Facile Synthesis of Sea-Urchin-like VN as High-Performance Anode for Lithium-Ion Batteries

Abstract

Lithium-ion batteries are still the main theme of the contemporary market. Commercial graphite has struggled to meet the demand of high energy density for various electronic products due to its low theoretical capacity. Therefore, exploring for a new anode with high capacity is important. Vanadium nitride has attracted widespread attention due to its high theoretical specific capacity and good chemical/thermal stability. However, vanadium nitride is accompanied by huge volume expansion and nanoparticle agglomeration during the electrochemical reaction, which limits its application. Herein, sea-urchin-like vanadium nitride (SUK-VN) was successfully prepared with a simple hydrothermal method combined with an annealing strategy to boost the actual capacity of the vanadium nitride. The special sea-urchin-like morphology effectively suppresses the agglomeration of vanadium nitride nanoparticles and exposes more reactive sites, which facilitates the electrochemical performance of electrode materials. In the half-cells, sea-urchin-like vanadium nitride exhibits a specific capacity of 361.5 mAh g⁻¹ at 0.1 A g⁻¹ after 60 cycles, and even still achieves a specific capacity of 164.5 with a Coulomb efficiency of approximately 99.9% at 1 A g⁻¹ after 500 cycles. Such a strategy provides the potential to enhance the electrochemical properties of vanadium nitride anodes in terms of solving the nanoparticle agglomeration.

1. Introduction

With the growing demand for consumer electronics, the steady development of electric vehicles, and the increasing demand for safe and environmentally friendly energy storage devices, lithium-ion batteries (LIBs) are still the main theme of the contemporary market [1,2]. Commercial graphite is the most commonly used anode material in LIBs. However, due to its low theoretical capacity (372 mAh·g⁻¹) and poor rate capability, it struggles to meet the demand for high-energy-density batteries for various electronic products and electric vehicles. Therefore, exploring new anode electrodes with high actual capacity and excellent cycling stability is extremely important [3].

Among various anode candidates, transition metal nitrides (TMNs) have attracted a great deal of attention due to their high melting point, low density, excellent thermal conductivity, good corrosion resistance, and remarkable chemical stability. TMNs show great promise for application as LIB anodes. In addition, the introduction of nitrogen into the transition metal lattice results in an electron density increase, so TMNs have an electronic structure similar to that of noble metals, which makes them a promising candidate for electrode materials. Therein, vanadium nitride (VN) with high theoretical capacity (~1200 mAh g⁻¹) has attracted much focus owing to its low molecular weight and high valence of V (+3) that can transfer three electrons per formula unit [4,5,6]. VN is often accompanied by a huge volume expansion (~240%) in the electrochemical reaction process, which makes the structural framework of material extremely unstable and leads to a rapid capacity decay [7]. For Li-ion batteries, it is known that electrode materials with smaller particle size distribution have a larger specific surface area, which facilitates the transfer of electrons and lithium ions. Additionally, the reduction in particle size to the nano scale can reduce the mechanical stress during the electrochemical reaction. Hence, an effective way to alleviate the volume expansion of VN is to treat it at the nano scale [8]. However, nanoscale VN tends to attract adjacent nanoscale VN to form agglomerates due to the surface of nanoscale VN having large surface energy via its treatment at the nano scale, which decreases the active sites, reduces the effective utilization of VN, and results in lower actual discharge capacity. So far, VN with different morphologies and microstructures has been fabricated with different techniques and has shown unique electrochemical properties as anodes for LIBs. Although the preparation of VN has received attention, its synthesis technology is complicated, costly or environmentally polluting, and not suitable for large-scale production. Therefore, it is imperative to develop a novel and simple synthesis strategy to prepare VN nanoparticles at a lower cost. Fortunately, a special morphology synthesized using the hydrothermal method can suppress the agglomeration of VN nanoparticles and increase the active sites of lithium-ion storage to enhance the electrochemical performance of electrode materials [9,10].

Herein, a sea-urchin-like VN (SUK-VN) composite was prepared easily using the hydrothermal method combined with the annealing treatment strategy. The SUK-VN composite with a sea-urchin-like special morphology suppressed the agglomeration of VN nanoparticles and increased the specific surface of the material, which increased the direct contact area between the electrode material and electrolyte, reduced the transmission distance of the Li-ions/electrons, and improved the kinetic reactions in the electrochemical process [11,12]. In addition, more active sites were exposed, enhancing the actual capacity of the material [13]. Correspondingly, the SUK-VN composite displayed a specific capacity of 361.5 mAh g⁻¹ at a current density of 0.1 A g⁻¹ after 60 cycles, and even still, obtained a specific capacity of 164.5 with a Coulomb efficiency (CE) of approximately 99.9% at a current density of 1 A g⁻¹ after 500 cycles. The SUK-VN composite with its synergistic structure and superior electrochemical performance will demonstrate a new strategy for the optimized design of transition metal nitrides in energy application fields.

