The introduction of gallium ions into V₂O₅ interlayers for highly reversible Zn ion batteries

Abstract

It is very important to construct energy storage systems with high safety and excellent electrochemical performance. In particular, aqueous Zn ion batteries (AZIBs) possess the characteristics of low-cost and environmental benignity. However, there are few cathode materials that match well with the zinc anode. Herein, Ga³⁺ pre-intercalation into V₂O₅ layers promotes the insertion/extraction kinetics of zinc ions. The assembled Zn/V₂O₅–0.1Ga battery with 3 M Zn(CF₃SO₃)₂ electrolyte shows a specific capacity of 512.07 mA h g⁻¹ at a current density of 0.1 A g⁻¹. It delivers an energy density of 281.64 W h kg⁻¹ at a power density of 55 W kg⁻¹. It can also provide a reversible capacity of 110 mA h g⁻¹ at 10 A g⁻¹ with a retention rate of 91.43% after 5000 cycles, revealing its potential applications in future energy storage devices.

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1. Introduction

In the past few years, the excessive depletion of fossil fuels has caused a serious energy crisis and environmental pollution. It is necessary to design and develop many sustainable energy sources.^1–4 Among them, Li-ion batteries (LIBs) have been utilized in smartphones, electric vehicles and laptops. Nevertheless, their future development is still restricted considering the high cost and toxic organic electrolyte.^5,6 Thus, it is crucial to seek an alternative to LIBs, such as rechargeable aqueous multivalent ion batteries (Na, K, Ca, Zn, and Mg).^7–10 Aqueous zinc ion batteries (AZIBs) attract wide attention owing to their high theoretical capacity (820 mA h g⁻¹), and the abundant resources and low redox potential of Zn²⁺/Zn (−0.76 V vs. SHE).^11–13 However, it is still an urgent task to explore suitable cathode materials, such as Prussian blue analogues, and Mn-based and vanadium-based materials.^14,15 Prussian blue analogues show low capacity and short cycle life.¹⁶ Manganese-based materials still need to be improved owing to low rate performance and unstable structures.^17,18

Recently, various V₂O₅ structures have been widely investigated as cathode materials for AZIBs,¹⁹ owing to their diverse oxidation states (V²⁺, V³⁺, V⁴⁺, and V⁵⁺) and unique crystal features.^20,21 The crystal structure of V₂O₅ connects the distorted VO₅ square cones through the shared edges and corners to form a 2D layered structure with weak van der Waals interactions between the layers. For instance, Chen et al. prepared V₂O₅ samples supported on carbon cloth by a template route. The fabricated cells delivered a capacity of 370 mA h g⁻¹ at 0.2 A g⁻¹.²² The Zn/V₂O₅@CNT devices presented a capacity of 485.8 at 0.1 A g⁻¹.²³ However, the capacity of the reported V₂O₅ electrode materials is far from their theoretical capacity (589 mA h g⁻¹). The strong electrostatic repulsion between the zinc ions and the host structure results in a slow electrochemical kinetic behavior. Therefore, some strategies have been proposed to modify the cathode material of AZIBs, such as metal ion pre-embedding, defect engineering and coating of other materials.

Metal ion pre-embedding is thought to be an efficient strategy to enhance the electrochemical performance of AZIBs. Many metal cations have been introduced into target materials, such as Zn, K, and Ca. Zhang et al. introduced La³⁺ into a layered V₂O₅ structure. The assembled device shows a capacity of 405 mA h g⁻¹ at 0.1 A g⁻¹ and energy density of 227.5 W h kg⁻¹ at 55 W kg⁻¹.²⁴ The Zn/V₂O₅ cells deliver a discharge capacity of 350 mA h g⁻¹ at 0.1 A g⁻¹ by the intercalation of PANI.²⁵ Liu and coworkers used K⁺ ions as a structure pillar for introduction into V₂O₅ interlayers. The resulting material possessed a capacity of 479.8 mA h g⁻¹ at 0.2 A g⁻¹ and maintained 91.3% of its initial capacity at 10 A g⁻¹ after 3000 cycles.²⁶

In this work, we prepare Ga³⁺-intercalated V₂O₅ nanobelts by a simple hydrothermal route. Gallium possesses natural advantages, such as its strong electronegativity forming stable chemical bonds. It has a small radius compared to other cations, making it easier to approach the target sample. The introduction of metal ions promotes the transfer of Zn ions, and maintains structural stability of the electrode materials. A series of zinc ion batteries are assembled using the obtained V₂O₅–0.1Ga samples as cathodes. The cells deliver a discharge capacity of 512.07 mA h g⁻¹ at 0.1 A g⁻¹. They maintain 91.43% of the original capacity at 10 A g⁻¹ after 5000 cycles. The batteries present an energy density of 281.64 W h kg⁻¹ at a power density of 55 W kg⁻¹. This work provides new ideas in designing cathode materials that match well with Zn anodes.

