Suppresses Dissolution and Enhanced Conductivity in Core–Shell VO2@Ni3N as a High‐Capacitance and Improved Cycling Anode Materials for Supercapacitor

Vanadium dioxide, as one of superior pseudocapacitive electrode candidates in supercapacitors, suffers from the detrimental dissolution in aqueous electrolytes and poor intrinsic electronic conductivity. Herein, an ultrathin Ni3N shell is deposited on the VO2 surfaces (VO2@Ni3N) to inhibit the inevitable dissolution during charge/discharge processes. Meanwhile, the Ni3N coating layer also provides sufficient electron transport pathways for the rapid surface faradic reactions of VO2, promoting the electrochemical reaction kinetics. The designed VO2@Ni3N anode exhibits significant energy storage ability, especially at large current density (2058 F g−1 at 1 A g−1, 711 F g−1 at 7 A g−1), and the considerable cycling lifespans (capacitance retention of 91.3% after 5000 cycles). The asymmetric supercapacitor presents an energy density of 41.73 Wh kg−1 at a power density of 1176.7 W kg−1. This study may enlighten the design of advanced pseudocapacitive materials for practical electrochemical energy storage technologies.


Introduction
The growing demand of renewable and sustainable energy society has triggered the prosperous development of advanced energy storage technologies with intrinsic safety, [1] high energy/power density, and long lifespans.Among them, supercapacitors (SCs) with distinct high power density and ultralong lifespans have been practically applied into many scenes such as consumer electronics, voltage stabilization, and microgrid as an energy storage unit to support short-term high power output. [2]nfortunately, the state-of-the-art commercial SCs are assembled based on low-capacitance activated carbon electrodes (storing energy through physical adsorption), and hardly employed as main energy supply system due to low energy density (<10 Wh kg À1 ). [3]In comparison, the pseudocapacitive electrode materials (e.g., metal oxides, conductive polymers, and metal organic frameworks), which store energy through rapid surface faradic reactions, have been explored, showing about 10-100 times higher than that of the commercial SCs due to the distinguish energy storage mechanism. [4]Hence, using high-capacitance pseudocapacitive materials as electrodes can be a promising way to improve the insufficient energy density of SCs.
Among developed pseudocapacitive materials, vanadium dioxide with various crystal structures, low cost, and high capacitance presents promising perspectives as electrodes for SCs. [5,6]Nevertheless, the inevitable dissolution of vanadium dioxide in aqueous solutions renders the poor cycling stability and undesirable specific capacity, limiting its further development for use in SCs. [7]Another bottleneck for the practical application of vanadium dioxide is the poor intrinsic electronic conductivity, which renders sluggish reaction kinetics during rapid charge/discharge process. [8]Recent studies suggest that coating highly conductive and robust materials on vulnerable materials surface can effectively improve the structural durability of the materials and may induce some unexpected synergistic effects, leading to superior electrochemical performances. [9]In this regard, it is necessary to construct a high capacitance, excellent structural durability, and high electronic conductivity surface coating layer to enhance the specific capacitance and cycling stability of vanadium dioxide.
Nickel nitride (Ni 3 N) has been regarded as a potential electrode materials due to its excellent electrochemical activities and high specific capacity, especially in SC and electrocatalysis.Shalom et al. pointed out the presence of nitrogen strongly influences the electronic properties of the metal by increasing the density of electrons on the surface, resulting in higher activities in reduction reactions. [10]The Ni 3 N lattice is essentially a metallic structure with disordered nitrogen atoms occupying interstitial positions, showing a combination of metallic, covalent, and ionic bonds.According to knowledge on the electronic structures of Ni 3 N, it can be employed as stable and conductivity protect layer to enhance the electrochemical performance of electrode materials. [11]Atomic layer deposition (ALD) technique is an effective strategy to controllably coat a uniform protect layer on the materials surface, forming a core-shell composites. [12]For example, Warren et al. used the ALD technique to uniformly coat highly reactive ruthenium oxide onto vertically aligned carbon nanotubes as pseudocapacitive electrodes for energy storage applications. [13]Their results show that the electrode after the ALD coating and further electrochemical oxidation delivers 100 times and 170 times higher specific capacitance, respectively, as compared with that of pure CNT electrodes. [14]erein, VO 2 @Ni 3 N core-shell was prepared by employing ALD technology.Ni 3 N was considered as a protect layer with high conductivity and stability to restrain the dissolution of VO 2 in aqueous electrolyte, avoiding the negative effects brought by the interaction between electrolyte and electrode.Meanwhile, VO 2 served as a pseudocapacitive material to provide plentiful electrochemical active sites. [15]The results demonstrated that the VO 2 @Ni 3 N core-shell electrode delivers superior specific capacitance of 2058 F g À1 at 1 A g À1 , and impressive capacitance retention of 91.3% after 5000 charge/discharge cycles.
