In Situ Electrochemically Transforming VN/V2O3 Heterostructure to Highly Reversible V2NO for Excellent Zinc Ion Storage

Achieving aqueous zinc‐ion batteries (AZIBs) with high capacity and long lifetime remains challenging because the intense charge repulsion of multivalent ions causes structural instability and sluggish kinetics. The electrochemical activity brought by in situ structure optimization has dramatically improved the electrochemical performance. Hereinto, the nanocomposites consisting of VN/V2O3 heterostructure composited with carbon (VN/V2O3@C) by a self‐template strategy are synthesized. The VN/V2O3 heterostructure undergoes an in situ electrochemical activation phase transition to highly reversible V2NO after the first cycle. The interface of V2O3 and VN induces ion displacement polarization under the action of the applied electric field, making it easier for oxygen and nitrogen atoms to dope into the crystal structure of VN and V2O3, contributing to V2NO phase formation. Furthermore, theory calculations demonstrate that V2NO can provide favorable adsorption for reversible Zn2+ storage. The V2NO@C electrode thus delivers high reversible capacities of 490.2 mAh g−1 after 310 cycles at 200 mA g−1 and impressive long‐cycle stability over 6000 cycles at 10 A g−1. Herein, it sheds new light on the mechanism of in situ electrochemical phase transition from heterostructures into one phase, which is a great revolution in designing cathode materials for AZIBs.


Introduction
[11][12] In addition, aqueous electrolytes have higher ionic conductivity (%1 S cm À1 ) and faster ion transfer than organic electrolytes (%1-low intrinsic conductivity causes slow kinetics. [42]Despite many efforts, its electrochemical performance is far from meeting people's needs.
The fierce electrostatic interaction between Zn 2þ and with host lattice of vanadium-based compounds is inclined to ignite the difficult Zn 2þ migration with a higher energy barrier. [44]hese materials commonly experienced the "trapping" of Zn 2þ in lattice or even phase transition into another zinc-involved phase.Such phase transition often plays a role in electrochemical activation during cycling and has dramatically improved the electrochemical performance of many cathode materials, such as V 2 O 3 .The present literature reported the transformation of V 2 O 3 into structures favorable for Zn 2þ storage, such as V d -V 2 O 3 to Zn-doped V 2 O 3 , [10] V 2 O 3 to Zn 0.4 V 2 O 5Àm •H 2 O, [26] and V 2 O 3 to V 2 O 5Àx •nH 2 O. [42] And the phase transformation mechanism of V 2 O 3 has also been investigated in the above works.It follows that the transition of V 2 O 3 is complicated depending on the synthesis and its structural/ compositional environments.It also infers that the transition has not been explored completely, considering the diverse synthetic methodologies and elaborate structures.
Herein, we successfully synthesized the porous VN/V 2 O 3 @C heterostructure by a facile self-template strategy.According to the results of X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS), the VN/V 2 O 3 heterostructure was determined to undergo a complete transition to the V 2 NO phase through in situ electrochemical activation to boost the electrochemical activity of the electrode.This is the first report of heterostructure-derived phase transition, different from the cases of previously reported vanadium oxide (i.e., from one to another phase). [10,26,42,45]The phase transition mechanism was also studied and proposed here.In addition, the calculations demonstrate that the appropriate adsorption energy of V 2 NO promotes favorable zinc ion storage, circumvents the trapping of Zn 2þ in the lattice, and ensures the continuous insertion/ extraction of zinc ions.Coupled with the high capacitance contribution beyond 92%, the V 2 NO@C electrode enhances the zinc ion storage performance.

Results and Discussion
The VN/V 2 O 3 @C heterostructure nanosheets were synthesized by the conventional two-step method.As schematically illustrated in Figure 1a, the VO 2 nanosheets were first hydrothermally synthesized as precursors.Then, a post-calcination treatment was done with C 3 N 4 as a nitrogen source to obtain VN/V 2 O 3 @C.Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were conducted to characterize the morphology of the synthesized products.The precursor VO 2 exhibits uniform nanosheet morphology with a smooth homogeneous surface (Figure S1a-e, Supporting Information).High-resolution TEM (HRTEM) image reveals lattice-resolved fringes with a distance of 0.31 nm, resulting from the (002) crystal planes of the VO 2 phase (Figure S1f, Supporting Information).The elemental mapping images of VO 2 nanosheets in Figure S1g-i, Supporting Information, confirm the uniform distribution of V and O elements.After calcination at 800 °C under an argon atmosphere, the final product VN/V 2 O 3 @C maintains the nanosheet morphology of the paternal precursor VO 2 (Figure S1, Supporting Information).SEM images of VN/V 2 O 3 @C show the typical nanosheet morphology with a low aspect ratio and remarkable porous structure (Figure 1b-d).
