Applications of Solution Process for Li₄SnS₄-Based Solid Electrolytes to All-Solid-State Lithium-Ion Batteries

As the application of lithium-ion batteries (LIBs) has been expanded to large-scale systems such as electric vehicles and energy storage systems, their safety becomes the most important characteristic. However, conventional LIBs contain flammable liquid electrolyte which can cause serious safety concerns. In this regard, all-solid-state lithium-ion batteries (ASLBs) using inorganic solid electrolytes (SEs) are the most promising candidate to compete with the conventional LIBs. Especially, bulk-type ASLBs are considered to be highly competitive in terms of their potential for large-scale manufacturing, which is contrasted by high cost in vacuum-deposition-based fabrication for thin-film-type ASLBs. For the bulk-type ASLBs, inorganic sulfide SEs such as Li10GeP2S12 and glass-ceramic Li2S-P2S5 are appropriate because they show high ionic conductivity and deformability. The bulk-type ASLBs employing these sulfide SEs have demonstrated promising electrochemical performances.1 However, the severe discrepancy between the obtained performances and the expected ones from the extremely high conductivity values of sulfide SEs has been overlooked. The governing origin for this discrepancy is the poor ionic contacts in the composite electrode structures. As all components in bulk-type ASLB are solid, the surfaces of active materials are not fully covered by the SEs as far as the electrodes are fabricated by the mixing process.2 As efforts to circumvent this problem, direct coating of SEs by laser vapor deposition3 was reported. The laser vapor deposition technique should be, however, ruled out for scale-up because of the high cost. In this regard, development of the wet SE-coating process can be thus more attractive. However, the wet process for sulfide SEs has been hindered by instability of sulfide materials with solvents, which leads to too low ionic conductivity of the resulting precipitated materials. Also, the most sulfide SE with various solutions did not form coatable homogeneous solutions and the used solvents were not environmentally friendly. Recently, our group reported new solution processable SE, LiI-Li4SnS4, which was prepared from methanol-based solution. The resulting glass phase LiI-Li4SnS4 showed the high ionic conductivity of 0.41 mS cm-1 and the excellent dry-air stability.2 In this presentation, we will report facile electrode fabrication procedure employing the solution process of Li4SnS4, and its electrochemical performances will be presented. References Y. S. Jung, D. Y. Oh, Y. J. Nam, K. H. Park, Israel J. Chem. 55, 472 (2015). K. H. Park, D. Y. Oh, Y. E. Choi, Y. J. Nam, L. Han, J.-Y. Kim, H. Xin, F. Lin, S. M. Oh, Y. S. Jung, Adv. Mater. 28, 1874 (2016). A. Sakuda, A. Hayashi, M. Tatsumisago, Sci. Rep. 3 , 2261 (2013).