The practical use of Ni-rich NCM cathode materials has been plagued by their poor thermal stability, especially at high-temperature (60 °C). In this study, we introduce lithium tetrafluoro(fluoromalonato)phosphate (LFMP) as a dual-functional electrolyte additive to improve the thermally stability of both LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathode and graphite anode materials. LFMP additive enables excellent cyclability, storage performance, and mitigated gas evolution of NCM811/graphite cells at 60 °C compared with its boron analogue, lithium difluoro(fluoromalonato)borate (LFMB). The underlying origin of the benefits of LFMP are elucidated to be two folds: (i) on the NCM811 cathode, LFMP derives a cathode electrolyte interphase (CEI) that suppresses electrolyte decomposition and gas evolution more effectively than LFMB-derived CEI, (ii) on the graphite anode, LFMP derives a LiF-rich solid electrolyte interphase (SEI) that is resistant to the attack by HF and PF₅, compared to an organic-rich, LFMB-derived SEI. Our first-principles calculations corroborate that LFMP is superior to LFMB owing to the strong binding with a superoxide radical, the weak binding with PF₅, and the favorable LiF formation upon electron reductions. Considering its formidable advantages, LFMP is an outstanding electrolyte additive for thermally stable NCM811/graphite batteries.