysts for ORR and OER have been attracting extensive interest. Recent reports have shown that atomically dispersed metals (such as Mn, Fe and Co) within nitrogen-doped carbons, exhibit excellent electrocatalytic performance towards ORR and may even surpass the corresponding nanoparticle counterparts. In both experimental and theoretical studies, Fe single atom catalysts (SACs) have been recognized as the most active catalysts towards ORR in alkaline media. Unfortunately, for the Fe-based SACs, Fenton reaction involving the Fe center and ORR byproduct H2O2 produces hydroxyl and oxygen radicals, which not only affect the durability of the catalysts by changing its chemical structure, but also damage the battery devices by corroding the ion membranes. Additionally, theoretical and experimental studies have shown that the Fe-based SACs performs poorly towards OER, as compared to other 3d transition metals, such as Co and Ni. Therefore, it is critical to improve the ORR durability and OER activity of Fe SACs such that they may be used as bifunctional oxygen catalysts in rechargeable Zn–air battery. Towards this end, the incorporation of Co atomic sites into Fe SACs represents a unique strategy. First of all, the weak Fenton activity between Co and H2O2 is anticipated to markedly enhance the ORR stability of Fe SACs. In addition, prior studies have shown that Co-doped carbon exhibits a low overpotential and small Tafel slope towards OER. Thus, it is envisioned that atomically dispersing Co atoms into Fe-doped carbon may achieve a bimetal catalyst with excellent ORR/OER activity and enhanced durability, in comparison to the monometal counterparts.

What's the cathode?

Metal sulfides having various micro-/nanostructures and compositions (Fig. 2), such as spherical/non-spherical hollow materials, high surface area/porosity/selectivity structures, hierarchical carbon-based hybrids, and unique functional materials, have been successfully prepared in recent years through various synthetic approaches. These material systems offer the possibility to study the influence of these features on the electrochemical properties of Li–S batteries. Generally, there are two typical strategies to building metal sulfides and sulfur cathode composites: (i) one-step in situ fabrication and (ii) stepwise synthesis, which often involves injection of sulfur into a host via melt/solution-diffusion methods. Synthetic strategies involving the use of metal sulfides to protect Li anodes, modify separators, and engineer interlayers, are more flexible than those related to the use of metal sulfides and sulfur cathode composites, owing to the removal of the sulfur injection process. We consider the fundamental challenges facing Li–S batteries and discuss synthetic strategies to highlight their attractive features. Accordingly, the methods can be broadly categorized into five groups: (i) building buffer space to accommodate volume fluctuations; (ii) enhancing the binding ability for LiPSs; (iii) improving the electronic conductivity of the active material; (iv) increasing the sulfur loading; and (v) designing other specialized functions.

What's the cathode?

sulfur

Metal sulfides having various micro-/nanostructures and compositions (Fig. 2), such as spherical/non-spherical hollow materials, high surface area/porosity/selectivity structures, hierarchical carbon-based hybrids, and unique functional materials, have been successfully prepared in recent years through various synthetic approaches. These material systems offer the possibility to study the influence of these features on the electrochemical properties of Li–S batteries. Generally, there are two typical strategies to building metal sulfides and sulfur cathode composites: (i) one-step in situ fabrication and (ii) stepwise synthesis, which often involves injection of sulfur into a host via melt/solution-diffusion methods. Synthetic strategies involving the use of metal sulfides to protect Li anodes, modify separators, and engineer interlayers, are more flexible than those related to the use of metal sulfides and sulfur cathode composites, owing to the removal of the sulfur injection process. We consider the fundamental challenges facing Li–S batteries and discuss synthetic strategies to highlight their attractive features. Accordingly, the methods can be broadly categorized into five groups: (i) building buffer space to accommodate volume fluctuations; (ii) enhancing the binding ability for LiPSs; (iii) improving the electronic conductivity of the active material; (iv) increasing the sulfur loading; and (v) designing other specialized functions.

What's the anode?

Li

Metal sulfides having various micro-/nanostructures and compositions (Fig. 2), such as spherical/non-spherical hollow materials, high surface area/porosity/selectivity structures, hierarchical carbon-based hybrids, and unique functional materials, have been successfully prepared in recent years through various synthetic approaches. These material systems offer the possibility to study the influence of these features on the electrochemical properties of Li–S batteries. Generally, there are two typical strategies to building metal sulfides and sulfur cathode composites: (i) one-step in situ fabrication and (ii) stepwise synthesis, which often involves injection of sulfur into a host via melt/solution-diffusion methods. Synthetic strategies involving the use of metal sulfides to protect Li anodes, modify separators, and engineer interlayers, are more flexible than those related to the use of metal sulfides and sulfur cathode composites, owing to the removal of the sulfur injection process. We consider the fundamental challenges facing Li–S batteries and discuss synthetic strategies to highlight their attractive features. Accordingly, the methods can be broadly categorized into five groups: (i) building buffer space to accommodate volume fluctuations; (ii) enhancing the binding ability for LiPSs; (iii) improving the electronic conductivity of the active material; (iv) increasing the sulfur loading; and (v) designing other specialized functions.

What's the cathode?

sulfur

Based on the above theory, the self-charging process of the flexible SSCFB was illustrated as follows (Fig. 7). In the initial stage, the device suffers from no external strains/deformations and an electrochemical equilibrium state is present (Fig. 7a). In the second stage, external strains/deformations are applied on the device, and the piezoelectric gel-electrolyte in the device generates a piezoelectric field with a positive potential close to the cathode and negative potential around the anode (Fig. 7b). As a result, Na+ migration is carried out through the piezoelectric gel-electrolyte from the cathode to the anode, followed by a charging reaction in the device (cathode: Na3V2(PO4)3 → NaV2(PO4)3 + 2Na+ + 2e−; anode: xC + 2Na+ + 2e− → Na2Cx) (Fig. 7c). Impressively, when the external force is removed, the piezoelectric field doesn't disappear immediately, and the self-charging process can proceed by virtue of the internal residual strains (Fig. 7d). Once the piezoelectric potential is balanced by the re-distribution of Na+, a new equilibrium of the electrodes is formed (Fig. 7e). Finally, the piezoelectric field disappears along with the complete loss of residual strain, and a reverse migration of Na+ to the original position is conducted (Fig. 7f). Once the external force is applied on the device again, the self-charging process can be repeated.

What's the cathode?

Na3V2(PO4)3

Based on the above theory, the self-charging process of the flexible SSCFB was illustrated as follows (Fig. 7). In the initial stage, the device suffers from no external strains/deformations and an electrochemical equilibrium state is present (Fig. 7a). In the second stage, external strains/deformations are applied on the device, and the piezoelectric gel-electrolyte in the device generates a piezoelectric field with a positive potential close to the cathode and negative potential around the anode (Fig. 7b). As a result, Na+ migration is carried out through the piezoelectric gel-electrolyte from the cathode to the anode, followed by a charging reaction in the device (cathode: Na3V2(PO4)3 → NaV2(PO4)3 + 2Na+ + 2e−; anode: xC + 2Na+ + 2e− → Na2Cx) (Fig. 7c). Impressively, when the external force is removed, the piezoelectric field doesn't disappear immediately, and the self-charging process can proceed by virtue of the internal residual strains (Fig. 7d). Once the piezoelectric potential is balanced by the re-distribution of Na+, a new equilibrium of the electrodes is formed (Fig. 7e). Finally, the piezoelectric field disappears along with the complete loss of residual strain, and a reverse migration of Na+ to the original position is conducted (Fig. 7f). Once the external force is applied on the device again, the self-charging process can be repeated.

What's the anode?

C

A superfast and stable ssZIB based on a CNF–PAM hydrogel electrolyte and a Mg0.23V2O5·1.0H2O cathode was successfully developed from this work. The designed CNF–PAM hydrogel shows high stretchability and robust mechanical stability. Moreover, the porous CNF–PAM hydrogel electrolyte provides efficient pathways for the transportation of zinc ions. And the robust layered structure of V2O5·1.0H2O pillared with Mg2+ ions and water supports the fast insertion/extraction of zinc ions in the lattice. The prepared ssZIB shows remarkably high rate capability and long-term cycling performance. At a high current density of 5 A g−1, the ssZIB provides a high specific capacity of about 216 mA h g−1 within a charging time of only three minutes for over 2000 cycles, maintaining 98.6% of the initial capacity. Furthermore, with the designed CNF–PAM hydrogel electrolyte, a spring ssZIB was also obtained. The spring ssZIB is still working under repeated stretching. Even under some critical states, such as repeated bending, freezing, and heating, the ssZIB shows high stability and reliability. The ssZIB shows extraordinary electrochemical performance, robust stability, and high stretchability, which helps bring new opportunities for using ZIBs in practical large-scale storage devices.

What's the cathode?

Mg0.23V2O5·1.0H2O

A superfast and stable ssZIB based on a CNF–PAM hydrogel electrolyte and a Mg0.23V2O5·1.0H2O cathode was successfully developed from this work. The designed CNF–PAM hydrogel shows high stretchability and robust mechanical stability. Moreover, the porous CNF–PAM hydrogel electrolyte provides efficient pathways for the transportation of zinc ions. And the robust layered structure of V2O5·1.0H2O pillared with Mg2+ ions and water supports the fast insertion/extraction of zinc ions in the lattice. The prepared ssZIB shows remarkably high rate capability and long-term cycling performance. At a high current density of 5 A g−1, the ssZIB provides a high specific capacity of about 216 mA h g−1 within a charging time of only three minutes for over 2000 cycles, maintaining 98.6% of the initial capacity. Furthermore, with the designed CNF–PAM hydrogel electrolyte, a spring ssZIB was also obtained. The spring ssZIB is still working under repeated stretching. Even under some critical states, such as repeated bending, freezing, and heating, the ssZIB shows high stability and reliability. The ssZIB shows extraordinary electrochemical performance, robust stability, and high stretchability, which helps bring new opportunities for using ZIBs in practical large-scale storage devices.

What's the electrolyte?

CNF–PAM hydrogel

Several routes can be envisaged to improve the mechanical response of the composite cathode, none of which is likely to be very easy. This includes the optimization of cathode morphology, cathode chemistry/structure, mechanical properties of the solid electrolyte, and use of external pressure. Reducing the cathode surface displacement is critical for lowering the mechanical strain during cycling, which can be achieved by decreasing the cathode particle size or cathode strain. For a cathode material with a specific strain, a smaller particle size would result in less surface displacement and therefore reduced stress at the cathode/solid electrolyte interface. However, reducing the cathode particle size could negatively affect the cathode utilization and energy density of SSBs. Reducing the cathode strain during cycling is another way to minimize mechanical issues. Different cathode chemistries and structures have been shown to lead to different volume change during cycling. The current results motivate the search for high-energy-density electrode materials with small volume change or “zero-strain”. Different solid-electrolyte materials will also result in different mechanical degradation behavior. For example, the rapid development of cracks in the first few cycles was observed in SSBs with an oxide-based solid electrolyte, which is more rigid than the sulfide solid electrolyte used in the current study. In contrast, polymer-based composite solid electrolytes can accommodate larger strain and have been shown to improve the cycling stability. Therefore, developing a solid electrolyte with higher elasticity is an additional route to mitigate mechanical instability. Finally, applying a large external pressure (stack pressure) during cycling could in principle reduce the contact loss at the cathode/solid electrolyte interface, but such a large stack pressure may not be practical for large format cells.

What's the cathode?