The asymmetric SC using commercial LiNiCoAlO 2 (NCA) cathode and the as-prepared VO 2 @Ni 3 N anode exhibits an energy density of 41.73 Wh kg À1 at a power density of 1176.7 W kg À1 , and a capacitance retention of 96% after 5000 charge/discharge cycles.

Results and Discussion
As illustrated in Figure 1a, well-crystalline VO 2 nanosheets were synthesized by a hydrothermal process of V 2 O 5 powders, H 2 O 2 , and H 2 C 2 O 4 .Then, the VO 2 nanosheets were coated by the amorphous NiO via ALD technology.Finally, the powders (VO 2 @NiO) were calcined in ammonia gas to get Ni 3 N layer, resulting in the preparation of core-shell VO 2 @Ni 3 N.The thickness of Ni 3 N and NiO can be regulated by changing the deposited cycles of ALD.The micromorphology of various VO 2 -based samples were characterized by scanning electron microscope (SEM), as shown in Figure 1b,c and S1-S4, Supporting Information.It can be observed that various VO 2 @NiO samples inherit the nanosheets morphology from VO 2 with irregular size, which can effectively increase the contact area between the material and the electrolyte, benefiting the diffusion of ions and providing more active sites.Additionally, Figure S5, Supporting Information, is the proportion of various elements of VO 2 @NiO in different ALD deposited cycles and VO 2 @Ni 3 N.It can be seen that the content of Ni elements gradually increases with the increase in the number of ALD deposited cycles.For VO 2 @Ni 3 N, the detection of N elements indicated that a part of oxygen is instead by nitrogen during calcining processes.Intuitively, the metal oxides layer deposited by ALD is likely to be amorphous.This result is confirmed by transmission electron microscope (TEM) images of VO 2 @NiO (Figure S6, Supporting Information) and X-ray diffraction (XRD) patterns (Figure S8, Supporting Information).In the high-resolution TEM (HRTEM) image (Figure S6b, Supporting Information), an ultrathin and uniform amorphous layer with a thickness of about 10 nm was characterized on the surface of VO 2 nanosheets.
Figure 1d shows the SEM image of VO 2 @Ni 3 N, suggesting that the high-temperature nitriding process promotes the transformation of the nanosheets into a sea urchin-like structure, while the spheres formed by VO 2 @Ni 3 N are more compact.It is difficult to observe the pore structure and the separate nanosheet structure.As previously reported, the generation of core-shell structures significantly enhances the ability of embedding and extracting of ions. [16]Figure 1e,f shows the TEM images of VO 2 @Ni 3 N, which demonstrates that the boundary between crystal VO 2 (brown area in Figure 1f ) and amorphous Ni 3 N is clear, and the lattice spacing of VO 2 is measured to be about 0.352 nm corresponding to the (110) crystal plane of VO 2 .The elemental distribution mapping of the core-shell VO 2 @Ni 3 N is shown in Figure 1g and S7, Supporting Information, which suggests the uniform distribution of V, O, Ni, and N elements on the surface of the core-shell materials.
