Unlocking Four‐electron Conversion in Tellurium Cathodes for Advanced Magnesium‐based Dual‐ion Batteries

Abstract Magnesium (Mg) batteries hold promise as a large‐scale energy storage solution, but their progress has been hindered by the lack of high‐performance cathodes. Here, we address this challenge by unlocking the reversible four‐electron Te 0 /Te 4+ conversion in elemental Te, enabling the demonstration of superior Mg//Te dual‐ion batteries. Specifically, the classic magnesium aluminum chloride complex (MACC) electrolyte is tailored by introducing Mg bis(trifluoromethanesulfonyl)imide (Mg(TFSI) 2 ), which initiates the Te 0 /Te 4+ conversion with two distinct charge‐storage steps. Te cathode undergoes Te/TeCl 4 conversion involving Cl − as charge carriers, during which a tellurium subchloride phase is presented as an intermediate. Significantly, the Te cathode achieves a high specific capacity of 543 mAh g Te −1 and an outstanding energy density of 850 Wh kg Te −1 , outperforming most of the previously reported cathodes. Our electrolyte analysis indicates that the addition of Mg(TFSI) 2 reduces the overall ion‐molecule interaction and mitigates the strength of ion‐solvent aggregation within the MACC electrolyte, which implies the facilized Cl − dissociation from the electrolyte. Besides, Mg(TFSI) 2 is verified as an essential buffer to mitigate the corrosion and passivation of Mg anodes caused by the consumption of the electrolyte MgCl 2 in Mg//Te dual‐ion cells. These findings provide crucial insights into the development of advanced Mg‐based dual‐ion batteries.