Lithium ion cells with longer lifetime, higher energy density and better rate capability are of interest for electric vehicle (EV) makers. The use of electrolyte additives is one of the most effective ways of improving cell performance^1–3. In this work, LiPO₂F₂ was used as an electrolyte additive to promote the performance of Li[Ni_0.5Co_0.2Mn_0.3]O₂ (NMC532)/graphite pouch cells. LiPO₂F₂ was graciously provided by Guangzhou Tinci New Materials Technology Co. and Shenzhen Capchem Technology Co.

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.

References:

1. M. Nie, J. Xia, L. Ma, and J. R. Dahn, J. Electrochem. Soc., 162, A2066–A2074 (2015).

2. D. Aurbach, K. Gamolsky, B. Markovsky, Y. Gofer, M. Schmidt, and U. Heider, Electrochimica Acta, 47, 1423–1439 (2002).

3. K. Xu, Chem. Rev., 114, 11503–11618 (2014).

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.