Bis(trimethylsilyl) phosphorofluoridate (BTSPFA) additive to improve the electrochemical performance of high capacity lithium-ion batteries

Abstract

As the demand for electric vehicles (EVs) has recently increased, there is a need to develop a high-performance lithium-ion battery(LIB) that exhibits high energy density, high power, high safety, and long life. One of the ways to realize the high energy densityof LIB is the high voltage and high capacity of cathode materials, and Ni-rich NCM materials with more than 60% Ni content are attracting attention. Ni-rich cathodes have a higher theoretical capacity, wider operating voltage, and lower manufacturing cost than commercialized cathode material, but there are performance degradation and safety issues caused by cation mixing, microcrack, transition metal dissolution, irreversible phase transition. As Ni content increases, the problem of transition metal dissolution becomes serious. Transitionmetal dissolution causes the loss of battery capacity due to the loss of cathode active material. Also, the transition metal ions are migrated to the anode and electro deposited to the anode surface, preventing the intercalation of lithium ions, causing serious capacity loss of the LIBs. Transition metal dissolution is further accelerated by the attack of Hydrogen fluoride (HF). In this work, we conducted a study on BTSPFA additive, which scavenges to HF while forming a stable cathode electrolyte interphase(CEI) layer on the NCM811 cathode. As the BTSPFA additive has a HF removal functional group, Trimethylsilyl (TMS) group is attached, it suppresses the dissolution of transition metals and electrolyte decomposition by HF removal, and improves the stability of the electrolyte-cathode interface by forming a BTSPFA based film on the Ni-rich cathode. As a result, the cycle performance at 45℃ improved by 11.9% compared to the reference electrolyte, and the 60℃ high-temperature storage performance for 30 days improved by 6.8%. The effect of inhibiting transition metal dissolution was confirmed through ICP, TOF-SIMS, and XPS analysis, and the effect of delaying the deterioration of the cathode surface structure was confirmed through XRD and TEM. We also observed that BTSPFA additive decomposes at Ni-rich cathode through SEM and XPS to form a stable film based on the additive.