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The blended slurry was then cast onto a clean current collector (Al foil for the cathode and Cu foil for the anode) and dried at 90 °C under vacuum overnight.

What is the cathode?

Al foil

The blended slurry was then cast onto a clean current collector (Al foil for the cathode and Cu foil for the anode) and dried at 90 °C under vacuum overnight. Finally, the obtained electrodes were cut into desired shapes on demand. It should be noted that the electrode mass ratio of cathode/anode is set to about 4, thus achieving the battery balance.

What is the anode?

Cu foil

In conclusion, the SiC/RGO nanocomposite, integrating the synergistic effect of SiC flakes and RGO, was synthesized by an in situ gas–solid fabrication method. Taking advantage of the enhanced photogenerated charge separation, large CO2 adsorption, and numerous exposed active sites, SiC/RGO nanocomposite served as the cathode material for the photo-assisted Li–CO2 battery.

What is the cathode?

SiC/RGO nanocomposite

The calculated diffusion coefficient of Zn2+ was of the order of 10−9–10−10 cm−2 s−1 (Fig. 4f), which is comparable to that of the reported V-based materials (Table S3†). These results clearly demonstrate that the NV NSs@ACC cathode allows the stable and fast migration of Zn2+, leading to good rate capability.

What is the cathode?

NV NSs@ACC

Li-ion batteries (LIBs) are expected to have a cell-level specific capacity of >350 W h kg−1 by 2025 to meet the market demanded driving range of an electric vehicle (EV). Among the state-of-the-art cathode materials, Ni-rich layered oxides, LiNixCoyAlzO2 (NCA) and LiNixCoyMnzO2 (NCM) with x + y + z = 1 and x ≧ 0.8, have an unbeatable high capacity of ∼200 mA h g−1.

What is the cathode?

Ni-rich layered oxides, LiNixCoyAlzO2 (NCA) and LiNixCoyMnzO2 (NCM)

Mg-doped P2-NMM10 and Zn-doped P2-NMZ10 cathode materials are synthesized by a solid state method, whose P2-type layered structures are confirmed by X-ray diffraction (XRD) (Fig. S1† and a summary of the crystallographic data is given in Tables S1–S4†). Their compositions are verified by energy dispersive X-ray spectroscopy (EDS) (Fig. S2†), showing that all the dopants are successfully introduced into the bulk materials and uniformly distributed.

What is the cathode?

Mg-doped P2-NMM10 and Zn-doped P2-NMZ10

Li and co-authors applied PTCDA-derived polymer P10 as the anode for aqueous K-ion batteries. Saturated solutions of K2SO4 (∼0.69 M) or KNO3 (∼3.75 M) were tested as the electrolytes. The potassium nitrate solution enabled better rate capabilities with P10 as the active material, owing to the higher conductivity. Particularly, ∼90 mA h g−1 was delivered at 5.4 A g−1, which was about 70% of the value achieved at 0.36 A g−1. A full cell with the potassium hexacyanoferrate cathode had an energy density of 24.2 W h kg−1 (per mass of both electrodes), could reach a high power of 2.08 kW kg−1, and retained 74% of the capacity after 300 cycles.

What's the cathode?

potassium hexacyanoferrate

Li and co-authors applied PTCDA-derived polymer P10 as the anode for aqueous K-ion batteries. Saturated solutions of K2SO4 (∼0.69 M) or KNO3 (∼3.75 M) were tested as the electrolytes. The potassium nitrate solution enabled better rate capabilities with P10 as the active material, owing to the higher conductivity. Particularly, ∼90 mA h g−1 was delivered at 5.4 A g−1, which was about 70% of the value achieved at 0.36 A g−1. A full cell with the potassium hexacyanoferrate cathode had an energy density of 24.2 W h kg−1 (per mass of both electrodes), could reach a high power of 2.08 kW kg−1, and retained 74% of the capacity after 300 cycles.

What's the anode?

PTCDA-derived polymer P10

Li and co-authors applied PTCDA-derived polymer P10 as the anode for aqueous K-ion batteries. Saturated solutions of K2SO4 (∼0.69 M) or KNO3 (∼3.75 M) were tested as the electrolytes. The potassium nitrate solution enabled better rate capabilities with P10 as the active material, owing to the higher conductivity. Particularly, ∼90 mA h g−1 was delivered at 5.4 A g−1, which was about 70% of the value achieved at 0.36 A g−1. A full cell with the potassium hexacyanoferrate cathode had an energy density of 24.2 W h kg−1 (per mass of both electrodes), could reach a high power of 2.08 kW kg−1, and retained 74% of the capacity after 300 cycles.

What's the electrolyte?

K2SO4 (∼0.69 M) or KNO3 (∼3.75 M)

Electrolytic water splitting in an electrochemical cell can produce both hydrogen and oxygen through the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode, respectively. The electrolysis process can be expressed by eqn (1)–(3).

What's the cathode?

Electrolytic water splitting in an electrochemical cell can produce both hydrogen and oxygen through the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode, respectively. The electrolysis process can be expressed by eqn (1)–(3).

What's the anode?

Lithium-ion/sulfur (Li-ion/S) batteries consisting of metallic lithium-free anodes and sulfur cathodes are promising energy storage solutions. Anode prelithiation enables the Li-ion/S battery assembly with the extensively-developed sulfur cathodes. However, it’s very challenging owing to the low lithiation potentials of anode materials (e.g. Al, 0.32 V vs. Li/Li+). Here, a free-standing LiAl alloy anode (c-LiAl) is prepared via an easy-to-implement chemical prelithiation, by using a newly exploited reagent of lithium 9,9-dimethylfluorene (Li-DiMF) with a lower redox potential of 0.22 V vs. Li/Li+. Compared with the LiAl anode prepared by electrochemical prelithiation (e-LiAl) and the lithium metal anode by electrodeposition (e-Li/Cu), the c-LiAl displays a superior cyclability in half cell test and high resistance towards polysulfide or ambient-air corrosion. When paired with a sulfur cathode, the resulting Li-ion/S battery with c-LiAl demonstrates a much better cycling performance than the Li-ion/S battery with e-LiAl and the lithium/sulfur battery with e-Li/Cu.

What's the cathode?

sulfur

Lithium-ion/sulfur (Li-ion/S) batteries consisting of metallic lithium-free anodes and sulfur cathodes are promising energy storage solutions. Anode prelithiation enables the Li-ion/S battery assembly with the extensively-developed sulfur cathodes. However, it’s very challenging owing to the low lithiation potentials of anode materials (e.g. Al, 0.32 V vs. Li/Li+). Here, a free-standing LiAl alloy anode (c-LiAl) is prepared via an easy-to-implement chemical prelithiation, by using a newly exploited reagent of lithium 9,9-dimethylfluorene (Li-DiMF) with a lower redox potential of 0.22 V vs. Li/Li+. Compared with the LiAl anode prepared by electrochemical prelithiation (e-LiAl) and the lithium metal anode by electrodeposition (e-Li/Cu), the c-LiAl displays a superior cyclability in half cell test and high resistance towards polysulfide or ambient-air corrosion. When paired with a sulfur cathode, the resulting Li-ion/S battery with c-LiAl demonstrates a much better cycling performance than the Li-ion/S battery with e-LiAl and the lithium/sulfur battery with e-Li/Cu.

What's the anode?

LiAl alloy

Lithium-ion/sulfur (Li-ion/S) batteries consisting of metallic lithium-free anodes and sulfur cathodes are promising energy storage solutions. Anode prelithiation enables the Li-ion/S battery assembly with the extensively-developed sulfur cathodes. However, it’s very challenging owing to the low lithiation potentials of anode materials (e.g. Al, 0.32 V vs. Li/Li+). Here, a free-standing LiAl alloy anode (c-LiAl) is prepared via an easy-to-implement chemical prelithiation, by using a newly exploited reagent of lithium 9,9-dimethylfluorene (Li-DiMF) with a lower redox potential of 0.22 V vs. Li/Li+. Compared with the LiAl anode prepared by electrochemical prelithiation (e-LiAl) and the lithium metal anode by electrodeposition (e-Li/Cu), the c-LiAl displays a superior cyclability in half cell test and high resistance towards polysulfide or ambient-air corrosion. When paired with a sulfur cathode, the resulting Li-ion/S battery with c-LiAl demonstrates a much better cycling performance than the Li-ion/S battery with e-LiAl and the lithium/sulfur battery with e-Li/Cu.

What's the anode?

LiAl

The data for organic-based anode materials are provided in Table 2. Only a few reports, which were discussed in Section 3.3, were dedicated to aqueous batteries. For this reason, the summary and outlook will be focusing on the non-aqueous systems. As in the case of cathode materials (Section 2.7), we chose several inorganic benchmarks, which showed superior capacities, rate capabilities or cycling stability. These benchmarks include graphite, porous carbon (p. C), N-doped carbon (N-d. C), bismuth, antimony, iron disulfide and red phosphorous.

What's the cathode?

The data for organic-based anode materials are provided in Table 2. Only a few reports, which were discussed in Section 3.3, were dedicated to aqueous batteries. For this reason, the summary and outlook will be focusing on the non-aqueous systems. As in the case of cathode materials (Section 2.7), we chose several inorganic benchmarks, which showed superior capacities, rate capabilities or cycling stability. These benchmarks include graphite, porous carbon (p. C), N-doped carbon (N-d. C), bismuth, antimony, iron disulfide and red phosphorous.

What's the anode?

organic-based

We explore a novel ether aided superconcentrated ionic liquid electrolyte; a combination of ionic liquid, N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (C3mpyrFSI) and ether solvent, 1,2 dimethoxy ethane (DME) with 3.2 mol/kg LiFSI salt, which offers an alternative ion-transport mechanism and improves the overall fluidity of the electrolyte. The molecular dynamics (MD) study reveals that the coordination environment of lithium in the ether aided ionic liquid system offers a coexistence of both the ether DME and FSI anion simultaneously and the absence of ‘free’, uncoordinated DME solvent. These structures lead to very fast kinetics and improved current density for lithium deposition-dissolution processes. Hence the electrolyte is used in a lithium metal battery against a high mass loading (~12 mg/cm2) LFP cathode which was cycled at a relatively high current rate of 1mA/cm2 for 350 cycles without capacity fading and offered an overall coulombic efficiency of >99.8 %. Additionally, the rate performance demonstrated that this electrolyte is capable of passing current density as high as 7mA/cm2 without any electrolytic decomposition and offers a superior capacity retention. We have also demonstrated an ‘anode free’ LFP-Cu cell which was cycled over 50 cycles and achieved an average coulombic efficiency of 98.36%. The coordination chemistry and (electro)chemical understanding as well as the excellent cycling stability collectively leads toward a breakthrough in realizing the practical applicability of this ether aided ionic liquid electrolytes in lithium metal battery applications, while delivering high energy density in a prototype cell.

What's the cathode?

LFP

We explore a novel ether aided superconcentrated ionic liquid electrolyte; a combination of ionic liquid, N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (C3mpyrFSI) and ether solvent, 1,2 dimethoxy ethane (DME) with 3.2 mol/kg LiFSI salt, which offers an alternative ion-transport mechanism and improves the overall fluidity of the electrolyte. The molecular dynamics (MD) study reveals that the coordination environment of lithium in the ether aided ionic liquid system offers a coexistence of both the ether DME and FSI anion simultaneously and the absence of ‘free’, uncoordinated DME solvent. These structures lead to very fast kinetics and improved current density for lithium deposition-dissolution processes. Hence the electrolyte is used in a lithium metal battery against a high mass loading (~12 mg/cm2) LFP cathode which was cycled at a relatively high current rate of 1mA/cm2 for 350 cycles without capacity fading and offered an overall coulombic efficiency of >99.8 %. Additionally, the rate performance demonstrated that this electrolyte is capable of passing current density as high as 7mA/cm2 without any electrolytic decomposition and offers a superior capacity retention. We have also demonstrated an ‘anode free’ LFP-Cu cell which was cycled over 50 cycles and achieved an average coulombic efficiency of 98.36%. The coordination chemistry and (electro)chemical understanding as well as the excellent cycling stability collectively leads toward a breakthrough in realizing the practical applicability of this ether aided ionic liquid electrolytes in lithium metal battery applications, while delivering high energy density in a prototype cell.

What's the anode?

