Comparison of Single-Ion Conducting Polymer Gel Electrolytes for Sodium, Potassium, and Calcium Batteries: Influence of Polymer Chemistry, Cation Identity, Charge Density, and Solvent on Conductivity

Abstract

From the standpoint of material diversification and sustainability, the development of so-called “beyond lithium-ion” battery chemistries is important for the future of energy storage. Na, K, and Ca are promising as the basis for battery chemistries in that these elements are highly abundant. Here, a series of single-ion conducting polymer electrolytes (SIPEs) for Na, K, and Ca batteries are synthesized and investigated. The two classes of metal cation neutralized SIPEs compared are crosslinked poly(ethylene glycol) dimethacrylate-x-styrene sulfonate (PEGDMA-SS) and poly(tetrahydrofuran) diacrylate-x-4-styrenesulfonyl (trifluoromethylsulfonyl)imide (PTHFDA-STFSI); three cation types, three charge densities, and four swelling states are examined. The impact on conductivity of all of these parameters is studied, and in conjunction with small angle X-ray scattering (SAXS), it is found that promoting ion dissociation and preventing the formation of dense ionic aggregates facilitates ion transport. These results indicate many of the lessons learned from the Li SIPE literature can be translated to beyond Li chemistries. At 25 °C, the best performing Na/K and Ca exchanged polymers yield active cation conductivity on the order of 10−4 S/cm and 10−6 S/cm, respectively, for ethylene carbonate:propylene carbonate gelled SIPEs, and 10−5 S/cm and 10−7 S/cm, respectively, for glyme gelled SIPEs.

Chemistry‐Performance Relationships of Polymer Gel‐Electrolytes for Mg−S and Li−S Batteries: Influence of Network Cation Solvation Capacity on Polymer‐Polysulfide Interactions

“…It is noted that the underlying mechanics of our past results regarding cation transport are reproduced here. Previously, we showed that decreasing the network cation solvation capacity, by decreasing the solvation site density, lead to a lower cation‐polymer interaction and a subsequent increase in conductivity [9,19] . This relationship between conductivity and crosslinking macromonomer chemistry extends to divalent cations as well as monovalent, with the caveat that divalent cations are significantly more difficult for a network to solvate.…”

“…Previously, we showed that decreasing the network cation solvation capacity, by decreasing the solvation site density, lead to a lower cationpolymer interaction and a subsequent increase in conductivity. [9,19] This relationship between conductivity and crosslinking macromonomer chemistry extends to divalent cations as well as monovalent, with the caveat that divalent cations are significantly more difficult for a network to solvate. Here, the same result is achieved by lowering the solvation capacity via decreased solvation site strength.…”

“…By changing to a lower dielectric constant network such as poly(tetrahydrofuran) diacrylate (PTHFDA), polymer-cation interactions can be reduced, enhancing the gel polymer electrolyte cationic conductivity. [9,10] The reduction of polymer-cation interactions are a result of diminished cation solvation sites present in networks with lower ether oxygen density.…”

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

“…2,3 This has inspired analogous calcium electrolytes which use common carbonate-4-6 or ether-7 based solvents and predominately ether-based polymer backbones, 8-10 with ionic moieties being one exception. [11][12][13] In the case of polymer electrolytes, however, ether-containing backbones result in low ionic…”

“…Followed by this, Ford and coworkers 13 were the first to report a gel-type single-ion conducting polymer for calcium ion conduction. We note that since the anions are covalently tethered to the polymer chains in single-ion conducting polymers, they do not partake in ion transport.…”