Stack Pressure-Independent Side-Reaction-Dominant Nanoscale Inactive Mg Loss in Rechargeable Mg Metal Batteries

Inactive magnesium, including electrochemically formed nanoscale Mg²⁺ ions in the solid electrolyte interphase (SEI Mg²⁺) and electrically isolated unreacted nano metallic Mg (Mg⁰), contributes to poor capacity and cycle life in magnesium metal batteries. Nevertheless, the precise quantification of nanoscale SEI Mg²⁺ versus inactive Mg⁰, as well as their formation mechanisms and relationship with the anode cycling reversibility, remains elucidated, thereby hindering progress in anode optimization design. Here, a magnesium-targeted acid-assisted continuous titration-collection-gas chromatography (AAC-TCGC) technique is developed to precisely quantify the percentage of nanolevel inactive SEI Mg²⁺ and Mg⁰ in Mg anode, revealing that the predominant contributor to Mg loss is the nanolevel inactive SEI Mg²⁺, which is different from the well-known inactive metal-dominant loss mechanism in Li/Zn battery. We find that the nanoscale SEI Mg²⁺ is mainly from the side reaction of the Mg anode with electrolyte anions/solvents or contaminants. We also discover a phenomenon that uniaxial stack pressure has no effect on altering the performance or morphology in the Mg metal anode (also distinct from Li/Zn anode behavior), highlighting the importance of nanoscale SEI Mg²⁺ loss tuning for magnesium metal battery construction. This study offers theories and approaches concerning the quantification and formation mechanism of inactive magnesium, crucial for developing high-performance magnesium metal batteries.