Cs-Induced Phase Transformation of Vanadium Oxide for High-Performance Zinc-Ion Batteries

Rechargeable aqueous zinc‐ion batteries are promising candidate for grid‐scale energy storage. However, the development of zinc‐ion batteries has been plagued by the lack of cathode materials with high specific capacity and superior lifespan. Herein, hexagonal Cs0.3V2O5 cathode is fabricated and investigated in zinc‐ion batteries. Compared with the traditional vanadium oxides, the introduction of Cs changes the periodic atomic arrangements, which not only stabilizes the open framework structure but also facilitates the Zn2+ diffusion with a lower migration energy barrier. Consequently, high specific capacity of 543.8 mA h g−1 at 0.1 A g−1 is achieved, which surpasses most of reported cathode materials in zinc‐ion batteries. The excellent cycle life is achieved over 1000 cycles with about 87.8% capacity retention at 2 A g−1. Furthermore, the morphological evolution and energy storage mechanisms are also revealed via a series of techniques. This work opens up a phase engineering strategy to fabricate the hexagonal vanadium oxide and elucidate the application of phase‐dependent cathodes in zinc‐ion batteries.


Introduction
Aqueous zinc-ion batteries (ZIBs) stand out for the facile manufacture, good safety, and environmental friendliness in the grid-scale energy storage fields. [1][2][3][4][5] The aqueous electrolytes can provide the higher ionic conductivities ( $ 1 S cm À1 ) by three orders of magnitude than that of organic electrolytes, which favors the high rate capability and high power density. [6][7][8] The abundant reserves and mass production of Zn result in the low cost of Zn anode (USD $2 kg À1 ). [9][10][11][12][13] Furthermore, the metallic Zn as the anode delivers a high theoretical capacity (820 mA h g À1 ), high energy density (5851 mA h mL À1 ), a high overpotential for hydrogen evolution compared with other negative electrodes in aqueous electrolyte, and a low negative potential (À0.762 V vs standard hydrogen electrode). [14][15][16][17] However, it is still a great challenge to develop the high-performance cathode materials, which can provide both high capacity and good structural stability. So far, numerous polymorphs (a-, b-, c-, d-, and k-) of MnO 2 have been successfully applied in aqueous ZIBs. [18][19][20][21] Afterward, Prussian blue analogs with cubic open framework structure were explored as the Zn 2+ host. [22][23][24][25] Unfortunately, these cathode materials undergo either limited capacity or poor cycling stability, which still cannot meet the requirement of large-scale energy storage. [26][27][28][29] Recently, the V-based oxides have been reported as promising cathode materials for aqueous ZIBs. [30][31][32] The rich chemistry of V makes multi-electron transfer reaction available, when the open structure V-O polyhedral allows for the fast ion migration. [33][34][35][36] In addition, the abundant polymorphs offer the great space to modulate the atomic structures and electrochemical performance in ZIBs. [37] Phase engineering is a promising route for structural modulation in exploiting the advanced cathode materials. [38] The physic-chemical properties, such as the magnetic, electrical, catalytic, and optical properties, are dominantly influenced by the crystal phase with a defined arrangement and organization of atoms and/or building blocks. [39] Therefore, any subtle structural changes may lead to different properties. To date, phase engineering on the V-based oxide is rarely researched in ZIBs. [40] Apart from the amorphous V 2 O 5 , most of the reported V-based oxide (A x V m O n , A = metal cation) cathodes are monoclinic (Na 0.33 V 2 O 5 [41] and Ag 0.4 V 2 O 5 [42] ) and orthorhombic phase (Li 0.06 V 2 O 5 [43] ) in ZIBs. [44][45][46] Conventionally, phase modulation can be more feasibly achieved in nanomaterials than in their bulk counterparts by the defects (point-, line-, and plane-) and surface modification (ligand exchange and surface coating). [47] Under certain synthetic conditions, it is feasible to fabricate nanomaterials with unconventional phase by tuning the reaction kinetics. The driving force for the phase transition stems from the coordination environment of the incorporated ions. [22] Through a tolerance factor-like behavioral trend, Furio group shows that the phase stability and ionic migration barrier for V 2 O 5 is affected by the species and concentration of the dopants. [48] Therefore, the study of crystal polymorphs is essential to the applications of Vbased oxide in ZIBs.