2.1.5 Electrochemical performance measurements. The electrochemical performances of an individual electrode, Fe-intercalated ML Ti3C2Tx anode, and graphite cathode were examined in a three-electrode system in an Ar-filled glove box with O2 and H2O levels below 0.1 ppm, using platinum foil as the counter electrode and a silver-ion electrode as the reference electrode. For full device testing, a two-electrode electrolyzer was employed, with an Fe-intercalated ML Ti3C2Tx anode and graphite cathode. The electrolyte was 1 M EMIm+[PF6]− ionic liquid (1.2 mL) dissolved in PC (1.5 mL) and EMC (3 mL) solvent. CV and GCD experiments were performed using a CHI660E electrochemical workstation (Chenhua, China). EIS was conducted at open-circuit potential, with an amplitude of 5 mV and frequencies ranging from 10 mHz to 100 kHz.

What's the cathode?

graphite

2.1.5 Electrochemical performance measurements. The electrochemical performances of an individual electrode, Fe-intercalated ML Ti3C2Tx anode, and graphite cathode were examined in a three-electrode system in an Ar-filled glove box with O2 and H2O levels below 0.1 ppm, using platinum foil as the counter electrode and a silver-ion electrode as the reference electrode. For full device testing, a two-electrode electrolyzer was employed, with an Fe-intercalated ML Ti3C2Tx anode and graphite cathode. The electrolyte was 1 M EMIm+[PF6]− ionic liquid (1.2 mL) dissolved in PC (1.5 mL) and EMC (3 mL) solvent. CV and GCD experiments were performed using a CHI660E electrochemical workstation (Chenhua, China). EIS was conducted at open-circuit potential, with an amplitude of 5 mV and frequencies ranging from 10 mHz to 100 kHz.

What's the anode?

Fe-intercalated ML Ti3C2Tx

2.1.5 Electrochemical performance measurements. The electrochemical performances of an individual electrode, Fe-intercalated ML Ti3C2Tx anode, and graphite cathode were examined in a three-electrode system in an Ar-filled glove box with O2 and H2O levels below 0.1 ppm, using platinum foil as the counter electrode and a silver-ion electrode as the reference electrode. For full device testing, a two-electrode electrolyzer was employed, with an Fe-intercalated ML Ti3C2Tx anode and graphite cathode. The electrolyte was 1 M EMIm+[PF6]− ionic liquid (1.2 mL) dissolved in PC (1.5 mL) and EMC (3 mL) solvent. CV and GCD experiments were performed using a CHI660E electrochemical workstation (Chenhua, China). EIS was conducted at open-circuit potential, with an amplitude of 5 mV and frequencies ranging from 10 mHz to 100 kHz.

What's the cathode?

graphite

The increased environmental hazards from the use of carbon-emitting fuels and the rapid depletion of conventional fossil fuels have triggered the research community to find alternatives to fossil-fuel-based technologies. In many innovative approaches to address these challenges, electrocatalytic water splitting driven by a renewable energy input to produce clean H2 has been widely viewed as a promising strategy as hydrogen is produced from renewable sources (such as solar and wind energy), which can be stored, transported and consumed without generating any carbon-based byproducts. Electrolysers are a crucial factor for water electrolysis. Currently, great efforts are being made to develop new electrocatalysts with high efficiency for overall water splitting. In addition to the advancement of electrocatalysts, another equally important aspect of water electrolysis is the development of novel electrolyzer design. The conventional cell design of a alkaline water electrolyser is illustrated in Scheme 1a. Both electrodes are dipped into an aqueous alkaline electrolyte with a separator (ion exchange membrane or porous diaphragm) between them. H2 and O2 are produced on the electrodes via two half-cell reactions: the hydrogen evolution reaction (HER), and the oxygen evolution reaction (OER). In the lab, these electrolysers are used directly, while in industry, these electrolysers are used in stacks to scale-up H2 production. For electrolysers, major costs are due to the fabrication and material for the three main components: anodes, cathodes and separators. The existence of a separator not only increases the cost of the electrolyser, but also induces extra overpotentials for water splitting. A new electrolyser with a simple, condensed, membrane-free design is highly desired since it not only requires lower overpotentials, but also needs smaller areas and fewer materials, which reduces the overall cost of the electrolyser cells and promotes the electrochemical water splitting.

What's the cathode?

The increased environmental hazards from the use of carbon-emitting fuels and the rapid depletion of conventional fossil fuels have triggered the research community to find alternatives to fossil-fuel-based technologies. In many innovative approaches to address these challenges, electrocatalytic water splitting driven by a renewable energy input to produce clean H2 has been widely viewed as a promising strategy as hydrogen is produced from renewable sources (such as solar and wind energy), which can be stored, transported and consumed without generating any carbon-based byproducts. Electrolysers are a crucial factor for water electrolysis. Currently, great efforts are being made to develop new electrocatalysts with high efficiency for overall water splitting. In addition to the advancement of electrocatalysts, another equally important aspect of water electrolysis is the development of novel electrolyzer design. The conventional cell design of a alkaline water electrolyser is illustrated in Scheme 1a. Both electrodes are dipped into an aqueous alkaline electrolyte with a separator (ion exchange membrane or porous diaphragm) between them. H2 and O2 are produced on the electrodes via two half-cell reactions: the hydrogen evolution reaction (HER), and the oxygen evolution reaction (OER). In the lab, these electrolysers are used directly, while in industry, these electrolysers are used in stacks to scale-up H2 production. For electrolysers, major costs are due to the fabrication and material for the three main components: anodes, cathodes and separators. The existence of a separator not only increases the cost of the electrolyser, but also induces extra overpotentials for water splitting. A new electrolyser with a simple, condensed, membrane-free design is highly desired since it not only requires lower overpotentials, but also needs smaller areas and fewer materials, which reduces the overall cost of the electrolyser cells and promotes the electrochemical water splitting.

What's the anode?

One should note that in this first demonstration of the nanomesh-based cathodes, the thickness of the electrodes of few micrometres remains insufficient for practical application in Li-ion batteries that require thicker electrodes, in the range of tens of micrometres. Thus, the next natural step is the realization of thicker electrodes, which should be feasible given the up-scalable character of their electro-fabrication. Also, the cracks observed in the electrodes after thermal activation of LMO could somewhat affect the performance and integration of the electrodes in batteries. Such cracking originates from the expanding volume of the drop-cast LiOH during its solidification, the thermomechanical stress exerted on the nanowires during the relatively rapid ramp-up phase of annealing or the capillary forces acting on the nanomesh structure during the final drying. Further studies could address this by changing the lithium precursor to a gaseous one, pre-annealing of the nanowires to relieve the internal stress, lowering the heating rate during the conversion or optimization of drying (e.g. through supercritical drying). Furthermore, the highly nanostructured character of our cathodes comes at an expense of a lower thermal budget during the integration of the current collector with the active material, a sloping potential during the discharge and an increased rate of side-reactions. Although compared to other 3D cathodes the nanomesh cathode shows a similar cycling stability (78% of initial capacity after 50 cycles at 1.2C), it will need to be further increased for practical device applications. A detailed post-mortem analysis of the electrodes could be helpful in choosing an appropriate route for mitigating capacity fading. The potential methods include the application of thin protective coatings (such as TiO2) or integration of the electrode with a solid electrolyte (which is, anyway, inherent for application in microbatteries). A candidate electrolyte could be one of the highly-conductive solid composite electrolytes which are obtained through controlled hydrolysis of lithiated silicon alkoxides from the liquid phase and, thus, could well penetrate and impregnate the porous nanowire electrode. Also, the active electrode material could be changed to a compound with a higher cycling stability than LixMnO2, provided it can be synthesized at sufficiently low temperatures to avoid oxidation of the current collector. This could be done by, for example, electrodeposition from eutectic melts reported by Zhang et al., which was demonstrated to produce dense, high quality LiCoO2 and highly stable LiMn2O4 3D coatings at low temperatures of 260–300 °C. Contrary to the alternative low temperature sol–gel or solvothermal syntheses, molten-salt electrolysis avoids the excessive gas evolution and pressure build-up which can be detrimental to the nanoporous electrodes. Perhaps, such electrodeposition could also be extended in the future to other attractive cathode materials, allowing integration of the nanomesh with e.g. high voltage lithium nickel manganese cobalt oxides (NMC).

What's the cathode?

nanomesh-based

For full cell testing, sulfur–carbon composite cathodes were used. It is well known that S, lithium disulfide and other reaction intermediates have poor electronic conductivity. To improve this, S nanoparticles were impregnated into a nitrogen doped carbon matrix (from now on labelled ‘S/NC’); a detailed account on the synthesis of the cathode was given in our previous report. The carbon composite matrix with good electronic conductivity and a high surface area (700 m2 g−1) contained graphene and multiwall carbon nanotubes in an equal weight ratio. The morphology of the cathode material is illustrated in the SEM image shown in Fig. S4.† Further, thermo-gravimetric analysis (TGA) shows a S loading of ∼55 wt% in the S/NC cathode (Fig. S5, ESI†).

What's the cathode?

sulfur–carbon composite

For full cell testing, sulfur–carbon composite cathodes were used. It is well known that S, lithium disulfide and other reaction intermediates have poor electronic conductivity. To improve this, S nanoparticles were impregnated into a nitrogen doped carbon matrix (from now on labelled ‘S/NC’); a detailed account on the synthesis of the cathode was given in our previous report. The carbon composite matrix with good electronic conductivity and a high surface area (700 m2 g−1) contained graphene and multiwall carbon nanotubes in an equal weight ratio. The morphology of the cathode material is illustrated in the SEM image shown in Fig. S4.† Further, thermo-gravimetric analysis (TGA) shows a S loading of ∼55 wt% in the S/NC cathode (Fig. S5, ESI†).

What's the cathode?

S/NC

Recently, potassium manganese oxides (KMOs) have gained more attention as positive materials due to their high operating voltage, high conductivity and controllable flexible interlayer spacing, which are beneficial for K+ ion diffusion and offer more ion intercalation channels over the whole exposed surface. For example, Kim et al. reported a P3-type layered structure of K0.5MnO2 as the cathode material for non-aqueous potassium-ion batteries, which exhibited a specific capacity of ≈100 mA h g−1 with good capacity retention. Zhao et al. synthesized AlF3-coated K1.39Mn3O6 microspheres that showed a highly reversible capacity of 110 mA h g−1 at 10 mA g−1, excellent cycling stability and rate capability. Xia et al. prepared the high K-content K0.77MnO2·0.23H2O for high-performance non-aqueous K-ion batteries with 134 mA h g−1 at a current density of 100 mA g−1.

What's the cathode?

K0.5MnO2

The data for organic-based cathode materials are summarized in Table 1. All these compounds were reported for non-aqueous batteries, which should be mainly due to the mismatch between their working potentials and water stability voltage window. Some n-type materials, such as quinones or aromatic imides, can operate without decomposition of water, especially with highly concentrated electrolytes. However, their working potentials are close to the onset of the hydrogen evolution reaction, which makes them more suitable for anodes rather than for cathodes (see Section 3.3). There are several reports on aromatic amines and nitroxyl radicals as cathodes for aqueous Li-, Na-, Zn- and ammonium-based cells. It might be expected that similar K-based aqueous dual-ion batteries will become successful in the future.

What's the cathode?

organic-based

The data for organic-based cathode materials are summarized in Table 1. All these compounds were reported for non-aqueous batteries, which should be mainly due to the mismatch between their working potentials and water stability voltage window. Some n-type materials, such as quinones or aromatic imides, can operate without decomposition of water, especially with highly concentrated electrolytes. However, their working potentials are close to the onset of the hydrogen evolution reaction, which makes them more suitable for anodes rather than for cathodes (see Section 3.3). There are several reports on aromatic amines and nitroxyl radicals as cathodes for aqueous Li-, Na-, Zn- and ammonium-based cells. It might be expected that similar K-based aqueous dual-ion batteries will become successful in the future.

What's the anode?

