We report a low-cost water-in-salt electrolyte (WiSE), of 30 m ZnCl2, which enables a dendrite-free Zn metal anode to possess a high coulombic efficiency.

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 ZnCl 2 "water-in-salt" electrolyte (WiSE) addresses this capacity fading problem to a large extent by facilitating unprecedented performance of a Zn battery cathode of Ca 0.20 V 2 O 5 •0.80H 2 O. Upon increasing the concentration of aqueous ZnCl 2 electrolytes from 1 m to 30 m, the capacity of Ca 0.20 V 2 O 5 •0.80H 2 O 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.

Potassium-ion batteries (KIBs) are a promising sustainable energy storage technology due to the high abundance and low cost of potassium. Carbon anode materials for KIBs have seen great successes, but the development of cathode materials is yet to catch up. In this study, poly(anthraquinonyl sulfide) (PAQS) is evaluated as a cathode material for KIBs. It exhibits a high reversible capacity of 200 mAh/g, which is the highest value for a potassium storage cathode material. The cell shows two slopes averaged at 2.1 and 1.6 V vs. K + /K. It shows a good cycling performance with the capacity retention of 75% after 50 cycles at a rate of C/10. These preliminary results indicate that PAQS is a promising cathode material for KIBs.