The XRD patterns of VO 2 , VO 2 @NiO, and VO 2 @Ni 3 N are shown in Figure 2a.The samples exhibit sharp diffraction peaks, indicating the well-crystalline structure.Moreover, all the diffraction peaks are pointed to monoclinic VO 2 (JCPDS Card No. 81-2392).No additional peaks are indexed to NiO or Ni 3 N, which can be attributed to the amorphous structure and minimal content of NiO and Ni 3 N in the samples.Although the deposited cycles are up to 400, the intrinsic crystalline structure was not destroyed after treatment by ALD and nitridation processes, as displayed in Figure S8, Supporting Information.The similar conclusion can also be deduced from the results of Raman, as displayed in Figure 2b.For VO 2 @NiO, the characteristic peaks at 541.7, 731.4,and 946.7 cm À1 belong to the stretching mode of the Ni─O bond, demonstrating the successful deposition of VO 2 @NiO.For VO 2 @Ni 3 N, the peak at 138.7 cm À1 represents the oxidation of the VO 2 to V 2 O 5 . [17]In addition, the characteristic peaks at 191.1 and 280.4 cm À1 correspond to the bending and vibrational modes of the V─O─V bond, respectively. [18]The stretching mode of the V─O─V bond locates at 406.4 cm À1 , and the appearance of the peak at 692.2 cm À1 is attributed to the coordination of the vanadium atom with the three oxygen atoms, and the peak at 990.2 cm À1 belongs to the stretching of the vanadium-oxygen double bond. [19]The characteristic peaks of typical Ni 3 N appear at 202.3, 230.4,and 516.1 cm À1 , demonstrating the successfully synthesis of the core-shell VO 2 @Ni 3 N. [20] The X-ray photoelectron spectroscopy (XPS) full-spectrum of VO 2 @Ni 3 N is shown in Figure S10, Supporting Information, which demonstrates the presence of Ni, V, O, and N elements in the sample.Figure 2c-f shows the high-resolution XPS spectra of different elements.The V 2p (Figure 2c) orbital with binding energies of 517.2 and 524.8 eV corresponds to 2p 3/2 and 2p 1/2 of V 5þ , [21] which can be explained by the oxidation of VO 2 .The binding energy at 515.9 and 523.5 eV coincides with the V 4þ 2p 3/2 and V 4þ 2p 1/2 . [22]The characteristic peaks with binding energies of 533.1, 531.7, 529.9, and 529.4 eV in the O 1s spectra (Figure 2d) can be ascribed to the formation of N─O, O─H, V─O, and Ni─O bonds, respectively, where the presence of Ni─O bonds is possibly derived from the bonding of Ni to O in VO 2 .The peaks at 400.1 and 398.5 eV in the N 1s spectrum (Figure 2e) can be attributed N─Ni and N─O bonds, respectively.The existence of Ni─N bonds indicates the presence of Ni 3 N in the core-shell materials, further demonstrating the successful synthesis of the VO 2 @Ni 3 N. Figure 2f shows the Ni 2p spectrum, where the Ni─O bond peak is derived from the bonding of Ni to O in VO 2 , consistent with the O 1s spectrum.In addition, the characteristic peaks are located at 870.9 and 852.9 eV, representing the formation of N─Ni bonds, indicating the presence of Ni 3 N, which is consistent with the N 1s spectrum.
The electrochemical performance of the prepared electrodes was tested in a three-electrode cell configuration with 1 M KOH as the electrolyte, Pt foil as the counter electrode, Hg/HgO as the reference electrode, and the as-prepared samples as the working electrode.For the VO 2 @NiO samples, the cyclic voltammetry (CV) and galvanostatic chargeÀdischarge (GCD) curves are shown in Figure S11, Supporting Information.VO 2 @NiO electrode shows the best electrochemical performance among the VO 2 @NiO samples, so VO 2 @Ni 3 N is directly derived from VO 2 @Ni 3 O.CV curves of VO 2 , VO 2 @NiO, and VO 2 @Ni 3 N at a scan rate of 100 mV s À1 are presented in Figure 3a.It can be observed that the VO 2 @Ni 3 N after hightemperature nitriding shows a rectangular-like shape with no obvious redox peaks, indicating that its energy storage mechanism is still pseudocapacitive.Furthermore, the largest closed area of the VO 2 @Ni 3 N in CV curves is clearly identified, which indicate a higher specific capacity of the electrode.The GCD curves at a current density of 1 A g À1 are shown in Figure 3b.The longest discharge time was obtained by VO 2 @Ni 3 N, suggesting the more superior energy storage ability, which is consistent with CV results.Figure 3c shows the CV curves of VO 2 @Ni 3 N at different scan rates with good symmetry and no obvious distortion even at 100 mV s À1 , indicating excellent electrochemical response.According to GCD curves (Figure 3d,e), the calculated massspecific capacity of the electrode is 2058, 1626, and 1290, 957 and 711 F g À1 at current densities of 1, 2, 3, 5 and 7 A g À1 , respectively, exhibiting the highest specific capacity among all electrodes.Compared to VO 2 @NiO, VO 2 @Ni 3 N forms nitrides after high-temperature nitriding, which facilitate the embedding and de-embedding of ions and minimize the dissolution of VO 2 , while the sea urchin shape also provides a richer range of active sites and shows the highest specific capacity.
