Ultrafast anode for high voltage aqueous Li-ion batteries

Clustered Mo sulfide Mo6S8 (Chevrel phase) demonstrates outstandingly high rate capability in contact with concentrated aqueous Li2SO4 solutions. Slow galvanostatic cycling can be performed with good Faradaic efficiency at a moderate capacity of 32.1 mAhg− 1 (1e− reduction of Mo6S8), which in combination with 1e− oxidation process in Li x Mn2O4 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 Mo6S8 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 Wh kg−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.