The length of nanosheets can extend to hundreds of nanometers with well-defined widths of 60-100 nm, as evidenced in TEM images (Figure 1e,f ).Compared with VO 2 nanosheets, the VN/V 2 O 3 @C is a homogeneous porous structure favorable for increasing the specific surface area.The rich pores can increase the contact area between the electrode material and the electrolyte (Figure 1g).In Figure 1h, we observe the remarkable lattice fringes with a spacing of 0.24 nm, corresponding to the V 2 O 3 (110) lattice planes.Then, other lattice fringe domains with a spacing of 0.21 nm are observed, corresponding to VN (200) lattice planes.Lots of defects are found in V 2 O 3 and VN domains.Significantly, there is a prominent heterostructure boundary between V 2 O 3 and VN, confirming the presence of heterointerface in VN/V 2 O 3 @C.Figure 1i,j (taken from Figure 1h) shows the line profile images of the crystal planes of V 2 O 3 and VN, respectively.That is consistent with the results of Figure 1h.
To further obtain the crystal structural information of the above two samples, we made a further XRD analysis.In Figure 2a, the XRD pattern of VN/V 2 O 3 @C can be well indexed to the cubic VN (JCPDF: 97-064-4847) and trigonal V 2 O 3 (JCPDF: 97-009-5760), whereas the synthesized precursor is ascribed to the pure monoclinic VO 2 phase (JCPDF: 97-007-3855).Subsequently, to verify the importance of calcination temperature for targeted synthesis, we used VO 2 precursor and C 3 N 4 with a mass ratio of 1:1 under an argon atmosphere at a heating rate of 5 °C min À1 at different calcination temperatures.The XRD patterns (Figure S2, Supporting Information) show that the VN/V 2 O 3 @C phase cannot be formed at 600, 700, or 750 °C.A high calcination temperature of 800 °C favors the structural transformation from VO 2 .
The specific surface area and pore size distribution are usually characterized by the N 2 physical sorption method.As shown in Figure 2b, the derived Brunauer-Emmett-Teller (BET) specific surface area of VN/V 2 O 3 @C is as large as 144.3 m 2 g À1 , which is much higher than that of VO 2 (%27 m 2 g À1 ).The pore-size distribution in Figure 2b inset obtained from the Barrett-Joyner-Halenda (BJH) method reveals the mesoporosity of VN/V 2 O 3 @C.It indicates that the majority of pores concentrate in a narrow pore-size range of 3-4 nm and have higher pore volume than VO 2 .Furthermore, the generated porous network structure resulted in a noticeable increase in the specific surface area of VN/V 2 O 3 @C, providing more contact between the electrode and the electrolyte and thus conducive to enhancing mass transfer. [10]ven though the morphologies exhibited in FESEM and TEM images of VN/V 2 O 3 @C and VO 2 are similar, is there amorphous carbon in VN/V 2 O 3 @C?The samples were analyzed by thermogravimetric-differential scanning calorimetry (TG-DSC) and Raman spectroscopy.The TG-DSC analysis was performed under an air atmosphere at a 5 °C min À1 ramping rate.Figure S3 and S4, Supporting Information, show that the weight loss below 210 °C is due to the dehydration of physically absorbed water.Based on this, it could be estimated that there is about 7.5 and 4.3% water in VN/V 2 O 3 @C and VO 2 , respectively.
According to Figure S3, Supporting Information, the VN/V 2 O 3 @C still has about 1.9% weight loss from 300 to 350 °C, which probably corresponds to the carbon content.In order to confirm the above analysis, VN/V 2 O 3 @C sample was calcined in an air atmosphere at different temperatures.From the XRD results (Figure S5, Supporting Information), it can be seen that VN/V 2 O 3 @C starts to be oxidized to V 2 O 5 at 300 °C (PDF: 97-064-7638).Moreover, the products obtained above 350 °C are all V 2 O 5 .So, the following chemical equation was obtained.