When cycled at 4.5 V, as shown in Fig. 6(a), P2-NMZ10 has 68% capacity retention after 200 cycles, while P2-NMM10 only has 51% capacity retention after 200 cycles, indicating that the Zn-doped sample possesses superior cyclability. Our detailed structural analysis reveals that the dopant precipitates in P2-NMZ10 samples are more stable upon continuous cycling, while the precipitates in P2-NMM10 samples are not stable. As shown in Fig. 6(b–d), the morphology of Mg-enriched precipitates changes obviously with increasing cycle numbers, especially from 100 cycles to 200 cycles, and finally high density intragranular cracks and nano-voids (revealed in Li-rich layered cathodes ) are developed after 200 cycles. Note that bulk degradations initiate from the grain interior after prolonged cycles (see more examples in Fig. S11(a and b)†). For the P2-NMZ10 sample, the precipitates are quite stable once formed. As illustrated in Fig. 6(e–g), the lamellar morphology and the density of the precipitates remain similar up to 200 cycles. More P2-NMZ10 examples are shown in Fig. S11(c and d).† The stabilized structure enables the stable cyclability of P2-NMZ10. Therefore, a good dopant should form high-density stable nano-precipitates so that the bulk structure does not degrade upon prolonged cycling.

What's the cathode?

Li-rich layered

A very rapid increase in separation between the ‘charge-averaged’ discharge and charge voltages, within the first few cycles (i.e., ∼10 cycles), can be observed for the ‘no’ Ti-containing Na-TM-oxide (i.e., ‘0.5 Mn/0 Ti’) (see Fig. 4a and b). This is a direct reflection of the rapid increment in corresponding voltage hysteresis (see Fig. 3a), and also ties-up with the rapid capacity fading observed in these very first 10 cycles (see Fig. 3d). By contrast, the changes in ‘charge-averaged’ voltages in case of the fully Ti-substituted Na-TM-oxide are insignificant, even up to the entire duration of galvanostatic cycling (viz., 100 cycles). Interestingly, despite the notable difference in behavior between the ‘0.5 Mn/0 Ti’ and ‘0 Mn/0.5 Ti’ Na-TM-oxide cathodes in the context of variations of the ‘charge-averaged’ discharge and charge voltages during cycling, the corresponding net-average voltages (viz., average of Vdischarge and Vcharge) do not appear to change during cycling in both the cases, with the net-average voltage for the fully Ti-substituted counterpart being consistently greater than that for the ‘control’ Na-TM-oxide cathode by ∼0.25 V. A decrease and increase in Vdischarge and Vcharge, respectively, during electrochemical cycling, are primarily manifestations of build-up of electrode/cell impedance (as also suggested by Jung et al.), whereas any notable variation of the net-average voltage during cycling (as not observed here) is more likely to be a manifestation of irreversible change in the structure (even if just at the surface) of the concerned electrode material. Accordingly, the above observations (viz., Fig. 4) indicate that replacement of Mn-ions by Ti-ions in the Na-TM-oxide cathode considerably suppresses the build-up of electrode/cell impedance upon electrochemical cycling. These will be discussed in more detail in the next two sections.

What's the cathode?

‘0.5 Mn/0 Ti’ and ‘0 Mn/0.5 Ti’ Na-TM-oxide

In this work, we use focused ion beam-scanning electron microscope (FIB-SEM)-based tomography with sub-100 nm spatial resolution to visualize and quantify the loss of mechanical contact in an SSB cathode composite after cycling. Quantitative analysis of the microstructural evolution and interface separation reveals a correlation between the capacity fade and the loss of physical contact between the cathode particles and solid electrolyte upon cycling.

What's the cathode?

SSB

• A high sulfur loading is essential to produce practical Li–S batteries. The metal sulfides for the design of 3D free-standing and sandwich-type cathodes contribute to a high sulfur loading without sacrificing rate capacities and cycling stability.

What's the cathode?

The working electrode was composed of carbon paper, the as-prepared electrocatalysts (70 wt%), super P (20 wt%) and a PVDF binder (10 wt%). The mass loading of the electrocatalysts on the carbon paper was 0.6–0.8 mg cm−2. The electrolyte was 1 M lithium bis (trifluoromethane) sulfonimide, LiTFSI, in dimethyl sulfoxide (DMSO). 2032-type coin LOBs, which included a lithium metal anode (Ø = 15 mm), a glass-fiber separator (GFC, Whatman, Ø = 19 mm) impregnated with an electrolyte, and a well-prepared cathode (Ø = 14 mm), were assembled in a glove box filled with high purity Ar. The electrochemical performance of the assembled LOBs was measured in a sealed home-made container filled with high purity O2. The galvanostatic discharge–charge tests were carried out on a LAND BT 2000 battery testing system. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted on a CHI 604B electrochemical workstation. Before each electrochemical test, the as-assembled LOBs were first equilibrated for several hours. In this work, all the current densities and specific capacities of LOBs were normalized by the actual mass loading of the active materials.

What's the cathode?

The working electrode was composed of carbon paper, the as-prepared electrocatalysts (70 wt%), super P (20 wt%) and a PVDF binder (10 wt%). The mass loading of the electrocatalysts on the carbon paper was 0.6–0.8 mg cm−2. The electrolyte was 1 M lithium bis (trifluoromethane) sulfonimide, LiTFSI, in dimethyl sulfoxide (DMSO). 2032-type coin LOBs, which included a lithium metal anode (Ø = 15 mm), a glass-fiber separator (GFC, Whatman, Ø = 19 mm) impregnated with an electrolyte, and a well-prepared cathode (Ø = 14 mm), were assembled in a glove box filled with high purity Ar. The electrochemical performance of the assembled LOBs was measured in a sealed home-made container filled with high purity O2. The galvanostatic discharge–charge tests were carried out on a LAND BT 2000 battery testing system. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted on a CHI 604B electrochemical workstation. Before each electrochemical test, the as-assembled LOBs were first equilibrated for several hours. In this work, all the current densities and specific capacities of LOBs were normalized by the actual mass loading of the active materials.

What's the anode?

lithium metal

In this review, we have provided a comprehensive overview of the design, synthesis, and application of metal sulfides in rechargeable Li–S batteries. We aimed to summarize important concepts to boost the practical development of high-energy-density Li–S batteries. As a promising polar substrate, metal sulfides have considerable benefits, including a moderately strong affinity for active sulfide binding, high electronic conductivity, the ability to modulate redox reaction kinetics, and a high activity capacity contribution. Through the use of metal sulfides, great progress has been made in improving sulfur cathodes, design of interlayers, modification of separators, and protection of lithium anodes. Despite these achievements, challenges remain in terms of developing affordable large-scale synthesis techniques, elucidating an in-depth mechanism of binding LiPSs, enhancing the rationality and reliability of theoretical simulations, and building stable high-energy-density Li–S batteries over routine LIBs. We hope that this review will inspire greater interest in the use of metal sulfides and other related materials with similar effects as polar substrates for the exploitation of advanced Li–S batteries.

What's the cathode?

sulfur

In this review, we have provided a comprehensive overview of the design, synthesis, and application of metal sulfides in rechargeable Li–S batteries. We aimed to summarize important concepts to boost the practical development of high-energy-density Li–S batteries. As a promising polar substrate, metal sulfides have considerable benefits, including a moderately strong affinity for active sulfide binding, high electronic conductivity, the ability to modulate redox reaction kinetics, and a high activity capacity contribution. Through the use of metal sulfides, great progress has been made in improving sulfur cathodes, design of interlayers, modification of separators, and protection of lithium anodes. Despite these achievements, challenges remain in terms of developing affordable large-scale synthesis techniques, elucidating an in-depth mechanism of binding LiPSs, enhancing the rationality and reliability of theoretical simulations, and building stable high-energy-density Li–S batteries over routine LIBs. We hope that this review will inspire greater interest in the use of metal sulfides and other related materials with similar effects as polar substrates for the exploitation of advanced Li–S batteries.

What's the anode?

lithium

Long cyclic lifespan under high-current density is an important factor to assess the possibility of practical application. Fig. 3e presents the GCD result at a high current density of 2.0 A g−1. Approximately 85% retention of the capacity over 1000 cycles (the initial capacity was 183 mA h g−1) and CEs of the NV NSs@ACC electrode more than 99% were measured, demonstrating the outstanding stability and efficacy of rapid electron transfer during the charge/discharge processes. By contrast, the NV-ACC electrode only shows a capacity of 45 mA h g−1 with obvious long-time active phenomenon (Fig. S3c†). The worsening performance of the NV-ACC electrode is mainly caused by the loose binding force between the active materials and the ACC substrate as well as the non-active binder, resulting in low conductivity and instability. This kind of traditional electrode needs more cycles to activate the cathode materials, which could be attributed to the inefficient electronic and mechanical contact between the active materials and the electrolyte, compared with the binder-free 2D ultrathin nanosheets grown on the ACC. To better verify this hypothesis, electrochemical impedance spectroscopy (EIS) was employed to investigate the ion transport property within the NV NSs@ACC electrode, as shown in Fig. S4a,† where a lower charge-transfer resistance (Rct) can be clearly observed compared to that of NV-ACC.

What's the cathode?

As a supplementary experiment to further confirm this advantageous effect of the DETA CB powders, we fabricated a coin-type (2032R) half cell (printed LCO cathode/liquid electrolyte (1 M LiPF6 in EC/PC (1/1 (v/v)))-filled polyethylene separator/Li metal anode) and investigated its electrochemical performance. The cell containing the DETA CB powders showed the higher discharge rate capability compared to its counterpart containing the pristine CB powders at a fixed charge current density of 0.1C (Fig. S4B†). Moreover, the cell with the DETA CB powders exhibited the higher charge rate capability at a fixed discharge current density of 0.1C (Fig. S4C†). It should be noted that this superior charge rate capability enabled by the DETA CB powders is expected to improve the storage efficiency of the electric energy generated by the cSiPV module.

What's the cathode?

LCO

As a supplementary experiment to further confirm this advantageous effect of the DETA CB powders, we fabricated a coin-type (2032R) half cell (printed LCO cathode/liquid electrolyte (1 M LiPF6 in EC/PC (1/1 (v/v)))-filled polyethylene separator/Li metal anode) and investigated its electrochemical performance. The cell containing the DETA CB powders showed the higher discharge rate capability compared to its counterpart containing the pristine CB powders at a fixed charge current density of 0.1C (Fig. S4B†). Moreover, the cell with the DETA CB powders exhibited the higher charge rate capability at a fixed discharge current density of 0.1C (Fig. S4C†). It should be noted that this superior charge rate capability enabled by the DETA CB powders is expected to improve the storage efficiency of the electric energy generated by the cSiPV module.

What's the electrolyte?

1 M LiPF6 in EC/PC (1/1 (v/v))

However, because of the larger sodium ionic radius (rNa+ 1.02 Å vs. rLi+ 0.76 Å), it is necessary to explore suitable cathode materials and modification methods that could accommodate sufficient sodium ions as well as ensure reversible and fast Na+ insertion/extraction. Various types of cathode materials have been proposed, including layered transition-metal oxides (NaxMO2), Prussian blue analogues (PBAs, NaxMy[Fe(CN)6]), polyanion-type compounds, and organic compounds. Due to the strong P–O covalent bond of the PO43− group, combined with the high electronegativity of the fluoride anion (F−), fluorophosphate-based polyanion compounds generally have robust crystal frameworks and tunable high redox potentials, which makes them get more attention.

What's the cathode?

layered transition-metal oxides (NaxMO2), Prussian blue analogues (PBAs, NaxMy[Fe(CN)6]), polyanion-type compounds, and organic compounds

Firstly, the effect of the KMnO4 concentration on the MnO2 loading and the capacity of the battery was studied. As shown in Fig. 3e, the loading of MnO2 on the electrode gradually increases with the increase of KMnO4 concentration. When the concentration of KMnO4 is 0.1 mM, a low MnO2 loading of 4.9 mg cm−3 (weight percentage of 21.1%) is obtained. And the capacity of the corresponding battery can be as high as 369.7 mA h g−1 (based on the mass of the active material of MnO2), superior to that of most of the earlier reported Zn–MnO2 batteries and aqueous zinc batteries (see Table S1†), such as Zn–MnO2 batteries (225 mA h g−1), Zn–ZnMn2O4 batteries (150 mA h g−1), Zn–PTO batteries (336 mA h g−1), Zn–NiCo2O4 batteries (183.1 mA h g−1), Zn–MoS2 batteries (202.6 mA h g−1), Ni–Zn batteries (265 mA h g−1), and Zn–V2O5@PEDOT batteries (360 mA h g−1). When the concentration of KMnO4 increases to 10 mM, the mass loading of MnO2 reaches 51.0 mg cm−3 (weight percentage of 79.1%), and the corresponding capacity remains at 306.7 mA h g−1. Meanwhile, the volumetric capacity of the battery increases accordingly at the beginning, and reaches its highest value (15.6 mA h cm−3) when the concentration of KMnO4 is about 10 mM, which is chosen as the optimal concentration for the preparation of cathode materials. Notably, a high loading of MnO2 seems to have a negligible effect on the ion diffusion as elucidated from the Nyquist plots in Fig. S9.† The linear slopes for the two electrodes are nearly the same, indicative of their similar ion diffusion resistance. High mass loading and high volumetric capacity are desirable in energy storage devices because of the advantages in maximizing the packing density of the electrode materials while lowering the manufacturing cost by reducing the number of inactive material layers.

