Prevention of surface and bulk degradation of Co-free Ni-rich layered cathodes by primary-particle-level Li2SO4 coating

Elevating the cutoff voltage is a method that can be employed to harness the high theoretical capacity of nickel-rich (Ni-rich) layered oxide cathodes. However, deep charging leads to rapid decays in voltage and capacity, primarily due to hydrogen fluoride (HF) attack on the surface and the lattice strain in the bulk. To improve the performance of cobalt-free (Co-free), nickel-rich (Ni-rich) layered oxide cathode material, LiNi 0.96 Mn 0.04 O 2 (LNMO), we propose a novel primary-particle-level lithium sulfate (Li 2 SO 4 ) coating method. This involves soaking the hydroxide precursor in an aqueous nickel sulfate solution before the calcination process, allowing the primary-particle-level Li 2 SO 4 coating layer to form during calcination. In contrast to conventional coating methods, the proposed Li 2 SO 4 coating can prevent both bulk and surface degradation. Firstly, the Li 2 SO 4 coating layer on primary particles serves as a barrier against primary particle agglomeration during calcination, resulting in cathode materials with reduced primary-particle-size. Reducing the primary particle size lowers the lithium ion (Li + ) concentration gradient, which in turn reduces lattice strain. Consequently, the formation of diffusion-impeding, diffusion-induced dislocations is significantly suppressed. Second, the Li 2 SO 4 layer consumes the transition metal dissolutive HF and prevents the deposition of insulating metal fluoride. Subsequently, the Li 2 SO 4 -coated LNMO (SLNMO) exhibits superior electrochemical performance, demonstrating a high initial discharge capacity of 224.5 mAh g −1 and a high initial Coulombic efficiency of 94.2 % at a cutoff voltage of 4.5 V. Furthermore, the Li 2 SO 4 -coated LNMO maintains a discharge capacity of 181.9 mAh g −1 with a high energy efficiency of 94.9 % at the 100th cycle. A one-step coating process is developed to reduce the primary particle size of the LiNi 0.96 Mn 0.04 O 2 (LNMO) cathode material and to form a thin Li 2 SO 4 coating on the primary particles. Due to the short Li + diffusion pathway in the small primary particles, the formation of diffusion-induced dislocations is suppressed during cell operation, and the Li 2 SO 4 layer protects the material from HF attack. • A novel bifunctional primary-particle-level Li 2 SO 4 coating method was developed. • The coating layer serves as a barrier against agglomeration during calcination. • The coating layer also consumes HF and prevents transition metal dissolution. • The coated material demonstes a high initial discharge capacity of 224.5 mAh g −1 . • The coated cathode also shows 81.0 % retention after 100 cycles.