Fe Species Intercalation Confined by the Interlayer Environment of V₂CT_x MXene for Lithium-Ion Storage

Abstract

This work successfully achieved pre-intercalation of Fe species in V2CTx MXene through an annealing method. The crystallographic structure, microscopic morphology, and functional groups of the samples before and after pre-intercalation were analyzed by XRD, SEM, and FTIR, and the electrochemical performance of MXene electrodes was studied. Research has shown that the interlayer spacing of pre-intercalated MXene increases with an increase in annealing temperature. The interlayer spacing of MXene annealed at 800 °C is 13.1% higher than that of the original MXene. However, the morphology of the samples was damaged by excessively high annealing temperatures, which also weakened the lithium-ion storage performance. In contrast, the cycling performance of MXene electrodes annealed at 400 °C showed the greatest improvement, reaching 71.65%. This is because iron species, acting as a pillar support structure, expand the interlayer spacing and broaden the transport channels for lithium ions. Meanwhile, high-temperature annealing generates more oxygen-containing functional groups, which provide additional active sites for lithium-ion transport, promote the kinetics of electrode reactions, and thus enhance its lithium-ion storage performance.

1. Introduction

With the development and utilization of renewable energy sources such as solar, wind, and tidal power, developing reliable and environmentally friendly energy storage methods is one of the major challenges for humanity in the 21st century. Electrochemical energy storage, due to its high energy density, high cycle efficiency, and flexible application, is considered an ideal method. Currently, among various energy storage devices, secondary batteries and supercapacitors play a crucial role. Lithium-ion batteries, with their high operating voltage, high energy density, long cycle life, low self-discharge rate, and lightweight design, have been widely used in portable electronic devices and energy storage systems [1,2,3]. However, issues such as high costs, safety concerns, and resource limitations are also critical factors that need attention during their development. Currently, most commercialized lithium-ion batteries use graphite as the anode material. However, graphite anodes have the problem of a relatively low theoretical specific capacity (372 mAh/g). In contrast, although silicon-based anodes have a high theoretical specific capacity (~4200 mAh/g), they undergo severe volume changes during the charge–discharge process [4]. Therefore, in terms of anodes, the development of new high-capacity electrode materials remains a key aspect in the development of lithium-ion batteries.

The two-dimensional materials have attracted extensive attention from researchers due to their unique structure completely different from bulk materials and excellent physical and chemical properties [5,6]. So far, more than 100 kinds of two-dimensional materials have been successfully prepared and reported [7,8,9], such as carbon nitride, black phosphorus, layered double hydroxides, transition metal oxides, transition metal sulfides, as well as transition metal carbides, nitrides and carbonitrides (referred to as MXenes). Among various two-dimensional materials, MXenes have been widely used in energy storage, catalysis, adsorption, biomedicine and other fields due to their advantages such as unique layered structure, large specific surface area, high electrical conductivity, good mechanical properties, and low ion diffusion coefficient [10]. Especially in the field of secondary batteries, MXenes show excellent application potential as an electrode material [11,12,13,14].

The general chemical formula of MXenes is M_n+1X_nT_x, where M represents a transition metal, X is C, N, or CN, n = 1, 2, 3, and T_x denotes surface terminal groups such as -O, -OH, and -F [15]. At present, MXenes are mostly prepared by the selective etching of MAX phases; that is, MXenes can be obtained by etching away the “A” layer of the MAX phase through a specific reaction. With the deepening of research on MXenes, the preparation methods have become diversified, such as the electrochemical method [16], molten salt method [17], chemical vapor deposition method [18], alkali-assisted hydrothermal method [19], etc. So far, more than 30 kinds of MXenes with definite structures and properties have been successively prepared [20].

