Three-Dimensional Vanadium and Nitrogen Dual-Doped Ti3C2 Film with Ultra-High Specific Capacitance and High Volumetric Energy Density for Zinc-Ion Hybrid Capacitors

Zinc-ion hybrid capacitors (ZICs) can achieve high energy and power density, ultralong cycle life, and a wide operating voltage window, and they are widely used in wearable devices, portable electronics devices, and other energy storage fields. The design of advanced ZICs with high specific capacity and energy density remains a challenge. In this work, a novel kind of V, N dual-doped Ti3C2 film with a three-dimensional (3D) porous structure (3D V-, N-Ti3C2) based on Zn-ion pre-intercalation can be fabricated via a simple synthetic process. The stable 3D structure and heteroatom doping provide abundant ion transport channels and numerous surface active sites. The prepared 3D V-, N-Ti3C2 film can deliver unexpectedly high specific capacitance of 855 F g−1 (309 mAh g−1) and demonstrates 95.26% capacitance retention after 5000 charge/discharge cycles. In addition, the energy storage mechanism of 3D V-, N-Ti3C2 electrodes is the chemical adsorption of H+/Zn2+, which is confirmed by ex situ XRD and ex situ XPS. ZIC full cells with a competitive energy density (103 Wh kg−1) consist of a 3D V-, N-Ti3C2 cathode and a zinc foil anode. The impressive results provide a feasible strategy for developing high-performance MXene-based energy storage devices in various energy-related fields.

Although ZICs exhibit good energy storage features compared to other conventional supercapacitors, some key technology parameters (e.g., low specific capacity and energy density) need to be further improved.The exploration of new cathode materials with high specific capacity and fast ion transport speed is urgently required [28][29][30][31][32].At present, the cathode materials of ZICs mainly include carbon-based materials (activated carbon, hollow carbon spheres, and porous carbon) [33,34], TiN [35], conductive polymers [36], transition metal compounds [37], etc.However, the low specific capacity of these reported cathode materials severely limits the energy density of ZICs.In an attempt to improve the energy storage performance of carbon-based materials, Yan et al. designed N-doped porous carbon with a capacity of 136.8 mAh g −1 [38] for Zn//N-HPC zinc-ion devices with an extraordinary energy density (191 Wh kg −1 ).Lu et al. fabricated ZICs with both ultra-high energy density (107.3 mAh g −1 ) and excellent cycle lifetime (99.7% retention after 20,000 cycles) by utilising N-doped hierarchically porous carbon [39].
In recent years, an emerging two-dimensional material, MXene, has been investigated as an extremely promising cathode material for ZICs due to its intercalation pseudocapacitance mechanism, metallic-like conductivity, and variable surface modification [40][41][42][43][44].The inevitable agglomerate stacking of MXene nanosheets reduces electrochemically active sites, limiting electrolyte ion diffusion, and prolongs the ion transport distance.To date, the aggregation and stacking problems of MXene have been solved by constructing a 3D structure, and the introduction of additional pseudocapacitance by heteroatom doping can improve MXene energy storage performance.For example, Zhang et al. demonstrated a 3D H-MXene hierarchical pore-structured film using H + crosslinking.The specific capacity of the assembled Zn//3D-PHMF capacitor is 105 mAh g −1 at 0.2 mA g −1 [45].Mateen et al. presented N-functionalization Ti 3 C 2 T x in situ by means of hydrothermal reaction.The N-Ti 3 C 2 T x exhibited an unrivalled specific capacitance of 583 F g −1 at 1 A g −1 [46].N doping improved the surface wettability and electrical conductivity of MXene and greatly enhanced its electrochemical performance in supercapacitors.Gao et al. synthesized V-doped Ti 3 C 2 T x using a hydrothermal method, which presented excellent specific capacitance (365.9F g −1 ) at 10 mV s −1 and excellent stability of 95% (after 5000 cycles) [47].Vanadium doping did not change the 2D morphology of MXene and resulted in stronger alkali metal-ion-O interactions on the MXene surface, introducing more active sites.Although these efforts have achieved the increased specific capacity of Ti 3 C 2 T x , the relatively low specific capacity and low energy density have severely hindered the practical applications of Ti 3 C 2 in ZICs.
Larger layer spacing can significantly increase the shuttle depth of electrolyte ions in Ti 3 C 2 , and ion pre-intercalation can expand the interlayer spacing, which is favorable for ion diffusion.Therefore, we present a kind of V, N dual-doped Ti 3 C 2 film with a 3D porous structure (3D V-, N-Ti 3 C 2 ) based on Zn 2+ pre-intercalation via a three-step synthetic process involving HCl etching, hydrothermal doping, and Zn 2+ -induced gel.The prepared 3D V-, N-Ti 3 C 2 film as a cathode displays an outstanding specific capacitance of ~855 F g −1 (309 mAh g −1 ) at 0.3 A g −1 with an excellent cycling stability.Additionally, the assembled Zn//3D V-, N-Ti 3 C 2 capacitors deliver an ultra-high energy density (103 Wh kg −1 ), which is almost optimal compared with the currently reported ZICs.This work highlights the potential of the MXene-based materials in metal-ion capacitor systems and other energy storage fields.

