Heterogeneous catalysts with Co3O4 and liquid redox mediators were utilized for the morphological control of discharged products in SABs. With Co3O4 nanowires/C as air cathodes, the discharge product tended to be like nanoflakes. However, after the addition of ferrocene to the electrolyte, the discharge product tended to be like nanofilms and the cyclic performance can achieve 570 cycles.

A high-quality sized FeFe(CN)6 was synthesized as a cathode material for a non-aqueous potassium-ion battery. The electrode delivered a reversible capacity of 124 mA h g−1 at the current rate of 0.5C and still retained a reversible capacity of 93 mA h g−1 after 500 cycles at 5C with a columbic efficiency of 100%. Structural evolution and redox couples of low and high spin FeIII/FeII were investigated by ex situ X-ray diffraction, Mössbauer spectroscopy, and X-ray photoelectric spectroscopy. The negligible volume change during the electrochemical process should be responsible for the excellent cyclic stability.

Chromium-based layered cathode materials suffer from the irreversible disproportionation reaction of Cr4+ to Cr3+ and Cr6+, which hinders the reversible multi-electron redox of Cr ions in layered cathodes, and limits their capacity and reversibility. To address this problem, a novel O3-type layer-structured transition metal oxide of NaCr1/3Fe1/3Mn1/3O2 (NCFM) was designed and studied as a cathode material. A high reversible capacity of 186 mA h g−1 was achieved at a current rate of 0.05C in a voltage range of 1.5 to 4.2 V. X-ray diffraction revealed an O3 → (O3 + P3) → (P3 + O3′′) → O3′′ phase-transition pathway for NCFM during charge. X-ray absorption, X-ray photoelectron and electron energy-loss spectroscopy measurements revealed the electronic structure changes of NCFM during Na+ deintercalation/intercalation processes. It is confirmed that the disproportionation reaction of Cr4+ to Cr3+ and Cr6+ can be effectively suppressed by Fe3+ and Mn4+ substitution. These results demonstrated that the reversible multi-electron oxidation/reduction of Cr ions can be achieved in NCFM during charge and discharge accompanied by CrO6 octahedral distortion and recovery.

A novel small-molecule compound of lithium iodine and 3-hydroxypropionitrile (HPN) has been successfully synthesized. Our combined experimental and theoretical studies indicated that LiIHPN is a Li-ion conductor, which is utterly different from the I–-anion conductor of LiI(HPN)2 reported previously. Solid-state lithium–air batteries based on LiIHPN as the electrolyte exhibit a reversible discharge capacity of more than 2100 mAh g–1 with a cyclic performance over 10 cycles. Our findings provide a new way to design solid-state electrolytes toward high-performance lithium–air batteries.

High rate capability and long cycle life are challenging goals for the development of room temperature sodium-ion batteries. Here we report a new single phase quaternary O3-type layer-structured transition metal oxide Na(NiCoFeTi)1/4O2 synthesized by a simple solid-state reaction as a new cathode material for sodium-ion batteries. It can deliver a reversible capacity of 90.6 mA h g−1 at a rate as high as 20C. At 5C, 75.0% of the initial specific capacity can be retained after 400 cycles with a capacity-decay rate of 0.07% per cycle, demonstrating a superior long-term cyclability at high current density. X-ray diffraction and absorption characterization revealed reversible phase transformations and electronic structural changes during the Na+ deintercalation/intercalation process. Ni, Co and Fe ions contribute to charge compensation during charge and discharge. Although Ti ions do not contribute to the charge transfer, they play a very important role in stabilizing the structure during charge and discharge by suppressing the Fe migration. In addition, Ti substitution can also smooth the charge–discharge plateaus effectively, which provides a potential advantage for the commercialization of this material for room temperature sodium-ion batteries.

Metal–air batteries are important power sources for electronics and vehicles because of their remarkable high theoretical energy density and low cost. In this work, we firstly investigate the electrochemical properties of sodium air batteries (SABs) with the addition of ferrocene in the electrolyte. Combining charge–discharge measurements with field-emission transmission electron microscopy images of the discharged air cathodes, our results have demonstrated that two different pathways with ferrocene as an electrocatalyst and involved in the electrochemistry during the charge process might be governed by the morphological features of the electrode caused by the deposition of Na2O2 in SABs. The SAB with ferrocene as a catalyst exhibits a high cycling performance of up to 230 cycles with a high capacity of 1000 mA h g−1.