The two-dimensional layered structure and surface metal active sites of MXenes provide abundant spatial channels for the storage and transport of lithium ions [21,22]. However, in practical applications, MXene layers are prone to stacking and aggregation, resulting in a significant reduction in their specific surface area. In addition, collapse and stacking can cause a significant increase in the vertical interlayer resistivity of MXenes, further hindering the transport of Li⁺ and leading to a decrease in its electrochemical lithium storage performance [23]. In order to further improve the lithium storage capacity of MXenes, many research groups have conducted extensive studies [24,25,26,27]. Surface modification of MXene layers is an effective way to suppress their stacking and aggregation, and further increase the interlayer spacing of MXenes to enhance the electrochemical lithium storage capacity. Luo et al. [28] prepared PVP Sn (IV) @ Ti₃C₂ material and successfully intercalated Sn⁴⁺ into the interlayer of Ti₃C₂ through ion exchange and electrostatic adsorption, increasing the interlayer spacing. By utilizing the synergistic effect between Sn⁴⁺ and Ti₃C₂, excellent lithium storage performance has been achieved. Zhang et al. [29] prepared SnS/Ti₃C₂T_x composite materials by combining hydrothermal reactions with subsequent annealing treatment. The introduced SnS nanoparticles were uniformly dispersed between Ti₃C₂T_x layers, increasing the specific surface area of the material and providing an effective electron transport pathway. The specific capacity of the material reached 255.9 mAh/g at a current density of 1000 mA/g. Duan et al. [30] adopted a hydrothermal synthesis strategy to prepare a porous Co₃O₄ nanoneedle/MXene composite through in situ growth and self-assembly. By utilizing the synergistic effect between MXene and Co₃O₄, it not only shortens the ion/electron transport pathways within the MXene matrix but also mitigates the agglomeration and volume expansion of Co₃O₄ nanoneedles. The as-prepared Co₃O₄/MXene electrode maintained a specific capacity of 632 mAh/g after 1000 cycles at a high current density of 1 A/g. Wang et al. [31] designed and synthesized a Ti₃C₂@VO₂ composite with a sandwich-like architecture. The VO₂ nanoparticles grown between the Ti₃C₂ layers act as pillars, preventing the stacking and structural collapse of multi-layer Ti₃C₂ nanosheets. Consequently, the Ti₃C₂@VO₂ composite exhibits superior electrochemical performance and slight volume expansion compared to pure VO₂. After 100 cycles at a current density of 100 mA/g, it achieves a capacity of 365.6 mAh/g with a volume expansion rate of 18.3%. Lu et al. [32] realized the improvement of electrochemical performance by using the V₂CT_x MXene-based lithium-ion battery pre-embedded with cations, expanded the layer spacing of V₂CT_x and enhanced the ion migration kinetics of V₂CT_x in the process of lithium-ion intercalation and delamination.

So far, most research on MXenes focuses on Ti₃C₂T_x, mainly due to its relatively more mature preparation process. As another important member of the MXene family, V₂CT_x MXene exhibits high electrical conductivity and low ion transport barriers. Moreover, the vanadium element with multiple oxidation states endows it with pseudocapacitive properties, thus showing great potential in electrochemical energy storage [33]. Theoretical studies have shown that the theoretical lithium storage capacity of V₂CT_x reaches 940 mAh/g, which is much higher than that of Ti₃C₂T_x (320 mAh/g) [34]. Moreover, V2CT_x and its modified materials demonstrate more excellent performance [35]. However, the difficulty in preparation caused by the high formation energy of V₂CT_x MXene and its structural instability severely restricted its development. If the preparation process conditions of V₂CT_x MXene materials can be further optimized and their advantages of high conductivity and high stability can be fully utilized, they are expected to play an important role in next-generation anode materials for lithium-ion batteries.

At present, the hydrothermal synthesis method, as a common method to adjust the interlayer spacing of MXene and optimize its performance through pre-intercalation, has a relatively low reaction temperature, usually below 200 °C. The level of reaction temperature usually affects the composition, morphology, and size of pre-intercalated particles, which in turn leads to differences in the interlayer spacing changes of MXene after pre-intercalation.

