Reversible Mn2+/Mn4+ double-electron redox in P3-type layer-structured sodium-ion cathode

Abstract

The balance between cationic redox and oxygen redox in layer-structured cathode materials is an important issue for sodium batteries to obtain high energy density and considerable cycle stability. Oxygen redox can contribute extra capacity to increase energy density, but results in lattice instability and capacity fading caused by lattice oxygen gliding and oxygen release. In this work, reversible Mn²⁺/Mn⁴⁺ redox is realized in a P3-Na_0.65Li_0.2Co_0.05Mn_0.75O₂ cathode material with high specific capacity and structure stability via Co substitution. The contribution of oxygen redox is suppressed significantly by reversible Mn²⁺/Mn⁴⁺ redox without sacrificing capacity, thus reducing lattice oxygen release and improving the structure stability. Synchrotron X-ray techniques reveal that P3 phase is well maintained in a wide voltage window of 1.5–4.5 V vs. Na⁺/Na even at 10 C and after long-term cycling. It is disclosed that charge compensation from Co/Mn-ions contributes to the voltage region below 4.2 V and O-ions contribute to the whole voltage range. The synergistic contributions of Mn²⁺/Mn⁴⁺, Co²⁺/Co³⁺, and O²⁻/(O_n)²⁻ redox in P3-Na_0.65Li_0.2Co_0.05Mn_0.75O₂ lead to a high reversible capacity of 215.0 mA h g⁻¹ at 0.1 C with considerable cycle stability. The strategy opens up new opportunities for the design of high capacity cathode materials for rechargeable batteries.