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Fig. 4d shows the comparison of the rate performances and coulombic efficiencies of the three types of cathodes under different current densities (0.1, 0.2, 0.5, 1.0, and 2.0C). All three cathodes show a coulombic efficiency over 98%, which could be due to the highly conductive CNF skeletons. And, with increasing the current density from 0.1C to 2.0C, the CNF@V2S3/S cathodes can retain a specific capacitance of 922 mA h g−1 (78.9% of the initial value), which is much higher than those of VS2-based LSBs. However, the CNF@VO0.9/S and CNF/S cathodes can retain 390 mA h g−1 (55.1%) and 592 mA h g−1 (68.8%). Here, to gain better understanding of the effect of V2S3 on the rate performances of cathodes, the in situ reaction resistances were first derived from the charge/discharge curves (see the ESI for details†). As shown in Fig. 4e, the reaction resistances of all three cathodes increase with the increasing specific capacity, which is caused by the low conductivity of intermediate LiPSs. Compared to the CNF@VO0.9/S and CNF/S cathodes, the CNF@V2S3/S ones show a much lower reaction resistance along the whole electrochemical processes. And similar results can also be obtained from Electrochemical Impedance Spectroscopy (EIS) spectra (Fig. S14†). This decrease in resistances often leads to fast redox kinetics on the CNF@V2S3/S cathodes. Besides, galvanostatic intermittent titration technique (GITT) tests were further carried out to further study the origins for fast transfer kinetics of CNF@V2S3/S cathodes (Fig. S15†). The diffusivities of Li ions between the cathode and electrolyte were estimated, as listed in Table S2.† The diffusion coefficient of Li+ ions in the CNF@V2S3/S/electrolyte system is 2.857 × 10−6 cm2 s−1, which is much higher than those of CNF@VO0.9/S and CNF/S ones (1.554 × 10−7 and 1.645 × 10−7 cm2 s−1) in this work. This greatly improved diffusivities of Li ions would result in the high specific capacity and the high rate capability of CNF@V2S3/S cathodes. Also, a similar conclusion can be obtained from EIS spectra in the low-frequency region (Fig. S14†). It can be seen that the curve slope of CNF@V2S3/S cathodes is much larger than those of the CNF@VO0.9/S and CNF/S ones (Table S3†).

What's the electrolyte?

Fig. 4d shows the comparison of the rate performances and coulombic efficiencies of the three types of cathodes under different current densities (0.1, 0.2, 0.5, 1.0, and 2.0C). All three cathodes show a coulombic efficiency over 98%, which could be due to the highly conductive CNF skeletons. And, with increasing the current density from 0.1C to 2.0C, the CNF@V2S3/S cathodes can retain a specific capacitance of 922 mA h g−1 (78.9% of the initial value), which is much higher than those of VS2-based LSBs. However, the CNF@VO0.9/S and CNF/S cathodes can retain 390 mA h g−1 (55.1%) and 592 mA h g−1 (68.8%). Here, to gain better understanding of the effect of V2S3 on the rate performances of cathodes, the in situ reaction resistances were first derived from the charge/discharge curves (see the ESI for details†). As shown in Fig. 4e, the reaction resistances of all three cathodes increase with the increasing specific capacity, which is caused by the low conductivity of intermediate LiPSs. Compared to the CNF@VO0.9/S and CNF/S cathodes, the CNF@V2S3/S ones show a much lower reaction resistance along the whole electrochemical processes. And similar results can also be obtained from Electrochemical Impedance Spectroscopy (EIS) spectra (Fig. S14†). This decrease in resistances often leads to fast redox kinetics on the CNF@V2S3/S cathodes. Besides, galvanostatic intermittent titration technique (GITT) tests were further carried out to further study the origins for fast transfer kinetics of CNF@V2S3/S cathodes (Fig. S15†). The diffusivities of Li ions between the cathode and electrolyte were estimated, as listed in Table S2.† The diffusion coefficient of Li+ ions in the CNF@V2S3/S/electrolyte system is 2.857 × 10−6 cm2 s−1, which is much higher than those of CNF@VO0.9/S and CNF/S ones (1.554 × 10−7 and 1.645 × 10−7 cm2 s−1) in this work. This greatly improved diffusivities of Li ions would result in the high specific capacity and the high rate capability of CNF@V2S3/S cathodes. Also, a similar conclusion can be obtained from EIS spectra in the low-frequency region (Fig. S14†). It can be seen that the curve slope of CNF@V2S3/S cathodes is much larger than those of the CNF@VO0.9/S and CNF/S ones (Table S3†).

What's the cathode?

CNF@VO0.9/S and CNF/S

Gas diffusion electrodes (GDEs) were fabricated by spraying a catalyst ink made from polynorbonene tetrablock copolymer powder ionomers (GT32 and GT73) and Pt-based electrocatalysts onto Toray TGP-H-060 gas diffusion layers (GDLs) with 5% PTFE. Commercially available 40% Pt/C (Alfa Aesar HiSPEC 4000, Pt nominally 40% wt, supported on Vulcan XC-72R carbon) was used at the cathode and 60% Pt–Ru/C (Alfa Aesar HiSPEC 10000, Pt nominally 40 wt%, and Ru, nominally 20 wt%, supported on Vulcan XC-72R carbon) was used as the anode. The detailed procedure for ink formulation and GDE fabrication has been previously reported, though a brief description is provided below.

What's the cathode?

Surface modification using some metal oxides is an appropriate approach to overcome such critical drawbacks of LMO. Metal oxides, such as Ta2O5, ZrO2, Al2O3, ZnO, MgO, and TiO2, have been used as coating layers for LMO cathode material powders. The coating layer minimizes direct contact between the cathode and the electrolyte solution, avoids unwanted interfacial reactions, and thus improves the structural stability and phase transitions. To some extent, these coating films can also improve the cycling performance of the material. Nevertheless, the reported high surface coating using metal oxides usually results in agglomerated nanoparticles and leads to increased resistance. Furthermore, the modest improvement could be a result of increased interfacial resistance due to the insulating nature of surface coating layers.

What's the cathode?

LMO

Moreover, the formation of metal sulfides with a non-crystalline structure also reveals enhanced adsorption/bonding of LiPSs owing to abundant defects and dangling bonds. For example, amorphous TiS4 has been successfully prepared by Sakuda et al. through a mechanical milling process using crystalline TiS2, sulfur, and acetylene black. When used as the cathode in a Li–S battery, the dissolution of LiPSs into the electrolyte is significantly suppressed because amorphization reduces the symmetry of the lattice, leading to the possibility of overlapping electron clouds between sulfur and titanium atoms, resulting in chemical bonding between the elements. Liu et al. reported the synthesis of a S@amorphous NiS2 composite via a coprecipitation reaction using Na2S8 and NiCl2 ethanol solution. Here the soluble sulfide ions can permeate into the loose amorphous structure, and thus the S particle would nucleate and grow in the inner space of NiS2, ensuing the high immobilization of S species. Very recently, Yu et al. meticulously designed hollow-amorphous N-doped carbon/MoS3 nanoboxes (NC/MoS3 NBs) as an advanced sulfur host for Li–S batteries. In a typical synthesis, crystalline α-Fe2O3 nanocubes are first coated with a layer of polydopamine, which are then calcined in a N2 atmosphere and etched in HCl solution to obtain N-doped carbon (NC) shells and hollow NC nanoboxes, respectively. Finally, the NC/MoS3 NB host is prepared through heterogeneous nucleation using CTAB as the surfactant and (NH4)2MoS4 as the precursor. Here, the amorphous MoS3 with unsaturated coordination Mo and electron-rich S not only has strong binding capability to LiPSs but also exhibits a catalytic effect on polysulfide conversion, which are verified by both experimental investigations and theoretical calculations. More interestingly, this synthesis strategy can be generalized to fabricate other amorphous metal chalcogenides nanoboxes such as CoSx and WSx, which is also promising to be applied in the fields of energy storage and conversion.

What's the cathode?

LiPSs

Moreover, the formation of metal sulfides with a non-crystalline structure also reveals enhanced adsorption/bonding of LiPSs owing to abundant defects and dangling bonds. For example, amorphous TiS4 has been successfully prepared by Sakuda et al. through a mechanical milling process using crystalline TiS2, sulfur, and acetylene black. When used as the cathode in a Li–S battery, the dissolution of LiPSs into the electrolyte is significantly suppressed because amorphization reduces the symmetry of the lattice, leading to the possibility of overlapping electron clouds between sulfur and titanium atoms, resulting in chemical bonding between the elements. Liu et al. reported the synthesis of a S@amorphous NiS2 composite via a coprecipitation reaction using Na2S8 and NiCl2 ethanol solution. Here the soluble sulfide ions can permeate into the loose amorphous structure, and thus the S particle would nucleate and grow in the inner space of NiS2, ensuing the high immobilization of S species. Very recently, Yu et al. meticulously designed hollow-amorphous N-doped carbon/MoS3 nanoboxes (NC/MoS3 NBs) as an advanced sulfur host for Li–S batteries. In a typical synthesis, crystalline α-Fe2O3 nanocubes are first coated with a layer of polydopamine, which are then calcined in a N2 atmosphere and etched in HCl solution to obtain N-doped carbon (NC) shells and hollow NC nanoboxes, respectively. Finally, the NC/MoS3 NB host is prepared through heterogeneous nucleation using CTAB as the surfactant and (NH4)2MoS4 as the precursor. Here, the amorphous MoS3 with unsaturated coordination Mo and electron-rich S not only has strong binding capability to LiPSs but also exhibits a catalytic effect on polysulfide conversion, which are verified by both experimental investigations and theoretical calculations. More interestingly, this synthesis strategy can be generalized to fabricate other amorphous metal chalcogenides nanoboxes such as CoSx and WSx, which is also promising to be applied in the fields of energy storage and conversion.

What's the electrolyte?

To further analyze the electrochemical difference between the pristine and treated LMO, the initial cycle curves for the two samples are plotted together in Fig. 2a. For pristine LMO, the plateau above 4.5 V appears as the typical behavior of the Li-rich cathode, which corresponds to the delithiation, charge compensation and oxygen release process. Upon discharge, an S-shaped curve was observed with a discharge capacity of around 200 mA h g−1. Interestingly, the characteristic charge plateau (above 4.5 V) was shortened in the T-LMO sample. Simultaneously, plateaus appeared at around 4.1 V for the charged state and 4.0 V/2.8 V for the discharged state, which can be seen clearly in the dQ/dV plots (Fig. 2b), indicating that the spinel phase may exist in T-LMO.

What's the cathode?

Li-rich

Metal–air batteries with high energy density have emerged as key players in the energy storage sector. They operate on two underlying processes, namely, the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). It provides the impetus to design efficient, earth-abundant and economic bifunctional electrocatalysts vis-à-vis precious metal-based catalysts. In this perspective, a few polyanionic battery insertion materials have been reported as potential electrocatalysts. In the current work, metal fluorophosphate (Na2MPO4F, M = Fe/Co/Mn) family of sodium insertion materials have been shown as a new class of bifunctional electrocatalysts with robust structural stability. In particular, Na2CoPO4F was found to exhibit superior catalytic performance with an onset potential of 0.903 V (vs. RHE) for the ORR and an overpotential of 380 mV (vs. RHE) for the OER. The underlying mechanism and kinetics were explored using ab initio computational studies. Overall, polyanionic transition metal fluorophosphates were explored for the first time as bifunctional electrocatalysts capable of working as potential cathode materials in hybrid metal–air batteries.

What's the cathode?

Imide-functionalized small molecules are represented by 3,4,9,10-perylene-tetracarboxylic diimide (PTCDI 10), recently proposed by Fan et al. It has lower solubility compared to PTCDA, which should have improved its cycling stability. Nevertheless, with 1 M and 3 M KFSI solutions in EC:DMC, rapid capacity fading associated with the material dissolution was observed. The cathode dissolution was suppressed by using 5 M KFSI in EC:DMC, which ensured ∼0% capacity decay over 100 cycles at 100 mA g−1 and ∼90% retention after 600 cycles at 4 A g−1. The reported Qm at 100 mA g−1 was 157 mA h g−1, which is higher than the theoretical value of 137 mA h g−1; the additional capacity (∼27 mA h g−1) originated from the carbon additive. Impressive capacities of 137 and 80 mA h g−1 were observed at 1 and 10 A g−1, respectively. The rate performance of PTCDI was reported to be superior compared to PTCDA under the same conditions. PTCDI-based full cells, which had pre-potassiated graphite as the anode, delivered a Qm of 140 and 80 mA h g−1 at 50 and 500 mA g−1, respectively. The output voltage was 2 V and ∼50% of the capacity retained after 500 cycles at 500 mA g−1.

What's the cathode?

Later, Wang et al. published a similar study, using annealed PTCDA as the anode material and 30 M KFSI aqueous solution as the electrolyte. The reversible capacity of PTCDA reached 134 mA h g−1 at 0.2 A g−1. Decent rate and cycling capabilities were demonstrated. A full cell with potassium iron(II) hexacyanoferrate as the cathode delivered 39 mA h g−1 based on the combined mass of the electrodes, and it was almost constant up to 4 A g−1 current density. The capacity retention of the full cell was 89% after 1000 cycles at 2 A g−1.

