Elsevier

Solid State Ionics

Volume 361, March 2021, 115563
Solid State Ionics

Synthesis and electrochemical properties of self-doped solid polymer electrolyte based on lithium 4-styrene sulfonate with BF3-THF

https://doi.org/10.1016/j.ssi.2021.115563Get rights and content

Highlights

  • The crystallite size and melting temperature of polymer electrolyte decreased after BF3-THF addition.

  • The ionic conductivity increased 15 times after BF3-THF was added.

  • The electrochemical stability up to 4.5 V was observed.

  • The lithium ion transference number decreased from 0.9 to 0.59 after addition of BF3-THF.

Abstract

Poly(lithium 4-styrene sulfonate)-based self-doped solid polymer electrolytes were prepared by radical polymerization and ion-exchange reactions of sodium 4-styrene sulfonate and oligo(ethylene oxide) methyl ether methacrylate. The crystalline melting temperature of ethylene oxide was reduced by increasing the lithium-ion concentration, which resulted in a reduction in the size of the crystalline domains due to the improved coordination between lithium ions and oxygen. Alternating current (AC) impedance measurements showed that the ionic conductivity was 15 times higher after the introduction of boron trifluoride-tetrahydrofuran (BF3-THF), and up to 1.22×10−5 S cm−1 was achieved at 25 °C in all the solid-state self-doped polymer electrolytes. Furthermore, although some oxidation current was observed due to the reaction with lithium metal after the addition of BF3-THF, electrochemical stability up to 4.5 V was obtained. For the same reason, the lithium-ion transference number decreased from 0.9 to 0.59 after the addition of BF3-THF. However, since the anion is immobilized in the polymer chain, it can be expected as a single-ion conductor regardless of the addition of BF3-THF.

Introduction

Over the past three decades, many self-doped polymer electrolytes (SDPEs) have been developed for lithium-ion battery applications [[1], [2], [3], [4]]. Their advantage is that they have good electrochemical stability and a high lithium-ion transference number because the reactive anions are immobilized in the polymer chains or the anion mobility is very limited through the polymer matrix [[5], [6], [7]]. In particular, electrochemical stability has become very important in recent studies aimed at increasing the operating voltage above 4.2 V to develop high-energy-density lithium-ion batteries [[8], [9], [10], [11]]. Similarly, it is expected that the anion-immobilized SDPE will not have a concentration gradient of lithium salts that may occur in the salt-doped polymer electrolyte system during the charge-discharge cycles [12,13]. SDPEs are generally obtained by the polymerization of monomers containing lithium salts, such as lithium methacrylate [14,15], lithium acrylate [16], lithium 4-styrene sulfonate [17], and lithium acrylamido sulfonate [18,19]. However, SDPEs usually exhibit significantly low ionic conductivity because the anions are immobilized through the polymer backbone chain and thus cannot contribute to the total ion conduction properties. In addition, the formation of a strong bond between the lithium cation and the counter anion may further reduce the ionic conductivity. Fortunately, improved ionic conductivity in SDPEs has been achieved by adding Lewis acids, such as BF3 or AlCl3 [[20], [21], [22], [23]]. Reducing the interaction between ions through complexation of anions and Lewis acids improved the dissociation of lithium cations, resulting in relatively high ionic conductivity at room temperature. Nevertheless, the demand for new SDPE development remains a strong challenge. Indeed, it must have an ionic conductivity of at least 1×10−5 S cm−1 at room temperature for commercial applications in lithium-ion battery industries [2]. For that reason, detailed studies on the chemical composition and lithium salt concentrations are required for preparing SDPEs with electrochemical stability up to 4.5 V and ionic conductivity above 1×10−5 S cm−1 at room temperature.

In this paper, we described the preparation of poly[(lithium 4-styrene sulfonate)-co-(ethylene oxide)methyl ether methacrylate)], poly(LSS-co-POEM) with different compositions. Subsequently, the effects of BF3-THF incorporation on the thermal and electrochemical properties were described. The electrochemical properties of the obtained polymer electrolytes were investigated in terms of ionic conductivity, electrochemical stability, and lithium-ion transference number measurement. Due to the optimized composition of the polymer electrolyte and the Lewis acid incorporation, a clear increase in the ionic conductivity was observed in the poly(lithium 4-styrene sulfonate)-based SDPE.

Section snippets

Materials

Sodium 4-styrene sulfonate (SSS), poly(ethylene oxide) methyl ether methacrylate (POEM, Mn: 1100 g mol−1), boron trifluoride-tetrahydrofuran (BF3-THF) complex, potassium persulfate, lithium hydroxide monohydrate (LiOH·H2O), and THF were purchased from Aldrich and used without further purification. Dimethyl sulfoxide (DMSO) and n-hexane, used as a solvent, were kindly supplied by Samchun chemical company.

Preparation of polymer electrolytes

Poly[(sodium 4-styrene sulfonate)-co-(ethylene oxide)methyl ether methacrylate)], poly(SSS-co

Synthesis of polymer electrolytes

Poly(LSS-co-POEM)s were synthesized by the radical copolymerization of POEM and SSS monomers and the subsequent ion-exchange reaction of sodium sulfonate with LiOH·H2O in DMSO as illustrated in Fig. 1. Subsequently, poly(LSS-co-POEM) with BF3-THF was prepared by the inclusion of the corresponding BF3-THF complex. Fig. 2 shows the FT-IR spectra for PSSS, POEM homopolymer, poly(SSS-co-POEM), poly (LSS-co-POEM), and poly(LSS-co-POEM) with BF3-THF. The PSSS and POEM homopolymers were additionally

Conclusions

In this study, poly(LSS-co-POEM) with various compositions was synthesized by radical polymerization and ion-exchange reaction to evaluate the effect of the lithium-ion concentration on the physical and electrochemical properties. As the lithium-ion concentration decreased, the crystalline size increased as shown by XRD, and consequently, the crystalline melting temperature of the POEM increased. The introduction of BF3-THF into the solid polymer electrolyte reduced the melting temperature of

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References (36)

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