2. Experimental section

All the purchased chemicals were used without any purification. Typically, 4 mmol V₂O₅ (Alfa Aesar) powder was dissolved into 50 mL of de-ionized water and stirred for 30 min at 50 °C. Then, 1 mL of 30% H₂O₂ was added into the above solution. After that, a certain amount of HCl (Codow) solution is uniformly dispersed into the above mixture to adjust the pH to 1. Subsequently, 0.1 mmol Ga(NO₃)₃·xH₂O (Macklin) was added into the prepared solution with stirring for half an hour. Then, the solution was placed into an 80 mL Teflon-lined stainless-steel autoclave and maintained at 180 °C for 48 h. After cooling to room temperature, the solution was washed with alcohol and deionized water several times and dried under vacuum at 60 °C for 12 h. For comparison, different amounts of Ga(NO₃)₃·xH₂O (0, 0.05, and 0.2 mmol) were also added to similar solutions and the prepared samples were labeled as V₂O₅, V₂O₅–0.05Ga, and V₂O₅–0.2Ga, respectively.

2.1. Structural characterization

The crystallographic information of the as-prepared samples was characterized by powder X-ray diffraction (XRD, 7000, Shimadzu, Cu Kα radiation, λ = 0.1541 nm, 40 kV), scanning electron microscopy (SEM, Gemini 300-71-31), and X-ray photoelectron spectroscopy (XPS, Thermo Kalpha). The specific surface area was studied by the Brunauer–Emmett–Teller (BET, Micromeritics ASAP, JW-TB200) method.

2.2. Electrochemical measurements

The cathode materials consist of the active materials, carbon black (Power Origin Limited) and polyvinylidene fluoride (Power Origin Limited) with a mass ratio of 7 : 2 : 1. Then a certain amount of N-methyl-L-2-pyrrolidone (Damao) was added to form a slurry, which was evenly pressed onto graphite paper. A piece of zinc foil was used as the anode with a thickness of 0.2 mm. The coin cells (CR2032) were assembled by using 3 M Zn(CF₃SO₃)₂ (Bidepharm) as the electrolyte. The average loading mass of the cathode is about 1.5 mg. The galvanostatic intermittent titration technique (GITT), galvanic charge–discharge (GCD) analysis and cycling stability tests were performed at a voltage range of 0.4–1.5 V using a Neware battery tester (CT-4008T-5V6A-164). The CV curves and Nyquist plots of the cells were measured by using an electrochemical workstation (Shanghai Chenhua, CHI660E).

3. Results and discussion

First, the crystal structures of the samples are investigated by XRD, as shown in Fig. 1a. It can be observed that the diffraction peaks match well with the V₂O₅ phase (PDF# 41-1426). The V₂O₅–0.1Ga sample presents sharp and strong diffraction peaks. The main peaks at 15.30, 20.32, 21.68, 26.16, 31.04, and 41.36° belong to the (200), (001), (101), (110), (301) and (002) crystal planes, respectively. By enlarging the XRD pattern, it can be observed that the (101) crystal plane shifts to the right. This demonstrates the successful doping of Ga ions into the V₂O₅ host structure. The lattice parameters a = 11.516 Å, b = 3.566 Å, and c = 4.373 Å. Then XPS is used to investigate the valence states and elemental composition of the electrode materials. Fig. 1b presents the full spectrum of the V₂O₅–0.1Ga sample, revealing the existence of the V, O, Ga and C elements. From Fig. 1c, the binding energies at 516.28/523.08 eV and 517.18/524.78 eV are in accordance with V⁴⁺ and V⁵⁺.²⁷ According to previous reports, V₂O₅ samples with mixed valences (V⁴⁺ and V⁵⁺) possess fast reaction kinetics and low polarization intensity.²⁸ The O 1s spectrum can be fitted into three peaks, as shown in Fig. 1d. The peaks at 529.98, 530.68 and 532.68 eV can be indexed to oxygens in the metal oxide,²⁹ V=O bonds,³⁰ and O–H from surface-absorbed water,³¹ respectively. The Ga 3d spectrum in Fig. 1e shows the signal peak at 20.8 eV, which confirms the presence of Ga³⁺ in the V₂O₅ samples.³²Fig. 1f shows nitrogen adsorption and desorption isotherms of the V₂O₅ and V₂O₅–0.1Ga samples. The specific surface area of V₂O₅–0.1Ga is 27.42 m² g⁻¹, which is larger than that of the V₂O₅ product (24.09 m² g⁻¹). In addition, the total pore volumes of the V₂O₅–0.1Ga and V₂O₅ samples are 0.061 and 0.052 cm³ g⁻¹, respectively. The results demonstrate that the addition of Ga element can increase the specific surface area and the active sites.

