A LiPF6-electrolyte-solvothermal route for the synthesis of LiF/LixPFyOz-coated Li-rich cathode materials with enhanced cycling stability†
Li-rich layered oxide materials are promising cathode materials for lithium ion batteries because of their high capacity and low cost. Nevertheless, the lack of a stable surface structure to suppress surface side reactions and layered-to-spinel transitions and thus rapid capacity fading is a critical problem that hinders the widespread practical applications of Li-rich layered oxide materials. In this work, a stable coating layer of LiF/LixPFyOz was introduced on the surface of Li-rich particles by a LiPF6-electrolyte-solvothermal process followed by post-sintering treatment. In the solvothermal process, the LiPF6-based electrolyte reacts with surface residues (such as H2O and Li2CO3) moderately at 80 °C, and then a thin LiF/LixPFyOz protective layer is formed. The LiF/LixPFyOz protective layer is tightly chemically bonded to the surface of Li-rich particles inhibiting surface side reactions and layered-to-spinel transitions during cycling as confirmed by dQ/dV, EIS, TEM, and XPS measurements. The obtained LiF/LixPFyOz-coated Li-rich cathode (sample LFP4) delivers an enhanced reversible discharge capacity of 210.7 mA h g−1 at 100 mA g−1 with 90.7% capacity retention after 100 cycles, while the uncoated sample LFP0 suffers from a rapid capacity fading and the capacity retention ratio is only 75.3%. Even at a higher current density of 1000 mA g−1, the discharge capacity of LiF/LixPFyOz-coated sample LFP4 is still as high as 188 mA h g−1 with a capacity retention ratio of 91.2% (much higher than 55.7% for uncoated sample LFP0) after 100 cycles, showing improved cycling stability and superior rate capability. This work demonstrates that LiF/LixPFyOz coating through the LiPF6-electrolyte-solvothermal route is a successful strategy for the modification of Li-rich oxide electrodes for the next-generation of high-energy Li-ion batteries.