Preparation of Delaminated Ti 3 C 2 T x
Delaminated Ti 3 C 2 T x suspensions were produced from Ti 3 AlC 2 powder using fluoride etching and an ultrasonic exfoliation process.Briefly, 1.6 g of LiF (Aladdin, 99%) was dissolved into 20 mL 9 M HCl and stirred for 5 min.Then, 1 g Ti 3 AlC 2 powder was slowly added to the above mixed solution, and an oil bath at 60 • C was used for 48 h to remove Al from the Ti 3 AlC 2 phase.The solution was then brought to room temperature and washed with deionized water using centrifugation until the pH was close to 7. After ultrasonic treatment under Ar for 2 h and centrifugation at 3500 rpm for 1 h, dark green supernatant was collected.The concentration of Ti 3 C 2 was determined by drying a certain volume of the solution and weighing its mass.

Preparation of 3D V-, N-Ti 3 C 2 Film
The 3D V-, N-Ti 3 C 2 film was obtained via a hydrothermal method and freeze-drying process.Firstly, 20 mg Ti 3 C 2 was dissolved in 10 mL DI water to obtain Ti 3 C 2 suspension.Then, 2 g urea and 2 mg ammonium metavanadate (NH 4 VO 3 ) was added into the Ti 3 C 2 suspension; it was stirred for 15 min, and then transferred to a 15 mL Teflon-lined autoclave and maintained at 120 • C for 24 h.The solution was centrifuged and rinsed with deionized water several times.The V, N double-doped Ti 3 C 2 film (V-, N-Ti 3 C 2 ) was then obtained by vacuum filtration and vacuum drying.V-doped Ti 3 C 2 film (V-Ti 3 C 2 ) was prepared in the absence of urea according to the above procedure.Subsequently, 20 mg V-, N-Ti 3 C 2 was dispersed into 20 mL deionized water and 0.6 mL ZnSO 4 (1 mg mL −1 ) was added; the solution was then stirred for 4 h.The 3D V-, N-Ti 3 C 2 film was obtained by means of vacuum filtration and freeze-drying.For comparison, 3D Ti 3 C 2 film was also obtained by replacing V-, N-Ti 3 C 2 with pure Ti 3 C 2 .
2.3.Assembly of 3D V-, N-Ti 3 C 2 //Zn Device Typical 3D V-, N-Ti 3 C 2 //Zn devices were constructed using sample 3D V-, N-Ti 3 C 2 film as the cathode, Zn metallic sheet as the anode, and a glass fiber membrane as the separator.Here, 2 M ZnSO 4 was used as the electrolyte in the experiments.

Results and Discussion
The schematic fabrication process of 3D V-, N-Ti 3 C 2 film is presented in Figure 1.Firstly, few-layered Ti 3 C 2 T x nanosheets were synthesized by etching Al from Ti 3 AlC 2 in LiF and HCl mixed solution, and then ultrasonic exfoliation was performed, the SEM images of which are shown in Figure S1.Subsequently, the exfoliated Ti 3 C 2 was then doped with V, N elements via the hydrothermal method in the presence of NH 4 VO 3 and urea.V-, N-Ti 3 C 2 was obtained by washing with deionized water.To increase the contact area, we added ZnSO 4 to V-, N-Ti 3 C 2 suspension to enable Zn 2+ ions to destroy the electrostatic repulsive forces between the nanosheets so as to link them together, forming a stable 3D structure.Subsequently, the 3D V-, N-Ti 3 C 2 film was obtained using vacuum filtration and freeze-drying processes.

Preparation of 3D V-, N-Ti3C2 Film
The 3D V-, N-Ti3C2 film was obtained via a hydrothermal method and freeze-drying process.Firstly, 20 mg Ti3C2 was dissolved in 10 mL DI water to obtain Ti3C2 suspension.Then, 2 g urea and 2 mg ammonium metavanadate (NH4VO3) was added into the Ti3C2 suspension; it was stirred for 15 min, and then transferred to a 15 mL Teflon-lined autoclave and maintained at 120 °C for 24 h.The solution was centrifuged and rinsed with deionized water several times.The V, N double-doped Ti3C2 film (V-, N-Ti3C2) was then obtained by vacuum filtration and vacuum drying.V-doped Ti3C2 film (V-Ti3C2) was prepared in the absence of urea according to the above procedure.Subsequently, 20 mg V-, N-Ti3C2 was dispersed into 20 mL deionized water and 0.6 mL ZnSO4 (1 mg mL −1 ) was added; the solution was then stirred for 4 h.The 3D V-, N-Ti3C2 film was obtained by means of vacuum filtration and freeze-drying.For comparison, 3D Ti3C2 film was also obtained by replacing V-, N-Ti3C2 with pure Ti3C2.

