Molecular design of highly Li-ion conductive cathode-electrolyte interface enabling excellent rate performance for lithium-ion batteries

The structural stability and ionic conductivity of the cathode electrolyte interface (CEI) film at high voltages are crucial to the nickel-rich layered cathode in Li-ion batteries (LIBs). These characteristics are largely determined by electrolyte components. Herein, an air-stable organic cyclophosphate salt named lithium 1, 3, 2-dioxaphosphinan-2-olate-2-oxide (LiDOP) is designed and synthesized as a CEI forming additive. It preferentially oxidizes and undergoes opening-ring polymerization under charging to form an organic–inorganic hybrid polymers with –LiPO4 segments and –(CH2)3– alternately connected on the cathode surface. Such a structure endows CEI film with high ionic conductivity, flexibility and mechanical strength, which can ameliorate the structure phase transformation, suppress the transition metal ion dissolution and enhance the interfacial stability at high cut-off voltage. The electrolyte with 0.2 wt% LiDOP endows 4.3 V-charged Li||LiNi0.8Co0.1Mn0.1O2 battery with 83.2 % capacity retention after 200 cycles at 5C rate. The graphite||NCM811 pouch cell shows a capacity retention of 85.6 % after 150 cycles under a 4.5 V voltage, while the cell without LiDOP shows only 23.5 %. Our work demonstrates the effectiveness of covalently coupled P-O-Li compounds as CEI film in stabilizing the interface, which provides a guidance for the electrolyte design in high-voltage LIBs.