English

• Zinc-ion batteries (ZIBs) are attracting numerous attentions as a promising candidate for future energy storage devices ascribing to the abundant zinc resource, high safety and low cost [1-6]. In particular, quasi-solid-state ZIBs with better flexibility, high compatibility with flexible electronics and higher inhibition of Zn dendrites have become the hotspot of research [7-10]. Nevertheless, the strong electrostatic forces experienced by the moving divalent Zn²⁺ ions in the cathode materials raise difficulties in the repetitive Zn²⁺ insertion/extraction reaction and cause structural distortion/failure, and thus resulting in unsatisfying electrochemical performance [11]. Therefore, seeking and optimizing cathode materials with rigid structure stability and excellent Zn²⁺ (de)intercalation kinetics has become a priority as well as a challenge for the development of ZIBs.

  Up to now, the researches of cathode materials for ZIBs are mainly focused on Mn-based materials [12-15], Prussian blue analogs [16-19], and V-based materials [20-24]. Among them, the Mn-based materials always suffer from rapid capacity attenuation during the repeated charging and discharging processes, deriving from the irreversible phase transition and Mn dissolution [25-27]. As for Prussian blue analogs, the capacity (< 100 mAh/g) is too low to meet the requirements for practical application. Considering these facts, great devotion has been paid to the study of V-based materials as cathodes for ZIBs. V-based materials can exhibit a large Zn storage capacity of ~400 mAh/g due to the variable chemical states of vanadium ions between +5 and +3. And they usually possess layered or tunnel structures with large inner spaces that allow rapid diffusion of Zn²⁺ ions. Additionally, the vanadium element is very rich in natural resources, which further reduces the cost.

  Among all the V-based materials, the layered hydrated vanadium pentoxide (V₂O₅·mH₂O, named as HVO) is of great potential as cathode for ZIBs, in which the expanded interlayer distances (~12 Å) caused by structural water molecules efficiently promote the diffusion and storage of Zn²⁺ ions [28-30]. However, the HVO material is still beset by poor structural stability during the process of repeated Zn²⁺ ions insertion/extraction [31, 32]. For instance, Sun et al. developed a binder-free V₂O₅·nH₂O/reduced graphene oxide film as the solid-state ZIBs cathode [33], which showed a rapid capacity decay when cycling at 0.1 A/g (capacity retention of 28% after 100 cycles). Xu et al. fabricated a 3D flower-like V₂O₅·nH₂O/MXene composite as aqueous ZIBs cathodes [34]. Although electrical conductivity of the composite was enhanced, the electrochemical property was still hampered by the sluggish Zn²⁺ ion diffusion and ineffective utilization (capacity retention of 69% after 50 cycles at 0.1 A/g). To address this inherently low structural stability and enhance the Zn²⁺ ion diffusion kinetics of HVO, a foreign cation pre-intercalation method is proposed. The foreign cations can act as "pillar" that not only stabilize the layered structure of the material, but also enhance its electrical conductivity. HVO with various foreign cations pre-intercalated in the interlayer spacing have been investigated, such as Na⁺ [35], Ca²⁺ [36], Zn²⁺ [37], Mg²⁺ [29], Mn²⁺ [30], Ni²⁺ [38], NH₄⁺ [39], La³⁺ [40]. For example, Mg_0.34V₂O₅·nH₂O nanobelts were designed and prepared by Ming et al. [29], which showed an enhanced cycling performance (negligible capacity decay at 0.1 A/g during 200 cycles, and ~97% capacity retention at 5 A/g after 2000 cycles). Liu et al. proposed Mn(II)-cation-expanded hydrated vanadium pentoxide [30], delivering a ultrastable cycling performance (96% capacity retention at 4 A/g after 2000 cycles). All these researches proved the effectiveness in enhancing the electrochemical performance of HVO by introduction of foreign cations and points out the direction for further in-depth investigations.