Based on this, we adjusted the experimental method for pre-intercalation in the early hydrothermal synthesis and innovatively adopted a one-step high-temperature annealing method to achieve the pre-intercalation of Fe species into V₂CT_x MXene at 200, 400, and 800 °C, respectively. By means of the intercalation mechanism where foreign substances are embedded between MXene layers, a pillar-supported two-dimensional heterostructure was constructed through a simple experimental method. The structural evolution of V₂CT_x MXene induced by the pre-intercalation of Fe species at high temperatures and its lithium-ion storage performance were investigated.

2. Results and Discussion

2.1. Microstructure and Morphology

Figure 1 shows the XRD spectrum of V₂CT_x MXene pre-intercalated with Fe species with different annealing temperatures. The diffraction peaks of the etched sample are completely different from those of V₂AlC (JCPDS No. 29-0101). They mainly consist of two sets of diffraction peaks. Among them, the characteristic peaks at 2θ = 9.0°, 35.6° and 41.2° correspond to the (002), (100) and (113) crystal planes of V₂CT_x, respectively [36], indicating that V₂CT_x MXene has been successfully prepared by the selective etching of V₂AlC. The positions of the remaining diffraction peaks are consistent with those of NaV₆O₁₅ (JCPDS No. 77-0146) [36]. Since there is no sodium element in the raw materials used in the experimental reaction, and considering the presence of ammonium fluoride in the raw materials, it can be inferred that the position of sodium element in the product structure is replaced by ammonium ions, and the ammonium ions are located at the position where sodium ions are in the structure of this standard card. This may be attributed to the fact that the V₂C dispersion formed after the etching reaction of the raw material V₂AlC undergoes a partial hydrothermal reaction under the condition of an appropriate amount of dilute hydrochloric acid, causing V₂C to be oxidized to generate a small amount of HV₆O₁₅, which then undergoes an ion exchange reaction with the ammonium ions present in the reaction solution, ultimately forming NH₄V₆O₁₅. The samples annealed at high temperatures of 200, 400 and 800 °C all retain the (002) characteristic peak, indicating that the pre-embedding of Fe species controlled by annealing does not induce changes in the crystal structure of V₂CT_x MXene [37].

Moreover, many new diffraction peaks appear in the XRD pattern of the sample after high-temperature annealing, which mainly correspond to V₂O₅ (JCPDS No. 65-0131), Fe₃O₄ (JCPDS No. 79-0416) and Fe₂O₃ (JCPDS No. 21-0920) formed during the annealing process. The production of these oxides mainly results from the decomposition of NH₄V₆O₁₅ and the oxidation reactions between intercalated Fe atoms with the -O functional groups on the MXene surface during the high-temperature annealing process. Among them, NH₄V₆O₁₅ decomposes at a high temperature to form V₂O₅. FeCl₃ undergoes a dissociation reaction and oxidizes with O in oxygen-containing functional groups to first form Fe₃O₄ at a relatively low temperature. Then, Fe₃O₄ can continue to be oxidized to Fe₂O₃ as the temperature rises. This oxidation phenomenon is also common in the annealing treatment of Ti₃C₂T_x MXene [38,39]. With an increase in annealing temperature, the amount of Fe₃O₄ decreases while the amount of Fe₂O₃ increases, which indicates that the oxidation of Fe is more sufficient at high temperatures, and Fe transforms from a low valence state to a high valence state.

At the same time, it can be observed that the (002) diffraction peak gradually shifts to low angles with increasing temperature. The interlayer spacings of each sample calculated by the Bragg equation (2dsinθ = nλ) are as follows: 9.6044 Å (original V₂CT_x MXene), 9.8096 Å (200 °C), 9.8324 Å (400 °C), and 10.8589 Å (800 °C). A detailed data comparison can be seen in

Table 1. Peak position and interlayer spacing of (002) diffraction peak of V₂CT_x MXene pre-intercalated with Fe species with different annealing temperatures.