2. Materials and Methods

2.1. Synthesis of SUK-VN Composite

0.4 g of thioacetamide (C₂H₅NS, AR) was added to a reactor containing 70 mL of anhydrous ethanol and stirred for 1 h until the solute was dissolved. Subsequently, 0.2 g of ammonium metavanadate (NH₄VO₃, AR) was added to the above solution and stirred for 1 h. Then, 2 mL of hydrochloric acid (HCl, AR) was added to the mixture and stirred for 0.5 h. After that, the reactor was hydrothermally treated in a drying oven at 180 °C for 30 h. The as-prepared sample was obtained by washing with deionized water and alcohol, centrifuging, and drying. Finally, the obtained product was heat-treated at 700 °C for 2 h in a NH₃ atmosphere (heating rate, 3 °C min⁻¹) and naturally cooled to room temperature to obtain the SUK-VN composite. For comparison with the SUK-VN composite, NH₄VO₃ was heat-treated at 700 °C for 2 h in a NH₃ atmosphere (heating rate, 3 °C min⁻¹) and naturally cooled to room temperature to obtain pure VN.

2.2. Characterization

The morphology was observed and characterized using a scanning electron microscope (SEM) from Hitachi SU80100 and transmission electron microscope (TEM) from FEI Talos 200S. X-ray diffraction (XRD) was employed using a Rigaku MiniFlex600 apparatus. Raman spectra were explored using a 532 nm laser source of an inVia Reflex Raman spectroscope. The contents of VN were investigated via thermogravimetric analysis (TGA) from NECTICH TG209F3. X-ray photoelectron spectroscopy (XPS) was employed using a Thermo Fisher Scientific ESLAB 250Xi apparatus to measure the element valence of the samples. The surface area and pore volume were analyzed using a Beishide device (3H-2000PS1).

2.3. Electrochemical Measurement

The active materials (SUK-VN composite or VN) and carbon nanotubes were mixed in DMF at a ratio of 7:3 and sonicated for 3 h to form a homogeneous solution. After the mixture was sonicated for 3 h, the solution was filtered and dried at 60 °C for 12 h under a vacuum environment to obtain the SUK-VN anode and pure VN anode. The loading amount of the active substance about the SUK-VN anode and pure VN anode was about 1.5 mg cm⁻². Li metal was designed as the counter electrode. Lithium hexafluorophosphate (LiPF₆) in ethylene carbonate/diethyl carbonate (EC/DEC, v/v, 1:1) at a concentration of 1.0 M was selected as the electrolyte. Celgard 2400 was used as the separator. Electrochemical tests of the SUK-VN anode and pure VN anode were performed using the LAND CT2001A test system (Wuhan, China). The cyclic voltammetry (CV) was obtained using the CHI760E electrochemical workstation (Chenhua, Shanghai, China).