What's the cathode?

In voltage research, the advantages of cathode exfoliation and anode exfoliation can be combined by using the voltage of the changing direction, and the process of electrolyte intercalation and gas expansion can be brought into full play.

What's the cathode?

In voltage research, the advantages of cathode exfoliation and anode exfoliation can be combined by using the voltage of the changing direction, and the process of electrolyte intercalation and gas expansion can be brought into full play.

What's the anode?

The vanadium loss of the cathode is measured using inductive coupled plasma optical emission spectroscopy (ICP OES) after 7, 20 and 40 cycles in the discharged state as it is difficult to accurately quantify the small amount of dissolved vanadium after the first cycle. It can be seen from Fig. 9 that a considerable amount of V is lost from the cathode. The extent of vanadium dissolution is roughly proportional to the upper potential limit used in the cycling, which indicates a correlation between the V loss and the anionic redox activity. The vanadium dissolution allows the V4+ ions and partially oxidized oxygen ions (e.g., O− ions, peroxides and superoxides) to remain in the structure upon the reinsertion of Li+ based on the charge neutrality. The formation of anionic redox species and its reversibility are closely linked to the vanadium dissolution, although their causalities remain unclear (i.e., whether the vanadium dissolution leads to the formation of anionic redox species and reduce its reversibility, or vice versa). We note that a lack of causality is also possible as they may occur simultaneously to stabilize each other due to their opposite contribution to the net charge.

What's the cathode?

Areal capacities for organic-based cathodes range from ∼0.07 to ∼2 mA h cm−2 (Chart 1d). As in the case with the electrode composition, no optimization of the mass loading is typically performed. For PTCDA 9, where such optimization took place, record high areal capacity was demonstrated.

What's the cathode?

organic-based

In order to get rid of templates, many efforts have been attempted. CP nanowire electrochemically polymerized and assembled onto two biased electrodes (anode and cathode) immersed in aqueous monomer solutions. The essence of this method is an electrode-wire-electrode or electrode-wire-target assembly. For instance, CP nanowires are prepared by an electro-deposition within channels between two electrodes on the surface of silicon wafers. By using this way, Chouvy et al. prepared oriented PPy nanowires and found the diameter and the length of the nanowires can be increased when the solution contained a high concentration of weak-acid anions and a low concentration of non-acidic anions. Since electrochemical polymerization is a shared and controlled method for preparing CPs and their nanostructures, electro-deposition within channels between two electrodes has received attention in fabricating molecule devices.

What's the cathode?

In order to get rid of templates, many efforts have been attempted. CP nanowire electrochemically polymerized and assembled onto two biased electrodes (anode and cathode) immersed in aqueous monomer solutions. The essence of this method is an electrode-wire-electrode or electrode-wire-target assembly. For instance, CP nanowires are prepared by an electro-deposition within channels between two electrodes on the surface of silicon wafers. By using this way, Chouvy et al. prepared oriented PPy nanowires and found the diameter and the length of the nanowires can be increased when the solution contained a high concentration of weak-acid anions and a low concentration of non-acidic anions. Since electrochemical polymerization is a shared and controlled method for preparing CPs and their nanostructures, electro-deposition within channels between two electrodes has received attention in fabricating molecule devices.

What's the anode?

Herein, with the aid of an electrochemistry-driven method, Fe ions were pre-intercalated into multilayered Ti3C2Tx (ML Ti3C2Tx) using cyclic voltammetry (CV) technology. Systemic characterization of the interlayer environment was conducted. The location of Fe ions was analyzed, combined with DFT simulations. The EMIm+ storage abilities of the Fe pre-intercalated MXene were confirmed using a dual-ion energy storage system including a graphite cathode. Meanwhile, the intercalation behavior of EMIm+ was studied via ex situ XRD and XPS. The complex relationship between the interlayer, EMIm+, and confined Fe ions was investigated.

What's the cathode?

graphite

Spherical [Ni0.5Mn0.5](OH)2 precursors were synthesized by the co-precipitation method. Stoichiometric amounts of NiSO4·6H2O and MnSO4·H2O (Samchun Chemical, Korea) were used as starting materials for [Ni0.5Mn0.5](OH)2. The metal solution concentration (2 mol L−1 for the metal solution), pH (∼11.0), temperature (45 °C), and stirring speed of the mixture in the batch-type reactor were carefully controlled. At the same time, aqueous NaOH (Samchun, Korea; NaOH/transition metal molar ratio = 2:1) and aqueous NH4OH (Junsei, Japan; NH4OH/transition metal molar ratio = 1.1:1) as chelating agents were separately fed into the reactor. The precursor powders were obtained by filtering, washing and vacuum drying at 110 °C overnight. For the Na[Ni0.5Mn0.5]O2 cathode, the obtained spherical precursors were mixed with Na2CO3 (Na:[Ni + Mn] molar ratio = 1.05:1, 5% excess Na2CO3), calcined at 800 °C for 24 h in an oxygen atmosphere, and quenched under vacuum conditions. For Ca-substituted Na1−2xCax[Ni0.5Mn0.5]O2 cathodes, an appropriate amount of Ca(OH)2 was mixed with the [Ni0.5Mn0.5](OH)2 precursor and Na2CO3 (Na:Ca:[Ni + Mn] molar ratio = 1.05–2x:x:1, x = 0.01, 0.02, and 0.03) in the same calcination process.

What's the cathode?

Na[Ni0.5Mn0.5]O

Spherical [Ni0.5Mn0.5](OH)2 precursors were synthesized by the co-precipitation method. Stoichiometric amounts of NiSO4·6H2O and MnSO4·H2O (Samchun Chemical, Korea) were used as starting materials for [Ni0.5Mn0.5](OH)2. The metal solution concentration (2 mol L−1 for the metal solution), pH (∼11.0), temperature (45 °C), and stirring speed of the mixture in the batch-type reactor were carefully controlled. At the same time, aqueous NaOH (Samchun, Korea; NaOH/transition metal molar ratio = 2:1) and aqueous NH4OH (Junsei, Japan; NH4OH/transition metal molar ratio = 1.1:1) as chelating agents were separately fed into the reactor. The precursor powders were obtained by filtering, washing and vacuum drying at 110 °C overnight. For the Na[Ni0.5Mn0.5]O2 cathode, the obtained spherical precursors were mixed with Na2CO3 (Na:[Ni + Mn] molar ratio = 1.05:1, 5% excess Na2CO3), calcined at 800 °C for 24 h in an oxygen atmosphere, and quenched under vacuum conditions. For Ca-substituted Na1−2xCax[Ni0.5Mn0.5]O2 cathodes, an appropriate amount of Ca(OH)2 was mixed with the [Ni0.5Mn0.5](OH)2 precursor and Na2CO3 (Na:Ca:[Ni + Mn] molar ratio = 1.05–2x:x:1, x = 0.01, 0.02, and 0.03) in the same calcination process.

What's the cathode?

Na1−2xCax[Ni0.5Mn0.5]O2

In addition, the galvanostatic discharge curves of the hybrid battery at different current densities are studied. As displayed in Fig. 6b, a capacity of 785 mA h gZn−1 is achieved at the current density of 2 mA cm−2 based on the mass of Zn consumed. The result is close to the theoretical value (820 mA h gZn−1). It demonstrates the high capacity of the hybrid battery, taking advantage of the Zn–air battery. When the active species in the cathode and the Zn consumed in the anode are both considered, a high gravimetric energy density of 950 W h kg−1 is achieved. Therefore, the results demonstrate the high capacity and energy density of the hybrid Zn battery. The stability of the hybrid battery was another crucial criterion for practical applications. As displayed in Fig. 6c, the hybrid battery exhibits good electrochemical durability with stable delivered capacity and charge/discharge profiles during cycling at 2 mA cm−2. After one thousand cycles, the voltage of the redox reaction and the ORR/OER process are still well retained (Fig. 6d), demonstrating the good long-term cycling stability of the hybrid zinc battery. In addition, high energy efficiency of 72% is achieved after one thousand cycles, which demonstrates the high efficiency of the hybrid battery.

What's the anode?

Zn

The two-cycled coin-type cells were disassembled in the argon-filled glove box. The cathode electrodes were carefully extracted from the cells and then immediately rinsed in dimethyl carbonate (DMC) solvent to remove electrolyte residues and dried under vacuum overnight. The obtained electrodes were then transferred from the glove box into an oven and treated with different temperatures (210 °C, 280 °C, 350 °C) in the air for four hours. The weight loss during the high-temperature treatment was below 3% of that of the electrode, which primarily resulted from the decomposition of carbon and binder. For comparison, one of the obtained electrodes was kept in the argon-filled glove box at room temperature. After thermal treatment/being kept in the glove box, the electrodes were reassembled into coin-type cells with a fresh Li metal as the counter electrode.

What's the cathode?

dimethyl carbonate (DMC)

Recently, Fan and co-authors proposed a PTCDI derivative with anthraquinone moieties (PTCDI-DAQ 11). This compound has lower solubility than PTCDI due to the increased molecular weight, as well as higher theoretical capacity of 200 mA h g−1. The observed Qm was up to 220 mA h g−1, again higher than the theoretical value. A decent capacity retention of 73% was demonstrated in the electrolyte with relatively low salt concentration (1 M KPF6 in DME). Moreover, a remarkable capacity of 137 mA h g−1 was obtained at 20 A g−1. A full cell with pre-potassiated terephthalate (K4TP) anode had a high energy density of 295 W h kg−1 (213 mA h g−1 × 1.38 V, calculated per cathode mass), excellent rate capabilities (94 mA h g−1 at 10 A g−1), and cycling stability (49% and 32% retention after 10000 and 30000 cycles, respectively).

What's the cathode?

Inspired by the superior ORR and OER electrocatalytic activities as well as excellent faradaic redox performance of the DBHF fibers, hybrid Zn batteries were fabricated. In the HZB, the DBHF fibers act as the cathode, Zn nanosheets on carbon cloth serve as the anode and a mixed solution of 6 M KOH and 0.2 M Zn(Ac)2 serves as the electrolyte. The charge/discharge performance of the hybrid Zn battery is investigated. As displayed in Fig. 6a, a smooth and flat voltage plateau at ∼1.21 V is detected in the initial discharging process, which indicates the ORR process of a typical Zn–air battery. In the subsequent charging process, the two voltage plateaus at ∼1.85 V and ∼2.05 V correspond to the M–O oxidation and the OER process, respectively. During the second discharge process, two voltage plateaus appear at ∼1.74 V and ∼1.21 V corresponding to the reduction reaction of M–O–OH and the ORR process, respectively. The charge/discharge results are in good agreement with the CV curves (Fig. S7†). Both results demonstrate that the fabricated hybrid battery is able to reversibly store and deliver charges via double sets of electrochemical reactions. They are the ORR/OER reactions in the Zn–air battery and the M–O/M–O–OH redox reaction in the Zn-ion battery assembled in one battery. Such a combination of complementary strength leads to a powerful hybrid Zn battery based on the Zn–air and Zn-ion systems.

What's the cathode?

DBHF fibers

Besides lithium, a modern LIB typically contains non-widespread transition metals, such as copper (anode current collector), nickel and cobalt (cathode materials). The shortage in cobalt is especially acute. If cobalt-free batteries remain underdeveloped, the production of Co will have to increase up to an order of magnitude to satisfy the future demand. Additionally, extraction and processing of transition metals might be harmful to the environment and is typically energy-intensive.

What's the cathode?

nickel and cobalt

Besides lithium, a modern LIB typically contains non-widespread transition metals, such as copper (anode current collector), nickel and cobalt (cathode materials). The shortage in cobalt is especially acute. If cobalt-free batteries remain underdeveloped, the production of Co will have to increase up to an order of magnitude to satisfy the future demand. Additionally, extraction and processing of transition metals might be harmful to the environment and is typically energy-intensive.

