Research PaperComb-like solid polymer electrolyte based on polyethylene glycol-grafted sulfonated polyether ether ketone
Introduction
Lithium ion batteries (LIBs) have a higher energy density, higher open-circuit voltage, and longer lifetime compared with other rechargeable systems such as nickel-cadmium and nickel-metal hydride batteries. Therefore, LIBs are considered as one of the most promising power systems [1], [2]. As one of the critical components, the electrolyte plays a vital role in preparing safe and high-performance LIBs. In most of the current commercial LIBs, liquid electrolytes based on lithium salts containing volatile organic solvents and separators have been widely used because of their high ionic conductivity and low electrode–electrolyte contact impedance. However, performance decay and safety problems of liquid electrolytes, including leakage, explosion, and flammability, present obstacles for the large-scale commercialization of LIBs [3], [4], [5]. Therefore, seeking alternatives to liquid electrolytes has been the major focus in LIB research in recent decades.
Solid polymer electrolytes (SPEs) with excellent safety, good mechanical performance, and shape flexibility have drawn considerable attention as important alternatives to liquid electrolytes [6], [7], [8]. Since Wright et al. [9], [10] firstly proved the ionic conduction ability in polyethylene oxide (PEO)-lithium ion complexes, the use of many polymers, such as PEO [11], [12], [13], polypropylene oxide (PPO) [14], [15] and polyethyleneimine (PEI) [16], as SPEs has been widely studied. As the earliest and most extensively studied systems, PEO-based SPEs stand out for their strong Li+ solvating ability, good chemical stability, wide electrochemical window, excellent film-forming property, and toughness [13]. In PEO-based electrolytes, lithium ions are mainly conducted through the ether oxygen atoms, which can act as coordination sites and thus promote the association/dissociation of lithium salts. However, although PEO-based systems show relatively high ionic conductivities at high temperatures, the crystallization of PEO at low temperatures severely hinders the ion transport through the electrolyte system because the ion conduction mainly occurs in the amorphous phase of PEO [17], [18], [19]. Furthermore, the preference for crystallization, especially for linear PEO, may result from the long polymer chain packing of PEO during usage, thus decreasing the ionic conductivity. Hence, finding ways to reduce the crystallinity, thus to improve the ionic conductivity, while maintaining the appropriate mechanical properties, is still a key issue for the practical application of PEO-based SPEs.
Many strategies, including blending [20], [21], modifying [22], [23], and preparation of PEO derivatives [24], [25], have been utilized to solve the above problems of PEO. Among these approaches, fabricating graft copolymers by introducing PEO side chains can effectively decrease the crystallinity of PEO. Zuckermann et al. [26] prepared three new comb-like peptoid polymers with ethylene oxide (EO)n side chains of varying lengths. In these copolymers, polypeptoids with longer (EO)n side chains exhibited rapid segmental motion, and the maximum conductivity could reach 2.6 × 10−4 S cm−1 at 100 °C. Recently, they reported the follow-up work on the structure–conductivity relationships of two types of ethyleneoxy-containing block copolypeptoids pNeh-b-pNte (poly[N-(2-ethyl)hexylglycine]-b-poly[N-2-(2-(2-methoxyethoxy)ethoxy)ethylglycine]) and pNdc-b-pNte (poly[N-decylglycine]-b-poly[N-2-(2-(2-methoxyethoxy)-ethoxy)ethylglycine]) [27]. These two copolymers have the same conducting block, pNte-containing EO side chains, but different non-conducting blocks, amorphous pNeh and crystalline pNdc, respectively. It was observed that the same conducting block exhibited different crystallization behaviors and thus resulted in different ionic conductivities within the two copolymers, providing new ways for tailoring the ionic transport properties based on these PEO copolymer electrolytes. In addition, star-shaped [28], [29] and comb-like [30] copolymers with flexible PEG or PEO branches are also a favorable solution to decrease the crystallinity of PEO. Niitani et al. [28] synthesized star-shaped poly[styrene]-b-poly[poly(ethylene glycol) methyl ethyl methacrylate] copolymer, which possesses the characteristics of a SPE for an application in LIBs. This SPE system showed high ionic conductivity and improved LIB performance. Zaghib et al. [30] demonstrated another possible candidate for SPEs based on the comb-like copolymers poly(styrene-co-4-vinylanisole) (PS–VA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA). The ionic conductivities of these copolymers can be higher than 10−5 S cm−1 at room temperature depending on their PEGMA/PS–VA ratios.
Recently, we also synthesized PEO-based star polymers with arms of different chain lengths through atom transfer radical polymerization [31]. These star polymers were blended with PVDF to fabricate microporous polymer electrolytes. This star structure can effectively destroy the crystallization of PEO and lead to fast molecule motion. The blended microporous membranes exhibited good pore distribution and connectivity, which resulted in a significantly improved ionic conductivity. Based on this, we designed and synthesized herein the novel comb-like copolymer SPEEK-g-PEG through an epoxide ring opening reaction between partially hydroxylated SPEEK and PEG-epoxide. The PEG side chains in this comb-like copolymer impeded the crystallization, while the highly rigid SPEEK main chain ensured high thermal stability and mechanical strength of the copolymer. We fully characterized structure and crystallization behavior of the SPEEK-g-PEG copolymer and evaluated the effects of temperature and PEG grafting ratio on the SPEEK-g-PEG SPEs.
Section snippets
Materials
Polyetheretherketone (PEEK, 450PF, MW = 38,200) was obtained from Victrex Inc. (England). Methoxypolyethylene glycol (PEG, MW = 350 and 550, Aladdin Reagent Inc., China) was used as received. LiClO4 (Sinopharm Chemical Reagent Co. Ltd., China) was dried at 150 °C under vacuum for 24 h and stored in an argon-filled glove box before use. Concentrated sulfuric acid, sodium borohydride (NaBH4), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), epichlorohydrin, triethanolamine, triphenylphosphine
Synthesis and Characterization of SPEEK-g-PEG copolymer
Preparation of comb-like graft copolymers is an effective way to inhibit the crystallization of PEO due to their looser aggregation structure when compared with linear PEO [32]. Here, we chose SPEEK as the main chain of the comb-like copolymer due to the following reasons: (1) PEEK is an excellent engineering polymer possessing high chemical resistance, thermal stability, mechanical strength, and toughness, which make it suitable for applications in the electronic industries, membrane
Conclusion
In order to effectively inhibit the crystallization of PEG, the novel comb-like copolymer SPEEK-g-PEG was synthesized and its possible application as polymer electrolyte in LIBs was evaluated. FTIR and 1H NMR results confirmed the successful grafting of PEG onto the SPEEK main chain. TGA and DSC investigations showed that the thermal stability of SPEEK-g-PEG meets the requirements of LIBs, while it was mostly in an amorphous state at room temperature. All polymer electrolyte membranes prepared
Acknowledgments
The authors are grateful to the National Natural Science Foundation of China (Grant Nos. 51622303, 51473056, and 51703080) and Fundamental Research Funds for the Central Universities (2016JCTD112) for support of this work. The authors also gratefully acknowledge the Analytical and Testing Center of HUST for the use of measurement facilities.
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These authors contributed equally to this work.