Li-Ion Transport in Three-Layer Electrolyte of Ionic Liquid/Solid-State Electrolyte (SSE)/Ionic Liquid
Authors: Fujishiro Miki , Tatara Ryoichi , Ueno Kazuhide , Watanabe Masayoshi , Dokko Kaoru 
Published: December 2020

In conventional Li-ion batteries, organic liquid electrolytes are widely used due to their high ionic conductivity and wettability to the porous composite electrodes. On the other hand, future high-energy battery may require utilizing inorganic solid-state electrolyte (SSE) to obtain superior safety and higher Li+ transport property (as Li+ transference number of inorganic solid electrolyte is theoretically unity). [1] However, since the oxide-based inorganic solid electrolyte is hard and brittle, it is difficult to achieve the intimate contact between the electrode and electrolyte. The poor contact causes a high interfacial resistance, resulting in the slow electrochemical reaction rate in a solid-state battery. To address this issue, the use of an organic electrolyte as an interlayer between an electrode and an SSE is a promising approach. [2] In this case, in addition to the interface between the electrode and the organic electrolyte, the interfacial phenomena at the organic electrolyte and SSE should be considered. In the present study, we prepared a three-layer electrolyte of organic electrolyte/SSE/organic electrolyte, and the Li+ ion transport property of the three-layer electrolyte was elucidated. We used an ionic liquid (IL) as the organic electrolyte, and the IL was prepared by mixing 1-ethyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide (P12TFSA) and lithium bis(fluorosulfonyl)amide (LiFSA) in a molar ratio of 1:1. A NASICON-type Li-ion conducting glass ceramic of Li1+x+yAlx(Ti, Ge)2-xSiyP3-yO12 (LATP) was purchased from OHARA and used as the SSE layer. The three-layer electrolyte was assembled by sandwiching a LATP glass ceramic plate with cellulose separators (20 μm thick, TBL46-20, NKK) impregnated with the IL. Li metal electrodes (2 cm2) were attached to both sides of the three-layer electrolyte to fabricate a symmetric cell of Li / three-layer electrolyte / Li. Electrochemical impedance spectroscopy (EIS) was performed for the symmetric cell, and the interfacial phenomena were analyzed. Nyquist plots of the symmetric cell are shown in Figure 1(a). The total interfacial resistance in the cell was estimated to be 123 ohm. This resistance is originated from the Li/IL interface (R
Li/IL) and the IL/LATP interface (R
IL/LATP). To evaluate the interfacial resistance of R
Li/IL, EIS measurement was performed for another symmetric cell of Li / IL / Li, and the R
Li/IL was estimated to be 34 ohm. Therefore, R
IL/LATP in the Li / three-layer electrolyte / Li was calculated to be 123-34 = 90 ohm. We also evaluated the apparent Li+ transference number (t
Li+) of the three-layer electrolyte using a DC polarization method. [3] Figure 1(b) shows the chronoamperogram of the Li / three-layer electrolyte / Li. From the steady-state current and R
Li/IL, the apparent t
Li+ was estimated to be 0.52. Although t
Li+ of LATP should be close to 1, t
Li+ of IL is as low as 0.24. In addition, Li+ ion should pass through the interface of IL/LATP. Therefore, apparent t
Li+ of the three-layer electrolyte is affected by t
Li+ of IL, the thickness of the IL layer, and the R
IL/LATP. We will also report the battery performance of the three-layer electrolyte.
Acknowledgement
This study was supported in part by the JSPS KAKENHI (Grant Nos. 16H06368, 18H03926, and 19H05813) from the Japan Society for the Promotion of Science (JSPS).
References
[1] J. C. Bachman et al., Chem. Rev., 116, 140-162 (2016).
[2] M. R. Busche et al., Nat. Chem., 8, 426-434 (2016).
[3] P. G. Bruce, C. A. Vincent, J. Electroanal. Chem. Interf. Electrochem., 225, 1-17 (1987).

Figure 1