Homogenous coating of Li+ conductive Li5AlO4 on single-crystalline nonstoichiometric Li1.04Ni0.92Al0.04O2 for rechargeable lithium-ion batteries

Cobalt-free, nickel-rich LiNi1-xAlxO2 (x ≤ 0.1) is an attractive cathode material because of high energy density and low cost but suffers from severe structural degradation and poor rate performance. In this study, we propose a molten salt-assisted synthesis in combination with a Li-refeeding induced aluminum segregation strategy to prepare Li5AlO4-coated single-crystalline slightly Li-rich Li1.04Ni0.92Al0.04O2. The symbiotic formation of Li5AlO4 from reaction between molten lithium hydroxide and doped aluminum in the bulk ensures a high lattice matching between the Ni-rich oxide and the homogenous conductive Li5AlO4 that permits high Li+ conductivity. Benefiting from mitigated undesirable side reactions and phase evolution, the Li5AlO4-coated single-crystalline Li1.04Ni0.92Al0.04O2 delivers a high specific capacity of 220.2 mA h g-1 at 0.1 C and considerable rate capability (182.5 mA h g-1 at 10 C). Besides, superior capacity retention of 90.8% is obtained at 1/3 C after 100 cycles in a 498.1 mA h pouch full cell. Furthermore, the particulate morphology of Li1.04Ni0.92Al0.04O2 remains intact after cycling at a cutoff voltage of 4.3 V, whereas slightly Li-deficient Li0.98Ni0.97Al0.05O2 features intragranular cracks and irreversible lattice distortion. The results highlight the value of molten salt-assisted synthesis and Li-refeeding induced elemental segregation strategy to upgrade Ni-based layered oxide cathode materials for advanced Li-ion batteries.