Aging investigations of a lithium-ion battery electrolyte from a field-tested hybrid electric vehicle

Abstract

The electrolyte of a used lithium-ion battery from a hybrid electric vehicle (HEV) was investigated. The liquid electrolyte was collected through the pressure valve of these 5 Ah cells. It consists of (29.8 ± 0.2) wt.% dimethyl carbonate (DMC), (21.7 ± 0.1) wt.% ethyl methyl carbonate (EMC), (30.3 ± 0.3) wt.% ethylene carbonate (EC) and (2.2 ± 0.1) wt.% cyclohexyl benzene (CHB) which were identified with GC–MS and quantified with GC–FID. Li+ (1.29 ± 0.04) mol L−1 and PF6− were determined with IC as the main ionic species in the solution. Furthermore, BF4− was clearly identified with IC–ESI–MS, IC–ICP–MS and 11B NMR and quantified to a concentration of (120.8 ± 8.3) mg L−1 with ICP–OES. The presence of POF3 (detected with GC–MS), F−, PO2F2−, HPO3F− and H2PO4− (determined with IC–ESI–MS) can be attributed to the reaction of the conducting salt LiPF6 via PF5 with traces of water. HPO3F− and H2PO4− could only be observed in cells which were opened in a laboratory hood under exposure of air humidity. This experiment was done to simulate escaping electrolyte from an HEV battery pack. Furthermore, several alkyl phosphates (identified with GC–MS and IC–ESI–MS) are present in the solution due to further reaction of the different fluorinated phosphates with organic carbonates.