Hybrid Aqueous/Organic Electrolytes Enable the High-Performance Zn-Ion Batteries

Rechargeable aqueous zinc ion batteries (ZIBs) are considered as one of the most promising systems for large-scale energy storage due to their merits of low cost, environmental friendliness, and high safety. The utilization of aqueous electrolyte also brings about some problems such as low energy density, fast self-discharge, and capacity fading associated with the dissolution of metals in water. To combat the issues, we utilize a freestanding vanadium oxide hydrate/carbon nanotube (V2O5·nH2O/CNT) film as the cathode and probe the performance in aqueous/organic hybrid electrolytes. The corresponding structural and morphological evolution of both V2O5·nH2O/CNT cathode and Zn anode in different electrolytes is explored. The integrity of electrodes and the suppression of zinc dendrites during cycles are largely improved in the hybrid electrolytes. Accordingly, the battery in hybrid electrolyte exhibits high capacities of 549 mAh g−1 at 0.5 A g−1 after 100 cycles and 282 mAh g−1 at 4 A g−1 after 1000 cycles, demonstrating an excellent energy density of 102 Wh kg−1 at a high power of 1500 W kg−1 based on the cathode.


Introduction
Electrolyte is a key component in the batteries. Based on the solvent in the electrolyte, batteries could be divided into two types, i.e., aqueous-based and organic-based. Batteries utilizing organic electrolytes possess great advantages in the energy densities thanks to the wide electrochemical window. The most successful example is Li-ion batteries where carbonatebased electrolytes are adopted, enabling an operating voltage of close to 5 V for attaining an energy density of over 200 Wh kg -1 [1,2]. In comparison, the aqueous electrolyte has a narrow electrochemical window of~1.23 V arising from the thermodynamic stability of water [3,4]. Consequently, the batteries making use of aqueous electrolyte have a typical energy density of less than 100 Wh kg -1 [5][6][7][8]. Nevertheless, aqueous electrolyte-based batteries still occupy a large proportion of energy storage markets. The reason lies on not only the low cost but also the improved safety due to the nonflammable nature of the electrolyte [3,9]. Current research interests in aqueous batteries are focused mainly on designing high-capacity cathode to improve the energy density.
Zn-based aqueous batteries are dominant in primary battery market owing to the high capacities (5851 mAh mL -1 ) and low redox potential of Zn (-0.76 V vs. standard hydrogen electrode, SHE) as well as their natural abundance [8][9][10][11][12][13][14]. Recent progress shows rechargeable Zn batteries are enabled with appropriate cathodes such as metal oxides and Prussian analogues [8,[10][11][12][15][16][17][18][19]. Among them, vanadium-based materials are reported to present high capacity to narrow the energy density gap between organic electrolyte-based batteries [12,16,17]. Significant progress has been made on the electrode material design to expand the layer distance for stable Zn ion insertion and extraction [12,20,21]. However, the intrinsic issues associated with aqueous electrolyte, such as fast self-discharge and dissolution of vanadium, remain largely unexplored. Attempts on the organic Zn-ion batteries have been discouraged by the high interfacial resistance on the electrolyte/electrode interface [17,22]. An intriguing question is whether a synergistic effect on the cost, safety, and performance could be achieved through manipulation of the electrolyte system. In this study, a flexible vanadium oxide hydrate/carbon nanotube (V 2 O 5 ·nH 2 O/CNT) freestanding film is adopted as a model electrode to probe the electrolyte effect. The analysis on structural and morphological evolution shows the benefits of hybrid aqueous/organic electrolyte in stabilizing the cyclic performance compared to neat aqueous one while avoiding the slow kinetics in organic electrolyte.

