Atomic insight into the layered/spinel phase transformation in charged LiNi_0.80Co_0.15Al_0.05O₂ cathode particles

Layered LiNi_0.80Co_0.15Al_0.05O₂ (NCA) holds great promise as a potential cathode material for high energy density lithium ion batteries. However, its high capacity is heavily dependent on the stability of its layered structure, which suffers from a severe structure degradation resulting from a not fully understood layered → spinel phase transformation. Using high-resolution transmission electron microscopy and electron diffraction, we probe the atomic structure evolution induced by the layered → spinel phase transformation in the NCA cathode. We show that the phase transformation results in the development of a particle structure with the formation of complete spinel, spinel domains, and intermediate spinel from the surface to the subsurface region. The lattice planes of the complete and intermediate spinel phases are highly interwoven in the subsurface region. The layered → spinel transformation occurs via the migration of transition metal (TM) atoms from the TM layer into the lithium layer. Incomplete migration leads to the formation of the intermediate spinel phase, which is featured by tetrahedral occupancy of TM cations in the lithium layer. The crystallographic structure of the intermediate spinel is discussed and verified by the simulation of electron diffraction patterns. (Figure Presented).