A well-crystallized single-phase quinary layer transition metal oxide of NaNi1/4Co1/4Fe1/4Mn1/8Ti1/8O2 was successfully synthesized. It exhibited excellent cycle performance and high rate capability as a cathode material for sodium-ion batteries.

A CoS2/multi-walled carbon nanotube (MWCNT) nanocomposite was synthesized and its sodium storage performances in ether-based electrolyte and commonly used carbonate-based electrolyte were investigated for the first time. A high capacity of 568 mA h g−1 after 100 cycles in ether-based electrolyte can be achieved.

A Na–air battery with NaI dissolved in a typical organic electrolyte could run up to 150 cycles with a capacity limit of 1000 mA h g−1. The low charge voltage plateau of 3.2 V vs. Na+/Na in a Na–air battery should mainly be attributed to the oxidation reaction of active iodine anions.

In order to improve the electrochemical performance of Na/CFx cell, fluorinated multi-walled carbon nanotubes (F-MWCNTs) and a new composite consisting of CFx, graphene nanosheets (GNS) and FeF3 (labeled as CGF for short) were employed as cathode materials in rechargeable room-temperature sodium batteries for the first time. The polarizations of Na/F-MWCNTs and Na/CGF cell were found to be 1600 and 800 mV, respectively. These values were much lower than that of Na/CFx cell (2000 mV). MWCNTs formed after the discharge process with intrinsic high surface area, high catalytic activity and “cage” type structure enhance charge reaction activity. Analogically, the combination of GNS and FeF3 with CFx provides more active sites to facilitate the decomposition of NaF during charge process. Moreover, the CGF electrode may have specific channels to avoid the dissolution of fluorine into the electrolyte similarly as the structure function of the F-MWCNTs electrode, leading to much better cycling performance. Overall, our results have demonstrated that the electrochemical performance of Na/CFx cell can be greatly improved by using MWCNTs and FeF3–GNS as catalysts.

•We have fabricated FeSe and CuWO4 thin films by R.F. sputtering.•We have examined the electrochemical behaviors of FeSe and CuWO4 thin film electrodes with sodium.•We have proposed the conversion and displacement reaction mechanisms for FeSe/Na and CuWO4/Na cells.Transition metal compounds of FeSe and CuWO4 thin films have been successfully fabricated by using R.F. sputtering method. Although two kinds of transition metal compounds of FeSe and CuWO4 thin films can react with sodium electrochemically, they exhibit different electrochemical features. The nanosized metal Fe is highly dispersed into Na2Se matrix and metal Cu is extruded from Na2WO4 mixture after the FeSe/Na and CuWO4/Na cells are discharged, respectively. The conversion reaction mechanism between FeSe and Na2Se is proposed for the FeSe/Na cell. While the displacement reaction mechanism for CuWO4/Na cell is proposed for the first time based on the transmission electron microscopy (TEM) and selected area electron diffraction (SAED) data. These various mechanisms make transition metal compounds interesting materials for rechargeable sodium ion batteries.

Here we demonstrate for the first time that CFx cathodes show rechargeable capability in sodium ion batteries with an initial discharge capacity of 1061 mAh g–1 and a reversible discharge capacity of 786 mAh g–1. The highly reversible electrochemical reactivity of CFx with Na at room temperature indicates that the decomposition of NaF could be driven by carbon formed during the first discharge. The high reversible capacity made it become a promising cathode material for future rechargeable sodium batteries.Keywords: CFx; high specific capacity; NaF; rechargeable sodium battery; room temperature;

•In this paper, we fabricated a composite electrode of NiCo2O4 nanosheets/Ni foam by a solvothermal method.•We examined its electrochemical behaviors in the Na–air battery.•We observed Na2O2 nanosheets after the discharge of Na–air battery.•We proposed NiCo2O4 nanosheets/Ni foam as a promising air electrode for rechargeable sodium–air batteries.NiCo2O4 nanosheets supported on Ni foam were synthesized by a solvothermal method. A composite of NiCo2O4 nanosheets/Ni as a carbon-free and binder-free air cathode exhibited an initial discharge capacity of 1762 mAh g− 1 with a low polarization of 0.96 V at 20 mA g− 1 for sodium–air batteries. Na2O2 nanosheets were firstly observed as the discharged product in sodium–air battery. High electrocatalytic activity of NiCo2O4 nanosheets/Ni made it a promising air electrode for rechargeable sodium–air batteries.

