Effective Stabilization of Ncm622 Cathodes in Aqueous/Non-Aqueous Hybrid Electrolytes by Adding a Phosphazene Derivate as Co-Solvent

Abstract

Recent progress in “water-in-salt” electrolytes (WiSEs) and “hybrid aqueous/non-aqueous electrolytes (HANEs)” made broader choices of active material in aqueous Li-ion batteries (ALIBs), because of their compared to standard aqueous electrolytes expanded electrochemical stability windows (ESWs). Exploring high energy density ALIBs is a consequently meaningful research topic. However, the formation of an ineffective interphase layer on the cathode surface in aqueous electrolyte obstructs the utilization of layered high energy cathode materials. Herein, a new organic compound, ethoxy-(pentafluoro)-cyclotriphosphazene (PFN) is introduced into HANEs as co-solvent. The addition of PFN can decrease the viscosity of highly concentrated HANEs and influence the solvation structure of Li+, facilitating the formation of uniform cathode electrolyte interphase (CEI). PFN forms the CEI component monoester phosphate, which betters the performance of ALIBs based on NCM622 || TiO2@LiTi2(PO4)3 (P:N=1.5:1) that exhibit enhanced cycling stability with 79.3% capacity retention after 200 cycles at 1 C in the voltage range of 1.1-2.7 V, indicating that PFN can be a useful species to stabilize NCM622 operation in HANEs.

Keywords: ethoxy-(pentafluoro)-cyclotriphosphazene (PFN), aqueous electrolytes, hybrid electrolytes, high concentration, high energy density aqueous lithium ion batteries