Optimized ion-conductive pathway in UV-cured solid polymer electrolytes for all-solid lithium/sodium ion batteries
Graphical abstract
Introduction
Lithium-ion batteries (LIBs) with their high energy density and large potential range have been studied for decades and are now the leading choice for industrial energy storage systems in electronic mobile devices and vehicles [[1], [2], [3], [4], [5], [6]]. LIBs are composed of electrodes (a cathode and an anode), an electrolyte, and a separator, and most studies focus on the electrodes because the electrodes determine the theoretical charging/discharging capacity of LIBs [[7], [8], [9], [10], [11]]. Non-aqueous electrolytes, composed of lithium salts and organic solvents, are commonly used in LIBs because of their fast ion conduction from the liquid phase. However, organic liquid electrolytes have serious drawbacks, such as leakage problems and flammability issues resulting from their volatility [[12], [13], [14]]. Thus, the development of substitutable electrolytes that are not explosive is essential for the reliable operation of LIBs.
Gel electrolytes, which contain polymeric materials for entrapping liquid electrolytes, are an alternative to conventional organic liquid electrolytes [15,16]. Poly (vinylidene fluoride-cohexafluoropropylene) (PVdF-HFP) copolymer, a representative gel electrolyte, has good entrapping properties and thermal/electrochemical stability for applications in LIBs [15,17,18]. Amorphous voids in the semicrystalline region of the HFP constituents in the PVdF-HFP contribute to its excellent entrapping ability, showing favorable interactions with organic liquid electrolytes, and this material could advance gel electrolytes and partially solve the leakage problem, improving the electrochemical characteristics of LIBs [19]. However, gel electrolytes involve a substantial amount of liquid and have poor mechanical durability, and thus safety concerns due to their volatility remain.
Solid polymer electrolytes with ion conductive properties have emerged recently as a strategy or avoiding these problems because solid polymer electrolytes have the advantages of reasonable flexibility and better interfacial compatibility with electrodes [[20], [21], [22]]. Polyethylene oxide (PEO) is a typical polymer for solid polymer electrolytes because its ethylene oxide (EO) unit has high chain flexibility and Li+ donor number, causing improved Li+ conduction [[23], [24], [25]]. However, there is a limit to how much PEO can be used in practical applications due to its low Li+ ionic conductivity because of its high crystallinity. To address this challenge [25], many studies on PEO have been reported, and their Li ion conduction has been improved by reducing the crystalline regions [26,27] and regulating PEO derivatives [28]. Another approach to preparing solid polymer electrolytes is the curing method by adding an external energy source such as UV irradiation to a monomer that possesses a cross-linking site. Various studies for functionalizing solid polymer electrolytes have involved the complexation of different polymers with a polymeric matrix, such as PEO and trimethylolpropane ethoxylate triacrylate (ETPTA), enabling strong linkages in the polymer electrolyte with tolerable solidity and great flexibility with the aid of a plasticizer [[29], [30], [31], [32], [33]]. In addition, inorganic materials can be utilized in solid polymer electrolytes to improve the solidity of the electrolyte to suppress side reactions from the interphase between the electrolyte and electrodes [34,35]. Nonetheless, these solid polymer electrolytes exhibit insufficient Li+ ionic conductivity for practical applications, and boosting Li ion conduction is required for solid-polymer electrolytes; solid-polymer electrolytes incorporated with Li+ solvated by gel polymers are thus an ideal solution. Li+ solvated by PVdF-HFP gel polymer, as mentioned above, can provide excellent ion pathways, and PVdF-HFP gel polymer electrolytes combined with a UV-curable polymer have been demonstrated by several reports [[36], [37], [38]]. However, PVdF-HFP gel polymers in UV-curable polymers still contain substantial fractions of liquid. Hence, further research on solid polymer electrolytes with minimal liquid contents is needed to bring these materials to the level of conventional liquid electrolyte systems.
In this study, the formation of an ion-conductive pathway in an ETPTA-based UV-curable polymer semi-interpenetrating polymer network (semi-IPN) matrix via a simple UV irradiation procedure was optimized. The UV-cured solid polymer electrolyte (USPE) consisted of a UV-curable hard matrix as a backbone film, and it showed negligible ionic conductivity, as well as an optimized ionic channel with Li+ solvated by gel polymers with minimal liquid content to boost Li+ conduction, as shown in Fig. 1. Note that the incorporated ions solvated by PVdF-HFP in the matrix may contain a small portion of liquid solvent, but after the curing step, no liquid spread out even after the strong squeezing process. The USPE with superior flexibility (Video S1) exhibited dramatically improved ionic conductivity via an additional ionic pathway for better Li+ conduction from the ions solvated by PVdF-HFP polymer, which penetrated the rigid cured ETPTA film. This ultraflexibility of the USPE can further enhance the interfacial properties of the electrode, leading to remarkable charging/discharging behavior under various conditions. Moreover, both the PVdF-HFP/ETPTA ratio and the HFP content in the PVdF-HFP are confirmed to be critical gel polymer additives for generating reinforced Li ion pathways in the USPE.
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Section snippets
Preparation of UV-cured solid polymer electrolyte
The precursor solution for USPE in LIB cell was prepared by mixing trimethylolpropane ethoxylate triacrylate (ETPTA, Sigma Aldrich) and 2-hydroxy-2-methylpropiophenone (HMPP, photoinitiator, Sigma Aldrich) with a liquid electrolyte (1 M LiPF6 in 1/1 ethylene carbonate (EC)/diethyl carbonate (DEC) by volume ratio, PuriEL, Soulbrain Co. Ltd., Korea). PVdF-HFP with different compositions (HFP contents: 0, 3, 6, 9, and 12%) was dissolved in the precursor solution as an additive at various weight
Results and discussion
The surface morphologies of the UV-cured solid polymer electrolytes (USPEs) with different ratios of PVdF-HFP in the ETPTA/PVdF-HFP mixture (note that the HFP content in PVdF-HFP is 6 wt%, hereafter PVdF-HFP-6) are shown in Fig. 2. Different weight ratios of PVdF-HFP in the ETPTA/PVdF-HFP-6 mixture from 0 wt% to 30 wt% were tested. To prepare free-standing USPE films, the precursor liquid solution was prepared by mixing ETPTA, PVdF-HFP-6, 2-hydroxy-2-methylpropiophenone (a photoinitiator) and a
Conclusion
In this work, we reported a solid polymer electrolyte with a supplementary ionic pathway via a UV-curing technique for polymerization. PVdF-HFP as an ion-solvated gel electrolyte that allows the surface of the USPE to be smoother with fewer grain boundaries, which improves its compatibility with electrodes. In addition, USPE with additional ionic pathways via the ion-solvated PVdF-HFP gel electrolyte could show improved electrochemical characteristics, leading to improved charging/discharging
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 was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (2018M3D1A1058624, 2019R1A2C3010479).
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