In-situ formed hybrid phosphates coating layer enabling co-free Li-rich layered oxides with stable cycle performance

Abstract

Li-rich layered oxides (LLOs) with high specific capacities over 250 mAh/g, are thought to be promising cathode materials for high energy density lithium-ion batteries. However, sluggish kinetics, irreversible oxygen release, and irreversible evolution of the surface/interface have led to poor cyclic properties, which are the principal barriers to commercial applications. In this work, an in-situ hybrid phosphates coating layer that combines the advantages of superior ionic conductivity (Li3PO4) and structural stability (LiTMPO4, TM = Ni, Mn) is proposed by an ion exchange and heat treatment method. Remarkably, the hybrid phosphates (Li3PO4-LiTMPO4) coating is not only deposited on the secondary particles of polycrystalline Co-free LLO surface, but also incorporated into the pores and edges of secondary particles. Based on the above advantages, the treated LLO exhibits an initial coulombic efficiency of 92.8%, excellent capacity retention, outstanding rate capability, and thermal stability. Advanced in/ex-situ measurements ascertain that the improvement mechanism of hybrid phosphates coating can be attributed to the synergistic effect of Li3PO4 and LiTMPO4, which transports lithium ions while reducing surface side reactions, and the strong P–O bond inhibiting irreversible oxygen release, thereby stabilizing the surface structure.