Multifunctional polymer electrolyte membrane networks for energy storage via ion-dipole complexation in lithium metal battery

Abstract

A novel concept of energy storage is presented involving ion-dipole complexation within multifunctional polymer electrolyte membrane (PEM), consisting of polyethylene glycol diacrylate (PEGDA) and succinonitrile (SCN) plasticizer and lithium bis-trifluoromethane sulfonyl imide (LiTFSI) salt. A similar complexation of lithium ions with ether oxygen and amine moieties occurs in the thermally cured poly(ethylene glycol) diglycidyl ether (PEGDGE) - polyether amine (Jeffamine D400, 2-arm) co-network. By virtue of the network functional groups, the ion transport is hindered which may be viewed as temporally holding of the Li ions, reminiscent of ion storage. Although some Li ions may be partially immobilized due to ion-dipole complexation, upon prelithiation via deep discharging the lithium metal/PEM/lithium iron phosphate (LFP) battery assembly to the low potential range of 0.01–0.5 V (i.e., oxidation at the Li metal anode), excess mobile Li ions become available that can travel to the cathode through the Li+ saturated complexation sites at ease, exhibiting high ionic conductivity with energy storage capability within the PEM matrix. Another way of supplying Li ions to PEM is through diffusion driven by ion concentration gradient between the two-layer PEM composite composed of a high salt concentration layer and a no-salt layer. The CV measurement was carried out as a function of aging time on the bilayer composite in the PEM potential range of 0.01 to 2.5 V, which indeed revealed a systematic emergence of redox reaction peaks, thereby validating the energy storing concept of the multifunctional PEM network.