Thermoelectric GNP-Integrated MoS ₂ /S Cathodes for Mg–S Batteries with a Li ₂ CO ₃ -Modified Halogen-Free Electrolyte

Magnesium–sulfur (Mg–S) batteries are attractive energy-storage systems but remain limited by sluggish Mg 2+ transport, interfacial instability, and polysulfide shuttling. Herein, a Li 2 CO 3 -modified halogen-free electrolyte based on Mg(NO 3 ) 2 in an acetonitrile/tetraethylene glycol dimethyl ether (ACN/G4) solvent system was combined with a MoS 2 /sulfur/graphene nanoplatelet (MoS 2 /S/GNP) composite cathode. The modified electrolyte exhibited an ionic conductivity of 1.4 × 10 –4 S cm –1, representing a ∼17% increase relative to the pristine electrolyte, together with an enhanced Mg 2+ transference number of 0.867. Symmetric Mg cells showed improved Mg plating/stripping stability and reduced polarization, indicating enhanced Mg/electrolyte interfacial compatibility. The MoS 2 /S/GNP cathode provided a conductive framework and catalytic sites for sulfur conversion. However, full-cell studies revealed substantial transport and interfacial limitations during cycling. The charge-transfer resistance increased from 237 to 2516 Ω after electrochemical activation, while the Mg 2+ diffusion coefficient decreased from 1.00 × 10 –14 to 1.26 × 10 –17 cm 2 s –1 . Galvanostatic cycling showed rapid capacity fading from ∼ 310 mAh g –1 in the first cycle to near-zero values after ∼15 cycles, accompanied by Coulombic efficiencies exceeding 100% during the early cycles, indicating significant parasitic reactions and polysulfide shuttling. These results demonstrate that Li 2 CO 3 modification influences Mg 2+ transport and interfacial behavior but is insufficient to overcome the fundamental degradation processes limiting halogen-free Mg–S batteries. The study provides insight into electrolyte–cathode interactions and highlights key challenges for future Mg–S battery development.