What's the anode?

copper

Interestingly, in the presence of Ti-ions as a partial substitute for Mn-ions (as in the case of ‘0.2 Mn/0.3 Ti’ Na-TM-oxide), Mn was found to be completely in the +4 oxidation state (i.e., Mn4+ and not Mn3+) in the pristine electrode, unlike for the ‘0.5 Mn/0 Ti’ counterpart (see ESI Fig. S7d†). Such observations suggest that the presence of Ti-ions suppresses the reduction of Mn4+ to the deleterious Mn3+ during electrode preparation and, thus, improves the stability to some extent. However, similar to the case with ‘0.5 Mn/0 Ti’, Mn3+ did form and the content progressively increased during electrochemical cycling (as inferred from post-cycling XPS data, ESI Fig. S4e†), thus, not totally addressing the problem concerning cycling (in)stability. Of course, in the total absence of Mn-ions, as for the completely Ti-substituted ‘0 Mn/0.5 Ti’ Na-TM-oxide, the oxidation state of Ti-ions remained only as +4 in the as-prepared electrodes, as well as in those cycled for even 100 electrochemical cycles, as can be seen from Fig. 5c and d. In contrast to those observed for the ‘control’ ‘0.5 Mn/0 Ti’ Na-TM-oxide cathode, neither any notable variation of Vdischarge and Vcharge with cycling could be seen, nor any change in colour of the separator could be noted in the case of the fully Ti-substituted (for Mn-ion) Na-TM-oxide cathode, which indicates non-occurrence of TM-dissolution. Not invoking here, the possible additional contribution towards capacity fading from Jahn–Teller distortion of Mn3+ ( ) during electrochemical cycling, the oxidation states of TM-ions at the surface of the Na-TM-oxides, in particular the presence/formation of Mn3+ and its likely dissolution into the electrolyte, presents a strong justification for the capacity fading in the presence of Mn-ions and mitigation of the same upon Ti-substitution. In this case, electrode impedance is also likely to be considerably affected by the replacement of Mn-ions by Ti-ions, as discussed in the following sections.

What's the cathode?

Ti-substituted (for Mn-ion) Na-TM-oxid

Later, Chen et al. used K2TP under similar conditions as the anode for hybrid supercapacitors. Activated carbon was used as the cathode. Energy of 101 W h kg−1 and power of 2.16 kW kg−1 based on the mass of two electrodes were demonstrated. If the electrolyte mass was included, the values reached 41.5 W h kg−1 and 885 W kg−1, respectively. The capacity retention of the supercapacitor was 97.7% in 500 cycles. In half cells, the capacity of K2TP at 5 A g−1 was 124 mA h g−1.

What's the cathode?

carbon

Later, Chen et al. used K2TP under similar conditions as the anode for hybrid supercapacitors. Activated carbon was used as the cathode. Energy of 101 W h kg−1 and power of 2.16 kW kg−1 based on the mass of two electrodes were demonstrated. If the electrolyte mass was included, the values reached 41.5 W h kg−1 and 885 W kg−1, respectively. The capacity retention of the supercapacitor was 97.7% in 500 cycles. In half cells, the capacity of K2TP at 5 A g−1 was 124 mA h g−1.

What's the anode?

K2TP

To obtain strong immobilization and fast conversion kinetics for polysulfides, the concept of constructing highly adsorptive and catalyzing heterostructures has been widely developed, such as TiN–TiO2, MoN–VN, and WS2–WO3. In fact, the anchoring of polysulfides as well as Li2S precipitation are part of a continuous multielectron redox reaction in a working battery, and thus an integrated platform, such as 1T MoS2 NDs in this work, enabling smooth “immobilization–diffusion–conversion” is highly appealing. The unique advantages for 1T MoS2 ND can be summarized as following: (i) the edge-rich NDs with an intrinsically strong affinity to polysulfides serve as anchoring sites for LPSs and accelerate their reduction, which contributes to the long cycle life and high sulfur utilization; (ii) the highly conductive and catalytic nature of 1T MoS2 NDs uniformly loaded on conductive substrates can facilitate Li–S reaction kinetics (Fig. S14, ESI†) and reduce the cell resistance, which benefits the high rate performance of MoS2 ND-modified cathodes; (iii) the well-defined morphology and phase of MoS2 maximize the active sites to fully demonstrate their potential for Li–S chemistry, which enables a boost of the electrochemical performance with a small amount of electrochemical inert catalysts. In addition, post-mortem analyses revealed that MoS2 NDs/porous carbon remained stable after cycling (Fig. S15, ESI†). To further determine whether the excellent catalytic performance of the MoS2 NDs could be retained during long-term cycling, the MoS2 ND/porous carbon/Li2S6 cathode was prolonged to 500 cycles at 0.5C (Fig. S16, ESI†), and exhibited only a low capacity fading rate of 0.08% per cycle, directly indicating the stability of the MoS2 NDs as a catalyst in long-term Li–S batteries. Finally, thanks to the facile fabrication method and the low content needed to improve LSB performance, the large-scale application of 1T MoS2 NDs in high-energy LSBs should not be problematic.

What's the cathode?

MoS2 ND-modified

To obtain strong immobilization and fast conversion kinetics for polysulfides, the concept of constructing highly adsorptive and catalyzing heterostructures has been widely developed, such as TiN–TiO2, MoN–VN, and WS2–WO3. In fact, the anchoring of polysulfides as well as Li2S precipitation are part of a continuous multielectron redox reaction in a working battery, and thus an integrated platform, such as 1T MoS2 NDs in this work, enabling smooth “immobilization–diffusion–conversion” is highly appealing. The unique advantages for 1T MoS2 ND can be summarized as following: (i) the edge-rich NDs with an intrinsically strong affinity to polysulfides serve as anchoring sites for LPSs and accelerate their reduction, which contributes to the long cycle life and high sulfur utilization; (ii) the highly conductive and catalytic nature of 1T MoS2 NDs uniformly loaded on conductive substrates can facilitate Li–S reaction kinetics (Fig. S14, ESI†) and reduce the cell resistance, which benefits the high rate performance of MoS2 ND-modified cathodes; (iii) the well-defined morphology and phase of MoS2 maximize the active sites to fully demonstrate their potential for Li–S chemistry, which enables a boost of the electrochemical performance with a small amount of electrochemical inert catalysts. In addition, post-mortem analyses revealed that MoS2 NDs/porous carbon remained stable after cycling (Fig. S15, ESI†). To further determine whether the excellent catalytic performance of the MoS2 NDs could be retained during long-term cycling, the MoS2 ND/porous carbon/Li2S6 cathode was prolonged to 500 cycles at 0.5C (Fig. S16, ESI†), and exhibited only a low capacity fading rate of 0.08% per cycle, directly indicating the stability of the MoS2 NDs as a catalyst in long-term Li–S batteries. Finally, thanks to the facile fabrication method and the low content needed to improve LSB performance, the large-scale application of 1T MoS2 NDs in high-energy LSBs should not be problematic.

What's the cathode?

MoS2 ND/porous carbon/Li2S6

To investigate the failure mechanism of the overcharge protection of the TDAC, a scaled-up NCA/graphite cell was employed to reclaim enough electrolyte and electrode materials for a post-mortem analysis. The reclaimed electrolyte after overcharge was in a brownish color, which could be contributed by the dissolution of the transition metals (Co and Ni) from the cathode, the decomposition products of the electrolyte solvent, or the TDAC molecules in the harsh overcharge environment. MS, NMR and CV were used to analyse the reclaimed electrolyte in comparison with the fresh electrolyte. Fig. 5a shows a peak at a 283.2 mass/charge ratio (m/z) for both electrolytes. The MS data confirmed the existence of TDAC cation species in both electrolytes. The similar intensity of the peaks indicated that the concentration of the TDAC in the overcharged electrolyte was about the same as that in the fresh one. NMR analysis as shown in Fig. 5b also confirmed that most of the TDACs remained intact since no TDAC decomposed compounds were detected. The CV profile in Fig. 5c reveals a pair of reversible redox peaks at ∼1.35 V vs. Fc+/Fc, exhibiting no discrepancy against the fresh TDAC electrolyte. The reclaimed electrolyte was then injected into a cell with new NCA and graphite sheets. As displayed in Fig. 5d, the potential plateau at ∼4.5 V reappeared in the reassembled cell despite a lower CE and a decreased capacity.

What's the cathode?

For full cells, commercial lithium iron phosphate (LFP) was used as the cathode material. A mixture of LFP powder, acetylene black (conductive additive) and polyvinylidene fluoride (PVDF) with a mass ratio of 8:1:1 in N-methyl-2-pyrrolidone (NMP) was first coated on aluminum foil, dried in a vacuum oven at 80 °C overnight and used as the LFP cathode. The areal mass loading of LFP was around 5 mg cm−2. The charge–discharge voltage was recorded from 2.0 to 4.0 V while the current rate varied from 0.2 to 2.0C (1C is equals to 170.0 mA h g−1). Electrochemical impedance spectroscopy (EIS) measurements were performed on an IVIUM electrochemical workstation with a frequency range of 105–0.01 Hz.

What's the cathode?

lithium iron phosphate (LFP)

For full cells, commercial lithium iron phosphate (LFP) was used as the cathode material. A mixture of LFP powder, acetylene black (conductive additive) and polyvinylidene fluoride (PVDF) with a mass ratio of 8:1:1 in N-methyl-2-pyrrolidone (NMP) was first coated on aluminum foil, dried in a vacuum oven at 80 °C overnight and used as the LFP cathode. The areal mass loading of LFP was around 5 mg cm−2. The charge–discharge voltage was recorded from 2.0 to 4.0 V while the current rate varied from 0.2 to 2.0C (1C is equals to 170.0 mA h g−1). Electrochemical impedance spectroscopy (EIS) measurements were performed on an IVIUM electrochemical workstation with a frequency range of 105–0.01 Hz.

What's the cathode?

LFP

Lithium–sulphur (Li–S) batteries with high theoretical capacity, cost-effectiveness and environmental sustainability have attracted tremendous interest as the next generation rechargeable batteries. However, the performance of Li–S batteries is constrained by sulphur's low conductivity and large volume change during lithiation/delithiation. As the cathode matrix for the sulphur cathode, h-BNNS with nitrogen vacancies (v-BN) were found to improve the cycling performance of Li–S batteries. Compared with the pristine h-BN electrode, the v-BN electrode demonstrated significantly improved capacity. The engineered vacancies were believed to improve the kinetics of polysulphide conversion, lithium-ion diffusion rate, and protection of the Li anode from corrosion.

What's the cathode?

sulphur

The hybrid Na–air battery was assembled in a sandwich structure. Initially, the anode part of the hybrid Na–air cell was fabricated in a glove box. The metallic sodium was cut and pressed into a sheet with a diameter of 10 mm as the anode, followed by the injection of organic electrolyte (1 M NaClO4 in tetraethylene glycol dimethyl ether) into the anode chamber. The solid electrolyte NASICON was sealed as the separator for the anode and cathode chamber, ensuring that one side of the NASICON was in contact with the organic electrolyte and the other side was exposed to air. The NASICON solid electrolyte was chosen to avoid direct contact between the aqueous electrolyte and Na anode, and specifically allow Na ions to shuttle between the anode and cathode during the reactions. The cathode compartment was built under ambient air by directly immersing the air electrode in the O2-saturated 0.1 M NaOH electrolyte. The as-prepared electrocatalysts coated on NF (diameter 10 mm) were directly used as air cathodes. For comparison, the 20% Pt/C (Macklin) and RuO2 catalysts cast on NF (diameter 10 mm) were prepared with a mass loading of 1 mg cm−2.

What's the cathode?

air

Yao et al. presented a series of aqueous systems with quinone-based anodes and industrially established cathodes, which showed excellent stability, kinetics and energy density. Particularly for K-based batteries, PAQS P1 was paired with Ni(OH)2 in alkaline media (10 M KOH). The anode showed a capacity of 200 mA h g−1. At r.t., the full cell energy density (79 W h kg−1/138 W h L−1) was smaller than for a nickel metal hydride battery (180 W h kg−1/597 W h L−1), partially due to a relatively high anode potential. However, the capacity retention (88% after 1300 cycles) was better than for the best metal hydrides. Moreover, the capacity of PAQS decreased only by 7% as the temperature decreased from 25 to −25 °C. It is in contrast with Ni-MmH, whose energy density significantly decreases at lower temperatures.

