An Inorganic-Dominate Molecular Diluent Enables Safe Localized High Concentration Electrolyte for High-Voltage Lithium-Metal Batteries

The development of high-voltage Lithium-metal batteries (LMBs) is hindered by suitable electrolytes that are simultaneously compatible with both high-voltage cathodes and Li anodes. Herein, a novel localized high-concentration electrolyte with ethoxy(pentafluoro)cyclotriphosphazene (PFPN) as a nonflammable diluent is developed. The inorganic-dominate diluent improves the safety of the organic electrolyte, and helps to construct robust passivation interphases on both electrodes. Specifically, PFPN accelerates the complete reduction of anions, leading to a stable anion-derived interphase layer on Li anode. Meanwhile, PFPN and anions co-participate in the formation of cathode-electrolyte interphase, suppressing side reactions and the structural damage of high-voltage Ni-rich cathodes. As a result, PFPN-based electrolyte prolongs the cycling life of LMBs based on high-voltage LiNi0.6Co0.2Mn0.2 (NCM622) and LiNi0.8Co0.1Mn0.1 (NCM811) cathodes. Specially, 25-mu m-thick Li paired with NCM622 with a N/P ratio of 1.3 (4.4 V) exhibits excellent capacity retention of above 90% after 200 cycles. This work highlights the important role of diluent in tailoring electrode/electrolyte interphases and provides a new strategy for designing high-safety electrolytes toward high-voltage LMBs.