Results
2.1. Electrode Materials. The V 2 O 5 ·nH 2 O nanowires were prepared through a hydrothermal method. CNTs were incorporated to increase the conductivity. X-ray diffraction (XRD) patterns in Figure 1( [23]. The thermogravimetric and differential thermal analysis (TGA-DTA) profiles of neat V 2 O 5 ·nH 2 O ( Figure S1) confirm the presence of 1.4 wt.% water, corresponding to a chemical composition of V 2 O 5 ·nH 2 O (n ≈ 0:14). The surface areas of V 2 O 5 ·nH 2 O and V 2 O 5 ·nH 2 O/CNT are 13.5 and 43.6 m 2 g -1 , respectively ( Figure S2). The high surface area of the latter mainly comes from the incorporation of 33.7 wt.% CNTs. The scanning electron microscopy (SEM) and optical images of the V 2 O 5 ·nH 2 O/CNT film are exhibited in Figure 1 (Figures 2(a) and 2(b)). The voltage profiles in the two electrolytes have similar shapes whose sloping features suggest the solid-solution reaction processes associated with Zn ion intercalation/deintercalation [12,24]. It is noted the irreversible charge capacity in the first cycle for the ZIB in Zn-H 2 O may be due to the intrinsic dissolution of V 2 O 5 ·nH 2 O in H 2 O, similar to the shuttle effect in lithium-sulfur batteries [25]. For the cyclic test, it shows a fast capacity degradation of the cell in aqueous electrolyte. The capacity decreases from 435 mAh g -1 in the first cycle to 70 mAh g -1 after 40 cycles. In contrast, the capacity of the cell in Zn-EC/EMC slightly decreases in the initial cycles and stabilizes at 382 mAh g -1 after 40 cycles, demonstrating a much more stable cyclic stability. The crystal structure evolution was probed by ex situ XRD at different charge/discharge depths to explore the reasons for the stability in organic electrolyte, as shown in Figure 2(c). There is no obvious change of the phase after discharging to 0.9 V. Several new peaks appear starting from 0.5 V, and the intensity increases with further discharging to 0.2 V. These peaks located at around 9.2°, 12.2°, 33.2°, and 35.9°correspond to a new phase, similar to previous report [8,24], but their detailed composition is hard to be determined at this stage. Two peaks at 15.4°and 31.0°present downshifts to 14.1°and 28.6°, respectively. The merging of two peaks (20.3°and 21.8°) into one at 20.5°and the presence of a minor peak at 18.8°are due to the increased Zn ion content intercalated into V 2 O 5 ·nH 2 O [16,20]. Upon charging, the shifted peaks recover to 15.4°and 31.0°at 1.0 V and the additional peaks disappear with a continuous charge to 1.6 V, indicating the reversibility of phase transitions. The structural change in aqueous electrolyte shows a similar trend with that in the organic electrolyte ( Figure S3). However, the diffraction peaks in the cycled electrode in Zn-EC/EMC have much higher intensities than those in Zn-H 2 O, implying better crystalline degree after the full charge in organic electrolyte. This phenomenon becomes more apparent with   increasing cycle numbers. The XRD patterns of the electrode after 100 cycles in aqueous electrolyte fully lose the crystallinity, compared to intact crystal structure for the electrode cycled in organic electrolyte ( Figure 2(d)). It is noted the electrode in Zn-EC/EMC maintains its freestanding property after 100 cycles and could be peeled off from the titanium current collector for XRD test, whereas the one cycled in Zn-H 2 O shows the collapsed structure and closely attaches to the titanium foil. Therefore, Ti peaks are observed in the XRD pattern of Zn-H 2 O electrode. To examine whether the organic electrolyte affects the charge storage mechanism, X-ray photoelectron spectroscopy (XPS) measurement was further conducted for the electrodes cycled in Zn-EC/EMC.   as observed in Figures S5a-5c. The continuous amorphization with repeated cycling leads the completed collapse of the fiber structure, which is in agreement with the XRD results ( Figure S5d). Therefore, the morphologies of V 2 O 5 ·nH 2 O/CNT electrodes after 100 cycles in Zn-H 2 O and Zn-EC/EMC were examined to further investigate the stability as displayed in Figures 3(i) and 3(j). It is noted that the electrode in Zn-EC/EMC maintains the nanowire structure, whereas the one cycled in Zn-H 2 O almost disappears. The energy-disperse spectroscopy (EDS) mapping results in Figure 3(k) indicate the one-dimensional structure of V 2 O 5 ·nH 2 O in Zn-H 2 O is completely collapsed and the active materials are merged with the CNT network.

