Decomposition of Non-Aqueous Electrolytes on High-Voltage Cathodes of Lithium-Ion Batteries

Abstract

Introduction Lithium-ion batteries play a key role in the future renewable energy society, and considerable effort has been devoted to improve the battery performance. High-voltage cathode materials mainly for EVs, which operate above electrochemical window of conventional non-aqueous electrolytes, have recently developed. However, the electrolytes are easily decomposed on the cathode and the reaction mechanism has not been clarified yet. In this study, we investigated the reaction products and their stereochemistry of decomposition of the conventional non-aqueous electrolytes on the high-voltage lithium cobalt oxide electrodes using gas chromatography and mass spectrometry to elucidate the decomposition mechanism. Experimental Cathode composite electrodes made of LiCoO2 powder, acetylene black, and PVDF were separated from cells charged at 4.2 V. The composite electrode powder including the Li x CoO2 (x≤ 0.5) was mixed with 1:1 (by volume) solution of EC and DMC, and then the mixture was kept at 50 °C. Reaction products in the mixture were analyzed using gas chromatograph (GC) and mass spectrometer. Results and discussion One of the main products from the EC, DMC, and Li x CoO2 mixture was identified as vinylene carbonate (VC). When EC and DMC mixed solution was heated without Li x CoO2, VC was not detected. The results indicate that EC was oxidized to VC by Li x CoO2 as shown in Figure 1a. It is well known that a typical way of organic chemical synthesis from EC to VC is radical reaction mechanism [1, 2]. To understand our EC oxidation reaction mechanism, the kinetic isotope effects (KIEs) were discussed. Reactions of EC and EC-d4 with DMC and Li x CoO2 were carried out and the amounts of VC-d2 and VC were measured by the GC, respectively. Figure 2 shows the time evolution of the amounts of VC-d2 and VC. Reaction ratios, k D and k H, were estimated by an equation as follows; k D = d[VC-d2]/dt or k H = d[VC]/dt The relative ratio of these independently determined reaction rate constant (k H/k D) was decided as 2.7 [3]. This value suggests that our reaction is not via a radical reaction, but via a new process; two–electron oxidative decomposition reaction, which will be a key to elucidate the mechanism for formation of VC. Ethylene glycol (EG) was formed in EC and DMC mixed solution without Li x CoO2, while it was not found in the EC, DMC, and Li x CoO2 mixture. When EG was added in the EC, DMC, and Li x CoO2 mixture, the amount of EG was decreased depending on reaction time, which indicates that EG is intermediate product of the reaction of EC on Li x CoO2. The results suggest that EG, which is hydrolyzate of EC, is oxidized on Li x CoO2as shown in Figure 1b. In the presentation, we discuss details of the oxidation reaction mechanism as well.