Preparation and Evaluation of Polymer Electrolytes with Low-Solubility of Lithium-Polysulfide
Authors: Miyauchi Hibiki , Inaba Kohei , Takahashi Keitaro , Seki Shiro 
Published: December 2020

Background

Lithium-Sulfur (Li-S) batteries are attracting attention as high energy density rechargeable battery. Elemental sulfur (S8) have 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), and thermal and ionic conductive properties of the samples were evaluated. Furthermore, Li-S batteries containing GPEs were prepared and evaluated by charge-discharge measurements.

Experimental

Sulfolane (SL), LiN(SO2CF3)2 (LiTFSA), and polyether-based P(EO/PO) macromonomer 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 each weight ratio of electrolyte solution : polymer = y : (10-y) (y=7, 8 and 9, respectively), and added DMPA as photo initiator. After that, these materials were radical polymerized by UV irradiation. In this research, GPEs are expressed as (x),yEl+(10-y)Po. Thermal property and ionic conductivity of prepared GPEs were investigated by TG-DTA, DSC and AC impedance measurements. Coin cells were assembled by Li metal negative electrode, GPE ((1.5),8El+2Po) and S8-based positive electrode. At the time, small amount of electrolyte solution (x=1.5) were dropped onto S8 based positive electrode. Assembled cells were evaluated by constant current charge-discharge tests at 303 K, and current density was set C/18.

Results & Discussion

Fig.1 shows ionic conductivity of prepared GPE samples. All GPEs exhibited high ionic conductivity (σ) compared withgenuine solid polymer electrolyte (black circle) at all measured temperature. From results, σ increased with decrease of Li salt concentration (inverse of x). Also, σ increased with electrolyte solution composition at low temperature region. Especially, lowest polymer composition (y=9) samples exhibited specific behavior. At x=2 and 1.5, lowest polymer composition samples exhibited higher σ compared with other samples at 303 K. On the other hands, samples were contrary tendency at x=1. Therefore, ionic transport mechanism should be affected by Li salt concentration and electrolyte solution amount into GPE.

Fig.2 shows charge and discharge profiles of prepared cell until 100 cycles. In the case of discharge curves, 1st plateau and 2nd plateau were observed at between 2.2 to 2.3 V and 2.0 to 2.1 V respectively. Therefore, appropriate reaction between S8 and Li+ should be occurred. Initial discharge capacity indicated 1,146 mAhg-1. Charge and discharge capacities decreased until 20 cycles, and the capacities stabilized approximately 520 mAhg-1. Coulombic efficiency also exhibited remarkable behavior, which indicated 99.7 % at 100 cycles. Therefore, sulfolane based GPE may restrict dissolution of Li2S
x
and improve performance of Li-S batteries.

Reference

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

Figure 1