  Inspired by these intriguing properties, we developed a simple microwave-assisted hydrothermal route to synthesize pre-potassiated hydrated vanadium pentoxide (K_xV₂O₅·nH₂O, named as KHVO) nanobelts, which are intertwined with each other to form a wrinkled three-dimensional (3D) structure. The KHVO material shares a similar layered structure with hydrated vanadium pentoxide, and the K⁺ ions in the interlayer serve as "pillars" to stabilize the lamellar structure. It possesses a large interlayer spacing (~10.4 Å) along c axis, which makes the diffusion of Zn²⁺ ions effortless. Supported by all these advantages, the KHVO cathode achieves an enhanced zinc storage property in quasi-solid-state ZIBs, specifically a relatively high reversible capacity of ~300 mAh/g at 100 mA/g, acceptable rate capability of 65 mAh/g at 5 A/g and ultrastable long-term cycling performance (> 1500 cycles at 2 A/g).

  Through a simple microwave-assisted hydrothermal route, the KHVO material was prepared. Firstly, 3 mmol of vanadium pentoxide (V₂O₅) and 3 mL of 30% hydrogen peroxide (H₂O₂) solution were dissolved in deionized water (30 mL) and magnetic stirred for 20 min. After a homogeneous reddish-brown solution was formed, 6 mmol of potassium chloride (KCl) was weighed and poured into the above solution and kept stirring for a while. Afterward, the obtained solution was sealed in reactors and reacted for 2 h at 180 ℃. After this hydrothermal process, the obtained product was washed with deionized water and absolute ethanol repeatedly and dried for 12 h at 60 ℃ vacuum oven.

  The control HVO material was prepared by a simple hydrothermal according to the previous literature [31]. 0.32 g of ammonium metavanadate (NH₄VO₃) was blended with 80 mL of mixed solvent of deionized water and ethanol (9:1, v/v), and the pH value of mixture was reduced to 2 using hydrochloric acid (HCl). After that, the resulting solution was loaded into a 100 mL reactor and kept at 160 ℃ for 12 h to form V₂O₅·mH₂O.

  The phase of the sample was analyzed by X-ray powder diffraction (XRD) measurement (Bruker, AXS D8 advance, Cu Kα radiation). The field-emission scanning electron microscopy (FESEM, JEOL, Model JSM-7600F) and the transmission electron microscopy (TEM, JEOL, Model JEM-2100) technologies were applied to investigated the morphology and nanostructure of the sample. The valence states of various constituent elements were investigated by the X-ray photoelectron spectroscopy (XPS, Thermo Fisher, Model ESCALAB 250 Xi). The ratio of pre-intercalated potassium element to vanadium element in the sample was confirmed by inductively coupled plasma (ICP, Thermo Fisher, Model ICAP RQ). For the target of measuring the content of crystal water, thermal gravimetric analyzer (TGA, Mettler Toledo, Model DSC3+) was applied.

  The Zn²⁺ ion storage properties of KHVO materials were evaluated in the quasi-solid-state zinc ion batteries. The cathode slurry was made of active material, carbon nanotube (CNT) and poly-(vinyldifluoride) (PVDF) in the mass ratio of 7:2:1 in N-methylpyrrolidone (NMP) solvent. Then, the electrode was prepared by coating the above slurry on titanium foil (diameter: 12 mm) and drying it in vacuum oven under 70 ℃ for more than 12 h. The mass loading of active material is controlled to ~1 mg/cm². The quasi-solid-state polymer membrane was prepared as follows: (1) adding 750 mg of PVDF polymer and 1 mmol of Zn(ClO₄)₂ salt in 10 mL of N-dimethylformamide (DMF) solvent under magnetic stirring to form homogeneous solution; (2) pouring the obtained solution into a mold with a diameter of 90 mm and drying it at vacuum oven for 8 h under 80 ℃; (3) cutting the dried membrane into 16 mm diameter discs and soaking them into 0.5 mol/L Zn(ClO₄)₂ in propylene carbonate (PC) solution for 24 h; (4) removing the excess electrolyte completely before the usage. The quasi-solid-state ZIBs were assembled with obtained KHVO cathode, zinc foil and above polymer membrane in the air. The galvanostatic charging-discharging tests were carried out using NEWARE battery system during the voltage range of 0.4–1.4 V vs. Zn²⁺/Zn. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests within the frequency ranging from 100 kHz to 0.01 Hz were evaluated by CHI660E electrochemical station. For comparison, the liquid-state ZIBs was also evaluated using glass fiber and 0.5 mol/L Zn(ClO₄)₂ in PC solvent as membrane and electrolyte, respectively.