The structural evolution of layered NaCrO2 cathodes for sodium-ion batteries during charge was investigated using synchrotron-based in situ X-ray diffraction and ex situ X-ray absorption spectroscopy. Three solid solution phases with expanding ‘c’ and contracting ‘a’/‘b’ lattice parameters were observed. The coordination changes of Cr and Na during sodium extraction were also studied.

A Cu2Se electrode on a copper grid substrate has been directly fabricated by a facile post-selenized method and tested as a positive material for sodium ion batteries. Cu2Se exhibits large reversible capacities (about 250 mA h g−1), good cyclic stabilities and low polarization. These results indicate that Cu2Se is a promising candidate as a cathode material for sodium ion batteries.

Graphene nanosheets (GNS) were employed as an air electrode for a sodium–air battery (SAB). High discharge capacity of 9268 mA h g−1 with low overpotential was achieved, indicating its superiority to a normal carbon film electrode. Our results indicate that GNS as air electrodes could improve the electrochemical performance of rechargeable SABs.

A double-layer structural air cathode consisting of diamond like carbon (DLC) active layer and CoOx catalytic layer is designed and its catalytic effect for Li-air batteries both in carbonate and ether based electrolytes are tested. Ethylene carbonate (EC)/dimethyl carbonate (DMC) based Li-air cell using this double-layer electrode as air cathode exhibits significant improvement in discharge/charge electrochemical performance but suffers from electrolyte decomposition as proved by Fourier transform infrared (FTIR) and secondary ion mass spectrometry (SIMS) measurements. In 1,2-dimethoxyethane (DME) based electrolyte, CoOx/DLC double layer electrode shows high catalytic activity towards oxygen evolution reaction (OER). Furthermore, over 30 cycles is achieved with a capacity of more than 2000 mAh g−1 by using CoOx/DLC double layer electrode, which makes it a promising air electrode for Li-air batteries.Highlights► A CoOx/diamond like carbon (DLC) double layer thin film electrode was fabricated. ► We examined electrochemical properties of CoOx/DLC electrode for Li-air batteries. ► CoOx/DLC electrode exhibited a bifunctional catalytic activity for Li-air batteries. ► Spatial distribution of discharge products were investigated by SIMS. ► Bifunctional effect of CoOx layer for Li-air battery was revealed.

Extensive research is underway to yield greater insights into the intrinsic properties of electrode materials for lithium storage. Presently, nanostructured thin-film electrodes without any additives and binders used in powder-based electrodes have been employed as the “ideal” system for fundamental research because of their low resistance, cleanliness and purity. This review summarizes the research on, and progress in such nanostructured thin-film electrode materials for lithium storage and for all-solid-state thin film batteries. Nanostructured thin film electrodes with various electrochemical reaction mechanisms based on nanometer-size effects, chemical composition and structure are summarized. Thin film electrodes used in all-solid-state thin film batteries are also described.Highlights► We reviewed nanostructured thin film electrode for lithium storage and film lithium batteries. ► We summarized thin film electrodes with various electrochemical reaction mechanisms. ► We described thin film electrodes used in all-solid-state thin film batteries.

Boron (B) has a high theoretical lithium storage capacity of 3046 mAh g−1 for forming Li5B4 phase. Li–B alloy has been used in high temperature thermal lithium batteries. In this work, a new tetragonal boron (B50) thin film with a thickness of 80 nm has been deposited on a vanadium coated glass substrate by a pulse laser deposition (PLD) method. It is found that this electrode film shows only a lithium storage capacity of 43 mAh g−1 in a nonaqueous electrolyte at room temperature. According to a first-principles calculation, the B50 is a metallic conductor. Therefore, the very poor activity could be related to poor lithium ion diffusion property since the diffusion barrier in this tetragonal B50 lattice is calculated as 2.59 eV. This consists well with the fact that the lithium storage capacity in the B50 thin film is improved significantly to 268 mAh g−1 at 85 °C.Highlights► Polycrystalline B50 thin film is prepared by PLD method. ► Li-storage in the B50 thin film is observed at room temperature and 80 °C. ► Structure of the B50 thin film before and after lithiation are observed by TEM and SAED. ► Dynamic parameters and electronic structure of the B50 are calculated. ► Diffusion barrier of lithium in the B50 lattice is calculated.

