Gel Polymer Electrolytes Based on Cross-Linked Poly(ethylene glycol) Diacrylate for Calcium-Ion Conduction

Complex Hydride-Based Gel Polymer Electrolytes for Rechargeable Ca-Metal Batteries

Rechargeable Ca batteries offer the advantages of high energy density, low cost, and earth-abundant constituents, presenting a viable alternative to lithium-ion batteries. However, using polymer electrolytes in practical Ca batteries is not often reported, despite its potential to prevent leakage and preserve battery flexibility. Herein, a Ca(BH4)2-based gel-polymer electrolyte (GPE) is prepared from Ca(BH4)2 and poly(tetrahydrofuran) (pTHF) and tested its performance in Ca batteries. The electrolyte demonstrates excellent stability against Ca-metal anodes and high ionic conductivity. The results of infrared spectroscopy and 1H and 11B NMR indicate that the terminal ─OH groups of pTHF reacted with BH4 - anions to form B─H─(pTHF)3 moieties, achieving cross-linking and solidification. Cyclic voltammetry measurements indicate the occurrence of reversible Ca plating/stripping. To improve the performance at high current densities, the GPE is supplemented with LiBH4 to achieve a lower overpotential in the Ca plating/stripping process. An all-solid-state Ca-metal battery with a dual-cation (Ca2+ and Li+) GPE, a Ca-metal anode, and a Li4Ti5O12 cathode sustained >200 cycles, confirming their feasibility. The results pave the way for further developing lithium salt-free Ca batteries by developing electrolyte salts with high oxidation stability and optimal electrochemical properties.

A sodium ion conducting gel polymer electrolyte with counterbalance between 1-ethyl-3-methylimidazolium tetrafluoroborate and tetra ethylene glycol dimethyl ether for electrochemical applications

For sodium batteries, the development of gel polymer electrolytes (GPEs) with remarkable electrochemical properties is in its early stage and persists to be a challenge. In this report we have synthesized a series of GPEs containing a poly(vinyllidene fluoride-co-hexafluoropropylene) (PVdF-HFP) and poly (methyl methacrylate) (PMMA) as blend polymer, sodium perchlorate (NaClO4) as ion-conducting salt and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) and tetra ethylene glycol dimethyl ether (TEGDME) as molecular solvents. The counter balance between EMIM-BF4 and TEGDME is maintained by the electrolyte, which is formed through the optimal weight ratio of 2 : 1. GPEs have an advantageous set of properties, including stability window of 5 V, Na+ transference number of 0.20, and a room-temperature ionic conductivity of 5.8 × 10-3 S cm-1. According to enthalpy and entropy calculations, optimized GPE yields the highest amount of disorder or amorphicity and contributes to greatest conductivity. XRD analysis supports this argument. Thermal investigations show that optimized GPE may preserve gel phase up to 125 °C. The prototype sodium cell fabricated with optimize GPE has a specific capacity of 281 mA h g-1 and open circuit voltage of 2.5 V. The optimized GPE exhibits potential for future electrochemical applications.

Polyvinylidene Fluoride-Based Gel Polymer Electrolytes for Calcium Ion Conduction: A Study of the Influence of Salt Concentration and Drying Temperature on Coordination Environment and Ionic Conductivity

Calcium-ion batteries emerged as a potential sustainable alternative energy storage system; however, there remains the need to further develop electrolytes to improve their performance. We report a gel polymer electrolyte (GPE)-based on polyvinylidene fluoride (PVDF) for calcium ion conduction. The gel electrolyte was synthesized by combining a PVDF polymer host, Ca(TFSI)2 salt, and N-methyl-2-pyrrolidone (NMP) solvent. Using Fourier transform infrared spectroscopy, we analyze the effect of salt concentration and drying temperature on the degree of salt dissociation in the electrolyte. Our results show that the concentration of free cations in the electrolyte is primarily coordinated with NMP as well as PVDF, generating a suitable network for ion transport, i.e., a liquid electrolyte encompassed within a polymer matrix. We find that processing conditions such as drying temperature, which varies solvent content, play a critical role in developing polymer electrolytes that demonstrate optimal electrochemical performance. The GPEs are semicrystalline and stable up to 120 °C, which is critical for their use in applications such as in electric vehicles and renewable energy storage systems. The ionic conductivity of the GPEs exhibit Arrhenius-type behavior, and the total ionic conductivity at room temperature is suitable for applications, with values of 0.85 × 10-4 S/cm for 0.5 M and 3.56 × 10-4 S/cm for 1.0 M concentrations. The results indicate that the GPE exhibits high conductivity and good stability, making it a promising candidate for use in high-performance calcium ion batteries.

