LAUSR

Zn batteries potentially offer the highest energy density among aqueous batteries that are inherently safe, inexpensive, and sustainable. However, most cathode materials in Zn batteries suffer from capacity fading, particularly at a low current rate. Herein, it is shown that the ZnCl2 “water‐in‐salt” electrolyte (WiSE) addresses this capacity fading problem to a large extent by facilitating unprecedented performance of a Zn battery cathode of Ca0.20V2O5∙0.80H2O. Upon increasing the concentration of aqueous ZnCl2 electrolytes from 1 m to 30 m, the capacity of Ca0.20V2O5∙0.80H2O rises from 296 mAh g−1 to 496 mAh g−1; its absolute working potential increases by 0.4 V, and most importantly, at a low current rate of 50 mA g−1, that is, C/10; its capacity retention increases from 8.4% to 51.1% over 100 cycles. Ex situ characterization results point to the formation of a new ready‐to‐dissolve phase on the electrode in the dilute electrolyte. The results demonstrate that the Zn‐based WiSE may provide the underpinning platform for the applications of Zn batteries for stationary grid‐level storage.