Experiments were performed using a high temperature (60°C) storage system, the ultra high precision charger (UHPC) at Dalhousie University, electrochemical impedance spectroscopy (EIS), in-situ gas evolution measurements and long term cycling tests (40°C and 20°C). LiPO2F2 containing cells demonstrated excellent capacity retention at both 40°C and 20°C, better impedance control, gas reduction and good rate capability at room temperature. The results of advanced characterization methods such as XPS and NMR to elucidate the impact of LiPO2F2 on the solid electrolyte interface layers will be reported, if available.
Figures 1a - 1c) show that LiPO2F2-containing NMC532/graphite pouch cells have better capacity retention, lower polarization growth and higher energy efficiency during long term cycling between 3.0 and 4.3 V at 40°C. Figures 1d - 1f) show that LiPO2F2-containing NMC532/graphite pouch cells have less voltage drop, less gas production and better impedance control during storage testing at high temperature (60°C). The results here suggest that LiPO2F2 is a very promising electrolyte additive for improving cell performance from many perspectives. The synergetic effect between LiPO2F2 and other useful electrolyte additives needs to be explored carefully.
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Figure 1(a-c) Normalized discharge capacity (a), the difference between average charge voltage and average discharge voltage (b) and energy efficiency versus cycle number for NMC532/graphite pouch cells with or without LiPO2F2 in 1.2M LiPF6 EC/EMC (3/7) during constant-voltage-constant-current (CCCV) cycling between 3.0 and 4.3 V at 40°C with a current corresponding to C/3 (cut-off current is C/20). Figures 1d – 1f show voltage versus time (d), gas production (e) and impedance change (f) during storage testing (500 hours) at 4.4 V for NMC532/graphite pouch cells with or without LiPO2F2 in 1.2M LiPF6 EC/DMC (3/7) at 60°C. In Figure 1f, the solid lines represent the results before storage and the dashed lines represent the results after storage.