Ultrafast anode for high voltage aqueous Li-ion batteries

Clustered Mo sulfide Mo ₆S ₈ (Chevrel phase) demonstrates outstandingly high rate capability in contact with concentrated aqueous Li ₂SO ₄ solutions. Slow galvanostatic cycling can be performed with good Far-adaic efficiency at a moderate capacity of 32.1 mAhg ^-1 (1e ^- reduction of Mo ₆S ₈), which in combination with 1e ^- oxidation process in Li _xMn ₂O ₄ cathode, results in equilibrium cell voltage of 1.5 V. Higher rates of charge and discharge have been also achieved within a considerably extended apparent electrochemical stability window involving second two-electron reduction of Mo ₆S ₈ which leads to apparent cell voltage of 1.85 V, specific capacity about 74.7 mAhg ^-1 at 90 % Faradaic efficiency, specific energy of 74 Whkg ^-1 (related to both electrodes) at extremely high charge-discharge rate of 60 C. Our study highlights the generic feature of Li-ion aqueous cells, namely, a high rate capability coupled with a relatively fast self-discharge which necessitates a more profound understanding of the nature of self-discharge in Li-ion insertion hosts in contact with aqueous solutions.