First-Principles Study on Li-Ion Desolvation Process in Electrode-Electrolyte Interface
Authors: Ohwaki Tsukuru , Ikeshoji Tamio , Duy Truong Vinh Truong , Ozaki Taisuke , Imai Hideto , Otani Minoru 
Published: February 2020

Introduction

Si-based anode materials have received a great deal of attention due to their higher capacity for lithium-ion rechargeable batteries. Many research efforts have been made to solve the issue of high modulus of volume change of the materials in their conversion reactions with Li, because it causes low endurance of the Si-based materials for charge-discharge cycle. Toward the commercialization of the materials, detailed investigations and profound understanding of electrochemical reactions in the interface between Si-based anode and electrolyte are also indispensable for realization of higher charging rate of the materials, which is generally important especially in case of application of high-capacity electrode materials like Si-based anode materials.
In this study, we focused on a Li-ion desolvation process, one of the factors affecting the charging rate through interface resistances. The key in establishing a technology for suppressing the interface resistances is a molecular-level understanding of how activation free energies of Li-ion deposition processes correlate with anode-electrolyte interface structures and solvation structures of Li ions. Therefore, we investigated the details of the Li-ion desolvation process in an electrode-electrolyte interface by adopting large-scale first-principles (FP) molecular dynamics (MD) with a bias control scheme.

Methodology

The FPMD calculation was carried out with an electrode-electrolyte interface model consisting of a hydrogen-terminated Si(111) slab, propylene carbonate (PC) molecules on it and one Li atom under the periodic boundary conditions (see Fig.). The FP calculation code OpenMX[1] was applied to our calculation and analysis. The DFT calculation was implemented with O(N) (linear-scaling) method based on the Krylov-subspace method [2]. The O(N)-DFT calculation was performed using a generalized gradient approximation (GGA-PBE) and norm-conserving type pseudopotentials. For imposing the bias corresponding to the charging condition on the anode-electrolyte interface, the effective screening medium (ESM) method [3,4] was used. The MD calculation was carried out in an NVT ensemble at 400 K. The free-energy analysis was carried out based on the blue-moon ensemble method. The free-energy profile of the Li-ion adsorption process was obtained by integrating the mean force onto the Li ion in the z-direction. The mean forces on the Li ion were calculated at different 10 z-coordinates. At each point, the MD calculation was carried out with constraining the z-coordinate of the Li ion for about 10 ps to obtain the time average.

Results and Discussion

From our blue-moon ensemble calculations for two different imposed biases (no imposed bias and 0.2 V), we obtained the free-energy profiles as a function of the reaction coordinate of the Li deposition (the z-coordinate of the Li atom). According to our analysis on the coordination number of the Li ion as a function of the z-coordinate of Li, the number of the PC molecules solvating the Li ion approaching the anode surface decreases after the Li ion goes through the energy barrier, indicating that the energy barriers found in the free-energy profiles are the ones for the Li-ion desolvation process. The activation energy for the case of no imposed bias was found to be 0.5 eV, which decreases by 0.2 eV compared with the imposed bias corresponding to the charging condition. Our calculated activation energies are within the range of the experimentally observed values.
Details of the mechanism of the Li-ion desolvation and the transition-state structure of the Li-PC complex will be discussed in our presentation.

Acknowledgements

This study was supported by the Strategic Programs for Innovative Research (SPIRE), MEXT, and the Computational Materials Science Initiative (CMSI), Japan. The computation in this work was performed using the K computer at the Advanced Institute for Computational Science, RIKEN, and the XC30 machine at the Research Center for Advanced Computing Infrastructure, Japan Advanced Institute of Science and Technology (JAIST).

References

[1] OpenMX: Open source package for Material eXplorer, http://www.openmx-square.org/
[2] T. Ozaki, Phys. Rev. B 74, 245101 (2006).
[3] M. Otani and O. Sugino, Phys. Rev. B, 73, 115407 (2006).
[4] T. Ohwaki, M. Otani, T. Ikeshoji, and T. Ozaki, J. Chem. Phys, 136, 134101 (2012).

Figure 1