Recent studies have suggested that Co-free, Ni-rich layered cathodes (e.g., doped LiNiO2) can provide promising battery performance for practical applications. However, these layered cathodes suffer from significant surface instability during various stages of the sample history, which generates inherent challenges for achieving stable battery performance and obtaining statistically representative characterization results. To reliably report the surface chemistry of these materials, delicate controls of stepwise sample preparation are required. In this study, we aim to illustrate how the surface chemistry of LiNiO2 based materials changes with various environments, including human exhalation, sample storage, sample preparation, electrochemical cycling, and surface doping. Our results demonstrate that the surface of these materials is highly reactive and prone to alter at various stages of sample handling and characterization. The sensitive surface could impact the interpretation of the surface chemical and structural information, including surface carbonate formation, transition metal reduction and dissolution, and surface reconstruction. Importantly, the heterogeneity of the surface degradation calls for a consolidation of nanoscale, high-resolution characterization and ensemble-averaged methods in order to improve statistical representation. Furthermore, the doping chemistry can effectively mitigate the surface degradation and improve overall battery performance due to the enhanced surface oxygen retention. Our study highlights the necessity of strict measurements through complementary characterization at multiple length scales to eliminate unintentional biased conclusions.

What's the cathode?

Co-free, Ni-rich layered

To examine the catalytic and capture property of 1T MoS2 NDs in a working Li–S cell, MoS2 ND/porous carbon/Li2S6 cathodes were prepared and subjected to synchrotron in situ XRD and in situ EIS characterizations. Fig. 4 shows the contour plot of the in situ XRD patterns collected during the first two cycles. Before discharging, no crystalline peaks were observed, confirming the high purity of the polysulfide catholyte and the low content of MoS2 NDs. When the cell was discharged to the plateau at 2.1 V, peaks referring to the (111) and (200) planes of cubic Li2S (PDF No. 023-0369, marked with a black dashed-line) appeared and reached their maximum intensity at the end of lithiation (bottom of Fig. 4). Upon charging, the intensity of Li2S decreased gradually, followed with no discernible XRD peaks and then the generation of monoclinic S8 (PDF No. 071-0137, marked with white dashed-lines), illustrating the solid (Li2S)–liquid (polysulfides)–solid (S8) reactions during the charging process. During the 2nd discharge/charge, reversible transitions between sulfur and Li2S were observed. When we compare the current in situ XRD results with peer studies, two findings can be extracted. First, MoS2 NDs propel the formation of Li2S crystals. Nelson et al. and Yang et al. both argued that the sluggish kinetics for solid (Li2S2)–solid (Li2S) conversion prevents Li2S formation upon the full discharge of sulfur/carbon electrodes, resulting in undetectable Li2S crystals during in situ XRD studies. The incomplete reduction accounted for the deficient sulfur utilization and low reversible capacities. Fortunately, the powerful MoS2 ND catalysts enabled the reversible formation of crystalline Li2S in our operando XRD studies. Second, Ye et al. reported that the poor catalytic capability of MoN led to occasioned residual S8 XRD peaks for the MoN/sulfur electrode after full discharging, which was also observed for conventional 2H MoS2 flakes-modified porous carbon/Li2S6 cathodes in this work (Fig. S9, ESI†). In contrast, no residual sulfur/Li2S crystals were observed for MoS2 ND/porous carbon/Li2S6 after full discharging/charging, respectively, again indicating the high catalytic property of the 1T MoS2 NDs.

What's the cathode?

MoS2 ND/porous carbon/Li2S6

To examine the catalytic and capture property of 1T MoS2 NDs in a working Li–S cell, MoS2 ND/porous carbon/Li2S6 cathodes were prepared and subjected to synchrotron in situ XRD and in situ EIS characterizations. Fig. 4 shows the contour plot of the in situ XRD patterns collected during the first two cycles. Before discharging, no crystalline peaks were observed, confirming the high purity of the polysulfide catholyte and the low content of MoS2 NDs. When the cell was discharged to the plateau at 2.1 V, peaks referring to the (111) and (200) planes of cubic Li2S (PDF No. 023-0369, marked with a black dashed-line) appeared and reached their maximum intensity at the end of lithiation (bottom of Fig. 4). Upon charging, the intensity of Li2S decreased gradually, followed with no discernible XRD peaks and then the generation of monoclinic S8 (PDF No. 071-0137, marked with white dashed-lines), illustrating the solid (Li2S)–liquid (polysulfides)–solid (S8) reactions during the charging process. During the 2nd discharge/charge, reversible transitions between sulfur and Li2S were observed. When we compare the current in situ XRD results with peer studies, two findings can be extracted. First, MoS2 NDs propel the formation of Li2S crystals. Nelson et al. and Yang et al. both argued that the sluggish kinetics for solid (Li2S2)–solid (Li2S) conversion prevents Li2S formation upon the full discharge of sulfur/carbon electrodes, resulting in undetectable Li2S crystals during in situ XRD studies. The incomplete reduction accounted for the deficient sulfur utilization and low reversible capacities. Fortunately, the powerful MoS2 ND catalysts enabled the reversible formation of crystalline Li2S in our operando XRD studies. Second, Ye et al. reported that the poor catalytic capability of MoN led to occasioned residual S8 XRD peaks for the MoN/sulfur electrode after full discharging, which was also observed for conventional 2H MoS2 flakes-modified porous carbon/Li2S6 cathodes in this work (Fig. S9, ESI†). In contrast, no residual sulfur/Li2S crystals were observed for MoS2 ND/porous carbon/Li2S6 after full discharging/charging, respectively, again indicating the high catalytic property of the 1T MoS2 NDs.

What's the cathode?

porous carbon/Li2S6

Based on eqn (12), the effective stiffness can be determined if the wave velocity and the material density are known. The wave velocity can be determined from the first arrival of the wave and cell thickness. To confirm that the measured wave velocity is accurate, calibration metals of known thicknesses and wave velocities were tested. Table 4 indicates metal foil thicknesses above 500 μm in thickness are accurately measured, whereas foil thicknesses less than 250 μm are underestimated. We attribute this error to the greater impact of the acoustic gel couplant at these lower thicknesses. The liquid gel couplant, which is necessary to induce low acoustic attenuation at the interface, is of a finite thickness and should be accounted for. Liquid gel couplant typically has a relatively lower wave velocity of around 1500 m s−1 (similar to water) and would therefore result in a significant underestimation of the wave velocity for thin metal foils where the couplant contributes to a greater proportion of the total propagation path. Fortunately, pouch cells are 500 μm thick at the minimum and can be accurately measured. To confirm the consistency of results regardless of battery thickness, pouch cells were constructed with n = 1 to n = 30 layers, with one layer (n = 1) being defined as: anode + separator + cathode. The subsequent layer is then: cathode (other side) + separator + anode. n = 30 is the full 210 mA h LiCoO2/graphite pouch cell, with 15 double-sided cathodes and 16 double-sided anodes. A schematic of the configuration is demonstrated in Fig. 1. In Fig. 3b, the last data point corresponds to n = 34, which was obtained from a slightly thicker commercial pouch cell of the same chemistry and configuration. The resulting thickness vs. first break (Fig. 3b) shows a linear relationship, indicating a constant wave velocity of approximately 1700 m s−1 and resulting in a calculated effective stiffness of 4.76 GPa (Fig. 3a). Therefore, the measured wave velocity and the resulting effective stiffness is confirmed to be the same regardless of how many repeating cell layers there are, and thicker cell stacks do not slow down the wave velocity. The measurement of 4.76 GPa is comparable to a prior ex situ study by Knehr and Hodson, where a digital caliper was used to measure the pouch cell thickness. The careful calibration and confirmatory studies here demonstrate the reliability of the acoustic measurement not only for relative shifts but also in calculating an intrinsic material stiffness.

What's the cathode?

Correction for ‘Identifying the anionic redox activity in cation-disordered Li1.25Nb0.25Fe0.50O2/C oxide cathodes for Li-ion batteries’ by Mingzeng Luo et al., J. Mater. Chem. A, 2020, 8, 5115–5127, DOI: 10.1039/C9TA11739C.

What's the cathode?

Li1.25Nb0.25Fe0.50O2/C oxide

5.3.2 Coexisting substrates and contaminants. The investigated substrates and contaminants for oily water separation were mainly strong electrolytes such as NaCl, HCl and NaOH. Considering these coexisting chemicals would not significantly change the surface properties of titanate, the added electrolytes were found to have no apparent effects for oil/water separation.

What's the electrolyte?

NaCl, HCl and NaOH

Small molecules. Perylenetetracarboxylic dianhydride (PTCDA 9) was relatively widely studied in potassium batteries. It was first proposed for K-based cells by Hu et al., who tested it with a carbonate-based electrolyte. The material delivered a Qm of 130 mA h g−1 at 10 mA g−1 with an average discharge potential of 2.4 V, while 66% of that capacity was retained after 200 cycles. At 500 mA g−1, the capacity dropped to 73 mA h g−1.

What's the electrolyte?

carbonate-based

After testing the nanomesh electrodes, we compared our results to the performance of various 3D-nanostructured core–shell cathodes reported in the literature (Fig. 6). To do that, we first constructed a Ragone plot to compare volumetric capacity and rate performance of the electrodes (Fig. 6a). Note that in this comparison, the capacities and currents are related to the total 3D volume of the cathodes, that is, including the volume of the 3D current collector, active material and the remaining porosity, but excluding any planar substrate.

What's the cathode?

Finally, the versatility in designing nickel nanomesh with different pore sizes, porosities and surface areas can be used to further optimize the structure of nanomesh cathodes for either very high capacities or yet a higher rate performance. For example, in the electrodes prepared in this work, the 3D current collector occupied 24% of the electrode volume. Although this is in the normal range for 3D cathodes (that have up to 40% volume lost to the current collector, Table S3†), it leaves room for improvement. Using nanomesh with yet a higher porosity (currently, up to 89%) may allow packing more active material into the structure and further increasing its volumetric capacity. If the nanomesh is designed with a proper combination of the nanowire diameter and spacing, the high surface area of the current collector can be preserved even for such highly porous nanomesh, maintaining its benefits in high utilization of active Li-ion materials. Because of its customizable structure, nanomesh can also be used to study the fundamental effects of a 3D structure on the properties of Li-ion electrodes. Knowing that nanomesh can also be fabricated as free-standing and flexible foil, the current work paves the way for future free-standing and stackable nanomesh-based cathodes. This, combined with the upscalable and generally feasible method of their fabrication, can make the cathodes attractive for practical application in high capacity and fast charging Li-ion batteries.

What's the cathode?

nanomesh

Finally, the versatility in designing nickel nanomesh with different pore sizes, porosities and surface areas can be used to further optimize the structure of nanomesh cathodes for either very high capacities or yet a higher rate performance. For example, in the electrodes prepared in this work, the 3D current collector occupied 24% of the electrode volume. Although this is in the normal range for 3D cathodes (that have up to 40% volume lost to the current collector, Table S3†), it leaves room for improvement. Using nanomesh with yet a higher porosity (currently, up to 89%) may allow packing more active material into the structure and further increasing its volumetric capacity. If the nanomesh is designed with a proper combination of the nanowire diameter and spacing, the high surface area of the current collector can be preserved even for such highly porous nanomesh, maintaining its benefits in high utilization of active Li-ion materials. Because of its customizable structure, nanomesh can also be used to study the fundamental effects of a 3D structure on the properties of Li-ion electrodes. Knowing that nanomesh can also be fabricated as free-standing and flexible foil, the current work paves the way for future free-standing and stackable nanomesh-based cathodes. This, combined with the upscalable and generally feasible method of their fabrication, can make the cathodes attractive for practical application in high capacity and fast charging Li-ion batteries.

