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

Abstract

Calcium batteries are promising alternatives to lithium batteries owing to their high energy density, comparable reduction potential, and mineral abundance. However, to meet practical demands in high-performance applications, suitable electrolytes must be developed. Here, we report the synthesis and characterization of polymer gel electrolytes for calcium-ion conduction prepared by the photo-cross-linking of poly(ethylene glycol) diacrylate (PEGDA) in the presence of solutions of calcium salts in a mixture of ethylene carbonate (EC) and propylene carbonate (PC) solvents. The results show room-temperature conductivity between 10 −5 and 10 −4 S/cm, electrochemical stability windows of ∼3.8 V, full dissociation of the salt, and minimal coordination with the PEGDA backbone. Cycling in symmetric Ca metal cells proceeds but with increasing overpotentials, which can be attributed to interfacial impedance between the electrolyte and calcium surface, which inhibits charge transfer. Calcium may still be plated and stripped yielding high-purity deposits and no indication of significant electrolyte breakdown, indicating that high overpotentials are associated with an electrically insulating, yet ion-permeable solid electrolyte interface (SEI). This work provides a contribution to the study and understanding of polymer gel materials toward their improvement and application as electrolytes for calcium batteries.

Current Status and Challenges of Calcium Metal Batteries

“…Although recent several reports presented plating and stripping signals and even Ca deposits, the large polarization overpotentials and severe kinetics hysteresis have not been improved for these calcium salts, with plating/stripping persisting on for only several cycles. [59,61,[71][72][73]…”

Vinylimidazole–based Polymer Electrolytes with Superior Conductivity and Promising Electrochemical Performance for Calcium Batteries

“…Complete salt dissolution and a higher degree of crosslinking in the polymer leads to increase in the oxidative stability of the electrolyte. 12,14 This increased conversion and crosslinking, indicated by thermal analysis, imparts slightly better oxidative stability to PVIm-1.0. For reference, in our previous work using an imidazolium-based ionic liquid trapped within a PEGDA matrix 14 , we observed a similar oxidative stability of ~3 V. We note that the use of calcium metal as the reference and counter electrode can shift the potential and current density observed during voltammetry measurements.…”

“…12,14 This increased conversion and crosslinking, indicated by thermal analysis, imparts slightly better oxidative stability to PVIm-1.0. For reference, in our previous work using an imidazolium-based ionic liquid trapped within a PEGDA matrix 14 , we observed a similar oxidative stability of ~3 V. We note that the use of calcium metal as the reference and counter electrode can shift the potential and current density observed during voltammetry measurements. Yet, calcium metal was used in this work owing to the standard practice of doing so in the literature and the difficulty of placing a separate reference electrode given the size of the polymer samples investigated.…”

“…conductivity and difficulties with achieving high redox rates in electrochemical cells. 14 This is fundamentally associated, in part, to the calcium ion's combined large ionic radius and greater charge density resulting in strong coordination to oxygen, as compared to other cations 15,16 in addition to the large hopping distance between neighboring coordinating sites for a PEO chain. 17 Incorporating polarizable units in the polymer backbone and softening cation-polymer interactions are some of the potential strategies to overcome challenges encountered with ether-based multivalent polymer electrolytes.…”

“…Furthermore, this also signifies the formation and presence of a stable solid electrolyte interface (SEI) layer on the calcium metal working electrode surface, which facilitates stable galvanostatic deposition and dissolution of calcium metal. It appears that the SEI forms in the initial 3 cycles 6,11,14 wherein the overpotentials are ca. -0.5 V and +0.5 V, following which the overpotentials stabilize at higher values.…”