Composition Engineering of Mn-Based Disordered Rocksalt for Understanding Electrode Degradation Behavior

Cation-disordered rocksalt (DRX) cathodes have emerged as promising candidates owing to their high theoretical capacity and use of earth-abundant transition metals (TMs), such as Mn, Ti, and Fe. Li+ transport occurs through a three-dimensional percolation network of 0-TM diffusion pathways. However, the practical application of Mn-based DRX cathodes is often hindered by structural instability and irreversible oxygen redox during cycling. In this study, we investigate the role of the stoichiometric Li/TM ratio on the electrochemical performance and degradation behavior of Li0.9+xMn1.1-2xTixO2 (x = 0.2, 0.3, 0.4) cathodes. The stoichiometric balance among Li, Mn, and Ti determines the dominant redox reactions and the subsequent structural evolution. The Mn-rich and Li-deficient Li1.1Mn0.7Ti0.2O2 delivers an activation-induced discharge capacity of 250.1 mAh g-1 at 10 mA g-1 with superior cycling stability, which is attributed to a facilitated bulk spinel-like phase transformation. In contrast, the Li-excess and Mn-deficient Li1.3Mn0.3Ti0.4O2 exhibit pronounced capacity fading due to severe irreversible oxygen redox. To elucidate the origin of these differences, a multi-scale analysis was conducted to resolve structural evolution across different probing depths. Ex situ XRD, Raman, and XPS analyses reveal that Mn-rich and Li-deficient cathodes maintain structural integrity through stable bulk transformation. In contrast, excessive Li content promotes detrimental surface reconstruction characterized by thick cathode-electrolyte interface (CEI) formation. These results indicate that the specific composition of Li and TM is a primary factor affecting cyclic stability and surface degradations induced by the CEI.