What's the cathode?

Compared with graphite, Bi, Sb and P, organic-based materials are less attractive in terms of the potentials. The electrode capacities are also generally lower, although several materials with high demetalation potentials showed a Qe of above 250 mA h g−1. Areal capacities (Chart 5c) are mostly comparable for organic-based materials and the benchmarks, except for graphite, for which very high loadings (up to 28.56 mg cm−2, corresponding to ∼7.4 mA h cm−2) were demonstrated. Similar to the cathodes (Section 2.7), the material loading of the anodes is typically left unoptimized.

What's the cathode?

The determination of metal and polymer material moduli are straightforward and taken from literature values. To verify the estimated moduli of the composite electrodes, they were stacked in various layers and acoustically interrogated. As shown in Fig. 3a, the graphite/Cu electrode has a modulus of 10.7 GPa, and the LiCoO2/Al electrode has a modulus of around 27.8 GPa, as calculated from the measured wave velocities (Fig. 3c) and the respective weighted densities of the double-sided electrodes. These values are slightly higher than the respective Hashin–Shtrikman bounds because the measured electrodes include the metal current collector which has a higher modulus. The values are lower than the single particle graphite or LiCoO2 because the composite electrodes are weighted down by the softer materials. The change in thickness of these individual electrodes during cycling can also be estimated from average peak-to-peak spacing of the intensity line profiles generated from TXM micrographs (Fig. S3†). These peak-to-peak intensities vary because of pouch cell manufacturing tolerances: the electrodes are not perfectly aligned within the cell, causing signal blurring at electrode boundaries. This percent error in thickness calculation is minimized by introducing threshold values for peak discrimination. The results of the peak spacing analysis (Fig. S4†) show that the average single layer expands upon charge and contracts upon discharge, and that the degree of hysteresis grows with current rate. The initial average electrode thickness of 170 μm confirms the ex situ digital caliper measurements of individual electrodes (180 μm for LiCoO2/Al and 200 μm for graphite/Cu) and the moduli estimation of 27.8 GPa for LiCoO2/Al and 10.7 GPa for graphite/Cu. Unfortunately, the differences in thickness changes between LiCoO2/Al and graphite/Cu during cycling are hard to discern due to the imperfect alignment of the electrode layers with the X-ray detector. For future studies, improved spatial resolution at the single electrode length scale would be aided by tests of single-layer pouch cells, where thicknesses of the single anode and cathode could be measured more accurately without stack distortion.

What's the cathode?

The determination of metal and polymer material moduli are straightforward and taken from literature values. To verify the estimated moduli of the composite electrodes, they were stacked in various layers and acoustically interrogated. As shown in Fig. 3a, the graphite/Cu electrode has a modulus of 10.7 GPa, and the LiCoO2/Al electrode has a modulus of around 27.8 GPa, as calculated from the measured wave velocities (Fig. 3c) and the respective weighted densities of the double-sided electrodes. These values are slightly higher than the respective Hashin–Shtrikman bounds because the measured electrodes include the metal current collector which has a higher modulus. The values are lower than the single particle graphite or LiCoO2 because the composite electrodes are weighted down by the softer materials. The change in thickness of these individual electrodes during cycling can also be estimated from average peak-to-peak spacing of the intensity line profiles generated from TXM micrographs (Fig. S3†). These peak-to-peak intensities vary because of pouch cell manufacturing tolerances: the electrodes are not perfectly aligned within the cell, causing signal blurring at electrode boundaries. This percent error in thickness calculation is minimized by introducing threshold values for peak discrimination. The results of the peak spacing analysis (Fig. S4†) show that the average single layer expands upon charge and contracts upon discharge, and that the degree of hysteresis grows with current rate. The initial average electrode thickness of 170 μm confirms the ex situ digital caliper measurements of individual electrodes (180 μm for LiCoO2/Al and 200 μm for graphite/Cu) and the moduli estimation of 27.8 GPa for LiCoO2/Al and 10.7 GPa for graphite/Cu. Unfortunately, the differences in thickness changes between LiCoO2/Al and graphite/Cu during cycling are hard to discern due to the imperfect alignment of the electrode layers with the X-ray detector. For future studies, improved spatial resolution at the single electrode length scale would be aided by tests of single-layer pouch cells, where thicknesses of the single anode and cathode could be measured more accurately without stack distortion.

What's the anode?

However, in the extreme case, when Qm of a dual-ion battery cathode is 300 mA h g−1 and the average potential is 5.0 V, Ecorrm is only ∼500 W h kg−1 if KPF6 is used as the supporting salt (Chart 2b). Switching to lighter salts, such as KClO4, KBF4 and particularly KF might significantly boost the energy density (Chart 2c). For potassium fluoride, an Ecorrm of ∼500 W h kg−1 is achieved already at a reasonable ∼200 mA h g−1 and 3.6 V. Optimization of the salt composition, however, has not been reported yet. Considering low solubility of KF in aprotic electrolytes, it is especially intriguing if it may succeed as the supporting salt.

What's the cathode?

Uniform doping has been recognized as an effective approach to improve the cycling performance of many layered cathodes. Herein, we propose non-uniform doping as a more effective approach to enhance the cycling stability of layered cathodes via the precipitation strengthening mechanism. In this work, we investigate four doped (Cu, Ti, Mg, and Zn) P2-layered cathodes for sodium ion batteries and validate that cycling induced dopant segregation can substantially enhance the cyclability due to mitigation of bulk cracking. Our comprehensive analysis indicates that dopant evolution is quite diverse during electrochemical cycling, not only depending on the nature of each dopant but also the cycling conditions applied. The migration and segregation behaviors of inactive dopants demonstrate the complex dynamics within grain bulk during battery cycling, which also offers us chances to engineer the physicochemical properties of layered cathodes. Non-uniform doping opens a new avenue for designing battery materials with superior mechanical properties.

What's the cathode?

doped (Cu, Ti, Mg, and Zn) P2-layered

To probe the surface chemistry of MoS2 NDs before and after LPS adsorption, XPS measurements were carried out. Fig. S7 (ESI†) shows the deconvoluted Mo 3d and S 2p spectra of the pristine and polysulfides-mixed MoS2 NDs. In comparison with the pristine MoS2 NDs, the Mo 3d peaks of 3d3/2 and 3d5/2 for the MoS2 ND/polysulfide were overall downshifted by ∼0.15 eV, which could be attributed to the intense interaction of the exposed Mo atoms with the surrounding strong electronegative sulfur ligand. In the S 2p spectra, peaks centered at 163.2, 162.6, and 161.3 eV, corresponding to the S 2p1/2 and 2p3/2 components of the original MoS2 ND, were almost intact after mixing with the polysulfides, possibly due to the weak interaction between the S atoms in the MoS2 NDs and polysulfides. Interestingly, new features located at 159.8 and 165.2–169.4 eV appeared for the MoS2 ND/polysulfide, which were considered due to the formation of the polythionate moiety. It is reasonable to deduce that the “sulfiphilic” surface for MoS2 NDs would significantly promote the retention of polysulfides in the cathode region. More importantly, the edge-rich MoS2 NDs increased the Mo–S bridge between the metallic catalyst and polysulfides, propelling electron transfer across the MoS2 NDs to the polysulfides, and thus significantly catalyzing the chemical redox conversion of the polysulfides.

What's the cathode?

MoS2 NDs

Overall, crosslinking the sensitizers under copper-free conditions lead to an increase of the photovoltaic performances of all the DSSCs with both electrolytes. For NiO photocathodes, the Jsc is decreased owing to most probable iodide association with triazole, but it is compensated with the increase of Voc and ff. Crosslinking of the dyes with copper(I) catalysis is also possible, but the resulting solar cells exhibit lower performances, showing that copper has poisoned the devices.

What's the cathode?

NiO

Compounds derived from hexaazatriphenylene (HAT) might reversibly accept up to six electrons per HAT unit and have excellent cycling capabilities. They might withstand 10000–50000 cycles with small capacity fading. Kapaev et al. showed that HAT-based polymer P14 delivered a Qm of up to 245 mA h g−1 at 50 mA g−1 in a 0.9–3.4 V vs. K potential range. Interestingly, the reversible capacity continuously increased upon cycling at 10 A g−1 from 150 to 169 mA h g−1 after 4600 cycles. This slow activation, of which the reasons are yet unknown, makes P14 the most stable among all reported cathode materials for non-aqueous K-based batteries. After 4600 cycles, the cells short-circuited due to dendrite formation at the potassium anode.

What's the cathode?

P14

Volumetric capacity and density are usually unreported for organic materials used in potassium batteries. Density for solid polar organic compounds containing only C, H, N and O ranges between roughly 1.1 and 1.7 g cm−3, and it is smaller than for inorganic cathode materials (calc. 2.3 g cm−3 for KMHCF, calc. 3.1 g cm−3 for KVPO4F, and 3.35 g cm−3 for V2O5 ( )). This limits the applicability of organic cathode materials where the low device volume is crucial, e.g., in portable electronics. However, in some electric vehicles and large-scale stationary applications the volume is supposed to be less restricted.

What's the cathode?

KMHCF

Volumetric capacity and density are usually unreported for organic materials used in potassium batteries. Density for solid polar organic compounds containing only C, H, N and O ranges between roughly 1.1 and 1.7 g cm−3, and it is smaller than for inorganic cathode materials (calc. 2.3 g cm−3 for KMHCF, calc. 3.1 g cm−3 for KVPO4F, and 3.35 g cm−3 for V2O5 ( )). This limits the applicability of organic cathode materials where the low device volume is crucial, e.g., in portable electronics. However, in some electric vehicles and large-scale stationary applications the volume is supposed to be less restricted.

What's the cathode?

KVPO4F

High energy density lithium ion batteries have been applied as the main energy storage technology in portable devices and green transportation. LiCoO2, a layered oxide cathode in lithium ion batteries, has obtained unprecedented success since its commercialization in the 1990s. LiNiO2, isostructural to LiCoO2, can deliver a higher capacity within a relatively low upper cutoff voltage. However, its practical application has been hindered due to the inherent structural instability. Thus, cobalt, as an essential element, has been incorporated into the LiNiO2 system to form LiNi1−x−yMnyCoxO2 (NMC) and LiNi1−x−yCoxAlyO2 (NCA) materials to release the strong magnetic moment and ultimately stabilize the structure. At present, researchers are pursuing the theoretical capacity of NMCs either by elevating the high cutoff voltage or increasing the nickel concentration. However, these strategies have inevitably provoked some of the degradation pathways of these materials, including transition metal reduction/dissolution, surface reconstruction, gas evolution, electrolyte oxidation/decomposition, and chemomechanical breakdown. Additionally, most of these materials are synthesized in the polycrystalline form and have heterogeneous chemistry, resulting in high heterogeneous degradation patterns.

What's the cathode?

LiCoO2

Against this backdrop, the presently reported work focuses on designing a composition (viz., combination of TM- and non-TM-ions) and suitably tuning some structural features of O3-type Na-TM-oxide based cathode materials to address the main issue concerning stability against hydration and also improve the electrochemical cyclic stability, as well as rate capability. While doing so, a strong correlation between the composition, structure, stability against hydration and electrochemical behavior/performance for such oxides has been established. As will be demonstrated, an air/water-stable (despite long-term exposures under stringent conditions) O3-type Na-TM-oxide has been developed, which is devoid of all the problems mentioned above, so much so that it allows the successful development of stable electrodes using a cost-effective and environment/health friendly ‘water based binder’ (i.e., Na-alginate) and water as solvent during electrode preparation (viz., ‘aqueous processing’). To the best of our knowledge, such excellent stability against hydration and electrochemical performance, despite being ‘aqueous-processed’, have not been reported earlier for O3-type Na-TM-oxide based cathodes.

What's the cathode?

O3-type Na-TM-oxide based

Against this backdrop, the presently reported work focuses on designing a composition (viz., combination of TM- and non-TM-ions) and suitably tuning some structural features of O3-type Na-TM-oxide based cathode materials to address the main issue concerning stability against hydration and also improve the electrochemical cyclic stability, as well as rate capability. While doing so, a strong correlation between the composition, structure, stability against hydration and electrochemical behavior/performance for such oxides has been established. As will be demonstrated, an air/water-stable (despite long-term exposures under stringent conditions) O3-type Na-TM-oxide has been developed, which is devoid of all the problems mentioned above, so much so that it allows the successful development of stable electrodes using a cost-effective and environment/health friendly ‘water based binder’ (i.e., Na-alginate) and water as solvent during electrode preparation (viz., ‘aqueous processing’). To the best of our knowledge, such excellent stability against hydration and electrochemical performance, despite being ‘aqueous-processed’, have not been reported earlier for O3-type Na-TM-oxide based cathodes.

