Revealing the degradation pathways of layered Li-rich oxide cathodes

Layered Li-rich transition metal oxides (LRTMO) are one of the most promising cathode candidates for high energy density lithium batteries due to the redox contributions from transition metal (TM) cations and oxygen (O) anion. However, their practical application is hindered by gradual capacity fading and voltage decay. Although oxygen loss and phase transformation have been widely recognized as primary factors for these drawbacks, the structural deterioration and chemical rearrangement of LRTMO during battery operations, and the kinetic and thermodynamic evolution, remain unclear. Herein, we comprehensively investigate the morphological, structural, and oxidation state evolutions from the individual atoms to secondary particles. By means of nano- to micro-scale characterizations, distinct structural changing pathways associated with different intra-particle heterogeneous reactions are identified. Substantial O-defects are formed through the particle by slow electrochemical activation, accompanied with oxygen release triggering progressive phase transformation on surface and formation of nano-voids in bulk. The ultra-fast heterogeneous Li- (de)intercalation often leads to O-distortion dominated lattice displacement, TM-ions dissolution, and Li-sites variation. These inhomogeneous and irreversible structural changes are responsible for first-cycle Coulombic inefficiency, and ongoing particle cracking and expansion in the following cycles.