To interpret the enhancement of charge storage ability, electrochemical impedance spectroscopy (EIS) of VO 2 , VO 2 @NiO, and VO 2 @Ni 3 N electrode was carried out.As shown in Figure 3f, each EIS curve was composed of a slope line at low-frequency region related to the ion diffusion and a semicircle at the high-frequency region associated to the charge transfer.The VO 2 @Ni 3 N electrode shows the smallest radius of semicircle region, demonstrating faster ions transport and electrons transfer.The enhanced conductivity of VO 2 @Ni 3 N generates from NiN layer.The cycling durability of the VO 2 @Ni 3 N core-shell structure was evaluated, as plotted in Figure 3i.The core-shell electrode exhibited a capacity retention rate of 91.3% after 5000 charge/discharge cycles, which was much higher than the VO 2 (50.1%) and VO 2 @NiO (80.6%).Evidently, the result demonstrated that the VO 2 @Ni 3 N electrode displayed more outstanding cycling stability than VO 2 and VO 2 @NiO electrode.The improved capacity, rate performance, and cycling stability of VO 2 @Ni 3 N electrode can be attributed to the following.First, the unique sea urchin-like structure is more stable than the secondary stacked nanosheets of VO 2 and VO 2 @NiO, improving the structural stability during charging-discharging processes.Second, the Ni 3 N with good electrical conductivity coated on the surface of samples not only accelerated the electron transfer but also inhibited the dissolve of vanadium oxide, resulting in the enhancement of rate performance and cycling stability.Finally, the purpose of VO 2 core in VO 2 @Ni 3 N electrode is to transport charges more efficiently, which can provide the pseudocapacitance.The Ni 3 N shell, which is coating on the core as a ultrathin   layer, provides a high surface area for charging and a large number of active sites for conducting faradic redox reactions. [23]herefore, the synergistic effects between core and shell are benefit to electrochemical reaction kinetics, delivering a high specific capacity and long-term cycling stability.
To further understand the storage kinetics in the VO 2 @Ni 3 N electrode, the contribution of capacitance was evaluated according to the Formula 5.As displayed in Figure 3g and S12a, Supporting Information, the contribution of the pseudocapacitance of the VO 2 @Ni 3 N electrode is 97.85% of the total capacitance at 100 mV s À1 , which is higher than that of the VO 2 @NiO electrode (70.54%).The capacitive contribution ratio is increasing with the increase of scan rate for both electrode, probably because at high scan rates the diffusion of ions in the electrolyte and interior is limited.However, the capacitive contribution ratio of VO 2 @Ni 3 N electrode is always higher than that of VO 2 @NiO at various scan rates, as shown in Figure 3h and S12b, Supporting Information, which can be explained by the fact that the type of VO 2 @Ni 3 N electrode had more active sites than the type of VO 2 @NiO.Generally, the higher the pseudocapacitance contribution at same scan rate, the more ions insert into electrode materials to provide capacitance, [24] demonstrating that the introduction of Ni 3 N was conducive to the ion diffusion in the electrode for SC.
To further investigate the practical application potentiality of VO 2 @Ni 3 N, an asymmetric SC was assembled using a commercial NCA electrode as the positive electrode, VO 2 @Ni 3 N as the negative electrode, and a KOH/PVA gel electrolyte, as illustrated in Figure 4a, while following the same approach only replace the negative electrode with VO 2 @NiO to make VO 2 @NiO//NCA ASC as a comparison (Figure S13, Supporting Information).The CV curves of NCA positive electrode and VO 2 @NiO negative electrode are shown in Figure S13a, Supporting Information.It can be seen that the CV curves of NCA electrode show obvious redox peaks during charging and discharging, indicating that it is belong to battery-type electrode based on redox reactions.There Figure 3. a) CV curves at 100 mV s À1 and b) GCD curves at 1 A g À1 of VO 2 , VO 2 @NiO, and VO 2 @Ni 3 N. c) CV curves at various scan rates and d) GCD curves at different current density of VO 2 @Ni 3 N. e) Rate performance and f ) EIS of VO 2 , VO 2 @NiO, and VO 2 @Ni 3 N. Pseudocapacitance contribution g) at 100 mV s À1 and h) at different scan rate of VO 2 @Ni 3 N. i) Cycling performance of VO 2 , VO 2 @NiO, and VO 2 @Ni 3 N.