P2-phase Na0.74CoO2 cathode material prepared by a solid-state method exhibits the specific discharge capacity of 107 mAh g−1 at 0.1 C with good cycling performance for rechargeable sodium ion batteries. The voltage polarization between charging and discharging at 0.1 C rate is about 150–250 mV and the coulombic efficiency in each cycle is about 89%. The expansion and compression in c-axis of the NaxCoO2 unit cell during the Na intercalation/deintercalation is revealed by ex situ XRD. XPS and in situ XAS data directly confirm that deintercalation/intercalation of Na ions from/into the layered structure proceeds with the Co3+/Co4+ redox reaction.Highlights► In this paper, we synthesized Na0.74CoO2 as cathode for rechargeable sodium ion batteries. ► We examined its electrochemical performance. ► A reversible capacity of 107 mAh/g with 0.1% capacity fading for the first 40 cycles was obtained. ► The deintercalation/intercalation reactions of Na ions from/into Na0.74CoO2 proceeded while Co3+/Co4+ redox.

A 3D NiCo2O4 nanowire array/carbon cloth (NCONW/CC) was employed as the cathode for Li–air batteries with a non-aqueous electrolyte. After its discharge, novel porous ball-like Li2O2 was found to be deposited on the tip of NiCo2O4 nanowires. The special structure of Li2O2 and active sites of catalysts are also discussed.

The electrochemical properties of nanocomposite SnO2–Se thin film prepared by pulsed laser deposition method have been investigated by cyclic voltammetry and charge/discharge measurements. A large reversible specific capacity of 958.8 mAh g−1 in SnO2–Se/Li cell cycled between 0.01 and 3.0 V is achieved. Our results have demonstrated that nanocomposite SnO2–Se exhibits larger capacity and better cycle performance than pure SnO2 and SnSe. The electrochemical reaction mechanisms of SnO2–Se with lithium are examined by X-ray diffraction, high resolution transmission electron microscopy, selected-area electron diffraction and spectroelectrochemical measurements. The reversible oxidation/reduction reaction of SnO2 and selenidation/reduction reaction of SnSe are revealed.Highlights► In this paper, nanocomposite SnO2–Se thin film electrode was fabricated by pulsed laser deposition. ► We examined the electrochemical properties of nanocomposite SnO2–Se electrode. ► SnO2–Se/Li cells exhibited a large reversible specific capacity of about 958.8 mAh g−1. ► The reversible oxidation/reduction reaction mechanism of SnO2–Se with lithium was revealed.

Amorphous Sn2P2O7 thin film electrodes have been successfully fabricated by radio frequency (r.f.) magnetron sputtering. Reversible lithiation and delithiation processes occurring at 1.47 V and 2.01 V versus lithium are observed in both discharge/charge curves and cyclic voltammograms of the amorphous Sn2P2O7 thin film electrodes for the first time. Large reversible capacity around 887 mAh g−1 is achieved when the cell is cycled between 0.01 V and 4.0 V, corresponding to 9.2 Li per Sn2P2O7 unit. Based on X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) evidences, the reaction mechanism involving the reversible decomposition and regeneration of amorphous Sn2P2O7 as well as alloying and de-alloying of tin with lithium is proposed.Highlights► In this paper, we fabricated amorphous Sn2P2O7 thin film electrodes by radio frequency magnetron sputtering. ► We examined their lithium electrochemical properties of Sn2P2O7 by charge/discharge measurement and cyclic voltammograms. ► We found a large reversible specific capacity of Sn2P2O7 thin film electrode of 887 mAh g−1 with excellent capacity retention. ► The electrochemical reaction mechanisms of amorphous Sn2P2O7 with Li involve the reversible decomposition and regeneration of amorphous Sn2P2O7. ► These findings provide a new possibility for further clarifying the intrinsic properties of Sn2P2O7 electrode.