3. Results and Discussion

The SUK-VN composite was synthesized using a hydrothermal method combined with an annealing treatment strategy, as shown in Figure 1a. NH₄VO₃, C₂H₅NS, and HCl were mixed into a homogeneous solution in anhydrous ethanol undergoing a hydrothermal reaction at 180 °C for 30 h to prepare vanadium oxide precursors of sea-urchin-like VN. After that, the as-prepared precursors were heat-treated at 700 °C for 2 h (heating rate, 3 °C min⁻¹) in a NH₃ atmosphere to synthesize the SUK-VN composite. Compared with the SUK-VN composite, pure VN was synthesized via the direct nitration of NH₄VO₃ at 700 °C for 2 h (heating rate, 3 °C min⁻¹) in a NH₃ atmosphere. Figure S1 shows an X-ray diffraction (XRD) pattern of pure VN. The diffraction peaks are fully consistent with the standard card of VN (JCPDS No. 73-0528) and no other impurities were observed, indicating the successful preparation of VN. In order to determine the morphological characteristics of vanadium–oxygen precursors, the SUK-VN composite, and the pure VN, the morphology was observed using scanning electron microscopy (SEM). Due to their large specific surface energy, the agglomerated bulk of pure VN nanoparticles was observed (Figure S2), which is not conducive to the implementation of high capacity and reduces the effective utilization of VN. Thankfully, this problem can be effectively solved by synthesizing special morphologies through hydrothermal methods combined with an annealing treatment strategy. Vanadium oxide precursors with a special sea-urchin-like morphology are exhibited in Figure S3a,b. The sea-urchin-like vanadium–oxygen precursors exhibited a uniform spherical shape with a diameter of about 20 μm and a spike length of about 10 μm. Moreover, the spikes in the sea-urchin-like vanadium–oxygen precursors were uniformly distributed, which facilitated electrolyte infiltration and shortened the ion–electron transport distance, improving the electrochemical performance of the electrode. According to the analysis of the XRD results in Figure S3c, the composition of the vanadium–oxygen precursor was determined to be V_8.18O₁₆·1.46H₂O because of the characteristic peaks produced by the vanadium oxide precursors coinciding with the standard card of V_8.18O₁₆·1.46H₂O (JCPDS No. 50-1797). As expected, the sea-urchin-like special shape from V_8.18O₁₆·1.46H₂O was well inherited in the SUK-VN composite after the annealing treatment under an ammonia atmosphere in Figure 1b, indicating that the SUK-VN composite had good structural stability. Figure 1c displays the surface of the SUK-VN composite which appears to have a porous structure, which is attributed to the decomposition of C₂H₅NS. The porous structure of SUK-VN composite exposes more active sites, which helps to improve the lithium storage performance of the material. The sea-urchin-like structure of the SUK-VN composite was further revealed via transmission electron microscopy (TEM). The porous structure of the SUK-VN composite is proven once again in Figure 1d, which facilitates the increase in specific surface area and exposes more active sites for lithium ions to participate in the reaction [14]. More active sites for enhanced Li-ion storage performance can increase the actual capacity of the electrode material. The high-resolution TEM (HRTEM) images of the SUK-VN composite are displayed in Figure 1e. A large number of lattice strips are clearly observable. Furthermore, the size of the lattice spacing was analyzed to be 0.206 nm, corresponding to the (200) crystal plane of VN (JCPDS No. 73-0528) (Figure 1f,g), which demonstrates the successful synthesis of SUK-VN [15,16]. The uniform distribution of elements of C, N, and V on the SUK-VN composite structure was confirmed by the mapping in Figure 1h. Moreover, the contents of C, N, and V elements were measured as 3.4%, 48.29%, and 48.31% (Table S1). The ratio of element V to element N was approximately 1:1, indicating the successful synthesis of VN. Among them, the reason for the slightly higher content of the N element than V element can be attributed to the following: a small amount of carbon derived from the pyrolysis of C₂H₅NS was present in SUK-VN composite. During heat treatment in an ammonia atmosphere, the derived carbon was nitrogen-doped, resulting in an increase in the elemental N content. Additionally, partial oxidation occurred on the surface of the SUK-VN composite when it was exposed to air, which resulted in the appearance of V-N-O and V-O on the SUK-VN composite, leading to a decrease in the content of N element. Therefore, the reason for the slightly higher N content than V content was the result of the combined effect of the above two phenomena.

