Malonatophosphate as an SEI- and CEI-forming additive that outperforms malonatoborate for thermally robust lithium-ion batteries

Ni-rich nickel-cobalt-manganese layered oxides have been widely used as advanced cathode materials; however, they exhibit poor thermal stability at elevated temperatures. Therefore, in this study, we synthesized lithium tetrafluoro(fluoromalonato)phosphate (LFMP) as a dual-functional electrolyte additive to improve the thermal stability of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathode and graphite anode materials. The LFMP additive imparts excellent cyclability, storage performance, and mitigated gas evolution of NCM811/graphite cells at 60°C compared to its boron analog, lithium difluoro(fluoromalonato)borate (LFMB). The vast advantages of the LFMP additive can be attributed to the following two-fold origin: (i) On the NCM811 cathode, LFMP derives a cathode electrolyte interphase (CEI) that suppresses electrolyte decomposition and gas evolution more effectively than LFMB; (ii) On the graphite anode, LFMP induces a LiF-rich solid electrolyte interphase (SEI) that is more resistant to attack by phosphorus pentafluoride (PF₅) than an LFMB-induced organic-rich SEI. Our first-principles calculations corroborate that the CEI and SEI layers derived by LFMP are superior to those of LFMB owing to the strong binding with a superoxide radical (O₂^▪−) and weak binding with PF₅ and favorable LiF formation, respectively. Considering its advantages, LFMP is an outstanding electrolyte additive for thermally stable NCM811/graphite batteries. This study opens a new avenue for using malonatophosphates as a new class of electrolyte additives for enhanced surface stability of Ni-rich cathodes and graphite anodes.