Developing ester-based fluorinated electrolyte with LiPO2F2 as an additive for high-rate and thermally robust anode-free lithium metal battery

The electrolyte of 1 M LiPF6 in FEC/TTE/EMC (FTE 352) has been demonstrated to increase cycle life, safety, and energy density of anode-free lithium metal battery (AFLMB) due to its high oxidative stability, nonflammability, and capability to form lithium fluoride (LiF) rich solid electrolyte interphase (SEI). Methyl acetate (MA), a linear ester, is selected to reduce the use of expensive FEC and expand the adaptability of an electrolyte for high rates and lower temperature applications. Due to its lower LUMO value, a complementary additive LiPO2F2 was applied to tune the SEI. In this work, 1 M LiPF6 in FEC/TTE/EMC/MA (1/5/2/2 vol ratio) + 1 wt% LiPO2F2 was thus developed, which has high ionic conductivity, high oxidative stability (>5 V), high-rate capability, thermal stability, and tolerance to low temperature (0 °C). Using an anode-free configuration, this electrolyte delivers an average coulombic efficiency of 97.5% at room temperature and 96% at 0 °C. In contrast, the average coulombic efficiency of 1 M LiPF6 in FTE 352 is 94.4% at 0 °C. This electrolyte also holds capacity retention of 48% under a current density of 2 mA/cm2 using Cu||NMC111, while the reference electrolyte's capacity retention is 35%.