Assembly of 3D V-, N-Ti3C2//Zn Device
Typical 3D V-, N-Ti3C2//Zn devices were constructed using sample 3D V-, N-Ti3C2 film as the cathode, Zn metallic sheet as the anode, and a glass fiber membrane as the separator.Here, 2 M ZnSO4 was used as the electrolyte in the experiments.

Results and Discussion
The schematic fabrication process of 3D V-, N-Ti3C2 film is presented in Figure 1.Firstly, few-layered Ti3C2Tx nanosheets were synthesized by etching Al from Ti3AlC2 in LiF and HCl mixed solution, and then ultrasonic exfoliation was performed, the SEM images of which are shown in Figure S1.Subsequently, the exfoliated Ti3C2 was then doped with V, N elements via the hydrothermal method in the presence of NH4VO3 and urea.V-, N-Ti3C2 was obtained by washing with deionized water.To increase the contact area, we added ZnSO4 to V-, N-Ti3C2 suspension to enable Zn 2+ ions to destroy the electrostatic repulsive forces between the nanosheets so as to link them together, forming a stable 3D structure.Subsequently, the 3D V-, N-Ti3C2 film was obtained using vacuum filtration and freeze-drying processes.The XRD patterns of 3D V-, N-Ti3C2 and other samples are presented in Figures 2a  and S1b.The Ti3C2 film has an intense (002) peak at around 6°, which confirms that Ti3C2 was successfully synthesized from Ti3AlC2.The XRD pattern of 3D V-, N-Ti3C2 shows that the doping and structural design had minimal effect on Ti3C2 phase structure.The SEM images (Figures S1-S4   The dense structure severely limits the exposure of surface active sites and ion diffusion, which in turn affects the electrochemical properties of the films.As for the 3D Ti 3 C 2 film (the thickness is about 900 nm) and the 3D V-, N-Ti 3 C 2 film (Figures 2b and S3), stable 3D structures (the thickness is about 45 µm) are constructed by linking zinc ions (Zn 2+ ) and by using freeze-drying treatment.The stable 3D structure provides abundant ion channels for membrane electrodes, which dramatically improves their energy storage property.In addition, the nitrogen adsorption-desorption isotherms of V-, N-Ti 3 C 2 , and 3D V-, N-Ti 3 C 2 samples were of type I and type IV, respectively, as shown in Figure S5, indicating the presence of mesopores in V-, N-Ti 3 C 2 and both micropores and mesopores in 3D V-, N-Ti 3 C 2 .The surface areas of V-, N-Ti 3 C 2 and 3D V-, N-Ti 3 C 2 were 1.23 m 2 g -1 and 45.17 m 2 g -1 , respectively.The 3D V-, N-Ti 3 C 2 film had a larger specific surface area and provided abundant ion adsorption sites and transport channels.Then, TEM was used to observe the morphology and structure of 3D V-, N-Ti 3 C 2 , as shown in Figure S6a.The 3D V-, N-Ti 3 C 2 nanosheets were intertwined and connected with each other.The HRTEM image (Figure 2c) indicated that the crystal plane spacing of 3D V-, N-Ti 3 C 2 was 1.23 nm, which was consistent with the (002) plane.The EDS elemental mapping analysis in Figure 2d confirms the coexistence of V, N, and Zn elements in the sample.The uniform distribution of these elements proves that V and N were successfully doped into Ti 3 C 2 .
the films are formed by the dense packing of Ti3C2 nanosheets.The V-Ti3C2 film and V-, N-Ti3C2 film are structurally dense and have small amounts of TiO2 on the nanosheets' surface.The dense structure severely limits the exposure of surface active sites and ion diffusion, which in turn affects the electrochemical properties of the films.As for the 3D Ti3C2 film (the thickness is about 900 nm) and the 3D V-, N-Ti3C2 film (Figures S3 and 2b), stable 3D structures (the thickness is about 45 µm) are constructed by linking zinc ions (Zn 2+ ) and by using freeze-drying treatment.The stable 3D structure provides abundant ion channels for membrane electrodes, which dramatically improves their energy storage property.In addition, the nitrogen adsorption-desorption isotherms of V-, N-Ti3C2, and 3D V-, N-Ti3C2 samples were of type I and type IV, respectively, as shown in Figure S5, indicating the presence of mesopores in V-, N-Ti3C2 and both micropores and mesopores in 3D V-, N-Ti3C2.The surface areas of V-, N-Ti3C2 and 3D V-, N-Ti3C2 were 1.23 m 2 g -1 and 45.17 m 2 g -1 , respectively.The 3D V-, N-Ti3C2 film had a larger specific surface area and provided abundant ion adsorption sites and transport channels.Then, TEM was used to observe the morphology and structure of 3D V-, N-Ti3C2, as shown in Figure S6a.The 3D V-, N-Ti3C2 nanosheets were intertwined and connected with each other.The HRTEM image (Figure 2c) indicated that the crystal plane spacing of 3D V-, N-Ti3C2 was 1.23 nm, which was consistent with the (002) plane.The EDS elemental mapping analysis in Figure 2d confirms the coexistence of V, N, and Zn elements in the sample.The uniform distribution of these elements proves that V and N were successfully doped into Ti3C2.XPS was employed to investigate the elemental content and chemical state of 3D V-, N-Ti3C2.The survey of XPS spectra and the high-resolution V 2p spectra of 3D V-, N-Ti3C2 are given in Figures S7 and 2e, respectively; they demonstrate the successful doping of V into Ti3C2.The peaks at 516.4 and 524.5 eV correspond to 2p3/2 and 2p1/2 of V 4+ , respectively [47].The peaks at a binding energy of 514.6 and 522.5 eV are assigned to 2p3/2 and 2p1/2 of V 3+ , respectively.The N 1s spectra are shown in Figure 2f, wherein the peaks at 400.0 and 401.6 eV correspond to pyrrolic-N and graphitic-N, respectively, indicating that N was successfully doped into Ti3C2 [46,48,49].Furthermore, a high-resolution O 1s spectrum XPS was employed to investigate the elemental content and chemical state of 3D V-, N-Ti 3 C 2 .The survey of XPS spectra and the high-resolution V 2p spectra of 3D V-, N-Ti 3 C 2 are given in Figures S7 and 2e, respectively; they demonstrate the successful doping of V into Ti 3 C 2 .The peaks at 516.4 and 524.5 eV correspond to 2p 3/2 and 2p 1/2 of V 4+ , respectively [47].The peaks at a binding energy of 514.6 and 522.5 eV are assigned to 2p 3/2 and 2p 1/2 of V 3+ , respectively.The N 1s spectra are shown in Figure 2f, wherein the peaks at 400.0 and 401.6 eV correspond to pyrrolic-N and graphitic-N, respectively, indicating that N was successfully doped into Ti 3 C 2 .[46,48,49].Furthermore, a high-resolution O 1s spectrum can be simulated to synthesize three peaks; the peaks at 529.6, 530.5, and 532.1 eV correspond to O-V, O-Ti, and HO-V/Ti (Figure 2g), respectively [45].The peak located at 282.0 eV in the C 1s spectrum (Figure S8b) can be attributed to the C-V, which provides evidence that V is successfully doped into Ti 3 C 2 .
