Origin of Structural Degradation in Li-Rich Layered Oxide Cathode

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Li and Mn-rich cathode materials (LMR) that utilize both cation and anion redox can yield substantial increases in battery energy density.1-3 However, even though voltage decay issues cause continuous energy loss and impedes its commercialization, prerequisite driving force for this phenomena are a mystery3-6 Here, with in-situ nanoscale sensitive coherent X-ray diffraction imaging (BCDI) techniques, we reveal that nanostrain and lattice displacement accumulate continuously during cell operation. Evidence unequivocally shows that this effect is the driving force for both structure degradations and oxygen loss that triggers the well-known rapid voltage decay in LMR. By further leveraging micro to macro length characterizations that span atomic structure, primary particle, multi-particle and electrode level, we demonstrate that the heterogeneous nature of LMR cathodes inevitably causes pernicious phase displacement/strain which cannot be eliminated by conventional doping or coating methods. These results affirm that lattice displacement and nano-strain, which represent commonly occurred but less detectable dynamic structure evolutions, play an undeniable role in structure decomposition and voltage fade. With these fundamental discoveries, we propose mesostructural design as a strategy to mitigate lattice displacement and inhomogeneous electrochemical/structural evolutions, thereby achieving stable voltage and capacity profiles. These findings highlight the significance of lattice strain/displacement in causing voltage decay and will inspire a wave of efforts to unlock the potential of the broad-scale commercialization of LMR cathode material.