Potential dependent degradation of spinel LiMn2O4 (LMO) and related structures assessed via manganese- and oxygen-sensitive scanning electrochemical microscopy†

Abstract

Manganese dissolution has been a long-standing problem that limits the widespread application of Mn-based Li-ion battery (LIB) cathodes, despite their low cost and high Li storage capacity. The accurate detection and quantification of species generated during the degradation of Mn-based cathodes, such as dissolved Mn and evolved lattice oxygen as a function of potential and/or state of charge, are essential for designing better cathode materials and interfaces. Here, we utilize mercury-based scanning electrochemical microscopy (SECM) probes that enable the real-time quantitative investigation (∼1 μM limit of detection) of Mn dissolution near the surface of spinel LiMn₂O₄ cathodes. Combined with SECM oxygen detection using Au probes, we characterize both oxygen and Mn loss from the cathode as a function of cathode potential. Our study reveals two distinct potential regions for Mn dissolution, where the degradation in the latter region is accelerated by both Mn and oxygen loss from the cathode. Our methodology also demonstrates that an electrolyte additive, tributyl phosphate (TBP), successfully suppresses Mn dissolution in the first region at low cathode potential, further supporting the idea of distinct degradation mechanisms in each region. This work elucidates the complex interplay of acid-base, interphase formation, and oxygen loss in Mn dissolution mechanisms in operating cathodes as a function of potential. It also establishes a methodology to investigate degradation processes in a variety of existing and future Mn-based cathodes and their related structures.