It should be noted that the sample reaches a constant weight at 400 °C, and the amorphous carbon coated with VN/V 2 O 3 @C is completely oxidized in the test process.Figure S4, Supporting Information, shows a significant decrease after 200 °C, which can be attributed to the oxidization of residual surfactant in the VO 2 sample.It also provides the carbon source for the subsequent product VN/V 2 O 3 @C.After 350 °C, there is an apparent weight rise, which mainly results from the complete oxidation to the V 2 O 5 .The local structure information can be analyzed from Raman spectra of VN/V 2 O 3 @C and VO 2 samples, as shown in Figure S6, Supporting Information.For VN/V 2 O 3 @C, two characteristic peaks at about 1389 and 1585 cm À1 represent the D-band (the defects) and G-band (graphitized carbon), respectively, which is valid evidence of the presence of amorphous carbon in VN/V 2 O 3 @C nanosheets.The carbon will significantly improve the electrochemical performance of the VN/V 2 O 3 @C electrode. [46,47]The Raman spectrum of the VO 2 nanosheets shows no signal peaks in the range of 1200-1800 cm À1 , indicative of no carbon present.The functional groups have been analyzed using Fourier transform infrared (FTIR) spectroscopy (Figure 2c) in the range of 4000-400 cm À1 using the KBr pellet as the control.From the FTIR spectrum, a free stretching peak appears at around 3438 cm À1 and is attributed to the O-H elongation mode of H 2 O. [48] The peaks related to the asymmetrical stretching vibration of gaseous CO 2 are also visible from 2377 to 2314 cm À1 . [49]urthermore, a strong peak at 1098 cm À1 is assignable to the C-N stretching vibrations. [50]The FTIR absorption band at 953 cm À1 is related to V═O vibrations. [51]In addition, we found a band at 1039 cm À1 corresponding to the C─O bond. [52]ompared with the FTIR spectrum of the VO 2 sample, the sharp peak located at 960 cm À1 is the characteristic peak of VN. [53] The XPS characterizations were also performed to clarify the surface information of both samples (Figure S7, Supporting Information).The chemical environments of elements in the samples are critical information.The survey XPS spectra (Figure S7, Supporting Information) present the existence of V, O, N, and C elements consistent with the XRD results.The sharp V 2p 3/2 and V 2p 1/2 bands in both samples can be observed (Figure 2d and S8, Supporting Information).More specifically, the distance V 2p 3/2 peak of VN/V 2 O 3 @C at 517.8 eV corresponds to the V 5þ ─O bond, indicating its surface was partially oxidized. [43]Also, another obvious peak at 516.8 eV corresponds to the V─N─O bond. [53]In addition, the signal of V─N is evident at 514 eV. [54]The presence of the peak at 397 eV in the N 1s spectrum (Figure 2e) also directly proves the formation of VN in the phase. [54]As for VO 2 , the V 4þ and V 5þ signals locate at 516.3 and 517.4 eV (Figure S8, Supporting Information), [29] demonstrating that the surface was also partially oxidized when exposed to air.As shown in the high-resolution O 1s spectrum (Figure 2f ), the VN/V 2 O 3 @C exhibits O─V (530.3 eV), O═C (530.8 eV), and O─N─V bond (531.6 eV), while VO 2 (Figure S9, Supporting Information) displays O-V (529.9 eV), O═C (530.8 eV), and O─H (532.4 eV). [25,29,31,54]The N 1s spectrum in VN/V 2 O 3 @C contains N-V (397 eV), pyridinic N (397.7 eV), pyrrolic N (399.8eV), and graphitic N (401.8eV), while VO 2 (Figure S10, Supporting Information) exhibited pyrrolic N (399.9eV) and amines (401.4 eV) from surfactant. [43,54]The C 1s spectrum is another essential piece of information that attracted our attention.We found that the C 1s in VN/V 2 O 3 @C (Figure S11, Supporting Information) contains C-C (284.8 eV), C-N (286.1 eV), C═O (288.4 eV), and C═C (282.9 eV).As for VO 2 , the high-resolution C 1s spectrum (Figure S12, Supporting Information) can be resolved into three components centered at 284.8, 285.9, and 287.8 eV, representing the C-C, C-N from surfactant, and C═O, respectively. [25,43,52]o evaluate the electrochemical performance, ZIBs were fabricated into 2032 typed coin cells in air, employing the obtained samples as the cathode, zinc foil as the anode, 3 M Zn(CF 3 SO 3 ) 2 as the aqueous electrolyte, and glass fiber as the separator.The cyclic voltammetry (CV) plots of the VN/V 2 O 3 @C electrode at 0.1 mV s À1 from 0.2 to 1.4 V versus Zn 2þ /Zn, as shown in Figure 3a.The integrated area of the first cycle CV curve is significantly smaller than that of the last four cycles, indicating that the electrochemical activity is improved after the first cycle, increasing the corresponding capacity. [55]The electrochemical activation process resulted in another structure more suitable for Zn 2þ insertion/extraction analyzed later.The overlap of CV curves for the last four cycles indicates a highly reversible Zn 2þ insertion/extraction process. [27]Clearly, three pairs of main redox peaks can be seen related to the Zn 2þ insertion/extraction.In contrast, three pairs of redox peaks can be seen in the CV curves of the VO 2 electrode, as shown in Figure S13, Supporting Information, also illustrating a three-step reaction related to the Zn 2þ insertion/extraction.The initial five cycles present that the CV curves do not overlap well, indicating the worse reversibility of the VO 2 cathode.