What's the cathode?

nanomesh-based

Similarly, the cycled S-C(PAN) electrodes paired with bare and coated LMO cathodes were investigated by XPS to confirm the SEI layer components and whether there was dissolved manganese deposited on the anode surface. Fig. 7 shows the XPS spectra of the Mn 2p and O 1s peaks on the S-C(PAN) anodes cycled at 25 °C (bare LMO and LMO-30 min) and 55 °C (LMO-30 min) after 100 cycles using 0.4C-rate. Fig. 7a shows the deposition of manganese on the anode surface when a cell was cycled using a bare LMO cathode and it displays two peaks at 640.1 (Mn2+) and 641.3 eV (Mn3+) for Mn 2p3/2. The decomposition of organic compounds from the electrolyte devastates the original SEI layer and the formation of a new SEI layer on the negative electrode surface in the form of MnxOy occurs. In the case of the LMO-30 min cathode, as shown in Fig. 7b and c, the deposition of manganese on the cycled S-C(PAN) electrode was hardly observed. In addition to XPS, EDS analysis and elemental mapping were also performed. No manganese content was detected on the anode surface when the cell was cycled with the LMO-30 min cathode at 25 °C (Fig. S6†) and LMO-30 min cathode at 55 °C (Fig. S7†). However, the cycled S-C(PAN) anode paired with bare LMO shows Mn distribution in Fig. S8.† As mentioned above, the PVDF@LGLZNO fibrous film protects the cathode and reduces the direct contact of the LMO cathode and the electrolyte. The peak of Mn was absent in the PVDF@LGLZNO coated LMO cathodes. DFT calculation further suggests that the dissolved manganese ion can be adsorbed on the garnet surface, whereby the adsorption energy of Mn2+ on the surface of the garnet was calculated as −53.067 kJ mol−1. Details about the estimation can be referred to in the ESI and Fig. S9.†

What's the anode?

S-C(PAN)

Similarly, the cycled S-C(PAN) electrodes paired with bare and coated LMO cathodes were investigated by XPS to confirm the SEI layer components and whether there was dissolved manganese deposited on the anode surface. Fig. 7 shows the XPS spectra of the Mn 2p and O 1s peaks on the S-C(PAN) anodes cycled at 25 °C (bare LMO and LMO-30 min) and 55 °C (LMO-30 min) after 100 cycles using 0.4C-rate. Fig. 7a shows the deposition of manganese on the anode surface when a cell was cycled using a bare LMO cathode and it displays two peaks at 640.1 (Mn2+) and 641.3 eV (Mn3+) for Mn 2p3/2. The decomposition of organic compounds from the electrolyte devastates the original SEI layer and the formation of a new SEI layer on the negative electrode surface in the form of MnxOy occurs. In the case of the LMO-30 min cathode, as shown in Fig. 7b and c, the deposition of manganese on the cycled S-C(PAN) electrode was hardly observed. In addition to XPS, EDS analysis and elemental mapping were also performed. No manganese content was detected on the anode surface when the cell was cycled with the LMO-30 min cathode at 25 °C (Fig. S6†) and LMO-30 min cathode at 55 °C (Fig. S7†). However, the cycled S-C(PAN) anode paired with bare LMO shows Mn distribution in Fig. S8.† As mentioned above, the PVDF@LGLZNO fibrous film protects the cathode and reduces the direct contact of the LMO cathode and the electrolyte. The peak of Mn was absent in the PVDF@LGLZNO coated LMO cathodes. DFT calculation further suggests that the dissolved manganese ion can be adsorbed on the garnet surface, whereby the adsorption energy of Mn2+ on the surface of the garnet was calculated as −53.067 kJ mol−1. Details about the estimation can be referred to in the ESI and Fig. S9.†

What's the anode?

S-C(PAN)

Liquid rechargeable Zn–air batteries were assembled with a homemade cell. A zinc sheet (purity 99.9 wt%) was used as the anode, which was polished with sandpaper before use. A mixed solution of 6 M KOH and 0.2 M Zn(Ac)2 was used as the electrolyte. The air cathode was composed of three layers, a catalyst layer, a nickel foam layer and a gas diffusion layer. The catalytic layer was fabricated by homogeneously mixing the NCAG/Fe–Co catalyst, acetylene black, PTFE (60 wt%) at the mass ratio of 6:1:3. The nickel foam was treated with 0.2 M HCl, water and ethanol for 20 min, successively, and then vacuum dried at 60 °C before use. The three layers were compressed with a roll press to obtain the cathode, which was then vacuum dried at 60 °C for 3 h.

What's the cathode?

Liquid rechargeable Zn–air batteries were assembled with a homemade cell. A zinc sheet (purity 99.9 wt%) was used as the anode, which was polished with sandpaper before use. A mixed solution of 6 M KOH and 0.2 M Zn(Ac)2 was used as the electrolyte. The air cathode was composed of three layers, a catalyst layer, a nickel foam layer and a gas diffusion layer. The catalytic layer was fabricated by homogeneously mixing the NCAG/Fe–Co catalyst, acetylene black, PTFE (60 wt%) at the mass ratio of 6:1:3. The nickel foam was treated with 0.2 M HCl, water and ethanol for 20 min, successively, and then vacuum dried at 60 °C before use. The three layers were compressed with a roll press to obtain the cathode, which was then vacuum dried at 60 °C for 3 h.

What's the anode?

zinc sheet

Liquid rechargeable Zn–air batteries were assembled with a homemade cell. A zinc sheet (purity 99.9 wt%) was used as the anode, which was polished with sandpaper before use. A mixed solution of 6 M KOH and 0.2 M Zn(Ac)2 was used as the electrolyte. The air cathode was composed of three layers, a catalyst layer, a nickel foam layer and a gas diffusion layer. The catalytic layer was fabricated by homogeneously mixing the NCAG/Fe–Co catalyst, acetylene black, PTFE (60 wt%) at the mass ratio of 6:1:3. The nickel foam was treated with 0.2 M HCl, water and ethanol for 20 min, successively, and then vacuum dried at 60 °C before use. The three layers were compressed with a roll press to obtain the cathode, which was then vacuum dried at 60 °C for 3 h.

What's the electrolyte?

A mixed solution of 6 M KOH and 0.2 M Zn(Ac)2

The non-aqueous gel polymer electrolyte (GPE) with dual redox additives was prepared by the “solution-cast” technique. Solid pellets of the host polymer PVdF-HFP (1 g) were put in 20 ml acetone and allowed to dissolve properly at room temperature by continuous stirring on a magnetic stirrer for 12 hours. Thereafter, 4 g IL was added in the polymer solution and stirred for another 12 hours. Redox additives (0.04 g KI) and (0.04 g DPA) were added in the PVdF-HFP/IL solution and stirred for further 12 hours at room temperature to obtain a homogeneous and clear solution. The solution was then poured in a glass petri dish and the common solvent acetone was allowed to evaporate slowly at room temperature. Upon complete evaporation of acetone, a free-standing, flexible and mechanically stable GPE film was obtained. The thickness of the dual redox-active GPE film was ∼400 μm. For the comparison, GPEs of compositions PVdF-HFP:IL (20:80 w/w), and PVdF-HFP/IL/DPA and PVdF-HFP/IL/KI with different amounts of redox additives were also prepared by the same procedure, as described above. These GPEs were stored in a dry atmosphere to avoid moisture adsorption. These GPE films were characterized by electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and thermogravimetric analysis (TGA). The room temperature ionic conductivity of all the GPEs was measured by EIS in the frequency range from 105 Hz to 1 Hz. The electrochemical stability window (ESW) for each GPE film was measured by LSV at a scan rate of 5 mV s−1. EIS and LSV were performed on an electrochemical analyzer (CHI660E, CH Instruments, USA). TGA was performed on a Perkin Elmer (USA) TGA-7 system from room temperature to 600 °C at a heating rate of 10 °C min−1 in a N2 atmosphere.

What's the electrolyte?

non-aqueous gel polymer

The non-aqueous gel polymer electrolyte (GPE) with dual redox additives was prepared by the “solution-cast” technique. Solid pellets of the host polymer PVdF-HFP (1 g) were put in 20 ml acetone and allowed to dissolve properly at room temperature by continuous stirring on a magnetic stirrer for 12 hours. Thereafter, 4 g IL was added in the polymer solution and stirred for another 12 hours. Redox additives (0.04 g KI) and (0.04 g DPA) were added in the PVdF-HFP/IL solution and stirred for further 12 hours at room temperature to obtain a homogeneous and clear solution. The solution was then poured in a glass petri dish and the common solvent acetone was allowed to evaporate slowly at room temperature. Upon complete evaporation of acetone, a free-standing, flexible and mechanically stable GPE film was obtained. The thickness of the dual redox-active GPE film was ∼400 μm. For the comparison, GPEs of compositions PVdF-HFP:IL (20:80 w/w), and PVdF-HFP/IL/DPA and PVdF-HFP/IL/KI with different amounts of redox additives were also prepared by the same procedure, as described above. These GPEs were stored in a dry atmosphere to avoid moisture adsorption. These GPE films were characterized by electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and thermogravimetric analysis (TGA). The room temperature ionic conductivity of all the GPEs was measured by EIS in the frequency range from 105 Hz to 1 Hz. The electrochemical stability window (ESW) for each GPE film was measured by LSV at a scan rate of 5 mV s−1. EIS and LSV were performed on an electrochemical analyzer (CHI660E, CH Instruments, USA). TGA was performed on a Perkin Elmer (USA) TGA-7 system from room temperature to 600 °C at a heating rate of 10 °C min−1 in a N2 atmosphere.

What's the electrolyte?

GPE

Carbon surfaces often exhibit poor electrochemical characteristics and therefore need special pretreatment, frequently termed as “activation”. For surface sensitive redox couples the electron transfer at carbon electrodes may depend on edge plane exposure, surface functional groups and cleanliness. Porous electrode structures, such as carbon felts and carbon papers composed of randomly oriented CFs, also suffer from poor electrolyte accessibility, high pressure drops within the cell, and non-uniform electrolyte velocities. These issues limit the mass transport, for instance in redox flow cells, and thus their electrochemical performance. Some improvements have been achieved by modifying the cell architecture (by adding flow fields) and the electrode design. Another strategy is to employ electrodes that allow for control of the orientation of their fibres along the electrolyte flow.

What's the electrolyte?

Owing to their attractive high carrier mobility, ambient stability, superior mechanical flexibility, large band gap and excellent optical properties, group IV–V compounds, as a new kind of 2D materials, show a promising potential application for optoelectronic devices. Herein, 2D GeP nanosheets were exfoliated by a facile LPE method and a photoelectrochemical (PEC)-type photodetector employing a doctor blade deposited 2D GeP nanosheet electrode on an ITO-coated glass was fabricated. Ultrafast carrier dynamics was carefully probed by transient absorption spectroscopy, and the parameters of the photodetector, such as the voltage and electrolyte concentration, were highly optimized. It shows remarkable performance with a responsivity of 187.5 μA W−1, a detectivity of 2.14 × 1012 Jones and an EQE of 61.3% at an ultraviolet wavelength of 380 nm. The proposed strategy avoids complicated material preparation and device fabrication and facilitates large-area photodetection.

What's the electrolyte?

In the context of reversibility, an excellent 1st cycle coulombic efficiency (CE) of ∼91% could be obtained with the MWCNT containing electrode, which is much higher than a CE of ∼78% recorded for the non-MWCNT containing counterpart (as reported in Section 3.3). Here, it may be mentioned that in another recently published study from our group (viz., ) addition of similarly functionalized MWCNTs to Na-titanate in the same way also enhanced the CE and cyclic stability in significant terms. In that study it was observed that the MWCNT network tends to uniformly ‘wrap’ the electrode-active particles due to favourable surface interaction, which, in turn, suppressed the occurrence of deleterious irreversible surface reactions with the electrolyte and, thus, improved the electrochemical performances. Even though looking more closely into such aspects here is beyond the scope of the presently reported work per se, it is not unlikely that the above mechanism may also be relevant to our Na-TM-oxide – MWCNT electrodes. On a slightly different note, a closer look at Fig. 3a, 7a and S9 (in the ESI†) indicates that the overpotential (or polarization) associated with the initiation of electrochemical desodiation gets lowered in the presence of MWCNTs. This may, in turn, be a manifestation of the improved conductivity bestowed by the MWCNT network, which assumes greater importance here due to the presence of d0 TM-ions in the concerned Na-TM-oxide.

