Nonflammable and Stable Quasi-Solid Electrolytes: Demonstrating the Feasibility of Application in Rechargeable Solid-State Magnesium Batteries

Quasi-solid electrolytes (QSEs) are an interesting research area for rechargeable magnesium batteries (RMBs). A solid-state electrolyte is favored in battery research as it can offer several processing advantages apart from addressing the safety issues pertaining to high-energy-density batteries. Herein, we showcase a unique QSE that can be easily synthesized from triethyl-phosphate (TEP) as the host medium and 1.5 M magnesium perchlorate (Mg(ClO4)2) as the electrolyte. The gelation and physico-chemical interactions between Mg2+ and TEP solvent molecules leading to the formation are comprehensively analyzed by 1H-, 13C-, 31P-NMR, and FTIR. The QSE is not only thermally stable, but also the electrochemical properties assessed by impedance spectroscopy in combination with d.c. polarization studies, linear sweep, and cyclic voltammetry techniques reveal that it does satisfy all of the pre-requisites to support excellent ionic conduction. The ionic conductivity of ∼4.0 × 10–3 S cm–1 at 25 °C and an estimated cationic transport number of ca. 0.37 for Mg2+ ions along with electrochemical stability >2.7 V and nonflammability underscore its suitability to be explored for RMB applications. In our attempts to justify, a prospective cathode material, δ-MnO2, was concurrently synthesized, characterized, and paired with the QSE and evaluated for its test-cell performance. The assembled δ-MnO2|QSE|Mg cells were appraised by employing conventional cyclic voltammetry, impedance spectroscopy, and galvanostatic charge–discharge studies, wherein an apparently impressive initial capacity of 65 mAh·g–1 at a current density 10 mA·g–1 displays a steady performance decline over multiple cycles. Our extensive follow-up investigations employing XRD, FESEM, EDS, and XPS to analyze the observed performance inadequacy reveal that this unexpected deviation is potentially related to a transformation of the active-material morphology and was not a case of electrolyte instability or unwarranted side reactions at the electrode–electrolyte interface. The QSE remains a feasible option for use in rechargeable Mg2+-ion batteries possibly paired with alternate electrodes.