In the present work, we design a novel cathode material, hexagonal Cs 0.3 V 2 O 5 , by incorporating Cs into the lattice of vanadium oxide. The Rechargeable aqueous zinc-ion batteries are promising candidate for gridscale energy storage. However, the development of zinc-ion batteries has been plagued by the lack of cathode materials with high specific capacity and superior lifespan. Herein, hexagonal Cs 0.3 V 2 O 5 cathode is fabricated and investigated in zinc-ion batteries. Compared with the traditional vanadium oxides, the introduction of Cs changes the periodic atomic arrangements, which not only stabilizes the open framework structure but also facilitates the Zn 2+ diffusion with a lower migration energy barrier. Consequently, high specific capacity of 543.8 mA h g À1 at 0.1 A g À1 is achieved, which surpasses most of reported cathode materials in zinc-ion batteries. The excellent cycle life is achieved over 1000 cycles with about 87.8% capacity retention at 2 A g À1 . Furthermore, the morphological evolution and energy storage mechanisms are also revealed via a series of techniques. This work opens up a phase engineering strategy to fabricate the hexagonal vanadium oxide and elucidate the application of phase-dependent cathodes in zinc-ion batteries.
introduction of Cs not only stabilizes the hexagonal framework but also improves the conductivity compared with the orthorhombic V 2 O 5 . In addition, the open framework structure facilitates the Zn 2+ diffusion with a lower migration energy barrier. As a result, the hexagonal Cs 0.3 V 2 O 5 delivers a specific capacity of 543.8 mA h g À1 at 0.1 A g À1 and still maintains 421.7 mA h g À1 at 2 A g À1 , which surpasses most of reported cathode materials in ZIBs. The excellent cycle life is achieved over 1000 cycles with about 87.8% capacity retention at 2 A g À1 . In addition, the morphological evolution and energy storage mechanisms are revealed via a series of techniques. This work provides a phase engineering strategy to fabricate the hexagonal vanadium oxide and elucidate the application of phase-dependent cathodes in ZIBs.

Results and Discussion
The Cs 0.3 V 2 O 5 sample was synthesized via a facile hydrothermal and annealing treatment methods using vanadium pentoxide as the vanadium source and cesium chloride as the cesium source. The detailed preparation process was shown in Figure S1, Supporting Information. The product after the hydrothermal process shows the uniform green suspension. The X-ray diffraction (XRD) pattern for the product after hydrothermal treatment was shown in Figure S2, Supporting Information, showing the poor crystallinity. The energy-dispersive spectroscopy (EDS) mapping images and EDS spectrum in Figure S3, Supporting Information show the uniform distribution of the elemental Cs, V, and O. The thermalgravimetric-differential scanning calorimeter (TG-DSC) curves in Figure S4, Supporting Information show the dominating weight loss below 150°C and the exothermic reaction below 400°C due to the loss of water for the product after the hydrothermal treatment. The endothermic behavior above 400°C is induced by the fusion process. The crystalline Cs 0.3 V 2 O 5 sample was obtained after the annealing process at 400°C in Ar atmosphere. The morphology was investigated by the scanning electron microscopy. In Figure S5, Supporting Information, the SEM images show the nano-sized Cs 0.3 V 2 O 5 sample. Figure 1a, Figures S6 and S7, Supporting Information show the atomic crystalline structures for hexagonal Cs 0.3 V 2 O 5 and orthogonal V 2 O 5 . There are two crystallographically independent five coordinated V atoms in VO 5 square pyramids linked by edges to form zigzag strings running in the c direction. The Cs shows a coordination number of 12, and the mean Cs-O distance is 3.37 A. [49] In Figure 1b Figure 1c reflects the corresponding FFT pattern. Figure S10, Supporting Information further confirms the detailed crystal plane information of Cs 0.3 V 2 O 5 . Figure 1d shows the XRD patterns of the synthesized Cs 0.3 V 2 O 5 (JCPDS #70-0325) and the commercial V 2 O 5 (JCPDS #41-1426), which are completely consistent with the standard data. [49] The EDS mapping images from V 2 O 5 in Figure S11, Supporting Information show the uniform distribution of the elemental V and O. Figure S12a, Supporting Information shows the TEM image of V 2 O 5 . In Figure S12b, Supporting Information, the selected area electron diffraction (SAED) shows a series of diffraction rings, which is coincident with that of V 2 O 5 . As shown in Figure 1e, the Cs 0.3 V 2 O 5 sample shows strong signals in the electron paramagnetic resonance (EPR) spectra, when that of the commercial V 2 O 5 only display a flat line. The EPR results indicate the presence of unpaired electrons and low-valence V for Cs 0.3 V 2 O 5 sample. [50] According to the ultraviolet diffuse reflectance spectra (UV-vis DRS) in Figure 1f, Cs 0.3 V 2 O 5 shows strong absorption between 550 and 800 nm. The corresponding Tauc plots are shown in Figure S13a, Supporting Information. As is calculated, Cs 0.3 V 2 O 5 shows a band gap of about 2.2 eV, while that of V 2 O 5 is about 2.4 eV. According to the four-probe test, the resistivity of Cs 0.3 V 2 O 5 is 7386 ΩÁcm and that of V 2 O 5 is 13 992 ΩÁcm, indicating the improved electronic conductivity. The infrared (IR) spectra in Figure S13b, Supporting Information show that the stretching vibration of V=O locates at 996 and 1018 cm À1 for Cs 0.3 V 2 O 5 , while that of V 2 O 5 locates at 1024 cm À1 . [51] This IR study further confirms the different V-O arrangement for the Cs 0.3 V 2 O 5 and V 2 O 5 .