What's the electrolyte?

FEC and DMC were purchased from SoulBrain Corp. LiPF6 was purchased from BASF Corp. Li chips (450 μm) were purchased from MTI Corp. The baseline electrolyte is 1 M LiPF6 + DMC/FEC (v/v = 8/2). The high fluorine content can provide a relatively robust electrolyte/electrode interface toward aggressive chemistries under high potentials. The electrolyte used for the protection failure study was commercial carbonate electrolyte: 1 M LiPF6 + EC/EMC (v/v = 4/6). TDAC·PF6 crystal powder was dissolved in baseline electrolyte to fabricate 0.2 M TDAC electrolyte. The NCA cathode, LMO cathode and graphite anode were composed of the active material, polyvinylidene fluoride (PVDF) binder and Super C carbon in a mass ratio of 90:5:5. The areal capacity of the NCA cathode, LMO cathode and graphite anode is 2.2 mA h cm−2. The NCA cathode and silicon–graphene (Si–C) anode sheets, with an areal capacity of ∼3 mA h cm−2, were kindly provided by NanoGraf Corp. The formulation of the anode sheet was the Si–C active material (mass ratio of 45:55), PVDF binder and Super C carbon in a mass ratio of 75:5:20. The loadings of the NCA cathode and Si–C anode were ∼18–19 mg cm−2 and ∼3–4 mg cm−2, respectively. All electrode sheets were dried under vacuum at 80 °C overnight before use.

What's the cathode?

LMO

FEC and DMC were purchased from SoulBrain Corp. LiPF6 was purchased from BASF Corp. Li chips (450 μm) were purchased from MTI Corp. The baseline electrolyte is 1 M LiPF6 + DMC/FEC (v/v = 8/2). The high fluorine content can provide a relatively robust electrolyte/electrode interface toward aggressive chemistries under high potentials. The electrolyte used for the protection failure study was commercial carbonate electrolyte: 1 M LiPF6 + EC/EMC (v/v = 4/6). TDAC·PF6 crystal powder was dissolved in baseline electrolyte to fabricate 0.2 M TDAC electrolyte. The NCA cathode, LMO cathode and graphite anode were composed of the active material, polyvinylidene fluoride (PVDF) binder and Super C carbon in a mass ratio of 90:5:5. The areal capacity of the NCA cathode, LMO cathode and graphite anode is 2.2 mA h cm−2. The NCA cathode and silicon–graphene (Si–C) anode sheets, with an areal capacity of ∼3 mA h cm−2, were kindly provided by NanoGraf Corp. The formulation of the anode sheet was the Si–C active material (mass ratio of 45:55), PVDF binder and Super C carbon in a mass ratio of 75:5:20. The loadings of the NCA cathode and Si–C anode were ∼18–19 mg cm−2 and ∼3–4 mg cm−2, respectively. All electrode sheets were dried under vacuum at 80 °C overnight before use.

What's the anode?

graphite

FEC and DMC were purchased from SoulBrain Corp. LiPF6 was purchased from BASF Corp. Li chips (450 μm) were purchased from MTI Corp. The baseline electrolyte is 1 M LiPF6 + DMC/FEC (v/v = 8/2). The high fluorine content can provide a relatively robust electrolyte/electrode interface toward aggressive chemistries under high potentials. The electrolyte used for the protection failure study was commercial carbonate electrolyte: 1 M LiPF6 + EC/EMC (v/v = 4/6). TDAC·PF6 crystal powder was dissolved in baseline electrolyte to fabricate 0.2 M TDAC electrolyte. The NCA cathode, LMO cathode and graphite anode were composed of the active material, polyvinylidene fluoride (PVDF) binder and Super C carbon in a mass ratio of 90:5:5. The areal capacity of the NCA cathode, LMO cathode and graphite anode is 2.2 mA h cm−2. The NCA cathode and silicon–graphene (Si–C) anode sheets, with an areal capacity of ∼3 mA h cm−2, were kindly provided by NanoGraf Corp. The formulation of the anode sheet was the Si–C active material (mass ratio of 45:55), PVDF binder and Super C carbon in a mass ratio of 75:5:20. The loadings of the NCA cathode and Si–C anode were ∼18–19 mg cm−2 and ∼3–4 mg cm−2, respectively. All electrode sheets were dried under vacuum at 80 °C overnight before use.

What's the electrolyte?

1 M LiPF6 + DMC/FEC

FEC and DMC were purchased from SoulBrain Corp. LiPF6 was purchased from BASF Corp. Li chips (450 μm) were purchased from MTI Corp. The baseline electrolyte is 1 M LiPF6 + DMC/FEC (v/v = 8/2). The high fluorine content can provide a relatively robust electrolyte/electrode interface toward aggressive chemistries under high potentials. The electrolyte used for the protection failure study was commercial carbonate electrolyte: 1 M LiPF6 + EC/EMC (v/v = 4/6). TDAC·PF6 crystal powder was dissolved in baseline electrolyte to fabricate 0.2 M TDAC electrolyte. The NCA cathode, LMO cathode and graphite anode were composed of the active material, polyvinylidene fluoride (PVDF) binder and Super C carbon in a mass ratio of 90:5:5. The areal capacity of the NCA cathode, LMO cathode and graphite anode is 2.2 mA h cm−2. The NCA cathode and silicon–graphene (Si–C) anode sheets, with an areal capacity of ∼3 mA h cm−2, were kindly provided by NanoGraf Corp. The formulation of the anode sheet was the Si–C active material (mass ratio of 45:55), PVDF binder and Super C carbon in a mass ratio of 75:5:20. The loadings of the NCA cathode and Si–C anode were ∼18–19 mg cm−2 and ∼3–4 mg cm−2, respectively. All electrode sheets were dried under vacuum at 80 °C overnight before use.

What's the cathode?

NCA

FEC and DMC were purchased from SoulBrain Corp. LiPF6 was purchased from BASF Corp. Li chips (450 μm) were purchased from MTI Corp. The baseline electrolyte is 1 M LiPF6 + DMC/FEC (v/v = 8/2). The high fluorine content can provide a relatively robust electrolyte/electrode interface toward aggressive chemistries under high potentials. The electrolyte used for the protection failure study was commercial carbonate electrolyte: 1 M LiPF6 + EC/EMC (v/v = 4/6). TDAC·PF6 crystal powder was dissolved in baseline electrolyte to fabricate 0.2 M TDAC electrolyte. The NCA cathode, LMO cathode and graphite anode were composed of the active material, polyvinylidene fluoride (PVDF) binder and Super C carbon in a mass ratio of 90:5:5. The areal capacity of the NCA cathode, LMO cathode and graphite anode is 2.2 mA h cm−2. The NCA cathode and silicon–graphene (Si–C) anode sheets, with an areal capacity of ∼3 mA h cm−2, were kindly provided by NanoGraf Corp. The formulation of the anode sheet was the Si–C active material (mass ratio of 45:55), PVDF binder and Super C carbon in a mass ratio of 75:5:20. The loadings of the NCA cathode and Si–C anode were ∼18–19 mg cm−2 and ∼3–4 mg cm−2, respectively. All electrode sheets were dried under vacuum at 80 °C overnight before use.

What's the anode?

silicon–graphene (Si–C)

FEC and DMC were purchased from SoulBrain Corp. LiPF6 was purchased from BASF Corp. Li chips (450 μm) were purchased from MTI Corp. The baseline electrolyte is 1 M LiPF6 + DMC/FEC (v/v = 8/2). The high fluorine content can provide a relatively robust electrolyte/electrode interface toward aggressive chemistries under high potentials. The electrolyte used for the protection failure study was commercial carbonate electrolyte: 1 M LiPF6 + EC/EMC (v/v = 4/6). TDAC·PF6 crystal powder was dissolved in baseline electrolyte to fabricate 0.2 M TDAC electrolyte. The NCA cathode, LMO cathode and graphite anode were composed of the active material, polyvinylidene fluoride (PVDF) binder and Super C carbon in a mass ratio of 90:5:5. The areal capacity of the NCA cathode, LMO cathode and graphite anode is 2.2 mA h cm−2. The NCA cathode and silicon–graphene (Si–C) anode sheets, with an areal capacity of ∼3 mA h cm−2, were kindly provided by NanoGraf Corp. The formulation of the anode sheet was the Si–C active material (mass ratio of 45:55), PVDF binder and Super C carbon in a mass ratio of 75:5:20. The loadings of the NCA cathode and Si–C anode were ∼18–19 mg cm−2 and ∼3–4 mg cm−2, respectively. All electrode sheets were dried under vacuum at 80 °C overnight before use.

What's the electrolyte?

0.2 M TDAC

In summary, we demonstrated the use of a multicomponent electrolyte containing sodium perchlorate, urea, N,N-dimethylformamide (DMF) and water, enabling us to immensely decrease the amount of water, forming a complex solvent sheath and a uniform SEI layer composed of complexes with inorganic salt Na2CO3 and other organic components between the interface of the NTP anode and electrolyte. Such an electrolyte provides a voltage window up to 2.8 V and ensures the feasibility of stable and reversible operation of a NVP/NTP sodium ion battery. This battery exhibits an excellent flat voltage platform of about 1.2 V and achieves 86% capacity retention after 100 cycles at the 10C rate. Simultaneously, a NiHCF//NTP full cell with the MCAE displays 80% capacity retention after 2000 cycles at 2C rate. In addition, this MCAE displays high safety, a wide operating temperature range (−50 °C to 50 °C) and high ionic conductivity because of the roles of urea and DMF additives. And above all, the fluorine-free MCAE makes the battery much safer and more environmentally friendly than the existing battery systems with highly concentrated aqueous electrolytes. Meanwhile, we provide a new perspective that can be expanded to other systems of aqueous sodium-ion full cells, which promotes the application of safe, environmentally friendly and stable AISBs for large-scale energy storage.

What's the anode?

NTP

In summary, we demonstrated the use of a multicomponent electrolyte containing sodium perchlorate, urea, N,N-dimethylformamide (DMF) and water, enabling us to immensely decrease the amount of water, forming a complex solvent sheath and a uniform SEI layer composed of complexes with inorganic salt Na2CO3 and other organic components between the interface of the NTP anode and electrolyte. Such an electrolyte provides a voltage window up to 2.8 V and ensures the feasibility of stable and reversible operation of a NVP/NTP sodium ion battery. This battery exhibits an excellent flat voltage platform of about 1.2 V and achieves 86% capacity retention after 100 cycles at the 10C rate. Simultaneously, a NiHCF//NTP full cell with the MCAE displays 80% capacity retention after 2000 cycles at 2C rate. In addition, this MCAE displays high safety, a wide operating temperature range (−50 °C to 50 °C) and high ionic conductivity because of the roles of urea and DMF additives. And above all, the fluorine-free MCAE makes the battery much safer and more environmentally friendly than the existing battery systems with highly concentrated aqueous electrolytes. Meanwhile, we provide a new perspective that can be expanded to other systems of aqueous sodium-ion full cells, which promotes the application of safe, environmentally friendly and stable AISBs for large-scale energy storage.

