Exceptionally highly stable cycling performance and facile oxygen-redox of manganese-based cathode materials for rechargeable sodium batteries

Highlights: • A clear occurrence of oxygen-redox is observed in P2-Na_2/3[Mn_0.7Zn_0.3]O₂. • Electron activity is derived from O 2p orbital during Na⁺ de/intercalation. • High capacity assisted by the oxygen redox is retained over 200 cycles. -- Abstract: In this study, the effect of Zn doping on the electrochemical properties of P2-Na_2/3[Mn_1−xZn_x]O₂ (x = 0.0, 0.1, 0.2, 0.3) is investigated for the first time. The P2-Na_2/3[Mn_0.7Zn_0.3]O₂ electrode deliveres a specific discharge capacity of approximately 190 mAh g⁻¹ based on the oxygen-redox reaction (O²⁻/O¹⁻), after which the Mn⁴⁺/Mn³⁺ redox reaction contributes to the capacity. The cycling performance of the P2-Na_2/3[Mn_0.7Zn_0.3]O₂ electrode is also greatly enhanced compared with that of the P2-Na_2/3MnO₂ electrode (capacity retention of 80% vs. 30% after 200 cycles). This improved cyclability is due to the suppression of cooperative Jahn–Teller distortion as well as stabilization of the structure by the electrochemically inactive Zn²⁺ ions. First-principle calculations and experimental analysis, including X-ray photoelectron spectroscopy and X-ray absorption near edge structure spectroscopy, clearly confirms that the Zn²⁺ substitution in P2-Na_2/3MnO₂ enables the O²⁻/O¹⁻ redox reaction. In addition, time-of-flight secondary ion mass spectroscopy analysis reveals that no sodium carbonates forms on the electrode surface. Our findings provide a potential new path to utilize cost-effective Mn-rich cathode materials for sodium-ion batteries via not only cationic redox but also anodic redox.