(002)	$2\mathit{\theta }$ (°)	Interlayer Spacing (Å)
V2CTx	9.200	9.6044
200 °C	9.007	9.8096
400 °C	8.987	9.8314
800 °C	8.135	10.8589

. Through comparison, it can be seen that the higher the annealing temperature, the larger the interlayer spacing. This result is contrary to the variation in the interlayer spacing of Ti₃C₂T_x MXene with annealing temperature reported in the literature [38]. It is worth noting that for the sample annealed at 800 °C, the full width at half maximum (FWHM) of the (002) characteristic peak increases, the intensity decreases significantly, and the peak becomes very flat, indicating that the order degree of MXene has been severely reduced. This is mainly because the excessively high heat treatment temperature destroys the two-dimensional nanolayered structure of MXene, causing severe collapse. This result can be confirmed by the morphology reflected in the SEM micrographs. This indicates that only an appropriate annealing temperature can ensure the integrity of the MXene nanolayered structure.

Figure 2 shows the SEM image of V₂CT_x MXene pre-intercalated with Fe species with different annealing temperatures. It can be seen that the original V₂CT_x MXene sample presents a typical accordion morphology. After the Fe species are pre-intercalated at 200 and 400 °C, the clear accordion morphology for the two V₂CT_x MXene samples can still be observed, and the MXenes still maintain the two-dimensional layered structure. It indicates that there is no obvious structure collapse after Fe species pre-intercalation. However, the morphology of the sample pre-intercalated with Fe species at 800 °C changed significantly, which was mainly due to the damage of its structure caused by the high-temperature environment. Obviously, the MXene layers are exfoliated, transforming from the original layered stacked structure to a non-layered disordered stacked flake surface structure, which also occurs during the excessively high-temperature annealing process of Ti₃C₂ MXene. And a dense crystalline layer can be formed on this flake-like surface, resulting in the loss of electrochemical active sites and a significant decline in performance [40].

Figure 3 shows the FTIR spectra of V₂CT_x MXenes pre-intercalated with Fe species with different annealing temperatures. As can be seen from the figure, characteristic peaks appear at 3422, 1624, 1346, 985, and 705 cm⁻¹, corresponding to the -OH, C=O, O-H, V=O, and V-O-V functional groups, respectively. It can be observed that the -OH functional groups at 3422 cm⁻¹ increase with the rise in temperature, reaching the maximum at 400 °C. This may be attributed to the high-temperature embedding of Fe species. However, the -OH functional groups decrease at 800 °C, which is probably due to the re-reaction of O and H in the sample to generate H₂ at 800 °C, leading to the reduction of -OH functional groups. The O-H functional groups at 1346 cm⁻¹ decrease with the increase in annealing temperature, because high-temperature calcination treatment results in the formation of more oxygen-containing functional groups. The presence of -O functional groups is more conducive to the excellent intercalation pseudocapacitive performance of V₂CTₓ MXene electrodes. In addition, during the annealing process, the surface of MXene sheets is oxidized, forming V₂O₅, Fe₂O₃, and Fe₃O₄ nanoparticles, which increases the proportion of surface oxygen-containing functional groups. This can also be confirmed by the increase in V=O and V-O-V functional groups with the rise in annealing temperature. According to relevant reports, most of the metal cations inserted between V₂CT_x layers are connected with V-O bonds [41,42]. It can be inferred that in this experiment, after Fe ions are intercalated, a certain degree of V-O-Fe is formed through the interaction with V-O. Moreover, Fe³⁺ has a stronger interaction with V-O than Fe²⁺ and is more easily oxidized by the oxygen-containing functional groups on the surface of V₂CT_x. Since the electronegativity of Fe is greater than that of V, the electron cloud density of V-O bonds decreases, and the binding energy increases. As a result, the content of oxygen functional groups in V₂CT_x increases significantly. Compared with -F and -OH functional groups, the =O functional group has higher energy, causes the least hindrance during ion transport, and can provide stable electrochemical active sites. Moreover, as an important adsorption site for lithium ions, oxygen functional groups will be beneficial to the lithium-ion storage of V₂CT_x MXene electrodes [43].