Fig. 1 Structural characterizations of the samples. (a) XRD patterns. (b) XPS survey spectrum. (c) V 2p, (d) O 1s, and (e) Ga 3d+O 2s. (f) N₂ adsorption–desorption isotherms; inset represents the pore size distributions of the V₂O₅–0.1Ga products.

SEM is then utilized to observe the morphologies of the samples. As described in Fig. 2a–d, the growth process of the V₂O₅ samples is investigated by controlling the content of Ga³⁺. In Fig. 2b, the V₂O₅–0.1Ga electrode possesses a belt-like shape. In contrast, the other three samples show uneven microcrystals. Using V₂O₅–0.1Ga samples as a cathode, the exposed surface is expected to provide abundant active sites to obtain superior rate performance. As seen in Fig. 2e, the elemental mapping confirms a uniform distribution of three elements of V, O and Ga along the surface of the nanobelts, which demonstrates that Ga³⁺ is doped into the V₂O₅ host structure.

Fig. 2 SEM images of (a) V₂O₅–0.05Ga, (b) V₂O₅–0.1Ga, (c) V₂O₅–0.2Ga and (d) V₂O₅ samples. (e) The corresponding elemental mapping of the V₂O₅–0.1Ga samples.

To study the electrochemical performance of the samples, some CR2032 coin cells are assembled with 3 M Zn(CF₃SO₃)₂ as the electrolyte. Fig. 3a shows the first five cycle CV curves of the V₂O₅–0.1Ga sample at 0.2 mV s⁻¹. The curves maintain the same shapes well, indicating their high reversibility of the redox reactions. Furthermore, there are two pairs of redox peaks located at 1.19/0.96 V and 0.77/0.62 V, which are related to the multi-step intercalation/de-intercalation of Zn²⁺. Fig. 3b presents the cycling stability of the samples. After many cycles of activation, the discharge capacity of the V₂O₅–0.1Ga sample delivers a specific capacity of 526.8 mA h g⁻¹ at 0.2 A g⁻¹ and maintains a retention rate of 95.8% after 130 cycles. The GCD curves of the V₂O₅–0.1Ga products show several charge–discharge platforms (Fig. 3c), which are in accordance with the corresponding CV curves shown in Fig. 3a.

Fig. 3 Electrochemical performance. (a) CV curves in the first 5 cycles at 0.2 mV s⁻¹, (b) cycling performance at 0.2 A g⁻¹, (c) GCD curves of the first five cycles at 0.1 A g⁻¹, (d) long-term cycling at 10 A g⁻¹, (e) rate capability at different current densities and (f) GCD curves at various current densities.

The cycling stability of the cells is also evaluated at 10 A g⁻¹, as depicted in Fig. 3d. The capacity of the V₂O₅–0.1Ga sample reaches 110 mA h g⁻¹ with a retention rate of 91.43% after 5000 cycles. Fig. 3e shows the corresponding rate performance of the batteries. The discharge specific capacities of the V₂O₅–0.1Ga electrodes are 512.07, 514.03, 499.67, 479.18, 437.59, 360.66, 244.64, are 148.40 mA h g⁻¹ at current densities from 0.1 to 10 A g⁻¹. When it returns to 0.1 A g⁻¹, the battery still obtains a capacity of 540.57 mA h g⁻¹. This demonstrates that the moderate amount of Ga³⁺ is conducive to the enhancement of the specific capacity and rate capability. On the contrary, excessive Ga³⁺ incorporation may lead to the disruption of the crystal structure and volume expansion of the V₂O₅ samples, thus producing adverse effects. Furthermore, the GCD curves (Fig. 3f) of the Zn/V₂O₅–0.1Ga batteries show that the capacity decreases with increasing current density. Table 1 lists the zinc-ion storage performance of several electrode materials.^33–41 It demonstrates the excellent electrochemical performance of Zn/V₂O₅–0.1Ga cells.

