The polymerization capability of alkenyl phosphates and application as gel copolymer electrolytes for lithium ion batteries with high flame-retardancy
Graphical abstract
Introduction
The resent decades have witnessed tremendous progress in the field of electrochemical energy storage. Lithium ion batteries (LIBs) have attracted the most attention due to their wide range of applications such as portable electronic devices, electrical vehicles and stationary power grid storage [1,2]. There is no doubt that LIBs bring great changes and convenience to people's lives, but at the same time, the safety problems are still a significant challenge and become more rigorous especially for the increasing development of power LIBs with a higher energy density [3,4]. When it comes to the safety issue of LIBs, it is widely considered to be closely related to the internal components including cathode material, anode material, separator and electrolyte. Among these, electrolyte is the blood of LIBs and plays a decisive role on the safety performance [5,6]. It is known that the use of traditional organic liquid electrolyte consisting of carbonate solvents with volatility and relatively low flashing point has brought risks of leakage and combustion. Therefore, it can be considered that the flammable electrolyte is the key factor affecting thermal stability of the whole LIBs system. For the past decades, a great deal of research works on the development of safer electrolytes including adding electrolyte additives, adopting nonflammable electrolyte solvents, synthesizing polymer and solid state electrolytes spring up [[7], [8], [9]].
Organic phosphorus-containing solvents especially for the alkyl phosphates have been used with organic carbonates to form flame-retarding or nonflammable electrolytes [[10], [11]]. The phosphorus flame retardants can form a dense carbon layer on the substrate surface to protect materials from further degradation. Wang et al. [11] reported that a concentrated electrolyte using a popular flame-retardant solvent of trimethyl phosphate allowed stable charge−discharge cycling with negligible degradation in cell performance. Cao et al. [12] also achieved great stability by using nonflammable electrolytes with high salt-to-solvent ratios in lithium batteries. Zhang et al. reported a promising approach by formulating a nonflammable localized high-concentration electrolyte based on triethyl phosphate, ethylene carbonate, and a partially fluorinated ether. This electrolyte readily generates a robust solid electrolyte interphase on the electrode and shows better cycling performance [13].
Compared with traditional liquid electrolytes, gel polymer electrolytes (GPEs) [14,15] and solid-state electrolytes [16,17] are highly preferred due to their wide electrochemical window and capability to tackle the safety issue by reducing flammability and improving stability. Effective and environment-friendly fire-retardant polymer electrolytes are key materials for safer operation of lithium batteries. Polymer electrolytes systems with fire-retardant polymer matrixes have been investigated in only few cases. The safe and non-flammable phosphorus-based polymer electrolytes composed of phosphate were used in lithium batteries [[18], [19], [20]], but the ionic conductivity of the such solid polymer electrolytes is very low at room temperature (10−5–10−8 S cm−1). In comparison with solid polymer electrolytes, GPEs combine the advantages of both the liquid and solid components and possess higher ionic conductivity at room temperature [21]. In GPEs, the liquid electrolyte is immobilized in a polymeric matrix and can reduce the risk of leakage compared to commercial separator. Furthermore, in order to improve the safety of the battery, the GPEs with higher safety properties is also important for high-energy-density LIBs. Therefore, the nonflammable GPEs have become one of the most desirable alternatives among various electrolytes for the fabrication of advanced LIBs. In consideration of the excellent flame-retarding property and the wide application in safe electrolytes, the phosphorus containing gel polymer electrolytes were designed and prepared in rare case. Kim et al. synthesized the tris(2-(acryloyloxy)ethyl) phosphate based cross-linking gel polymer electrolytes and used in lithium ion batteries [22]. Our team reported two novel phosphonate cross-linkers based gel copolymer electrolytes which were used for sodium ion batteries and showed excellent long term cycle performance [23,24]. Herein we report an alkenyl phosphate based gel polymer electrolyte with high flame retardancy for lithium ion battery, which was prepared by in situ thermal polymerization with commercial cross-linker. The polymerization capacity and cross-linking degree of the polymer matrix of GPE are crucial to the performance of GPE battery. Therefore, the polymerization capability of different structural alkenyl phosphonate/phosphates was also investigated and the selective phosphate was used as preferable monomer to form safer GPE for lithium ion battery.
Section snippets
Materials and intermediates
Allyl bromide, triethylphosphite, 2-hydroxyethyl acrylate, methyl methacrylate (MMA), ethyl acrylate (EA), styrene, 2,2-azobisisobutyronitrile (AIBN) and sulfuryl chloride were purchased from Shanghai Aladdin Bio-chem Technology Co. LTD. Allyl alcohol, triethylene glycol dimethacrylate (TEGDMA), 1,4-butanediol dimethacrylate (BDDMA) and pentaerythritol tetraacrylate (PET4A) were supplied by Yantai YK Chemical Engineering Co., Ltd. Commercial MMA, EA, styrene, TEGDMA, BDDMA and PET4A with double
The polymerization capability of alkenyl phosphonate/phosphates
The alkenyl phosphonate of DEAP, alkenyl phosphates of ADEP, AEDEP and MADBP were prepared (Scheme 2) and characterized by 1H NMR, 13C NMR, 31P NMR, FT-IR and HRMS. The radical thermal polymerizations of these alkenyl phosphonate/phosphates were studied and the molecular weights of the homopolymers and copolymers of these compounds were tested and listed in Table 1. The DEAP and ADEP have poor homopolymerization ability with the lower molecular weight less than 5000, while the AEDEP has good
Conclusion
The polymers of different alkenyl phosphonate/phosphates were synthesized and the polymerization capabilities were investigated and compared. The (acryloyloxyethyl) dialkyl phosphates have good polymerization ability and tend to form alternative copolymers with other polymerizable monomers. The (acryloyloxyethyl) dialkyl phosphates containing polymers have good thermal stability and excellent flame retardant. The GPEs containing MADBP have good stability, high conductivity, good electrochemical
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgements
The authors greatly acknowledge the financial support from the National Natural Science Foundation of China (No. 21771164) and (No. U1804129).
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