Synthesis and Electrochemical Performance of Ni-Doped VO₂(B) as a Cathode Material for Lithium Ion Batteries

Abstract

Ni-doped VO₂(B) samples (Ni_xVO₂(B)) were fabricated by a facile one-step hydrothermal method. When evaluated as a cathode material for lithium ion batteries (LIBs), these Ni-doped VO₂(B) exhibited improved lithium storage performance as compared to the pure VO₂(B). In particular, when the doping amount is 3%, Ni_xVO₂(B) showed the highest lithium storage capacity, best cycling stability, smallest electrochemical reaction resistance, and largest lithium diffusion coefficient. For example, after 100 cycles at a current density of 32.4 mA/g, Ni_xVO₂(B) delivered a high specific discharge capacity of 163.0 mAh/g, much higher than that of the pure VO₂(B) sample (95.5 mAh/g). Therefore, Ni doping is an effective strategy for enhancing the lithium storage performance of VO₂(B).

1. Introduction

Rechargeable lithium-ion batteries (LIBs) have been widely used as power sources for portable electronics, electric vehicles (EV), and hybrid vehicles (HEV) [1,2,3]. To meet the ever-increasing demand of high energy density LIBs, it is very crucial to develop high-capacity electrode active materials [4,5]. Among the various candidate cathode materials, vanadium oxides have attracted great attentions due to their high energy density, low cost, and abundant sources [6,7]. In particular, metastable mono-clinic VO₂(B) stands out from these vanadium oxides because of its unique VO₂(B) bilayer structure, large lattice spacing of edge-sharing VO₆ octahedron, high theoretical capacity, and fast ion-transfer rate [8,9]. Unfortunately, the practical capacity of VO₂(B) is significantly lower than its theoretical capacity (324 mAh·g⁻¹) and the cycling stability is poor. To solve these issues, many strategies, such as nanocrystallization [9,10,11] and carbonization [12,13], were adopted and these materials indeed exhibit improved electrochemical performance. However, the cycling performance is still not satisfied enough for practical applications, due to collapse of the active material and the increased charge transfer resistance upon repeated cycling [10,11,12,13].

It has been demonstrated doping alien metal cations could enhance the structural stability and improve the electrical conductivity of electrode materials, which are crucial parameters for determining the cyclability of electrodes [14,15]. Hou et al. synthesized Al³⁺ doped VO₂(B), which delivered a high initial discharge capacity (282 mAh/g at 32.4 mA/g) and high capacity retention rate (71.6% after 50 cycles) [16]. Han et al. reported Ag⁺ doped VO₂(B); this Ag⁺ doped VO₂(B) delivered higher initial discharge capacity (340.5 mAh/g at 32.4 mA/g), which was 151.9 mAh/g higher than the undoped one; but the capacity retention rate was only 27.1% after 100 cycles, significantly lower than the undoped one (41.6%) [17]. Since the ion radius of Ni²⁺ (0.055 nm) is very close to that of V⁴⁺ (0.058 nm), Ni²⁺ can easily enter the lattice of VO₂(B) and substitute V⁴⁺, resulting in oxygen vacancies and improved rate performance. Therefore, Ni²⁺ was doped in LiFeBO₃/C, Na₃V₂(PO4)₃/C, LiMn₂O₄, MnO₂/CNT and other cathode materials to improve their electrochemical performance [18,19,20]. It should be noted that there are few reports on Ni-doped VO₂(B).

Herein, Ni²⁺ doped VO₂(B) was successfully synthesized from V₂O₅, nickel nitrate, and maltose by a facile hydrothermal method. The effect of Ni²⁺ doping on the microstructure and electrochemical properties of VO₂(B) were investigated by XRD, SEM, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge/discharge tests.