What's the electrolyte?

sodium perchlorate, urea, N,N-dimethylformamide (DMF) and wate

The electrochemical properties of the prepared α-MnS@NS-C samples were evaluated with lithium metal as the reference electrode. For electrochemical measurements, the obtained α-MnS@NS-C material was mixed with carbon black (SuperP) and polyacrylic acid binder in the stoichiometric ratio of 80:10:10. This slurry was coated on Cu foil and vacuum dried at 120 °C for 12 h to form the anode. An active material loading of 1.5 mg cm−2 was used. A 2032-coin type cell comprising a cathode and lithium metal anode separated by a polymer membrane (Celgard 2400) and a glass fiber (Whatman GF/B) was fabricated in an Ar-filled glove box and aged for 12 h before electrochemical measurements. The electrolyte employed was a 1:1 mixture of ethylene carbonate and dimethyl carbonate containing 1 M LiPF6. Galvanostatic measurements were performed on test coin cells using a programmable battery tester (BTS-2004H, Nagano, Japan) over the potential range of 0.01–3.0 V versus Li+/Li. Cyclic voltammetry (CV) measurements were performed using a Biologic instrument within the voltage range of 0.01–3.0 V and a scan rate of 0.1–1.0 mV s−1.

What's the cathode?

The electrochemical properties of the prepared α-MnS@NS-C samples were evaluated with lithium metal as the reference electrode. For electrochemical measurements, the obtained α-MnS@NS-C material was mixed with carbon black (SuperP) and polyacrylic acid binder in the stoichiometric ratio of 80:10:10. This slurry was coated on Cu foil and vacuum dried at 120 °C for 12 h to form the anode. An active material loading of 1.5 mg cm−2 was used. A 2032-coin type cell comprising a cathode and lithium metal anode separated by a polymer membrane (Celgard 2400) and a glass fiber (Whatman GF/B) was fabricated in an Ar-filled glove box and aged for 12 h before electrochemical measurements. The electrolyte employed was a 1:1 mixture of ethylene carbonate and dimethyl carbonate containing 1 M LiPF6. Galvanostatic measurements were performed on test coin cells using a programmable battery tester (BTS-2004H, Nagano, Japan) over the potential range of 0.01–3.0 V versus Li+/Li. Cyclic voltammetry (CV) measurements were performed using a Biologic instrument within the voltage range of 0.01–3.0 V and a scan rate of 0.1–1.0 mV s−1.

What's the anode?

lithium metal

The electrochemical properties of the prepared α-MnS@NS-C samples were evaluated with lithium metal as the reference electrode. For electrochemical measurements, the obtained α-MnS@NS-C material was mixed with carbon black (SuperP) and polyacrylic acid binder in the stoichiometric ratio of 80:10:10. This slurry was coated on Cu foil and vacuum dried at 120 °C for 12 h to form the anode. An active material loading of 1.5 mg cm−2 was used. A 2032-coin type cell comprising a cathode and lithium metal anode separated by a polymer membrane (Celgard 2400) and a glass fiber (Whatman GF/B) was fabricated in an Ar-filled glove box and aged for 12 h before electrochemical measurements. The electrolyte employed was a 1:1 mixture of ethylene carbonate and dimethyl carbonate containing 1 M LiPF6. Galvanostatic measurements were performed on test coin cells using a programmable battery tester (BTS-2004H, Nagano, Japan) over the potential range of 0.01–3.0 V versus Li+/Li. Cyclic voltammetry (CV) measurements were performed using a Biologic instrument within the voltage range of 0.01–3.0 V and a scan rate of 0.1–1.0 mV s−1.

What's the electrolyte?

1:1 mixture of ethylene carbonate and dimethyl carbonate containing 1 M LiPF6

Fig. 1a schematically shows the composition of the light-assisted Li–CO2 battery, which consisted of a Li anode, a non-aqueous electrolyte, a separator, and a cathode with the photo-electro-catalyst. Upon discharging under illumination (Fig. 1b), the photocatalyst is excited to generate holes and electrons after absorbing photons. Since the conduction band potential (VCB) of the photocatalyst is more negative than the standard redox potential of 2.8 V, photoexcited electrons could reduce CO2 by a photocatalytic reaction. Notably, the solar light provides significant energy for CO2 reduction in sharp contrast with the conventional Li–CO2 battery, in which the carbon dioxide reduction reaction (CO2RR) process is driven by the electrocatalytic reaction. Meanwhile, photoexcited holes capture the electrons from the Li anode through the external circuit due to the more positive valence band potential (VVB) than 2.8 V, ensuring the continuous supply of photoexcited electrons for the CO2RR process. During the charging process (Fig. 1c), the inverse reaction 4Li + 3CO2 → 2Li2CO3 + C is promoted by the photoexcited holes on the cathode on account of the positive VB position compared with 2.8 V. Simultaneously, the photoexcited electrons transfer to the anode through the external circuit to reduce Li+ to Li metal. In this photoassisted charging process, the photovoltage generated on the photo-catalyst is utilized to compensate the charging potential of the Li–CO2 battery.

What's the cathode?

Fig. 1a schematically shows the composition of the light-assisted Li–CO2 battery, which consisted of a Li anode, a non-aqueous electrolyte, a separator, and a cathode with the photo-electro-catalyst. Upon discharging under illumination (Fig. 1b), the photocatalyst is excited to generate holes and electrons after absorbing photons. Since the conduction band potential (VCB) of the photocatalyst is more negative than the standard redox potential of 2.8 V, photoexcited electrons could reduce CO2 by a photocatalytic reaction. Notably, the solar light provides significant energy for CO2 reduction in sharp contrast with the conventional Li–CO2 battery, in which the carbon dioxide reduction reaction (CO2RR) process is driven by the electrocatalytic reaction. Meanwhile, photoexcited holes capture the electrons from the Li anode through the external circuit due to the more positive valence band potential (VVB) than 2.8 V, ensuring the continuous supply of photoexcited electrons for the CO2RR process. During the charging process (Fig. 1c), the inverse reaction 4Li + 3CO2 → 2Li2CO3 + C is promoted by the photoexcited holes on the cathode on account of the positive VB position compared with 2.8 V. Simultaneously, the photoexcited electrons transfer to the anode through the external circuit to reduce Li+ to Li metal. In this photoassisted charging process, the photovoltage generated on the photo-catalyst is utilized to compensate the charging potential of the Li–CO2 battery.

What's the anode?

Li

Fig. 1a schematically shows the composition of the light-assisted Li–CO2 battery, which consisted of a Li anode, a non-aqueous electrolyte, a separator, and a cathode with the photo-electro-catalyst. Upon discharging under illumination (Fig. 1b), the photocatalyst is excited to generate holes and electrons after absorbing photons. Since the conduction band potential (VCB) of the photocatalyst is more negative than the standard redox potential of 2.8 V, photoexcited electrons could reduce CO2 by a photocatalytic reaction. Notably, the solar light provides significant energy for CO2 reduction in sharp contrast with the conventional Li–CO2 battery, in which the carbon dioxide reduction reaction (CO2RR) process is driven by the electrocatalytic reaction. Meanwhile, photoexcited holes capture the electrons from the Li anode through the external circuit due to the more positive valence band potential (VVB) than 2.8 V, ensuring the continuous supply of photoexcited electrons for the CO2RR process. During the charging process (Fig. 1c), the inverse reaction 4Li + 3CO2 → 2Li2CO3 + C is promoted by the photoexcited holes on the cathode on account of the positive VB position compared with 2.8 V. Simultaneously, the photoexcited electrons transfer to the anode through the external circuit to reduce Li+ to Li metal. In this photoassisted charging process, the photovoltage generated on the photo-catalyst is utilized to compensate the charging potential of the Li–CO2 battery.

What's the electrolyte?

Fig. 1a schematically shows the composition of the light-assisted Li–CO2 battery, which consisted of a Li anode, a non-aqueous electrolyte, a separator, and a cathode with the photo-electro-catalyst. Upon discharging under illumination (Fig. 1b), the photocatalyst is excited to generate holes and electrons after absorbing photons. Since the conduction band potential (VCB) of the photocatalyst is more negative than the standard redox potential of 2.8 V, photoexcited electrons could reduce CO2 by a photocatalytic reaction. Notably, the solar light provides significant energy for CO2 reduction in sharp contrast with the conventional Li–CO2 battery, in which the carbon dioxide reduction reaction (CO2RR) process is driven by the electrocatalytic reaction. Meanwhile, photoexcited holes capture the electrons from the Li anode through the external circuit due to the more positive valence band potential (VVB) than 2.8 V, ensuring the continuous supply of photoexcited electrons for the CO2RR process. During the charging process (Fig. 1c), the inverse reaction 4Li + 3CO2 → 2Li2CO3 + C is promoted by the photoexcited holes on the cathode on account of the positive VB position compared with 2.8 V. Simultaneously, the photoexcited electrons transfer to the anode through the external circuit to reduce Li+ to Li metal. In this photoassisted charging process, the photovoltage generated on the photo-catalyst is utilized to compensate the charging potential of the Li–CO2 battery.

What's the anode?

Li

The energy and power of all the cells are also compared in the form of Ragone plots (i.e., the plots of specific energy against effective power density), as shown in Fig. 7f. All the capacitor cells show a standard variation of Ragone plots, which are generally observed for the power sources like supercapacitors. The effect of incorporating redox additives (DPA/KI, individually or dual) in the electrolyte component can be directly observed on the energy and power of the capacitor cells. It may be noticed that the enhancement in specific energy due to the incorporation of redox additives has been observed for each value of power density (Fig. 7f). The optimum value of Esp is found to be ∼73 W h kg−1 at a Peff of ∼568 W kg−1 for Cell#4 (with the redox-active GPE containing dual redox additives). The high specific energy of this carbon supercapacitor is not only due to high specific capacitance owing to dual redox activity at the interfaces (according to Scheme (3), mentioned above), it is also due to relatively high operating voltage window (∼2.5 V) of electrochemically stable IL-incorporated, non-aqueous gel polymer films as electrolytes. The specific energy values of the present capacitor cell (Cell#4) are substantially higher than or equivalent to that of many reported carbon supercapacitors based on redox-active non-aqueous electrolytes including GPEs. A comparison of the present system with similar devices, reported in the literature, is given in Table 3.

What's the electrolyte?

non-aqueous gel polymer films

To test the cyclability of columnar lithium metal during cell operation, operando GISAXS was used to probe the morphology of lithium metal deposits during cycling. GISAXS, being sensitive to length scales on the order of a few to a few hundred nanometers, exhibits a distinct set of evenly spaced intensity oscillations in the qy direction (Fig. 12) arising from the highly monodispersed columnar microstructure, with column diameters of ∼100–200 nm depending on current density (Fig. S8†); the periodicity Δqy of these oscillations is related to the column diameter d by Δqy = 2π/d. Oscillations in the qz direction arise from interference between X-rays scattered from the lithium–electrolyte and lithium–copper interfaces, complicated by multiple scattering and indicate the top surface of the film is very smooth. At the end of plating 1 mA h cm−2 of lithium, strong intensity oscillations are apparent in the qy direction, indicative of a columnar morphology (Fig. 12c). After stripping half of the plated capacity (0.5 mA h cm−2), the intensity oscillations are still present, though broader and less pronounced, indicating the columnar morphology is maintained but is less monodisperse in size and shape (Fig. 12d). At the end of the second plating half cycle the oscillations are even less pronounced (Fig. 12e), suggesting that cycling a large percentage of the plated 1 mA h cm−2 capacity, in this case 50%, with a columnar morphology leads to a loss of uniformity in the microstructure. This is further evidenced by the fact that if all the columnar lithium is stripped to a 1 V vs. Li/Li+ cutoff voltage, the second plating half cycle does not produce a columnar morphology (Fig. S9†). This is consistent with the report from Zhang et al. which demonstrated using ex situ SEM that lithium is stripped from both the tops and sides of lithium nanorods, enabled by the high lithium-ion diffusivity of the SEI, disrupting uniform re-plating onto the high aspect ratio columns. This limits the cycle life of practical batteries which must uniformly plate and strip high capacities of lithium during each charge/discharge cycle, particularly in “anode-free” configurations. Thus, further understanding the impact of additional conditions, including electrochemical profile and mechanical compression (stack pressure), which could help promote the growth of columns with a lower aspect ratio (disc-like rather than rod-like), will be crucial to maintaining morphology control after hundreds of cycles and enhancing the long term cyclability of lithium metal batteries. The effects of compression, the presence of a separator, and lower volume of electrolyte, in particular, require additional investigation as the behavior observed using the flooded open cell design employed in this study may not be directly transferable to a commercial-type battery.

