Very-Low Solid-Electrolyte/Electrode Interface Resistance in Li(Ni0.5Mn1.5)O4-Based Thin-Film Lithium Battery

Abstract

All-solid-state lithium batteries with improved safety and larger capacity have been anticipated as a next-generation rechargeable battery. In particular, 5 V-class cathode material, LiNi0.5Mn1.5O4 (LNMO), has received considerable attention for the expected high power density. In all-solid-state LBs using LNMO, however, high interface resistance between an electrode and electrolyte hinders high power density. Thus, the reduction of the interface resistance and the investigation of the origins are of important issues. Recent progress in thin-film deposition techniques of electrodes and electrolytes has made it possible to fabricate all-solid-state thin film batteries with atomically controlled interfaces, and quantitative studies of the interface resistance values have been reported. In this study, we fabricated all-solid-state thin-film lithium batteries using LNMO epitaxial thin films and Li3PO4 solid electrolyte, and demonstrated a very low Li3PO4/LNMO interface resistance. Figure 1 shows a structure of all-solid-state thin films battery (inset), and its charge and discharge curves. The fabricated thin film batteries consisted of an epitaxial cathode LNMO(100) film, an amorphous solid-electrolyte Li3PO4 film, a metallic anode Li film, a current collector LaNiO3(100) film, and Nb-doped SrTiO3(100) substrate. To ensure a clean Li3PO4/LNMO interface in the thin films batteries, the ultrahigh vacuum chambers used for the film depositions and characterizations were all directly connected each other. The LNMO thin-film battery exhibits good battery performance. The charge and discharge curves show redox reactions of LNMO (Ni2+/Ni4+ and Mn3+/Mn4+), and the capacity did not degrade even up to the 100th cycle. We also evaluated the interface resistance using impedance spectroscopy, and found a very low interface resistance of ~5 Ωcm2. A Nyquist plot (Fig. 2) displays one semicircle at a higher frequency region (104~106 Hz), originating from the resistance of Li3PO4 (5×10-7 S/cm). In addition, a smaller semicircle at a lower frequency region (102~104 Hz) is attributed to the interface resistance at the Li3PO4/LNMO interface. The very low interface resistance of ~5 Ωcm2 is one order of magnitude smaller than those reported before, and implies that space charge layer formed at the Li3PO4/LNMO interface hardly results in high interface resistance. This study was supported by NEDO, JST-ALCA, JST-CREST, and Kakenhi. The authors also acknowledge the support of Toyota Motor Corporation. [1] S. Shiraki, H. Oki, Y. Takagi, T. Suzuki, A. Kumatani, R. Shimizu, M. Haruta, T. Ohsawa, Y. Sato, Y. Ikuhara, T. Hitosugi, J. Power Sources 267 (2014) 881-887. [2] M. Haruta, S. Shiraki, T. Suzuki, A. Kumatani, T. Ohsawa, Y. Takagi, R. Shimizu, and T. Hitosugi, Nano Lett. 15 (2015) 1498-1502. Figure 1