  The HVO and KHVO were fabricated through a facile traditional and microwave-assisted hydrothermal route, respectively, whose structures were confirmed by X-ray powder diffraction (XRD) measurements. The HVO sample (Fig. S1 in Supporting information) depicts a set of diffraction peaks corresponding to a set of planes along the z-axis of the layered V₂O₅·mH₂O, revealing a preferred orientation along the (001) plane [29-31, 33]. While for the XRD pattern of KHVO nanobelts in Fig. 1a, a few additional peaks are detected other than the major peaks along the (00l) direction ascribing to the intercalation of K⁺ ions in the interlayers forming K_xV₂O₅·mH₂O phase as reported in previous studies [28, 38, 41, 42]. As illustrated in the inset of Fig. 1a, the KHVO has a layered structure, where the bilayered chains of VO₅ pyramids are connected by the corner-sharing O atoms and accommodate the K⁺ ions and H₂O molecular between the interlayers. And according to Bragg's law, the diffraction peak at 8.5° for the (001) plane is calculated with an interlayer distance to be as large as 10.4 nm, providing sufficient spaces for rapid diffusion of Zn²⁺ ions in the KHVO sample.

  Figure 1

  

  Figure 1.  (a) XRD pattern of KHVO (inset: schematic diagram of crystal structure, green and red balls stand for V and O, respectively, and blue balls stand for K and H₂O molecule). (b) XPS survey of KHVO and respective high-resolution spectra of (c) O 1s and V 2p and (d) K 2p.

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  To explore the elemental compositions and chemical states of the KHVO sample, X-ray photoelectron spectroscopy (XPS) measurements were carried out. The survey confirms the existence of V, O and K elements (Fig. 1b). Fig. 1c shows the high-resolution spectrum of O 1s and V 2p region for the KHVO sample. Two states of O 1s at 531.6 eV for H–O and 530.7 eV for V–O can be differentiated, deriving from crystal water and VO layer, respectively [42]. In the V 2p_3/2 region, the main characteristic peak located at 517.8 eV is identified to be V⁵⁺. A relatively weak characteristic peak located at 516.7 eV is also detected, which indicates the presence of V⁴⁺ resulted from the K⁺ ions pre-intercalation [43]. And K 2p region of KHVO sample is also provided in Fig. 1d, which shows two peaks located at 295.9 and 293.1 eV, belonging to the K 2p_1/2 and K 2p_3/2, respectively [44, 45].

  Furthermore, to measure the content of K element and crystal water in the KHVO sample, inductively coupled plasma (ICP) and thermal gravimetric analysis (TGA) were conducted. According to the ICP test, the atomic ratio of K to V is determined to be 0.26:1, corresponding to a formula of K_0.52V₂O₅·nH₂O. The TG analysis results are given in Fig. S2 (Supporting information). The rapid weight loss of both samples under 100 ℃ comes from the volatilization of physically absorbed water. As the temperature increases further, for KHVO sample, 2.3% of weight loss occurs, which is caused by the loss of structural water, implying that n is ~0.29 and thus K_0.52V₂O₅·0.29H₂O. In the same way, the crystal water content in HVO sample is calculated to be 8.8%, fixing the formula as V₂O₅·0.94H₂O.

  Scanning electron microscope (SEM) and transmission electron microscope (TEM) tests were conducted to observe the morphology and microstructure of both samples. The low-magnification SEM image (Fig. S3a in Supporting information) of the HVO sample displays the uniformly distributed nanobelt morphology with a length of tens of micrometers. An enlarged view in Fig. S3b (Supporting information) reveals the width of the HVO nanobelts to be 60–100 nm. While for the KHVO sample, a wrinkled three-dimensional (3D) structure morphology (Figs. 2a and b) is observed, in which the micron-sized nanobelts are cross-linked and intertwined. Moreover, the TEM images (Figs. 2c and d, Fig. S4 in Supporting information) reveal the width of the nanobelts is around 20–50 nm. Zooming in the nanobelt in the marked area in Fig. 2d, the high-resolution TEM (HRTEM) picture in the inset shows a lattice fringe of 0.24 nm, corresponding to the diffraction peak at ~34° (311) in XRD pattern. Additionally, the dark-field scanning TEM (STEM) image and its respective energy-dispersive X-ray spectroscopy (EDS) element mapping images in Fig. 2e exhibit the uniform distribution of K, V and O elements in KHVO nanobelt.

