During the charge/discharge cycling processes in lithium ion batteries (LIBs), degradation reactions of organic solvents would be inevitable at the electrode-electrolyte interface because of the instability of electrolyte components,^1,2 leading to electrode degradation, capacity fade, poor cycling stability, and even safety issues. Since the reductive decomposition mechanisms of electrolytes at the anode-electrolyte interface have been extensively studied by theoretical simulations,³ it is critical to understand the interfacial reaction mechanisms at the cathode-electrolyte interface to provide more effective strategies to help design interfacial layer in LIBs, alleviating capacity fading and improving the cycle performance of batteries.

First-principles calculations were conducted to study the oxidative decomposition reactions of ethylene carbonate (EC) on the (110) surface of LiNi_1/3Co_1/3Mn_1/3O₂ (NCM333). All the possible oxidative decomposition reaction steps of EC on cathode surfaces, including H-abstraction reaction and ring-opening reactions caused by C_c-O_e and/or C_e-O_e bond cleavage, were analyzed from both thermodynamic and kinetic aspects. We found that EC decompositions are initiated by the ring-opening reaction (C_c-O_e bond cleavage) as the first step with an activation barrier of 0.57 eV and reaction energy of -0.70 eV. Afterwards, the H-abstraction reaction is quite easily to occur with an activation energy of only 0.26 eV. Figure 1 shows the reaction energies (ΔE) and activation energies (Barriers) for four possible reaction pathways as the first decomposition step for EC on NCM333 (110) surfaces. Another layered cathode material LiCoO₂ (LCO) was also considered, and the similar decomposition steps were found for EC decomposition on (110) surfaces.

In a further study, lithium salt anions (PF₆^-) and solvent molecules (EC) as well as Mn cations were introduced into the model. These molecules showed significant effects on the reaction pathways.

References

(1) Aurbach, D.; Gofer, Y.; Langzam, J. Journal of the Electrochemical Society 1989, 136, 3198.

(2) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Energy & Environmental Science 2011, 4, 3243.

(3) Martinez de la Hoz, J. M.; Leung, K.; Balbuena, P. B. ACS Applied Materials & Interfaces 2013, 5, 13457.

Figure 1. Reaction energies (ΔE) (black filled circles) and activation energies (Barriers) (red empty circles) for four possible reaction pathways as the first decomposition step for EC on NCM333 (110) surfaces. Color code: lithium, green; carbon, brown; oxygen, red; hydrogen, white; Co, dark blue; Ni, silver gray; Mn, purple.