Interphase Engineering of LiNi0.88Mn0.06Co0.06O2 Cathodes Using Octadecyl Phosphonic Acid Coupling Agents

Abstract

Ni-rich layered oxides are proven high-energy cathode materials for Li-ion batteries, but their characteristic short cycle life remains a challenge for implementation on a wider scale. The surface reactivity of Ni-rich layered oxides is the driving force for several capacity fading mechanisms, and a common strategy to combat these issues is to apply a protective surface coating. In this work, LiNi0.88Mn0.06Co0.06O2 (NMC) is surface-modified using octadecyl phosphonic acid (OPA) as a coupling agent through a wet-chemical process. Post-treatments of the coated NMC material at 350 and 450 °C in an O2 atmosphere are also evaluated. Physical characterization confirms the presence of a surface coating and confirms that the surface modification processing has a negligible effect on the bulk material structure. The bare NMC material shows an initial discharge capacity of 199 mAh/g in NMC||LTO cells. The coated NMC material shows a slightly lower initial discharge capacity of 188 mAh/g, but the capacity retention after 210 cycles improves from 86 to 95%. The coated NMC material shows higher discharge capacities than the bare NMC material beyond cycle 75. Postmortem characterizations indicate that the surface reactivity is reduced by the OPA coating as less fluorinated byproducts are formed. The high-temperature post-treatments of the coated NMC material change the surface chemistry but do not improve the electrochemical performance.