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Air‐stable layered structured cathodes with high voltage and good cycling stability are highly desired for the practical application of Na‐ion batteries. Herein, we report a P2‐Na2/3Ni2/3Te1/3O2 cathode that is stable in ambient air with an average operating voltage of ~3.8 V, demonstrating excellent cycling stability with a capacity retention of more than 92.7% after 500 cycles at 20 mA g−1 and good rate capability with 91.9% capacity utilization at 500 mA g−1 with respect to capacity at 5 mA g−1 between 2.0 and 4.0 V. When the upper cutoff voltage is increased to 4.4 V, P2‐Na2/3Ni2/3Te1/3O2 delivers a reversible capacity of 71.9 mAh g−1 and retains 91.8% of the capacity after 100 cycles at 20 mA g−1. The charge compensation during charge/discharge is mainly due to the redox couple of Ni2+/Ni3+ in the host with a small amount of contribution from oxygen. The stable structure of the material without phase transformation and with small volume change during charge‐discharge allows it to give excellent cycle performance especially when the upper cutoff voltage is not higher than 4.2 V. [ABSTRACT FROM AUTHOR]

Abstract Traditional sodium-ion battery cathodes utilize the change in oxidation state of the transition metal(s) in the structure to accommodate the electron transfer during charge and discharge. Recent researches on sodium-rich compounds such as Na 2 RuO 3 and Na 2 IrO 3 suggest that anionic reaction with oxygen can be an additional source of electrons during electrochemical reactions. Here we demonstrate for the first time that stoichiometric NaVO 3 , despite the valence of vanadium of 5+, delivers a reversible capacity by activating it beyond 4.5 V. Elemental analysis confirms Na removal and insertion from/into NaVO 3 during charge/discharge. X-ray photoelectron spectroscopy and X-ray absorption near-edge spectroscopy results show that the oxidation state of vanadium remains unchanged, while oxygen is likely to compensate for the charge transfer during charge/discharge. Theoretical calculation on spin density of electrons in the lattice also supports the involvement of oxygen during sodium removal. Our demonstration of the unique behaviors of NaVO 3 provides a new exciting direction for research in sodium-ion battery cathodes. Highlights • NaVO 3 can be charged despite vanadium in its highest oxidation state. • Capacity of NaVO 3 depends strongly on particle size and electrical conductivity. • NaVO 3 undergoes reversible charge-discharge after activation. • NaVO 3 maintains its crystal structure upon charge and discharge. • Theoretical calculation suggests the involvement of oxygen during sodium removal. [ABSTRACT FROM AUTHOR]

P2-type Na 0.7 MnO 2 , with high capacity and excellent Na ion conductivity, is a promising cathode material for Na-ion batteries. However, its rapid capacity decay upon repeated cycles restricts its practical application. In this study, we demonstrate a facile method to surface coat and dope P2-Na 0.7 MnO 2 with a P2-Na 0.7 Ni 0.33 Mn 0.67 O 2 layer in a single step to enhance its cycle stability. The coating suppresses the dissolution of Mn ions into electrolyte, and the Ni dopant suppresses orthorhombic distortion, inhibits Na + /vacancy ordering and improves structural stability upon cycling. As a consequence, the coating enhances capacity retention from 62.2% (Na 0.7 MnO 2 ) to 87.7% (Na 0.7 MnO 2 /20 wt% Na 0.7 Ni 0.33 Mn 0.67 O 2 ) over 50 cycles, and from 20.7% (Na 0.7 MnO 2 ) to 68.9% (Na 0.7 MnO 2 /20 wt% Na 0.7 Ni 0.33 Mn 0.67 O 2 ) over 100 cycles without sacrificing the initial discharge capacity. In addition, the air-stable Na 0.7 Ni 0.33 Mn 0.67 O 2 surface layer protects Na 0.7 MnO 2 against air exposure. [ABSTRACT FROM AUTHOR]

Spinel Li 4 Mn 5 O 12 , a well-known 3 V Li-ion battery (LIB) material with excellent cycling stability and good rate capability, is examined as Na-ion battery (NIB) cathode for the first time. Electrochemical studies clearly show that Na ions can be reversibly inserted into and extracted from the three-dimensional spinel structure. However, unlike in LIB, the available capacity in NIB is strongly dependent on the particle size and current rate due to the sluggish Na-ion transport in solid phase. Cycle performance of Li 4 Mn 5 O 12 in NIB is also inferior to that in LIB. Ex-situ X-ray diffraction study indicates a gradual loss of crystallinity with cycling, and that the crystal lattice undergoes an irreversible expansion during the initial 20 cycles. Inductively coupled plasma spectroscopy shows a decrease of Li/Mn ratio in Li 4 Mn 5 O 12 with cycling. The results suggest that Li ions are removed from the material during the charging process. The charge-discharge mechanism is also discussed in the paper. [ABSTRACT FROM AUTHOR]

• A novel sodium-ion-intercalated vanadium-based material coated with PEDOT was prepared. • Expanded interlayer spacing of NVO@PEDOT allows facile diffusion of Zn2+. • PEDOT coating layer protects active material from dissolving into electrolyte. • Zn||NVO@PEDOT shows good stability of maintaining 137.2 mAh/g over 600 cycles. Vanadium pentoxide (V 2 O 5) garners attention as cathode in zinc ion batteries (ZIBs) for its multi-electron redox reactions and high reversible capacity (>300 mAh/g), cost-effectiveness, and diverse crystal structures. However, inferior cycling stability stemming from limited electron conductivity and sluggish divalent Zn2+ diffusion in crystal structure largely hinders its practical application. In this study, a sodium-ion-embedded vanadium pentoxide (Na 2 V 6 O 16) cathode material (NVO) was synthesized via a simple hydrothermal method to expand the interlayer spacing, which allows facile diffusion of Zn2+. Furthermore, surface coating with poly(3,4-ethylenedioxythiophene) (PEDOT) was conducted to improve conductivity and protect active material from dissolving into electrolyte during charge-discharge. As a result, the assembled Zn||NVO@PEDOT exhibited a reversible discharge capacity of 301.0 mAh/g at 1 A/g, maintaining 137.2 mAh/g over 600 cycles, outperforming the Zn batteries with commercial V 2 O 5. Even under a high current density of 10 A/g, NVO@PEDOT still delivered a high capacity of 147.4 mAh/g after 150 cycles. This study provides innovative insights aimed at improving the electrochemical capacity and stability of cathode materials for aqueous zinc batteries. [ABSTRACT FROM AUTHOR]