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

Abstract 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.