Jonas Welch: Advancing Non-Flammability and Thermal Stability of Sodium-ion Battery Electrolytes: From Characterization to Application

Abstract

Battery energy storage is an important technology for helping a global transition away from fossil fuels, in balancing intermittent power sources such as solar and wind, and enabling electrified transport. The rapid growth of battery production reinforces the need for more low-cost and sustainably sourced battery materials.

The sodium-ion battery, which can be made without lithium, copper and cobalt, is a more sustainable alternative to lithium-ion batteries. Although it can utilize electrode materials based on earth-abundant elements, such as the Na-Fe-C-N-based Prussian white for positive electrodes, sodium electrolytes still rely on toxic and flammable solutions with limited thermal stability, such as sodium hexafluorophosphate in carbonate solvents. This thesis demonstrates the use of elimination, inclusion and substitution as complementary tactics to design electrolyte chemistries that are non-flammable, which potentially increase battery safety, and more thermally stable, enabling use in higher-temperature conditions.

Physicochemical measurements (viscosity, density, ionic conductivity, flammability) and characterizations using nuclear magnetic resonance spectroscopy and x-ray photoelectron spectroscopy provided insights into electrolyte chemistry and properties. Galvanostatic cycling of pouch batteries, including studies of voltage profiles, coulombic efficiency, rate capability and self-discharge, was used to evaluate battery performance. Electrochemical impedance spectroscopy and intermittent current interruption techniques were used to track internal resistance.

The use of the sodium bis(fluorosulfonyl)imide and nonfluorinated sodium bis(oxalato)borate (NaBOB) salts decreased the electrolyte decomposition at elevated temperatures. Non-flammable electrolytes based on flame-retarding triethyl phosphate, either as a cosolvent to a carbonate-based electrolyte or as a standalone solvent together with NaBOB and sulfur-based additives, promoted stable long-term battery cycling with high-mass-loading Prussian white and hard carbon electrodes, also at 55 °C. The low-gas-evolution-property of the latter electrolyte was exploited in cycling experiments to prove feasibility of delegating the formation cycling to the battery end customer, challenging the need for this to take place directly following cell assembly—a potentially cost-saving innovation.