Magnesium Polymer Electrolytes Based on the Polycarbonate Poly(2-butyl-2-ethyltrimethylene-carbonate)

Magnesium electrolytes based on a polycarbonate with either magnesium tetrakis(hexafluoroisopropyloxy) borate (Mg(B(HFIP)$_4$)$_2$) or magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)$_2$) for magnesium batteries were prepared and characterized. The side-chain-containing polycarbonate, poly(2-butyl-2-ethyltrimethylene carbonate) (P(BEC)), was synthesized by ring opening polymerization (ROP) of 5-ethyl-5-butylpropane oxirane ether carbonate (BEC) and mixed with Mg(B(HFIP)$_4$)$_2$ or Mg(TFSI)$_2$_ to form low- and high-salt-concentration polymer electrolytes (PEs). The PEs were characterized by impedance spectroscopy, differential scanning calorimetry (DSC), rheology, linear sweep voltammetry, cyclic voltammetry, and Raman spectroscopy. A transition from classical salt-in-polymer electrolytes to polymer-in-salt electrolytes was indicated by a significant change in glass transition temperature as well as storage and loss moduli. Ionic conductivity measurements indicated the formation of polymer-in-salt electrolytes for the PEs with 40 mol % Mg(B(HFIP)$_4$)$_2$ (HFIP40). In contrast, the 40 mol % Mg(TFSI)$_2$ PEs showed mainly the classical behavior. ... mehrHFIP40 was further found to have an oxidative stability window greater than 6 V vs Mg/Mg$^{2+}$, but showed no reversible stripping-plating behavior in an Mg||SS cell.