In Figure 3b, the porous VN/V 2 O 3 @C electrode reveals outstanding rate capability when the current density stepwise increases.The discharge capacities at 0.05, 0.2, 0.5, 1, 2, 5, and 10 A g À1 are 633, 540.5, 461.2, 411.8, 366.6, 312.8, and 262.8 mAh g À1 .The retention of capacities reaches 41.5% when the current density increases by 200 times.Moreover, the discharge capacity can restore 515 mAh g À1 when the current density returns to 0.05 A g À1 , demonstrating an excellent rate capability.The corresponding charge-discharge curves at different current densities from 0.05 to 10 A g À1 are displayed in Figure 3c, wherein a stable voltage platform pertains even at a current density as high as 10 A g À1 .In contrast, VO 2 displays a much poorer rate capability than VN/V 2 O 3 @C (Figure 3b), especially at high rates.Moreover, as the rate increases to 5 A g À1 , the reversible capacity is almost zero.The capacity retention is only 9.4%, with the current density from 0.05 to 10 A g À1 .The discharge capacity at high current density strongly requires an excellent charge transfer rate, including electrons and ions.Such low capacity of VO 2 implies the poor reaction kinetics caused by the high electron and ion transportation resistance.
The Nyquist plots of fresh VN/V 2 O 3 @C and VO 2 electrodes are shown in Figure 3d.The VN/V 2 O 3 @C electrode exhibits a smaller semicircle at high frequencies, indicating reduced electron transfer resistance and enhanced electrochemical reaction kinetics.Table S1, Supporting Information, shows the electrochemical impedance parameters obtained by date fitting with the electrical equivalent circuit.The charge transfer resistance R ct values of VN/V 2 O 3 @C and VO 2 are 159 and 220.7 Ω, respectively, indicating the higher conductivity of VN/V 2 O 3 @C to accelerate the transmission of electrons/ions, thereby improving the electrochemical performance. [32]he cyclability of the battery was tested at a low current density of 0.2 A g À1 (Figure 3e).The initial capacities of both electrodes approach, but the VN/V 2 O 3 @C electrode exhibits almost a stable capacity as the cycling progresses, and the reversible capacity is still 490.2mAh g À1 even after 310 cycles.While the VO 2 electrode has an obvious capacity decay after the initial 50 cycles, and the reversible capacity is only 63.9 mAh g À1 after 310 cycles.The capacity retention degradation during the long cycling ought to result from the structural destruction proved later.When tested at 1 A g À1 , the VN/V 2 O 3 @C electrode could deliver a capacity of 420.9 mAh g À1 after 900 cycles with a retention rate of 91.5%.In sharp contrast, the VO 2 electrode only retained 21.3% (87.5 mAh g À1 ), indicating low reversibility and poor cyclability (Figure S14, Supporting Information).Then, we tested the two electrodes at a high current density of 5 A g À1 .Remarkably, the VN/V 2 O 3 @C electrode still displays a long-term cycle life and offers a capacity of 305.8 mAh g À1 with 82.6% capacity retention after 2000 cycles (Figure S15, Supporting Information).These results powerfully demonstrate that VN/V 2 O 3 @C electrode has good stability and zinc storage performance even at a high current density.
The excellent electrochemical properties of the VN/V 2 O 3 @C electrode stimulated us further to explore their cycling stability at higher current density.As shown in Figure 3f, even at the ultrahigh current density of 10 A g À1 , a high reversible capacity of 204.2 mAh g À1 was delivered.The CE remained at about 100% even after 6000 cycles, indicating that VN/V 2 O 3 @C has good practicability under the high-rate charge-discharge process.Figure S16, Supporting Information, is the Ragone diagram of the energy and power density of the VN/V 2 O 3 @C electrode compared with other ZIBs cathodes.][58][59] At the same time, the cycling and rate performance of the VN/V 2 O 3 @C electrode have also been compared with other vanadium nitrides.As shown in Table S2, Supporting Information, it can be seen that the VN/V 2 O 3 @C electrode is better than most vanadium nitrides in terms of specific capacity and cycle stability.