X-ray photoelectron spectroscopy (XPS) is employed to demonstrate the chemical state of Cs, V, and O species. The XPS survey spectra of Cs 0.3 V 2 O 5 and V 2 O 5 are shown in Figure S14, Supporting Information. In Figure 1g, the two fitted peaks at 723.5 and 737.4 eV are attributed to the Cs 3d5/2 and Cs 3d3/2, respectively. Figure 1h,i show the XPS spectra of V and O from Cs 0.3 V 2 O 5 and V 2 O 5 . For Cs 0.3 V 2 O 5 , the fitted peaks at 517.0 and 524.6 eV are ascribed to V(V) 2p3/2 and V(V) 2p3/2, while the peaks at 515.6 and 523.4 eV are ascribed to V(IV) 2p3/2 and V(IV) 2p3/2, respectively. [52] The XPS data reveal that the ratio of V(V) and V(IV) is 6.5:1, which is close to that of the theoretical value The specific surface area and pore size were detected by the physical adsorption method. According to the Brunauer-Emmett-Teller (BET) method, the Cs 0.3 V 2 O 5 displays a high surface area of 81.7 m 2 g À1 and pore volume of 0.0182 cm 3 g À1 in Figure S15, Supporting Information, which can shorten the Zn 2+ diffusion path and facilitate the insertion/extraction of Zn 2+ . In addition, the large amount of microporous structure can further provide sufficient contact between the electrode and electrolyte. However, the commercial V 2 O 5 only shows a specific area of 49.1 m 2 g À1 and pore volume of 0.0117 cm 3 g À1 .
In order to investigate the Zn 2+ storage performance, the 2032 type coin cells were assembled using a zinc foil anode, a 3 M Zn(CF 3 SO 3 ) 2 electrolyte, and a non-woven fiber separator. The cyclic voltammetry (CV) curves are carried out at scan rates from 0.1 to 2 mV s À1 within a voltage window of 0.2-1.6 V versus Zn 2+ /Zn. Figure S16a, Supporting Information shows the photo image of Cs 0.3 V 2 O 5 and V 2 O 5 . The microporous membrane loading the electrode materials is shown in Figure S16b, Supporting Information. Figure S16c,d, Supporting Information show the contact angel tests for Cs 0.3 V 2 O 5 and V 2 O 5 , respectively. It is obvious that the membrane with the Cs 0.3 V 2 O 5 sample shows better contact with electrolyte, benefiting the rapid ion transport and rate performance. As shown in Figure 2a,b, both of Cs 0.3 V 2 O 5 and V 2 O 5 display two pairs of well-defined redox peaks, which can be assigned to the reversible and stepwise intercalation and extraction of Zn 2+ for the electrode materials. The CV shape and the peak position of the redox peaks for Cs 0.3 V 2 O 5 and V 2 O 5 are similar to those of previously reported vanadium oxides cathodes. [53] The integrated area of the Energy Environ. Mater. 2023, 6, e12502 2 of 8 CV curves for Cs 0.3 V 2 O 5 is larger than that for V 2 O 5 . This higher integrated area of the CV curves for Cs 0.3 V 2 O 5 indicates a larger specific capacity than that of V 2 O 5 . Cs 0.3 V 2 O 5 shows two cathode peaks at 0.44 and 0.91 V and two anode peaks at 0.69 and 1.18 V. In contrast, V 2 O 5 shows two cathode peaks at 0.40 and 0.76 V and two anode peaks at 0.83 and 1.07 V. The higher potential for the cathode peaks means the higher discharge platform. In Figure 2c, each peak interval for the two pairs of redox peaks is signed as D R1/O1 and D R2/O2 . The small peak interval means the small reaction barrier and rapid reaction kinetics. The values of D R1/O1 and D R2/O2 for Cs 0.3 V 2 O 5 are 0.25 and 0.27 V, while those for V 2 O 5 are 0.43 and 0.31 V, respectively. [54] Therefore, the introduction of Cs to V 2 O 5 lattice delivers the improved intercalation/extraction behavior of Zn 2+ , compared with that of V 2 O 5 . Through the CV curves with increased scanning speeds from 0.1 to 2 mV s À1 , the cathodic and anodic peaks shift toward the negative and positive direction due to the electrode polarization. Generally, the current response consists of the diffusion and capacitive portion, according