What's the cathode?

O3-type Na-TM-oxide based

This simple, yet methodical sonication method can be applied to evaluate the surface stability of inorganic materials. In particular, the agglomerated secondary particles require a dispersing process to prepare samples for the TEM analysis. In addition, an extended sonication may be necessary in the cycled electrodes because of the binder usage. Our results have also shown that the surface of layered oxide materials can be easily influenced by the sonication method. Furthermore, our previous study also demonstrated that the cathode surface can be transformed by simply soaking layered oxide powders in organic solvents, including typical electrolyte solvents. With the inherent surface fragility of Ni-rich oxide materials, it is crucial to take cautions in preparing samples for characterization. Alternatively, many studies have applied focused ion beam (FIB) to prepare thin samples for TEM experiments. In this case, the chemical and structural information around grain boundaries can be protected for TEM analysis. However, it is not clear how the surface region may be influenced by the typical FIB coating protection and sample handling.

What's the cathode?

Research focus on cathode materials for LIBs is currently going through a paradigm shift where the previously overlooked disordered rocksalt (DRS) materials are receiving an increasing level of interest. It was reported that Li-rich materials with a high Li-to-TM ratio allow the formation of percolating network with low energy barriers for Li diffusion, which leads to good transport properties while its structural integrity is retained. Several Li-rich transition metal oxides with a DRS structure such as Li1+xTi2xFe1−3xO2 (0 ≤ x ≤ 0.333), Li1.2Ni1/3Ti1/3Mo2/5O2, Li1.2Ti0.4Mn0.4O2, Li1.3Nb0.3Fe0.4O2 ( ) and Li1.3Nb0.43Ni0.27O2 ( ) have been studied, and their high reversible capacities showed a promising prospect of discovering new cathode material in much less explored domain. Li2VO2F is the first Li-rich DRS material where O was partially substituted by F. The substitution increases the performance of the cathode such as discharge capacity and nominal voltage.

What's the cathode?

Li2VO2F

To achieve aqueous zinc batteries with high capacity and superior durability, developing a scalable and efficient strategy for preparing an outstanding cathode is of pivotal importance. Herein, porous cobalt/nickel composite hydroxides supported on 3D Co–Ni foam (CNF) are used to construct a robust cathode (denoted as UTCNF) via a facile and cost-effective ultrasonic method that is suitable for scaling up to an industrial level. Attributed to the synergistic effects of the 3D foam architecture, the porous structure, and the in situ formation of electrochemically active cobalt/nickel composite hydroxides, the UTCNF//Zn battery exhibits excellent performance with a relatively high capacity (2.13 mA h cm−2 at 8 mA cm−2), excellent rate performance (47% capacity retention from 8 to 50 mA h cm−2), and superb stability (only ∼10% capacity loss after 30000 cycles at a high current density of 40 mA cm−2). Furthermore, a maximum energy density of 3.99 mW h cm−2 and a peak density of 320 mW cm−2 are also attained. Notably, a soft-package battery that uses the UTCNF as a cathode demonstrates great potential as a power supply. This work may provide valuable ideas for the achievement of large-scale aqueous zinc batteries with high performance.

What's the cathode?

UTCNF

To achieve aqueous zinc batteries with high capacity and superior durability, developing a scalable and efficient strategy for preparing an outstanding cathode is of pivotal importance. Herein, porous cobalt/nickel composite hydroxides supported on 3D Co–Ni foam (CNF) are used to construct a robust cathode (denoted as UTCNF) via a facile and cost-effective ultrasonic method that is suitable for scaling up to an industrial level. Attributed to the synergistic effects of the 3D foam architecture, the porous structure, and the in situ formation of electrochemically active cobalt/nickel composite hydroxides, the UTCNF//Zn battery exhibits excellent performance with a relatively high capacity (2.13 mA h cm−2 at 8 mA cm−2), excellent rate performance (47% capacity retention from 8 to 50 mA h cm−2), and superb stability (only ∼10% capacity loss after 30000 cycles at a high current density of 40 mA cm−2). Furthermore, a maximum energy density of 3.99 mW h cm−2 and a peak density of 320 mW cm−2 are also attained. Notably, a soft-package battery that uses the UTCNF as a cathode demonstrates great potential as a power supply. This work may provide valuable ideas for the achievement of large-scale aqueous zinc batteries with high performance.

What's the cathode?

UTCNF

Electrospun NiO with different nanostructures has also been widely studied in SCs. For instance, by electrospinning a PVP/nickel nitrate/citric acid polymer solution followed by a subsequent annealing process to remove PVP, NiO hollow nanofibers comprised of NiO sheets with a diameter of 17 nm were prepared. The resultant NiO nanofibers exhibited a higher pseudocapacitance (336 F g−1 at 5 mA cm−2) than the solid NiO nanofibers fabricated without citric acid (136 F g−1). In another reference, by using electrospun PAA as a sacrificial template, NiO hollow nanofibers consisting of NiO nanoparticles with diameters varying from 10 to 30 nm were synthesized through a direct ion-exchange process in oversaturated Ni(NO3)2 solution, followed by air annealing. The NiO nanofibers with a porous structure and empty space can accommodate large volume changes and exhibited a high specific capacitance of 700 F g−1 at 2 A g−1, as well as a remarkable rate capability (80% capacitance retention with the current density increasing from 1 to 5 A g−1). In addition, electrospun nickel acetate/PVP-derived NiO nanofibers were applied as cathode materials for ASCs (AC as the anode). The device delivered a specific capacitance of 141 F g−1 with an energy density of 43.75 W h kg−1 and power density of 7.5 kW kg−1.

What's the cathode?

nickel acetate/PVP-derived NiO nanofibers

Long-term cyclic stability of symmetric cells. One of the most promising applications of metal anodes is to combine them with a sulfur or air cathode with a high energy density. It is thus important to find an efficient way of infusing the alkali metal inside the anode scaffold. To introduce Na in the network, the anode was pre-assembled by stacking the CNF film on top of a pure Na foil. The Na foil was manually pressed into a thickness ranging between 200 and 400 μm. The use of this stacked design saved materials and processing time better than the classical sacrificial-cell electrodeposition method. The thickness of Na foil could be further reduced using industrial pressing tools. Fig. 6a presents a schematic of the stacking scheme of the assembled electrode which allowed Na to be effectively stripped and plated by removal and re-infiltration into the fiber network. Fig. 6b presents a side view of the CNF film stacked on top of a Na layer after 6 mA h cm−2 of Na was stripped and re-deposited at a current density of 1 mA cm−2. The image signifies successful re-infiltration of Na in the fiber network after the two-step process. The fiber diameter increased to ∼600–800 nm from the initial ∼150 nm, proving a large load of deposited Na. The final fiber thickness obtained with the stacked design was compatible with the thickness obtained with regular electrodeposition, indicating that a majority of Na was infiltrated from the initial film into the network. In view of the large variation in the measured Na thicknesses, however, it may be possible that some part of Na had been redeposited back onto the original Na film, as shown in the schematics. The above fiber network was likely to hinder any dendritic growth resulting from this scenario. The overall ZNO@CNF film thickness remained at ∼100 μm after infiltration, suggesting that the network could withstand the mechanical pressure inside of the cell while retaining its original shape. The smooth top view of the same anode shown in Fig. S26† indicates that the ZnO@CNF film was able to hold and control the deposited Na even with a high areal loading of 6 mA h cm−2.

What's the cathode?

sulfur

Long-term cyclic stability of symmetric cells. One of the most promising applications of metal anodes is to combine them with a sulfur or air cathode with a high energy density. It is thus important to find an efficient way of infusing the alkali metal inside the anode scaffold. To introduce Na in the network, the anode was pre-assembled by stacking the CNF film on top of a pure Na foil. The Na foil was manually pressed into a thickness ranging between 200 and 400 μm. The use of this stacked design saved materials and processing time better than the classical sacrificial-cell electrodeposition method. The thickness of Na foil could be further reduced using industrial pressing tools. Fig. 6a presents a schematic of the stacking scheme of the assembled electrode which allowed Na to be effectively stripped and plated by removal and re-infiltration into the fiber network. Fig. 6b presents a side view of the CNF film stacked on top of a Na layer after 6 mA h cm−2 of Na was stripped and re-deposited at a current density of 1 mA cm−2. The image signifies successful re-infiltration of Na in the fiber network after the two-step process. The fiber diameter increased to ∼600–800 nm from the initial ∼150 nm, proving a large load of deposited Na. The final fiber thickness obtained with the stacked design was compatible with the thickness obtained with regular electrodeposition, indicating that a majority of Na was infiltrated from the initial film into the network. In view of the large variation in the measured Na thicknesses, however, it may be possible that some part of Na had been redeposited back onto the original Na film, as shown in the schematics. The above fiber network was likely to hinder any dendritic growth resulting from this scenario. The overall ZNO@CNF film thickness remained at ∼100 μm after infiltration, suggesting that the network could withstand the mechanical pressure inside of the cell while retaining its original shape. The smooth top view of the same anode shown in Fig. S26† indicates that the ZnO@CNF film was able to hold and control the deposited Na even with a high areal loading of 6 mA h cm−2.

What's the cathode?

air

Ni foams, typical 3D porous substrates, are extensively employed as current collectors/supports in energy storage devices, because they not only afford large surface areas and easy electrolyte access, but also enable efficient ion diffusion and charge transfer. For example, Chen and coworkers designed hierarchical micro-nano sheet arrays of nickel–cobalt double hydroxide supported on Ni foam (NiCo-DH) via an etching-deposition-growth strategy. The corresponding Ni–Zn battery with a NiCo-DH electrode as the cathode delivered a high specific capacity of 329 mA h g−1. Wang et al. synthesized a Co3O4@NiV-LDH nanowires array grown on the surface of a Ni foam with a hierarchical structure. When used as the cathode, the assembled battery demonstrated a satisfactory capacity of ∼1.25 mA h cm−2. Despite the favorable electrochemical performance of these cathodes, the complicated or multi-step preparation processes required substantially impede their industrial application. Furthermore, the ex situ formed active materials deposited on the Ni foam are unavoidably peeled off during long-term charge–discharge tests, which may cause trouble for the cyclic stability of zinc batteries.

What's the cathode?

Co3O4@NiV-LDH

Typically, a certain amount of P(VDF-HFP) powder and BaTiO3 piezoelectric particles (4:1 in m/m) were firstly added into a mixture of N,N-dimethylformamide (DMF) and deionized (DI) water under constant stirring at ambient temperature, forming a well-dispersed solution. The solution was then spread on a clean glass substrate, followed by an immersion in a flowing water bath at 80 °C. Several minutes later, a homogeneous white BaTiO3-P(VDF-HFP) film was obtained and self-peeled off from the glass substrate. Next, the film was dried at 100 °C under vacuum to remove the containing water. Afterwards, the dried film was punched according to the size of the anode or cathode, followed by an appropriate polarization under a DC field of 3.5 kV mm−1 in air atmosphere at ambient temperature for 0.5 h. Finally, the polarized pieces were fully soaked in a liquid electrolyte of 1 mol L−1 NaClO4 dissolved in propylene carbonate (PC) with a 5 vol% fluorinated ethylene carbonate (FEC) additive in a glove box over 24 h, yielding the desired BaTiO3-P(VDF-HFP)-NaClO4 piezoelectric gel-electrolyte.

What's the cathode?

Typically, a certain amount of P(VDF-HFP) powder and BaTiO3 piezoelectric particles (4:1 in m/m) were firstly added into a mixture of N,N-dimethylformamide (DMF) and deionized (DI) water under constant stirring at ambient temperature, forming a well-dispersed solution. The solution was then spread on a clean glass substrate, followed by an immersion in a flowing water bath at 80 °C. Several minutes later, a homogeneous white BaTiO3-P(VDF-HFP) film was obtained and self-peeled off from the glass substrate. Next, the film was dried at 100 °C under vacuum to remove the containing water. Afterwards, the dried film was punched according to the size of the anode or cathode, followed by an appropriate polarization under a DC field of 3.5 kV mm−1 in air atmosphere at ambient temperature for 0.5 h. Finally, the polarized pieces were fully soaked in a liquid electrolyte of 1 mol L−1 NaClO4 dissolved in propylene carbonate (PC) with a 5 vol% fluorinated ethylene carbonate (FEC) additive in a glove box over 24 h, yielding the desired BaTiO3-P(VDF-HFP)-NaClO4 piezoelectric gel-electrolyte.