The Zn 2+ storage properties of 3D V-, N-Ti 3 C 2 and other electrode films were then investigated.Figure 3a displays the CV curves of Ti 3 C 2 , 3D Ti 3 C 2 , V-Ti 3 C 2 , V-, N-Ti 3 C 2 , and 3D V-, N-Ti 3 C 2 at 0-1.3 V when the scanning rate was 5 mV s −1 .These CV curves have similar shapes, and there are some redox peaks (around 0.2-0.4V and 0.7-1 V) during charge and discharge, indicating the reversible insertion/removal of Zn 2+ .However, under the same voltage window and scanning rate, the CV areas are in the order Ti 3 C 2 < 3D Ti 3 C 2 < V-Ti 3 C 2 < V-, N-Ti 3 C 2 < 3D V-, N-Ti 3 C 2 , which indicates that the capacity of 3D-structured MXene is higher than that of MXene film, and the optimal energy storage performance of 3D V-, N-Ti 3 C 2 is mainly due to the rational creation of the 3D structure that can expand the contact area with the electrolyte ions and then expose more active sites.Additionally, the V-Ti 3 C 2 and V-, N-Ti 3 C 2 showed an improved electrochemical performance compared with that of Ti 3 C 2 ; this is related to the dual-doping of V and N, which provide additional active sites.Figures S9-S13 show the CV curves of samples in 5~100 mV s −1 .With the increase in the scanning rate, the CV shape remains unchanged, indicating excellent high-speed and velocity performance.Figure 3b shows the GCD curves of the 3D V-, N-Ti 3 C 2 at 0.3 to 20 A g −1 .Then, the GCD curves of Ti 3 C 2 , 3D Ti 3 C 2 , V-Ti 3 C 2 , and V-, N-Ti 3 C 2 at different current density are shown in Figures S9-S12.The correlation between the gravimetric specific capacitance and current density for all the samples is given in Figure 3c.Similarly to the CV results, the 3D V-, N-Ti 3 C 2 delivers the highest gravimetric capacitance at 0.3 A g −1 , up to 855 F g −1 (309 mAh g −1 , Figure S13), and significantly more than the other samples.The gravimetric capacitance of 3D V-, N-Ti 3 C 2 up to 122 F g −1 at 15 A g −1 is also better than that of the other electrodes and previously reported Ti 3 C 2 -based electrode materials (Table S1).Results demonstrate that a stable 3D structure can provide a larger active specific surface area and more ion transport channels, while the V, N dualdoping can provide more active sites for the improved capacitance.Remarkably, the 3D V-, N-Ti 3 C 2 film electrode shows a maximum energy density of 200 Wh kg −1 at a power density of 195 W Kg −1 (Figure 3d), which is much higher than those of the previously reported zinc-ion capacitors [42, 43,45,49].Furthermore, the cycling stability of the 3D V-, N-Ti 3 C 2 film electrode was tested at a current density of 8 A g −1 (Figure 3e); 95.26% of its initial specific capacitance was retained after 5000 cycles, indicating robust long-term cycle stability.Figure S15 shows linear plots of the resultant charging current density and relevant scan rate for Ti 3 C 2 , 3D Ti 3 C 2 , V-Ti 3 C 2 , V-, N-Ti 3 C 2 , and 3D V-, N-Ti 3 C 2 , where the slopes are the electrochemical double layer capacitance (C dl ).The 3D V-, N-Ti 3 C 2 with hierarchical holes had a large ECSA of 7.7 mF/cm 2 , indicating that a large number of electrochemically active sites appeared in the material after V, N co-doping following Zn 2+ pre-layering.
To analyse the kinetics of the charge storage of the 3D V-, N-Ti 3 C 2 electrode, the CV curves of the 3D V-, N-Ti 3 C 2 at 5-100 mV s −1 are presented in Figure 4a.There is a pair of weak redox peaks on the CV curves, and the oxidation peaks (peak I) and reduction peaks (peak II) are gradually shifted towards the positive and negative directions with the increase in the scanning rate.The measured peak current (i) and scan rate (v) are measured based on the following relationship: where a and b are variable parameters and υ is the scan rate.The b value reflects the charge storage mechanism.If the b = 0.5, it is a diffusion-controlled process, whereas if the b = 1.0, the capacitor dominates the controlled process.The b value is the slope of linear fitting of log(i) versus log (v).As displayed in Figure 4b, the b value of peak I is 0.77, and of peak II it is 0.81, which suggests a hybrid capacitive and diffusion-controlled charge storage reaction mechanism.To analyse the kinetics of the charge storage of the 3D V-, N-Ti3C2 electrode, the CV curves of the 3D V-, N-Ti3C2 at 5-100 mV s −1 are presented in Figure 4a.There is a pair of weak redox peaks on the CV curves, and the oxidation peaks (peak Ⅰ) and reduction peaks (peak Ⅱ) are gradually shifted towards the positive and negative directions with the increase in the scanning rate.The measured peak current (i) and scan rate (v) are measured based on the following relationship: where a and b are variable parameters and υ is the scan rate.The b value reflects the charge storage mechanism.If the b = 0.5, it is a diffusion-controlled process, whereas if the b = 1.0, the capacitor dominates the controlled process.The b value is the slope of linear fitting of log(i) versus log (v).As displayed in Figure 4b, the b value of peak Ⅰ is 0.77, and of peak Ⅱ it is 0.81, which suggests a hybrid capacitive and diffusion-controlled charge storage reaction mechanism.We further analysed the electrochemical kinetic processes with EIS (Figure 4c).The intercepts on the X-axis indicate the electron transfer resistances (Rs) in the high-frequency region.The Rs of Ti3C2, 3D Ti3C2, V-Ti3C2, V-, N-Ti3C2, and 3D V-, N-Ti3C2 are measured as 28.2, 20.6, 18.9, 19.3 and 16.5 Ω, respectively.After constructing the 3D structure, the Rs of 3D Ti3C2 and 3D V-, N-Ti3C2 are decreased to 19.3 and 16.5 Ω, respectively.This indicates that the 3D structure provides more ion transport channels and a larger specific surface area, which effectively reduce solution resistance and electrode resistance.Moreover, V-Ti3C2 and V-, N-Ti3C2 show smaller Rs values (18.9 and 19.3 Ω) than Ti3C2 (28.2 Ω), an indication of an enhancement of the conductivity of Ti3C2 by means of V, N dual-doping.The diameter of the semicircle on the X-axis exhibits charge transfer resistance (Rct).The smaller Rct of 3D V-, N-Ti3C2 (37.1 Ω) indicates that the 3D structure provides more charge transfer channels, while the V/N dual-doping effectively reduces the Rct of Ti3C2.
To understand the electrochemical reaction mechanism of the 3D V-, N-Ti3C2 cathode in ZIC devices, we further investigated the structural changes in 3D V-, N-Ti3C2 during charge-discharge processes by performing ex situ XRD and ex situ XPS.As shown in Fig- ure 4e, based on the GCD curves of Zn//3D V-, N-Ti3C2 material and taking five Nanomaterials 2024, 14, x FOR PEER REVIEW 7 of 11 representative locations (A, B, C, D, and E) of the cathode, we extracted the surface changes of samples under different charging/discharging conditions.The device was first charged from state A (0 V) to 1.3 V (state C) and then discharged to 0 V (state E) under a constant charging-discharging current.Figure 4d shows the XRD in different states.Firstly, when charging to 0.8 V from 0 V, the peak located at 6.02° indicated that the (002) plane of 3D V-, N-Ti3C2 experienced a small shift towards a higher angle at 6.07°.Continuing to charge from 0.8 V (state B) to 1.3 V, the (002) peak shifted significantly from 6.07° to 6.29°.This was mainly due to H + /Zn 2+ de-intercalation during the charging process, which