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LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) cathodes paired with a graphite anode have emerged as a promising alternative for current power batteries. Unfortunately, the structural degradation of Ni-rich cathodes at high working voltages brings about serious capacity fading, sequentially hampering their practical use in lithium-ion batteries (LIBs). In this work, phenyl 4-fluorobenzene sulfonate (PFBS) is investigated as a multifunctional film-forming additive to suppress the structural degradation of NCM811 and alleviate the chemical decomposition of electrolyte solvents. Computational and experimental results prove that the PFBS molecule preferentially undergoes electrochemical reactions rather than the electrolyte solvents on both the cathode and anode to form a stabilized and uniform solid electrolyte interphase (SEI). The presence of 1.0 wt % PFBS is conducive to maintaining a stable SEI at the NCM811 cathode, thus mitigating the irreversible structural transformation and holding the stability of the SEI on the graphite surface. Due to the multifunctional feature of PFBS, the electrochemical performances of the NCM811//graphite pouch cell significantly improved at −20, 25, and 45 °C. Notably, the pouch cell with a PFBS additive achieved a capacity retention of 89.9% over 400 cycles at 1C at 25 °C, which is much superior to that of 29.3% for the PFBS-free one. Furthermore, the pouch cell with 1 wt % PFBS in electrolyte also achieved superior capacity retention at 45 °C (89.01%) and −20 °C (49.18%) at 1C. Theoretical calculation and X-ray photoelectron spectroscopy analysis reveal that the −OSO 2 − and −F functional groups of PFBS not only joined in the formation of a stable SEI but also facilitated the diffusion of Li ions. The excellent cycling performance achieved in a wide-temperature region with PFBS demonstrates that this functional molecule has prospects for application in power LIBs.

Enhancing the flame retardancy of electrolytes and the quality of interface films is of great significance to improve the safety performance of lithium-ion batteries (LIBs). In this work, the effects of (ethoxy)­pentafluorocyclotriphosphazene (PFPN), hexafluorocyclotriphosphazene (FPPN), and pentafluoro­(phenoxy)­cyclotriphosphazene (HFPN) as flame-retardant additives in the functional electrolyte on the performances of LiNi0.6Mn0.2Co0.2O2 (NCM622)/graphite pouch cells are comprehensively investigated. It is indicated that the contents of PFPN, FPPN, and HFPN, respectively, reach 5, 8, and 8% to achieve the purpose of flame retardancy in the 1 M LiPF6/EC + EMC (EC/EMC = 1:2 in weight) system. When the cells without and with PFPN, FPPN, and HFPN additives are, respectively, charge–discharged in the voltage range of 3.0–4.4 V at 1C, their capacity retentions are 36.1, 60.7, 74.7, and 73.8% after 150 cycles. The interface analysis and theoretical calculation show that PFPN, FPPN, and HFPN can facilitate the formation of stable interface films on two electrodes and subsequently improve the battery performance. The addition of cyclotriphosphazene flame retardant in the electrolyte ultimately improves the safety of NCM622/graphite pouch cells without sacrificing electrochemical performance. The as-prepared additive-containing electrolyte exhibits promising prospects in the application, and the evaluation method is also useful to develop the functional electrolyte with flame retardant and film-forming properties.

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