What's the electrolyte?

To investigate the electrochemical reaction mechanism of Zn2+ in the NV NSs@ACC//Zn battery, ex situ XRD and XPS analyses are carried out. Fig. 5a shows the different charge/discharge states during the 2nd cycle at 0.1 A g−1. During the discharge process, it can be observed that a small amount of (NH4)2V10O25·8H2O transforms to the new phase of Zn3(OH)2V2O7·2H2O (JCPDS no. 50-0570) (Fig. 5b), which could be attributed to H+/Zn2+-co-insertion into the VO layers. The peaks between the range of 10–35° for the samples cha.0.8 V and cha.1.4 V were more obvious than that for the discharge process. This is attributed to the XRD pattern of Zn(OH)2 impurity, which could arise from the deposited Zn(OH)2@glass fiber adhered on the surface of the sample. In addition, as can be seen in Fig. 5c, the peak for the (001) plane evidently shifts to a lower degree from 8.3 to 6.7° after immersion in the electrolyte for 1 h, corresponding to an increase in the interlayer spacing from 10.6 Å to 13.2 Å. Note that the interlayer distance during the subsequent discharge/charge process is always larger than that of the pristine sample, revealing that the expansion of the layer spacing upon immersion arose from water molecule intercalation, which can weaken the electrostatic attraction and thus facilitate the migration of multivalent Zn2+. More significantly, the diffraction peak (001) of NV NSs@ACC shifts to a lower degree during the discharge process and almost moves back to its original position during the charge process. The d-spacing of the (001) plane in different samples corresponding to the ex situ XRD patterns are presented in Table S4.† These results indicate the highly reversible Zn2+ intercalation/deintercalation of the NV host without the destruction of its open-framework during the subsequent discharge/charge process.

What's the electrolyte?

The small thickness of an active material and its high contact area with the current collector and electrolyte can be simultaneously realized using 3D-structured conductive substrates. The higher surface area of a 3D-current collector can allow distributing the active material over a thinner coating (or, similarly, increasing the loading of the active material while keeping its thickness low), simultaneously maintaining open porosity for the electrolyte. Such core–shell, structurally integral electrode design can be achieved with a variety of micro- and nanostructured 3D substrates, such as micropillars, nanoporous metals, inverse metal opals or interconnected metal nanowires, which have a big advantage over macroscopic 3D foams in effective utilization of the electrode volume. Compared to nanostructured electrodes based on nanoparticles embedded in binding and conductive additives, the electrodes based on monolithic 3D structures are typically less tortious (translating to lower polarization resistance) and can be often fabricated with high spatial precision (e.g. by lithography or area-selective electrodeposition), making them particularly attractive for all-solid-state microbatteries.

What's the electrolyte?

Fig. 2a shows the photographs of the PLB-CSE; the as-prepared membrane is free-standing and exhibits excellent ductility and flexibility. It is worth noting that the composite electrolyte membrane with such a high proportion of inorganic ceramic particles still exhibits great flexibility, and no cracks could be found in the electrolyte films upon bending, indicating an excellent mechanical capability. This flexible and strong PLB-CSE membrane can be used in flexible devices. To further analyze the microstructure morphologies and the distribution of LATP inorganic nanoparticles, the SEM image is presented in Fig. 2b. The SEM image shows a flat and homogeneous surface free of pores for the electrolyte membrane; the LATP ceramic fillers are uniformly dispersed in the polymer matrix. Fig. 2c displays the cross-sectional features of the PLB-CSE membrane, the thickness of which is about 60 μm, and the corresponding EDS mapping images of titanium (Ti), phosphorus (P), sulfur (S), and fluorine (F) are given in Fig. 2d–g. It can be observed that the LATP ceramic particles are uniformly dispersed in the electrolyte membrane, and the homogeneous LATP inorganic particles are in close contact with each other forming a continuous network structure, allowing Li-ion transfer between LATP ceramic grains, enabling a fast and efficient conductive pathway, which can promote ion transportation within the composite electrolyte.

What's the electrolyte?

Zero-valent nonmetal engineering can also be extended to P-regulated metal nitrides and carbides. Fu et al. selected small-sized tungsten nitride (WN; ∼3 nm) as the model to investigate the function of P dopants. The experimental results confirmed that P modification could lead to significant enhancements in catalytic activity and stability for HER in acidic media, particularly with low onset overpotential of 46 mV, Tafel slope of 54 mV dec−1, large exchange current density of 0.35 mA cm−2, and long-term stability. The enhanced performance could be attributed to the interaction of P with WN, which can increase the work function. Different from the earlier reported uncontrolled P-doping strategy, Tang et al. fabricated hierarchical nanowires comprising P-doped MoC2 nanoparticles via the controlled carbonization of ternary organic–inorganic (MoOx–phytic acid–polyaniline) nanohybrids. A series of samples with different contents of P in the range of 0–3.4 wt% were successfully prepared by varying the introduced amount of polyaniline. With an increase in the P content, the XPS peaks for Mo 3d shifted to lower binding energies, signifying that the electron density of Mo2+ in MoC2 was enriched by doping P. In addition, P doping could enrich the electrons around the Fermi level in MoC2, resulting in weakened Mo–H and promoted Hads desorption in HER kinetics. Combining optimal Hads desorption energy on Mo2C and hierarchical structure, the P–Mo2C@C nanowires with controlled P doping delivered benchmark HER activity and good stability in acidic electrolytes.

What's the electrolyte?

Among the many proposed applications of graphene-based materials, supercapacitors, also known as electrochemical capacitors, have been an active area of research for the past decade. Compared with secondary batteries, graphene-based supercapacitors are electrochemical energy storage devices that promise outstanding power density, charge/discharge rate, cycling stability, and operational safety. Supercapacitors are often utilized individually or in tandem with batteries for energy storage and supply (Scheme 1a(i)). They can be classified into two types: the electric double-layer capacitor (EDLC) (Scheme 1a(ii)) and the pseudocapacitor (Scheme 1a(iii)). The energy storage mechanism of the EDLC involves a simple charge separation at the interface between the conductive electrode and the electrolyte. Because there is no chemical transformation at the EDLC electrodes, the system is quite stable, although the specific capacitance is relatively low. In comparison, charge storage in a pseudocapacitor is predominantly achieved via the redox or faradic transformation of capacitive electrode materials, such as metal oxides and conductive polymers. The specific capacitance of the pseudocapacitor is generally high, but gradually decays because of structural collapse and degradation. A composite of a conductive substance and a pseudocapacitive material is desirable to enhance the overall capacitance, charge/discharge rate, and cycling lifetime. Graphene is a promising conductive component because it has a large surface area, and excellent electronic and mechanical properties. To develop a high-performance pseudocapacitor, we focused on developing a conductive polymer as an electrode counterpart to graphene. Various graphene–polymer composites with non-covalent and covalent interactions have been synthesized, and high capacitive performances have been achieved. The non-covalent interaction between graphene and polymers limits charge transfer at their interface, and hence constrains the cycling stability (Scheme 1b(i)). To address this issue, we propose the formation of the covalent bond between graphene and redox-active polymers.

What's the electrolyte?

In conclusion, a new synthesis approach is presented wherein doped La2Zr2O7 pyrochlore nanocrystals are synthesized with a composition that will result in the correct stoichiometry to form Li-conducting garnets based on Li7La3Zr2O12. La and Ta co-doped pyrochlores with a La:Zr:Ta stoichiometry of 3:1.4:0.6 are demonstrated to readily form garnet-type Li6.4La3Zr1.4Ta0.6O12 between 400–550 °C in a ternary mixture of molten LiNO3–LiOH–Li2O2, with unprecedentedly low reaction temperatures. Pyrochlores can also be used as quasi-single-source precursors and blended with a Li source and reactively sintered in 2 hours at 1200 °C to form highly dense, highly conducting garnet ceramics with a microstructure similar to that obtained through advanced sintering techniques, providing a unique and successful approach to dense garnet electrolytes that can be easily extended to more advanced ceramic forming techniques such as tape-casting or additive manufacturing, potentially improving processability of garnet solid-electrolytes for solid-state lithium batteries.

What's the electrolyte?

The chemical composition and structure of the SEI after 1.0 mA h cm−2 deposition at 0.2 mA cm−2, 0.5 mA cm−2 and 1.0 mA cm−2 were further probed by XPS depth profiling. The XPS depth profiles of the N 1s, Li 1s, F 1s, C 1s, and O 1s spectra are shown in Fig. 6a–i and S13–S15.† At 0.2 mA cm−2 (Fig. 6a–c), the major inorganic components comprising Li3N and LiF were distributed homogeneously in the depth direction of the SEI. Li3N, as the reduction product of LiNO3, is a well-known Li ion conductor with a conductivity of 10−3 S cm−1 at room temperature; LiF, either as an electrolyte additive or SEI component, can improve the diffusion of lithium ions at the electrode surface. The mixed Li3N and LiF layer can facilitate rapid lithium ion diffusion, stabilize the SEI and provide uniform electrodeposition. For the high current densities, especially 1.0 mA cm−2 (Fig. 6g–i), the SEI lacked the inorganic components Li3N and LiF; instead, organic ROLi species were observed. The poor ionic conductivity of ROLi impeded sufficient transport of Li ions towards the electrode surface at high current densities and induced preferential deposition. The very different SEI composition resulting from the repeated SEI breakdown and repair processes also reinforced the rupturing and re-forming cycles of the SEI. In addition, the XPS chemical composition results are in accordance with the mechanical properties obtained by AFM force probing. The SEI formed at a low current density contained a higher content of inorganic components and greater mechanical strength, while the SEI formed at a high current density contained a mixed organic–inorganic layer and was relatively soft (Fig. 6j–l).

What's the electrolyte?

Polyanionic materials are a series of compounds containing polyhedral anionic structural units, which are linked to construct a three-dimensional network structure by strong covalent bonds. The unique structure endows the compounds with a stable crystal structure, moderate capacity, high operating voltage, and safety. Among them, monoclinic LiV2(PO4)3 (discharges to be Li3V2(PO4)3) is one of the most attractive cathode materials for primary batteries, which has stable voltage plateaus between 3.0 V and 4.3 V (vs. Li+/Li) and a suitable theoretical specific capacity (133 mA h g−1) corresponding to the insertion of two Li ions per formula unit. Most importantly, the unique NASICON structure of Li3V2(PO4)3 facilitates Li+ conduction and delivers faster Li+ diffusion kinetics when compared with traditional cathode materials such as LiFePO4, LiCoO2, and LiNi1/3Co1/3Mn1/3O2, ensuring superior rate capability and low-temperature performance. Thus, LiV2(PO4)3 has great potential for application as a cathode material for lithium primary batteries. For practical application, the shelf life of the Li/LiV2(PO4)3 batteries should be long enough; this means that the self-discharge rate should be as low as possible. Since the Li/LiV2(PO4)3 primary battery possesses a high open-circuit voltage (OCV) of about 4.1 V, the side reactions among electrode materials, electrolyte, and even current collectors can severely affect the shelf life of these batteries. In addition, the corrosion of the current collector leads to self-discharge. Therefore, stabilizing the interface between the electrode and electrolyte is essential.

