Unveiling the Reversibility and Stability Origin of the Aqueous V2O5–Zn Batteries with a ZnCl2 “Water‐in‐Salt” Electrolyte

Abstract Aqueous V2O5–Zn batteries, an alternative chemistry format that is inherently safer to operate than lithium‐based batteries, illuminates the low‐cost deployment of the stationary energy storage devices. However, the cathode structure collapse caused by H2O co‐insertion in aqueous solution dramatically deteriorates the electrochemical performance and hampers the operation reliability of V2O5–Zn batteries. The real‐time phase tracking and the density functional theory (DFT) calculation prove the high energy barrier that inhibits the Zn2+ diffusion into the bulk V2O5, instead the ZnCl2 “water‐in‐salt electrolyte” (WiSE) can enable the dominant proton insertion with negligible lattice strain or particle fragment. Thus, ZnCl2 WiSE enables the enhanced reversibility and extended shelf life of the V2O5–Zn battery upon the high temperature storage. The improved electrochemical performance also benefits by the inhibition of vanadium cation dissolution, enlarged voltage window, as well as the suppression of the Zn dendrite protrusion. This study comprehensively elucidates the pivotal role of a concentrated ZnCl2 electrolyte to stabilize the aqueous batteries at both the static storage and dynamic operation scenarios.

a) SEM image of the milled V 2 O 5 cathode particle. b-d) The elemental maps of C, O, and V corresponding to Figure S1a.

Figure S2
The post-mortem morphology of the V 2 O 5 electrode after 50 cycles in 1 M ZnSO 4 at a) a low magnification and b) a higher magnification.

Figure S3
Price comparison of various "water-in-salt" electrolytes.
We have elaborated the characterizations of discharge product of V 2 O 5 in 30 m ZnCl 2 . The new peak in Figure 2 can be indexed to the (002) peak of H x V 2 O 5 . To give more convincing evidence, we supplied the ex-situ XRD result of the V 2 O 5 electrode at fully discharge state after 5 cycles in 30 m ZnCl 2 . It displays that the all the peaks of the discharge product can be indexed to the H x V 2 O 5 phase.
Combine with the electrochemical analysis and TEM result, the Zn ion does not insert into V 2 O 5 cathode but precipitate on the cathode electrode. The analogous phenomenon is also reported for the MnO 2 cathode due to the H + insertion. Therefore, the discharge product should be assignable to the H x V 2 O 5 .

Figure S4
The ex-situ XRD pattern of the V 2 O 5 cathode at the 6 th discharge state in 30 m ZnCl 2 .
To further probe the charge storage mechanism during the dynamic process, the Zn-V 2 O 5 cells with WiSE electrolyte were cycled three times and disassembled at specific voltages (0.4, 0.6, 0.8, 1.0, 1.2 V) upon fourth discharge process, as shown in the Figure S5. According to the reviewer's suggestion, ICP tests were conducted to obtain the atomic ratio of Zn to V in the cathode at the different voltage. According to the Faraday formula, 1 mol electron transfer would cause 147.4 mAh g -1 capacity. The ratio of Zn to the transferred electron aroused from the electrochemical reaction was calculated by discharge capacity and the Zn/V ratio. As for the zinc intercalation reaction, 1 mol zinc atom corresponds to 2 mol electron transfer. As shown in Figure S5a, the Zn/e-value is about 0.5 for 1 M ZnSO 4 electrolyte, suggesting the energy storage process is dominated by zinc intercalation reaction. Besides, the zinc intercalated into the V 2 O 5 lattice would cause a gradually increased Zn/V value. As for the proton insertion mechanism, the precipitate can be depicted as Zn x (OH) 2x ·Zn y Cl 2y . The Zn/e-value should be higher than 0.5 considering that the amount of OHshould equal the transferred electron. The results show that the Zn/evalue is about 0.61 at different voltages for WiSE. Thus, the proton insertion reaction runs through the entire energy storage process. We do not find the peak that indexed to the zinc hydroxide chloride species no matter in the operando XRD or ex-situ XRD spectra. HRTEM was conducted to investigate the more specific information of the zinc hydroxide chloride species. As shown in Figure S7, the Zn and Cl elements overlapped and covered on the V 2 O 5 , suggesting the OHinduced zinc hydroxide chloride precipitate.
We have selected a specific region marked by red rectangle, in which only Zn and Cl elements exist.
The Figure S6b, corresponding to the zinc hydroxide chloride deposits, demonstrated no lattice fringe and the corresponding fast Fourier transform (FFT) pattern only displayed ambiguous rings. Thus, we think that no signal of zinc hydroxide chloride species can be observed in XRD test due to its relative amorphous nature.

