Dioxolanone-Anchored Poly(allyl ether)-Based Cross-Linked Dual-Salt Polymer Electrolytes for High- Voltage Lithium Metal Batteries

Vijayakumar V, Diddens D, Heuer A, Kurungot S, Winter M, Nair JR

Abstract

Novel cross-linked polymer electrolytes (XPEs) are synthesizedby free-radical copolymerization induced by ultraviolet (UV)-lightirradiation of a reactive solution, which is composed of a difunctionalpoly(ethylene glycol) diallyl ether oligomer (PEGDAE), a monofunctionalreactive diluent 4-vinyl-1,3-dioxolan-2-one (VEC), and a stock solutioncontaining lithium salt (lithium bis(trifluoromethanesulfonyl)imide, LiTFSI)in a carbonate-free nonvolatile plasticizer, poly(ethylene glycol) dimethylether (PEGDME). The resulting polymer matrix can be represented as alinear polyethylene chain functionalized with cyclic carbonate (dioxolanone)moieties and cross-linked by ethylene oxide units. A series of XPEs areprepared by varying the [O]/[Li] ratio (24 to 3) of the stock solution andthoroughly characterized using physicochemical (thermogravimetric analysis−mass spectrometry, differential scanning calorimetry, NMR, etc.) andelectrochemical techniques. In addition, quantum chemical calculations are performed to elucidate the correlation betweenthe electrochemical oxidation potential and the lithium ion−ethylene oxide coordination in the stock solution. Later, lithiumbis(fluorosulfonyl)imide (LiFSI) salt is incorporated into the electrolyte system to produce a dual-salt XPE that exhibitsimproved electrochemical performance, a stable interface against lithium metal, and enhanced physical and chemicalcharacteristics to be employed against high-voltage cathodes. The XPE membranes demonstrated excellent resistance againstlithium dendrite growth even after reversibly plating and stripping lithium ions for more than 1000 h with a total capacity of 0.5mAh cm−2. Finally, the XPE films are assembled in a lab-scale lithium metal battery configuration by using carbon-coatedLiFePO4 (LFP) or LiNi0.8Co0.15Al0.05O2 (NCA) as a cathode and galvanostatically cycled at 20, 40, and 60 °C. Remarkably, at20 °C, the NCA-based lithium metal cells displayed excellent cycling stability and good capacity retention (>50%) even after1000 cycles.