caused a reduction in interlayer spacing.Furthermore, the (002) peak shifted from 6.29° to 6.09° with discharge from 1.3 to 0.8 V (state D), then, with discharge from 0.8 V to 0 V (state E), the (002) peak shifted from 6.09° to 6.05°.This was mainly caused by H + /Zn 2+ intercalation during the discharge process [50].To capture more details of the reaction mechanism, the analysis was performed with ex situ XPS O 1s spectra (Figure 4f).The peaks at 532.6 and 531.6 eV at initial state belong to the V/Ti-OH and V/Ti-O groups, respectively.In the charge steps, the characteristic V/Ti-OH peak intensity at 532.6 eV undergoes a significant decrease from 0 to 0.8 V.As the charge process rises to 1.3 V, there is a slight decrease in the intensity of the characteristic peak of V/Ti-OH.In the discharge process, the intensity of the V/Ti-OH characteristic peak decreases slightly when the discharge voltage ranges from 1.3 V to 0.8 V, then manifests a sharp decrease following a further discharge to 0 V, which is related to the redox reaction of H + .To further investigate the contribution of Zn 2+ in the charge/discharge pro- We further analysed the electrochemical kinetic processes with EIS (Figure 4c).The intercepts on the X-axis indicate the electron transfer resistances (R s ) in the high-frequency region.The R s of Ti 3 C 2 , 3D Ti 3 C 2 , V-Ti 3 C 2 , V-, N-Ti 3 C 2 , and 3D V-, N-Ti 3 C 2 are measured as 28.2, 20.6, 18.9, 19.3 and 16.5 Ω, respectively.After constructing the 3D structure, the R s of 3D Ti 3 C 2 and 3D V-, N-Ti 3 C 2 are decreased to 19.3 and 16.5 Ω, respectively.This indicates that the 3D structure provides more ion transport channels and a larger specific surface area, which effectively reduce solution resistance and electrode resistance.Moreover, V-Ti 3 C 2 and V-, N-Ti 3 C 2 show smaller R s values (18.9 and 19.3 Ω) than Ti 3 C 2 (28.2 Ω), an indication of an enhancement of the conductivity of Ti 3 C 2 by means of V, N dual-doping.The diameter of the semicircle on the X-axis exhibits charge transfer resistance (R ct ).The smaller R ct of 3D V-, N-Ti 3 C 2 (37.1 Ω) indicates that the 3D structure provides more charge transfer channels, while the V/N dual-doping effectively reduces the R ct of Ti 3 C 2 .
To understand the electrochemical reaction mechanism of the 3D V-, N-Ti 3 C 2 cathode in ZIC devices, we further investigated the structural changes in 3D V-, N-Ti 3 C 2 during charge-discharge processes by performing ex situ XRD and ex situ XPS.As shown in Figure 4e, based on the GCD curves of Zn//3D V-, N-Ti 3 C 2 material and taking five representative locations (A, B, C, D, and E) of the cathode, we extracted the surface changes of samples under different charging/discharging conditions.The device was first charged from state A (0 V) to 1.3 V (state C) and then discharged to 0 V (state E) under a constant charging-discharging current.Figure 4d shows the XRD in different states.Firstly, when charging to 0.8 V from 0 V, the peak located at 6.02 • indicated that the (002) plane of 3D V-, N-Ti 3 C 2 experienced a small shift towards a higher angle at 6.07 • .Continuing to charge from 0.8 V (state B) to 1.3 V, the (002) peak shifted significantly from 6.07 • to 6.29 • .This was mainly due to H + /Zn 2+ de-intercalation during the charging process, which caused a reduction in interlayer spacing.Furthermore, the (002) peak shifted from 6.29 • to 6.09 • with discharge from 1.3 to 0.8 V (state D), then, with discharge from 0.8 V to 0 V (state E), the (002) peak shifted from 6.09 • to 6.05 • .This was mainly caused by H + /Zn 2+ intercalation during the discharge process [50].
To capture more details of the reaction mechanism, the analysis was performed with ex situ XPS O 1s spectra (Figure 4f).The peaks at 532.6 and 531.6 eV at initial state belong to the V/Ti-OH and V/Ti-O groups, respectively.In the charge steps, the characteristic V/Ti-OH peak intensity at 532.6 eV undergoes a significant decrease from 0 to 0.8 V.As the charge process rises to 1.3 V, there is a slight decrease in the intensity of the characteristic peak of V/Ti-OH.In the discharge process, the intensity of the V/Ti-OH characteristic peak decreases slightly when the discharge voltage ranges from 1.3 V to 0.8 V, then manifests a sharp decrease following a further discharge to 0 V, which is related to the redox reaction of H + .To further investigate the contribution of Zn 2+ in the charge/discharge process, the ex situ XPS Zn 2p spectra are shown in Figure 4g.Herein, the characteristic peak intensity of Zn 2p and the Auger lines of Zn decline in the charge process (from state A to state C) and then gradually increase during the discharge stage (from state C to state E).This phenomenon illustrates the electrochemical reaction of Zn 2+ with the oxygen terminals on the surface of 3D V-, N-Ti 3 C 2 during the charging-discharging process [42].In general, the charge storage mechanism of 3D V-, N-Ti 3 C 2 includes the pseudocapacitive behavior of H + and chemical absorption/desorption of Zn 2+ .Based on ex situ XRD and XPS, the charge storage mechanism of the 3D V-, N-Ti 3 C 2 cathode probably behaves as follows: Discharge: To exemplify the viability of the 3D V-, N-Ti 3 C 2 film for practical applications, ZIC devices were assembled with zinc sheet and 3D V-, N-Ti 3 C 2 as anode and cathode, respectively (Figure 5a).The CV curves of the 3D V-, N-Ti 3 C 2 //Zn show a rectangular shape with weak redox peaks at 5 mV s −1 to 100 mV s −1 in Figure 5b, indicating the pseudo-capacitance feature of the 3D V-, N-Ti 3 C 2 //Zn ZIC.The GCD curves (Figure 5c) show symmetric charge/discharge processes at 0.5 to 10 A g −1 , implying a high coulombic efficiency.The specific capacitance and specific capacity were able to reach up to 441 F g −1 and 159 mAh g −1 , respectively, at 0.5 A g −1 , implying an excellent energy storage performance (Figure 5d).Furthermore, we compared the energy density and power density of the 3D V-, N-Ti 3 C 2 //Zn with previously reported ZICs.As shown in Figure 5e, the 3D V-, N-Ti 3 C 2 //Zn ZIC 103 Wh kg −1 energy density was at a power density of 325 W kg −1 , which was significantly superior to recently reported ZICs [29,34,45,46,51].In addition, the 3D V-, N-Ti 3 C 2 //Zn ZIC full cells exhibited a favorable cyclic stability, which exhibited 86.6% capacity retention after 3000 cycles at 10 A g −1 (Figure S14).To further verify the viability of the 3D V-, N-Ti 3 C 2 //Zn ZIC, we extended the working voltage window and capacity by connecting two ZIC devices in parallel and in series.as depicted in Figure 5b.As anticipated, the voltage window widened to 2.6 V when two devices were connected in series, and then two devices in parallel were able to double the capacity.A small red LED bulb was able to be successfully powered by two 3D V-, N-Ti 3 C 2 //Zn ZICs connected in series, indicating that the ZICs can work effectively in practical applications.

