Halogen-Free Ionic-Liquid Electrolytes and Sulfur/Microalgae-Derived Hard Carbon Cathode for Magnesium Batteries

Abstract

Rechargeable magnesium batteries are promising candidates for next-generation energy storage systems due to their intrinsic safety, natural abundance, and high volumetric capacity. However, their practical application remains limited by sluggish Mg2+ transport, electrolyte instability, and low cathode utilization. In this work, a halogen-free electrolyte (HFE) based on Mg(NO3)2 in an acetonitrile/tetraethylene glycol dimethyl ether (ACN/G4) solvent system is modified using the ionic liquid 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]) to form HFE_IL, with the aim of enhancing ionic transport and interfacial stability. In parallel, a sustainable sulfur cathode integrated with microalgae-derived hard carbon (S_C) is developed to improve electronic conductivity and suppress polysulfide shuttling. Structural and spectroscopic analyses confirm that the incorporation of the ionic liquid preserves the electrolyte framework while tuning the solvation environment. Electrochemical characterization (EIS, CV, LSV, GCD, and Mg stripping/plating measurements) reveals that HFE_IL exhibits reduced bulk and interfacial resistances, a significantly lower activation energy (0.0173 eV compared to 0.14 eV), and an increased Mg2+ transference number (~0.8). Furthermore, enhanced Mg2+ diffusion (~10−13 cm2 s−1) and improved charge-transfer kinetics are achieved compared to the pristine electrolyte. Symmetric Mg‖Mg cells demonstrate stable stripping/plating behavior with reduced polarization over 100 h. In full Mg‖electrolyte‖S_C cells, the HFE_IL system delivers a higher discharge capacity (~575 mAh g−1) compared to the pristine electrolyte (~437 mAh g−1), indicating improved reversibility and Mg2+ utilization. This study demonstrates that ionic-liquid modification of halogen-free electrolytes, combined with sustainable carbon–sulfur cathodes, provides an effective strategy to enhance Mg2+ transport, interfacial stability, and overall electrochemical performance in magnesium batteries.