Composite Electrolytes Based on Sulfide-Based Solid Electrolytes and Highly Concentrated Electrolytes
Authors: Saiki Shiori , Ock Jiyoung , Tatara Ryoichi , Dokko Kaoru , Watanabe Masayoshi , Ueno Kazuhide 
Published: December 2020

Lithium-ion batteries (LIBs) have been widely used as power sources for electronic devices because of their high energy densities. LIBs are typically composed of a transition metal oxide cathode, a graphite anode, and an organic electrolyte. Flammability, volatility and liquid leakage issues of conventional organic electrolytes lead to safety concerns. As an alternative to organic liquid electrolytes, inorganic solid electrolytes are considered to be promising since they impart nonflammablity, and single ion conducting property [1],[2]. Among inorganic solid electrolytes, sulfide-type solid electrolytes show high ionic conductivity and softer mechanical properties compared to oxide-type solid electrolytes [3]. However, there is still large contact loss at the interfaces unlike liquid electrolyte systems, which results in high interfacial resistance between the electrode and electrolyte. Additionally, it is well-known that sulfides react with trace moisture in the atmosphere, resulting in generation of toxic H2S gas. The contact between the electrode active material and the solid electrolyte interface can be improved by mixing a small amount of electrolyte solution or gel electrolyte with the solid electrolyte. Among the liquid electrolytes, highly concentrated electrolytes with a high concentration of lithium salt show properties such as non-flammability and non-volatility. In our previous works, the concentrated electrolyte using sulfolane as a solvent shows a high lithium ion transference number of ~0.7. Therefore, it can be expected that a good interface is formed without sacrificing the intrinsic high Li ion transport property of the solid electrolyte such as lithium ion transference number of unity when a small amount of the concentrated electrolyte is impregnated.
In this study, we prepared a composite electrolyte by mixing an Li7P3S11 (LPS) and a concentrated liquid electrolyte [Li(SL)2][TFSA] which is a 1:2 molar mixture of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and sulfolane (SL), with different weight ratios of [Li(SL)2][TFSA]. After compression molding of the obtained composite, the ionic conductivity was determined by AC impedance measurement using an electrochemical cell.
Figure 1 shows the ionic conductivity of the composite electrolytes with different weight ratios of the concentrated electrolyte added. As the ratio of concentrated electrolyte increased, the ionic conductivity in the composite electrolyte decreased. For the eletrolytes with the weight ratio of [Li(SL)2][TFSA] less than 20% ( i.e., LPS-rich composite electrolytes), the conductivity value was ~ 1 mS cm-1, which is close to the ionic conductivity of LPS (1.4 mS cm-1). On the other hand, when the weight fraction of concentrated electrolyte increases up to 40%, the conductivity value approached 0.42 mS cm-1 which is the ionic conductivity of [Li(SL)2][TFSA]. This result shows that Li ion conduction between LPS particles is a dominant route for the LPS-rich composite electrolyte, whereas the liquid phase appears to be the main conduction pathway for [Li(SL)2][TFSA]-rich composite electrolytes. An the intermediate weight ratios of the concentrated electrolyte of ~30%, the ionic conductivity significantly decreased. These results suggest that both LPS and the concentrated electrolyte serve as ion conducting pathway depending on the weight ratios and the interfacial resistance between the solid electrolyte and the liquid electrolyte is not a fatal problem in these composite electrolytes.

Reference

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Figure 1