Nanocomposite ZnO–Se thin film has been successfully fabricated by pulsed laser deposition method. The electrochemical behavior is investigated by galvanostatic cycling and cyclic voltammetry. The reversible specific capacity of ZnO–Se/Li cells cycled between 0.01 and 3.5 V is found to be 505 mAh g−1 with good capacity retention. Nanocomposite ZnO–Se thin film electrode exhibits much better electrochemical performance than pure ZnO. The reversible oxidation/reduction reaction of ZnO and selenidation/reduction reaction of ZnSe are revealed in the electrochemical reaction mechanism of nanocomposite ZnO–Se with lithium.

The physical and electrochemical behaviors of the carbon-coated NaCrO2 electrodes for sodium-ion batteries are investigated for the first time. The discharge capacity of carbon-coated NaCrO2 after the 40th cycle remains 110 mAh g− 1. The carbon-coated NaCrO2 shows better performances in terms of the larger discharge capacity and improved cycle stability than naked NaCrO2. Our results have demonstrated the feasibility of the carbon-coating in improving the cyclic stability of electrode materials for sodium ion batteries.Highlights► In this paper, we fabricated carbon-coated NaCrO2 composite by a solid-state reaction process. ► We examined the physical and electrochemical behaviors of the carbon-coated NaCrO2 electrodes with sodium. ► We found the discharge capacity of carbon-coated NaCrO2 of 110 mAh g− 1 after the 40th cycle. ► The carbon-coated NaCrO2 showed larger specific capacity and better cycle performance than those of the naked one. ► The carbon layer on the surface of NaCrO2 could effectively improve the cycling performance.

NASICON-type Fe2(MoO4)3 thin films have been fabricated by magnetron sputtering and are firstly investigated as positive electrode materials for sodium ion battery. It shows a higher reversible sodium storage capacity of approximately 91 mAh g− 1 and better cycling stability compared to those of bulk Fe2(MoO4)3 electrode. Our results have demonstrated that Fe2(MoO4)3 in the form of nanostructure such as thin film can significantly improve its electrochemical performance for sodium ion battery.Highlights► In this paper, we fabricated NASICON-type Fe2(MoO4)3 thin films by magnetron sputtering. ► We examined their electrochemical behaviors with sodium. ► We found a higher reversible sodium storage capacity of approximately 91 mAh/g. ► Fe2(MoO4)3 thin film can significantly improve its electrochemical performance for sodium ion battery.

Al2O3-doped ZnO (AZO) films have been prepared by radio frequency (rf) magnetron sputtering. The electrical properties and electrochemical behavior are investigated by Hall measurements, galvanostic cycling, and cyclic voltammograms. The result demonstrates that doping with a small amount of Al2O3 (<3 wt %) can improve the electrochemical performance of ZnO significantly. Among all of the AZO films, AZO2 (2 wt % Al2O3) film shows the best behavior with a large reversible specific capacity around 590 mAh g–1 and excellent capacity retention. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) measurements confirm the formation of LiAl and nanosized Al2O3 during the first discharge and charge processes, respectively. The electrochemical reaction mechanism of AZO with lithium is proposed. It is believed that the nanosized Al2O3 formed after the charge process in AZO films plays an important role in the improvement of electrochemical performance.

The electrochemical behavior of magnetron sputtered Sb2O4 thin film as anode materials for rechargeable sodium ion batteries was investigated for the first time. Sb2O4 thin film electrodes exhibited a large reversible capacity of 896 mAh g− 1. The reversible conversion reactions involving both alloying/dealloying and oxidation/reduction processes of antimony were revealed during the electrochemical reaction of Sb2O4 film electrode with sodium. The high reversible capacity and good cyclibility of Sb2O4 electrode made it become a promising anode material for future rechargeable sodium ion batteries.Highlights► In this paper, we fabricated Sb2O4 thin films by magnetron sputtering. ► We examined the electrochemical reaction of Sb2O4 with Sodium ► We found a large reversible capacity of Sb2O4 of 896 mAh g− 1 with good cycling performance retention. ► The reversible conversion reactions involving both alloying/dealloying processes and oxidation/reduction processes of nano sized metal antimony were revealed. ► The high reversible capacity and good cycle ability of Sb2O4 electrode made it become a promising anode material for future rechargeable sodium batteries.