What's the cathode?

potassium iron(II) hexacyanoferrate

Later, Wang et al. published a similar study, using annealed PTCDA as the anode material and 30 M KFSI aqueous solution as the electrolyte. The reversible capacity of PTCDA reached 134 mA h g−1 at 0.2 A g−1. Decent rate and cycling capabilities were demonstrated. A full cell with potassium iron(II) hexacyanoferrate as the cathode delivered 39 mA h g−1 based on the combined mass of the electrodes, and it was almost constant up to 4 A g−1 current density. The capacity retention of the full cell was 89% after 1000 cycles at 2 A g−1.

What's the anode?

PTCDA

Later, Wang et al. published a similar study, using annealed PTCDA as the anode material and 30 M KFSI aqueous solution as the electrolyte. The reversible capacity of PTCDA reached 134 mA h g−1 at 0.2 A g−1. Decent rate and cycling capabilities were demonstrated. A full cell with potassium iron(II) hexacyanoferrate as the cathode delivered 39 mA h g−1 based on the combined mass of the electrodes, and it was almost constant up to 4 A g−1 current density. The capacity retention of the full cell was 89% after 1000 cycles at 2 A g−1.

What's the electrolyte?

30 M KFSI aqueous solution

Determined by a continuous discharging test over 27 h, the recorded specific capacity of the battery with the CNT@CNP catalyst is 846.7 mA h gZn−1, and the relative energy density is 1059.92 mW h gZn−1 at an average voltage of 1.2 V, as shown in Fig. 7(d). These outcomes are better than those of most current Fe–N–C catalysts shown in Table 2. Moreover, the rechargeable performance of the battery, equipped with the CNT@CNP catalyst and commercial 20 wt% IrO2 on carbon paper as an air cathode, has been tested by us. As observed in Fig. 7(e), the novel battery shows remarkably improved ORR and OER properties, compared with the reference of the battery equipped with the commercial Pt/C and IrO2 counterpart. At a constant current density of 20 mA cm−2 for 10 h, the cycling of charge and discharge reaches 2 times with respect to the reference battery, as shown in Fig. 7(f). The battery almost maintains a stable discharge potential of 0.83 V and a recharge voltage of 2.25 V even after 60 cycles, which is much better than that of the reference battery shown in Fig. 7(f). So, the synthesized catalyst shows a very good power density in comparison with the corresponding results available in the literature shown in Table 2. Therefore, the optimal catalytic performance of the carbon interpenetrating networks may be attributed to the formation of abundant edge Fe–Nx moieties as active sites at the boundaries and interfaces in the CNT@CNP composites with a smart structure. If so, developing Fe–N–C with interpenetrating network structures may be major breakthroughs of non-PGM materials serving as next-generation catalysts for the ORR.

What's the cathode?

air

Fig. 1c shows the main fabrication processes of high-performance LSBs based on CNF@V2S3/S composite cathodes. Electrospun nanoscale fibrous skeletons were often used as conductive substrates for active materials in energy-storage devices. During the annealing under Ar, many V atoms were forced out to the surface of electrospun nanofibers under the Kirkendall effect, and formed into VO0.9 nanocrystals. Then, the formed VO0.9 nanocrystals will further be vulcanized into V2S3 nanocrystals with an ultralow rate during the vulcanization reaction in a S/Ar environment. As a result, many V2S3 nanocrystals are stably connected to the surface of CNFs, which benefit the transport of electrons through the CNF/V2S3 composites. And these tiny V2S3 nanocrystals can also show a high specific surface area and a high catalytic activity, which can benefit the chemisorption and the transformation of sulfur species during the charge/discharge processes in LSBs.

What's the cathode?

CNF@V2S3/S composite

F-doped h-BNNS were used as the cathode material for magnesium batteries, a potential alternative to Li-ion batteries due to their higher safety and lower cost. It was reported that fluorine can be grafted via a reaction between h-BNNS and fluoroboric acid. Fluorination effectively improves the electrical conductivity of h-BNNS and their electrochemical performance for Mg batteries.

What's the cathode?

F-doped h-BNNS

Recently, a high energy density (97.6 W h kg−1) along with impressive cycling steadiness (73% retention after 5000 cycles at 1 A g−1) was reported by Yang et al. for a lithium-ion capacitor fabricated by using PAN/hydrothermal-made V3O7·H2O nanowire derived CNFs/V2O3 hybrids as the anode and commercial activated carbon (AC) as the cathode. Additionally, the prepared CNFs/V2O3 nanocomposites with internal void spaces displayed a high capacity of 569.1 mA h g−1 at 0.1 A g−1 as well as unexpected rate capability (238.5 mA hg−1 at 10.0 A g−1) in half-cell tests. By electrospinning a PAN/VO(acac)2 precursor solution, followed by calcination, V–O–C nanocomposites with a specific surface area of 587.9 m2 g−1 were obtained. An atomic level dispersion of vanadium within the composite nanofibers enabled the resultant V–O–C nanocomposites to deliver a specific capacitance of 463 F g−1 at 1 A g−1 with good electronic conductivity and electrolyte penetration. Moreover, the electrochemical properties of V5+ are superior to those of V3+ and V4+ when vanadium oxide-embedded carbon fibers are applied in a supercapacitor. Bai et al. produced CNFs/V2O3, CNFs/VO2–V2O5 and CNFs/V2O5via electrospinning a PAN/VO(acac)2 precursor solution, followed by different calcination processes. The results showed that the electrochemical performance of CNFs/V2O5 is better than that of CNFs/V2O3 and CNFs/VO2–V2O5 when they were applied to SCs. In Tang's work, CNFs/VO/VOx web-like nanocomposites were obtained by electrospinning a polymer-based solution containing PAN, PVP, hybrid vanadium precursors (VO2, V2O5, and VO(acac)2) and DMF, followed by carbonization treatment. The formation of quasi-metallic VO (∼102 Ω−1 cm−1) in the CNF enhanced the rate of electron transfer in the web-like electrode. Besides, the well-developed pore structure and rich vanadium redox couples also promoted the rapid ion transfer rate. As a consequence, the SC equipped with symmetric CNFs/VO/VOx electrodes exhibited a specific capacitance of 325.7 F g−1 at 1 A g−1 with 92% retention at 4 A g−1 after 5000 cycles.

What's the cathode?

activated carbon (AC)

Second, the electrode capacity was calculated (eqn (4)) to be 453 mA h g−1 and third, by integrating the potential profile (eqn (5)), the gravimetric electrode energy density was estimated to be 570 W h kg−1. Both values are here for the organic redox-active part (AQ4), while if we take into account the entire electrode (AQnC72), we arrive at significantly more modest values: 222 mA h g−1 and 279 W h kg−1 (Table S8†). Neither of these measures are totally fair to be compared with traditional electrodes as we also assume a role as current collector for the graphene. With this caveat, the former measure provides twice the experimentally measured capacity of pure AQ, reported to be 217 mA h g−1 ( ) and a theoretical gravimetric energy density comparable to the cathode active materials LiFePO4 (544 W h kg−1) and LiMn2O4 (548 W h kg−1).

What's the cathode?

LiFePO4

During the initial discharge on the cathode:

What's the cathode?

In the last few decades, the demand for batteries has rapidly increased in various application fields. Although rechargeable batteries (such as lithium-ion batteries) have become a focus of attention, primary batteries still occupy a large market share in many civilizations and military applications such as portable power sources, signal lights, and space and ocean exploration. As a result, efforts to develop primary batteries with high energy density, high power density, and low cost have never stopped ever since the invention of volt batteries in 18th century. Especially in recent years, large number of batteries with high energy density have been developed, such as the Zn–O2, Mg–O2, Al–O2, Na–O2, and Li–O2 systems. In addition, primary batteries, such as the currently developed Li–SO2, Li–SOCl2, Li–MnO2, and Li-CFx systems, have also been developed under isolated conditions for applications such as ocean and out space. Primary batteries are usually required to have high energy density, long shelf life, stable discharge voltage, and low self-discharge rate and should be readily available for use. Currently, the Li/MnO2 batteries account for more than 80% share of the lithium primary battery market due to their low cost, high safety, and natural abundance of Li and Mn. Moreover, the Li/MnO2 batteries possess a high practical specific energy of 308 W h kg−1 and a long shelf life of about 10 years. However, the application of Li/MnO2 batteries is limited by their poor electrochemical performance at high power output and low temperature, which is ascribed to several issues including low electronic conductivity, low lithium diffusion coefficient, and structural susceptibility of the MnO2 cathode. To address these issues, numerous methods, such as structural design and composite, coating, and metal cation doping, have been studied to optimize the performance of the MnO2 cathode. In addition, to overcome the intrinsic limitations of the current cathode materials, exploring novel cathode materials is a potential strategy for promoting the performance of lithium primary batteries.

What's the cathode?

MnO2

In the last few decades, the demand for batteries has rapidly increased in various application fields. Although rechargeable batteries (such as lithium-ion batteries) have become a focus of attention, primary batteries still occupy a large market share in many civilizations and military applications such as portable power sources, signal lights, and space and ocean exploration. As a result, efforts to develop primary batteries with high energy density, high power density, and low cost have never stopped ever since the invention of volt batteries in 18th century. Especially in recent years, large number of batteries with high energy density have been developed, such as the Zn–O2, Mg–O2, Al–O2, Na–O2, and Li–O2 systems. In addition, primary batteries, such as the currently developed Li–SO2, Li–SOCl2, Li–MnO2, and Li-CFx systems, have also been developed under isolated conditions for applications such as ocean and out space. Primary batteries are usually required to have high energy density, long shelf life, stable discharge voltage, and low self-discharge rate and should be readily available for use. Currently, the Li/MnO2 batteries account for more than 80% share of the lithium primary battery market due to their low cost, high safety, and natural abundance of Li and Mn. Moreover, the Li/MnO2 batteries possess a high practical specific energy of 308 W h kg−1 and a long shelf life of about 10 years. However, the application of Li/MnO2 batteries is limited by their poor electrochemical performance at high power output and low temperature, which is ascribed to several issues including low electronic conductivity, low lithium diffusion coefficient, and structural susceptibility of the MnO2 cathode. To address these issues, numerous methods, such as structural design and composite, coating, and metal cation doping, have been studied to optimize the performance of the MnO2 cathode. In addition, to overcome the intrinsic limitations of the current cathode materials, exploring novel cathode materials is a potential strategy for promoting the performance of lithium primary batteries.

What's the cathode?

MnO2

In order to further study the chemisorption of LiPSs on the electrodes, we carried out the visual adsorption experiment of Li2S6 on the CNF@V2S3 cathodes. As shown in Fig. 3g, the dark-yellow colour of Li2S6 solution containing CNF@V2S3 fades in the visible region obviously after 24 h; while the solution containing CNFs, by contrast, almost shows no change in its colour, directly demonstrating that CNF@V2S3 cathodes can anchor LiPSs strongly. Moreover, we also compare the colour change of the polypropylene separators used in the assembled LSBs. As shown in Fig. S6,† after 100 cycles, the separator in the LSBs based on CNF@V2S3 composites almost retains its own white colour; while that based on CNF@V2S3 and CNFs cathodes show an apparent change in colour from white to light yellow, especially that based on CNFs. These visual comparisons of the change in colour again indicate that the CNF@V2S3 composites could greatly inhibit the shuttling of LiPSs between the electrodes. Furthermore, first-principles simulations based on density functional theory (DFT) have been applied to theoretically study the chemisorption behaviours of LiPSs. Fig. 2h shows the adsorption conformation of LiPSs (Li2Sn: n = 2, 4 and 6) on the (204) plane of V2S3. And the Eads between V2S3 and Li2S/Li2S4/Li2S6 were estimated to be 1.29, 0.81 and 1.02 eV, which are much higher than those values of the Eads between C and LiPSs.

What's the cathode?

CNF@V2S3

In order to further study the chemisorption of LiPSs on the electrodes, we carried out the visual adsorption experiment of Li2S6 on the CNF@V2S3 cathodes. As shown in Fig. 3g, the dark-yellow colour of Li2S6 solution containing CNF@V2S3 fades in the visible region obviously after 24 h; while the solution containing CNFs, by contrast, almost shows no change in its colour, directly demonstrating that CNF@V2S3 cathodes can anchor LiPSs strongly. Moreover, we also compare the colour change of the polypropylene separators used in the assembled LSBs. As shown in Fig. S6,† after 100 cycles, the separator in the LSBs based on CNF@V2S3 composites almost retains its own white colour; while that based on CNF@V2S3 and CNFs cathodes show an apparent change in colour from white to light yellow, especially that based on CNFs. These visual comparisons of the change in colour again indicate that the CNF@V2S3 composites could greatly inhibit the shuttling of LiPSs between the electrodes. Furthermore, first-principles simulations based on density functional theory (DFT) have been applied to theoretically study the chemisorption behaviours of LiPSs. Fig. 2h shows the adsorption conformation of LiPSs (Li2Sn: n = 2, 4 and 6) on the (204) plane of V2S3. And the Eads between V2S3 and Li2S/Li2S4/Li2S6 were estimated to be 1.29, 0.81 and 1.02 eV, which are much higher than those values of the Eads between C and LiPSs.

What's the cathode?

