Flame-retardant gel polymer electrolyte and interface for quasi-solid-state sodium ion batteries
Graphical abstract
Introduction
Sodium ion batteries (SIBs) have gained extensive interests due to the abundant sodium resources and achieved remarkable development in electrode materials [1], [2], [3], [4]. However, safety issues, such as fire and explosion hazards of the batteries when using conventional nonaqueous liquid electrolytes with high volatility and flammability hinder the application of SIBs in large-scale energy storage system. Replacing nonaqueous liquid electrolytes with solid state electrolytes is one of the effective strategies to address the safety issues [5], [6], [7]. Thereinto, polymer electrolytes developed since 1973 are favored by people because of superior flexibility, low interface impedance and easy device integration [8], [9], [10], [11], [12].
Mostly, polymer electrolyte membranes are formed by solution-casting or hot-pressing techniques. The former one uses volatile solvents to dissolve alkali salt and commercial polymer at first, and afterwards volatilize the toxic solvent such as acetonitrile and acetone for a homogeneous and dry membrane. And the latter one is carried out through mixing alkali salt and thermoplastic polymer by ball-milling, followed by compression molding [13], [14]. Both of above film forming methods are time-consuming and energy-consuming. Moreover, the utilization rate of active material is low in solid-state batteries with a sandwich configuration if the electrolyte membrane is ex-situ premade outside the battery. In recent years, the in-situ polymerization (photoinduced/thermal-cured) is well utilized due to the simple preparation process and as well as the ease of constructing a solid-state battery that integrates the electrode and electrolyte into a whole [15], [16], [17], [18]. Generally, the cross-linked copolymers consisting of conductive branched segments and rigid matrixes are applicable to the in-situ polymerization techniques, which makes a tradeoff between the ionic conductivity and mechanical strength [19]. As one of the most common monomers to prepare a crosslinked polymer, Poly(ethylene glycol) methyl ether methacrylate (PEGMA) exactly is equipped with ethoxy pendant groups for ion conduction and the branched conformation is helpful to inhibit crystallization [20], [21]. It is reported that a branched-graft copolymer-based free-standing electrolyte film synthesized by the co-polymerization (PEGMA, ethylene glycol dimethylacrylate (EGDMA) and methacrylate polyhedral oligomeric silsesquioxane (MA-POSS)) acquired an ionic conductivity of 1.6 × 10-4 S cm−1 at 60 °C [22]. Xue [23] et al. fabricated a flexible composite polymer electrolyte by polymerization of PEGMA and poly(ethylene glycol) diacrylate (PEGDA) under ultraviolet light irradiation, which delivered an ionic conductivity of 3.76 × 10-5 S cm−1 at 30 °C. The cross-linked polymer is generally tough but fragile. Besides, the low ionic conductivity of solid polymer electrolytes (SPEs) at room temperature is still difficult to satisfy practical requirement although unremitting efforts have been attached to increase the ionic conductivity.
Gelation of polymer electrolytes is the most direct and effective way to increase the ionic conductivity to a value higher than 10-4 S cm−1 [24], [25]. In past years, conventional carbonate solvents, such as ethylene carbonate [26], dimethyl carbonate [27], propylene carbonate [28], etc. and ionic liquids [29], [30], [31], [32], [33], [34], were applied to plasticize the polymer electrolytes, anticipating an increased ionic conductivity. However, either the flammable organic solvents or costly ionic liquids are inapplicable in large-scale energy storage system. Recently, nonflammable organic phosphates with low cost were chosen as solvents for electrolyte owing to their low viscosity, wide electrochemical window, and good solvating ability [35], [36], [37]. Cao [38] et al. dissolved NaClO4 in triethyl phosphate (TEP) solvent as a nonflammable electrolyte with a wide electrochemical window of 0–5 V (vs. Na/Na+) for SIBs, which exhibited outstanding battery performance. The excellent compatibility with the electrode materials and nonflammable property of phosphate-based electrolytes offer a good application prospect in secondary batteries.
Herein, in-situ gelation strategy is adopted to prepare a branched but free of cross-linking gel polymer electrolytes for QSIBs. In a typical preparation of the flexible gel polymer electrolytes (GPEs), a mixture of PEGMA monomer, initiator, TEP plasticizer and sodium salt were injected into a rigid and high absorptive glass fiber before polymerization. In addition to the high ionic conductivity at room temperature, stable interface between polymer and Na metal anode was constructed to guarantee a long cycling life. This gel polymer electrolyte shows a great potential in assembling high performance QSIBs that integrates the electrode and electrolyte into a whole with low interface impedance.
Section snippets
Preparation of electrolytes
Solid-state polymer electrolytes were prepared via in-situ thermal induced polymerization as depicted in Fig. 1. At first, NaTFSI was dissolved in poly(ethylene glycol methyl ether methacrylate) (PEGMA, Mn = 475 g mol−1) to prepare a NaTFSI solution with a molar concentration of 1 mol L-1. 2, 2-azoisobutyronitrile (AIBN, 2 mg mL−1) was dissolved in the NaTFSI solution and stirred at 25 °C for 3 h to make a clear and transparent precursor solution. Then, a given volume (100 uL, 150 uL, 200 uL,
Results and discussion
As shown in Fig. 1a, PEGMA with low molecular weight is a freely moveable liquid. After free radical polymerization via thermal cured, the CC bond of PEGMA is opened to form a solid but brittle branched polymer (PPEGMA) with ethoxy pendants. In order to get a self-supported and flexible electrolyte membrane, glass fiber membrane used in common liquid SIBs is selected as a matrix. In this way, the glass fiber loading precursor liquid was sealed in a coin cell to get a closed environment for free
Conclusion
Flame-retardant GPEs were prepared by in-situ thermal polymerization for quasi-state sodium ion batteries. The as-prepared flexible and self-supported GPE, PGT32-5%, delivers a high ionic conductivity of 9.1 × 10-4 S cm−1 at room temperature and a high oxidation degradation potential of 4.8 V vs. Na+/Na. A stable interface was built after addition of FEC to protect Na metal anode from the corrosion of TEP and residual PEGMA. And benefitting from the stable interface, the quasi-solid-state
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.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (Grant No. 21975026), and Beijing Natural Science Foundation (Grant No. L182056).
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Contributed equally to this work.