The Effects of Small Amounts of Cobalt in LiNi1−xCoxO2 on Lithium-ion Diffusion

Abstract

Cobalt substitution for nickel in the positive electrode material LiNi1-xCoxO2 at 0 ≤ x ≤ 0.10 is investigated to determine the impact of cobalt on Li diffusivity, measured using the Atlung Method for Intercalant Diffusion (AMID) in coin cells. Cobalt was found to have little to no impact on Li diffusivity in the intermediate voltage range (4.2 V to 3.7 V). At high voltage (4.3 V to 4.2 V), 0 to 10% cobalt incrementally suppresses the H2–H3 phase transition and enables faster lithium diffusion. Additionally, at low voltage in the kinetic hindrance region (3.7 V to 3.0 V) cobalt can improve lithium diffusion by reducing cation mixing (nickel in the lithium layer). However, cation mixing can also be minimized through synthesis conditions, improving diffusivity without using cobalt. Cobalt was found to have minimal impact on the following material properties of LiNi1-xCoxO2: crystallinity, surface impurities, particle size, and electronic conductivity. Cobalt substituted for nickel from 0% to 10% was found to decrease first cycle discharge capacity in the voltage range between 4.3 V to 3.0 V and improve capacity retention in coin cell cycling vs Li metal negative electrodes. The latter impact is most likely due to the suppression of the H2–H3 phase transition as Co is added.