Figure 6. a) Galvanostatic charge/discharge curves, and b) cycle stability of Na 2 Mn 3 O 7 between 1.5 and 4.7 V at a rate of C/20.

Reversibility of an electrode reaction is important for energy-efficient rechargeable batteries with a long battery life. Additional oxygen-redox reactions have become an intensive area of research to achieve a larger specific capacity of the positive electrode materials. However, most oxygen-redox electrodes exhibit a large voltage hysteresis >0.5 V upon charge/discharge, and hence possess unacceptably poor energy efficiency. The hysteresis is thought to originate from the formation of peroxide-like O22− dimers during the oxygen-redox reaction. Therefore, avoiding O-O dimer formation is an essential challenge to overcome. Here, we focus on Na2-xMn3O7, which we recently identified to exhibit a large reversible oxygen-redox capacity with an extremely small polarization of 0.04 V. Using spectroscopic and magnetic measurements, the existence of stable O−• was identified in Na2-xMn3O7. Computations reveal that O−• is thermodynamically favorable over the peroxide-like O22− dimer as a result of hole stabilization through a (σ + π) multiorbital Mn-O bond.

Nonbonding oxygen 2p orbitals during oxygen-redox reaction are monitored using resonant inelastic X-ray scattering (RIXS).

Kinetic formation of the peroxo-like O22− dimer is identified as the origin of a voltage hysteresis in oxygen-redox battery electrodes.

It is desirable for secondary batteries to have high capacities and long lifetimes. This paper reports the use of Na2FeS2 with a specific structure consisting of edge‐shared and chained FeS4 as the host structure and as a high‐capacity active electrode material. An all‐solid‐state sodium cell that uses Na2FeS2 exhibits a high capacity of 320 mAh g−1, which is close to the theoretical two‐electron reaction capacity of 323 mAh g−1, and operates reversibly for 300 cycles. The excellent electrochemical properties of all‐solid‐state sodium cells are derived from the anion–cation redox and rigid host structure during charging/discharging. In addition to the initial one‐electron reaction of NaxFeS2 (1 ≤ x ≤ 2) activated Fe2+/Fe3+ redox as the main redox center, the reversible sulfur redox further contributes to the high capacity. Although the additional sulfur redox affects the irreversible crystallographic changes, stable and reversible redox reactions are observed without capacity fading, owing to the local maintenance of the chained FeS4 in the host structure. Sodium iron sulfide Na2FeS2, which combines low‐cost elements, is one of the candidates that can meet the high requirements of practical applications.