A Review of the Relationship between Gel Polymer Electrolytes and Solid Electrolyte Interfaces in Lithium Metal Batteries

Lithium metal batteries (LMBs) are a dazzling star in electrochemical energy storage thanks to their high energy density and low redox potential. However, LMBs have a deadly lithium dendrite problem. Among the various methods for inhibiting lithium dendrites, gel polymer electrolytes (GPEs) possess the advantages of good interfacial compatibility, similar ionic conductivity to liquid electrolytes, and better interfacial tension. In recent years, there have been many reviews of GPEs, but few papers discussed the relationship between GPEs and solid electrolyte interfaces (SEIs). In this review, the mechanisms and advantages of GPEs in inhibiting lithium dendrites are first reviewed. Then, the relationship between GPEs and SEIs is examined. In addition, the effects of GPE preparation methods, plasticizer selections, polymer substrates, and additives on the SEI layer are summarized. Finally, the challenges of using GPEs and SEIs in dendrite suppression are listed and a perspective on GPEs and SEIs is considered.

Vinylimidazole-Based Polymer Electrolytes with Superior Conductivity and Promising Electrochemical Performance for Calcium Batteries

Calcium batteries are next-generation energy storage technologies with promising techno-economic benefits. However, performance bottlenecks associated with conventional electrolytes with oxygen-based coordination chemistries must be overcome to enable faster cation transport. Here, we report an imidazole-based polymer electrolyte with the highest reported conductivity and promising electrochemical properties. The polymerization of vinylimidazole in the presence of calcium bis(trifluoromethanesulfonyl)imide (Ca(TFSI)₂) salt creates a gel electrolyte comprising a polyvinyl imidazole (PVIm) host infused with vinylimidazole liquid. Calcium ions effectively coordinate with imidazole groups, and the electrolytes present room temperature conductivities of >1 mS/cm. Reversible redox activity in symmetric Ca cells is demonstrated at 2 V overpotentials, stable cycles at 0.1 mA/cm², and areal capacities of 0.1 mAh/cm². Softer coordination, polarizability, and closer coordinating site distances of the imidazole groups can explain the enhanced properties. Hence, imidazole is a suitable chemical benchmark for the future design and advancement of polymer electrolytes for calcium batteries.

Comonomer-Tuned Gel Electrolyte Enables Ultralong Cycle Life of Solid-State Lithium Metal Batteries

Rechargeable lithium metal batteries (LMBs) are considered the "holy grail" of energy storage systems. Unfortunately, uncontrollable dendritic lithium growth inherent in these batteries has prevented their practical applications. The benefits of solid-state electrolyte for LMBs are limited due to the common compromise between ionic conductivity and mechanical property. This work proposes a mechanism for simultaneous improvement in ionic conductivity and mechanical strength of gel polymer electrolyte (GPE) which is based on tunable cross-linked polymer network through adjusting monomer ratios. With increasing bisphenol A ethoxylate dimethacrylate (E2BADMA) and poly(ethylene glycol) diacrylate (PEGDA) mass ratios in GPE precursors, the formed polymer network experienced a composition evolution from a 3D cross-linked mono PEGDA network to triple PEGDA, E2BADMA, and PEGDA/E2BADMA networks and then to dual E2BADMA and PEGDA/E2BADMA networks, accompanied by the increase in both storage modulus (from 6 to 37 MPa) and ionic conductivity (from 0.06 to 0.44 mS cm^-1). As a result, the E2BADMA/PEGDA mass ratio of 2:1 facilitates the successful fabrication of a dual-network-supported GPE (PEEPL-12) with a mechanical strength of 37 MPa and superior electrochemical properties (a high ionic conductivity of 0.44 mS cm^-1 and a wide electrochemical stability window of 4.85 V vs Li/Li⁺). Such polymer electrolyte-based symmetric lithium metal batteries delivered a long cycle life (2000 h at 0.1 mA cm^-2 and 0.1 mAh cm^-2), and the Li|PEEPL-12|LiFePO₄ cell delivered a high capacity of 140 mAh g^-1 at the 100th cycle at the current density of 0.1 C (1 C = 170 mAh g^-1). A more thorough investigation indicated the formation of a stable solid electrolyte interphase layer on a lithium metal anode. These extraordinary features open up a venue for fabrication of advanced polymer electrolyte for long-cycle-life lithium metal batteries.