CNF@V2S3

In order to understand the relationship between self-discharge and electrolyte composition, the Li/LiV2(PO4)3 batteries were disassembled after one-week storage. The images of the Al current collectors and metallic Li of the disassembled battery are shown in Fig. 2. As shown in Fig. 2a and j, the Al current collector was obviously corroded, and many black precipitates were formed at the edge of the metallic Li electrode with the LPE-EC electrolyte. However, the appearances of the Al current collector and metallic Li electrode of the batteries with the LPE-PC and LPE-PC–LiBOB electrolytes were significantly better, as displayed in Fig. 2b, c, k and l, showing smooth Al current collectors and clean metallic Li surfaces. SEM was performed in order to verify the actual condition of the Al surface. As shown in the SEM images of the Al current collectors (Fig. 2d and g), the surface morphologies of the corroded Al current collector of the battery with the LPE-EC electrolyte displayed a high pit density, whereas those in the case of the batteries with the LPE-PC and LPE-PC–LiBOB electrolytes were smoother. The pits on the Al current collector were probably caused by the electrochemical oxidation of EC at high potentials generates a proton and triggers the chemical corrosion of aluminum foils. Pitting corrosion of the Al current collector can cause serious problems: passivation of cathode materials, increase of electrical resistance, contamination of the electrolyte, and reduction reactions of dissolved Al3+ on the anode side. Thus, the observed black precipitates on the metallic Li electrode could be the Li–Al alloy (LixAl) or a mixture of metallic Li and Li–Al alloy (cathodic: Al → Al3+ + 3e−; anodic: Al3+ + 3e− + Li → LixAl). The growth of precipitates can lead to separator piercing and internal short circuit of a battery, which is responsible for the rapid voltage drop and self-discharge of the Li/LiV2(PO4)3 batteries with the LPE-EC electrolyte (Fig. 1b). SEM images of the surface of the Al current collector of a battery with the LPE-PC and LPE-PC–LiBOB electrolytes after one-week storage are shown in Fig. 2e and h and Fig. 2f and i, respectively. Almost no obvious pitting corrosion was observed on the surfaces of the Al current collectors, confirming the better shelf stabilities of the Li/LiV2(PO4)3 batteries with the LPE-PC and LPE-PC–LiBOB electrolytes than that of the Li/LiV2(PO4)3 battery with the LPE-EC electrolyte (Fig. 1c and d), respectively. Therefore, the electrolyte composition can affect the stability of the current collector. The corrosion of the current collectors is a key reason for the self-discharge behavior of batteries.

What's the cathode?

The FIB-SEM tomography and electrochemical testing results together strongly suggest that the severe mechanical degradation is the main cause of the rapid capacity fade in the SSB composite electrode, particularly in later cycles. In current study, the crack development is mostly observed near the cathode/solid electrolyte interface, which suggests that the cathode particle volume change is the origin of the mechanical failure during cycling. Thus, minimizing this mechanical degradation is critical to extending the cycle life of SSBs.

What's the cathode?

When used as cathode materials, anhydrides and imides of tetracarboxylic aromatic acids undergo two-electron reversible reduction, as depicted in Scheme 2. The reduction of the remaining carbonyls occurs at low voltages; it is generally considered irreversible, although some data on using these structures as anode materials indicate the opposite (see Section 3.3).

What's the cathode?

anhydrides and imides of tetracarboxylic aromatic acids

Nowadays, a variety of multifunctional electrocatalysts are being developed while there is still a lack of suitable design to fully realize their multifunctionalities. Here, we propose a general, simple, two-component design of an electrolyser to replace the traditional three-component design for decoupled water splitting. Trifunctional (OER, HER and ORR) electrocatalysts (such as nickel sulfide foams with surface grown N-doped carbon nanotube arrays) are used as the gas evolution electrode to replace both the cathode and the anode, while materials with suitable redox activities (NaTi2(PO4)3 or commercial Ni(OH)2) are used as the relay electrode. In such a design, the H2/O2 evolution can be switched by reversing the current polarity, and the ORR before the HER consumes the residual O2 left in the electrolyser, guaranteeing the high purity (∼99.9%) of the as-obtained H2. With NaTi2(PO4)3 as the relay electrode and the nickel sulfide foam as the gas evolution electrode, owing to the high decoupling efficiency of the NaTi2(PO4)3 relay (97%) and the low HER/OER overpotentials of the trifunctional nickel sulfide foam, an energy conversion efficiency of up to 94.3% can be obtained for the as-assembled electrolyser at a current density of 10 mA cm−2. When combined with a commercial Si PV module with an efficiency of 14.4%, the as-designed PV-electrolysis system showed a solar-to-hydrogen conversion efficiency of up to 10.4%. Utilization of trifunctional electrocatalysts greatly reduces the complexity of the electrolyser and the overall cost for electrochemical H2 production, and these electrolysers may potentially be used to construct highly competitive water splitting systems for continuous H2 production and green energy harvesting. Our research may also bring new insights into the utilization of multifunctional electrocatalysts in other devices, such as metal–air batteries and fuel cells.

What's the cathode?

Nowadays, a variety of multifunctional electrocatalysts are being developed while there is still a lack of suitable design to fully realize their multifunctionalities. Here, we propose a general, simple, two-component design of an electrolyser to replace the traditional three-component design for decoupled water splitting. Trifunctional (OER, HER and ORR) electrocatalysts (such as nickel sulfide foams with surface grown N-doped carbon nanotube arrays) are used as the gas evolution electrode to replace both the cathode and the anode, while materials with suitable redox activities (NaTi2(PO4)3 or commercial Ni(OH)2) are used as the relay electrode. In such a design, the H2/O2 evolution can be switched by reversing the current polarity, and the ORR before the HER consumes the residual O2 left in the electrolyser, guaranteeing the high purity (∼99.9%) of the as-obtained H2. With NaTi2(PO4)3 as the relay electrode and the nickel sulfide foam as the gas evolution electrode, owing to the high decoupling efficiency of the NaTi2(PO4)3 relay (97%) and the low HER/OER overpotentials of the trifunctional nickel sulfide foam, an energy conversion efficiency of up to 94.3% can be obtained for the as-assembled electrolyser at a current density of 10 mA cm−2. When combined with a commercial Si PV module with an efficiency of 14.4%, the as-designed PV-electrolysis system showed a solar-to-hydrogen conversion efficiency of up to 10.4%. Utilization of trifunctional electrocatalysts greatly reduces the complexity of the electrolyser and the overall cost for electrochemical H2 production, and these electrolysers may potentially be used to construct highly competitive water splitting systems for continuous H2 production and green energy harvesting. Our research may also bring new insights into the utilization of multifunctional electrocatalysts in other devices, such as metal–air batteries and fuel cells.

What's the anode?

Therefore, the above results demonstrate the superior energy/power densities and high efficiency of the DBHF sample, which, based on the above discussions, are associated with the “fiber-in-tube” hierarchical structure, hollow and defective oxide bubbles and conductive and porous carbon network of the DBHF fibers. The combined effects of the features are favourable for the generation of more active sites, superior catalytic activities and enhanced kinetics of the DBHF cathode in the hybrid batteries.

What's the cathode?

DBHF

To better understand the good cycling stability of the full cell in the Na–H2O–urea–DMF electrolyte, inductively coupled plasma emission spectrometry-atomic emission spectroscopy (ICP-AES) analysis of the vanadium concentration in the Na–H2O–urea–DMF electrolyte and analysis of electrochemical impedance parameters were carried out before and after cycling. As shown in Fig. 4b and Table S3,† it is found that the concentration of V-ions in the Na–H2O–urea–DMF electrolyte before cycling is 0.05 μg mL−1. After 50 cycles and after 100 cycles, the concentrations of V-ions in the Na–H2O–urea–DMF electrolyte are 16.65 μg mL−1 and 20.12 μg mL−1, respectively. In 1 M Na2SO4 solution, the concentrations of V-ions are 0.08 μg mL−1, 60.68 μg mL−1 and 72.34 μg mL−1, respectively. Compared to 1 M Na2SO4 solution, the Na–H2O–urea–DMF electrolyte can obviously inhibit the dissolution of the electrode materials. Upon comparison of the V-ion concentrations in NaClO4 solution, Na–H2O–DMF, and Na–H2O–urea electrolyte of the NVP//NTP full cell after 50 cycles (Fig. S14†), it is found that the Na–H2O–urea–DMF electrolyte can suppress the dissolution of V-ions in the charge–discharge process. This result is consistent with the cycling performances of the NVP//NTP full cell in different electrolytes (Fig. S13†). Thus, it can be concluded that the NVP/C cathode is much more stable in the Na–H2O–urea–DMF electrolyte, which is beneficial for the long lifespan of the full cell. Similarly, as illustrated in Fig. S15,† the NTP anode in the Na–H2O–urea–DMF electrolyte displays high coulombic efficiency and good cycling stability relative to that in 17 M NaClO4 and 1 M Na2SO4 solution.

What's the cathode?

NVP/C

To better understand the good cycling stability of the full cell in the Na–H2O–urea–DMF electrolyte, inductively coupled plasma emission spectrometry-atomic emission spectroscopy (ICP-AES) analysis of the vanadium concentration in the Na–H2O–urea–DMF electrolyte and analysis of electrochemical impedance parameters were carried out before and after cycling. As shown in Fig. 4b and Table S3,† it is found that the concentration of V-ions in the Na–H2O–urea–DMF electrolyte before cycling is 0.05 μg mL−1. After 50 cycles and after 100 cycles, the concentrations of V-ions in the Na–H2O–urea–DMF electrolyte are 16.65 μg mL−1 and 20.12 μg mL−1, respectively. In 1 M Na2SO4 solution, the concentrations of V-ions are 0.08 μg mL−1, 60.68 μg mL−1 and 72.34 μg mL−1, respectively. Compared to 1 M Na2SO4 solution, the Na–H2O–urea–DMF electrolyte can obviously inhibit the dissolution of the electrode materials. Upon comparison of the V-ion concentrations in NaClO4 solution, Na–H2O–DMF, and Na–H2O–urea electrolyte of the NVP//NTP full cell after 50 cycles (Fig. S14†), it is found that the Na–H2O–urea–DMF electrolyte can suppress the dissolution of V-ions in the charge–discharge process. This result is consistent with the cycling performances of the NVP//NTP full cell in different electrolytes (Fig. S13†). Thus, it can be concluded that the NVP/C cathode is much more stable in the Na–H2O–urea–DMF electrolyte, which is beneficial for the long lifespan of the full cell. Similarly, as illustrated in Fig. S15,† the NTP anode in the Na–H2O–urea–DMF electrolyte displays high coulombic efficiency and good cycling stability relative to that in 17 M NaClO4 and 1 M Na2SO4 solution.

What's the anode?

NTP

To better understand the good cycling stability of the full cell in the Na–H2O–urea–DMF electrolyte, inductively coupled plasma emission spectrometry-atomic emission spectroscopy (ICP-AES) analysis of the vanadium concentration in the Na–H2O–urea–DMF electrolyte and analysis of electrochemical impedance parameters were carried out before and after cycling. As shown in Fig. 4b and Table S3,† it is found that the concentration of V-ions in the Na–H2O–urea–DMF electrolyte before cycling is 0.05 μg mL−1. After 50 cycles and after 100 cycles, the concentrations of V-ions in the Na–H2O–urea–DMF electrolyte are 16.65 μg mL−1 and 20.12 μg mL−1, respectively. In 1 M Na2SO4 solution, the concentrations of V-ions are 0.08 μg mL−1, 60.68 μg mL−1 and 72.34 μg mL−1, respectively. Compared to 1 M Na2SO4 solution, the Na–H2O–urea–DMF electrolyte can obviously inhibit the dissolution of the electrode materials. Upon comparison of the V-ion concentrations in NaClO4 solution, Na–H2O–DMF, and Na–H2O–urea electrolyte of the NVP//NTP full cell after 50 cycles (Fig. S14†), it is found that the Na–H2O–urea–DMF electrolyte can suppress the dissolution of V-ions in the charge–discharge process. This result is consistent with the cycling performances of the NVP//NTP full cell in different electrolytes (Fig. S13†). Thus, it can be concluded that the NVP/C cathode is much more stable in the Na–H2O–urea–DMF electrolyte, which is beneficial for the long lifespan of the full cell. Similarly, as illustrated in Fig. S15,† the NTP anode in the Na–H2O–urea–DMF electrolyte displays high coulombic efficiency and good cycling stability relative to that in 17 M NaClO4 and 1 M Na2SO4 solution.

What's the electrolyte?

Na–H2O–urea–DMF

P. M. V. conceived the idea of the nanomesh-based cathodes and their development was supervised by S. P. Z. S. P. Z. developed the method for coating the nanomesh with MnO2 and its thermal activation and performed thermodynamic simulations and electrochemical testing. D. C. contributed to the development of the MnO2 coating, activation of the cathodes with Li citrate and electrochemical testing. D. C. also assisted in the in situ XRD experiments performed by F. Mattelaer in the group of C. Detavernier, University of Ghent. The manuscript was written by S. P. Z. and revised by P. M. V. All authors have given approval to the final version of the manuscript.

What's the anode?

nanomesh-based

Chemical reactions during storage generate electrically insulating carbonate which elevates internal cell resistance and also alters the surface morphology. The electronic and ionic insulation of carbonate induces a sequence of chemical/electrochemical reaction behaviors. For instance, the transformed Lewis base surfaces can react with electrolytic species, generating detrimental species such as HF and gases. Therefore, extra cautions are required when preparing or handling LiNiO2-based materials before the battery assembly (Fig. 1). Herein, we evaluate the battery performance of the LiNiO2 cathode after storage in the dry box (humidity: ∼30%) and Ar glove box (the water level was ∼0.5 ppm) for two weeks. Two weeks would be a reasonable time frame between the synthesis of cathode powders and their processing into batteries in the actual manufacturing.

