The application of pressurised carbon dioxide to recover electrolyte from lithium-ion batteries in electric vehicles

Jethwa, Shiva Jagdish (2024). The application of pressurised carbon dioxide to recover electrolyte from lithium-ion batteries in electric vehicles. University of Birmingham. Ph.D.

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Abstract

Electric vehicles (EVs) have experienced significant growth and market dominance over the past decade, contributing to a reduction in greenhouse gas emissions as they replace traditional internal combustion engine vehicles. However, the lithium-ion battery (LIB) technology powering these EVs degrades over time and eventually requires replacement. This has led to a growing accumulation of end-of-life LIB waste. Most commercial processes currently focus on recovering high-value materials from spent LIBs, often overlooking the recovery of organic materials, particularly the electrolyte component. This research thesis aims to address this gap by investigating the recovery of EV LIB electrolyte using pressurised carbon dioxide.

The initial study focused on the solubility of the primary solvent components of typical LIB electrolytes, namely, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and ethylene carbonate (EC). Each was established in binary and quaternary (1:1:1 wt) systems in carbon dioxide under temperatures of 298.2, 313.2, and 328.2 K, and pressures ranging from 0.12-14.1 MPa. Within the constraints of the system parameters explored, the linear carbonates required mild pressure and high temperature conditions to promote their solubility, whereas EC required the elevation of pressure and low temperature conditions to enhance its dissolution in carbon dioxide. Additionally, both linear carbonates were found to act as co-solvents to the cyclic carbonate, promoting its solubility in carbon dioxide.

Findings from the phase equilibria studies were applied to a pressurised fluid extraction process. Pressurised carbon dioxide was used to extract an artificially created LIB electrolyte mixture (DMC, EMC, and EC) in a 1:1:1 mass ratio, weighing 1.5 g. Initial optimisations focused on extraction duration and dynamic flowrate, and the most optimal conditions were identified to occur between 90-210 minutes at 2.4-2.6 L/min. The combined effects of pressure and temperature were investigated, and both conditions were optimised and evaluated using response surface methodology (RSM). The optimal extraction yield for the artificial LIB electrolyte was 70.2%, achieved at conditions of 12.0 MPa and 328.2 K. Key findings concluded that linear carbonates respond more effectively to temperature enhancement, highlighting the importance of vapour pressure, while the cyclic component demonstrated a strong association with fluid density.

Final investigations explored the analysis, processing, and extraction of commercial EV LIB pouch cells using supercritical carbon dioxide and solvent extraction techniques. The research found that more than 60% of the electrolyte was trapped in the electrodes and separator components, and substantial electrolyte loss was experienced due to the volatility of the linear carbonate, posing a challenge for collection. The electrolyte component was characterised before and after extraction using GC-MS, NMR, and ICP-OES techniques. The supercritical carbon dioxide extraction was performed at 12.0 MPa and 328.2 K, at a flowrate of 6 g/min for 60 minutes of dynamic and 45 minutes of static operation. The extraction produced satisfactory results, recovering a predominantly linear carbonate mixture with maximum recovery yields of 47.6% and 44.4% from the anode and separator materials, respectively. For solvent extraction, acetone was used under an HPV temperature of 323.2 K and a pump flowrate of 6.5 L/min for 165 minutes. The process achieved a maximum electrolyte yield of 96.7% from the anode material and proved effective in recovering the lithium conducting salt.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):