Li-ion Cell Electrolytes For Enhanced Performance at High Temperatures

Abstract

Producing Li-ion cells with long lifetimes is vital to reaching sustainable energy goals. This research initially involved cycling cells at 70 °C to accelerate cell testing. Electrolyte changes were tracked and compared to results cycled at lower temperatures to show the effects of elevated temperature conditions. Results are shown for LiNi0.5Mn0.3Co0.2O2 (NMC532)/graphite cells cycled at 70 °C up to 4.3 V upper cutoff potentials. In addition, after long-term cycling, some cells had a significant amount of degradation product within their electrolyte, dimethyl-2,5-dioxahexane carboxylate (DMOHC). This led to the inspiration to use DMOHC as a primary electrolyte solvent. Here, it was found that when lithium bis (fluoro sulfonyl)imide (LiFSI) salt, vinylene carbonate (VC) and ethylene sulphate (DTD) additives were used with DMOHC, cells of various NMC compositions could continually cycle for up to two years at 70 °C and 85 °C. These impressive results lead to the investigation of the physical properties of DMOHC, including viscosity and conductivity, for future Li-ion electrolyte design. Various electrolytes consisting of DMOHC or DMOHC blended with either dimethyl carbonate (DMC) or diethyl carbonate (DEC) were tested in NMC cells to an upper cut-off potential of 3.8 V, 3.9 V and 4.0 V to promote cell lifetimes and avoid Al-current collector corrosion caused by LIFSI utilization. Further work investigated four novel dicarbonate compounds synthesized as potential DMOHC alternatives to achieve lower viscosities and higher conductivities compared to DMOHC. Butane-2,3-diyl dimethyl bis(carbonate) (DMe-DMOHC) electrolytes showed the greatest capacity retention and cycle life in NMC cells, performing even better at an elevated 70 °C formation. A concluding study in this work presents DMOHC-containing electrolytes being tested for applications where Li-ion cells power surgical tools, such as battery-operated drills, which undergo multiple rounds of autoclave sterilization. A method to simulate the temperature conditions of an autoclave cycle is introduced. Here, it was shown that these DMOHC-containing pouch cells provide benefits in reducing gassing and increasing cell lifetime to pouch cells using traditional Li-ion cell electrolytes.