2.2. Electrochemical Properties

Figure 4 shows the CV curve of the first five cycles of V₂CT_x MXene electrodes pre-intercalated with Fe species with different annealing temperatures. It can be seen from the CV curves that there is a slight difference between the CV curve of the first cycle and those of other cycles. This is because irreversible reactions occur during the insertion and extraction of lithium ions in the first charge–discharge process of the battery, resulting in the formation of a solid electrolyte interphase (SEI film). The CV curves from the second to the fifth cycles basically overlap, indicating that the insertion and extraction processes of lithium ions inside the electrode material are reversible. All electrodes exhibit CV curves similar to rectangles, and at the same time, all CV curves have redox peaks. This indicates that NH₄V₆O₁₅ in the initial MXene, as well as V₂O₅, Fe₂O₅, and Fe₃O₄ generated by high-temperature annealing, are all involved in the charge–discharge reaction. It can be inferred from this that the storage mechanism of the V₂CT_x MXene electrode is the synergistic effect of two mechanisms: ion intercalation and surface redox [44]. The oxidation peaks appear between 1 and 2.5 V, and the reduction peaks appear between 0.5 and 1.7 V. The symmetric CV curves indicate that the Li⁺ insertion and extraction reactions are reversible. In addition, from the V₂CT_x MXene, 200 and 400 °C samples in the figure, it can be seen that the reduction peaks appearing at low potentials gradually decrease with the increase in temperature, indicating that the rate of charge transfer kinetics becomes faster, which promotes the insertion/extraction of lithium ions. However, the reduction peak of the 800 °C sample shifts significantly to the right, which may be due to the layered structural damage caused due to the high temperature of 800 °C, thus leading to a decrease in lithium-ion transfer kinetics.

Figure 5 shows the CV curve of V₂CT_x MXene electrodes pre-intercalated with Fe species with different annealing temperatures at different scanning speeds. As shown in the figure, with the increase in the scanning rate, the area enclosed by the curve gradually increases, which indicates that the specific capacity of the electrode is proportional to the scanning rate. Moreover, as the scanning rate increases, the peak current of each electrode gradually increases. At the same time, the oxidation peak gradually shifts to the direction of high potential, and the reduction peak gradually shifts to the direction of low potential, indicating that there is a certain polarization phenomenon in the electrodes treated at different annealing temperatures during the process of lithium-ion insertion and extraction. In addition, the peak separation of the CV curve of the tested sample begins to increase significantly at 2 mV/s without obvious distortion, indicating its excellent electrochemical performance.

Figure 6 shows the cyclic and rate performance. The specific capacity of each sample electrode is relatively high during the first discharge, followed by a certain degree of attenuation. This may be due to the occurrence of irreversible reactions and the formation of a SEI film during the initial cycle process. Since lithium ions cannot be fully intercalated between the V₂CT_x layers, the specific capacity decreases significantly. After 10 cycles, the discharge specific capacity begins to gradually increase. This is because the repeated intercalation and deintercalation of lithium ions between the layers lead to an increase in the interlayer spacing, which increases the active sites for lithium-ion storage and accelerates the transport rate of lithium ions, thereby improving the lithium storage specific capacity between the layers. In addition, the Coulombic efficiency of all V₂CT_x MXene electrodes is slightly higher than 100%, showing excellent cycle stability. The discharge capacity of V₂CT_x MXene electrodes annealed at 200 and 400 °C after 100 cycles is significantly higher than that of the original V₂CT_x MXene. Compared with the other three materials, the V₂CT_x MXene electrode annealed at 400 °C exhibits the most excellent electrochemical performance and the highest specific capacity. Its initial discharge capacity is 570.3 mAh/g, and after 100 cycles, the discharge capacity can still reach 493.6 mAh/g, which is a full 71.65% higher than the 287.6 mAh/g of the original V₂CT_x MXene. Compared with the pure V₂CT_x MXene materials prepared by the selective etching method reported in the current literature, its electrochemical performance has been significantly improved. Naguib et al. [45] reported that the specific capacities of V₂CT_x MXene after 50 and 150 cycles at 1 C are 288 and 260 mAh/g, respectively. Liu et al. [46] reported that the specific capacities of V₂CT_x MXene at 100 mA/g decreased from 295 mAh/g at the 2nd cycle to 257 mAh/g at the 140th cycle. It can be seen that the MXene pre-intercalation method is an effective means to expand the interlayer spacing, improve the interlayer environment, and enhance the electrochemical performance of materials.