C-Ti
To exemplify the viability of the 3D V-, N-Ti3C2 film for practical applications, ZIC devices were assembled with zinc sheet and 3D V-, N-Ti3C2 as anode and cathode, respectively (Figure 5a).The CV curves of the 3D V-, N-Ti3C2//Zn show a rectangular shape with weak redox peaks at 5 mV s −1 to 100 mV s −1 in Figure 5b, indicating the pseudo-capacitance feature of the 3D V-, N-Ti3C2//Zn ZIC.The GCD curves (Figure 5c) show symmetric charge/discharge processes at 0.5 to 10 A g −1 , implying a high coulombic efficiency.The specific capacitance and specific capacity were able to reach up to 441 F g −1 and 159 mAh g −1 , respectively, at 0.5 A g −1 , implying an excellent energy storage performance (Figure 5d).Furthermore, we compared the energy density and power density of the 3D V-, N-Ti3C2//Zn with previously reported ZICs.As shown in Figure 5e, the 3D V-, N-Ti3C2//Zn ZIC 103 Wh kg −1 energy density was at a power density of 325 W kg −1 , which was significantly superior to recently reported ZICs [29,34,45,46,51].In addition, the 3D V-, N-Ti3C2//Zn ZIC full cells exhibited a favorable cyclic stability, which exhibited 86.6% capacity retention after 3000 cycles at 10 A g −1 (Figure S14).To further verify the viability of the 3D V-, N-Ti3C2//Zn ZIC, we extended the working voltage window and capacity by connecting two ZIC devices in parallel and in series.as depicted in Figure 5b.As anticipated, the voltage window widened to 2.6 V when two devices were connected in series, and then two devices in parallel were able to double the capacity.A small red LED bulb was able to be successfully powered by two 3D V-, N-Ti3C2//Zn ZICs connected in series, indicating that the ZICs can work effectively in practical applications.