to the formula of i = av b . In this equation, v refers to the scan rate, both a and b are the adjustable parameters. A b value of 0.5 reveals that this process is controlled by the diffusion process, when a b value of 1.0 suggests that this process is controlled by the capacitive process. In Figure 2d,g, the b values for Cs 0.3 V 2 O 5 are 0.93, 1.02, 0.94, and 1.06, when those of V 2 O 5 are 0.77, 0.78, 0.72, and 0.61, respectively. [55] Thus, it is less restricted by diffusion-controlled process for Cs 0.3 V 2 O 5 compared with that of V 2 O 5 . In order to reveal the corresponding ratios of different types of capacity, the following formula is utilized: i = k 1 v + k 2 v 0.5 , where k 1 and k 2 are the coefficients of capacitive and diffusion contributions. According to Figure 2e,h, Figures S17 and S18, Supporting Information, the ratio of capacitive contribution increases with increased scan speed and reaches 97.6% at 2 mV s À1 for Cs 0.3 V 2 O 5 . Furthermore, the portion of capacitive contribution for Cs 0.3 V 2 O 5 is higher than that of V 2 O 5 at each scan speed, revealing the improved property of intercalation pseudocapacitance. The Zn 2+ intercalation/extraction kinetics can be revealed by E a . Therefore, the charge transfer resistance (R ct ) was investigated at a series of voltages under different temperatures. In Figure S19, Supporting Information, electrochemical impedance spectroscopy (EIS) was conducted in a frequency range from 10 mHz to 100 kHz with a current amplitude of 5 mV. According to Figure 2f,i, E a at 0.5 and 1.0 V can be calculated by fitting the R ct at different temperatures into the Arrhenius equation. As a result, respectively. This E a study further confirms that the Zn 2+ intercalation/extraction process is relatively more facile for Cs 0.3 V 2 O 5 than that of V 2 O 5 .
To further reveal the Zn 2+ diffusion kinetics, the galvanostatic intermittence titration technique (GITT) was executed. As illustrated in Figure 3a,b and Figure S20, Supporting Information, the diffusion coefficient of Zn 2+ for Cs 0.3 V 2 O 5 ranges from 10 À7.5 to 10 À8.5 cm 2 s À1 , when that of V 2 O 5 ranges from 10 À8 to 10 À9 cm 2 s À1 . In addition, the diffusion coefficient of Zn 2+ for Cs 0.3 V 2 O 5 is higher than most of reported vanadium oxides cathodes. [56] The high diffusion coefficient of Zn 2+ in Cs 0.3 V 2 O 5 suggests that the hexagonal crystalline structure facilitates the rapid Zn 2+ insertion/extraction, favoring the improved performance in ZIBs. The structure-electrochemical property relationship is further investigated in Figure 3c and Figures S21-S23, Supporting Information. Figure S21, Supporting Information shows the -OH stretching and bending vibration modes and -CF 3 SO 3 vibration modes, indicating the deposition layer of zinc triflate hydroxide hydrate. [57,58] According to the ex situ grazing incidence XRD (GIXRD) spectra in Figure S22, Supporting Information, the peaks of Cs 0.3 V 2 O 5 electrode show small shift, indicating its well-structural stability. The in situ XRD was utilized to reveal the structural evolution during the Zn 2+ insertion/extraction processes for Cs 0.3 V 2 O 5 cathode. During the first discharge and charge processes, the peaks assigned to Cs 0.3 V 2 O 5 from 10 to 40 degree exhibited almost negligible shift. In addition, all XRD peaks in terms of either position or intensity remained almost unchanged, implying a single-phase reaction in the whole electrochemical processes. [45] Through the in situ XRD evolution, there is only tiny shift for the peaks of Cs 0.3 V 2 O 5 . This result indicates that there is small structure distortion in the charge/discharge processes. [59] Consequently, the Cs 0.  