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

Abstract

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 Model1, 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, LiNi0.5Mn0.3Co0.2O2/graphite (NMC532), LiNi0. 83Mn0. 6Co0. 11O2/graphite (Ni83) and LiFePO4(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 LiPF6 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 LiPF6 salt, also cycled at 70°C. The same conclusion was found with NMC532 cells filled with DEOHC with LiFSI salt or LiPF6. 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 low voltage limits show unprecedented cycling lifetimes when using DMOHC electrolytes containing LIFSI while also exhibiting limited gassing. To mitigate the high viscosity of DMOHC, various cells were filled with electrolytes that used DMOHC and various percentages of diethyl carbonate (DEC) and dimethyl carbonate (DMC) with LiFSI as the salt. DMC/DEC was chosen for its low viscosity and its ionic conductivity. Even with DMC, the cycling performance showed improvements over traditional EC-based electrolytes. There were slight decreases in performance compared to a cell with a pure DMOHC electrolyte. In addition, various additives were tested, including prop-1-ene-1,3-sultone (PES), to see if gassing could be further limited. We propose DMOHC and DEOHC as new solvents for Li-ion cell electrolytes, possibly replacing EC, to enable long-lasting, high-temperature tolerant Li-ion cells. REFERENCES E. R. Logan, E. M. Tonita, K. L. Gering, and J. R. Dahn, J Electrochem Soc, 165, A3350–A3359 (2018). Figure 1. (a) Normalized capacity versus time for LFP (blue), NMC532 (red) and Ni83 (black) artificial graphite cells, using 1M LIFSI DMOHC 2%VC 1%DTD electrolyte cycling at C/20 and at 85°C. (b) corresponding voltage polarization data versus time. Figure 1