is serious polarization in CV curves of the assembled VO 2 @NiO//NCA ACS, as shown in Figure S13b,c, Supporting Information.According to the charges balance and CV curves (Figure S15, Supporting Information), the mass ratio of VO 2 @Ni 3 N negative and NCA positive is 1.63:1.The CV curves of the VO 2 @Ni 3 N//NCA ASC (Figure 4b) show significant polarization at 1.9 V, so it was concluded that the device could achieve the highest performance at a voltage window of 1.8 V.The CV curves of as-assembled VO 2 @Ni 3 N//NCA ASC are well maintained even at 100 mV s À1 , as plotted in Figure 4c, indicating the significant electrochemical reaction kinetics and rate performance.Moreover, the specific capacitance of the VO 2 @Ni 3 N//NCA ASC can be up to 61.1 F g À1 at 1 A g À1 and 30.56 F g À1 at 5 A g À1 (Figure 4d).Cycling stability performance was also evaluated, as shown in Figure 4e and S14, Supporting Information.The capacitance retention of the VO 2 @Ni 3 N//NCA ASC device remained at 96% (current density at 10 A g À1 ) relative to initial specific capacitance after 5000 continuous charge/discharge cycles, which is higher than the 93.7% of the VO 2 @NiO//NCA ASC device.Furthermore, three fully charged devices were connected in series as a power source to supply multiple LEDs, and the results are shown in the inset of Figure 4e.Finally, the energy and power densities of the asymmetric device were further calculated using Formula 3 and 4, with an energy density of 41.56 Wh kg À1 at a power density of 1176 W kg À1 , higher than the VO 2 @NiO// NCA ASC device with an energy density of 39.81 Wh kg À1 at a power density of 833.3 W kg À1 .The Ragone plot shown in Figure 4f indicates that the core-shell structure has unique advantages for increasing energy density and power density.

Conclusion
In summary, an ultrathin Ni 3 N is coated on the surface of VO 2 to inhibit the dissolution of V species in aqueous electrolyte and enhance the conductivity of the electrode.The mass-specific capacitance of the VO 2 @Ni 3 N core-shell electrode is as high as 2058 F g À1 at 1 A g À1 and still has a specific capacity of 711 F g À1 at a current density of 7 A g À1 .The high cycling stability of 91.3% is still maintained after 5000 charge/discharge cycles, much higher than that of pure VO 2 (50.1%).Ultimately, the VO 2 @Ni 3 N core-shell material was used as a SC negative electrode to achieve a dual improvement in electrode conductivity and reversibility, which minimizes the dissolution of VO 2 and improves both material stability and specific capacity and capacity retention.An asymmetric SC VO 2 @Ni 3 N//NCA ASC with a voltage window of 0-1.8 V delivers an energy density as high as 41.73 Wh kg À1 at a power density of 1176.7 W kg À1 , and the capacitance retention rate of this ASC was 96% after 5000 charge/discharge cycles, indicating that the strategy has great potential for the modification of electrode materials in SCs.(Ar ≥ 99%) and high purity ammonia (NH 3 ≥ 99%) were purchased from Harbin Grand Gas Co.

