Discharge-triggered electronic delocalization via asymmetric motifs suppresses Jahn–Teller distortion in Mn-based layered oxides

Abstract

Mn-based layered oxide cathodes with anionic redox activity are promising candidates for sodium-ion batteries (SIBs) owing to their high operating voltage and energy density. However, the inevitable Mn³⁺ generation after cycles results in severe Jahn–Teller distortion, damaging the long cycling stability of the cathodes. Herein, we present a metal-to-metal charge transfer (MMCT) mechanism that acts at the end of discharge to inhibit the generation of Mn³⁺, thereby improving the structural sustainability of the electrode. A P2-Na_0.69Li_0.23Ru_0.17Mn_0.6O₂ cathode is selected as a model. The doping of highly electronegative Ru regulates the covalency of asymmetrical Mn–O–Ru, ensuring the electron transfer between Mn³⁺ and Ru⁴⁺, which reduces Mn³⁺ content and mitigates the Jahn–Teller distortion. As a result, the electrode shows a robust structural stability with a minimal unit cell volume change of only 0.2%. Moreover, the cathode exhibits excellent electrochemical performance, maintaining capacity retention of over 90% after 900 cycles at 2–4.3 V. The findings provide fresh insights for inhibiting the Jahn–Teller effect, offering a new paradigm for the application of Mn-based layered cathodes with high energy density for sustainable SIBs.