2. Experimental

2.1. Synthesis of Ni-Doped VO₂(B)

Metastable VO₂(B) was synthesized by a hydrothermal method. Vanadium oxide (V₂O₅) powder was used as the V source and maltose (C₁₂H₂₂O₁₁) as the deoxidizer. All the chemical reagents were analytical grade

In a typical process, 0.5 g V₂O₅ powders, 0.25 gC₁₂H₂₂O₁₁, and a proper amount of nickel nitrate (Ni(NO₃)₂) were dissolved in 50 mL deionized water. The Ni/V molar ratio was controlled to be 0 at.%, 1 at.%, 2 at.%, 3 at.% and 4 at.%, respectively. After magnetic stirring at room temperature for 0.5 h, the mixed solution was transferred to an autoclave and heated at 180 °C for 24 h in an oven. After hydrothermal reaction, the resulting precipitate was collected by filtration, freeze-dried, and sintered at 350 °C for 1 h in nitrogen atmosphere to yield Ni⁻doped VO₂(B) samples. The samples with Ni/V molar ratio of 0 at.%, 1 at.%, 2 at.%, 3 at.% and 4 at.% were labeled as Ni0, Ni1, Ni2, Ni3, and Ni4, respectively.

2.2. Characterization

The phase structure of the samples was determined by X-ray powder diffraction (XRD, Rigaku RINT2400 with Cu Kα radiation), at a scan rate of 20°/min. The surface morphology of the samples was observed by a field emission scanning electron microscope (S-4800, Japan Hi-Tech Corporation, Tokyo, Japan). The composition of the samples was analyzed by an energy diffusion spectrometer (EDS, Oxford, UK, INCA IE 350).

2.3. Electrochemical Tests

The electrochemical performance of the samples were evaluated in coin-type cells (CR2025), which were assembled in a Mikrouna glove box filled with high purity argon. The working electrodes were fabricated by coating the slurry of Ni-doped VO₂(B), acetylene black, and polyvinylidene fluoride (PVDF) binder with a weight ratio of 7:2:1 dispersed in N-Methyl-2-pyrrolidone solvent on an aluminum foil. The obtained electrodes were then dried in a vacuum of 90 °C for 12 h and punched to the disks with the diameter of 16 mm. Lithium foil was used as the anode electrode and single layer polypropylene (PP) as the separator. 1 mol/L LiPF6 in a mixture solution of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC) (v/v/v = 1:1:1) was used as electrolyte.

Galvanostatic discharge/charge measurement was performed in a voltage range of 1.5–4.0 V (vs. Li/Li⁺). Both the electrochemical impedance spectroscopy (EIS) and the cyclic voltammetry (CV) were carried out using a CHI760D electrochemical workstation.

3. Results and Discussion

Figure 1 presents the XRD patterns of the series of Ni-doped VO₂(B) samples. It can be seen that the diffraction peaks of all the samples match well with the standard card of VO₂(B) (JCPDS No. 81-2392). No peaks of other phases or impurities were observed, implying that Ni²⁺ doping did not change the phase structure of VO₂(B). By a careful observation (shown in Figure 1b), one can see that the (110) diffraction peak shifts toward high angle with the increasing Ni²⁺ doping content. This change is likely due to the difference of ionic radius of Ni²⁺ (0.055 nm) and V⁴⁺ (0.058 nm). After doping, Ni²⁺ cations entered the crystal lattice and substitutes V⁴⁺, which changed the interplanar spacing and therefore leading to the shift of diffraction peaks. This verified that Ni²⁺ cations have been successfully incorporated into the VO₂(B) lattice.

In order to study the actual element composition of the series of Ni-diped VO₂(B), we performed EDS analysis on the samples and the results are listed in

Table 1. The theoretical atomic percentage and actual atomic percentage of the samples.

