A fundamental approach to solve energy-related issues is the development of a stable energy storage device. Lithium-ion batteries (LIBs) are one of the most promising candidates because of their high energy density and long cycle life. Charge transfer reactions at electrode/electrolyte interfaces is regarded as a main factor limiting the performance of LIBs.^1,2 In this study, we focus on Li insertion/desorption reaction R at the interface between graphite anode and 1 M LiPF₆ EC solution:

Li_nC_6m → Li_n-1C_6m + Li⁺(EC) (R).

Methods

We applied density functional theory (DFT) + effective screening medium (ESM) method³ combined with reference interaction site method (RISM);⁴ ESM-RISM calculation treats graphite surface (Li_xC₆ slab + Li⁺) and liquid solution (1 M LiPF₆ EC) by quantum mechanics and implicit solvation model, respectively.⁵ ESM-RISM calculations were performed on the configuration of a vacuum/slab/solvent system as shown in Figure 1, where the DFT slab domain is on the left-hand side, and the RISM solvents (EC, Li⁺, and PF₆⁻) treated by the RISM equations are on the right-hand side. The Li insertion/desorption path (reaction R) was assumed as a combination of one or two straight lines, as indicated in Figure 1. The reacting Li was moved from the stable Li site in the graphite layer (r = 0) to the interior region of the solution. In the ESM-RISM calculations, the distribution function of the solvation is automatically formulated for the solvation/desolvation structures.

Results

We successfully obtained the energy landscapes of reaction R under the various constant chemical potentials of electron; a profile of the grand potential Ω along the reaction R path as indicated in Figure 1. The activation energy of reaction R at the equilibrium potential is approximately 0.6 eV, which is consistent with the experimental measurements.^1,2 In the presentation, we will discuss changes of the activation energy and the surface charge density with changing the Li content x of Li_xC₆ (x=0, 0.5, 1) on two differently tilted graphite (30 and 90 degrees).

References

1. T. Abe, H. Fukuda, Y. Iriyama, Z. Ogumi, J. Electrochem. Soc. 151, A1120 (2004).
2. K. Xu, A. von Cresce, U. Lee, Langmuir 26, 11538 (2010).
3. M. Otani, O. Sugino, Phys. Rev. B 73, 115407 (2006).
4. F. Hirata, P. J. Rossky, Chem. Phys. Lett. 83, 329, (1981).
5. S. Nishihara, M. Otani, Phys. Rev. B 96, 115429 (2017).