What's the cathode?

LiNiO2

However, due to the slow kinetics of both HER at the cathode and UOR at the anode in urea–water electrolysis, electrocatalysts on the electrodes are necessary to speed up the reactions to achieve high energy efficiency. Traditional noble metal catalysts, such as Pt/C, RuO2, or IrO2, can effectively catalyze monolithic electrolysis, but their low abundance and high cost limit their wide application. Therefore, it is necessary to explore efficient non-noble metal electrocatalysts, such as transition metal compounds, transition metal oxides, macrocycles, nitrides, sulfides, phosphides, etc. Among these catalyst materials, the phosphides have been explored most recently, mainly due to their good conductivity and wettability, including MoP, CoP, FeP, and Ni2P. With regard to cost, abundance, catalytic activity, stability and applicable pH range, MoP has been shown to be a promising candidate.

What's the cathode?

The UTCNF cathode was prepared via a simple and efficient ultrasonic-treatment strategy. In detail, a piece of pre-cleaned CNF (1 cm × 3 cm × 0.15 cm), thoroughly washed with acetone and water, was immersed in 3 M HCl. Next, the solution was transferred to an ultrasonic apparatus and ultrasonicated for 10 min. After the ultrasonic treatment, the CNF was rinsed with distilled water twice and dried at 60 °C to obtain the UTCNF. The mass loading of the cobalt/nickel composite hydroxide, obtained by comparison of the weight change between UTCNF and CNF, is ∼6.0 mg cm−2.

What's the cathode?

UTCNF

Lithium–sulfur batteries have low material costs and high energy densities, which have attracted considerable research interest for application in next-generation energy-storage systems. However, the practical applications of Li–S batteries face challenges owing to their poor sulfur utilization, service lifetimes, and rate capability. Recently, great progress has been made in the design, synthesis, and application of micro/nanostructured metal sulfides to address obstacles facing Li–S batteries. This review aims to highlight valuable concepts from the latest reports. Major approaches to improve sulfur cathodes and strategies for preparing metal sulfide-based materials are first summarized with a particular focus on their main functions and useful properties. Then, the electrochemical activities of metal sulfides are classified and their applications in Li–S batteries are introduced to provide a fundamental understanding of the material interactions involved. In parallel, advancements in the use of interlayers, modification of separators, and protection of lithium anodes that involve metal sulfides are surveyed. Finally, special attention is paid to the general design principles, future prospects, and challenges facing metal sulfides for high-energy-density Li–S batteries.

What's the cathode?

sulfur

The technology of engineering nanostructured materials is usually applied to create numerous active sites, which can significantly improve the electrochemical performance. Recently, the 2D morphology of nanosheet materials with high specific area has been proved to effectively accelerate the electrochemical kinetics between the active materials and the electrolytes, which are therefore supposed to be promising materials for energy storage. Unfortunately, the serious aggregation behavior in 2D materials, derived from the van der Waals forces among individual nanosheets, will decrease the exposed active sites, thus leading to poor conductivity. Carbon cloth (CC) has been known to be an inexpensive substrate/current collector capable of providing continuous pathways for the transport of electrons. Following the above-mentioned discussions, it would be reasonable to believe that an intriguing way of overcoming the above problems is the in situ growth of 2D nanosheet compounds on a flexibly conductive substrate to obtain a free-standing cathode, aiming at achieving good rate capability, high capacity, and energy density simultaneously.

What's the cathode?

A carbon nanotube film was used as the oxygen cathode and Li foil with 0.5 mm thickness as the anode for LOBs. A glass fiber separator wetted with a 100 μL electrolyte composed of 1.0 M LiTFSI in TEGDME was employed to separate the cathode and anode. All assembled cells were measured on a LAND electrochemical testing system at 25 °C. The cycling test was conducted with the capacity limited at 1000 mA h g−1 at a current density of 300 mA g−1.

What's the cathode?

oxygen

A carbon nanotube film was used as the oxygen cathode and Li foil with 0.5 mm thickness as the anode for LOBs. A glass fiber separator wetted with a 100 μL electrolyte composed of 1.0 M LiTFSI in TEGDME was employed to separate the cathode and anode. All assembled cells were measured on a LAND electrochemical testing system at 25 °C. The cycling test was conducted with the capacity limited at 1000 mA h g−1 at a current density of 300 mA g−1.

What's the anode?

Li foil

Co-free Ni-rich cathode materials, particularly LiNiO2 and its doped analogs, face ongoing challenges as the need for commercialization rises. To address surface instability and its associated phenomena, a systematic and accurate understanding is highly required. In this work, we systematically studied the fragile surfaces of the LiNiO2-based cathode materials, with the goal of identifying how experimental conditions may influence characterization results. We presented the challenges in characterizing and analyzing the surface chemistry of LiNiO2-based materials using two comparative cathodes as a model platform, i.e., LiNiO2 versus Mg/Ti–LiNiO2. The surface lithium residuals are inevitable and highly dependent on various sample storage and handling conditions. Due to the highly reactive surfaces, we found that the sample preparation for electron microscopy and surface-sensitive X-ray analyses greatly influences the final observations, resulting in skewed surface chemistry analytical results. We also provided some recommendations regarding how to obtain representative characterization analyses. Using simple comparisons, we further illustrated the advantages of Mg/Ti dual dopants to enhance the surface structural resistance to many circumstances, while corroborating previously reported observations. However, we found that the surface of Mg/Ti–LiNiO2 material still encounters a series of problems caused by the fragile CEIs at elevated temperatures. Efforts should be devoted towards the development of highly stable CEIs either by cathode surface coating, doping, electrolyte modification or combining multiple strategies.

What's the cathode?

Co-free Ni-rich

Co-free Ni-rich cathode materials, particularly LiNiO2 and its doped analogs, face ongoing challenges as the need for commercialization rises. To address surface instability and its associated phenomena, a systematic and accurate understanding is highly required. In this work, we systematically studied the fragile surfaces of the LiNiO2-based cathode materials, with the goal of identifying how experimental conditions may influence characterization results. We presented the challenges in characterizing and analyzing the surface chemistry of LiNiO2-based materials using two comparative cathodes as a model platform, i.e., LiNiO2 versus Mg/Ti–LiNiO2. The surface lithium residuals are inevitable and highly dependent on various sample storage and handling conditions. Due to the highly reactive surfaces, we found that the sample preparation for electron microscopy and surface-sensitive X-ray analyses greatly influences the final observations, resulting in skewed surface chemistry analytical results. We also provided some recommendations regarding how to obtain representative characterization analyses. Using simple comparisons, we further illustrated the advantages of Mg/Ti dual dopants to enhance the surface structural resistance to many circumstances, while corroborating previously reported observations. However, we found that the surface of Mg/Ti–LiNiO2 material still encounters a series of problems caused by the fragile CEIs at elevated temperatures. Efforts should be devoted towards the development of highly stable CEIs either by cathode surface coating, doping, electrolyte modification or combining multiple strategies.

What's the cathode?

LiNiO2-based

In this paper, MoP and NiCo-LDH are used to synthesize a composite of MoP@NiCo-LDH, which is supported on nickel foam to form a bifunctional composite catalyst (abbreviated as MoP@NiCo-LDH/NF), as shown in Fig. 1. During the synthesis, different electrodeposition times are used to optimize the catalyst's performance toward both UOR and HER. Both instrumental characterization and electrochemical measurements confirm that the composition between MoP and NiCo-LDH as well as their electrodeposition onto the NF surface can increase the material's specific surface area and produce a synergetic effect toward high performance. To validate this bifunctional catalyst, a two-electrode electrolyser (MoP@NiCo-LDH/NF-20‖MoP@NiCo-LDH/NF-20) with this catalyst on both the anode and the cathode was designed and constructed, and the test results show that this catalyst has better catalytic performance than that of a Pt/C/NF‖IrO2/NF electrolyser, where noble metal materials (Pt and IrO2) are used as the catalysts.

What's the cathode?

In this paper, MoP and NiCo-LDH are used to synthesize a composite of MoP@NiCo-LDH, which is supported on nickel foam to form a bifunctional composite catalyst (abbreviated as MoP@NiCo-LDH/NF), as shown in Fig. 1. During the synthesis, different electrodeposition times are used to optimize the catalyst's performance toward both UOR and HER. Both instrumental characterization and electrochemical measurements confirm that the composition between MoP and NiCo-LDH as well as their electrodeposition onto the NF surface can increase the material's specific surface area and produce a synergetic effect toward high performance. To validate this bifunctional catalyst, a two-electrode electrolyser (MoP@NiCo-LDH/NF-20‖MoP@NiCo-LDH/NF-20) with this catalyst on both the anode and the cathode was designed and constructed, and the test results show that this catalyst has better catalytic performance than that of a Pt/C/NF‖IrO2/NF electrolyser, where noble metal materials (Pt and IrO2) are used as the catalysts.

What's the anode?

Fig. S3d† and 4d display cyclic voltammogram (CV) curves obtained using the S-C(PAN) anode with bare LMO and LMO-30 min in the potential range of 1.0–3.2 V at a scan rate of 0.1 mV s−1, respectively. One peak is observed in both positive and negative scans in both samples. For the LMO-30 min cathode, a more pronounced peak appearing at 2.63 V in the first positive scan is observed and the peak is shifted to 2.45 V in the subsequent cycles. The peak of the negative scan appears at 1.77 V as depicted in Fig. 4d, which shows that PVDF@LGLZNO does not practically involve in the redox reactions of the tested voltage range, thus the composite coating must be stable and did not contribute to the capacity. The large polarization between the peaks in the first and following cycles is due to SPAN activation energy and the SEI formation. This means that more energy is needed to dissociate the sulfur atom from the pyridine-derivative, which is usually termed as the unique activation process of sulfurized polyacrylonitrile.

What's the cathode?

LMO

Fig. S3d† and 4d display cyclic voltammogram (CV) curves obtained using the S-C(PAN) anode with bare LMO and LMO-30 min in the potential range of 1.0–3.2 V at a scan rate of 0.1 mV s−1, respectively. One peak is observed in both positive and negative scans in both samples. For the LMO-30 min cathode, a more pronounced peak appearing at 2.63 V in the first positive scan is observed and the peak is shifted to 2.45 V in the subsequent cycles. The peak of the negative scan appears at 1.77 V as depicted in Fig. 4d, which shows that PVDF@LGLZNO does not practically involve in the redox reactions of the tested voltage range, thus the composite coating must be stable and did not contribute to the capacity. The large polarization between the peaks in the first and following cycles is due to SPAN activation energy and the SEI formation. This means that more energy is needed to dissociate the sulfur atom from the pyridine-derivative, which is usually termed as the unique activation process of sulfurized polyacrylonitrile.

What's the anode?

S-C(PAN)

Fig. 3 shows the electrochemical performances of a half-cell in a three-electrode system and a NVP//NTP full cell. As shown in Fig. 3a and b, the NVP half-cell exhibits a pair of redox peaks with good symmetry at 0.68 V and 0.42 V vs. Ag/AgCl, which can be ascribed to the reversible conversion of the V3+/V4+ couple, corresponding to the reversible extraction/insertion of Na+ in the NVP electrode. The half-cell delivers a specific charge and discharge capacity of 110 mA h g−1 and 96 mA h g−1 in the first cycle, and exhibits good reversibility for the first time. The irreversible charge capacity mainly resulted from the dissolution of the V-ions until the appearance of O2 evolution which was recorded in CV curves. For NTP, a pair of Ti4+/Ti3+ redox peaks at −0.55 V and −0.82 V vs. Ag/AgCl can be found in the CV curves. As seen in Fig. 3c, the CV curves show that the hydrogen evolution reaction still does not occur when the potential is −1.1 V vs. Ag/AgCl, which suggests that the Na–H2O–urea–DMF electrolyte can inhibit the side reaction well. As shown in Fig. 3d, the NTP in the Na–H2O–urea–DMF electrolyte delivers a pristine discharge capacity of 118 mA h g−1, concurrently exhibiting a perfect platform and good reversibility, which is better than that of a NTP anode with cationic doping. As shown in Fig. S10 and S11,† the NTP anode exhibits an impressive cycling stability in a conventional non-aqueous electrolyte. However, the coulombic efficiency and cycling performance of the NVP cathode at 2C in a conventional non-aqueous electrolyte are poor due to the dissolution of V-ions and change of structure.

What's the cathode?

NVP

Fig. 3 shows the electrochemical performances of a half-cell in a three-electrode system and a NVP//NTP full cell. As shown in Fig. 3a and b, the NVP half-cell exhibits a pair of redox peaks with good symmetry at 0.68 V and 0.42 V vs. Ag/AgCl, which can be ascribed to the reversible conversion of the V3+/V4+ couple, corresponding to the reversible extraction/insertion of Na+ in the NVP electrode. The half-cell delivers a specific charge and discharge capacity of 110 mA h g−1 and 96 mA h g−1 in the first cycle, and exhibits good reversibility for the first time. The irreversible charge capacity mainly resulted from the dissolution of the V-ions until the appearance of O2 evolution which was recorded in CV curves. For NTP, a pair of Ti4+/Ti3+ redox peaks at −0.55 V and −0.82 V vs. Ag/AgCl can be found in the CV curves. As seen in Fig. 3c, the CV curves show that the hydrogen evolution reaction still does not occur when the potential is −1.1 V vs. Ag/AgCl, which suggests that the Na–H2O–urea–DMF electrolyte can inhibit the side reaction well. As shown in Fig. 3d, the NTP in the Na–H2O–urea–DMF electrolyte delivers a pristine discharge capacity of 118 mA h g−1, concurrently exhibiting a perfect platform and good reversibility, which is better than that of a NTP anode with cationic doping. As shown in Fig. S10 and S11,† the NTP anode exhibits an impressive cycling stability in a conventional non-aqueous electrolyte. However, the coulombic efficiency and cycling performance of the NVP cathode at 2C in a conventional non-aqueous electrolyte are poor due to the dissolution of V-ions and change of structure.

