Toward Stable, High-Energy, Partially Disordered Mn-Rich Spinel Cathodes by Revealing and Mitigating Surface Degradation.

Abstract

Mn-rich cathodes balance performance and sustainability but suffer from limited cyclability due to Mn dissolution and cathode-to-anode crosstalk. The Jahn-Teller (J-T) effect of Mn³⁺ is often linked to the above phenomena, such as in spinel LiMn₂O₄. However, in typical voltage ranges, significant Mn³⁺ only appears near the end of discharge, highlighting the need to reassess its role in driving Mn dissolution, structural degradation, and battery performance. Here, the spinel cathode's degree of disorder is tailored to expand the Mn redox range, enabling segmentation into J-T active and less active voltage ranges. Cycling at segmented voltage windows reveals surface degradation mechanisms with and without the major J-T effect. Despite a stronger J-T effect below 3.6 V vs. Li/Li⁺, Mn dissolution is less significant than above 3.6 V. Expanding the cycling window to 2.0-4.3 V causes severe degradation as the J-T active range induces a tetragonal phase and Mn²⁺-rich surface, driving Mn dissolution and consuming Li-ion inventory in full cells. Reducing electrolyte acidity minimizes Mn³⁺ disproportionation, enabling a stable dopant-free Mn-only cathode with a 250 mAh g^-1 specific capacity. These findings demonstrate that full cells using Mn-rich cathodes have the potential to avoid the notorious crosstalk problem through electrolyte engineering.