Tuning Li/Ni mixing by reactive coating to boost the stability of cobalt-free Ni-rich cathode

Cobalt-free cathode materials are attractive for their high capacity and low cost, yet they still encounter issues with structural and surface instability. AlPO4, in particular, has garnered attention as an effective stabilizer for bulk and surface. However, the impact of interfacial reactions and elemental interdiffusion between AlPO4 and LiNi0.95Mn0.05O2 upon sintering on the bulk and surface remains elusive. In this study, we demonstrate that during the heat treatment process, AlPO4 decomposes, resulting in Al doping into the bulk of the cathode through elemental interdiffusion. Simultaneously, PO43− reacts with the surface Li of material to form a Li3PO4 coating, inducing lithium deficiency, thereby increasing Li/Ni mixing. The suitable Li/Ni mixing, previously overlooked in AlPO4 modification, plays a pivotal role in stabilizing the bulk and surface, exceeding the synergy of Al doping and Li3PO4 coating. The presence of Ni2+ ions in the lithium layers contributes to the stabilization of the delithiated structure via a structural pillar effect. Moreover, suitable Li/Ni mixing can stabilize the lattice oxygen and electrode-electrolyte interface by increasing oxygen removal energy and reducing the overlap between the Ni3+/4+ eg and O2− 2p orbitals. These findings offer new perspectives for the design of stable cobalt-free cathode materials.