  Figure 2

  

  Figure 2.  (a, b) SEM images, (c, d) TEM images (inset: high-magnification TEM) and (e) dark-field STEM image and corresponding K, V and O elemental EDS mapping of KHVO.

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  Zinc storage performance of the KHVO as cathode materials was evaluated in quasi-solid-state ZIBs consisting of metallic zinc foil as anode and PVDF-Zn(ClO₄)₂-based polymer membrane as a quasi-solid electrolyte at a voltage window of 0.4–1.4 V vs. Zn²⁺/Zn. The electrochemical behavior of the KHVO electrode is firstly evaluated by the cyclic voltammetry (CV) technique, as shown in Fig. 3a. One distinct pair of redox peaks locating at 0.9 and 1.0 V are observed in all four CV cycles, relating to the Zn²⁺ ions intercalation and extraction process, respectively. The redox peaks experience a slight shift with the CV shapes remained intact and stabilize from the 3^rd cycle onwards, expressing high reversibility of the redox reactions for KHVO cathode. At a low current density of 100 mA/g the initial three galvanostatic charging-discharging cycles of KHVO cathode are displayed in Fig. 3b. It obtains a relatively high discharge capacity of 373 mAh/g and a charge capacity of 332 mAh/g in the first cycle, with a coulombic efficiency (CE) of 89%. The almost overlapping charging-discharging profiles for the 2^nd and 3^rd cycles display well-defined redox plateaus, suggesting the high reversibility of the redox reactions. Except for a slight decay during the initial few cycles, a stable and reversible specific capacity of around 300 mAh/g with 100% CE is achieved for KHVO in the following 100 cycles at 100 mA/g with a high capacity retention of 88% compared to the second cycle (Fig. 3c). In contrast, the HVO cathode shows a restricted zinc storage property at the same condition, only emerging a discharge capacity of 96 mAh/g after 100 cycles, resulting in a poor capacity retention of 45%. Furthermore, the KHVO cathode in liquid-state ZIBs also shows stable cycling performance (Fig. S5a in Supporting information), further implying the cycling stability of KHVO cathode.

  Figure 3

  

  Figure 3.  (a) The initial three cycles galvanostatic charging-discharging profiles of KHVO cathode at 100 mA/g. (b) Cycling stability at 100 mA/g. (c) The first four CV curves for KHVO at 0.1 mV/s. (d) Rate property at various current densities. (e) Cycling stability at 2 A/g up to 1500 cycles.

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  Moreover, the rate capability of the KHVO cathode was also evaluated, as shown in Fig. 3d. At current densities of 0.1, 0.2, 0.5, 1, 2 and 5 A/g the reversible capacities of 270, 215, 166, 135, 110 and 65 mAh/g are achieved, respectively. On the contrary, the pure HVO cathode exhibits a relatively poor rate capability (i.e., 250, 190, 133, 90 and 48 mAh/g at 0.1, 0.2, 0.5, 1 and 2 A/g, respectively) and capacity of the battery become almost undetectable when the current density increases to more than 2 A/g. In addition, the rate performance of the KHVO cathode in liquid-state ZIBs was also evaluated (Fig. S5b in Supporting information). It is can be seen that the KHVO cathode in quasi-solid-state ZIBs delivers an acceptable capacity retention, maintaining 70% capacity of liquid-state ZIBs even at a high rate of 5 A/g. The long-term cycling stability of both electrodes at a relatively high rate of 2 A/g are compared as well. As shown in Fig. 3e, KHVO electrode exhibits almost no capacity decay throughout 1500 cycles with almost 100% CE and maintains a capacity of 120 mAh/g after 1500 cycles. The galvanostatic charging-discharging profiles of KHVO at different cycles also show almost no change (Fig. S6 in Supporting information), indicating superior reversibility and stability. While a gradual capacity decay is observed for the HVO electrode with a capacity retention of 60% after 1500 cycles at 72 mAh/g.