In fact, after carefully examining the cycling and rate behaviors in Figure 3 above, it can be found that the reversible capacity and CE value of the first charge/discharge cycle are very low and significantly differs from the following cycles.The ex situ XRD measurements were done to determine the underlying matter and gain insight into the Zn 2þ storage mechanism.In Figure 4a, the electrode supplies a low first discharge capacity of 258.8 mAh g À1 but delivers a high charge capacity of 1473.8 mAh g À1 with a low CE of only 17.5%.Afterward, the CE is nearly 100% for the second charge and discharge cycle.Such discrepancy phenomenon between the initial two cycles also happens in reported works. [26,42]It can be rationally speculated that an irreversible phase transition occurs during the first cycle.From the ex situ XRD patterns corresponding to the selected stages during the initial two cycles, the VN/V 2 O 3 @C undergoes a distinct phase transition from VN/V 2 O 3 to V 2 NO (JCPDF card: 97-004-3182) with high reversibility after the first charge.The diffraction peaks of the V 2 O 3 phase tend to persist before charging to 1.2 V for the first cycle.Upon discharging in the second cycle, the characteristic pattern of the V 2 O 3 phase completely disappears, indicating the phase transition toward V 2 NO indexed by a standard JCPDF card.Furthermore, reversible cycling of V 2 NO@C was performed in the second and subsequent cycles (Figure 4b).
The new-formed V 2 NO phase can boost the electrochemical activity of the electrode for fast zinc ion storage.In the magnified (012) diffraction of V 2 O 3 (Figure 4c), a slight reduction of the interlayer distance during the first discharge.This phenomenon is because more intercalated Zn 2þ with a solvation effect will shield the electrostatic repulsion between layers as discharging proceeds.Moreover, the hydrogen bonds formed between Zn 2þ , H 2 O, and lattice oxygen/nitrogen make the crystal layers more compact. [60,61]This similar phenomenon also occurs in peaks of ( 104) and (110) of V 2 O 3 and (111) of VN (Figure S17, Supporting Information).While in the second discharge, activated V 2 NO improved the structural coordination due to the strong electrostatic interaction between the insertion of Zn 2þ and the host during the discharge process, so the (111) interplanar spacing of V 2 NO is expanded during discharge.After charging, a full recovery suggests good reversibility (Figure 4d). [53,62]ctually, the V 2 NO phase resembles the VN phase in terms of the XRD pattern.Therefore, to further confirm the formation of V 2 NO, the XRD patterns of both were carefully compared and analyzed.It can be seen from Figure S18, Supporting Information, that the (200) peak of V 2 NO is slightly shifted toward higher angles.V 2 NO can be considered to form by O atoms substituting N sites of the VN phase.When smaller O atoms enter pristine N sites, a slight decrease in interplanar spacing happens.Additionally, XPS tests were further performed on the activated V 2 NO electrode.In the V 2p spectrum (Figure S19, Supporting Information), the peaks at 516.2 and 517.6 eV, respectively, correspond to V─N─O and V─O bonds, [53] demonstrating the formation of V 2 NO.Compared with the V 2p spectrum of the VN/V 2 O 3 @C, the peak at 514 eV characteristic of V─N disappears, further verifying no VN in the post-activated electrode.We also used the TEM technique to detect the electrode after the first cycle.As shown in Figure 4e, the nanosheet structure was maintained.The inset image is the corresponding select area electron diffraction (SAED) pattern recorded from Figure 4e.The bright diffraction spot set confirms the single crystallinity of V 2 NO, verifying the phase transition from VN/V 2 O 3 .And the two adjacent noticeable spots can be indexed to be ( 200) and (220).Figure 4f describes the clear lattice fringes manifesting the excellent crystallinity of V 2 NO.The magnified HRTEM image in Figure 4g provides more detailed information on the V 2 NO@C microstructure.The lattice fringes correspond to (200) and (020) planes of the V 2 NO phase.Moreover, high-angle annular dark field-electron diffraction spectroscopy (HAADF-EDS) mappings declare the homogeneous distribution of V, N, and O elements over the nanosheets (Figure 4h).Overall, comprehensive XRD, XPS, HRTEM, and elemental mapping results all show that the activated product of phase transition is V 2 NO.