What's the anode?

NTP

Fig. 3 shows the electrochemical performances of a half-cell in a three-electrode system and a NVP//NTP full cell. As shown in Fig. 3a and b, the NVP half-cell exhibits a pair of redox peaks with good symmetry at 0.68 V and 0.42 V vs. Ag/AgCl, which can be ascribed to the reversible conversion of the V3+/V4+ couple, corresponding to the reversible extraction/insertion of Na+ in the NVP electrode. The half-cell delivers a specific charge and discharge capacity of 110 mA h g−1 and 96 mA h g−1 in the first cycle, and exhibits good reversibility for the first time. The irreversible charge capacity mainly resulted from the dissolution of the V-ions until the appearance of O2 evolution which was recorded in CV curves. For NTP, a pair of Ti4+/Ti3+ redox peaks at −0.55 V and −0.82 V vs. Ag/AgCl can be found in the CV curves. As seen in Fig. 3c, the CV curves show that the hydrogen evolution reaction still does not occur when the potential is −1.1 V vs. Ag/AgCl, which suggests that the Na–H2O–urea–DMF electrolyte can inhibit the side reaction well. As shown in Fig. 3d, the NTP in the Na–H2O–urea–DMF electrolyte delivers a pristine discharge capacity of 118 mA h g−1, concurrently exhibiting a perfect platform and good reversibility, which is better than that of a NTP anode with cationic doping. As shown in Fig. S10 and S11,† the NTP anode exhibits an impressive cycling stability in a conventional non-aqueous electrolyte. However, the coulombic efficiency and cycling performance of the NVP cathode at 2C in a conventional non-aqueous electrolyte are poor due to the dissolution of V-ions and change of structure.

What's the anode?

NTP

For the operando-XRD measurements self-standing cathode materials were prepared, using Li1.2Mn0.6Ni0.1Co0.1O2 powder, high-conductive carbon (ketjan black, Alfa Aesar, 99.99%) and PVDF (Kynar Flex, Arkema) dissolved in acetone (Poch czda) in a 70:10:20 wt% ratio, respectively. The slurry was spread on glass and dried in air for about 15 minutes at 80 °C in a glove box overnight as was discussed previously. For the tests, a home-made cell with a Berillium window, adjusted to a PANalytical Empyrean diffractometer, was assembled in an mBraun Unilab argon glovebox, using the same counter electrode and electrolyte as for the CR2032 cells. The operando-XRD measurements were conducted, using the PANalytical Empyrean diffractometer combined with s one-channel Biologic potentiostat/galvanostat with a C/50 current load applied, in the voltage regime of 2.0 V to 4.8 V.

What's the cathode?

Our study underlines the importance of gaining deeper insights into the anionic redox activities of Li-rich materials for designing a new cathode material for future LIBs. Promoting the reversible anionic redox contributions while suppressing the irreversible reactions is vital in designing a cathode material with high energy density and good cycling performance. The combined computational/experimental approach presented here can be applied to predict and verify the anionic redox activities in other Li-rich materials to accelerate the discovery of materials with high energy density and good cycling performance.

What's the cathode?

In this study, a flexible self-charging sodium-ion full battery (SCSFB) was feasibly designed by employing self-synthesized Na3V2(PO4)3@C as the cathode, commercial hard carbon as the anode, and a flexible BaTiO3-P(VDF-HFP) film immobilized with a liquid electrolyte of NaClO4 as the built-in piezoelectric gel-electrolyte (BaTiO3-P(VDF-HFP)-NaClO4). Owing to the excellent compatibilities of the electrodes, the flexible SCSFB delivers reasonable electrochemical performance including large specific capacity and stable cyclability. Besides, external mechanical energy from the ambient environment can also be simultaneously collected and stored via a persistent self-charging mode, whether with static compression, repeated bending or continuous palm patting. This work paves a feasible way to the achievement of sustainable sodium-ion full batteries with high flexibility and enhanced safety for wearable electronics that come into direct contact with human tissues.

What's the cathode?

Na3V2(PO4)3@C

In this study, a flexible self-charging sodium-ion full battery (SCSFB) was feasibly designed by employing self-synthesized Na3V2(PO4)3@C as the cathode, commercial hard carbon as the anode, and a flexible BaTiO3-P(VDF-HFP) film immobilized with a liquid electrolyte of NaClO4 as the built-in piezoelectric gel-electrolyte (BaTiO3-P(VDF-HFP)-NaClO4). Owing to the excellent compatibilities of the electrodes, the flexible SCSFB delivers reasonable electrochemical performance including large specific capacity and stable cyclability. Besides, external mechanical energy from the ambient environment can also be simultaneously collected and stored via a persistent self-charging mode, whether with static compression, repeated bending or continuous palm patting. This work paves a feasible way to the achievement of sustainable sodium-ion full batteries with high flexibility and enhanced safety for wearable electronics that come into direct contact with human tissues.

What's the anode?

hard carbon

ssZIBs are composed of four components including a cathode, an anode (zinc metal), a solid electrolyte and current collectors. The gel electrolyte is sandwiched between the cathode and anode. Each component has a significant effect on the performance of the whole battery. Currently, various gel electrolytes have been studied and have been applied in ZIBs. However, the performance of ssZIBs is still not comparable with that of the batteries using liquid electrolytes, especially high rate capability, due to the low ionic conductivities of gel electrolytes. To realize high ionic conductivity, an ideal gel electrolyte requires a high-water content and efficient ion migration channels, combined with reasonable mechanical properties. As one of the principal natural polymers, cellulose nanofibers (CNFs) are widely used as a sustainable reinforcing additive in various composites. In addition, the hydrophilic skeleton of CNFs and their three-dimensional fiber network with a large space can help stabilize the channels for charge transportation. Furthermore, due to the abundant hydroxyl groups on the surface of CNFs, it is facile to link the CNFs with polyacrylamide (PAM) molecular chains through hydrogen bonding. The combination of CNFs and PAM can serve as an effective solid electrolyte with good mechanical properties and high ionic conductivity. Due to the high thermal stability of hydrogel polymers, ssBs are also able to operate under both low and high temperature conditions, which is critical for the application of batteries in some harsh environments such as cold/frozen regions and flying airplanes in space.

What's the cathode?

ssZIBs are composed of four components including a cathode, an anode (zinc metal), a solid electrolyte and current collectors. The gel electrolyte is sandwiched between the cathode and anode. Each component has a significant effect on the performance of the whole battery. Currently, various gel electrolytes have been studied and have been applied in ZIBs. However, the performance of ssZIBs is still not comparable with that of the batteries using liquid electrolytes, especially high rate capability, due to the low ionic conductivities of gel electrolytes. To realize high ionic conductivity, an ideal gel electrolyte requires a high-water content and efficient ion migration channels, combined with reasonable mechanical properties. As one of the principal natural polymers, cellulose nanofibers (CNFs) are widely used as a sustainable reinforcing additive in various composites. In addition, the hydrophilic skeleton of CNFs and their three-dimensional fiber network with a large space can help stabilize the channels for charge transportation. Furthermore, due to the abundant hydroxyl groups on the surface of CNFs, it is facile to link the CNFs with polyacrylamide (PAM) molecular chains through hydrogen bonding. The combination of CNFs and PAM can serve as an effective solid electrolyte with good mechanical properties and high ionic conductivity. Due to the high thermal stability of hydrogel polymers, ssBs are also able to operate under both low and high temperature conditions, which is critical for the application of batteries in some harsh environments such as cold/frozen regions and flying airplanes in space.

What's the anode?

zinc metal

Recently, we successfully synthesized and deployed a new family of redox shuttle additives, aromatic cyclopropenium salts, for Na-ion batteries under harsh overcharging conditions. The cyclopropenium cation combines the elements of aromaticity and ionicity, leading to superior electrochemical stability and solubility over conventional neutral shuttle molecules. Together with the other enlightening studies of cyclopropenium salts in fields such as electrophotocatalysis and redox flow batteries, we envisioned the possibility of devising a high-potential cyclopropenium cation catered to Ni-rich cathodes.

What's the cathode?

Ni-rich

The electrochemical performances of bare LMO and LMO-30 min in a full cell (S-C(PAN)‖LMO) configuration at 1C-rate and 25 °C were compared as shown in Fig. 4a. The full cell (S-C(PAN)‖LMO) with the LMO-30 min cathode shows better capacity retention of 77% after 1000 cycles at 1C, corresponding to only 0.023% fading rate per cycle from the first cycle to the 1000th cycle. In contrast, the bare LMO delivers a retention capacity of 45%, about 0.055% capacity fading rate per cycle under the same conditions. The performance is noteworthy, since S. Wei et al. have reported 0.027% capacity decay rate per cycle calculated from the second to 1000th cycle, even at low current density (0.4C-rate) for the Li/S-C(PAN) cell.

What's the cathode?

To study the catalytic performance of MoS2, the potentiostatic reduction of Li2S8 (10 mM based on sulfur in tetraglyme solution) was conducted on CF-based current collectors. Here, 25 μL Li2S8 was dropped onto the CF, MoS2 sheet/CF, and MoS2 ND/CF current collectors as the cathodes. CR2032 coin cells were assembled using as-prepared cathodes, Li metal anodes, Celgard separators, and 25 μL 0.5 M LiTFSI tetraglyme electrolyte. The cells were discharged to 2.06 V at 0.112 mA, to reduce all the long chain polysulfides to Li2S4. Then, the cells were kept potentiostatically at 2.05 V to drive the nucleation and growth of Li2S until the current dropped below 10−5 A. The current–time curves were integrated based on Faraday's law to evaluate the capacities from the precipitation of Li2S on the various current collectors. To study the morphology of the precipitated Li2S, the operated cells were dissembled in a glovebox and washed with flooded DME before taking them for SEM observation.

What's the cathode?

CR2032 coin cells

To study the catalytic performance of MoS2, the potentiostatic reduction of Li2S8 (10 mM based on sulfur in tetraglyme solution) was conducted on CF-based current collectors. Here, 25 μL Li2S8 was dropped onto the CF, MoS2 sheet/CF, and MoS2 ND/CF current collectors as the cathodes. CR2032 coin cells were assembled using as-prepared cathodes, Li metal anodes, Celgard separators, and 25 μL 0.5 M LiTFSI tetraglyme electrolyte. The cells were discharged to 2.06 V at 0.112 mA, to reduce all the long chain polysulfides to Li2S4. Then, the cells were kept potentiostatically at 2.05 V to drive the nucleation and growth of Li2S until the current dropped below 10−5 A. The current–time curves were integrated based on Faraday's law to evaluate the capacities from the precipitation of Li2S on the various current collectors. To study the morphology of the precipitated Li2S, the operated cells were dissembled in a glovebox and washed with flooded DME before taking them for SEM observation.

What's the anode?

Li metal

To study the catalytic performance of MoS2, the potentiostatic reduction of Li2S8 (10 mM based on sulfur in tetraglyme solution) was conducted on CF-based current collectors. Here, 25 μL Li2S8 was dropped onto the CF, MoS2 sheet/CF, and MoS2 ND/CF current collectors as the cathodes. CR2032 coin cells were assembled using as-prepared cathodes, Li metal anodes, Celgard separators, and 25 μL 0.5 M LiTFSI tetraglyme electrolyte. The cells were discharged to 2.06 V at 0.112 mA, to reduce all the long chain polysulfides to Li2S4. Then, the cells were kept potentiostatically at 2.05 V to drive the nucleation and growth of Li2S until the current dropped below 10−5 A. The current–time curves were integrated based on Faraday's law to evaluate the capacities from the precipitation of Li2S on the various current collectors. To study the morphology of the precipitated Li2S, the operated cells were dissembled in a glovebox and washed with flooded DME before taking them for SEM observation.

What's the electrolyte?

25 μL 0.5 M LiTFSI tetraglyme

Later, Xu et al. investigated how the electrolyte affects the performance of 1a-based cathodes. It was shown that a KFSI solution in 1,2-dimethoxyethane (DME) enables much better rate capability and cycling stability of 1,5-AQDS, owing to a stable solid electrolyte interphase (SEI) forming in this electrolyte. For the optimized electrolyte composition, Qm was 84 mA h g−1 at 3C (390 mA g−1), and 80% of the capacity was retained after 1000 cycles.

What's the cathode?

