An effective method to reduce residual lithium compounds on Ni-rich Li[Ni_0.6Co_0.2Mn_0.2]O₂ active material using a phosphoric acid derived Li₃PO₄ nanolayer

The Ni-rich Li[Ni_0.6Co_0.2Mn_0.2]O₂ surface has been modified with H₃PO₄. After coating at 80 ℃, the products were heated further at a moderate temperature of 500 ℃ in air, when the added H₃PO₄ transformed to Li₃PO₄ after reacting with residual LiOH and Li₂CO₃ on the surface. A thin and uniform smooth nanolayer (< 10 nm) was observed on the surface of Li[Ni_0.6Co_0.2Mn_0.2]O₂ as confirmed by transmission electron microscopy (TEM). Time-of-flight secondary ion mass spectroscopic (ToF-SIMS) data exhibit the presence of LiP⁺, LiPO⁺, and Li₂PO₂⁺ fragments, indicating the formation of the Li₃PO₄ coating layer on the surface of the Li[Ni_0.6Co_0.2Mn_0.2]O₂. As a result, the amounts of residual lithium compounds, such as LiOH and Li₂CO₃, are significantly reduced. As a consequence, the Li₃PO₄-coated Li[Ni_0.6Co_0.2Mn_0.2]O₂ exhibits noticeable improvement in capacity retention and rate capability due to the reduction of residual LiOH and Li₂CO₃. Further investigation of the extensively cycled electrodes by X-ray diffraction (XRD), TEM, and ToF-SIMS demonstrated that the Li₃PO₄ coating layers have multi-functions: Absorption of water in the electrolyte that lowers the HF level, HF scavenging, and protection of the active materials from deleterious side reactions with the electrolyte during extensive cycling, enabling high capacity retention over 1, 000 cycles.