In order to visualize the microstructure changes of the VN/V 2 O 3 @C after the first cycle, the SEM technique was applied to the activated V 2 NO electrode.Compared with the pristine VN/V 2 O 3 @C electrode, the activated V 2 NO@C electrode (Figure S20, Supporting Information) maintained the porous nanosheet structure from maternal VN/V 2 O 3 @C, effectively ensuring the contact between the electrode and the electrolyte.Considering a complete phase transition, the structural transformation mechanism from the VN/V 2 O 3 heterostructure to V 2 NO was proposed here.Figure 4i shows the schematic diagram of the phase transition after the first activation cycle.As an early transition metal element, the V atom has a shortage of 3d electrons.Oxygen and nitrogen atoms at the interface between VN and V 2 O 3 have a strong ability to attract electrons.Under the action of an applied electric field, the mutual coupling of O atoms and VN as well as N atoms and V 2 O 3 at the interface happens, and the 3d electrons of the V atom transfer to the 2p orbital of the O and N atoms, which improves electron delocalization between V, N, and O atoms.During the charging process, ion displacement polarization occurs under an external electric field.The defects of VN and V 2 O 3 intensify the movement of O and N atoms, making it easier for them to be doped into the VN and V 2 O 3 lattices rendering the phase transition.Finally, a stable V 2 NO structure is formed when the electron density on the O, N, and V atoms is balanced.
Then, a series of electrochemical characterizations are applied to investigate the surface charge storage behavior of the V 2 NO@C electrode in a half cell. Figure 5a and S21a, Supporting Information, are CV curves of V 2 NO@C and VO 2 electrodes at different scan rates ranging from 0.
, where a is a variable parameter and b value can be obtained from the slope of log(v) versus log(i) curve.When the b value is 0.5, the dynamics of zinc ions are controlled by diffusion.When the b value is close to 1, it is the supercapacitor characteristic, and the capacitance of the surface controls the charge storage. [10]n Figure 5a, two pairs of obvious redox peaks of the V 2 NO electrode represent the typical insertion/extraction electrochemical characteristics and show good rate performance.As shown in Figure 5b, the b-values of the V 2 NO electrode are 0.97, 0.80, 0.90, and 1.00 for peaks 1, 2, 3, and 4, respectively.These values are close to 1, indicating that the main charge is stored on the The k 1 and k 2 are obtained from the slope and intercept of the v 1/2 versus i(v)/v 1/2 plots, respectively. [63]The capacitance and diffusion-controlled capacities are evaluated at various scanning rates, and the capacitance contribution is specified at the tiled area in the corresponding CV curve.The capacitive contributions are 92.4% and 74.9% for V 2 NO (Figure 5c) and VO 2 electrodes (Figure S21c, Supporting Information) at 0.3 mV s À1 , respectively.It should be noted that the capacitance-contributed fitting current is higher than the experimental CV curve, and thus some area is overflowed in Figure 5c.This deviation can be attributed to the flaws of the calculation method: [64,65] 1) the ohmic resistance was not taken into account during the calculation, resulting in a deviation of the experimental data of the redox peak potential from the fitting results; 2) the residual current was not considered when the CV scanning was reversed, and it also can render the fitted current higher than the experimental values; 3) the double-layer capacitance may to some extent increase the pseudocapacitance.Further, we compare the two electrodes at various scanning speeds.The results show that the ratio of pseudocapacitance to the total capacitance of V 2 NO@C is higher than that of VO 2 .The ratios are 92.5, 92.9, 94.0, and 96.3% for V 2 NO@C (Figure 5d) and 77.1, 81.3, 82.8, and 84.8% for VO 2 (Figure S21d, Supporting Information) at 0.5, 0.7, 0.9, and 1.1 mV s À1 , respectively.The in situ generated V 2 NO@C from VN/V 2 O 3 @C could obviously boost pseudocapacitive charge storage favoring the high rate capability.
After that, the galvanostatic intermittent titration technique (GITT) was adopted to study Zn 2þ solid-state diffusion kinetics in V 2 NO@C and VO 2 during cycling. [33]At the selected second cycle, the GITT-determined discharge/charge curves are shown in Figure S22 and S23, Supporting Information, at a current density of 30 mA g À1 .The Zn 2þ diffusion coefficient of both electrodes during the cycles ranges from 10 À9 to 10 À8 cm À2 s À1 (Figure S24, Supporting Information).V 2 NO exhibits a comparable diffusion rate to VO 2 , suggesting a similar Zn 2þ diffusion profile.Hence, the faster electrochemical kinetics and more excellent electrochemical performance of VN/V 2 O 3 @C mainly result from the higher capacitance contribution initiated by forming the porous nanosized V 2 NO via in situ electrochemical activation, consistent with the above charge contribution results.