Advantages of Hybrid Electrolytes.
It is speculated that the damage of the electrode structure is related to the dissolution of active materials. The solubility of V 2 O 5 in water is 0.8 g L -1 at 20°C [26]. In an effort to identify the stability of  [27]. The above finding signifies that the electrode in Zn-EC/EMC shows much better structural stability than that in Zn-H 2 O, so as to largely improve the capacity retention. In addition to the solubility, the electrolyte also affects the charge transfer. The electrochemical impedance spectroscopy (EIS) was conducted to examine the internal resistance of the cells in both two electrolytes. The characteristic resistance values calculated according to the equivalent circuit are shown in the inset of Figure 4(b) [28,29]. R e is the resistance of electrolyte; Rst//CPEst represents the interphase contact resistance; Rct//CPEdl is the charge transfer resistance, and W 0 refers to the diffusion resistance. The fresh cell in Zn-H 2 O presents much lower values of R ct and R st than the cell in Zn-EC/EMC, due to the better wettability of both anodes and cathodes and the lower desolvation energy of Zn ions in aqueous electrolytes [17]. To reduce the charge transfer resistance and maintain the stability, an aqueous/organic hybrid electrolyte is therefore designed with various ratios. We find that with 10% H 2 O in EC/EMC as solvent, both interfacial and charge transfer resistances are largely reduced from 580.6 and 561.3 Ω to 1.6 and 298.0 Ω, respectively, while the dissolution of V 2 O 5 ·nH 2 O is minimized as evidenced by low solubility of vanadium in Figure 4(a). Further increasing the water content is helpful in improving the kinetics, but leads to deterioration of the electrode. The stability of flexible V 2 O 5 ·nH 2 O/CNT electrodes in different electrolytes is then evaluated. The electrodes were firstly activated at a low current rate of 50 mA g -1 for two cycles, as shown in Figure 4(c), before charge/discharged at 500 mA g -1 . The cells have fast capacity degradations using the electrolytes of Zn-H 2 O and Zn-H 2 O-EC/EMC(4-6). The residual capacities are only 84 and 302 mAh g -1 after 100 cycles from initial capacities of 394 and 465 mAh g -1 , respectively. In contrast, for Zn-H 2 O-EC/EMC(1-9), the capacities of cells rise gradually in the initial cycles for material activation and stabilize at 549 mAh g -1 after 100 cycles, which is even higher than its initial capacity. As to Zn-EC/EMC, the cell delivers a continuously increased capacity along with cycles. The increase in the capacity is ascribed to the decreased resistance after cycling ( Figure S7, EIS), probably attributed to the close interphase contacts among anode, cathode, and electrolyte and the electrochemical polishing of passive film on the surface of Zn anode [3,17]. The morphology of the electrode after 100 cycles in Zn-H 2 O-EC/EMC(1-9) was also examined to further confirm the excellent structural stability of V 2 O 5 ·nH 2 O in the hybrid electrolyte, as shown in Figure S6a. More water contents in the electrolyte lead to in the loose and collapsed structures after cycles ( Figures S6b-S6e), which is consistent with the solubility result (Figure 4(a)). The comparison of rate capabilities in different electrolytes measured at increasing current rates from 0.1 to 0.2, 0.5, 1, and 2 A g -1 is given in Figure 4(d). The cell in Zn-H 2 O delivers fast capacity degradations at all the current rates. Even when the rate is reduced back to 0.1 A g -1 , the capacities still continuously decrease. Under the Zn-EC/EMC electrolyte, the V 2 O 5 ·nH 2 O/CNT electrode delivers capacities of 301, 268, 210, 169, and 129 mAh g -1 , respectively. When the rate is reduced back to 0.5 and 0.1 A g -1 , the capacities are recovered to 256 and 371 mAh