A recent article published in the Polymer Journal presented research on enhancing the mechanical properties of gel electrolytes and their application in lithium (Li) secondary batteries. The study focused on the chemical and physical synthesis of self-healing ion gels using ionic liquids (ILs) as electrolytes.
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Background
Gel electrolytes are soft materials that consist of a polymeric network swollen with an ion-conducting electrolyte. They can achieve quasi-solid states while retaining the electrolyte's electrochemical properties, providing stability and robustness. As a result, gel electrolytes are used in various electrochemical devices, including secondary batteries, capacitors, sensors, and actuators.
Ion gel electrolytes, which are gel electrolytes infused with ILs, function as room-temperature molten salts with desirable physicochemical properties such as nonflammability, nonvolatility, and high chemical and thermal stability. These are highly advantageous in electrochemical devices. Their self-healing properties are added advantages.
Self-healing polymeric materials demonstrate high durability through the spontaneous repair of mechanical damage. However, integrating these materials into wearable and flexible devices while maintaining mechanical strength and preserving the unique properties of ILs poses challenges. Consequently, various physical and chemical methods are being explored to design self-healing ion gels.
Chemical Synthesis Methods
Self-healing ion gels are generally prepared through supramolecular and dynamic covalent chemistry. For instance, an ion gel with high mechanical strength, self-standing ability, and rapid self-healing characteristics was created using multiple hydrogen bonds and a nanophase-separated block copolymer structure.
This ion gel, which comprises a jammed micellar network, demonstrated significantly improved mechanical strength and self-standing capability compared to those prepared solely with hydrogen-bonded copolymers. The hydrogen-bonded micellar structure facilitated rapid self-healing (within hours) at room temperature without external stimuli.
Slight variations in the anion and cation structures of the IL significantly influence the mechanical properties of ion gels. For example, an ion gel made with various ILs, such as polystyrene (PS), poly(N, N-dimethylacrylamider-acrylic acid) (P(DMAAm-r-AAc)), 1-ethyl-2,3- dimethylimidazolium ([C2mim]), and bis(trifluoromethanesulfonyl) imide ([TFSI]), exhibited no self-healing. This was ascribed to the low hydrogen-bond-donating capability of the [C2dmim] cation.
In contrast, replacing [TFSI] anions with methyl phosphonate anions, which possess strong hydrogen-bond-accepting characteristics, significantly reduced the ion gel’s storage modulus. Thus, regulating the competitive hydrogen bonding between anions, cations, and polymer chains is critical for achieving the desired mechanical properties of ion gels.
Physical Synthesis Methods
Synthesis of self-healing polymers through physical methods, such as polymer elasticity or chain entanglement, is challenging. Controlling the chemical crosslink density of hydrogels around the sol-gel transition point can enable self-healing behavior via the entanglement of dangling chains; however, this approach can compromise the mechanical strength of the hydrogels.
Conversely, while excessive physical chain entanglement enhances mechanical strength, it can reduce self-healing functionality due to longer relaxation times. Therefore, addressing the trade-off between self-healing capability and mechanical strength is a key challenge in developing self-healing polymeric materials through physical methods.
An ultrahigh-molecular-weight (UHMW) ion gel developed via the physical entanglement of poly(methyl methacrylate) polymers demonstrated exceptional mechanical properties and self-healing behavior.
Notably, radical polymerization of the methacrylate monomers in the IL at low initiator concentrations yielded UHMW polymers with nearly 100 % monomer conversion. In contrast, the monomer conversion was significantly lower at low initiator concentrations when a conventional organic solvent, toluene, was used, preventing the formation of UHMW polymers.
Application in Li-Secondary Batteries
Li metal is a promising anode material for next-generation high energy density Li secondary batteries due to its high theoretical capacity and low working potential. However, its high reactivity causes dendrite growth and the formation of dead Li during charging and discharging, raising battery safety concerns.
Efforts to control the Li dissolution-deposition process have led to the development of new electrolytes, functional separators, and artificial interphases. These efforts have particularly focused on suppressing Li dendrites and dead Li using self-healing polymeric materials with high toughness.
A solvated ionic liquid made through the regulation of competitive hydrogen bonding between a highly concentrated electrolyte and a hydrogen-bonded polymer exhibits excellent toughness. This highly stretchable concentrated gel electrolyte exhibits unique physicochemical properties, similar to ILs, attributed to few free solvent molecules in the electrolyte.
Such an electrolyte forms a stable solid-electrolyte interphase with the Li metal anode. Moreover, these mechanically functional gel electrolytes improve the cycling performance of next-generation high-capacity anodes, such as Li metal and silicon anodes.
Conclusion and Future Prospects
Polymeric materials are promising for next-generation Li secondary batteries. The tough and stretchable characteristics of gel electrolytes are expected to facilitate their application in flexible batteries, which are vital for Internet-of-Things devices.
However, current research on gel electrolytes is still in its infancy, facing several challenges that must be addressed before practical applications in battery systems. Foremost, careful evaluation and optimization of the electrolytes' thickness and quantity are necessary for improved battery performance. Additionally, integrating suitable cathode materials is crucial for enhancing overall battery efficiency.
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Journal Reference
Tamate, R. (2024). Development of functional polymer gel electrolytes and their application in next-generation lithium secondary batteries. Polymer Journal. DOI: 10.1038/s41428-024-00969-8, https://www.nature.com/articles/s41428-024-00969-8
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