First-Principles Study on LiCoO2/Sulfide Interfaces in All-Solid-State Li-Ion Battery: Space–Charge Layer and Interfacial Co Mixing
Authors: Haruyama Jun , Sodeyama Keitaro , Takada Kazunori , Tateyama Yoshitaka 
Published: February 2020

All-solid-state Li-ion batteries (ASS-LIBs), which contain inorganic solids as the electrolytes, have good properties such as high energy density, shelf life, and long cycle life. Recently, sulfide-based electrolytes attract considerable attention because some sulfide materials indicate high Li-ion conductivities, which have become comparable to those of organic-liquid electrolytes. However, large interfacial resistance is usually observed if we combine sulfide electrolytes and oxide cathode. [1,2] Rate-determining factor exists at the cathode/sulfide interfaces. In addition, the interfacial resistance is reduced dramatically by interposing an insulating buffer layer (LiNbO3 [3] or Li4Ti5O12[4]) to the interfaces. Therefore, microscopic investigations at the cathode/sulfide interfaces provide useful aspects to reduce the interfacial resistance and to design the components of ASS-LIBs. In this study, we focused on the effects of Li-ion space–charge layer (SCL) and transition metal leakage [5,6] because they are considered as the main origin of the interfacial resistance.

Interfaces between oxide cathode, buffer layer, and sulfide electrolyte have been treated by first-principles calculations. LiCoO2 (LCO), β-Li3PS4 (LPS), and LiNbO3 (LNO) are selected as the interface components, because they are typical materials for cathode, sulfide electrolyte, and buffer layer, respectively. We have conducted first-principles calculation and obtained the stable structures and the electronic states. In addition, we calculated Li chemical potentials based on Li-vacancy formation energies at these interfaces. The values indicate that Li-depleted layer (i.e. SCL) grows at the beginning of the charge process, and the interposition of the buffer layer suppresses the depletion. [7] Furthermore, we have evaluated the possibilities of Co diffusion by comparing exchange energies of Co and other cationic elements. The results show preferential Co and P mixing at the LCO/LPS interface, and the LNO interposition can suppress these mixing, which is consistent with previous experiments [5,6]. These understanding of the microscopic properties will contribute to the reduction of interfacial resistance.

[1] K. Takada, Acta Mater.,
61, 759 (2013).

[2] K. Takada, N. Ohta, L. Zhang, X. Xu, B. T. Hang, T. Ohnishi, M. Osada, T. Sasaki, Solid State Ionics,
225, 594 (2012).

[3] N. Ohta, K. Takada, L. Zhang, R. Ma, M. Osada, T. Sasaki, Adv. Mater., 18, 2226 (2006).

[4] N. Ohta, K. Takada, I. Sakaguchi, L. Zhang, R. Ma, K. Fukuda, M. Osada, T. Sasaki, Electrochem. Commun.,
9, 1486 (2007).

[5] A. Sakuda, A. Hayashi, M. Tatsumisago, Chem. Mater., 22, 949 (2010); T. Ohtomo, A. Hayashi, M. Tatsumisago, Y. Tsuchida, S. Hama, K. Kawamoto, J. Power Sources, 233, 231 (2013).

[6] J. H. Woo, J. E. Trevey, A. S. Cavanagh, Y. S. Choi, S. C. Kim, S. M. George, K. H. Oh, S.-H. Lee J. Electrochem. Soc., 159, A1120 (2012).

[7] J. Haruyama, K. Sodeyama, L. Han, K. Takada, Y. Tateyama, Chem. Mater., 26, 4248 (2014).