g -1 . The hybrid electrolyte is expected to achieve a synergistic effect in obtaining both the structural stability and superior rate capability. The organic solvent effectively enhances the stability of the electrode, and the incorporated water in the hybrid electrolyte improves the ionic conductivities as discussed before. Therefore, the electrodes in Zn-H 2 O-EC/EMC(4-6) and Zn-H 2 O-EC/EMC(1-9) show attractive capacities under all the current rates. The latter has higher value due to its advantage in inhibiting the dissolution of active materials (Figure 4(a)). The long-term cyclic test of the cell was therefore conducted in Zn-H 2 O-EC/EMC(1-9) at a high current density of 4 A g -1 as shown in Figure 4(e). The battery takes 50 cycles at 0.5 A g -1 to activate the materials and maintains a high capacity of 282 mAh g -1 after 1000 cycles. The efficiencies constantly sustain around 100% during the cyclic process. A comparison of the electrochemical performance between the current work and the vanadium-based electrodes is presented in Table S1. Among these studies, the current V 2 O 5 ·nH 2 O/CNT electrodes deliver equally excellent or even better capacities for long cycles due to the benefits of hybrid electrolytes.
Fast self-discharge is a general issue facing the aqueous batteries [30]. To explore the self-discharge rate with hybrid electrolyte, the performance of these cells is compared in Figure 5. All the batteries were cycled at 500 mA g -1 for 40 cycles to activate the electrode. They were then rested for 72 h before further discharging. The battery in Zn-H 2 O only releases a 35.1% capacity of the charged value, corresponding to a fast self-discharge rate of 0.9% per hour. This is not surprised considering the fast dissolution of V 2 O 5 ·nH 2 O in aqueous electrolyte. Along with time, active materials will be dissolved and shuttled to the anode, giving rise to a rapid self-discharge. Turning to the organic and hybrid electrolytes (10% H 2 O), the cells deliver identical discharge capacities to the charged ones. The dissolution phenomenon is totally inhibited in the two electrolytes for avoiding fast self-discharge.

Morphologies of Zn
Anodes. The formation of zinc dendrites on the anode during cycling is always a severe issue to influence the electrochemical performance of cells [31][32][33]. Continuous growth of dendrites may penetrate the separator, leading to the short circuit of batteries. This phenomenon is effectively alleviated in hybrid electrolyte. The cycled cells were disassembled as shown in Figure S8. The Zn anode is severely eroded in the aqueous electrolyte in contrast to the intact electrode in organic and hybrid electrolyte. The morphologies of Zn electrodes before and after 100 cycles were further examined by SEM. Figure 6 Figure S9). EC plays twofold effects. On the one hand, it increases the solubility of the salt in organic solvent thanks to its large dielectric constant. On the other hand, it facilities the miscibility of H 2 O, EC, and EMC mixture enabling the preparation of hybrid electrolyte [34,35]. All the cells were tested at a current density of 2 mA cm -2 with a Zn plating capacity limitation of 2 mAh cm -2 . After 50 cycles, Zn electrodes were examined by SEM, as shown in Figure S10. It shows severe Zn dendrites appear on the surfaces in Zn-H 2 O, Zn-H 2 O-EC, and Zn-EC, especially for Zn-EC where dendrites are most pronounced. However, with the presence of EMC, Zn dendrites are effectively alleviated. The suppression of dendrite growth is enhanced with increasing EMC contents, as presented in Figures S10d-S10f. The mechanism may reside in the blocking effect of alkyl groups on EMC that attaching and covering the surface of Zn electrodes, specifically the active sites where Zn dendrite grows rapidly [36].
