Sheet-Type All-Solid-State Batteries Using Electrode Sheets with Effective Electrode-Electrolyte Interface

Abstract

The all-solid-state lithium secondary batteries using sulfide-based solid electrolytes are highly anticipated as next generation batteries because of their potential of high power and energy density, long battery life, and higher safety compared with batteries with liquid electrolyte [1]. One of the issues to be solved for the commercialization is the development of a processing technology.Two types of all-solid-state batteries using inorganic solid electrolytes have been studied. One is thin-film batteries and the other is bulk-type batteries. Thin-film batteries consist of electrode and solid electrolyte films. The thickness of the electrode films is usually less than 10 micrometers. Bulk-type batteries consist of composite electrodes with active material and solid electrolyte particles and solid electrolyte separator layer. The bulk-type batteries are further classified into two configurations. One is pellet-type batteries which are constructed using powder compression die. Most of the bulk-type batteries reported heretofore belong to this configuration. The pellet-type batteries are useful for the study of the evaluation of the performance of electrode materials and composite electrode in all-solid-state cells. However, the thickness of the solid electrolyte layer prepared by this process is relatively thick because of the difficulty of the formation of homogeneous and thin solid-electrolyte layer.Sheet-type batteries, which consist of electrode sheets with current collector sheets as shown in Fig. 1, are more practicable battery configuration than pellet-type one. The reports on the sheet-type all-solid-state batteries are still few in number[2, 3]although the research and development of the sheet-type all-solid-state batteries are as important as the materials characterization in the pellet-type batteries.Here we report the practical slurry coating process for the construction of the sheet-type all-solid-state batteries. The charge-discharge performance of the all-solid-state batteries was evaluated.The coarse and small-size solid electrolyte particles were prepared by mechanical milling from crystalline Li2S (Mitsuwa Chemicals) and P2S5 (Aldrich) using heptane as a solvent. Positive and negative electrode sheets were prepared on aluminum or copper foils by coating the slurries consisting of LiNi1/3Co1/3Mn1/3O2 (NCM) or graphite as active materials, 75Li2S·25P2S5 (mol%) glassy solid electrolyte, acetylene black, styrene-butadiene-based binder and aprotic organic solvents. The solid electrolyte sheet was also prepared by slurry coating process.The electrode and electrolyte sheet were stacked and pressed at ca. 300 MPa for cell construction.Cross-sectional SEM images revealed that the electrode and solid electrolyte were inhomogeneously distributed in the electrode sheets prepared using a coarse solid electrolyte particles. The homogeneity in the electrode layer was improved by using smaller-sized solid electrolyte particles. As a result, all-solid-state cell with electrode sheets with the fine solid electrolyte showed higher discharge capacity and rate capability than the cell using the coarse solid electrolyte. The prepared all-solid-state cells are charged and discharged with the capacity of more than 100 mAh g-1 (-NMC) at 30°C. The prepared all-solid-state cell showed the energy density of more than 100 Wh kg-1. Thus, the particle size and its homogeneous distribution play an important role for fabricating sheet-type batteries with improved battery performance. Acknowledgement This research was financially supported by the Japan Science and Technology Agency (JST), Advanced Low Carbon Technology Research and Development Program (ALCA), Specially Promoted Research for Innovative Next Generation Batteries (SPRING) Project.