What's the cathode?

monoclinic LiV2(PO4)3 (discharges to be Li3V2(PO4)3

Polyanionic materials are a series of compounds containing polyhedral anionic structural units, which are linked to construct a three-dimensional network structure by strong covalent bonds. The unique structure endows the compounds with a stable crystal structure, moderate capacity, high operating voltage, and safety. Among them, monoclinic LiV2(PO4)3 (discharges to be Li3V2(PO4)3) is one of the most attractive cathode materials for primary batteries, which has stable voltage plateaus between 3.0 V and 4.3 V (vs. Li+/Li) and a suitable theoretical specific capacity (133 mA h g−1) corresponding to the insertion of two Li ions per formula unit. Most importantly, the unique NASICON structure of Li3V2(PO4)3 facilitates Li+ conduction and delivers faster Li+ diffusion kinetics when compared with traditional cathode materials such as LiFePO4, LiCoO2, and LiNi1/3Co1/3Mn1/3O2, ensuring superior rate capability and low-temperature performance. Thus, LiV2(PO4)3 has great potential for application as a cathode material for lithium primary batteries. For practical application, the shelf life of the Li/LiV2(PO4)3 batteries should be long enough; this means that the self-discharge rate should be as low as possible. Since the Li/LiV2(PO4)3 primary battery possesses a high open-circuit voltage (OCV) of about 4.1 V, the side reactions among electrode materials, electrolyte, and even current collectors can severely affect the shelf life of these batteries. In addition, the corrosion of the current collector leads to self-discharge. Therefore, stabilizing the interface between the electrode and electrolyte is essential.

What's the electrolyte?

Polyanionic materials are a series of compounds containing polyhedral anionic structural units, which are linked to construct a three-dimensional network structure by strong covalent bonds. The unique structure endows the compounds with a stable crystal structure, moderate capacity, high operating voltage, and safety. Among them, monoclinic LiV2(PO4)3 (discharges to be Li3V2(PO4)3) is one of the most attractive cathode materials for primary batteries, which has stable voltage plateaus between 3.0 V and 4.3 V (vs. Li+/Li) and a suitable theoretical specific capacity (133 mA h g−1) corresponding to the insertion of two Li ions per formula unit. Most importantly, the unique NASICON structure of Li3V2(PO4)3 facilitates Li+ conduction and delivers faster Li+ diffusion kinetics when compared with traditional cathode materials such as LiFePO4, LiCoO2, and LiNi1/3Co1/3Mn1/3O2, ensuring superior rate capability and low-temperature performance. Thus, LiV2(PO4)3 has great potential for application as a cathode material for lithium primary batteries. For practical application, the shelf life of the Li/LiV2(PO4)3 batteries should be long enough; this means that the self-discharge rate should be as low as possible. Since the Li/LiV2(PO4)3 primary battery possesses a high open-circuit voltage (OCV) of about 4.1 V, the side reactions among electrode materials, electrolyte, and even current collectors can severely affect the shelf life of these batteries. In addition, the corrosion of the current collector leads to self-discharge. Therefore, stabilizing the interface between the electrode and electrolyte is essential.

What's the cathode?

LiFePO4, LiCoO2, and LiNi1/3Co1/3Mn1/3O2

NaxTMO2 type [TM: transition metal(s)] ‘layered’ oxides, having an initial Na-content (x) of ∼1 (i.e., ‘O3’), are important as potential cathode materials for upcoming Na-ion battery systems. However, among other problems (viz., phase transformations during Na-removal/insertion, TM-dissolution etc.), such oxides suffer from severe instability against hydration in a generic sense, which in turn, negatively impacts the stability, electrochemical performances and environment/health friendliness. Against this backdrop, we have designed a composition (viz., combination of TM-/non-TM-ions) to address the aforementioned problems, in particular, air/water-instability. Partial/complete substitution of Ti-ions for Mn-ions eliminated the presence of Mn3+ (which dissolves in electrolyte) at the particle surface, suppressed the increment in impedance (as well as voltage hysteresis) during electrochemical cycling and, thus, significantly improved cyclic stability. More importantly, the air/water-stability improved drastically, such that no phase/structural change occurred even after 40 days of air-exposure and 12 h of stirring in water, unlike for predominately Mn-ion containing counterparts, which progressively evolved O′3 and P3 phases. In fact, water-stability enabled the usage of environment/health friendly, as well as less expensive, ‘aqueous-binder’ (viz., Na-alginate) and water (as solvent) for electrode preparation. The as-developed environment/health friendly ‘aqueous-processed’ electrode also exhibits excellent long-term cyclic stability, with capacity retention being ∼82% after 100 cycles (∼60% after 500 cycles and ∼56% after 750 cycles) at C/5, along with just ∼4% capacity fade over the last 250 cycles investigated (viz., cycles 500–750). Such an elimination of requirements for toxic/hazardous-cum-expensive ‘non-aqueous’ solvents/binders for electrode preparation and the associated learning, in terms of suitable combinations of TM-/non-TM-ions for these oxides in the pursuit of desired structural features, are significant steps towards the development of alkali metal-ion batteries in general.

What's the cathode?

NaxTMO2

NaxTMO2 type [TM: transition metal(s)] ‘layered’ oxides, having an initial Na-content (x) of ∼1 (i.e., ‘O3’), are important as potential cathode materials for upcoming Na-ion battery systems. However, among other problems (viz., phase transformations during Na-removal/insertion, TM-dissolution etc.), such oxides suffer from severe instability against hydration in a generic sense, which in turn, negatively impacts the stability, electrochemical performances and environment/health friendliness. Against this backdrop, we have designed a composition (viz., combination of TM-/non-TM-ions) to address the aforementioned problems, in particular, air/water-instability. Partial/complete substitution of Ti-ions for Mn-ions eliminated the presence of Mn3+ (which dissolves in electrolyte) at the particle surface, suppressed the increment in impedance (as well as voltage hysteresis) during electrochemical cycling and, thus, significantly improved cyclic stability. More importantly, the air/water-stability improved drastically, such that no phase/structural change occurred even after 40 days of air-exposure and 12 h of stirring in water, unlike for predominately Mn-ion containing counterparts, which progressively evolved O′3 and P3 phases. In fact, water-stability enabled the usage of environment/health friendly, as well as less expensive, ‘aqueous-binder’ (viz., Na-alginate) and water (as solvent) for electrode preparation. The as-developed environment/health friendly ‘aqueous-processed’ electrode also exhibits excellent long-term cyclic stability, with capacity retention being ∼82% after 100 cycles (∼60% after 500 cycles and ∼56% after 750 cycles) at C/5, along with just ∼4% capacity fade over the last 250 cycles investigated (viz., cycles 500–750). Such an elimination of requirements for toxic/hazardous-cum-expensive ‘non-aqueous’ solvents/binders for electrode preparation and the associated learning, in terms of suitable combinations of TM-/non-TM-ions for these oxides in the pursuit of desired structural features, are significant steps towards the development of alkali metal-ion batteries in general.

What's the electrolyte?

Although tremendous efforts have resulted in the great advances in recent years, O3-type layered oxide cathode materials have been plagued by the issue of poor thermal stability and structural stability against a humid atmosphere, which are favorable properties for practical application. To confirm the practical acceptability, we first checked the thermal stability and air-stability of the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode by comparing it with the Na[Ni0.5Mn0.5]O2 cathode. Differential scanning calorimetry (DSC) was performed to observe the exothermic reactions of the deeply de-sodiated electrodes (highly unstable state) collected from cells charged to 4.3 V (vs. Na/Na+) after the first cycle. The results are plotted in Fig. S15,† showing both onset temperature of the exothermic reactions and the total specific heat generation. Although a similar amount of Na+ ions (according to charge capacities) was extracted from the crystal structure, the thermal reaction between NaxCa0.01[Ni0.5Mn0.5]O2 and Nax[Ni0.5Mn0.5]O2 (x is close to 0) cathodes was different. In contrast, the Nax[Ni0.5Mn0.5]O2 electrode displayed an exothermic peak at 254.1 °C with a heat generation of 634.5 J g−1, the NaxCa0.01[Ni0.5Mn0.5]O2 electrode showed a lower heat generation of 352 J g−1 and an exothermic peak at 258.9 °C. It should be noted that the thermal reaction is related to the evolution of oxygen from the crystal structure because the oxygen released from the cathode materials can react with the electrolyte solution and undergo a redox reaction. Therefore, it is strongly believed that the improved thermal stability of the Ca-substituted cathode likely resulted from the strong interaction of immobile Ca2+ with O2−, which suppresses the rate of release of thermal oxygen from the highly oxidized cathode material.

What's the cathode?

O3-type layered oxide

Although tremendous efforts have resulted in the great advances in recent years, O3-type layered oxide cathode materials have been plagued by the issue of poor thermal stability and structural stability against a humid atmosphere, which are favorable properties for practical application. To confirm the practical acceptability, we first checked the thermal stability and air-stability of the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode by comparing it with the Na[Ni0.5Mn0.5]O2 cathode. Differential scanning calorimetry (DSC) was performed to observe the exothermic reactions of the deeply de-sodiated electrodes (highly unstable state) collected from cells charged to 4.3 V (vs. Na/Na+) after the first cycle. The results are plotted in Fig. S15,† showing both onset temperature of the exothermic reactions and the total specific heat generation. Although a similar amount of Na+ ions (according to charge capacities) was extracted from the crystal structure, the thermal reaction between NaxCa0.01[Ni0.5Mn0.5]O2 and Nax[Ni0.5Mn0.5]O2 (x is close to 0) cathodes was different. In contrast, the Nax[Ni0.5Mn0.5]O2 electrode displayed an exothermic peak at 254.1 °C with a heat generation of 634.5 J g−1, the NaxCa0.01[Ni0.5Mn0.5]O2 electrode showed a lower heat generation of 352 J g−1 and an exothermic peak at 258.9 °C. It should be noted that the thermal reaction is related to the evolution of oxygen from the crystal structure because the oxygen released from the cathode materials can react with the electrolyte solution and undergo a redox reaction. Therefore, it is strongly believed that the improved thermal stability of the Ca-substituted cathode likely resulted from the strong interaction of immobile Ca2+ with O2−, which suppresses the rate of release of thermal oxygen from the highly oxidized cathode material.

What's the electrolyte?

Although tremendous efforts have resulted in the great advances in recent years, O3-type layered oxide cathode materials have been plagued by the issue of poor thermal stability and structural stability against a humid atmosphere, which are favorable properties for practical application. To confirm the practical acceptability, we first checked the thermal stability and air-stability of the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode by comparing it with the Na[Ni0.5Mn0.5]O2 cathode. Differential scanning calorimetry (DSC) was performed to observe the exothermic reactions of the deeply de-sodiated electrodes (highly unstable state) collected from cells charged to 4.3 V (vs. Na/Na+) after the first cycle. The results are plotted in Fig. S15,† showing both onset temperature of the exothermic reactions and the total specific heat generation. Although a similar amount of Na+ ions (according to charge capacities) was extracted from the crystal structure, the thermal reaction between NaxCa0.01[Ni0.5Mn0.5]O2 and Nax[Ni0.5Mn0.5]O2 (x is close to 0) cathodes was different. In contrast, the Nax[Ni0.5Mn0.5]O2 electrode displayed an exothermic peak at 254.1 °C with a heat generation of 634.5 J g−1, the NaxCa0.01[Ni0.5Mn0.5]O2 electrode showed a lower heat generation of 352 J g−1 and an exothermic peak at 258.9 °C. It should be noted that the thermal reaction is related to the evolution of oxygen from the crystal structure because the oxygen released from the cathode materials can react with the electrolyte solution and undergo a redox reaction. Therefore, it is strongly believed that the improved thermal stability of the Ca-substituted cathode likely resulted from the strong interaction of immobile Ca2+ with O2−, which suppresses the rate of release of thermal oxygen from the highly oxidized cathode material.

What's the cathode?