After annealing at high temperatures of 200 and 400 °C, the V₂CTₓ MXene pre-embedded with Fe species still maintains a complete layered structure. Moreover, Fe₂O₃, Fe₃O₄, and V₂O₅ generated between the MXene layers not only play a pillar-supporting role to expand the interlayer spacing but also provide more active sites for the transport of lithium ions, thus significantly improving the electrode capacity. In comparison, the more Fe₂O₃ generated during the high-temperature annealing process, the more prominent the effect on expanding the interlayer spacing, and thus the best electrochemical performance is achieved. However, the V₂CT_x MXene electrode annealed at 800 °C has the lowest specific capacity, with an initial discharge specific capacity of 251.8 mAh/g, and the remaining capacity drops to 235.2 mAh/g after 100 cycles. This is mainly because the layered structure of the V₂CT_x MXene electrode is severely damaged at the high temperature of 800 °C, and some active sites are covered, which in turn hinders the insertion and extraction process of Li⁺ in the electrolyte, resulting in the lowest specific capacity. It can be seen that appropriate high-temperature annealing treatment can effectively improve the specific capacity of V₂CT_x MXene.

When the current density returns to 1 C, the specific capacity of the electrode also recovers to its original value accordingly, which indicates that all V₂CT_x MXene electrodes have good rate performance and cyclic reversibility. The V₂CT_x MXene electrode embedded with Fe species at 400 °C exhibits the best rate performance. At current densities of 0.2, 0.5, 1, 2, and 5 C, its mass specific capacities are 663.4, 575.1, 491.0, 401.2, and 278.2 mAh/g, respectively, as shown in Figure 6b. When the current density is restored to 1 C, its mass specific capacity increases to 450.0 mAh/g, which is consistent with the results of XRD and CV tests. This indicates that appropriate annealing temperature treatment can produce oxides of Fe and V between the layers, which can not only play the role of pillar support but also provide active sites. Therefore, the diffusion kinetics of electrolyte lithium ions in the V₂CT_x MXene electrode are the largest. However, excessively high annealing temperature leads to the destruction of the MXene layered structure and the masking of some active sites, ultimately severely reducing the electrochemical performance.

3. Experimental Section

3.1. Reagents and Materials

The reagents required for the experimental synthesis of samples mainly include hydrochloric acid (HCl, 40 wt %), V₂AlC powders, ferric chloride (FeCl₃), ammonium fluoride (NH₄F), N-methyl pyrrolidone (NMP), and lithium hexafluorophosphate (LiPF₆). Among them, V₂AlC MAX powders were purchased from Jilin 11 Technology Co., Ltd. (Changchun, China), and the rest were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the above chemicals used in this experiment are of analytical purity, and all the reagents were not further purified.

3.2. Preparation of V₂CT_x MXene

V₂CT_x MXene was prepared by the following method. A total of 40 mL of 40% diluted hydrochloric acid was poured into a polytetrafluoroethylene beaker, and 2 g of NH₄F was slowly added to it. After stirring for half an hour, V₂CT_x MAX phase ceramic powder with particle size of 400 meshes was added. After stirring evenly, it was put into a reactor and kept at 120 °C for 24 h. After cooling at room temperature, the dark brown solution generated after etching was poured out, centrifuged repeatedly for 5 min at a speed of 5000 rpm/min, and then the supernatant was poured out. In the above process, repeat the operation for 5 times until the pH value of the supernatant reaches above 6, add an appropriate amount of ethanol, and conduct ultrasonic treatment with a 750 W ultrasonic machine for 5 min, so that there are no obvious particles in the centrifuge tube, and the resulting sediment is multi-layer V₂CT_x MXene, which is loaded into the centrifuge tube for standby.