Experimental Section
Synthesis of VO 2 @Ni 3 N: VO 2 @Ni 3 N were prepared by high-temperature nitridation in an ammonia atmosphere based on VO 2 @NiO materials, which were obtained by hydrothermal production of vanadium dioxide as precursors and further by atomic layer deposition techniques.The detailed procedure is as follows: 5 mmol of C 2 H 2 O 4 and 1.5 mmol of V 2 O 5 were added to 30 mL of deionized water and stirred continuously at 75 °C until the color of the solution turned dark blue; 30 mL of anhydrous ethanol and 1.5 mL of 30% H 2 O 2 were then added to the above solution in turn and stirred continuously until the solution turned yellow.The solution was poured into 100 mL of PTFE (polytetrafluoroethylene) liner and sealed in an autoclave, which was kept at 180 °C for 5 h.After cooled down to room temperature, the black precipitate was collected and washed repeatedly by centrifugation with deionized water and ethanol.The VO 2 precursors were obtained after completely dried in a vacuum oven at 60 °C.The VO 2 @NiO were produced by depositing 100, 200, 300, and 400 cycles of NiO on the surface of VO 2 by using the ALD technique, denoted as VO 2 @NiO-1, VO 2 @NiO-2, VO 2 @NiO, and VO 2 @NiO-4, respectively.By testing the electrochemical properties and structural characterization of the above four samples, the VO 2 @NiO sample with the best performance was selected as the precursor to prepare VO 2 @Ni 3 N.In detail, the VO 2 @NiO sample was placed in a tube furnace under a mixture of ammonia and argon gas (Ar:NH 3 = 10:1).Then, the temperature was increased from room temperature to 350 °C at a heating rate of 5 °C min À1 and maintained for 2 h to obtain the VO 2 @Ni 3 N material.
Preparation of Electrode: The electrode was prepared by mixing as-synthesized materials, PVDF, and Super P in a mass ratio of 8:1:1.Then, 1-methyl-2-pyrrolidinone was dropped as the solvent with stirring at 500 r min À1 for 12 h to form a slurry.Finally, the slurry was uniformly coated on the nickel foam substrate and dried at 60 °C for 12 h to obtain an electrode.
Electrochemical Measurements: The electrochemical performance of the as-prepared samples was evaluated via CV, GCD, and EIS measurements on an electrochemical workstation (CHI 660) using the three-electrode configuration in a 1 M KOH electrolyte.The as-fabricated electrode materials were directly used as the working electrode, and the Pt foil and Hg/HgO served as the counter and reference electrodes, respectively.EIS measurements were taken between 10 À2 and 10 5 Hz with a voltage amplitude of 5 mV of open-circuit potential.
The specific capacitance of the electrode material can be calculated from the constant current charge/discharge curve according to Formula 1.To achieve the highest electrochemical performance of the asymmetric capacitor (ASC), it is necessary to achieve charge matching based on Equation (2).The energy density (E) and power density (P) of ASC devices can be calculated according to Equation ( 3) and (4).Equation ( 5) is used to calculate the diffusion contribution and the capacitance contribution to the total capacitance where C represents the specific capacitance of the material, m þ and m -are the mass of the active material in the positive and negative electrodes, respectively, g; C þ , C --are the specific capacitance values of the positive and negative electrodes, respectively, F g À1 ; and V þ , V --are the voltage windows of the positive and negative electrodes, respectively.E (Wh kg À1 ) and P (W kg À1 ) represent the energy density and power density of the device, respectively.And i(V ), V, k 1 V, and k 2 V 1/2 denote the current response, scan rate, capacitance process, and diffusion control process, respectively.
The Assembly Processes of Asymmetric Device: The commercial NCA, VO 2 @Ni 3 N, KOH/PVA gel was used as positive electrode, negative electrode, and electrolyte, respectively, and denoted as NCA//VO 2 @Ni 3 N.The active electrodes (both positive and negative) were immersed in the KOH/PVA gel electrolyte for 2 h to ensure that the electrolyte was fully infiltrated.To avoid short circuits, the electrodes were separated by glass fiber.After leaving for 10 h, the asymmetric NCA//VO 2 @Ni 3 N SC was obtained.The reparation of KOH/PVA gel electrolyte is as follows: polyvinyl alcohol (PVA, 1 g) mixed with 7 mL of deionized water, and stirred continuously at 85 °C until PVA completely dissolved.Then, the dissolved PVA gel was placed in an oven at 90 °C for 4-5 h.Subsequently, 1 M KOH (3 mL) was dropped in PVA gel at 60 °C with slow stirring.Finally, KOH/PVA gel was obtained.

Figure 4 .
Figure 4.The electrochemical performance of VO 2 @Ni 3 N//NCA ASC: a) Schematic diagram, b) CV curves at various voltage windows, c) CV curves at various scan rates, d) GCD curves, e) Cycling stability, and f ) Ragone plot.