Application of Single-Ion Conducting Gel Polymer Electrolytes in Magnesium Batteries

Abstract

The development of polymer electrolytes for magnesium batteries has been hindered by difficulties related to achieving sufficient magnesium ion conduction and magnesium electrodeposition. The high charge density of the magnesium cation causes it to interact with both the polar polymer matrix and any counteranions, challenging ion transport. Solvents and salts that are known to be incompatible with the magnesium electrode have widely been used in prior reports to increase ionic conductivity. Herein we report the use of a single-ion conducting magnesium gel polymer electrolyte consisting of a poly(ethylene glycol) dimethacrylate (PEGDMA) crosslinker that is copolymerized with an anionic monomer and subsequently swelled in solvents compatible with magnesium metal. Sufficient magnesium ion conductivities (>10 −4 S/cm at 25 °C) were able to be achieved with specific solvents and solvent mixtures. Constant potential holds were used to interrogate magnesium electrodeposition from these electrolytes. Small amounts of magnesium deposits were identified; however, the large interfacial impedances appear to impact the degree of deposition. It is hypothesized that the large interfacial impedance is due to the charge transfer resistance associated with the solvated magnesium cation. Thus, it is proposed that the solvated magnesium cation species that is transported in this electrolyte cannot effectively electrodeposit.

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

“…The ion exchange process was proved to be stoichiometric for PEGDMA based polymers, as established in our previous work [18,44,45]. For the PTHFDA polymers presented here, successful stoichiometric ion exchange was confirmed with the use of inductively coupled plasma-optical emission spectroscopy (ICP-OES) for PTHFDA1000_low_STFSIK, PTHFDA1000_med_STFSIK, PTHFDA1000_high_STFSIK, PTHFDA1000_low_STFSICa, and PTHFDA1000_med_STFSICa.…”

“…For EC:PC swelled PEGMDA-SS polymers, there was generally less of a difference in conductivity between Na and K exchanged polymers. In high dielectric solvents, ion dissociation is more facile, and the dissociated ions are coordinated more strongly with the solvent molecules over the polymer [44,46]. As was determined from the DEG/TEG case, ion dissociation and the transport of polymer-coordinated cations were more sensitive to the cation identity than transport of solvent-coordinated cations.…”

“…In doing so, design characteristics are identified for beyond Li-ion SIPEs, a crucial step for developing these new storage technologies and expanding the portfolio of rechargeable battery possibilities. This work builds on our recent reports on solvent-free lithium and beyond lithium SIPEs based on crosslinked PEG networks and gel SIPEs for Li and Mg batteries [18,[44][45][46]].…”

“…In reference to Equation (1), as the charge density is increased the maximum possible number of charge carriers is increased, which according to Equation (1) increases the conductivity. However, morphological changes induced in the polymer system as a function of increased charge density can lower the actual number of charge carriers as well as charge carrier mobility [44,45]. For the PEGDMA-SS systems and other sulfonated SIPEs, it has been shown that an increase in charge density leads to the formation of dense ionic aggregates, which reduce the number of ion pairs that are easily dissociable, as many ion pairs are trapped within the aggregate and inaccessible by solvent molecules [25,45].…”

“…A select subset of the SIPEs studied above were examined with SAXS in an attempt to contextualize the observed conductivity trends within terms of the polymer structure. It has been previously demonstrated that these types of SIPEs display a characteristic "ionomer peak" that is indicative of ionic aggregation [18,44,45].…”