  To find out the origin of the superior electrochemical performance of KHVO electrode comparing to HVO electrode, the apparent Zn²⁺ diffusion coefficients of both samples were calculated based on the CV technique at various scan rates (Figs. 4a and b, Fig. S7 in Supporting information). As indicated in Fig. 4a, when the scan rate increases from 0.1 mV/s to 1.0 mV/s, the position of the redox peaks for KHVO electrode shift slightly, but the overall shapes of CV curves are well maintained. As can be seen from Fig. 4b and Fig. S7b, the peak currents (i_p) are linearly related to the square root of sweeping rates (ν^1/2), demonstrating a typical diffusion-controlled process [46, 47]. After that, the diffusion coefficient of Zn²⁺ ions (D_Zn²⁺) will be obtained by the equation as follows:

  (1)

  Figure 4

  

  Figure 4.  (a) CV curves of KHVO cathode at various scanning rates. (b) The linear fitting of the peak current (i_p) as the y-axis and the square root of scanning rate (ν^1/2) as the x-axis. (c) GITT profiles of KHVO cathode. (d) The calculated diffusion coefficient of Zn²⁺ in KHVO cathode.

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  where n stands for the amount of charge involved in the reaction, which comes from the integral over the redox peak. S stands for the contacting area between the electrode and electrolyte. C_Zn²⁺ stands for the Zn²⁺ concentration in the electrode material, which is determined by the ratio of Zn²⁺ involved in the reaction to the molar volume of KHVO and HVO. The main oxidation peaks marked at Fig. 4a and Fig. S7a were chosen as a representative to explore the D_Zn²⁺. From the linear relationship between i_p and ν^1/2 for KHVO cathode (i = 0.00113ν^1/2 − 1.41231 × 10⁻⁴, as shown in Fig. 4b) and HVO cathode (i = 0.00581ν^1/2 + 4.31421 × 10⁻⁵, as shown in Fig. S7b), D_Zn²⁺ is calculated to be 2.27 × 10⁻¹² cm²/s and 1.14 × 10⁻¹³ cm²/s for KHVO and HVO electrodes, respectively.

  The galvanostatic intermittent titration technique (GITT) was also conducted, as shown in Fig. 4c and Fig. S8a (Supporting information). Based on the following equation [48-50]:

  (2)

  where m_B, V_M, M_B, S, τ, ΔE_s and ΔE stand for the mass of electrode material, molar volume, molecular weight, geometric area of electrode, the duration of the current pulse, change of quasi-equilibrium potential and the change of battery voltage, respectively. The apparent Zn²⁺ ions diffusion coefficient under different charge/discharge depth are calculated to be ~10⁻¹⁰–10⁻¹² cm²/s for KHVO cathode (Fig. 4d) and ~10⁻¹²–10⁻¹⁴ cm²/s for HVO cathode (Fig. S8b in Supporting information), respectively, which are well consistent with the above D_Zn²⁺ determined by CV technique and further indicates the fast Zn²⁺ ions diffusion coefficient in KHVO. Besides, according to the electrochemical impedance spectroscopy analysis (Fig. S9 in Supporting information), the charge-transfer resistance of the KHVO (213 Ω) is much lower than that of HVO (572 Ω). Hence, it is believed that pre-potassiated modification contributes to the fast electrochemical kinetics of the KHVO [51-53].

  Additionally, to verify the Zn²⁺ ions storage mechanism in the KHVO electrode, ex-situ XRD characterization at various charging-discharging states was carried out to explore the evolution of its crystal structure (Fig. 5a). The pristine KHVO electrode shows similar diffraction peaks of (001), (300), (511) and (405) planes to those of the KHVO powder, except for the peaks from carbon nanotubes (CNTs) (conducting material) and Ti foil (current collector). During the process of Zn²⁺ ions insertion/extraction, the same diffraction peaks as the pristine KHVO electrode are detected without generation of new diffraction peaks, evidencing the well-maintained layered KHVO crystal structure. Furthermore, a highly reversible variation of the (001) peak is evidenced, which is plotted in the enlarged (001) peak region. Upon the Zn²⁺ ions insertion, (001) peak shifts to the lower angles with an increase of the interlayer spacing. In the following Zn²⁺ ions extraction process, (001) peak gradually restores to its original position as well as the relaxation of the interlayer distance. This transition demonstrates a (de)intercalation reaction mechanism of the Zn²⁺ ion in KHVO material with superior reversibility, which can be expressed as follows: K_0.52V₂O₅·nH₂O + xZn²⁺ + xe⁻ ↔ Zn_xK_0.52V₂O₅·nH₂O. In addition, the XRD pattern of KHVO cathode after long cycling (Fig. S10 in Supporting information) shows no obvious change compared with original condition, indicating the superior cycling stability of KHVO electrode. To examine the vanadium chemical states at different electrochemical states the ex-situ XPS was also conducted (Fig. 5b). After fully discharge, only one set of characteristic peaks at 516.5 and 523.9 eV can be fitted, which belongs to the V 2p_3/2 and V 2p_1/2 of the V⁴⁺, respectively. And when the charging is completed, the vanadium returns to its pristine V⁵⁺ state with V 2p_3/2 and V 2p_1/2 at 517.6 and 516.4 eV, respectively. Thus, based on these ex-situ analyses, we believe that the high stability and reversibility of KHVO electrode during the electrochemical reaction process provide a solid foundation for superior Zn storage property.