Table 1 The electrochemical performance of the V₂O₅–0.1Ga electrode materials

Materials	Morphology	Current density (A g^−1)	Discharge capacity (mA h g^−1)	Electrolyte	Ref.
VO2@PPy	Hollow spheres	0.1	440	3 M Zn(CF3SO3)2	33
V2O5	Nanofibers	0.02	319	3 M Zn(CF3SO3)2	34
Mn-doped-VO2	Nanobelts	0.1	209.6	3 M Zn(CF3SO3)2	35
Ni0.25V2O5·nH2O	Nanoribbons	0.2	389	3 M ZnSO4	36
VO2·0.2H2O	Nanocuboids	0.25	423	2 M ZnSO4	37
V2O5x/PANI	Nanosheets	0.1	400	2 M ZnSO4	38
K1.15V5O13·1.3H2O	Nanobelts	0.2	461	3 M Zn(OTF)2	39
Ba1.2V6O16·3H2O	Nanobelts	0.1	321	2 M ZnSO4	40
V2O5	Hollow spheres	0.2	280	3.65 M ZnSO4	41
V2O5–0.1Ga	Nanobelts	0.1	512.07	3 M Zn(CF3SO3)2	This work

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Fig. 4a presents the CV curves of the Zn/V₂O₅–0.1Ga cells at various scan rates (0.2–1.0 mV s⁻¹). With the increasing of the sweep speed, the positive and negative peak currents move to high and low potentials, respectively. The shape of the curves remains almost unchanged, showing their excellent reversibility. For the CV curves, the relationship between peak current (i) and sweep speed (v) can be obtained through the equation as follows:

i = av^b (1)

where a and b are adjustable constants. The b value can be obtained by a linear fitting. The electrochemical reaction is determined by a diffusion-controlled process when the value of b is close to 0.5, while it is capacitance-controlled behavior if b ≥ 1. If the b value is 0.5–1, it indicates a pseudocapacitive contribution- and battery behavior-dominated reaction process. From Fig. 4b, the b values are 0.83, 1.00, 0.87 and 0.84, respectively. This confirms the capacitive contribution.

Fig. 4 Energy storage kinetics of the V₂O₅–0.1Ga electrodes for AZIBs. (a) CV curves, (b) the fitting plots of log(i) and log(v), (c) and (d) the capacitive contribution ratios at various scan rates, (e) GITT curves and (f) Nyquist plots.

In addition, the following equation can be utilized to calculate the pseudocapacitive and diffusion contributions:

i(V) = k₁v + k₂v^1/2 (2)

where k₁ and k₂ are fixed constants. The parameter k₁v is defined as the surface-controlled process, while k₂v^1/2 is the diffusion-controlled one. The capacitive process reaches 87.6% of the total capacity at 1.0 mV s⁻¹ (Fig. 4c). In Fig. 4d, the capacitance contribution ratio increases from 75.4% to 87.6% with the sweep rate increasing. The GITT curves are characterized to further evaluate the diffusion ability of zinc ions. From Fig. 4e, the D_Zn²⁺ values of the V₂O₅–0.1Ga sample are determined to be 10⁻⁶ and 10⁻⁸ cm² s⁻¹ in the discharge state. This demonstrates that the V₂O₅–0.1Ga nanobelts accelerate the Zn²⁺ migration. EIS is utilized to further investigate the zinc ion storage process in the Zn/V₂O₅–0.1Ga battery. The curves consist of a semicircle in the high frequency region and a straight line in the low one. The radius of the semicircle represents the charge transfer resistance (R_ct) between the electrode and electrolyte. As presented in Fig. 4f, V₂O₅–0.1Ga samples possess a lower charge transfer resistance (R_ct) than the other samples, indicating their excellent reaction kinetics. In addition, the V₂O₅–0.1Ga samples show an R_s value of 5.1 Ω.

