Lithium ion batteries (LIBs) have been widely used in electric vehicles. At the same time, high capacities of batteries are required desperately, which could be realized by increasing the operational voltage of LIBs. However, side reactions occur under these high voltages at the cathode-electrolyte interface because of the instability of electrolyte components^1-3, leading to electrode degradation, capacity fade and poor cycling stability of batteries and also some safety issues. Therefore, it is meaningful to understand the decomposition mechanism of organic solvents in LIBs, which could pave the way for electrode material design and battery design.

In this study, first-principles calculations were performed to investigate the initial stage of ethylene carbonate (EC) decomposition on the stable (110)⁴ surface of Li_xCoO₂ (LCO, x=0.67, 1). For the possible initial decomposition reactions of EC on cathode surfaces, H-abstraction reaction and ring opening reaction have been proposed^{5, 6} and investigated on Li_xMnO₂ surfaces^{7, 8}. For the adsorption models of EC on LCO (110) slab, parallel (Fig. 1a) and O_c-upward (Fig. 1b) configurations were adopted, and the corresponding reaction barriers of possible reactions in two configurations were calculated by density functional theory (DFT) calculations. Climbing image nudged elastic band (CINEB) method was used to search the transition state (TS) and minimum energy path (MEP) of certain reactions.

Based on the calculation results, H-abstraction reaction (ΔE_barrier = 0.96 eV) is prone to occurring in a parallel configuration on LCO (110), while ring opening reaction (ΔE_barrier = 0.89 eV) is favored in O_c-upward configuration. These predicted EC decomposition fragments may not be the final products of side reactions, which could also react with other adsorbed fragments, leading to the SEI formation. The following reactions are considered and discussed in the presentation.

References

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Fig. 1. Adsorption models of EC on LCO (110) in a parallel configuration (a) and O_c-upward configuration (b).