Figure 2a displays an X-ray diffraction (XRD) pattern of the SUK-VN composite to investigate the composition of the composite. The diffraction peaks were unambiguously assigned to the standard card of VN (JCPDS No. 73-0528), and no other stray peaks were found, implying that VN of the pure phase was synthesized [17,18]. The diffraction peaks of carbon were not identified, indicating that the carbon content was in a small amount and uniformly dispersed. In order to explore the composition and surface electronic state of the SUK-VN composite, X-ray photoelectron spectroscopy (XPS) spectra were analyzed. Figure 2b shows the XPS spectrum of the SUK-VN composite containing V, N, O, and C elements. Among them, element C was derived from C₂H₅NS, while element V was derived from NH₄VO₃. The high-resolution C 1s peak is divided into three peaks located at 284.8, 285.7, and 288.8 eV, corresponding to C-C/C=C, C-N, and C=O bonds, respectively (Figure 2c) [19,20]. Combined with the results of mapping and XRD (Figure 1h and Figure 2a), the analysis concludes that the SUK-VN composite contains a small amount of carbon. Among them, the oxygen-containing functional groups are beneficial to improving the wettability of the carbon surface, allowing the electrolyte to penetrate the whole electrode quickly, achieving the rapid diffusion of ions, accelerating the reaction kinetics, and making full use of the active material. The large number of C=C bonds indicate the presence of carbonaceous structures in the nanostructures. C-N/C-C bonds display successful nitrogen doping on a carbon substrate [21]. In addition, the electronegativity of a nitrogen atom is higher than that of a carbon atom, which improves the overall electrical conductivity. The high-resolution V 2p_3/2 showed three peaks at 514.2, 515.6, and 517.4 eV, which were attributed to V-N, V-N-O, and V-O bonds, respectively (Figure 2d) [22,23]. The presence of V-N bonds proves the successful synthesis of VN. The presence of V-N-O and V-O bonds was due to the partial oxidation of the surface of VN when the materials were exposed to air, which is normal for metal nitrides. Figure 2e shows the high-resolution N 1s spectra with three peaks at 397.6, 399.3, and 401.2 eV corresponding to the V-N, V-N-O, and C-N bonds [24,25], respectively, which are consistent with the above analysis. In order to investigate the specific surface area and pore size distribution of the SUK-VN composite and pure VN, Brunauer–Emmett–Teller (BET) curves were analyzed. As expected, the SUK-VN composite exhibited a specific surface area of 46.8 m² g⁻¹ higher than the pure VN, which is shown in Figure 2f (15.0 m² g⁻¹), which is thanks to the sea-urchin-like morphology of the SUK-VN composite. Obviously, the pore sizes of the SUK-VN composite and pure VN were concentrated in a mesoporous structure of 2–50 nm. The developed mesoporous structure of the SUK-VN composite effectively shortens the diffusion distance of Li-ions in the electrolyte and displays better transmission kinetics [26,27].

In order to investigate the mechanism of the electrochemical reaction during the charging/discharging of the SUK-VN composite, a CV test was performed at a sweep rate of 0.1 mV s⁻¹ in the potential range of 0.01–3.0 V (V vs. Li⁺/Li). Figure 3a shows that a reduction peak (approximately 0.7 V discharge potential) is observable during the electrochemical reaction of the first cycle and is not present on the CV curves afterward, which is attributed to the side reaction and the formation of a solid electrolyte (SEI) film [28]. In addition, the CV curves largely overlap in the subsequent turns, reflecting the excellent structural stability of the SUK-VN composite. A pair of reduction peak/oxidation peak appears at about 0.01/0.17 V, which is attributed to the reaction of the lithium-ion with the carbon material [29]. A couple of reduction/oxidation peaks on the reaction of Li-ions with VN appear at about 0.95/1.2 V (VN + xLi + xe⁻→ Li_xVN) [30,31,32,33]. Figure 3b displays the galvanostatic charge–discharge (GCD) curves of the SUK-VN composite for the first five turns at a current density of 0.1 A g⁻¹. It can be clearly seen that there is a large difference between the discharge specific capacity and the charge specific capacity in the first lap, which is due to the additional discharge capacity provided by the by-product decomposition reaction and the formation of SEI film during the first lap. In the subsequent laps, the GCD curves basically keep overlapping, indicating that the SUK-VN composite has good electrochemical cycling stability. As expected, all voltage platforms correspond to the peaks appearing in Figure 3b. To illustrate the structural advantages of the sea-urchin-like morphology of the SUK-VN composite synthesized using a hydrothermal method combined with an annealing treatment strategy, rate performances were performed for the SUK-VN composite and pure VN at different current densities (Figure 3c), respectively. The specific capacities of the SUK-VN composite at the current densities of 0.1, 0.2, 0.3, 0.5, 0.7, 1, 2, 3, and 5 A g⁻¹ reached 299, 218.7, 183.0, 158.3, 141.3, 127.0, 94.9, 77.5, and 58.5 mAh g⁻¹, respectively. Moreover, the specific capacity of SUK-VN composite was again obtained at 361.5 mAh g⁻¹ when the cycle was performed again at a current density of 0.1 A g⁻¹, while the specific capacities of the pure VN composite at 0.1, 0.2, 0.3, 0.5, 0.7, 1, 2, 3, and 5 A g⁻¹ reached 85.5, 74.6, 67.2, 57.4, 50.4, 44.5, 34.9, 29.0, and 21.7 mAh g⁻¹, respectively. When the current density returned to 0.1 A g⁻¹ again, the pure VN exhibited a specific capacity of 100.3 mAh g⁻¹. Compared with the pure VN, the SUK-VN composite exhibited better rate performance, which was attributed to the fact that the sea-urchin-like morphology provided a greater number of active sites and a larger specific surface area to enhance the actual specific capacity and reaction kinetics. An interesting phenomenon emerges in Figure 3c. It can be seen that the capacity of the SUK-VN composite decreased gradually at a current density of 0.1 A g⁻¹ due to the SEI film being formed gradually. Interestingly, the specific capacity of the SUK-VN composite demonstrated a gradual increase in subsequent cycles, which was owing to the electrochemical grinding and gradual activation of SUK-VN during the charging/discharging process. To compare the long cycle performance of the SUK-VN composite and pure VN, constant current charge/discharge tests were performed at a current density of 1 A g⁻¹. Figure 3d shows the specific capacity of 164.5 mAh g⁻¹, with approximately 99.9% Coulomb efficiency (CE) for the SUK-VN composite able to be achieved after 500 cycles at a current density of 1 A g⁻¹, while the pure VN only displayed a specific capacity of 64.2 mAh g⁻¹ at a current density of 1 A g⁻¹ after 500 cycles with approximately 99.9% CE. Compared with the rate performance and long cycle performance of pure VN, it can be seen that the electrochemical performance of the SUK-VN composite synthesized using the hydrothermal method combined with annealing treatment strategy is more advantageous thanks to the sea-urchin-like morphology of the SUB-VN composite, which can suppress the agglomeration of VN nanoparticles and relieve the volume expansion of VN to a certain extent, improve the effective area in contact with the electrolyte, increase the active sites of the reaction, and enhance the actual capacity of the electrode material. Moreover, the uniform distribution of the derived carbon in SUK-VN mitigates the volume expansion of VN and maintains the stability of the material structure.