Density functional theory (DFT) calculations were applied to explore the adsorption energy of zinc ions over different materials.Intriguingly, the adsorption energy is À1.44 eV toward the adsorbed Zn 2þ on the (111) surface, favoring the adsorption of Zn 2þ and improving the cycling stability of V 2 NO (Figure 5e).The feasible and sustainable insertion of Zn 2þ into V 2 NO affords the capacity and voltage.Surprisingly, the blocked adsorption of Zn 2þ into VO 2 on the (002) surface is observed due to the positive Gibbs free energy (þ0.98 eV) (Figure 5f ), demonstrating the poor Zn 2þ storage capability of VO 2 . [10]Furthermore, the direct insertion of Zn 2þ into VO 2 may cause structural instability.In contrast with the VO 2 , the high reversible structure of V 2 NO ensures high cycling stability during the insertion and extraction of zinc ions.Similarly, since the adsorption energy of Zn 2þ on the (104) crystal plane of V 2 O 3 is þ0.72 eV (Figure S25, Supporting Information), the insertion of Zn 2þ was hindered and led to structural instability. [10]The adsorption energy of Zn 2þ on the VN(111) crystal plane is À0.68 eV (Figure S26, Supporting Information), closer to thermal neutrality and favoring the (de)intercalation of Zn 2þ .In the VN/V 2 O 3 heterostructure, the structural instability of V 2 O 3 caused by Zn 2þ insertion tends to affect the VN structure because of the intact heterointerface between them, possibly triggering the as-observed phase transition of VN/V 2 O 3 .
Therefore, the excellent electrochemical performance of the activated V 2 NO@C is related to three aspects: 1) The structure shows a large porosity and high specific surface area, which can effectively expand the contact area between the electrode active substance and the electrolyte and shorten the migration distance of Zn 2þ .In addition, the small size of crystals and larger specific surface area enable Zn 2þ to have more storage sites on the surface, providing more pseudocapacitive contributions; 2) Due to the highly stable structure of V 2 NO, the stress generated by the volume expansion of Zn 2þ during the intercalation/extraction process is effectively relieved, and the cycle stability is effectively guaranteed; and 3) The appropriate adsorption energy favorably promotes the rapid intercalation/deintercalation of Zn 2þ .

Conclusion
This work reports the successful synthesis of VN/V 2 O 3 @C by one-step nitrogenization of precursor VO 2 nanosheets.As the cathode material of AZIBs, the VN/V 2 O 3 @C undergoes a structural transition to V 2 NO@C after the first cycle through an in situ electrochemically activation, demonstrated by the XRD, XPS, and HRTEM results.The possible structural transition mechanism was studied and proposed here.This phase transition can well boost the electrochemical activity of the electrode for fast zinc ion storage.Moreover, other structural features, including abundant pore structure, large capacitance contribution, suitable adsorption energy, and high electrical conductivity, can also favor zinc ion storage.The V 2 NO@C electrode achieved a high reversible capacity of 490.2 mAh g À1 after 310 cycles at 0.2 A g À1 and impressive long-cycle stability (over 6000 cycles at 10 A g À1 ).This in situ structural activation strategy involves heterostructureinduced phase transition, which provides a new perspective on the Zn 2þ storage mechanism and the influence of insertion/desertion of Zn 2þ on the structure of vanadium oxide.

Experimental Section
Material Synthesis: All chemical reagents were used as received without further purification.A simple hydrothermal treatment was used to synthesize VO 2 nanosheets.In a typical procedure, 1 mmol ammonium metavanadate (NH 4 VO 3 , 99%, Shanghai Macklin Biochemical Co., Ltd.) and 1 g polyvinylpyrrolidone (99%, Shanghai Macklin Biochemical Co., Ltd.) were added to 30 mL deionized water; then 1 mL nitric acid (1 mol L À1 ) was added to the above solution and stirred at room temperature till the solution turned blood red.Finally, the red solution was transferred to a 50 mL Teflon-lined stainless steel autoclave and heated for 40 h in an electric oven at 180 °C.The resulting product was centrifuged and washed with deionized water and ethanol three times, respectively.It was then dried at 60 °C for 12 h under a vacuum to get green powder, i.e., VO 2 .
We placed 10 g urea in a crucible and heated it in a muffle furnace at 550 °C for 2 h at a 5 °C min À1 heating rate to obtain a C 3 N 4 sample.
The VN/V 2 O 3 @C composite was obtained by mixing VO 2 and the as-obtained C 3 N 4 at a mass ratio of 1:1 and calcined at 800 °C for 2 h in an Ar atmosphere at a 5 °C min À1 heating rate.