Na0.98Ca0.01[Ni0.5Mn0.5]O2

On the other hand, the HER and OER are the two half-reactions involved in electrocatalytic water splitting. These hydrogen-based and oxygen-based alkaline electrochemical reactions are often hindered by sluggish kinetics, which leads to a large overpotential and low round-trip efficiency. Enormous effort has been devoted to the development of highly efficient electrocatalysts for the HER and OER based on the structure–activity relationship, to increase the reaction rate and efficiency of the total system. Accordingly, Liu et al. reported AuPtPdRhRu alloy nanoparticles for HER application by an ultrasonication-assisted wet chemical method (Fig. 5a and b). The as-synthesized quinary AuPtPdRhRu nanoparticles on carbon (AuPtPdRhRu/C), whose average diameter is 2.6 ± 0.3 nm, exhibited excellent HER activity and durability in a 1.0 M KOH electrolyte. The required overpotential of AuPtPdRhRu/C to drive a current density of −30 mA cm−2 was 190 mV, which is smaller than those of quaternary AuPtPdRh/C (260 mV) and ternary AuPtRh/C (600 mV) catalysts. AuPtPdRhRu/C exhibited a lower Tafel slope of 62 mV dec−1 compared to AuPtPdRh/C (91 mV dec−1), AuPtPd/C (177 mV dec−1), and benchmark Pt/C (77 mV dec−1) catalysts, indicating its superior activity toward the alkaline HER. Moreover, AuPtPdRhRu/C exhibited stable electrocatalytic performance over 8 h, verified by chronopotentiometry at a current density of 100 mA cm−2. Likewise, Qiu et al. reported noble-metal free nanoporous HEA catalysts for the OER under alkaline conditions. Quinary nanoporous NiCoFeAlX (np-NiCoFeAlX, X = V, Cr, Mn, Cu, Zr, Nb, Mo) and senary np-MoCuNiCoFeAl catalysts were fabricated by a mechanical alloying and chemical dealloying strategy. Among the catalysts, np-MoNiCoFeAl exhibited the lowest OER overpotential (240 mV) to drive a current density of 10 mA cm−2 in a 1.0 M KOH electrolyte. Moreover, the np-MoNiCoFeAl catalyst also showed a smaller Tafel slope of 46 mV dec−1, compared to ternary np-NiFeAl (62 mV dec−1) and quaternary np-NiCoFeAl (61 mV dec−1). Moreover, the quinary np-MoNiCoFeAl exhibited high durability compared to ternary and quaternary catalysts, and retained more than 95% of its initial activity after 2000 potential cycles. Due to the oxidative potential under OER conditions, the surface of np-MoNiCoFeAl might undergo oxidation, forming surface oxides on the HEA surfaces. After a chronopotentiometry test at 20 mA cm−2 for 50 h, however, the five elements were still uniformly distributed in np-MoNiCoFeAl, forming a high-entropy-oxide (HEO) skin that exhibits high-entropy driven stability.

What's the cathode?

1.0 M KOH

On the other hand, the HER and OER are the two half-reactions involved in electrocatalytic water splitting. These hydrogen-based and oxygen-based alkaline electrochemical reactions are often hindered by sluggish kinetics, which leads to a large overpotential and low round-trip efficiency. Enormous effort has been devoted to the development of highly efficient electrocatalysts for the HER and OER based on the structure–activity relationship, to increase the reaction rate and efficiency of the total system. Accordingly, Liu et al. reported AuPtPdRhRu alloy nanoparticles for HER application by an ultrasonication-assisted wet chemical method (Fig. 5a and b). The as-synthesized quinary AuPtPdRhRu nanoparticles on carbon (AuPtPdRhRu/C), whose average diameter is 2.6 ± 0.3 nm, exhibited excellent HER activity and durability in a 1.0 M KOH electrolyte. The required overpotential of AuPtPdRhRu/C to drive a current density of −30 mA cm−2 was 190 mV, which is smaller than those of quaternary AuPtPdRh/C (260 mV) and ternary AuPtRh/C (600 mV) catalysts. AuPtPdRhRu/C exhibited a lower Tafel slope of 62 mV dec−1 compared to AuPtPdRh/C (91 mV dec−1), AuPtPd/C (177 mV dec−1), and benchmark Pt/C (77 mV dec−1) catalysts, indicating its superior activity toward the alkaline HER. Moreover, AuPtPdRhRu/C exhibited stable electrocatalytic performance over 8 h, verified by chronopotentiometry at a current density of 100 mA cm−2. Likewise, Qiu et al. reported noble-metal free nanoporous HEA catalysts for the OER under alkaline conditions. Quinary nanoporous NiCoFeAlX (np-NiCoFeAlX, X = V, Cr, Mn, Cu, Zr, Nb, Mo) and senary np-MoCuNiCoFeAl catalysts were fabricated by a mechanical alloying and chemical dealloying strategy. Among the catalysts, np-MoNiCoFeAl exhibited the lowest OER overpotential (240 mV) to drive a current density of 10 mA cm−2 in a 1.0 M KOH electrolyte. Moreover, the np-MoNiCoFeAl catalyst also showed a smaller Tafel slope of 46 mV dec−1, compared to ternary np-NiFeAl (62 mV dec−1) and quaternary np-NiCoFeAl (61 mV dec−1). Moreover, the quinary np-MoNiCoFeAl exhibited high durability compared to ternary and quaternary catalysts, and retained more than 95% of its initial activity after 2000 potential cycles. Due to the oxidative potential under OER conditions, the surface of np-MoNiCoFeAl might undergo oxidation, forming surface oxides on the HEA surfaces. After a chronopotentiometry test at 20 mA cm−2 for 50 h, however, the five elements were still uniformly distributed in np-MoNiCoFeAl, forming a high-entropy-oxide (HEO) skin that exhibits high-entropy driven stability.

What's the cathode?

1.0 M KOH

Lithium sulfur batteries (LSBs) with high theoretical energy densities of 2600 W h kg−1 are considered promising candidates to take precedence over lithium ion batteries (LIBs) in meeting the emerging and demanding applications, such as electric vehicles and smart grids. However, the practical implementation of LSBs is plagued by several fundamental challenges, including the dissolution and shuttling of lithium polysulfides (LPSs), the insulating nature of sulfur and lithium sulfides, and the instability of Li metal anodes. In the past decade, progress in structural design and materials chemistry has induced discernible improvements in battery performance. For instance, a high sulfur utilization of 1620 mA h g−1 with long cycle life of 1000 cycles was reported for sulfur/graphene composite electrodes. However, statistical analysis of the reported cathodes indicates that the major cathodes tested under low sulfur loadings (<2 mg cm−2) and high electrolyte/sulfur (E/S) ratios (>10 μL mg−1) are unacceptable for practical applications. Based on theoretical estimations, LSBs should possess sulfur loadings of above 7 mg cm−2, areal capacities of over 6 mA h cm−2, and E/S ratios of less than 5 μL mg−1, to outperform current LIBs in terms of the practical energy density.

What's the electrolyte?

Over the past several decades, rechargeable lithium-ion batteries (LIBs) have been widely used in the energy storage market of portable electronic devices and electric vehicles. However, the scarcity and rising costs of Li raw materials have restricted the further large-scale commercialization of LIBs, and thus have spurred extensive research in seeking cost-effective and more reliable energy storage systems. Among various promising alternatives, Na-ion batteries (NIBs) have attracted considerable attention because of the high abundance and low cost of Na resources. Of various anode candidates for NIBs, Na metal anodes are prized for their high specific capacity (1166 mA h g−1) and low electrochemical potential (−2.714 V vs. standard hydrogen electrode). However, the naturally formed solid electrolyte interphase (SEI) that originates from the instantaneous reaction between the highly reactive Na and electrolyte solvents is generally non-uniform and fragile. The heterogeneous feature easily causes uneven Na deposition and the formation of Na dendrites, and thus brings about serious safety issues. Additionally, the fragile SEI cannot tolerate huge volume changes upon the repetitive plating/stripping process, which gives rise to repeated breakdown/repair of the SEI, leading to a low coulombic efficiency (CE) and short cycle life. These issues impede the practical applications of Na metal anodes.

What's the anode?

Na metal

Over the past several decades, rechargeable lithium-ion batteries (LIBs) have been widely used in the energy storage market of portable electronic devices and electric vehicles. However, the scarcity and rising costs of Li raw materials have restricted the further large-scale commercialization of LIBs, and thus have spurred extensive research in seeking cost-effective and more reliable energy storage systems. Among various promising alternatives, Na-ion batteries (NIBs) have attracted considerable attention because of the high abundance and low cost of Na resources. Of various anode candidates for NIBs, Na metal anodes are prized for their high specific capacity (1166 mA h g−1) and low electrochemical potential (−2.714 V vs. standard hydrogen electrode). However, the naturally formed solid electrolyte interphase (SEI) that originates from the instantaneous reaction between the highly reactive Na and electrolyte solvents is generally non-uniform and fragile. The heterogeneous feature easily causes uneven Na deposition and the formation of Na dendrites, and thus brings about serious safety issues. Additionally, the fragile SEI cannot tolerate huge volume changes upon the repetitive plating/stripping process, which gives rise to repeated breakdown/repair of the SEI, leading to a low coulombic efficiency (CE) and short cycle life. These issues impede the practical applications of Na metal anodes.

What's the electrolyte?

Over the past several decades, rechargeable lithium-ion batteries (LIBs) have been widely used in the energy storage market of portable electronic devices and electric vehicles. However, the scarcity and rising costs of Li raw materials have restricted the further large-scale commercialization of LIBs, and thus have spurred extensive research in seeking cost-effective and more reliable energy storage systems. Among various promising alternatives, Na-ion batteries (NIBs) have attracted considerable attention because of the high abundance and low cost of Na resources. Of various anode candidates for NIBs, Na metal anodes are prized for their high specific capacity (1166 mA h g−1) and low electrochemical potential (−2.714 V vs. standard hydrogen electrode). However, the naturally formed solid electrolyte interphase (SEI) that originates from the instantaneous reaction between the highly reactive Na and electrolyte solvents is generally non-uniform and fragile. The heterogeneous feature easily causes uneven Na deposition and the formation of Na dendrites, and thus brings about serious safety issues. Additionally, the fragile SEI cannot tolerate huge volume changes upon the repetitive plating/stripping process, which gives rise to repeated breakdown/repair of the SEI, leading to a low coulombic efficiency (CE) and short cycle life. These issues impede the practical applications of Na metal anodes.

What's the anode?

Na metal

To quantitatively evaluate the ECD performance, the difference (ΔT) in transmittance values in coloured (TC) and bleached (TB) states is defined as below: The measurement results regarding the visible light transmittance of the ECDs under coloured and bleached states according to various UV curing times of 0, 5, 10, and 20 minutes are presented in Fig. 3a and Fig. S4 (ESI†). To assess the ΔT value, the transmittance values at a wavelength of 550 nm were chosen (Fig. 3b). For the ECD with 5 min of curing, the ΔT was reduced from 54.7% to 51.7%, as shown in Fig. 3b and Table 1. As the UV curing time was delayed, the ΔT values were slightly decreased to approximately 50.0% primarily owing to the increased TC value. This suggests that the UV curing process can deteriorate the EC performance of ECDs. It could be inferred that immobile PMMA molecules block many of the reaction sites on the surfaces of both the electrodes. Fig. 3c shows the dynamic spectra of the UV-cured EDCs at a wavelength of 550 nm under a periodic bias voltage between −1.2 and 0 V. The reliabilities of all the UV-cured ECDs were maintained for the first five cycles, which is comparable to that of uncured ECD using the liquid electrolyte. Furthermore, their low driving voltage of −1.2 V and natural state (0 V) bleaching characteristics are beneficial for low-power devices such as Internet of Things (IoT) devices and wearable displays.

What's the electrolyte?