What's the anode?

In 2015, unique graphene-like Co9S8 with a high surface area and hierarchical porosity was first proposed by Nazar's group to afford superior LiPS adsorptivity for high-performance Li–S batteries. The Co9S8/S composite cathode not only manifests up to a factor of 10 increase of cycling stability (0.045% fading per cycle over 1500 cycles at C/2 at a sulfur loading of 1.5 mg cm−2) compared to standard porous carbons but also, more importantly, enables a high-loading sulfur electrode with up to 75 wt% sulfur and up to 4.5 mg cm−2 areal sulfur loading. First-principles calculations and spectroscopic studies consistently demonstrate that a synergistic dual interaction based on Sn2− → Coδ+ and Li+ → Sδ− exists between the metal sulfide-based host material and LiPSs. In particular, it is revealed that the nature of the crystallographic surface plays a very crucial role. The binding energy of 2.22 eV for Li2S2 on the (002) surface (Co/S = 1:4) increases to 3.24 eV for the (202) surface (Co/S = 5:4) and further to 6.06 eV for the purely Co-terminated (008) plane. This finding enables a better understanding of the interaction mechanism for metal sulfides and provides guidance for fabricating host materials with preferred crystal faces, but unfortunately, the mechanism by which the polar surface accelerates LiPS redox has not been clearly revealed yet. Soon after, Yuan et al. demonstrated that simply introducing half-metallic CoS2 microparticles into graphene electrodes largely amplified the electrochemical current response and reduced charge transfer resistance in LiPS symmetrical cells. More importantly, it is found that CoS2 serves as a mediator to provide strong CoS2–polysulfide interaction and to facilitate the liquid–liquid redox of polysulfides, further affecting the correlated liquid–solid transformation, viz., the prior formation of Li2S nuclei. With a sulfur loading of 0.4 mg cm−2, a high initial capacity of 1368 mA h g−1 at 0.5C, an increase of 10% in energy efficiency, and a low decay rate of 0.034% per cycle for 2000 cycles at 2.0C are realized through the incorporation of mechanically milled CoS2 with a low specific surface area. These observations suggest that manipulating the redox reactivity of polysulfides upon a conducting surface is a more effective strategy for improving the Li–S battery performance, in comparison to tuning the dissolvability and transport behavior of polysulfides in aprotic electrolytes. Given that the metal sulfide herein employed is in the bulk form, therefore, the performance enhancement can be further achieved by optimizing its structure and properties, including engineering the crystal facets to mediate the adsorbate–adsorbent interaction, reducing the dimension to maximize the exposed polar surface and introducing a strong coupling effect between substrates and supported metal sulfides for intriguing modulation of material properties.

What's the cathode?

Co9S8/S composite

In 2015, unique graphene-like Co9S8 with a high surface area and hierarchical porosity was first proposed by Nazar's group to afford superior LiPS adsorptivity for high-performance Li–S batteries. The Co9S8/S composite cathode not only manifests up to a factor of 10 increase of cycling stability (0.045% fading per cycle over 1500 cycles at C/2 at a sulfur loading of 1.5 mg cm−2) compared to standard porous carbons but also, more importantly, enables a high-loading sulfur electrode with up to 75 wt% sulfur and up to 4.5 mg cm−2 areal sulfur loading. First-principles calculations and spectroscopic studies consistently demonstrate that a synergistic dual interaction based on Sn2− → Coδ+ and Li+ → Sδ− exists between the metal sulfide-based host material and LiPSs. In particular, it is revealed that the nature of the crystallographic surface plays a very crucial role. The binding energy of 2.22 eV for Li2S2 on the (002) surface (Co/S = 1:4) increases to 3.24 eV for the (202) surface (Co/S = 5:4) and further to 6.06 eV for the purely Co-terminated (008) plane. This finding enables a better understanding of the interaction mechanism for metal sulfides and provides guidance for fabricating host materials with preferred crystal faces, but unfortunately, the mechanism by which the polar surface accelerates LiPS redox has not been clearly revealed yet. Soon after, Yuan et al. demonstrated that simply introducing half-metallic CoS2 microparticles into graphene electrodes largely amplified the electrochemical current response and reduced charge transfer resistance in LiPS symmetrical cells. More importantly, it is found that CoS2 serves as a mediator to provide strong CoS2–polysulfide interaction and to facilitate the liquid–liquid redox of polysulfides, further affecting the correlated liquid–solid transformation, viz., the prior formation of Li2S nuclei. With a sulfur loading of 0.4 mg cm−2, a high initial capacity of 1368 mA h g−1 at 0.5C, an increase of 10% in energy efficiency, and a low decay rate of 0.034% per cycle for 2000 cycles at 2.0C are realized through the incorporation of mechanically milled CoS2 with a low specific surface area. These observations suggest that manipulating the redox reactivity of polysulfides upon a conducting surface is a more effective strategy for improving the Li–S battery performance, in comparison to tuning the dissolvability and transport behavior of polysulfides in aprotic electrolytes. Given that the metal sulfide herein employed is in the bulk form, therefore, the performance enhancement can be further achieved by optimizing its structure and properties, including engineering the crystal facets to mediate the adsorbate–adsorbent interaction, reducing the dimension to maximize the exposed polar surface and introducing a strong coupling effect between substrates and supported metal sulfides for intriguing modulation of material properties.

What's the anode?

The Mg/Ti dual dopants also enhance the surface stability even upon deep electrochemical delithiation. Here we found that ex situ soft XAS surface analysis may provide misleading results if the charged electrodes are not handled properly. In principle, Ni should experience oxidation during charging and reduction during discharging. However, due to the cathode–electrolyte parasitic reaction, the surface nickel oxidation state at various charge/discharge states were lower than that of the bulk (Fig. S9†). Another phenomenon that we constantly observe is that the Ni oxidation states for the charged samples were lower than that at the pristine state in both the TEY and FY modes (Fig. S9†), which is counterintuitive. We believe that this might be due to the thermodynamically unstable Ni4+ continually reacting with the electrolyte during the cell teardown and subsequent sample handling. Thus, eliminating the cell resting time and removing the residual electrolyte are critical for maintaining the true Ni oxidation states for soft XAS analysis. We found that the best practice is to dissemble the cell and rinse the electrode immediately after the cell has reached the designated state of charge. This practice is particularly important for the charged electrodes. Here we rinsed the cycled electrode in an Ar filled glove box immediately after the cells were charged to 4.4 V vs. Li/Li+. The LiNiO2 delivered a higher charge capacity at this upper cutoff voltage than the Mg/Ti–LiNiO2 (Fig. S10†). If the surface was stable at the charged state, the Ni oxidation state at the surface should not exhibit significant difference in these two electrodes, because the charge capacity only differed by 22 mA h g−1 (i.e., the Ni oxidation state differed by 0.08). However, we found drastic differences between these electrodes, as explained below. The Ni L3-right/left peak ratios were averaged from three electrodes at the charged state. A significantly higher L3-right/left peak ratio in both FY and TEY modes was found in the Mg/Ti–LiNiO2 sample than that in the LiNiO2 sample (Fig. 4d), indicating enhanced surface stability of the Mg/Ti–LiNiO2 sample in the charged state. Moreover, the low L3-right/left peak ratio in the FY mode of the LiNiO2 electrodes also suggests that the surface reaction induced Ni reduction extended to the deep subsurface. The direct comparison between NMC622 and NMC811 (Fig. S11†) further supports that a higher nickel content results in more surface instability, which can potentially complicate the state of charge assessment using the surface sensitive soft XAS technique. As more studies have made use of the soft XAS technique for analyzing the surface chemistry of battery materials, our results clearly demonstrate that caution is needed when interpreting the data. Although the synchrotron characterization is not highly accessible, other surface-sensitive characterization methods, e.g., XPS, IR, Raman, TEM, ToF-SIMS, also encounter the similar challenges when preparing/handling samples. To improve the data representativeness, the effective error analysis, based on repeated measurements, is recommended.

What's the cathode?

As benchmark inorganic cathodes, we selected a number of materials for non-aqueous cells which, according to the reviews and to the best of our knowledge, are superior in terms of voltage, specific energy/power or cycling stability and are attributed to various material families (hexacyanoferrates, layered oxides, polyanionic compounds, and conversion-type cathodes). The list of benchmarks includes KVPO4F, K2[(VOHPO4)2(C2O4)] (KVPCO), K1.75Mn[Fe(CN)6]·0.16H2O (KMHCF), Fe[Fe(CN)6] (FeHCF), K1.81Ni[Fe(CN)6]0.97·0.086H2O (KNHCF), K0.7Mn0.5Fe0.5O2 (KMFO), K0.51V2O5 and K0.42V2O5·0.25H2O (KVO), and sulfur (S).

What's the cathode?

hexacyanoferrates, layered oxides, polyanionic compounds, and conversion-type

As benchmark inorganic cathodes, we selected a number of materials for non-aqueous cells which, according to the reviews and to the best of our knowledge, are superior in terms of voltage, specific energy/power or cycling stability and are attributed to various material families (hexacyanoferrates, layered oxides, polyanionic compounds, and conversion-type cathodes). The list of benchmarks includes KVPO4F, K2[(VOHPO4)2(C2O4)] (KVPCO), K1.75Mn[Fe(CN)6]·0.16H2O (KMHCF), Fe[Fe(CN)6] (FeHCF), K1.81Ni[Fe(CN)6]0.97·0.086H2O (KNHCF), K0.7Mn0.5Fe0.5O2 (KMFO), K0.51V2O5 and K0.42V2O5·0.25H2O (KVO), and sulfur (S).

What's the anode?

The cathodes were prepared by mixing 80 wt% active materials, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) binder with N-methyl-2-pyrrolidone (NMP) as the solvent. The mixture was coated on aluminum foil, pressed at 20 MPa, and dried at 120 °C in air for 2 h. The 2025 coin-type cells, consisting of the as-prepared cathode, Li metal as the anode and Celgard 2325 as the separator, were assembled in an argon-filled glove box. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (volume ratio 3:7). All cells were cycled between 2.0 and 4.8 V (versus Li/Li+) at a current density of 20 mA g−1 at room temperature on Land CT 2001A battery testers unless otherwise defined. The electrochemical impedance spectroscopy (EIS) measurements were performed from 100 kHz to 10 mHz with an amplitude of 10 mV using a multichannel potentiostat (PARSTAT MC).

What's the cathode?

In the context of voltage hysteresis, ‘charge-averaged’ discharge (Vdischarge), charge (Vcharge), and net-average (viz., further average of Vdischarge and Vcharge) voltages (as true average voltages) for the ‘control’ and fully Ti-substituted (for Mn-ion) Na-TM-oxide cathodes were estimated, as per the following relations, and presented in Fig. 4 as variations with cycle number.

What's the cathode?

Na-TM-oxide

In terms of anode fabrication, TiO2/C@NPSC-x was mixed sufficiently with conductive carbon and sodium carboxymethylated cellulose (CMC-Na) to form a uniform slurry (weight ratio: 7:2:1). Then, the Cu foil coated by the slurry was dried in a vacuum oven at 110 °C overnight. The anode electrodes for ex situ XRD tests were fabricated by coating the active materials/CMC-Na (weight ratio: 9:1) slurry on an Al current collector. For cathode electrodes, the home-made activated carbon (ZDPC), conductive carbon and PTFE were blended evenly in deionized water (weight ratio: 90:5:5) and painted on carbon-coated Al foil. The loading weight of each electrode was about 1.0 mg cm−2. Then, coin-type CR2032 cells were constructed inside a glove box filled with Ar gas. The electrolyte was 0.8 M KPF6 in DEC and EC (1:1, v/v). The separator was glass fibers. Fresh potassium foil served as the counter electrode. Before construction of the PIHC device, the TiO2/C@NPSC-700 anode was pre-activated at 0.1 A g−1 for 10 cycles.

What's the cathode?

carbon (ZDPC)

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@V2S3/S