Conclusions
In summary, the 3D V-, N-Ti 3 C 2 film electrode with a high specific capacity was rationally designed by constructing 3D structures and using V/N dual-doping.Thanks to the porous structure design, heteroatom doping, and metal-ion pre-intercalation, the fabricated 3D V-, N-Ti 3 C 2 based on Zn-ion pre-intercalation delivered a maximum capacitance of 855 F g −1 at 0.3 A g −1 and demonstrated 95.26% capacitance retention after 5000 charge/discharge cycles.The dual-ion energy storage mechanism of H + /Zn 2+ was revealed by using ex situ XRD and XPS.In addition, the as-assembled aqueous 3D V-, N-Ti 3 C 2 //Zn ZICs displayed 103 Wh kg −1 energy density at a power density of 325 W kg −1 and a favorable cycling durability.This work presents an illuminating insight into rational pore structural design and heteroatom doping to obtain a desirable specific capacity and energy density for MXene electrode materials in energy storage systems.

Figure 1 .
Figure 1.Schematic illustration of the synthesis process of the 3D V-, N-Ti3C2 film.
and 2b) showed the morphology of Ti3C2 film, 3D Ti3C2 film, V-Ti3C2 film, V-, N-Ti3C2 film, and 3D V-, N-Ti3C2 film.The SEM images of cross-sections of Ti3C2 film are shown in Figures S2 and S4a,b.V-Ti3C2 films and V-, N-Ti3C2 films show that