Simultaneously, the predicted structural changes of O3-Nax[Ni0.5Mn0.5]O2 during Na+ extraction/insertion are presented in Fig. 5a. With extraction of 0.75 mol of Na from O3-Na1[Ni0.5Mn0.5]O2, the c-lattice parameter gradually increased from ∼15.97 to ∼17.40 Å because of the reinforced repulsive force between O2− anions. Then, after an additional 0.25 mol Na extraction, it decreased to ∼15.24 Å owing to MeO2 slab gliding arising from the structural instability, which matched well with the structural change of other layered-type cathode materials for SIBs. Furthermore, as shown in Fig. 5b, we compared the c-lattice parameters of various O3-/P3-NaxCa0.01[Ni0.5Mn0.5]O2 verified through Rietveld refinement based on operando XRD results and those of O3/P3-Nax[Ni0.5Mn0.5]O2 predicted through the first-principles calculations. We confirmed that despite the occurrence of a partial two-phase reaction, the c-lattice parameter was increased in general during extraction of Na ions from O3-Na1[Ni0.5Mn0.5]O2 to P3-Na0.25[Ni0.5Mn0.5]O2. In the case of Na0[Ni0.5Mn0.5]O2, although two phases were detected in the XRD patterns of NaxCa0.01[Ni0.5Mn0.5]O2 from the operando XRD analysis, the average value of c-lattice parameters on the two phases was similar to the c-lattice parameter predicted through first-principles calculations. Moreover, this result implies that at the Na0[Ni0.5Mn0.5]O2 composition, coexistence of O3′ and O3′′ phases is more favorable than perfect phase transition to the O3′′ phase, which agrees with the experimental results of previous research studies.

What's the cathode?

The comprehensive electrochemical and theoretical investigations suggested that the MoS2 NDs with strong LPS absorptivity and high catalytic property would be an ideal choice for robust electrochemical performance for LSBs. Thus, we systematically performed electrochemical measurements of MoS2 ND/porous carbon/Li2S6, in comparison with porous carbon/Li2S6 and MoS2 sheet/porous carbon/Li2S6 cathodes. Fig. 6a shows the CV curves of MoS2 ND/porous carbon/Li2S6 between 1.7–2.8 V vs. Li+/Li at a scan rate of 0.1 mV s−1. Two cathodic peaks at 2.26 and 2.01 V were delivered during the first scan, which were assigned to the reduction of long-chain polysulfides to short-chain Li2S4 and further to solid Li2S. The reversible oxidation of Li2S to polysulfides and to sulfur were presented by two anodic peaks at 2.39 and 2.47 V, respectively. In the following three cycles, the overlap of the discharge/charge peaks evidently indicated the excellent reversibility of the MoS2 ND/porous carbon/Li2S6 electrode. Note that a slight negative shift of the cathodic peak from 2.26 V in the 1st sweep to 2.28 V in the following sweeps could be observed, possibly due to the increased polarization as the starting material changed from a liquid catholyte to solid sulfur from the 2nd cycle. The excellent reversibility was also confirmed by discharging/charging at 0.1C for 100 cycles (Fig. 6b). The MoS2 ND/porous carbon/Li2S6 electrode presented a capacity of 1107 mA h g−1 at the 2nd cycle and retained 1020 mA h g−1 after 100 cycles, rendering a low capacity degradation rate of 0.08% per cycle. In contrast, the porous carbon/Li2S6 and MoS2 sheet/porous carbon/Li2S6 presented much higher capacity fading rates of 0.5% and 0.19% per cycle, respectively (Fig. S11, ESI†).

What's the cathode?

carbon/Li2S6

To further probe the electrochemical performance of the UTCNF, a kinetic analysis was conducted. Fig. 4a shows the CV curves of the UTCNF at different scan rates. The redox electrochemical reactions occurring on the UTCNF may be described as the reversible reactions Ni(OH)2 + OH− ↔ NiOOH + H2O + e− and Co(OH)2 + OH− ↔ CoOOH + H2O + e−. According to the formula i = k1v + k2v1/2 (i and v are the current and scan rate), i at a given potential can be divided into capacitive limited effects (k1v) and diffusion-controlled effects (k2v1/2). As illustrated in Fig. 4b, the detailed capacitive fraction (blue region) was calculated to be 35.8% at 0.5 mV s−1. Moreover, the capacitive contributions at 0.1 and 1 mV s−1 were also determined (Fig. 4c) to be 20.6% and 40.4%, respectively, which indicates that the electrochemical process is controlled by both capacitive behavior and diffusion. Fig. 4d shows the Nyquist plots of the UTCNF//Zn battery and the CNF//Zn battery, and the fitting values are listed in Table S3.† The UTCNF//Zn battery shows a larger charge transfer resistance Rct (4.1 Ω) than that of the CNF//Zn battery (2.4 Ω), which may be due to the decreased conductivity due to in situ formation of cobalt/nickel composite hydroxides after ultrasonic treatment. Such a markedly enhanced performance of the UTCNF//Zn battery may be attributed to the formed surface-active materials and the unique porous structure of the UTCNF cathode.

What's the cathode?

UTCNF

The need to increase the energy density of Li-ion batteries leads to continual development of electrode material technology, including engineering of the physicochemical properties of cathode materials. Among them, Li-rich layered oxides are attracting considerable research attention as a candidate for high performance positive electrodes due to their high specific charge (capacity), exceeding 200 mA h g−1. This group of compounds, proposed firstly by Thackeray and Johnson, include pristine Li2MnO3 or Li2MnO3 with partial substitution of Mn with other transition metals (TMs) and is often described as: xLi2MnO3·(1 − x)LiMO2 (or alternatively Li[LiyMn1−y−zMz]O2) (M – transition metal). Although, initially Thackeray et al. reported that Li2MnO3 (theoretical capacity 468 mA h g−1) is electrochemically inactive because Mn4+ cannot be oxidized to higher states, further studies showed that by means of appropriate activation processes Li2MnO3 can deliver higher discharge capacities than LiMnO2. Further studies by Kobobuchi et al. suggested that upon delithiation the local electronic state of lattice oxygen in Li2MnO3 may change, affecting the material performance. XAS studies of the O K-edge and Mn L-edge showed that in the Li-rich NMC sample charged up to 4.7 V vs. Li+/Li, oxygen ions also contribute to the reduction of manganese via electron transition from O-1s to the hybridized bands composed of O-2p and Mn-3dx2−y2. As a result, the change of the local electronic state surrounding Mn4+ ions triggers their migration within TMs and/or into the Li layers, which may be reflected in phase transformation of the initial monoclinic (C2/m) layered structure into either a rhombohedral (Rm) and/or a spinel structure. As confirmed by Amalraj et al. phase transition of Li2MnO3 from layered to spinel-like structures occurs even at potentials as low as 4.3 V, while at 4.5 V Li2MnO3 by partial loss of Li2O forms Li2−xMnO3−x/2 with enhanced stability associated with the existence of a composite structure built up with Li2MnO3 domains in a LiMnO2 matrix.

What's the cathode?

C-based materials such as spheres, fibers, and cages have been reported to adsorb polysulfides. Their ability to trap LiPSs in cathodes suppresses the shuttle effect, and improves the cycling of Li–S batteries. To compare the LiPS adsorption on hollow Co5.47Nx–C and C spheres, visual adsorption tests were conducted using Li2S6 as the LiPS; the results are presented in Fig. 4a. The initially yellow Li2S6 solution containing hollow Co5.47Nx–C spheres became colorless after 8 h. In contrast, the equivalent test with C spheres showed slight fading of the yellow solution. This showed that the Co5.47Nx–C spheres adsorbed the LiPS more readily than the C spheres. This was attributed to both physical and chemical adsorption because of the Co5.47Nx nanoparticles and mesoporous hollow layer of the Co5.47Nx–C spheres.

What's the cathode?

Fig. 5b and d display the images of S-C(PAN) anodes cycled with bare LMO and LMO-30 min, respectively. Also, the images of S-C(PAN) anodes cycled with PLMO-10 min and LMO-10 min are shown in Fig. S4b and d.† The morphologies of cycled S-C(PAN) electrodes with bare LMO, PLMO-10 min, and LMO-10 min electrodes seem rough with obvious cracks. In contrast, Fig. 5d displays the smooth surface of the S-C(PAN) electrode with no severe cracking. The surface feature and integrity are believed to be related to the manganese dissolution of the LMO cathode and the re-deposition on the anode surface, which will be discussed in detail in the following text.

What's the cathode?

LMO

Fig. 5b and d display the images of S-C(PAN) anodes cycled with bare LMO and LMO-30 min, respectively. Also, the images of S-C(PAN) anodes cycled with PLMO-10 min and LMO-10 min are shown in Fig. S4b and d.† The morphologies of cycled S-C(PAN) electrodes with bare LMO, PLMO-10 min, and LMO-10 min electrodes seem rough with obvious cracks. In contrast, Fig. 5d displays the smooth surface of the S-C(PAN) electrode with no severe cracking. The surface feature and integrity are believed to be related to the manganese dissolution of the LMO cathode and the re-deposition on the anode surface, which will be discussed in detail in the following text.

What's the anode?

S-C(PAN)

Fig. 5b and d display the images of S-C(PAN) anodes cycled with bare LMO and LMO-30 min, respectively. Also, the images of S-C(PAN) anodes cycled with PLMO-10 min and LMO-10 min are shown in Fig. S4b and d.† The morphologies of cycled S-C(PAN) electrodes with bare LMO, PLMO-10 min, and LMO-10 min electrodes seem rough with obvious cracks. In contrast, Fig. 5d displays the smooth surface of the S-C(PAN) electrode with no severe cracking. The surface feature and integrity are believed to be related to the manganese dissolution of the LMO cathode and the re-deposition on the anode surface, which will be discussed in detail in the following text.

What's the anode?

S-C(PAN)

Here, we demonstrate a new method to modify the surface of an LMO electrode for the first time. A stabilized LMO cathode was achieved by the electrospun coating of PVDF and gallium (Ga) and niobium (Nb) co-doped LLZO (Li5.6Ga0.26La2.9Zr1.87Nb0.05O12, LGLZNO). The PVDF@LGLZNO fibrous film coating acts as an effective artificial CEI to suppress the manganese dissolution and significantly minimize the undesirable interfacial reaction between the cathode and electrolyte. The improved LMO was further verified by coupling with an S-C(PAN) composite anode to realize stable rate performance and long cycling stability.

What's the cathode?

LMO

Here, we demonstrate a new method to modify the surface of an LMO electrode for the first time. A stabilized LMO cathode was achieved by the electrospun coating of PVDF and gallium (Ga) and niobium (Nb) co-doped LLZO (Li5.6Ga0.26La2.9Zr1.87Nb0.05O12, LGLZNO). The PVDF@LGLZNO fibrous film coating acts as an effective artificial CEI to suppress the manganese dissolution and significantly minimize the undesirable interfacial reaction between the cathode and electrolyte. The improved LMO was further verified by coupling with an S-C(PAN) composite anode to realize stable rate performance and long cycling stability.

What's the anode?

S-C(PAN) composite

Here, we demonstrate a new method to modify the surface of an LMO electrode for the first time. A stabilized LMO cathode was achieved by the electrospun coating of PVDF and gallium (Ga) and niobium (Nb) co-doped LLZO (Li5.6Ga0.26La2.9Zr1.87Nb0.05O12, LGLZNO). The PVDF@LGLZNO fibrous film coating acts as an effective artificial CEI to suppress the manganese dissolution and significantly minimize the undesirable interfacial reaction between the cathode and electrolyte. The improved LMO was further verified by coupling with an S-C(PAN) composite anode to realize stable rate performance and long cycling stability.

What's the electrolyte?

Increasing the areal sulfur loading and decreasing the amount of electrolyte in cathodes are natural methods to meet the above requirements. However, thick sulfur electrodes suffer cracking and peeling off problems during slurry casting, thus damaging the electrode integrity. Another obstacle for thick electrodes is the poor immersion of the electrolyte, which restrains the effective sulfur utilization. A promising alternate approach is to use a polysulfide solution, designated as a catholyte, as the starting material. The liquid catholyte is ready to immerse and envelope the surface of the conductive host, thus facilitating rapid charge transfer at the electrode/electrolyte interface, and giving rise to fast reaction kinetics. In addition, highly concentrated catholytes possess an intrinsically low E/S ratio. For example, the E/S ratio of 1.5 M Li2S6 catholyte is only 3.5 μL mg−1, which is difficult to achieve in sulfur particle electrodes. However, the flooded amounts of LPSs in carbon/catholyte electrodes induce a severe shuttling effect due to the poor interaction between the nonpolar carbon host and polar LPSs, leading to a short cycle life. Thus, introducing polar electrocatalysts in carbon/catholyte becomes imperative.

What's the cathode?

Increasing the areal sulfur loading and decreasing the amount of electrolyte in cathodes are natural methods to meet the above requirements. However, thick sulfur electrodes suffer cracking and peeling off problems during slurry casting, thus damaging the electrode integrity. Another obstacle for thick electrodes is the poor immersion of the electrolyte, which restrains the effective sulfur utilization. A promising alternate approach is to use a polysulfide solution, designated as a catholyte, as the starting material. The liquid catholyte is ready to immerse and envelope the surface of the conductive host, thus facilitating rapid charge transfer at the electrode/electrolyte interface, and giving rise to fast reaction kinetics. In addition, highly concentrated catholytes possess an intrinsically low E/S ratio. For example, the E/S ratio of 1.5 M Li2S6 catholyte is only 3.5 μL mg−1, which is difficult to achieve in sulfur particle electrodes. However, the flooded amounts of LPSs in carbon/catholyte electrodes induce a severe shuttling effect due to the poor interaction between the nonpolar carbon host and polar LPSs, leading to a short cycle life. Thus, introducing polar electrocatalysts in carbon/catholyte becomes imperative.

