Alkyl Dicarbonates, Common Electrolyte Degradation Products, Can Enable Long-Lived Li-Ion Cells at High Temperatures

A common degradation product dimethyl-2,5-dioxahexane carboxylate (DMOHC) produced in Li-ion cell electrolytes after ageing is used here as an electrolyte solvent, allowing Li-ion cells to operate at high temperatures (70 °C and 85 °C) with excellent capacity retention and low impedance growth. Viscosity and conductivity values are reported for various DMOHC and diethyl-2,5-dioxahexane carboxylate (DEOHC) blends with dimethyl carbonate (DMC) and diethyl carbonate (DEC). Charge-discharge cycling data are reported for LiFePO₄/graphite (LFP), Li[Ni_0.5Mn_0.3Co_0.2]O₂/graphite (NMC3.8 V, balanced for 3.8 V cut-off), Li[Ni_0.6Mn_0.4Co₀₀]O₂/graphite (NMC640, balanced for 4.1 V cut-off) and Li[Ni_0.83Mn_0.06Co_0.11]O₂/graphite (Ni83, balanced for 4.06 V cut-off) pouch cells at 70 °C and 85 °C. Pouch cells with DMOHC electrolyte have extraordinarily long lifetimes at 70 °C and 85 °C Pouch cells containing DMOHC-based electrolytes produce little to no gas compared to traditional ethylene carbonate (EC) based electrolytes. Cells taken apart after testing showed uniform negative electrode lithiation and no differences in the cell components were observed when using DMOHC electrolytes compared to EC. Lastly, micro X-ray fluorescence spectroscopy analysis was performed to probe the degree of transition metal deposition on negative electrodes of cycled cells. Very low levels of transition metals were found on the negative electrode even for cells tested at 85 °C. DMOHC is a co-solvent that can enable Li-ion batteries with exceptional high temperature lifetimes.