  Figure 5

  

  Figure 5.  (a) Ex situ XRD patterns of the KHVO cathode during the first cycle. (b) Ex situ XPS spectrum of V 2p region at the full discharge/charge states.

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  In summary, K⁺ intercalation modified hydrated vanadium pentoxide (K_0.52V₂O₅·0.29H₂O, KHVO) sample with a wrinkled 3D structure assembled by nanobelts was constructed via a microwave-assisted hydrothermal route. When used as cathode for quasi-solid-state ZIBs, the KHVO achieves an enhanced electrochemical performance of high capacity (~300 mAh/g and 88% capacity retention at 100 mA/g after 100 cycles), acceptable rate capability (110 and 65 mAh/g at 2 and 5 A/g, respectively) and ultrastable cycling performance (at 2 A/g for more than 1500 cycles). Such superior performance mainly comes from the modification of intercalated K⁺, which stabilizes the lamellar structure as "pillars" and improves the Zn²⁺ ion diffusion kinetics of the electrode material.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  The authors gratefully acknowledge the National Natural Science Foundation of China (Nos. 51972067, 51802265, 51802044, 51902062 and 51802043), and the Guangdong Natural Science Funds for Distinguished Young Scholar (No. 2019B151502039).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.09.091.

  
  
• 1. [1]

     Q.F. Li, X.H. Rui, D. Chen, et al., Nano-Micro Lett. 12 (2020) 1267

  2. [2]

     F. Wang, O. Borodin, T. Gao, et al., Nat. Mater. 17 (2018) 543–549 doi: 10.1038/s41563-018-0063-z

  3. [3]

     Y. Bai, H. Zhang, B. Xiang, et al., J. Colloid Interface Sci. 597 (2021) 422–428 doi: 10.1016/j.jcis.2021.04.010

  4. [4]

     M. Song, H. Tan, D. Chao, H.J. Fan, Adv. Funct. Mater. 28 (2018) 1802564 doi: 10.1002/adfm.201802564

  5. [5]

     D. Xie, F. Hu, X. Yu, et al., Chin. Chem. Lett. 31 (2020) 2268–2274 doi: 10.1016/j.cclet.2020.02.052

  6. [6]

     L. Su, L. Liu, Y. Wang, et al., Chin. Chem. Lett. 31 (2020) 2358–2364 doi: 10.1016/j.cclet.2020.03.014

  7. [7]

     Y. Zeng, X. Zhang, Y. Meng, et al., Adv. Mater. 29 (2017) 1700274 doi: 10.1002/adma.201700274

  8. [8]

     D. Chao, C. Zhu, M. Song, et al., Adv. Mater. 30 (2018) 1803181 doi: 10.1002/adma.201803181

  9. [9]

     S. Huang, F. Wan, S. Bi, et al., Angew. Chem. Int. Ed. 58 (2019) 4313–4317 doi: 10.1002/anie.201814653

  10. [10]

      K. Leng, G. Li, J. Guo, et al., Adv. Funct. Mater. 30 (2020) 2001317 doi: 10.1002/adfm.202001317

  11. [11]

      B. Tang, L. Shan, S. Liang, J. Zhou, Energy Environ. Sci. 12 (2019) 3288–3304 doi: 10.1039/c9ee02526j

  12. [12]

      B. Yang, X. Cao, S. Wang, et al., Electrochim. Acta 385 (2021) 138447 doi: 10.1016/j.electacta.2021.138447

  13. [13]

      W. Fan, F. Liu, Y. Liu, et al., Chem. Commun. 56 (2020) 2039–2042 doi: 10.1039/c9cc08604h

  14. [14]

