DEGRADATION OF ELECTRIC VEHICLE LITHIUM-ION BATTERIES IN ELECTRICITY GRID SERVICES

Abstract

It has been proposed to repurpose retired electric vehicle (EV) lithium ion batteries (LIB) into stationary electricity grid services. This thesis investigates the performance characteristics of two popular leading commercial EV LIBs, with NMC+LMO and LFP positive electrodes. The energy electricity service of energy arbitrage (EA) and power electricity service of frequency regulation (FR) are compared. LIB performance was compared on the basis of energy capacity degradation throughout cycle life, round trip energy efficiency, and thermal response. Additional diagnostics methods including SEM, EDS, half-cell testing, impedance growth and differential voltage analysis were used to determine why each LIB degraded. The results give useful insight into which EV LIB chemistries are most appropriate for which electricity grid application. It was found that EA degrades LIBs twice as fast as FR. NMC+LMO degrades twice as fast as LFP but is more energy efficient.