Figure 1 .
Figure 1.Schematic illustration of the synthesis process of the 3D V-, N-Ti 3 C 2 film.The XRD patterns of 3D V-, N-Ti 3 C 2 and other samples are presented in Figures 2a and S1b.The Ti 3 C 2 film has an intense (002) peak at around 6 • , which confirms that Ti 3 C 2 was successfully synthesized from Ti 3 AlC 2 .The XRD pattern of 3D V-, N-Ti 3 C 2 shows that the doping and structural design had minimal effect on Ti 3 C 2 phase structure.The SEM images (Figures S1-S4 and 2b) showed the morphology of Ti 3 C 2 film, 3D Ti 3 C 2 film, V-Ti 3 C 2 film, V-, N-Ti 3 C 2 film, and 3D V-, N-Ti 3 C 2 film.The SEM images of cross-sections of Ti 3 C 2 film are shown in Figures S2 and S4a,b.V-Ti 3 C 2 films and V-, N-Ti 3 C 2 films show that the films are formed by the dense packing of Ti 3 C 2 nanosheets.The V-Ti 3 C 2 film and V-, N-Ti 3 C 2 film are structurally dense and have small amounts of TiO 2 on the nanosheets' surface.The dense structure severely limits the exposure of surface active sites and ion diffusion, which in turn affects the electrochemical properties of the films.As for the 3D Ti 3 C 2 film (the thickness is about 900 nm) and the 3D V-, N-Ti 3 C 2 film (Figures 2b and S3), . The Ti 3 C 2 film has an intense (002) peak at around 6 • , which confirms that Ti 3 C 2 was successfully synthesized from Ti 3 AlC 2 .The XRD pattern of 3D V-, N-Ti 3 C 2 shows that the doping and structural design had minimal effect on Ti 3 C 2 phase structure.The SEM images (Figures S1-S4 and 2b) showed the morphology of Ti 3 C 2 film, 3D Ti 3 C 2 film, V-Ti 3 C 2 film, V-, N-Ti 3 C 2 film, and 3D V-, N-Ti 3 C 2 film.The SEM images of cross-sections of Ti 3 C 2 film are shown in Figures S2 and S4a,b.V-Ti 3 C 2 films and V-, N-Ti 3 C 2 films show that the films are formed by the dense packing of Ti 3 C 2 nanosheets.The V-Ti 3 C 2 film and V-, N-Ti 3 C 2 film are structurally dense and have small amounts of TiO 2 on the nanosheets' surface.

Figure 3 .
Figure 3. (a) CV curves of the Ti3C2 film, 3D Ti3C2 film, V-Ti3C2 film, V-, N-Ti3C2 film, and 3D V-, N-Ti3C2 film at 5 mV s −1 .(b) GCD curves of the 3D V-, N-Ti3C2 film electrode from 0.3 to 20 A g −1 .(c) Specific capacitance of all film electrodes at various current densities.(d) Energy and power density profiles for all film electrodes.(e) Cycling stability of the 3D V-, N-Ti3C2 film at 8 A g −1 .

Figure 3 .
Figure 3. (a) CV curves of the Ti 3 C 2 film, 3D Ti 3 C 2 film, V-Ti 3 C 2 film, V-, N-Ti 3 C 2 film, and 3D V-, N-Ti 3 C 2 film at 5 mV s −1 .(b) GCD curves of the 3D V-, N-Ti 3 C 2 film electrode from 0.3 to 20 A g −1 .(c) Specific capacitance of all film electrodes at various current densities.(d) Energy and power density profiles for all film electrodes.(e) Cycling stability of the 3D V-, N-Ti 3 C 2 film at 8 A g −1 .

Figure 4 .
Figure 4. (a) CV curves of the 3D V-, N-Ti 3 C 2 at 5 to 100 mV s −1 ; (b) the determination of the b values of the peak I and peak II based on log(i) versus log(v) plots; (c) Nyquist plots of 3D V-, N-Ti 3 C 2 film, Ti 3 C 2 film, 3D Ti 3 C 2 film, V-Ti 3 C 2 film, and V-, N-Ti 3 C 2 film; (d) ex situ XRD patterns of the 3D V-, N-Ti 3 C 2 cathode; (e) GCD profile of 3D V-, N-Ti 3 C 2 ; (f) ex situ C 1s XPS spectra of the 3D V-, N-Ti 3 C 2 cathode; and (g) ex situ Zn 2p XPS spectra of the 3D V-, N-Ti 3 C 2 cathode.