Alkyl Dicarbonate-Based Electrolytes Can Enable Long-Lived Li-Ion Cells at High-Temperatures

Alkyl dicarbonates are known electrolyte degradation products produced in Li-ion cells that use ethylene carbonate (EC) and dimethyl carbonate (DMC) as electrolyte components. Also referred to as dimerization compounds, dimethyl 2,5-dioxahexanedioate (DMOHC) and diethyl 2,5-dioxahexanedioate (DEOHC), were investigated as a possible sole electrolyte solvent or and one component of a blended solvent mixture, when mixed with linear carbonates. The viscosities of DMOHC and DEOHC were measured in this report and compared to the predictions of the Advanced Electrolyte Model 1 , along with the two alkyl dicarbonates mixed with lithium salts and other common electrolyte solvents. Electrolytes based on DMOHC or DEOHC alone exhibit much higher viscosity than conventional EC-based electrolytes at room temperature. Thus, for testing, LiNi 0.5 Mn 0.3 Co 0.2 O 2 /graphite (NMC532), LiNi 0. 83 Mn 0. 6 Co 0. 11 O 2 /graphite (Ni83) and LiFePO 4 (LFP)/graphite cells with DMOHC were tested at 70 ° C and 85 ° C using C/20 charge and discharge rates. DMOHC and DEOHC were mixed with two different electrolyte salts. First, the common LiPF 6 salt and the second, lithium bis(fluorosulfonyl)imide (LiFSI), both with 2% vinylene carbonate (VC). These two electrolytes were tested in NMC532/graphite cells cycled to 4.3V at a C/20 charge/discharge rate and 70°C. Cells with DMOHC and LiFSI showed considerable improvements in capacity retention compared to those filled with an EC-based electrolyte with LiPF 6 salt, also cycled at 70°C. The same conclusion was found with NMC532 cells filled with DEOHC with LiFSI salt or LiPF 6 . Figure 1a shows the fractional capacity versus time for NMC532/graphite, Ni83/graphite and LFP/graphite pouch cells with 1.0 M LiFSI in DMOHC with 2% VC (vinylene carbonate) and 1% DTD (ethylene sulfate) additives tested at C/20 and 85°C. These cells were tested to upper cut-off potentials of 3.8, 3.9 and 3.65 V respectively. In addition, Figure 1b shows the corresponding voltage polarization results. Results show that Ni-containing cells experience exceptional lifetimes and low impedance growth despite the high cycling temperature. DMOHC-containing cells have since been tested up to 100°C. DMOHC was most advantageous for mitigating severe gassing at high temperatures. Ex-situ gas experiments show DMOHC-containing Ni83 cells produce the least amount of gas even at high voltage (4.0 V) compared to cells using EC-based electrolytes. Ni-containing cells operating to