Nanostructured NiSi thin films have been successfully fabricated by pulsed laser deposition. Their lithium electrochemical properties have been investigated by charge/discharge measurement and cyclic voltammograms. A large reversible specific capacity of 1220 mAh/g was found with excellent capacity retention. Ex situ HRTEM and SAED data have demonstrated that the electrochemical reaction mechanisms of nanostructured NiSi with lithium involve the reversible transformation between NiSi and Ni/Li22Si5. The results implied that Ni could be an “active” participant in the electrochemical reactions of NiSi alloy with lithium but not only an inactive volume change buffer. This kind of structural reversibility should be responsible for its excellent electrochemical performance.► In this paper, we fabricated nanostructured NiSi thin films by pulsed laser deposition. ► We examine their lithium electrochemical properties by charge/discharge measurement and cyclic voltammograms. ► We found a large reversible specific capacity of NiSi thin film electrode of 1220 mAh g–1 with excellent capacity retention. ► The electrochemical reaction mechanisms of the nanostructured NiSi with Li involve reversible transformation between NiSi and Ni/Li22Si5 during discharge and charge. ► This kind of reversible phase transition should be responsible for its good electrochemical performance.

FeOF thin film has been successfully fabricated by reactive pulsed laser deposition for the first time, and its electrochemical behavior was examined as a negative electrode active material in lithium-ion batteries. The electrochemical properties of the as-deposited FeOF thin film during the first charging and discharging have been investigated by the galvanostatic cycling and cyclic voltammetry measurements. By using ex situ X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected-area electron diffraction measurements (SAED), it can be found that FeOF was initially decomposed into Fe0, LiF, and Fe2O3 after discharging to 1.0 V. The newly formed Fe2O3 is then subsequently reduced into Li2O and Fe0 after further discharging to 0.01 V. In the subsequent cycle, the reduction peaks at 0.76 V and the oxidation–reduction peaks at 1.6 and 1.9 V could be attributed to the reversible decomposition and formation of Li2O with the conversion reaction of Fe2O3 into Fe.

The electrochemical properties of nanocomposite Fe2O3–Se thin film prepared by pulsed laser deposition (PLD) method have been investigated by cyclic voltammetry and charge/discharge measurements. A large reversible capacity of nanocomposite Fe2O3–Se thin film was found to be around 650 mAh g−1. A new couple of reduction and oxidation peaks at 1.4 and 1.8 V were observed from cyclic voltammogram for the first time. Our data demonstrated that nanocomposite Fe2O3–Se exhibit larger capacity and better cycle performance than pure Fe2O3. The electrochemical reaction mechanisms of Fe2O3–Se with lithium were examined by X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED). The reversible conversions reaction of nanosized metal Fe with Li2Se and Li2O formed after initial discharge process into FeSe and Fe2O3 respectively were revealed.

A Na–air battery with NaI dissolved in a typical organic electrolyte could run up to 150 cycles with a capacity limit of 1000 mA h g−1. The low charge voltage plateau of 3.2 V vs. Na+/Na in a Na–air battery should mainly be attributed to the oxidation reaction of active iodine anions.

Chromium-based layered cathode materials suffer from the irreversible disproportionation reaction of Cr4+ to Cr3+ and Cr6+, which hinders the reversible multi-electron redox of Cr ions in layered cathodes, and limits their capacity and reversibility. To address this problem, a novel O3-type layer-structured transition metal oxide of NaCr1/3Fe1/3Mn1/3O2 (NCFM) was designed and studied as a cathode material. A high reversible capacity of 186 mA h g−1 was achieved at a current rate of 0.05C in a voltage range of 1.5 to 4.2 V. X-ray diffraction revealed an O3 → (O3 + P3) → (P3 + O3′′) → O3′′ phase-transition pathway for NCFM during charge. X-ray absorption, X-ray photoelectron and electron energy-loss spectroscopy measurements revealed the electronic structure changes of NCFM during Na+ deintercalation/intercalation processes. It is confirmed that the disproportionation reaction of Cr4+ to Cr3+ and Cr6+ can be effectively suppressed by Fe3+ and Mn4+ substitution. These results demonstrated that the reversible multi-electron oxidation/reduction of Cr ions can be achieved in NCFM during charge and discharge accompanied by CrO6 octahedral distortion and recovery.