Materials Characterization: XRD (Smartlab 9KW, Rigaku) and FESEM (Gemini 300, Zeiss) were used to detect the crystal phase and structure information of all samples.Further, the microstructure characteristics of all samples were characterized by a 200 kV transmission electron microscope (FEI, TalosF200x).The pore profile was characterized on a Quantachrome adsorption instrument (MICROMERITICS, ASAP 2460-4MP).The element valence was characterized by an X-Ray photoelectron spectrometer (Thermo Fisher ESCALAB XIþ), and the corresponding composition information was characterized by FTIR spectroscopy (a Tensor II IR spectrometer) and Raman spectroscopy (HORIBA, LabRAM HR Evolution).In addition, TG-DSC analysis was carried out in the METTLER thermal analyzer TGA/DSC3þ.
Electrochemical Characterization: The working electrode was prepared by mixing the active material, conductive agent (acetylene black), and binder (polyvinylidene fluoride) at a weight ratio of 8:1:1, and then an appropriate amount of N-methyl pyrrolidone (NMP) was added.The above mixture was stirred at room temperature for 3 h.The slurry was uniformly applied to a titanium foil and vacuum dried for 12 h at 90 °C to remove the NMP solvent.After cooling to room temperature, they were tableted by a powder tableting machine, and the active material mass on each electrode sheet with a diameter of 14 mm was 1-1.5 mg cm À2 .CR2032 coin-shaped cells were applied, and the assembly process was carried out in the air atmosphere with zinc foil used as the anode, 3 M Zn(CF 3 SO 3 ) 2 aqueous solution as the electrolyte, and glass fiber (Whatman GF/F) as the separator.The cycle performance and galvanostatic charge-discharge cycling tests were completed using the LAND-CT2001A multichannel galvanostat of the Wuhan Blue Test System in a voltage window of 0.2-1.4V versus Zn 2þ /Zn and tested in a 25 °C incubator.CV and electrochemical impedance spectroscopy were tested on an electrochemical workstation (CHI-760E; Chen Hua) with a frequency range of 0.01 Hz-100 kHz at room temperature.
DFT Calculation: First-principles calculations were implemented by using DFT in the Vienna ab initio Simulation Package [60] based on the project augmented wave method.We used the DFT þ U (U = 3.25 eV) method to circumvent the over-delocalization of the 3 d-electrons in metal oxides. [66]In addition, the Perdue-Burke-Ernzerhof version of the generalized gradient approximation was performed to deal with the adsorption energy. [67]The Brillouin zone was sampled using 2 Â 3 Â 1 based on Monkhorst-Pack k-point mesh.A plane-wave basis with a cutoff energy of 500 eV efficiently guarantees total energy convergence.The convergence criterion for the change of total energy was set as 10 À5 eV, and the atoms in the structure were entirely relaxed with force below 0.01 eV Å À1 .

Figure 1 .
Figure 1. Materials synthesis and characterization of VN/V 2 O 3 @C nanosheets.a) Schematic illustration of the synthesis process.b-d) SEM, e-g) TEM, and h) HRTEM images.Line profile images displaying measurements of 10 atomic planes of the i) V 2 O 3 and j) VN taken from (h) as marked by white and yellow lines, respectively.

Figure 3 .
Figure 3. Electrochemical performance and behavior for VN/V 2 O 3 @C and VO 2 electrodes.a) CV curves of the VN/V 2 O 3 @C electrode at 0.1 mV s À1 .b) Rate capability and c) corresponding galvanostatic charge/discharge profiles.d) EIS of two fresh electrodes (the inset is the equivalent circuit diagram).Cyclic performances of VN/V 2 O 3 @C electrode at e) 0.2 and f ) 10 A g À1 .
3 to 1.1 mV s À1 .The general equation used to analyze the electrochemical kinetic process is I = av b .The equation can be changed in the following logarithmic form: log

Figure 4 .
Figure 4. a) Charge/discharge curves at selected states of the first and second cycles at a current density of 0.1 A g À1 .b) Ex situ XRD patterns and c,d) the corresponding magnified XRD patterns of different peaks of the VN/V 2 O 3 @C electrode.e,f ) TEM, g) HRTEM, and h) elemental mapping images of the V 2 NO@C.The inset in (e) is the SEAD pattern of V 2 NO@C.i) The schematic diagram of the phase transition of the VN/V 2 O 3 to V 2 NO after the first activation cycle.

Figure 5 .
Figure 5. a) CV curves at different scan rates from 0.3 to 1.1 mV s À1 of the V 2 NO@C electrode.b) Log (i, current density) versus log (v, scan rate) plots at the four peaks according to CV measurements for the V 2 NO electrode.c) Capacitance separation at 0.3 mV s À1 for the V 2 NO@C electrode.d) Capacity contribution ratios at different scan rates for the V 2 NO@C electrode.The adsorption profile and energy for Zn 2þ on the e) V 2 NO (111) and f ) VO 2 (002) surface.