Aqueous rechargeable zinc/sodium vanadate batteries with enhanced performance from simultaneous insertion of dual carriers

Rechargeable aqueous zinc-ion batteries are promising energy storage devices due to their high safety and low cost. However, they remain in their infancy because of the limited choice of positive electrodes with high capacity and satisfactory cycling performance. Furthermore, their energy storage mechanisms are not well established yet. Here we report a highly reversible zinc/sodium vanadate system, where sodium vanadate hydrate nanobelts serve as positive electrode and zinc sulfate aqueous solution with sodium sulfate additive is used as electrolyte. Different from conventional energy release/storage in zinc-ion batteries with only zinc-ion insertion/extraction, zinc/sodium vanadate hydrate batteries possess a simultaneous proton, and zinc-ion insertion/extraction process that is mainly responsible for their excellent performance, such as a high reversible capacity of 380 mAh g–1 and capacity retention of 82% over 1000 cycles. Moreover, the quasi-solid-state zinc/sodium vanadate hydrate battery is also a good candidate for flexible energy storage device.


Discussion of Zn deposition
According to the electrostatic shield mechanism, additive Na + would form a positively charged electrostatic shield around the initial growth Zn protuberances to limit the further deposition of Zn on them. 1 However, when the concentration of Na + is too high, there is massive and extra Na + around the surface of Zn negative electrode to occupy the active sites of Zn deposition during the charge process. Hence, the Zn deposition on the surface of Zn negative electrode will be limited. As a result, it tends to deposit on the protuberances due to their lower electrostatic shield. Therefore, when the Na 2 SO 4 concentration is up to 1.5 M, there is Zn dendrite deposition (Supplementary Figure 7).

Supplementary Note 2 Preparation of NVO nanorods
The NVO nanorods were prepared similar to the previously reported method. 2 Typically, 1.8 g of V 2 O 5 and 0.4 g of NaOH were first dissolved into 40 mL of deionized water, and then the solution was transferred into stainless steel autoclave (100 mL). The stainless steel autoclave was sealed and kept at 180 °C for 40 h. After that, the products were collected via washing the precipitates with deionized water for several times and freeze-drying.

Calculation process for half-redox products
Fully-discharged products: There are obvious two pairs of redox peaks from the mentioned CV and charge/discharge curves. To indicate the redox process clearly, we also checked the contents of Zn in half-charged/discharged state via XPS spectra and ICP-AES. The ratio of Zn in fully-discharged and half-discharged state is 1.9. According to the three equations, x, y, and z can be calculated to be 2.14, 0.1, and 0.357, respectively. Therefore, the half-discharged products are H 2.14 NaZn 0.2 V 3 O 8 •1.5H 2 O and 0.357Zn 4 SO 4 (OH) 6 •4H 2 O.

The dissolution of the fully discharged products in electrolyte
To evaluate the dissolution of H 3.9 NaZn 0.5 V 3 O 8 •1.5H 2 O and Zn 4 SO 4 (OH) 6 •4H 2 O in electrolyte, the positive electrode from the completely discharged cell were rinsed by deionized water and immersed into electrolyte that was used in the Zn/NVO batteries.
After 5h, the quantity of Zn 2+ and V 3+ in such electrolyte was measured by ICP-AES. 24 The concentrations of Zn 2+ and V 3+ in electrolyte without immersing fully discharged electrode were 1 M and 0 M, respectively. After 5h, the concentrations of Zn 2+ and V 3+ in electrolyte were 1.00007 M and 0.00019 M, respectively, indicating that the dissolution of H 3.9 NaZn 0.5 V 3 O 8 •1.5H 2 O and Zn 4 SO 4 (OH) 6 •4H 2 O in electrolyte can be ignored during the cycling.

Quantifying the inserted Zn 2+ in NVO
The contents of V and Zn were determined by ICP-AES, which was a quantified measurement (I =aC b , where I and C represent the intensity of characteristic spectra and the concentration of element, respectively; a and b are adjustable parameters). In ICP-AES, the standard curves of V and Zn were first obtained via measuring standard solutions with different concentrations of V and Zn. By fitting standard curves of V and Zn, the values of a and b were achieved. To avoid the effect of electrolytes or other deposits, the electrodes were first washed by deionized water more than 5 times and then used to characterize. After that, the as-prepared sample solutions were tested.
The contents of V and Zn would be finally quantified according to above formula based on calculated a and b values.