Decoupling Capacity Fade and Voltage Decay of Li-Rich Mn-Rich Layered Oxides by Regulating Surface Reconstruction Pathways

The exploitation of oxygen anion redox in Li- and Mn-rich layered oxides (LMR-NMCs) offers one of the highest capacities among cathode materials for Li-ion batteries (LIBs). However, long-term utilization remains a challenge due to continuous voltage and capacity decay, driven by irreversible phase transitions involving cation disordering and oxygen release. While the thermodynamic origins of cation disordering are well understood, the mechanisms behind oxygen loss and the resulting lattice densification are still unclear. Additionally, the formation of mixed spinel-rocksalt nanodomains after cycling complicates the degradation process.

In this study, we demonstrate a strong correlation between phase transition pathways and oxygen stability at the particle surface of LMR-NMCs through a comparative investigation using electrolyte modifications. By adjusting surface reconstruction processes, we can control the evolution of both phase and electrochemical mechanisms. Removing polar ethylene carbonate from the electrolyte significantly reduces irreversible oxygen loss at the cathode-electrolyte interface. This adjustment preferentially facilitates an in-situ layered-to-spinel phase transition, preventing the formation of the typical rocksalt phase.

The spinel-stabilized surface formed in situ enhances charge transfer kinetics by providing three-dimensional ion channels, thereby preserving the reversible redox activity of Ni, Mn, and O over 700 cycles. This is supported by findings from electron microscopy, X-ray absorption spectroscopy, and resonant inelastic X-ray scattering. The spinel surface phase also accelerates the bulk layered-to-spinel phase transition during deep delithiation and lithiation, causing thermodynamic voltage fade without capacity loss. In contrast, conventional electrolytes promote a layered-to-rocksalt surface reconstruction, hindering charge transfer and resulting in simultaneous capacity and apparent voltage fades.

Our study decouples the thermodynamic and kinetic factors contributing to voltage decay in LMR-NMCs and establishes a clear link between surface reconstruction, bulk phase transitions, and the electrochemistry of high-capacity cathodes utilizing both cation and anion redox couples. This research underscores the importance of stabilizing the electrochemical interface to advance Mn-rich cathode chemistries for future LIBs.