In situ synthesis of a nickel concentration gradient structure of Ni-rich LiNi0.8Co0.15Al0.05O2 with promising superior electrochemical properties at high cut-off voltage

Abstract

Nickel-rich layered cathode materials have aroused widespread interest due to their high discharge capacity, which is a basic requirement for next-generation high energy density lithium batteries. However, with the increase of nickel content, cathode materials face the serious challenge of capacity degradation, which is attributed to the formation of rock salt-type oxides such as NiO on the surface of cathode particles. To overcome this shortcoming, a novel Ni concentration gradient LiNi_0.8Co_0.15Al_0.05O₂ (NCG-NCA) cathode material was successfully synthesized using the characteristic reaction of Ni²⁺ and dimethylglyoxime. The final synthesized nickel concentration gradient material combines the advantages of high discharge capacity and excellent stability, which are attributed to the high nickel content in the core and high cobalt content on the surface of the material particles. The cycling stability of the NCG material is remarkably improved, exhibiting an excellent capacity retention of 75% after 200 cycles at a current density of 10C (1C = 160 mA g⁻¹) under a high cut-off voltage of 4.5 V, much higher than that of a pristine NCA (P-NCA) cathode without NCG (50%). The excellent cycling stability of NCG-NCA is due to formation of a stable surface, which is not prone to serious atomic rearrangement on the surface. More importantly, with the structural analysis of NCA materials by neutron diffraction, we find that the proportion of Li/Ni mixing of NCA is reduced by utilizing the NCG structure; in turn, the rate performance of NCG-NCA cathode materials is improved greatly.