Solid‐State Synthesis of Phase‐Pure Micron‐Sized Patrónite Vanadium Tetrasulfide toward Practical Magnesium Batteries

The Patrónite vanadium tetrasulfide (VS 4 ) follows a simultaneous cationic and anionic redox (SCAR) mechanism in magnesium batteries, which renders a delocalized electronic structure for fast kinetics and meanwhile enables multielectron reactions for delivering high capacity. In contrast to most research that focuses on the hydrothermal route resulting in VS 4 nanoparticles, herein a more industrially relevant synthesis route is targeted, utilizing a solid‐state approach leading to micron‐sized VS 4 . The obtained 2 × 10 μm VS 4 needles show good water/air stability, allowing an environmentally friendly coatings procedure using polyvinylpyrrolidon binder in isopropanol. Electrochemical investigation shows that the micron‐sized VS 4 cathode delivers a maximum capacity of 420 mAh g −1 in a tetrakis(hexafluoroisopropyloxy)borate (Mg[B(hfip) 4 ] 2 )‐based electrolyte, however suffers from fast capacity fading and overcharging. Targeting these issues, a concentrated Mg[B(hfip) 4 ] 2 electrolyte in bis(2‐methoxyethyl)ether is applied, which enables stable cycling for 300 cycles at the expense of a reduced capacity. Multimodal characterizations confirm a reversible SCAR mechanism of VS 4 during cycling. However, pulverization of the electrode and fragmentation of the VS 4 backbones are evident, leading to the leakage of active material into electrolyte and causing capacity decay. To avoid active material loss, crystal engineering or surface protection strategies should be developed toward practical Mg batteries.