A high-quality sized FeFe(CN)6 was synthesized as a cathode material for a non-aqueous potassium-ion battery. The electrode delivered a reversible capacity of 124 mA h g−1 at the current rate of 0.5C and still retained a reversible capacity of 93 mA h g−1 after 500 cycles at 5C with a columbic efficiency of 100%. Structural evolution and redox couples of low and high spin FeIII/FeII were investigated by ex situ X-ray diffraction, Mössbauer spectroscopy, and X-ray photoelectric spectroscopy. The negligible volume change during the electrochemical process should be responsible for the excellent cyclic stability.

Heterogeneous catalysts with Co3O4 and liquid redox mediators were utilized for the morphological control of discharged products in SABs. With Co3O4 nanowires/C as air cathodes, the discharge product tended to be like nanoflakes. However, after the addition of ferrocene to the electrolyte, the discharge product tended to be like nanofilms and the cyclic performance can achieve 570 cycles.

A CoS2/multi-walled carbon nanotube (MWCNT) nanocomposite was synthesized and its sodium storage performances in ether-based electrolyte and commonly used carbonate-based electrolyte were investigated for the first time. A high capacity of 568 mA h g−1 after 100 cycles in ether-based electrolyte can be achieved.

A well-crystallized single-phase quinary layer transition metal oxide of NaNi1/4Co1/4Fe1/4Mn1/8Ti1/8O2 was successfully synthesized. It exhibited excellent cycle performance and high rate capability as a cathode material for sodium-ion batteries.

Graphene nanosheets (GNS) were employed as an air electrode for a sodium–air battery (SAB). High discharge capacity of 9268 mA h g−1 with low overpotential was achieved, indicating its superiority to a normal carbon film electrode. Our results indicate that GNS as air electrodes could improve the electrochemical performance of rechargeable SABs.

A Cu2Se electrode on a copper grid substrate has been directly fabricated by a facile post-selenized method and tested as a positive material for sodium ion batteries. Cu2Se exhibits large reversible capacities (about 250 mA h g−1), good cyclic stabilities and low polarization. These results indicate that Cu2Se is a promising candidate as a cathode material for sodium ion batteries.

Metal–air batteries are important power sources for electronics and vehicles because of their remarkable high theoretical energy density and low cost. In this work, we firstly investigate the electrochemical properties of sodium air batteries (SABs) with the addition of ferrocene in the electrolyte. Combining charge–discharge measurements with field-emission transmission electron microscopy images of the discharged air cathodes, our results have demonstrated that two different pathways with ferrocene as an electrocatalyst and involved in the electrochemistry during the charge process might be governed by the morphological features of the electrode caused by the deposition of Na2O2 in SABs. The SAB with ferrocene as a catalyst exhibits a high cycling performance of up to 230 cycles with a high capacity of 1000 mA h g−1.

High rate capability and long cycle life are challenging goals for the development of room temperature sodium-ion batteries. Here we report a new single phase quaternary O3-type layer-structured transition metal oxide Na(NiCoFeTi)1/4O2 synthesized by a simple solid-state reaction as a new cathode material for sodium-ion batteries. It can deliver a reversible capacity of 90.6 mA h g−1 at a rate as high as 20C. At 5C, 75.0% of the initial specific capacity can be retained after 400 cycles with a capacity-decay rate of 0.07% per cycle, demonstrating a superior long-term cyclability at high current density. X-ray diffraction and absorption characterization revealed reversible phase transformations and electronic structural changes during the Na+ deintercalation/intercalation process. Ni, Co and Fe ions contribute to charge compensation during charge and discharge. Although Ti ions do not contribute to the charge transfer, they play a very important role in stabilizing the structure during charge and discharge by suppressing the Fe migration. In addition, Ti substitution can also smooth the charge–discharge plateaus effectively, which provides a potential advantage for the commercialization of this material for room temperature sodium-ion batteries.

The structural evolution of layered NaCrO2 cathodes for sodium-ion batteries during charge was investigated using synchrotron-based in situ X-ray diffraction and ex situ X-ray absorption spectroscopy. Three solid solution phases with expanding ‘c’ and contracting ‘a’/‘b’ lattice parameters were observed. The coordination changes of Cr and Na during sodium extraction were also studied.

A 3D NiCo2O4 nanowire array/carbon cloth (NCONW/CC) was employed as the cathode for Li–air batteries with a non-aqueous electrolyte. After its discharge, novel porous ball-like Li2O2 was found to be deposited on the tip of NiCo2O4 nanowires. The special structure of Li2O2 and active sites of catalysts are also discussed.