Unveiling decaying mechanism of non-flammable all-fluorinated carbonate electrolytes in lithium metal batteries with 4.6-V LiCoO2 cathodes at elevated temperatures

Highly fluorinated electrolytes have attracted much attention in lithium metal batteries (LMBs) because of their unique properties, such as increased anodic stability and altered coordination state of Li+ ions. However, the complex decaying mechanism of fluorinated electrolytes at elevated temperatures is not well understood, which is essential to assess their suitability for practical applications in harsh environments. Herein the influence of elevated temperatures on the stability of fluorinated electrolytes and the formation of interphases between the 4.6-V LiCoO2 cathode and electrolyte are systematically investigated. The results reveal that elevated temperatures significantly affect the stability of the electrolyte and the formation of interphases. In particular, 1,1,2,2tetrafluoroethyl-2,2,3,3-tetrafluoropropylether undergoes decomposition catalyzed by reactive oxygen from the delithiated cathode at high temperatures and 4.6 V. Meanwhile, the degradation of fluorinated electrolytes is primarily caused by the temperature-induced decomposition of fluoroethylene carbonate. Although methyl (2,2,2-trifluoroethyl) carbonate has the highest stability among the solvents in this fluorinated system, it still experiences degradation in harsh environments. These findings highlight the importance of considering highly fluorinated ethers and the synergistic effects between fluorinated carbonate solvents, lithium salts, and solvation structures when designing fluorinated electrolytes, especially for applications at elevated temperatures. This work sheds light on the design and optimization of highly fluorinated electrolytes, enabling the development of 4.6-V LiCoO2-based LMBs capable of withstanding challenging environments and operating conditions.