      L. Wang, Z. Cao, P. Zhuang, et al., ACS Appl. Mater. Interfaces 13 (2021) 13338–13346 doi: 10.1021/acsami.1c01405

  15. [15]

      K. Zhang, D. Kim, Z. Hu, et al., Nat. Commun. 10 (2019) 5203 doi: 10.1038/s41467-018-07646-4

  16. [16]

      Z. Li, T. Liu, R. Meng, et al., Energy Environ. Mater. 4 (2021) 111–116 doi: 10.1002/eem2.12108

  17. [17]

      Z. Liu, G. Pulletikurthi, F. Endres, ACS Appl. Mater. Interfaces 8 (2016) 12158–12164 doi: 10.1021/acsami.6b01592

  18. [18]

      H. Yi, R. Qin, S. Ding, et al., Adv. Funct. Mater. 31 (2021) 2006970 doi: 10.1002/adfm.202006970

  19. [19]

      M. Zhang, R. Liang, T. Or, et al., Small Struct. 2 (2021) 2000064 doi: 10.1002/sstr.202000064

  20. [20]

      X. Zhang, H. Chen, W. Liu, et al., Chem. Asian J. 15 (2020) 1430–1435 doi: 10.1002/asia.202000162

  21. [21]

      D. Chen, X. Rui, Q. Zhang, et al., Nano Energy 60 (2019) 171–178 doi: 10.1016/j.nanoen.2019.03.034

  22. [22]

      C. Xia, J. Guo, Y. Lei, et al., Adv. Mater. 30 (2018) 1705580 doi: 10.1002/adma.201705580

  23. [23]

      S. Chen, K. Li, K.S. Hui, J. Zhang, Adv. Funct. Mater. 30 (2020) 2003890 doi: 10.1002/adfm.202003890

  24. [24]

      W. Liu, L. Dong, B. Jiang, et al., Electrochim. Acta 320 (2019) 134565 doi: 10.1016/j.electacta.2019.134565

  25. [25]

      A. Konarov, N. Voronina, J.H. Jo, et al., ACS Energy Lett. 3 (2018) 2620–2640 doi: 10.1021/acsenergylett.8b01552

  26. [26]

      S. Zhao, B. Han, D. Zhang, et al., J. Mater. Chem. A6 (2018) 5733–5739 doi: 10.1039/C8TA01031E

  27. [27]

      G. Fang, C. Zhu, M. Chen, et al., Adv. Funct. Mater. 29 (2019) 1808375 doi: 10.1002/adfm.201808375

  28. [28]

      A. Moretti, S. Jeong, S. Passerini, ChemElectroChem 3 (2016) 1048–1053 doi: 10.1002/celc.201600040

  29. [29]

      F. Ming, H. Liang, Y. Lei, et al., ACS Energy Lett. 3 (2018) 2602–2609 doi: 10.1021/acsenergylett.8b01423

  30. [30]

      C. Liu, Z. Neale, J. Zheng, et al., Energy Environ. Sci. 12 (2019) 2273–2285 doi: 10.1039/c9ee00956f

  31. [31]

      Y.J. Zhang, X.Y. Yuan, T. Lu, et al., J. Colloid Interface Sci. 585 (2021) 347–354 doi: 10.1177/0959651820948648

  32. [32]

      S. Natarajan, S. -J. Kim, V. Aravindan, J. Mater. Chem. A 8 (2020) 9483–9495 doi: 10.1039/d0ta02852e

  33. [33]

      J. Sun, Y. Zhang, Y. Liu, et al., J. Colloid Interface Sci. 587 (2021) 845–854 doi: 10.3390/jmse9080845

  34. [34]

      G.S. Xu, Y.J. Zhang, Z.W. Gong, et al., J. Colloid Interface Sci. 593 (2021) 417–423 doi: 10.1016/j.jcis.2021.02.090

  35. [35]

      C. -Y. Lee, A.C. Marschilok, A. Subramanian, et al., Phys. Chem. Chem. Phys. 13 (2011) 18047–18054 doi: 10.1039/c1cp21658a

  36. [36]

      C. Xia, J. Guo, P. Li, et al., Angew. Chem. Int. Ed. 57 (2018) 3943–3948 doi: 10.1002/anie.201713291

  37. [37]

      D. Kundu, B.D. Adams, V. Duffort, et al., Nat. Energy 1 (2016) 16119 doi: 10.1038/nenergy.2016.119

  38. [38]

