Investigation of Ionic Conductive Behavior of SL-Based Gel Polymer Electrolyte and Interfacial Property of Li / Gel Polymer Electrolyte
Authors: Miyauchi Hibiki , Inaba Kohei , Takahashi Keitaro , Seki Shiro 
Published: July 2021

Introduction

Lithium-Sulfur (Li-S) batteries are attracting attention as high energy density rechargeable battery. Elemental sulfur (S8) has high theoretical capacity (1,672 mAhg-1) as positive electrode active material. However, S8 electrode is degraded by redox shuttle effect of lithium-polysulfide (Li2S
n
), which are generated during charge-discharge reactions. To solve this problem, many electrolyte designs were proposed. Sulfolane (SL) based high Li salt concentration electrolyte solution was reported as low solubility electrolyte of Li2S
n
[1]. The aim of this concept is chemically protection of Li2S
n
dissolution into electrolyte solution. Therefore, we proposed new polymer electrolyte, which protects chemically and physically dissolution and diffusion of Li2S
n
. In this study, we prepared SL-based solid gel polymer electrolytes (GPEs), ionic conductive property and time dependence of Li/GPEs interfacial resistance were evaluated.

Experimental

Sulfolane (SL), LiN(SO2CF3)2 (LiTFSA), and polyether-based P(EO/PO) macromonomer (EO/PO=8/2) were used as electrolyte materials for GPEs. First, electrolyte solution samples were prepared by mixing LiTFSA and SL at each molar ratio of SL : LiTFSA = x : 1 (x=1, 1.5 and 2, respectively). Next, prepared electrolyte solution and P(EO/PO) macromonomer were mixed at weight ratio of electrolyte solution is 70, 80, 90 wt%, respectively, and added DMPA as photo initiator. After that, these materials were radical polymerized by UV irradiation. Temperature dependence of ionic conductivity of prepared GPEs were investigated by AC impedance measurements (353～217 K). Impressed voltages were changed from 100mV to 500 mV under 258 K. Also, [Li | GPE | Li] symmetrical cells were assembled, and impedance measurements of these cells were carried out to investigation of Li/GPE interfacial resistance at 333 K.

Results & Discussion

Fig.1 shows temperature dependence of ionic conductivity (s) of GPEs. Conventional polyether-based solid electrolyte ([Li]/[O]=0.1: black circle) was showed for comparison. s of all GPEs indicated higher value compared with [Li]/[O]=0.1. s increased with electrolyte solution composition or decrease of Li salt composition. These should be related with mobility of polymer chain or microscopic viscosity of GPEs, like a T
gs. Also, s observed at around or less than T
gs by changing impressed voltage. At this area, temperature dependence was changed inconsecutively. Therefore, ionic transport mechanism changes with segmental motion of polymer chain might be occurred below T
g.

Fig.2 shows time dependence of Li/GPE interfacial resistance (R
Li) at 333 K. R
Li moderately increased over time, which wasn’t stabilized over 1000 h. (Measurement was interrupted temporary at broken line.) R
Li of general polyether based solid electrolyte reported as ca. 100 W cm2 [2],[3]. However, R
Li indicated quite low values of 30～40 W cm2 at 1000 h. Therefore, favorable SEI formation might be occurred between Li metal and GPE. In presentation, temperature dependence of R
Li and lithium cation transport number (t
Li+) will be discussed.

References

[1] A. Nakanishi et al., J. Phys. Chem. C, 123, 14229-14238 (2019).

[2] S. Seki et al., J. Power Sources,
146, 741-744 (2005).

[3] M. Kato et al., J. Electrochem. Soc.,
167, 070559 (2020).

Figure 1