What's the electrolyte?

Fig. 3(a) and (d) show the results of ion conductivity through EIS measurements of the porous NASICON pellets and epoxy-NASICON pellets, respectively. The NASICON ceramics sintered at 1100 °C had an ionic conductivity of 1.76 × 10−4 S cm−1. A cross-sectional SEM image of the NASICON pellet sintered at 1100 °C is shown in Fig. 3(b). Here, the solid electrolyte secures the ion transfer channel after sintering, leaving pores to remain inside. When the epoxy polymer is added to the sample, the polymer material sufficiently fills the internal pores (Fig. 3(e)). In the SEM image, it was confirmed that the empty space between the NASICON crystals was filled with a black material, which was identified as a polymer material containing C through the EDS analysis shown in Fig. 3(c) and (f). The detailed EDS analysis results for each component can be seen in ESI Fig. S4.† Comparing the components of Na, Si, Zr, and P, which are all contained in NASICON, with the components of C, which are only contained in the epoxy polymers, we can see that the epoxy polymer is completely contained in the inner pores. In the same sample, Na-ions only move through the connected ceramics, which means that adding epoxy polymers does not affect the ion conduction. The ionic conductivity after epoxy treatment of the NASICON pellet is determined to be 1.45 × 10−4 S cm−1. Therefore, epoxy polymer is not involved in the ion transport mechanism, so it can increase the physical strength while maintaining the existing ion conductivity.

What's the electrolyte?

Since the capacitance in redox supercapacitors is mainly produced by the fast faradaic reaction occurring near a solid electrode surface at an appropriate potential, a relatively short diffusion path can be provided by nanostructured materials to improve the power density of supercapacitors. Therefore, electrode materials composed of NCPs have recently received consideration for supercapacitors. In addition, porous nanostructures increase the contact area between electrolyte and active materials. These two features lead to fast reaction kinetics. Electrode materials in supercapacitors essentially involve processes at the interface between an electrode and an electrolyte solution. Increasing the area of the interface is expected to increase the rate of the process. Porous electrode materials with high surface area per unit volume, especially with structural elements on the nanometer scale, have received considerable attention. Thus, active electrodes with meso- or nanoporous structures and good conductivity are highly desirable for high power densities.

What's the electrolyte?

Electrolyte additives may modify lithium deposition in several different ways, e.g. by influencing initial SEI formation on the current collector, the nucleation stage of lithium metal deposition, the growth of lithium metal deposits, or some combination thereof. To investigate during which step of lithium metal electrodeposition HF plays an active role, a cell was first galvanostatically brought down to 0 V vs. the counter electrode (CE, Li/Li+) at 0.5 mA cm−2 in LP30 containing 100 ppm added HF to form the initial SEI on copper (Fig. 3a). The electrolyte was then removed, the cell rinsed with as-received LP30, and refilled with as-received LP30 for deposition of lithium metal (Fig. 3b). There is no voltage plateau ∼2 V indicative of HF reduction, but we observe an additional small amount of capacity below 1 V which is consistent with solvent reduction. Lithium metal with a highly monodispersed columnar morphology (Fig. 3c) was still deposited although there was no HF in the electrolyte during electroplating. This demonstrates that it is the initial SEI on copper which directs the microstructure and that HF does not play an active role beyond the initial SEI formation. This experimental evidence is in agreement with studies in the literature that report the formation of a columnar microstructure without additives when a LiF rich layer was deposited ex situ on copper prior to cell assembly.

What's the electrolyte?

The fabrication of the nanomesh-based cathodes is performed in several steps, starting from the formation of the AAO template, growth of the nanomesh, deposition of the active material precursor and finally its activation by reaction with a lithium source (Fig. 1). In our approach, we focused on lithiated manganese oxides (here, generally abbreviated as LMO) as the active cathode material since they can be synthesized at relatively low temperatures and their precursor (MnO2) can be easily conformally electrodeposited onto high aspect ratio structures. This made LMO the natural choice for demonstrating the potential of nanomesh-based electrodes for Li-ion batteries. It is worth noting that LMO also show a relatively high reduction potential vs. Li+/Li (for example, ∼2.9 V for the layered LiMnO2 and 4–3 V for spinel LiMn2O4 ( ), are cheap and not toxic, but suffer from limited stability when cycled in liquid electrolytes.

What's the cathode?

lithiated manganese oxides

The fabrication of the nanomesh-based cathodes is performed in several steps, starting from the formation of the AAO template, growth of the nanomesh, deposition of the active material precursor and finally its activation by reaction with a lithium source (Fig. 1). In our approach, we focused on lithiated manganese oxides (here, generally abbreviated as LMO) as the active cathode material since they can be synthesized at relatively low temperatures and their precursor (MnO2) can be easily conformally electrodeposited onto high aspect ratio structures. This made LMO the natural choice for demonstrating the potential of nanomesh-based electrodes for Li-ion batteries. It is worth noting that LMO also show a relatively high reduction potential vs. Li+/Li (for example, ∼2.9 V for the layered LiMnO2 and 4–3 V for spinel LiMn2O4 ( ), are cheap and not toxic, but suffer from limited stability when cycled in liquid electrolytes.

What's the cathode?

LMO

Acoustic wave velocity is determined by the speed of wave propagation through a medium with defined thickness. While the wave arrival time can be determined from the measured acoustic signal, thickness must be measured independently to verify the wave velocity. The expansion and contraction of a pouch cell during cycling can be imaged with transmission X-ray microscopy (TXM), which has sufficient range and pixel resolution to measure both the total cell thickness and the average layer thicknesses. TXM parameters were optimized with exposure time of 20 seconds, beam voltage of 140 kV, a 0.4× objective lens and 90° projection angle (Table 1). Fig. 1 illustrates the experimental configuration and example radiographs of the mechanical expansion of a pouch cell upon cycling. The commercial pouch cell chosen (LiCoO2/graphite, 210 mA h nominal capacity) has a total thickness of approximately 5.6 mm when fully charged. The measured thickness varies between 5.4 and 5.6 mm (∼4% change) at a rate of 1C. There is less variation (∼0.5%) at a rate of 3C because of less attainable capacity before hitting the 4.5 V voltage cutoff on charge. The thickness changes are dominated by the ∼10% volume expansion and contraction of graphite anodes upon lithiation/delithiation. With 15 double-sided anodes and cathodes, each of the 30 cell layers (one layer is defined as an anode, a cathode, with a separator layer in between each electrode) is approximately 170 μm in thickness as measured by average peak-to-peak spacing (additional information on pixel thresholds for thickness measurements can be found in Fig. S1–S5†).

What's the cathode?

Then, Na metal was placed on the surface of the UV curing polymer electrolyte, and the product of a cell test in which a Na metal/solid electrolyte/cathode was made was placed in a 2032-coin cell. All steps were processed in a glovebox with less than 10 ppm for both oxygen and moisture. When manufacturing the bipolar stacked cell, a coin cell was prepared by putting Al foil between the two solid electrolyte cells.

What's the cathode?

In order to evaluate the electrochemical performance of the obtained electrodes, two-electrode Zn–MnO2 batteries were assembled using COG@MnO2 as the cathode and COG@Zn as the anode in an aqueous electrolyte containing 2 M ZnSO4 and 0.1 M MnSO4. The typical cyclic voltammetry (CV) curves of the aqueous COG@MnO2//COG@Zn battery at different scan rates are shown in Fig. 3d. There are two reduction peaks at around 1.2 and 1.4 V, which should be ascribed to the zinc insertion into MnO2 and the subsequent reduction of Mn(IV) to the Mn(III)/Mn(II) states. Similarly, the oxidation peak and shoulder appear at 1.6–1.8 V, corresponding to Zn-extraction from the MnO2 cathode as the Mn(III)/Mn(II) states undergo oxidation to the Mn(IV) state. The overall reaction of the battery can be formulated as follows:

What's the cathode?

COG@MnO2

In order to evaluate the electrochemical performance of the obtained electrodes, two-electrode Zn–MnO2 batteries were assembled using COG@MnO2 as the cathode and COG@Zn as the anode in an aqueous electrolyte containing 2 M ZnSO4 and 0.1 M MnSO4. The typical cyclic voltammetry (CV) curves of the aqueous COG@MnO2//COG@Zn battery at different scan rates are shown in Fig. 3d. There are two reduction peaks at around 1.2 and 1.4 V, which should be ascribed to the zinc insertion into MnO2 and the subsequent reduction of Mn(IV) to the Mn(III)/Mn(II) states. Similarly, the oxidation peak and shoulder appear at 1.6–1.8 V, corresponding to Zn-extraction from the MnO2 cathode as the Mn(III)/Mn(II) states undergo oxidation to the Mn(IV) state. The overall reaction of the battery can be formulated as follows:

What's the anode?

COG@Zn

In order to evaluate the electrochemical performance of the obtained electrodes, two-electrode Zn–MnO2 batteries were assembled using COG@MnO2 as the cathode and COG@Zn as the anode in an aqueous electrolyte containing 2 M ZnSO4 and 0.1 M MnSO4. The typical cyclic voltammetry (CV) curves of the aqueous COG@MnO2//COG@Zn battery at different scan rates are shown in Fig. 3d. There are two reduction peaks at around 1.2 and 1.4 V, which should be ascribed to the zinc insertion into MnO2 and the subsequent reduction of Mn(IV) to the Mn(III)/Mn(II) states. Similarly, the oxidation peak and shoulder appear at 1.6–1.8 V, corresponding to Zn-extraction from the MnO2 cathode as the Mn(III)/Mn(II) states undergo oxidation to the Mn(IV) state. The overall reaction of the battery can be formulated as follows:

What's the electrolyte?

2 M ZnSO4 and 0.1 M MnSO4

The UTCNF//Zn and CNF//Zn batteries were assembled using UTCNF or CNF cathodes (an area of 1 × 1 cm2 and a thickness of 0.15 cm) and a Zn plate anode with 6 M KOH + 0.5 M Zn(Ac)2 as the electrolyte. The total mass of the UTCNF electrode was ∼32.3 mg and the corresponding hydroxide formation rate was ∼18.6%. Galvanostatic charge–discharge (GCD) experiments were performed using a multichannel battery testing system (Land CT 2001A). Cyclic voltammetry (CV) curves and electrochemical impedance spectra (EIS) were collected on an electrochemical workstation (PARSTAT MC).

What's the cathode?

CNF

In summary, active catholyte has been applied to assemble a high-performance hybrid supercapacitor, which relies on the reversible conversion between soluble Mn2+ and solid MnO2 at the cathode and the redox of Ti–O with the bonding/de-bonding of H3O+ at the 400-KOH-Ti3C2 anode. The well-separated potential window between the cathode and anode leads to a hybrid supercapacitor with a wide voltage window of 1.7 V. This hybrid supercapacitor achieves a high energy density of 43.4 W h kg−1, without using any ion-selective membrane, and super-long cycle life (75% retention after 20000 cycles). Furthermore, the hybrid supercapacitor can operate well even with the frozen electrolyte at −70 °C. These inspiring results provide a new strategy to design high-performance hybrid supercapacitors for low temperature applications.

What's the cathode?

MnO2

In summary, MoP was synthesized on nickel foam, on which NiCo-LDH was electrodeposited to form catalyst electrodes (MoP@NiCo-LDH/NF-x), where x represents the electrodeposition time period, and they were explored as bifunctional catalysts for both the hydrogen evolution reaction (HER) at the cathode and the urea oxidation reaction (UOR) or the oxygen evolution reaction (OER) at the anodes of urea–water or water electrolysis. The morphology and catalytic performance were investigated using a series of physical characterization and electrochemical tests. The results show that at an electrolysis current density of 100 mA cm−2, the anode potential of the MoP@NiCo-LDH/NF-20 electrode is 1.392 V (vs. RHE) for UOR, 233 mV less than that of an IrO2/NF electrode (1.625 V); and the cathode potential of the MoP@NiCo-LDH/NF-20 electrode is 0.255 V (vs. RHE) for HER, closest to that of IrO2/NF (170 mV). For the two-electrode electrolyser (MoP@NiCo-LDH/NF-20‖MoP@NiCo-LDH/NF-20), the cell voltage is as low as 1.405 V for urea–water electrolysis and 1.697 V for water electrolysis at a current density of 100 mA cm−2, even lower than those of a Pt/C/NF‖IrO2/NF cell. Moreover, after 20 hours of continuous electrolysis, the performance and morphology of the catalyst are almost unchanged, indicating that it has both high activity and stability. The results show that the developed MoP@NiCo-LDH/NF-20 is a promising bifunctional catalyst.

What's the cathode?

