Non-flammable Gel Polymer Electrolyte for Enhancing the Safety and High-Temperature Performance of Lithium-Ion Batteries.

As the applications of lithium-ion batteries (LIBs) have expanded, battery safety has emerged as a major concern because of the thermal runaway of LIBs arising from the use of flammable liquid electrolytes (LEs). Gel polymer electrolytes (GPEs) have been considered as potential candidates to replace LEs and improve the thermal safety of LIBs. In our study, a chemically cross-linked nonflammable GPE was synthesized and used in an LIB. A cross-linking agent, spirocyclic pentaerythritol diphosphate perfluorinated ether acrylate, comprising a phosphorus moiety and a fluoroether chain, was designed and synthesized to prepare a nonflammable cross-linked GPE. The obtained GPE effectively suppressed the deleterious reactions of the LE and imparted nonflammable characteristics. The pouch-type graphite/LiNi0.6Co0.2Mn0.2O2 cell with a nonflammable GPE delivered an initial discharge capacity of 146.7 mAh g-1 with a capacity retention of 71.1% after 300 cycles at 0.5 C and 55 °C. Moreover, the chemically cross-linked GPE exhibited excellent dimensional and thermal stability, which allowed for the safer operation of LIBs even under harsh conditions. This work provides guidelines for designing nonflammable electrolyte systems for advanced LIBs with high safety, enhanced thermal stability, and good cycling characteristics at elevated temperatures.

Cathode Electrolyte Interphase-Forming Additive for Improving Cycling Performance and Thermal Stability of Ni-Rich LiNixCoyMn1-x-yO2 Cathode Materials.

High-capacity Ni-rich LiNixCoyMn1-x-yO2 (NCM) has been investigated as a promising cathode active material for improving the energy density of lithium-ion batteries (LIBs); however, its practical application is limited by its structural instability and low thermal stability. In this study, we synthesized tetrakis(methacryloyloxyethyl)pyrophosphate (TMAEPPi) as a cathode electrolyte interphase (CEI) additive to enhance the cycling characteristics and thermal stability of the LiNi0.8Co0.1Mn0.1O2 (NCM811) material. TMAEPPi was oxidized to form a uniform Li+-ion-conductive CEI on the cathode surface during initial cycles. A lithium-ion cell (graphite/NCM811) employing a liquid electrolyte containing 0.5 wt % TMAEPPi exhibited superior capacity retention (82.2% after 300 cycles at a 1.0 C rate) and enhanced high-rate performance compared with the cell using a baseline liquid electrolyte. The TMAEPPi-derived CEI layer on NCM811 suppressed electrolyte decomposition and reduced the microcracking of the NCM811 particles. Our results reveal that TMAEPPi is a promising additive for forming stable CEIs and thereby improving the cycling performance and thermal stability of LIBs employing high-capacity NCM cathode materials.