Li/Cu synergistic stabilization surface in P2-type Mn-based layered oxides: inhibiting Jahn-Teller effect via...

Manganese-based layered oxides represent an attractive cathode option for sodium-ion batteries, owing to their low cost, environmental compatibility, and high capacity. However, structural degradation, irreversible phase transitions, and Jahn-Teller (J-T) distortion induced by transition metals significantly reduce the operating voltage and cycling stability. To address these issues, we propose a Li/Cu co-doping strategy and synthesize Na_0.67Ni_0.18Li_0.1Cu_0.05Mn_0.67O₂ (NMLC) via solid-state reaction. The low-valent Li⁺ modulates the valence distribution of transition metals, converting part of the high-spin Mn³⁺ into low-spin Mn⁴⁺. Meanwhile, the high electronegativity of Cu²⁺ shortens the MnO bond lengths in the MnO₆ octahedra, further alleviating J-T distortion. In situ XRD confirms that this co-doping effectively inhibits the irreversible P2-O2 phase transition, enabling the material to retain a pure P2 structure during cycling. Consequently, The NMLC cathode delivers a high reversible capacity of 147.9 mAh g^-1 within 1.5-4.2 V and maintains 73.2 mAh g^-1 at 15C, demonstrating excellent electrochemical performance. These results confirm that regulating the spin state of transition metals can mitigate the J-T effect, offering a viable route to design stable, high-performance cathodes for sodium-ion batteries.