MoP@NiCo-LDH/NF-20

In summary, MoP was synthesized on nickel foam, on which NiCo-LDH was electrodeposited to form catalyst electrodes (MoP@NiCo-LDH/NF-x), where x represents the electrodeposition time period, and they were explored as bifunctional catalysts for both the hydrogen evolution reaction (HER) at the cathode and the urea oxidation reaction (UOR) or the oxygen evolution reaction (OER) at the anodes of urea–water or water electrolysis. The morphology and catalytic performance were investigated using a series of physical characterization and electrochemical tests. The results show that at an electrolysis current density of 100 mA cm−2, the anode potential of the MoP@NiCo-LDH/NF-20 electrode is 1.392 V (vs. RHE) for UOR, 233 mV less than that of an IrO2/NF electrode (1.625 V); and the cathode potential of the MoP@NiCo-LDH/NF-20 electrode is 0.255 V (vs. RHE) for HER, closest to that of IrO2/NF (170 mV). For the two-electrode electrolyser (MoP@NiCo-LDH/NF-20‖MoP@NiCo-LDH/NF-20), the cell voltage is as low as 1.405 V for urea–water electrolysis and 1.697 V for water electrolysis at a current density of 100 mA cm−2, even lower than those of a Pt/C/NF‖IrO2/NF cell. Moreover, after 20 hours of continuous electrolysis, the performance and morphology of the catalyst are almost unchanged, indicating that it has both high activity and stability. The results show that the developed MoP@NiCo-LDH/NF-20 is a promising bifunctional catalyst.

What's the anode?

MoP@NiCo-LDH/NF-20 electrode

In summary, MoP was synthesized on nickel foam, on which NiCo-LDH was electrodeposited to form catalyst electrodes (MoP@NiCo-LDH/NF-x), where x represents the electrodeposition time period, and they were explored as bifunctional catalysts for both the hydrogen evolution reaction (HER) at the cathode and the urea oxidation reaction (UOR) or the oxygen evolution reaction (OER) at the anodes of urea–water or water electrolysis. The morphology and catalytic performance were investigated using a series of physical characterization and electrochemical tests. The results show that at an electrolysis current density of 100 mA cm−2, the anode potential of the MoP@NiCo-LDH/NF-20 electrode is 1.392 V (vs. RHE) for UOR, 233 mV less than that of an IrO2/NF electrode (1.625 V); and the cathode potential of the MoP@NiCo-LDH/NF-20 electrode is 0.255 V (vs. RHE) for HER, closest to that of IrO2/NF (170 mV). For the two-electrode electrolyser (MoP@NiCo-LDH/NF-20‖MoP@NiCo-LDH/NF-20), the cell voltage is as low as 1.405 V for urea–water electrolysis and 1.697 V for water electrolysis at a current density of 100 mA cm−2, even lower than those of a Pt/C/NF‖IrO2/NF cell. Moreover, after 20 hours of continuous electrolysis, the performance and morphology of the catalyst are almost unchanged, indicating that it has both high activity and stability. The results show that the developed MoP@NiCo-LDH/NF-20 is a promising bifunctional catalyst.

What's the electrolyte?

Recently, the atomic-level thickness and large surface area of 2D materials are beneficial to the generation of carriers on the surface and separation of photogenerated carriers. Typically, in comparison to their 3D counterparts, 2D materials present extremely numerous active sites, high carrier mobility, and controllable interface, which offer a large number of advantages for catalysts, electronics, optoelectronics, etc. Among them, penta-type materials have been developed as a series of novel 2D materials. For instance, Yang et al. reported a novel 2D material, penta-Pt2N4, with a large direct band gap, high carrier mobility and high Young's modulus, suggesting numerous potential applications in nanoelectronics. Xiao et al. found that penta-graphene shows desirable electrochemical performance and it may be a potential candidate for Li/Na-ion battery anodes. However, bulk pyrite-SiAs2 has been synthesized under high pressure and validated as a semiconductor decades ago. Meanwhile, this type of pyrite structure can form a 2D penta structure after cleaving the plane, but 2D penta-SiAs2 has not yet been explored.

What's the cathode?

Recently, the atomic-level thickness and large surface area of 2D materials are beneficial to the generation of carriers on the surface and separation of photogenerated carriers. Typically, in comparison to their 3D counterparts, 2D materials present extremely numerous active sites, high carrier mobility, and controllable interface, which offer a large number of advantages for catalysts, electronics, optoelectronics, etc. Among them, penta-type materials have been developed as a series of novel 2D materials. For instance, Yang et al. reported a novel 2D material, penta-Pt2N4, with a large direct band gap, high carrier mobility and high Young's modulus, suggesting numerous potential applications in nanoelectronics. Xiao et al. found that penta-graphene shows desirable electrochemical performance and it may be a potential candidate for Li/Na-ion battery anodes. However, bulk pyrite-SiAs2 has been synthesized under high pressure and validated as a semiconductor decades ago. Meanwhile, this type of pyrite structure can form a 2D penta structure after cleaving the plane, but 2D penta-SiAs2 has not yet been explored.

What's the anode?

penta-graphene

Recently, the atomic-level thickness and large surface area of 2D materials are beneficial to the generation of carriers on the surface and separation of photogenerated carriers. Typically, in comparison to their 3D counterparts, 2D materials present extremely numerous active sites, high carrier mobility, and controllable interface, which offer a large number of advantages for catalysts, electronics, optoelectronics, etc. Among them, penta-type materials have been developed as a series of novel 2D materials. For instance, Yang et al. reported a novel 2D material, penta-Pt2N4, with a large direct band gap, high carrier mobility and high Young's modulus, suggesting numerous potential applications in nanoelectronics. Xiao et al. found that penta-graphene shows desirable electrochemical performance and it may be a potential candidate for Li/Na-ion battery anodes. However, bulk pyrite-SiAs2 has been synthesized under high pressure and validated as a semiconductor decades ago. Meanwhile, this type of pyrite structure can form a 2D penta structure after cleaving the plane, but 2D penta-SiAs2 has not yet been explored.

What's the electrolyte?

Flexible supercapacitors are playing an increasingly important role in energy storage due to their viability in flexible intelligent wearable electronic devices, which require irregular power supply at different discharge rates. Two-dimensional (2D) nanomaterials have gradually become potential electrode materials for flexible supercapacitors due to their atomic thickness and electrochemically active surface, especially since they allow the development of binder-free flexible electrodes with improved capacitance. 2D materials can provide slit-shaped ion diffusion channels that enable fast movement of electrolyte ions into the electrode bulk.

What's the electrolyte?

To study the electrolyte reduction processes resulting in SEI formation, cyclic voltammetry (CV) was performed systematically as a function of added HF concentration (as-received, +10, +100, and +500 ppm) and voltage sweep rate. Fig. 5 shows a series of CV scans measured at 5 mV s−1 in electrolytes with varying concentrations of HF. The peak at ∼2 V, indicated by the dashed line, results from the electrocatalytic reduction of HF to form LiF (reaction (2)). The additional peak at higher potential in the as-received electrolyte may be attributed to either PF6− or POF3 reduction. The slight shift in peak position to higher potentials (from 1.9 to 2 V) with increasing HF concentration is consistent with the report by Strmcnik et al. Solutions with more HF exhibit higher peak currents and increased capacity going into SEI formation. If a uniform film of LiF alone were fully passivating the copper surface, this would be achieved at a specific capacity as further HF reduction would be prevented by the passivation, and increased HF in the electrolyte would not lead to increased SEI capacity. However, this data suggests that LiF alone does not passivate the copper surface at these HF concentrations and scan rates because increased HF concentration in the electrolyte results in a corresponding increase in the capacity of the HF reduction peak. Passivating LiF films can be formed under different conditions, however, namely at slower scan rates and on smoother single-crystalline substrates. We rationalize that HF reduction is limited by the availability of HF at this interface, which in turn determines the amount of LiF that is formed in the initial SEI.

What's the electrolyte?

We report, for the first time, that DPPT-TT is a lower performance channel material compared to P3HT and PBTTT. For DPPT-TT, the ON current (∼−1.0 μA), VON value (∼0.63 V), and transconductance (0.012 mS), are far below the performance of P3HT and PBTTT. Still, the addition of DBSA to the electrolyte is effective in enhancing its performance and results in a significant increase in the ON current and decrease in the operation voltage. The positive effect of adding DBSA to the electrolyte extends not only to DPPT-TT but also PBTTT, where it appears to have an even stronger effect.

What's the electrolyte?

Inspired by the remarkably active Mn4CaO5 clusters for water oxidation, Mn3O4 with high-concentration and stable Mn3+ has, therefore, been extensively researched as an efficient electrocatalyst for OER. In the spinel structure Mn2+Mn23+O4, Mn existed as Mn2+ and Mn3+; here, Mn3+ occupied the octahedral site and Mn2+ ion occupied the tetrahedral site. The coexistence of Mn2+ and Mn3+ offers excellent OER activity. Therefore, numerous studies have focused on exploring high-quality Mn3O4 catalysts for efficient electrocatalytic OER. Further, earlier studies have also demonstrated that Mn3O4 electrocatalysts are highly active toward OER in neutral and alkaline media. For example, Nam's group reported the fabrication of 4 and 8 nm Mn3O4 nanoparticles (Fig. 13a and b). Owing to the enlarged surface area and improved electrical conductivity, the as-obtained Mn3O4 nanoparticles showed apparently higher OER activity in comparison with other Mn oxides (Fig. 13c). Remarkably, the optimized 8 nm Mn3O4 nanoparticles that were loaded on the surface of NFs exhibited outstanding OER activity with overpotential of only 395 mV at 10 mA cm−2 under neutral conditions, which was much superior to those of Fe-, Co-, and Ni-based oxides. As demonstrated by Maruthapandian et al., Mn3O4 catalysts also showed excellent electrocatalytic performance for OER in basic solutions (Fig. 13d and e). In particular, they demonstrated the synthesis of Ni-doped Mn3O4 catalysts, which showed significantly enhanced OER electrocatalytic activity in 1 M KOH solution with overpotential of 283 mV at 10 mA cm−2 (Fig. 13f). Further, a mechanistic study revealed that the largely improved OER performance could be ascribed to the enhanced adsorption/desorption of hydroxide and oxygen atoms, boosting the formation of metal oxyhydroxide/hydroperoxo. Moreover, the efficient electron charge between the electrolyte and Mn3O4 and enlarged electrochemical surface active sites facilitated the promotion of electrocatalytic OER performance.

What's the electrolyte?

Typically, the working electrodes were fabricated by mixing 10 wt% of the binder polytetrafluoroethylene (PTFE), 10 wt% conductive agent (Super-P-Li) and 80 wt% of active materials (MnO2, K0.296Mn0.926O2 and active carbon). Ethanol was used to make the above materials into a slurry and Ni foam was employed as the current collector to coat the slurry with a mass loading of around 3 mg cm−2. Each electrode was dried for 6 hours in the oven and finally subjected to a pressure of 10 MPa. Electrochemical tests were performed on a three-electrode system with 1 M potassium sulfate as the electrolyte with the HgO/Hg as the reference electrode and Pt foil as the counter electrode. The CHI 660E Electrochemical Workstation was used to test the galvanostatic charge–discharge (GCD) and cyclic voltammetry (CV). The Zahner IM6 Electrochemical Workstation was employed to measure the electrochemical impedance spectroscopy (EIS). The LAND battery system was used to test the cycling life.

What's the electrolyte?

1 M potassium sulfate

In summary, the mechanism of Li dendrite formation in LLZTO was discussed in terms of energy band structure as well as defect states, and was investigated by REELS, SPEM, and Nano Q-DLTS. The experimental results corroborate that the higher defect densities at the GBs lower the SBH by 0.5 eV and makes metallic Li atoms propagate along the GBs in LLZTO. As a way of preventing Li dendrite formation in LLZTO, bandgap engineering by the laser annealing treatment was proposed. Amorphous LLZTO and Li2O2 layers formed after laser annealing hinder the Li dendrite formation by blocking the electron injection. The hybrid electrolyte cell comprising the laser-treated LLZTO sample exhibits significant improvement in cycling performance and stability. Consequently, the laser annealing treatment on the LLZTO electrolytes can direct the development of materials engineering toward high-performance solid-state batteries. Furthermore, the analytical approach in this study highlights the importance of the electronic band structure of LLZTO for its stability and provides the direction to study various solid electrolyte materials.

What's the electrolyte?

LLZTO

Anthraquinone-1,5-disulfonic acid disodium salt (1,5-AQDS, 1a) was first proposed for potassium batteries by Xu et al. In a carbonate-based electrolyte, the authors achieved the reversible capacity per material mass (Qm) of 95 mA h g−1 at 0.1C (13 mA g−1), and after a hundred cycles the capacity was 78 mA h g−1.

What's the electrolyte?

carbonate-based

The high-rate performance of 3Mg/Mg2Sn was mainly attributed to a low RCT. EIS spectra revealed that the RCT in 3Mg/Mg2Sn is significantly smaller than that in Mg2Sn (Fig. S5†). While the RCT values in 3Mg/Mg2Sn varied between 77 and 140 Ω, those in Mg2Sn varied in the range of 500–2500 Ω. This was not surprising because the apparent RCT is dependent on the surface area that is accessible by electrolytes. The larger BET surface area and micro-to-macroporosity of 3Mg/Mg2Sn, therefore, resulted in a significantly low RCT, which eventually contributed to the high-rate performance.

What's the electrolyte?

For electrochromic applications, the device shows rapid, self-powered color switching and multicolor display (color switching from light yellow to transparent, light red, dark green, dark blue and black, corresponding to the combination of the different states of the two films). As an energy storage device, the as-assembled device provides three different open-circuit potentials with an overall areal capacity of up to 933 mA h m−2. Meanwhile, the utilization of the mixed Al/Li-ion electrolyte and the addition of PEDOT:PSS into the inorganic materials greatly promote the cycle stability of the cathode films. Such a new design of the EES device with multicolor display, large charge capacity and high cycle stability can be promising for future color switching/energy storage applications, which may also provide new insights into the design of multifunctional devices.

What's the cathode?