Microporous polymer electrolyte based on PVDF/PEO star polymer blends for lithium ion batteries
Graphical abstract
Introduction
With rapid development and widespread use of electronic equipments, the rechargeable lithium ion batteries have been widely used as energy supply and storage for portable electronic devices due to their high energy density [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. In general, the lithium ion battery electrolytes can be divided into solid electrolytes and liquid electrolytes. Compared with traditional liquid electrolytes, solid electrolytes have high safety, good chemical stability and adaption of geometry [12], [13], [14], [15], [16].
Since polymer electrolytes were proposed for the first time by Wright et al. [17], [18], various polymers have been studied as candidates for solid electrolytes, such as polyvinyl chloride (PVC) [16], [19], [20], [21], poly(methyl methacrylate) (PMMA) [16], [19], [22], poly(ethylene oxide) (PEO) [16], [19], [22], [23], polyacrylonitrile (PAN) [16], [19], [22] and poly(vinylidene fluoride) (PVDF) [16], [19], [22], [24], [25], [26], [27], [28]. However, the large-scale commercial usage for polymer electrolytes is suppressed by low ionic conductivity caused by inevitable partial crystalline structures formed by the long polymer chain packing at room temperature. How to achieve the balance between conductivity and mechanical properties of a solid polymer electrolyte (SPE) is still an undergoing difficulty. Many improved methods have been proposed to overcome this problem, for example, using single ionic conductive polymers as polymer electrolyte membrane to increase the carrier concentration; [29], [30], [31] preparing microporous polymer electrolytes (MPEs) to provide channels for ion transport; [32], [33] adding organic/inorganic nanoparticles to decrease crystallinity of the polymer membranes [34], [35], [36] and so on. Among them, MPEs have attracted public attentions by high conductivity and good chemical stability of practical applications at room temperature. Polyethylene (PE), [37] polypropylene (PP), [38] poly (vinyl alcohol) (PVA), [39] PAN, [40] and poly(MMA-co-AN) [41] have been studied for MPEs. PVDF is considered as a good candidate for MPEs because of its strong electron-withdrawing groups (C–F) and high dielectric constant (ε≈8.4), [42] which promotes the dissolution of lithium salt. Moreover, good flexibility and corrosion resistance from PVDF provide guarantees for the safety of lithium ion battery.
The leakage of flammable solvents is a big threat in liquid electrolytes. A good method to solve this issue is the use of low liquid leakage of electrolytes, meanwhile, the low liquid leakage of electrolytes could keep the battery perform better. But electrolyte leakage is still a severe situation for MPEs which are made up of pure PVDF. Qiu et al. [43] prepared MPEs by blending linear PEO with PVDF, in which linear PEO could improve the formation of the hole within PVDF backbone. Thus the ion transport was promoted, which led to a significant conductivity increase by absorbing more conductive electrolytes. Zhang et al. [44] prepared MPEs by blending block copolymers of PEO and PMMA with PVDF through phase inversion technique. PMMA had the affinity with PVDF, and PEO had the affinity with glycerin, both of which could improve the pore structure, connectivity and uptake of electrolyte, thus high ionic conductivity could be obtained at room temperature. Very recently, we used organogelator to prepare gelled MPEs, the organogelator was dissolved in electrolyte and turned into the solid-like gel when the temperature cooled to room temperature, which significantly reduced electrolyte leakage and retained relatively high ionic conductivity.
In this study, a serial of PEO-based star polymers with different chain length arms were synthesized via atom transfer radical polymerization (ATRP). The star polymer with several arms is harder to crystallize and is able to lead to fast molecular motion than linear polymer, which is conducive to ionic conduction. At the same time, the star structure can facilitate the hole formation of polymer electrolyte film by better dispersion of glycerin in the PVDF/PEO star polymer. The MPEs were then prepared with the blends of PEO-based star polymers and PVDF by the phase inversion technique, in which the pore porosity and connectivity were improved significantly. Thermal property, ionic conductivity, and electrochemical stability were also investigated to figure out the correlation between the microphase structure and the electrochemical properties. The results show great potential application in the rechargeable lithium ion batteries.
Section snippets
Materials
LiClO4 (Sinopharm Chemical Reagent Co., Ltd., China) was dried at 150 °C in vacuum oven for 24 h and stored in glove box before use. Ethylene carbonate (EC) and propyl carbonate (PC) were purified by distillation at reduced pressure. Divinyl benzene (DVB), anisole, tetrahydrofuran (THF) and triethylamine (TEA) were purified by distillation under reduced pressure. CuBr (99%, Aladdin), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 99%, Aladdin), α-bromoisobutyryl bromide (98%, Aladdin),
Results and discussion
The PEO star polymers were prepared via “arm-first” method by ATRP according to procedures in the previous reports (Scheme 1) [45], [47]. ATRP [50] was often used for the synthesis of well-defined polymers with controlled molecular weight. Moreover, the classic “arm-first” method based on cross-linking of a linear macroinitiator with a divinyl compound is an efficient way to synthesized star polymers containing multiple arms and functionalities. The polymerizations were carried out using
Conclusion
In summary, a novel MPE was prepared with PEO-based star polymer and PVDF by the phase inversion technique, and the special construction of star polymer could help the formation of pores, and promote the absorption of the electrolyte, the highest conductivity at room temperature attained 3.03×10−3 S cm−1 when the MPE prepared by star polymer with mPEG-2000 (MPE-3). The MPE-3 also showed a good compatibility with lithium metal electrode as well as a wide electrochemical stability window of 5.0 V
Acknowledgments:
We are grateful to the National Natural Science Foundation of China (Grant nos. 51473056, 51203055, 51210004, and 51433002) for support of this work. Additionally, the authors thank the Analytical and Testing Center of Huazhong University of Science and Technology for SEM measurement.
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