Depth-Profiling Spectroscopic Investigation of The Activation Mechanism of LiMn₂O₄: Micro-SORS Method

Lithium manganese oxide (LiMn2O4, LMO) stands out as a promising material for lithium-ion batteries due to its high thermal stability, low cost, abundance, and environmental compatibility. However, the dynamics of LMO’s behavior in electrode is still unknown. In this study, we explore LMO as a storage electrode in photo-rechargeable batteries (PRB) using non-invasive depth profiling techniques, specifically micro-spatially offset Raman spectroscopy (micro-SORS). We delve into the Raman profiles of LMO layers, examining their variations based on depth after galvanostatic charging and discharging cycles of PRB. During these cycles, the oxidation state of manganese species shifts from Mn³⁺ to Mn^3.5+, accompanied by a phase transition from cubic to tetragonal. This transition results in increased capacity. To unravel this mechanism, we analyze depth-dependent Raman profiles corresponding to the phase transition throughout charging and discharging cycles. To validate our micro-SORS findings, we employ X-ray absorption fine structure (XAFS) and X-ray absorption near edge structure (XANES) techniques, confirming both oxidation states and phases. The XAFS and XANES data reveal the accumulation of an irreversible charged state (Li2Mn2O4, L2MO) over repeated charging-discharging cycles. By integrating micro-SORS analysis with this information, we pinpoint the specific site of accumulation in the electrode, elucidating the capacity increment mechanism through tracking this accumulation. While Raman analysis is common in battery research, this study marks the pioneering use of non-destructive, depth-dependent analysis using micro-SORS. This innovative approach not only provides valuable insights into battery degradation but also offers a revolutionary perspective, capturing macroscopic changes with microscopic precision.