The energy density and power density can be calculated based on eqn (3) and (4):

E = QU/2m (3)

P = iU/2m (4)

where E (W h kg⁻¹) and P (W kg⁻¹) refer to the energy density and power density, respectively. Q (A h) represents the discharge capacity, U (V) is the operating voltage, i (A) denotes the discharge current, and m (kg) refers to the mass of active material. Therefore, it can be concluded that the cells possess an energy density of 281.64 W h kg⁻¹ at a power density of 55 W kg⁻¹ at 0.1 A g⁻¹.

Ex situ XRD was performed to study the Zn²⁺ storage mechanism of the Zn/V₂O₅–0.1Ga batteries at different stages (Fig. 5a). The (001) crystal plane shifts to a high angle when the voltage drops from 1.5 V to 0.4 V. This is due to the strong electrostatic adsorption between the intercalated Zn²⁺ and V₂O₅ materials, which reduces the interplanar spacing of the (001) plane.⁴² After full charging, the plane returns to its original state, which is attributed to the reversible removal of Zn²⁺ in the host material. Besides, a new phase Zn₃(OH)₂V₂O₇·2H₂O appears during the reaction due to the insertion of Zn²⁺, demonstrating that Zn²⁺ is embedded successfully into the V₂O₅–0.1Ga samples. The generation of a new phase is related to H⁺ in the electrolyte. The electrochemical reaction process can be represented as below:

V⁵⁺ + 4H₂O → VO₂(OH)⁻ + 6H⁺ (5)

Zn → Zn²⁺ + 2e⁻ (6)

2VO₂(OH)⁻ + 3Zn²⁺ + 3H₂O → Zn₃(OH)₂V₂O₇·2H₂O + 4H⁺ (7)

Fig. 5 Structural and morphology characterization. (a) Ex situ XRD patterns at various charge/discharge states, (b) and (c) XPS spectra of V and Zn elements and (d) schematic diagram of the Zn²⁺ insertion mechanism on the V₂O₅–0.1Ga cathode.

Finally, XPS spectra are again obtained to study the composition and chemical valence of the V₂O₅–0.1Ga electrode during the charge and discharge states. In Fig. 5b, the V 2p_1/2 and V 2p_3/2 diffraction peaks can be attributed to V⁴⁺ (523.68/516.28 eV) and V⁵⁺ (524.98/517.38 eV), respectively.⁴³ In the charge state, the signal of the V⁴⁺ peaks decreases, while that of V⁵⁺ increases. This is caused by reversible insertion/extraction of Zn²⁺ in the redox reactions. As seen in Fig. 5c, there are no Zn²⁺ signals detected in the pristine state. When discharged to 0.4 V, the peaks at 1022.18 eV and 1045.38 eV belong to Zn 2p_3/2 and Zn 2p_1/2. The peak signal is lower during charging than discharging. It is worth noting that the intensity of the Zn signal peak does not disappear completely after charging, which is due to the irreversible de-embedding of some Zn²⁺. Fig. 5d illustrates an electrochemical reaction schematic of the reversible insertion/extraction of Zn²⁺.

4. Conclusions

In summary, we have prepared several Ga-ion pre-embedded V₂O₅ nanobelts by a hydrothermal strategy. The obtained samples show large specific surface areas, which is beneficial to increasing the reaction active sites. The pre-embedding of Ga³⁺ effectively maintains the stability of the electrode material during repeated charging and discharging. The assembled Zn/V₂O₅–0.1Ga possesses an outstanding specific capacity and long cycle life after 5000 cycles and ultrafast Zn²⁺ diffusion capability. Moreover, they also achieve high energy density and power density. The superior performance V₂O₅–0.1Ga cathode materials show great potential for advanced and safe aqueous zinc ion batteries.

Author contributions

Ming Zhao: conceptualization, software, data curation, writing – original draft preparation, and methodology, Shilong Li: visualization, and Xiang Wu and Abdukayum Abdukader: supervision, writing – reviewing and editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This project is supported by the Natural Science Foundation of China (no. 52172218) and the Tianshan Innovation Team Plan of Xinjiang Uygur Autonomous Region (2023D14002).

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