      J. Feng, Y. Wang, S. Liu, et al., ACS Appl. Mater. Interfaces 12 (2020) 24726–24736 doi: 10.1021/acsami.0c04199

  39. [39]

      L. Xu, Y. Zhang, J. Zheng, et al., Mater. Today Energy 18 (2020) 100509 doi: 10.1016/j.mtener.2020.100509

  40. [40]

      J. He, X. Liu, H. Zhang, et al., ChemSusChem 13 (2020) 1568–1574 doi: 10.1002/cssc.201902659

  41. [41]

      M. Tian, C. Liu, J. Zheng, et al., Energy Storage Mater. 29 (2020) 9–16 doi: 10.1016/j.ensm.2020.03.024

  42. [42]

      M. Ghosh, S. Dilwale, V. Vijayakumar, S. Kurungot, ACS Appl. Mater. Interfaces 12 (2020) 48542–48552 doi: 10.1021/acsami.0c13221

  43. [43]

      S. Mickevicius, V. Bondarenka, S. Grebinskij, et al., Micron 40 (2009) 126–129 doi: 10.1016/j.micron.2007.12.010

  44. [44]

      S. Li, M. Chen, G. Fang, et al., J. Alloy. Compd. 801 (2019) 82–89 doi: 10.15302/j-laf-20190107

  45. [45]

      Y. Liu, Y. Qiao, W. Zhang, et al., Nano Energy 5 (2014) 97–104 doi: 10.1016/j.nanoen.2014.02.010

  46. [46]

      Q. Li, D. Chen, H. Tan, et al., J. Energy Chem. 40 (2020) 15–21 doi: 10.1117/12.2573596

  47. [47]

      X.H. Rui, D.H. Sim, C. Xu, et al., RSC Adv. 2 (2012) 1174–1180 doi: 10.1039/C1RA00698C

  48. [48]

      B. Tang, G. Fang, J. Zhou, et al., Nano Energy 51 (2018) 579–587 doi: 10.1016/j.nanoen.2018.07.014

  49. [49]

      L. Deng, X. Niu, G. Ma, et al., Adv. Funct. Mater. 28 (2018) 1800670 doi: 10.1002/adfm.201800670

  50. [50]

      X. Guo, G. Fang, W. Zhang, et al., Adv. Energy Mater. 8 (2018) 1801819 doi: 10.1002/aenm.201801819

  51. [51]

      Q. Li, H. Zhu, Y. Tang, et al., Chem. Commun. 55 (2019) 12108–12111 doi: 10.1039/c9cc06362e

  52. [52]

      J. Liu, Y. Cao, J. Zhou, et al., ACS Appl. Mater. Interfaces 12 (2020) 54537–54544 doi: 10.1021/acsami.0c13835

  53. [53]

      Z. Yan, H.Y. Pan, J.Y. Wang, et al., Rare Metals 40 (2021) 1357–1365 doi: 10.1007/s12598-020-01494-2

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  Figure 1  (a) XRD pattern of KHVO (inset: schematic diagram of crystal structure, green and red balls stand for V and O, respectively, and blue balls stand for K and H₂O molecule). (b) XPS survey of KHVO and respective high-resolution spectra of (c) O 1s and V 2p and (d) K 2p.

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  Figure 2  (a, b) SEM images, (c, d) TEM images (inset: high-magnification TEM) and (e) dark-field STEM image and corresponding K, V and O elemental EDS mapping of KHVO.

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  Figure 3  (a) The initial three cycles galvanostatic charging-discharging profiles of KHVO cathode at 100 mA/g. (b) Cycling stability at 100 mA/g. (c) The first four CV curves for KHVO at 0.1 mV/s. (d) Rate property at various current densities. (e) Cycling stability at 2 A/g up to 1500 cycles.

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  Figure 4  (a) CV curves of KHVO cathode at various scanning rates. (b) The linear fitting of the peak current (i_p) as the y-axis and the square root of scanning rate (ν^1/2) as the x-axis. (c) GITT profiles of KHVO cathode. (d) The calculated diffusion coefficient of Zn²⁺ in KHVO cathode.

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  Figure 5  (a) Ex situ XRD patterns of the KHVO cathode during the first cycle. (b) Ex situ XPS spectrum of V 2p region at the full discharge/charge states.

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