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.
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