Low-Voltage Operation and Lithium Bis(fluorosulfonyl)imide Electrolyte Salt Enable Long Li-Ion Cell Lifetimes at 85 °C

LiFePO₄/graphite (LFP), Li[Ni_0.5Mn_0.3Co_0.2]O₂/graphite (NMC3.8 V, balanced for 3.8 V cut-off), and Li[Ni_0.83Mn_0.06Co_0.11]O₂/graphite (Ni83, balanced for 4.06 V cut-off) cells were tested at 85 °C. Three strategies were used to improve cell lifetime for all positive electrode materials at 85°C. First, low voltage operation (<4.0 V) was used to limit the parasitic reactions at the positive electrode. Second, LiFSI (lithium bis(trifluoromethanesulfonyl)imide) was used as the electrolyte salt for its superior thermal stability over LiPF₆ (lithium hexafluorophosphate). The low voltage operation avoids the aluminum corrosion seen at higher voltages with LiFSI. NMC3.8 V cells were operated at 6 C charge and 6 C discharge without issue for 2500 cycles and then moved to room temperature where normal operation was obtained. Finally, dimethyl-2,5-dioxahexane carboxylate (DMOHC) was used as a sole electrolyte solvent or mixed with dimethyl carbonate. μ-XRF data showed no detectable levels of transition metal deposition on the negative electrode of Ni83 and LFP cells, and DMOHC cells showed less gassing after testing compared to EC-based electrolytes. We found incredible capacity retention and cycle life for Ni83 and NMC3.8 V cells using DMOHC and LiFSI at 70 °C and at 85 °C in tests that ran for more than 6 and 5 months (and are still running), respectively.