To study the stability of Zn anodes during cycles, symmetrical cells with electrolytes of Zn-H 2 O, Zn-H 2 O-EC/EMC(1-9), and Zn-EC/EMC were electrochemically measured. Figure 6(e) presents a comparison of plating and stripping of Zn//Zn electrodes in different electrolytes at a current density of 2 mA cm -2 with a capacity limitation of 2 mAh cm -2 . In Zn-H 2 O, low overpotentials of about 59 mV are obtained in the first several cycles. However, some fluctuations with abrupt augments are observed arising possibly from the nonuniform plating/stripping. As to the cells with Zn-H 2 O-EC/EMC(1-9) and Zn-EC/EMC, although the overpotentials for the first cycle are as large as 216 and 217 mV, respectively, they gradually decrease with prolonging cycle numbers (Figures 6(f) and S11), as a result of the reduced internal resistance of batteries after cycles [17]. The above observations suggest the beneficial effect of hybrid electrolyte in stabilizing not only the cathode but also Zn anode.

Discussion
A freestanding V 2 O 5 ·nH 2 O/CNT electrode was studied as a cathode to explore the electrolyte effects on the performance of ZIBs. The zinc intercalation/deintercalation mechanism in aqueous and nonaqueous electrolyte was analyzed by ex situ XRD, revealing the similar phase transition in the discharge/charge process. However, the structure of V 2 O 5 ·nH 2 O cycled in aqueous electrolyte is broken up, whereas the electrode in organic electrolyte maintains a very stable nanowire structure. It is found that the organic electrolyte is beneficial to the inhibition of vanadium dissolution but increases the charge transfer resistance because of high desolvation energy. Therefore, combining the merits of both aqueous and organic electrolytes, the hybrid electrolyte enables the high-performance Zn-ion batteries with long cyclic stability and excellent rate capability as well as slow self-discharge. It is worth mentioning that the hybrid electrolyte keeps the advantages of nonsensitive to the ambient environment, as that in aqueous electrolyte, for decreasing the fabrication cost. We believe this work provides a novel electrolyte-based strategy in improving the performance of the Zn-ion batteries besides electrode optimization.   system (Rigaku SmartLab) with Cu Ka radiation source. To determine the water and CNT content in V 2 O 5 ·nH 2 O/CNT, thermogravimetric and differential thermal analysis (TGA-DTA) was conducted in the temperature range of 25-600°C at a heating rate of 10°C min -1 under N 2 and air atmosphere, respectively. X-ray photoelectron spectroscopy (XPS, PHI5600 by Physical Electronics, Inc.) was conducted using monochromatic Al Ka X-ray at 14 kV. The solubilities of V 2 O 5 ·nH 2 O in different electrolytes were measured by inductively coupled plasma mass spectrometry (ICP-MS).

Electrochemical
Tests. The V 2 O 5 ·nH 2 O/CNT films were cut into electrode discs of 12 mm in diameter,~1.6 mg cm -2 of active materials in weight, and~70 μm in thickness. CR2032 coin cells were assembled in the ambient using the Zn foil as the anode and the glass fiber (Whatman, GF/D) as the separator. A titanium foil was adopted as the current collector to avoid the corrosion. To investigate the solvent effect, 1 M zinc perchlorate (Zn(ClO 4 ) 2 ) in organic solvents (ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 1:1), DI water and water/organic hybrids were used as electrolyte. The coin cells with different electrolytes were charge/discharge cycled between 0.2 and 1.6 V on a LAND 2001 CT battery tester at room temperature. The electrochemical impedance spectra (EIS) were obtained at a constant perturbation amplitude of 5 mV in the frequency range between 0.01 Hz and 100 kHz on a VMP electrochemical workstation (Biologic, France).   Figure S7. EIS of the battery in Zn-EC/EMC before and after 40 cycles. Figure S8. Photographs of electrodes and separators in Zn-EC/EMC, Zn-H 2 O-EC/EMC(1-9), and Zn-H 2 O after 10 cycles. Figure S9. Photographs of H 2 O and EMC mixture and Zn(ClO 4 ) 2 in EMC. Figure S10.