To understand the reaction kinetic properties of the SUK-VN composite in the electrochemical process more clearly, CV tests at sweep speeds of 0.2–3 mV s⁻¹ were conducted. The characteristic peaks in the CV curves maintain a good shape as the sweep continues to become larger, and the potentials of the characteristic peaks just have a small shift, indicating that the SUK-VN composite has excellent cycling stability and satisfactory polarization, which facilitates the enhanced storage capacity of the SUK-VN composite for Li-ions (Figure 4a). Generally, the relationship between the peak electrochemical response current (i) and the scan rate (v) can be calculated by the following equation [34,35].

$$i=a{v}^b$$

(1)

$$i\left(V\right)=k_{1}v+k_2\cdot v^{1/2}$$

(2)

$$i\left(V\right)∕v^{1∕2}=k_1\cdot v^{1∕2}+k_2$$

(3)

where $i$, $k_{1}v$, $k_2\cdot v^{1/2}$, and $v$ denote the current density, pseudocapacitive control, diffusion control, and scan rate, respectively. $a$ and $b$ are the variable parameters. The value of b is related to the diffusion process or the capacitive behavior. The b-value of 0.5 indicates the reaction process is dominated by a diffusion-controlled process, while the b-value of 1 is regarded as a pseudocapacitive process. The b-value of the O₁ for SUK-VN is 0.92, and the b-value of the R₁ is 0.94 (Figure 4b), demonstrating that the reaction is jointly controlled by the pseudocapacitive process and diffusion-controlled process. Furthermore, the pseudocapacitance process plays a leading role. Figure 4c displays that the pseudocapacitor process provides 74.3% of the total capacity at a sweep of 3 mV s⁻¹. In addition, the percentages of the pseudocapacitor contribution at different sweeps from 0.3 to 2 mV s⁻¹ were calculated. The percentages of capacity provided by the pseudocapacitor process increases from 54.6% to 71.5% at sweep speeds of 0.3 to 2 mV s⁻¹. The agglomeration of VN nanoparticles is alleviated by the special sea-urchin-shaped morphology of the SUK-VN composite, increasing the specific surface area of the material, generating more pseudocapacitive reactions on the surface, and accelerating the reaction kinetics.

4. Conclusions

In summary, we report a hydrothermal method combined with an annealing treatment strategy to successfully synthesize a SUK-VN composite. The SUK-VN composite shows excellent lithium storage performance, summarizing the advantages as follows: the special sea-urchin-like morphology of the SUK-VN composite suppresses the degree of agglomeration of VN nanoparticles and increases the specific surface area, which facilitates the faster penetration of the electrolyte and shorter ion/electron transport distance. More active sites are exposed, offering the possibility to achieve higher electrochemical performance; a larger specific surface area allows more pseudocapacitive reactions to occur on the material surface, relieving the volume expansion of VN and accelerating the reaction kinetics. We believe that such a simple synthesis strategy with the unique sea-urchin-like structure and the outstanding electrochemical performance would give some inspiration to the relevant metal nitrides.