Information shows the detailed XPS spectra evolution when Cs 0.3 V 2 O 5 cathode was discharged at 0.7 and 0.2 V and charged at 0.7 and 1.6 V. During the discharge process, the intensity of V(V) peak becomes much weaker, whereas the intensity of V(IV) peak becomes much stronger. This result demonstrates that the reduction in V to lower valence is derived from the intercalation of Zn 2+ . During the charge process, the intensity of V(V) peak becomes much stronger, when that of V(IV) peak becomes much weaker. In addition, the XPS spectra of O also present a similarly reversible evolution, which is induced by the intercalation of Zn 2+ . The XPS spectra of Zn shows that there is an incomplete extraction of Zn 2+ in the Cs 0.3 V 2 O 5 lattice. The findings reveal the small degree of structural change and valence variation during the charge/ discharge processes. In Figures S28-S30, Supporting Information, the Cs 0.3 V 2 O 5 cathode at 0.2 V is characterized to further confirm the presence of Zn atoms. It is clear that there exists a large amount of Zn in Cs 0.3 V 2 O 5 cathode based on the EDS spectrum. In addition, the elemental Zn is uniformly dispersed in the host of Cs 0.3 V 2 O 5 , implying that Cs 0.3 V 2 O 5 can act as Zn 2+ host in ZIBs. Figures S31-S33, Supporting Information shows the Cs 0.3 V 2 O 5 cathode after charging to 1.6 V. According to the EDS spectrum, there is still a little Zn in the Cs 0.3 V 2 O 5 cathode after the charge process, consistent with the XPS result. This phenomenon is ascribed to the irreversible extraction of Zn 2+ , which has been revealed in most of reported V-based cathode. More importantly, the Cs 0.3 V 2 O 5 cathode retains the pristine morphology by the and 269.5 mA h g À1 at specific current density of 0.1, 0.2, 0.5, 1, 2, and 5 A g À1 , respectively. When the specific current density goes back from 5 to 0.1 A g À1 , the specific capacity of Cs 0.3 V 2 O 5 electrode is recovered to 533.0 mA h g À1 , suggesting the superior rate capability. Compared with the capacities at 0.1 A g À1 , the capacity retention of Cs 0.3 V 2 O 5 electrode is 77.5% at 2 A g À1 . In addition, the Coulombic efficiency is nearly 100% at each specific current density, demonstrating well reversibility in the charge/discharge processes. On the contrast, the V 2 O 5 electrode only shows the discharge capacities of 350.5, 346.9, 321.1, 296.8, 268.1, and 203.5 mA h g À1 at specific current density of 0.1, 0.2, 0.5, 1, 2, and 5 A g À1 , respectively. With the current density increasing from 0.1 to 5 A g À1 and then decreasing back to 0.1 A g À1 , V 2 O 5 electrode shows apparent capacity degrade. This result suggests that Cs 0.3 V 2 O 5 electrode facilitates the facile diffusion ability of Zn 2+ . Additionally, the long-term cycling performance of Cs 0.3 V 2 O 5 and V 2 O 5 is also evaluated in Figure 3e,g. Cs 0.3 V 2 O 5 electrode shows a capacity retention of 82.3% at 0.5 A g À1 after 300 cycles, when the rapid capacity degrade is observed for the V 2 O 5 electrode. In addition, Cs 0.3 V 2 O 5 electrode shows a high-capacity retention of 87.8% at 2 A g À1 compared with its initial capacity after 1000 cycles, when V 2 O 5 maintained only 40.6% of its initial capacity. The EIS plots before and after the cycling test have been shown in Figure S35, Supporting Information, which indicates the well stability of Cs 0.3 V 2 O 5 after the long cycling test. The superior cycle stability of Cs 0.3 V 2 O 5 can be attributed to the hexagonal phase structure, which acts as a well-substantial host for the insertion/extraction of Zn 2+ . As shown in Figure 3h and Table S1, Supporting Information, the energy density and specific capacity of Cs 0.3 V 2 O 5 cathode is significantly superior to most of the other reported V-based cathodes in ZIBs, indicating that Cs 0.3 V 2 O 5 is a very promising candidate for fabricating highperformance electrochemical energy storage devices.