Sample Designation	Ni:V Atomic Percentage (Starting Composition)	Ni:V Atomic Percentage (Actual Composition)
Ni0	0	0
Ni1	1%	0.45%
Ni2	2%	0.66%
Ni3	3%	1.50%
Ni4	4%	1.80%

. It can be seen that the Ni content in the samples increased with the increasing amount of Ni(NO₃)₂ during hydrothermal process. This also verified that Ni²⁺ cations were successfully doped into the VO₂(B) lattice. However, the actual Ni/V molar ratio in the samples was smaller the initial Ni/V molar ratio in the reaction solutions.

Figure 2 gives the SEM images of the Ni₀, Ni₁, Ni₂, Ni₃, and Ni₄ samples. All the samples had a rod-like morphology. After Ni-doping, the diameter of the VO₂(B) rod slightly decreased compared to the undoped sample, suggesting that the growth of the VO₂(B) crystal was suppressed. In particular, the size of the Ni₃ sample (with length in the range of 1–1.5 μm) was nearly half the length of the Ni₀ sample (in the range of 1–3 μm); in addition, the particle size of Ni₃ sample was more uniform than that of the Ni₀ sample. The uniform and smaller size of Ni₀ could provide a larger specific surface area [21], facilitate the transport of Li⁺ [22], and improve the electrochemical performance of the material.

Figure 3a shows the cycling performances of the series of samples at a current density of 32.4 mA/g. The first specific discharge capacities of Ni₀, Ni₁, Ni₂, Ni₃, Ni₄ were 216, 227, 228, 251, and 246 mAh/g, respectively. After 100 cycles, the corresponding specific capacities decreased to 95.5, 137.0, 163.0, 134.8, and 110.5 mAh/g, respectively; the capacity retention rates were 44.2%, 48.7%, 59.1%, 64.9%, and 55.7% for of Ni₀, Ni₁, Ni₂, Ni₃, and Ni₄, respectively. Generally, this capacity decay is mainly caused by the collapse of the crystal structure of VO₂(B), which hinders the intercalation/de-intercalation of Li⁺ during discharge/charge cycles. Obviously, the Ni doping increased first discharge capacity and improved the capacity retention rate of VO₂(B). That is, Ni²⁺ doping into can increase the structural stability of the V–O bond, thus stabilizing the crystal structure of VO₂(B) and increasing the capacity retention rate. At the same time, the substitution of Ni²⁺ for V⁴⁺ in VO₂(B) can generate additional oxygen vacancies, which can broaden the channels for the diffusion of lithium ions. Among the five samples, Ni₃ had the best lithium storage performance. With the increase of Ni doping amount, the first specific discharge capacity and capacity retention rate first increased and then decreased, indicating that an appropriate amount of Ni²⁺ doping can improve the electrochemical performance of VO₂(B). For the Ni₄ sample, the excessive substitution of Ni²⁺ for V⁴⁺ made the crystal lattice of VO₂(B) excessively deformed and unstable, and therefore hindered the diffusion of Li⁺. Figure 3b–f are the typical charge/discharge curves of Ni₀, Ni₁, Ni₂, Ni₃, and Ni₄ samples in the 1st, 50th, and 100th cycle. All charge/discharge curves showed very similar features, having two distinct plateaus (2.5 V and 1.7 V), corresponding to lithium de-intercalation and intercalation reactions. Among them, the discharge plateau at 2.5 V corresponded to the reduction of V⁵⁺ to V⁴⁺ [23], and the discharge plateau at 2.2 V corresponded to the reduction of V⁴⁺ to V³⁺ [24]. The separation between the charge and discharge plateaus decreased significantly after Ni doping, demonstrating that the electrochemical polarization of the electrode was reduced and the capacity was increased.