3.3. Preparation of V₂CT_x MXene Pre-Intercalated with Fe Species with Different Annealing Temperatures

Mix the prepared V₂CT_x MXene with a 3 M concentration of ferric chloride solution, centrifuge for 5 min at a speed of 5000 r/min, and then discard the upper clear liquid. Add an appropriate amount of ethanol with a concentration of 75% to it, sonicate with a 750 W ultrasonic machine for 30 min, pour out the upper layer of ethanol in the centrifuge tube, and dry the resulting precipitate in a vacuum drying oven at 80 °C for more than 12 h. Grind the obtained solid dry material until there are no obvious particles, and then vacuum seal it for future use. Divide the encapsulated powder into three parts and heat them separately at 200, 400, and 800 °C in a rapid annealing furnace under vacuum conditions. The resulting powder is V₂CT_x MXene pre-intercalated with Fe species by the high-temperature annealing method.

3.4. Microstructural Characterization

X-ray diffraction patterns (XRD) of V₂CT_x MXene powders were obtained using a Rigaku D/Max-2500 diffractometer (Tokyo, Japan). The test uses a CuKα (λ = 0.15406 nm) radiation source with a working voltage of 40 kV and a working current of 200 mA. The scanning test is conducted at a scanning speed of 10 °/min in the scanning range from 5 to 65°. The morphologies of V₂CT_x MXene powders were observed by scanning electron microscopy (SEM, JSM-7800F, JEOL, Tokyo, Japan). Nicolet iS50 Fourier transform infrared spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) was used to conduct FTIR tests on V₂CT_x MXene powders, the spectra in the range of 4000–500 cm⁻¹ were collected with a wavenumber accuracy of 0.01 cm⁻¹.

3.5. Electrochemical Measurements

Using V₂CT_x MXene pre-intercalated with Fe species as anode (which contributes the charge storage between 0.05 and 3 V), lithium sheet as a counter electrode/reference electrode (to match the electrochemical reaction mechanism of lithium-ion intercalation/deintercalation, construct a stable lithium-ion transport cycle, and reduce interfacial side reactions), and commercial lithium-ion battery electrolyte (1 M LiPF₆ in ethylene carbonate/diethyl carbonate/ethyl methyl carbonate (EC/DEC/EMC), 1:1:1 in volume) as an electrolyte, the battery system was assembled with a CR2032 button electrode shell to explore the performance and mechanism of the Fe species pre-intercalated V₂CT_x MXene electrode for lithium-ion storage at high temperatures. The cyclic voltammetry (CV) curves and electrode performance were collected using the electrochemical workstation (Metrohm, Autolab PGSTAT302N) and the battery test system (NEWARE, CT-4008T). During the testing process, the voltage window was set to 0.05–3 V, and the cyclic voltammetry curve of the lithium-ion battery was tested at a scanning speed of 1 mV/s. In addition, CV curves at different scan speeds (0.5, 1, 2, 5, and 10 mV/s) were tested and compared to further investigate the electrochemical performance of V₂CT_x MXene electrodes. The battery testing system was used to test the constant current charge discharge curve and cycling characteristics of the V₂CT_x MXene as the anode of lithium-ion batteries in the potential range of 0.05–3 V. At the same time, the rate characteristics of the V₂CT_x MXene as the anode of lithium-ion batteries were tested with current densities of 0.2, 0.5, 1, 2, and 5 C, and the changes in its cycling and rate performance were analyzed.

4. Conclusions

In this study, the two-dimensional heterostructure was constructed by pre-intercalating Fe species into V₂CT_x MXene at high temperature. The results show that the pre-intercalation of Fe species not only maintains the crystal structure and accordion-like morphology of V₂CT_x MXene, but also expands the interlayer spacing of MXene through a pillar support effect. The layer spacing increases with the increase in annealing temperature, providing more active sites for lithium-ion transport. The V₂CT_x MXene electrode pre-intercalated with Fe species at 400 °C has the highest capacity, which is 71.65% higher than the original V₂CT_x MXene electrode, showing very excellent electrochemical performance. However, annealing temperature up to 800 °C can destroy the MXene layered structure, which has a negative impact on the electrochemical performance.