Combining highly concentrated electrolytes with a polymer network is a valid approach to simultaneously achieve fast Li + ion transport, high thermal stability, and a wide electrochemical window in a quasi-solid-state form. In this work, flexible gel electrolytes comprising commercially available poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and highly concentrated electrolytes of Li salts/sulfolane (SL) were prepared by a simple solution casting method. The anionic effects of the gel electrolytes on the Li-ion conductivity and charge transfer kinetics at the gel/electrode interface were investigated. The SL-based gel electrolyte with lithium bis(fluorosulfonyl)amide (LiFSA) showed an ionic conductivity of 0.7 mS cm −1 and a high Li transference number (> 0.5) at 30 °C. The charge transfer resistance in a [Li/gel/LiCoO2] cell with LiFSA was lower than that of the cells with lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) or LiBF4, indicating faster interfacial charge transfer kinetics in the gel electrolyte with FSA. The Li/LiCoO2 cell with the LiFSA/SL gel electrolyte exhibited a higher capacity than that of the cells with the LiTFSA/SL and LiBF4/SL gel electrolytes. Hence, rationally designed gel electrolytes containing highly concentrated SL-based electrolytes enable the high rate performance of Li batteries.
Flexible solid-state electrolyte membranes are beneficial for feasible construction of solid-state batteries. In this study, a flexible composite electrolyte was prepared by combining a Li + -ion-conducting solid electrolyte Li 1.5 Al 0.5 Ti 1.5 (PO 4 ) 3 (LATP) and a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF−HFP) gel containing a highly concentrated electrolyte of Li[N-(SO 2 CF 3 ) 2 ] (LiTFSA)/sulfolane using a solution casting method. We successfully demonstrated the operation of Li/LiCoO 2 cells with the composite electrolyte; however, the rate capability of the cell degraded with increasing LATP content. We investigated the Li-ion transport properties of the composite electrolyte and found that the gel formed a continuous phase in the composite electrolyte and Li-ion conduction mainly occurred in the gel phase. Solid-state 6 Li magic-angle spinning NMR measurements for LATP treated with the 6 LiTFSA/sulfolane electrolyte suggested that the Li + -ion exchange occurred at the interface between LATP and 6 LiTFSA/sulfolane. However, the kinetics of Li + transfer at the interface between LATP and the PVDF−HFP gel was relatively slow. The interfacial resistance of LATP/gel was evaluated to be 67 Ω•cm 2 at 30 °C, and the activation energy for interfacial Li + transfer was 39 kJ mol −1 . The large interfacial resistance caused the less contribution of LATP particles to the Li-ion conduction in the composite electrolyte.
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
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