Mitigating Jahn–Teller Effect of Mn-Based Layered Oxide Cathodes for Sodium-Ion Batteries by Regulation of Coordination Chemistry

Abstract

P2-type Mn-based layered oxide cathode materials are competitive candidates for sodium-ion batteries (SIBs), which are expected to be widely used in large-scale electrochemical energy storage applications due to their easy availability. However, MnO₆ octahedra centered around Mn³⁺ are inclined to adverse phase transitions and lattice oxygen loss under high operating voltages, which markedly compromise the capacity and cycling stability. Here, a configurational entropy tuning strategy was proposed to optimize the P2-type Na_0.8Li_0.17Mg_0.18Mn_0.66O₂ (LMM) cathode. The as-synthesized cathode material, Na_0.8Li_0.17Ca_0.025Mg_0.12Ni_0.05Mn_0.66O₂ (LMCNM), conforms to the standard P63/mmc crystal phase. Impressively, this material exhibits a capacity retention rate of 92% after 100 cycles at a 0.4C rate (where 1C = 125 mA h g^–1) and demonstrates minimal volume change (0.94%) during charge–discharge cycles at higher working voltages (2.0–4.3 V). In situ X-ray powder diffraction (XRD), ex situ X-ray photoelectron spectroscopy (XPS), and computational analyses collectively indicate that through the charging and discharging processes of LMCNM, there is no obvious Jahn–Teller distortion, while there is clear evidence for charge compensation from Mn³⁺ to Mn⁴⁺. Furthermore, partial reversible anionic redox has been achieved through codoping with Ca and Ni to harmonize expressive stability and high capacity.