A Phosphorofluoridate-Based Multifunctional Electrolyte Additive Enables Long Cycling of High-Energy Lithium-Ion Batteries

Ni-rich layered oxides are regarded as key components for realizing post Li-ion batteries (LIBs). However, high-valence Ni, which acts as an oxidant in deeply delithiated states, aggravates the oxidation of the electrolyte at the cathode, causing cell impedance to increase. Additionally, the leaching of transition metal (TM) ions from Ni-rich cathodes by acidic compounds such as Brønsted-acidic HF produced through LiPF₆ hydrolysis aggravates the structural instability of the cathode and renders the electrode-electrolyte interface unstable. Herein, we present a multifunctional electrolyte additive, bis(trimethylsilyl) phosphorofluoridate (BTSPFA), to attain enhanced interfacial stability of graphite anodes and Ni-rich cathodes in Li-ion cells. BTSPFA eliminates the corrosive HF molecules by cleaving silyl ether bonds and enables the formation of a polar P-O- and P-F-enriched cathode electrolyte interface (CEI) on the Ni-rich cathode. It also promotes the creation of a solid electrolyte interphase composed of inorganic-rich species, which suppresses the reduction of the electrolyte during battery operation. The synergistic effect of the HF scavenging ability of BTSPFA and the stable BTSPFA-promoted CEI effectively suppresses the TM leaching from the Ni-rich cathode while also preventing unwanted TM deposition on the anode. LiNi_0.8Co_0.1Mn_0.1O₂/graphite full cells with 1 wt % BTSPFA exhibited an enhanced discharge capacity retention of 79.8% after 500 cycles at 1C and 45 °C. These unique features of BTSPFA are useful for resolving the interfacial deterioration issue of high-capacity Ni-rich cathodes paired with graphite anodes.