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