Figure 4a shows the CV curve of the series of samples measured at a scan rate of 0.1 mV/s in the voltage of 1.5–4.0 V. All the samples clearly presented a couple of redox peaks, representing the process of the intercalation/de-intercalation of Li⁺ [15]. With the increase of Ni doping amount, the oxidation peak reduction peak shifted to the lower potential side and higher potential side, respectively. This can be attributed to the structural change and depolarization after Ni doping [12]. The potential difference between oxidation peak and reduction peak (Δφ) of Ni₀, Ni₁, Ni₂, Ni₃, and Ni₄ were 0.99 V, 0.92 V, 0.54 V, 0.58 V, and 0.88 V, respectively. It is obvious that the potential difference decreased after Ni doping, which is indicative of decreased electrochemical polarization and better electrochemical reaction reversibility of the electrodes [25].

To investigate the electrochemical reaction kinetics of the electrode, EIS plots were tested after the electrodes were activated for three cycles, and the results are shown in Figure 4b. The charge transfer resistances of Ni₀, Ni₁, Ni₂, Ni₃, and Ni₄ were 860.0 Ω, 718.2 Ω, 615.0 Ω, 542.9 Ω, and 636.0 Ω, respectively. Obviously, the charge transfer resistance of VO₂(B) was decreased after Ni doping. That is, the substitution of Ni²⁺ for V⁴⁺ in VO₂(B) can effectively enhance the electrochemical reaction in VO₂(B) [22]. As shown in Figure 2, the Ni doped samples had a larger specific surface area, which can increase the electrochemical active sites and benefit the fast intercalation and deintercalation of Li⁺ [26]. To further understand the Li⁺ diffusion performance of the samples, Figure 4c gives the relationship between Z′ and ω^−1/2 according to the EIS plots in Warburg region. The lithium ion diffusion coefficient of the samples can be evaluated according to the following formula [27,28]:

$${\mathrm{D}}_{\mathrm{Li}}=\frac{{\left(\mathrm{RT}\right)}^2}{2{\left({\mathrm{An}}^2{\mathrm{F}}^2{\mathrm{C}}_{\mathrm{Li}}\mathsf{\sigma }\right)}^2}$$

(1)

where A is the surface area of the electrode and n is the number of electrons per mole in the oxidation reaction; F is the Faraday constant; C_Li is the concentration of Li⁺; σ is the Warburg coefficient, which can be obtained from the slope of Z′ vs. ω^−1/2 in Warburg region (Figure 4c); R is the gas constant; and T is kelvin degree (298 K).

The calculated lithium ion diffusion coefficients of Ni₀, Ni₁, Ni₂, Ni₃, Ni₄ were 2.82 × 10⁻¹⁵, 4.426 × 10⁻¹⁵, 5.24 × 10⁻¹⁵, 6.815 × 10⁻¹⁵, 3.13 × 10⁻¹⁵ cm²/s, respectively (

Table 2. σ and D_Li values of the samples.

Sample Designation	σ	DLi
Ni0	58.2773	2.82 × 10−15
Ni1	46.5196	4.426 × 10−15
Ni2	42.7479	5.24 × 10−15
Ni3	37.4864	6.815 × 10−15
Ni4	55.2987	3.13 × 10−15

). Apparently, the lithium ion diffusion coefficient increased after Ni doping, and Ni₃ had the largest lithium ion diffusion coefficient among the five samples.

4. Conclusions

In this work, Ni-doped VO₂(B) samples were synthesized by a hydrothermal method. The effect of Ni doping amount on the microstructure and lithium storage performance was investigated in detail. It was found that Ni doping facilitates the formation of uniform and smaller rod-like VO₂(B). Electrochemical performance tests demonstrated that Ni doping can increase the lithium ion diffusion coefficient, reduce the electrochemical reaction resistance and polarization of VO₂(B) electrode during charge/discharge process. In particular, when the doping amount was 3%, the sample exhibited the best lithium storage performance. After 100 cycles at a current density of 32.4 mA/g, this sample delivered a specific discharge capacity of 163.0 mAh/g, much higher than that of the pure VO₂(B) sample (95.5 mAh/g). The results reported in this work could provide clues for the rational structural design and performance optimization of vanadium oxides cathode materials.