An effective method to reduce residual lithium compounds on Ni-rich Li[Ni0.6Co0.2Mn0.2]O-2 active material using a phosphoric acid derived Li3PO4 nanolayer

Abstract

The Ni-rich Li[Ni0.6Co0.2Mn0.2]O-2 surface has been modified with H3PO4. After coating at 80 A degrees C, the products were heated further at a moderate temperature of 500 A degrees C 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[Ni0.6Co0.2Mn0.2]O-2 as confirmed by transmission electron microscopy (TEM). Time-of-flight secondary ion mass spectroscopic (ToF-SIMS) data exhibit the presence of LiP+, LiPO+, and Li2PO (2) (+) fragments, indicating the formation of the Li3PO4 coating layer on the surface of the Li[Ni0.6Co0.2Mn0.2]O-2. As a result, the amounts of residual lithium compounds, such as LiOH and Li2CO3, are significantly reduced. As a consequence, the Li3PO4-coated Li[Ni0.6Co0.2Mn0.2]O-2 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.