The effects of sulfur anion doping on the performance of LiNiO2 material for lithium secondary batteries

Abstract

The layered lithium nickel oxide LiNiO2 has been drawing much attention as a promising cathode material for lithium secondary batteries because of its comparatively low cost, large theoretical capacity (275 mAh/g), and environmental advantages over LiCoO2. However, LiNiO2 has some severe problems such as low discharge capacity (about 140-150 mAh/g) due to the difficulty in the synthesis of stoichiometric LiNiO2 and capacity fading due to the formation of NiO2 phase by the phase transition of LiNiO2 structure during intercalation/deintercalation of lithium ion. In this work, we have synthesized sulfur doped nickel oxide compounds and investigated the effect of sulfur-doping on the electrochemical performance. LiNiO2-ySy compounds were synthesized by partial substitution of oxygen for LiNiO2 with sulfur anion. The structural and electrochemical properties of the synthesized materials were investigated using various analytical techniques. LiNiO2 powders were first synthesized by a sol-gel method and then doped with sulfur powders by a solid state reaction under the flow of oxygen to prepare LiNiO2-ySy (y = 0 ∼ 0.3) powders. The LiNiO2 electrode showed a gradual decrease of discharge capacity with cycle number, the capacity retention rate of LiNiO2-ySy electrodes significantly improved. The initial capacity of the LiNiO2-ySy cells was lower than that of LiNiO2 cell and decreased with increasing the content of sulfur doped in LiNiO2-ySy. LiNiO2 powders were also doped with sulfur using Li2S powders to investigate the effect of sulfur source. The effect of the amount of sulfur used in the synthesis of LiNiO2-ySy compounds were examined to improve the LiNiO2 electrode performance.