Singlet Oxygen Reactivity with Carbonate Solvents Used for Li-Ion Battery Electrolytes

High degrees of delithiation of layered transition metal oxide cathode active materials (NCMs and HE-NCM) for lithium-ion batteries (LIBs) was shown to lead to the release of singlet oxygen, which is accompanied by enhanced electrolyte decomposition. Here, we study the reactivity of chemically produced singlet oxygen with the commonly used cyclic and linear carbonate solvents for LIB electrolytes. On-line gassing analysis of the decomposition of ethylene carbonate (EC) and dimethyl carbonate (DMC) reveals different stability toward the chemical attack of singlet oxygen, which is produced in situ by photoexcitation of the Rose Bengal dye. Ab initio calculations and on-the-fly simulations reveal a possible reaction mechanism, confirming the experimental findings. In the case of EC, hydrogen peroxide and vinylene carbonate (VC) are found to be the products of the first reaction step of EC with singlet oxygen in the reaction cascade of the EC chemical decomposition. In contrast to EC, simulations suggested DMC to be stable in the presence of singlet oxygen, which was also confirmed experimentally. Hydrogen peroxide is detrimental for cycling of a battery. For all known cathode active materials, the potential where singlet oxygen is released is found to be already high enough to electrochemically oxidize hydrogen peroxide. The formed protons and/or water both react with the typically used LiPF6 salt to HF that then leads to transition metal dissolution from the cathode active materials. This study shows how important the chemical stability toward singlet oxygen is for today's battery systems and that a trade-off will have to be found between chemical and electrochemical stability of the solvent to be used.