Ultra-water-tolerant electrolyte design coupled with multiscale mechanism understanding toward practical magnesium metal batteries

The high susceptibility of Mg anodes and electrolytes to water impurities usually leads to severe performance degradation in rechargeable magnesium batteries (RMBs). This bottleneck imposes stringent requirements on manufacturing conditions and processes. Here, we report a highly water-resistant magnesium triflate (Mg(OTf) 2 )-based electrolyte by incorporating tris(2,2,2-trifluoroethyl) borate (B(TFE) 3 ) and systematically reveal the interfacial electrode/electrolyte reaction mechanism. It is found that the B(TFE) 3 reacts with H 2 O molecules to form B(TFE) 3−x OH x and protonated H + . The preferential decomposition of B(TFE) 3−x OH x molecules increases the content of B-containing organics and reduces the Mg(OH) 2 fraction in the SEI layer, thus boosting interfacial Mg 2+ migration. In addition, two anomalous effects of H 2 O on electrode performance are observed. The adsorption of B(TFE) 3−x OH x molecules favors the preferential growth of Mg along the (002) face, enabling a prolonged cycling stability of 2500 h when 10,000 ppm H 2 O is added. Moreover, the co-intercalation of Mg 2+ /H + effectively enhances the specific capacity of the Mo 6 S 8 cathode. After introducing an Al 3+ -involved phytic acid film on the Mg anode to suppress H + reduction, a water-tolerant Mg metal battery prototype with 2500 cycles is successfully constructed. This work provides valuable insights into the development of high-performance water-tolerant RMBs and designing low-cost manufacturing processes.