Figure S7
a) The HAADF image of the discharged V 2 O 5 electrode cycled in WiSE. b) The HRTEM image of the selected region marked by red rectangle in Figure S7a. c) The FFT pattern obtained in Figure  S7b. The energy dispersive X-ray spectra corresponding to Figure S7a with the elemental maps of d) V, e) Cl, and f) Zn.  The capacity loss caused by vanadium dissolving was further discussed. As shown in Figure 4d, there are 0.5 g V 2 O 5 material in 5 mL electrolytes. The concentration of dissolved vanadium is 875.7 µg/ml. In other words, only 1.56% active material dissolved into the dilute electrolyte. We ascribe this large difference of vanadium dissolving amount in dynamic and static situations to the pulverization of the cathode particles upon cycling. Thus, the vanadium dissolution is the main reason of capacity decay, but it is also associated with the water co-intercalation process. To verify our hypothesis, the morphological evolution and statistical summary of the particle size distribution before and after static storage, corresponding to Figure S10 were evaluated. It can be observed that the V 2 O 5 particle maintained the original morphology upon the storage. And the average particle size was about 5 µm before and after storage, demonstrating no particle pulverization within dilute electrolyte. Therefore, the water-insertion induced particle fracture and the subsequent vanadium dissolution should be the main reason of the capacity loss in dilute electrolyte. The decent kinetics in different electrolytes were further confirmed by electrochemical impedance spectroscopy (EIS) measurements. As shown in Figure S11 and Table S1, the V 2 O 5 electrode cycled in 1 M ZnSO 4 displays very high charge-transfer resistance (R ct ) than electrode in 30 m ZnCl 2 before and after cycling. It is worth noting that there is an obvious decrease in the R ct for the V 2 O 5 in 1 M ZnSO 4 after ten cycles, which could be ascribed to the increased surface area due to particle fracture. Additionally, the electrode in 1 M ZnSO 4 has a faster ionic diffusion in the electrochemical processes, which is in agreement with the GITT results.

Figure S11
EIS spectra of V 2 O 5 electrode in different electrolytes.
As shown in Figure S12, the dilute aqueous electrolyte has a very high ionic conductivity. The increase of salt concentration accompanies with the decrease of the ionic conductivity from 34.35 ms cm -1 to 2.21 ms cm -1 , but this value maintained the same magnitude as compared to the conventional organic electrolyte (~ 1 ms cm -1 ).

Figure S12
The Nyquist plots of different electrolytes (1 M ZnSO 4 , 1 m ZnCl 2 , 10 m ZnCl 2 and 30 m ZnCl 2 ).    We test the coulombic efficiency of zinc metal to evaluate the side reactions occurred on zinc metal in asymmetric Zn||Zn cells with titanium foil as the current collector. For the repeated Zn plating/stripping tests of asymmetric cells, the capacity of Zn plating was fixed at 1 mAh cm -2 under 0.5 mA cm -2 , and the cutoff voltage of stripping process was set at 1.0 V. A pre-cycling of cells was carried out for three cycles to stabilize the cell performance. In this asymmetric cell, plating/stripping in 30 m ZnCl 2 demonstrates a much higher average CE of 98.4% than 92.3% in 1 M ZnSO 4 . Therefore, the H 2 evolution induced by the low pH value is not a critical concern in concentrated electrolyte.