The Vienna ab initio simulation package (VASP) is further employed to investigate the energy storage mechanisms of Cs 0.3 V 2 O 5 and V 2 O 5 in ZIBs. The calculation result of the density of states (DOS) in Figure 4a,b shows that the introduced Cs results in the unequal distribution between the spin-up and spin-down electrons for Cs 0.3 V 2 O 5 , which is in complete with the EPR result. Furthermore, the introduced Cs mainly contributes to the high-density state and the DOS profiles of Cs 0.3 V 2 O 5 pass through the Fermi level, indicating the half-metallic behavior and facilitating the electron transmission in electrochemical reactions. Compared with V 2 O 5 , the lowered orbitals for Cs 0.3 V 2 O 5 verify the stronger electron-acceptor character, which is in accordance with the higher discharge platform. Figure 4c shows the partial charge distribution around the mid-gap state above the Fermi level, which validates that the unoccupied states originate from the V atoms. The V atoms can provide acceptor level for electron donated by the Zn atoms. Figure S36, Supporting Information shows the charge density difference of the transition state of Zn migration in V 2 O 5 and Cs 0.3 V 2 O 5 . The Zn atom interacts with the O atoms, which exactly bond with the V atoms. The electrons tend to flow to the V atoms, leading to their decreased valence state. The abovementioned unoccupied orbitals can accept the extra electrons and then stabilize the transition state of the Zn migration. Figure S37, Supporting Information shows the transition state for the Zn-inserted Cs 0.3 V 2 O 5 and V 2 O 5 . The compound chemistry and crystal structure of electrode materials determine the ion migration and mobility. The coordination environment of Cs 0.3 V 2 O 5 differs from that of V 2 O 5 . The basic structural unit of Cs 0.3 V 2 O 5 is VO 5 , whereas that of V 2 O 5 is VO 6 . As shown in Figure S37, Supporting Information, Zn 2+ inserts the Cs 0.3 V 2 O 5 and forms ZnO 2 in the transition state. In addition, the distance between the other O atoms and Zn 2+ is larger than 3 A. Compared with the ZnO 4 structure in the transition state for V 2 O 5 , the introduction of Cs results in the hexagonal crystal structure, which reduces the activation barrier for ion hopping. [60] According to Figure 4d, the Zn-O bonds for Cs 0.3 V 2 O 5 (1.82 and 1.84 A) display smaller bond distance than that of V 2 O 5 (1.88 A), indicating the stronger combination ability toward Zn for Cs 0.3 V 2 O 5 . In addition, the Zn-inserted Cs 0.3 V 2 O 5 shows less structural distortion than that of V 2 O 5 , which is beneficial to enhanced cycling stability in ZIBs. Figure S38, Supporting Information shows the two pathways of Zn migration inside the Cs 0.3 V 2 O 5 and V 2 O 5 framework. Meanwhile, the intercalation and extraction energy barriers of Zn 2+ in Cs 0.3 V 2 O 5 structure is only about 1.16 eV, which is lower than most of reported data. Therefore, the Cs 0.3 V 2 O 5 can act as an excellent Zn 2+ host in ZIBs.

Conclusion
In summary, this work provides a phase engineering strategy to exploit the cathodes in ZIBs. The hexagonal phase Cs 0.3 V 2 O 5 is fabricated as an optimal cathode by introducing Cs in the lattice of vanadium oxide. The crystalline framework of Cs 0.3 V 2 O 5 serves an ideal Zn 2+ host, facilitating the rapid diffusion kinetics. Furthermore, there is no significant phase changes and structural collapse among the insertion/extraction process of Zn 2+ . The obtained Cs 0.3 V 2 O 5 shows the improved electronic conductivity, in favor of the charge transfer process. As a result, high specific capacity of 543.8 mA h g À1 at 0.1 A g À1 is achieved, which surpasses most of reported cathode materials in ZIBs. The excellent cycle life is achieved over 1000 cycles with about 87.8% capacity retention at 2 A g À1 . This phase engineering provides guidelines toward the design and synthesis of the desired phase-dependent cathodes for ZIBs.

Experimental Section
The material preparation method, the characterization of materials, electrochemical measurement and theoretical calculation method are mentioned in the supplementary data.