Chemical and Structural Evolution during the Synthesis of Layered Li(Ni,Co,Mn)O2 Oxides

The discovery of Li-containing transition-metal (TM) oxides has attracted broad interest and triggered intensive studies on these oxides as cathodes for lithium-ion batteries over decades. Unfortunately, a clear picture of how Li/TM/O ions are transported and electrons are transferred during the synthesis of these compounds is still missing, especially when cubic close-packed (ccp) anion sublattices are involved, as it is the case for spinel, layered, or rock-salt systems. In the present study, a series of layered Li(Ni,Co,Mn)O2 oxides were chosen as target materials to elucidate the underlying formation mechanism of these compounds during the high-temperature lithiation reaction. The consistent experimental results demonstrate that, as lithium ions are inserted from the surface to the bulk, some TM cations located within the bulk of crystallites are able to diffuse to the near-surface region. They create cation vacancies for the inserted lithium ions, and the mass transport behavior of these elements is driven by the chemical potential gradient. Concurrently, oxygen anions from lithium oxides and/or ambient oxygen are adsorbed and incorporated into the ccp oxygen lattice on the surface structure, connecting the relocated TM cations and the incorporated lithium ions by forming ionic bonds. This process is concomitant with crystal growth, surface reorganization caused by phase transformation, and occurrence and disappearance of pores.