Integrating molybdenum into zinc vanadate enables Zn₃V₂MoO₈ as a high-capacity Zn-supplied cathode for Zn-metal free aqueous batteries†

Gang Wang ^a, Quan Kuang *^a, Pan Jiang ^a, Qinghua Fan ^a, Youzhong Dong ^a and Yanming Zhao ^ab
aSchool of Physics and Optoelectronics, South China University of Technology, Guangzhou, 510641, P. R. China. E-mail: sckq@scut.edu.cn
^bSouth China Institute of Collaborative Innovation, Dongguan, 523808, P. R. China

First published on 3rd March 2023

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1. Introduction

With the increasing prominence of energy shortage and environmental pollution, clean energy is an effective solution to these problems. However, the utilization of clean energy, such as wind and solar energy, is limited in space and time, so we need to convert and store these energies. Energy storage technology has become an important part of promoting the use of clean energy.^1–3 The technology of rechargeable lithium-ion batteries (LIBs) is relatively mature and occupies a dominant position in the energy storage market, but its high cost and low safety limit its promotion and application in the field of large-scale energy storage.^4,5 Therefore, developing rechargeable aqueous batteries with low cost, high safety and environmental benignity has become a promising alternative for grid-scale electrochemical energy storage.^6–10

Due to the abundance of zinc resources and the low redox potential of zinc metal (−0.76 V vs. a standard hydrogen electrode), aqueous zinc-ion batteries (AZIBs) have become an attractive potential choice among rechargeable aqueous batteries.^11,12 However, owing to the lack of high-capacity cathode materials and the serious dendrite growth of zinc anodes, the mass production and practical application of AZIBs still face profound challenges. In fact, similar problems had also been encountered before the commercialization of LIBs.¹³ To solve these problems, many lithium-supplied cathode materials were developed, such as LiCoO₂, LiMn₂O₄, and LiFePO₄.¹⁴ Furthermore, the SONY Corporation proposed a strategy of using graphite to replace lithium metal as the anode,¹⁵ and realized the commercialization of LIBs based on a LiCoO₂||graphite system.

By drawing on the successful experience of LIBs, the development of Zn-supplied cathode materials is essential for the commercialization of AZIBs. However, most current studies on AZIBs are focused on the development of zinc-free cathode materials, such as manganese oxides,^16–18 vanadium oxides/phosphates,^19,20 Prussian blue analogs,²¹ and organic materials.^22–25 Very few Zn-supplied cathode materials have been reported so far. The Zn-supplied cathodes are mainly in the spinel family, such as ZnMn₂O₄, ZnAl_xCo_2−xO₄ and ZnNi_xMn_xCo_2−2xO₄, which are severely constrained by their poor reversible capacity.^26–28 For example, Zhang et al. reported a manganese-deficient ZnMn₂O₄/C cathode for AZIBs which obtained a discharge capacity of 150 mA h g⁻¹ at 50 mA g⁻¹.²⁶ Pan et al. found ZnAl_xCo_2−xO₄ (x = 0.67) exhibiting a high operating voltage and cycle stability (114 mA h g⁻¹ after 100 cycles).²⁷ For vanadium-based cathode materials, ZnV₂O₄ exhibited a high reversible capacity of 312 mA h g⁻¹,²⁹ which is due to the intercalation/extraction of exogenous Zn⁺ from the Zn-metal anode, but not from the cathode side. This was the situation until 2021, when our group first presented a vanadium-based Zn-supplied cathode for AZIBs, Zn₃V₃O₈, which delivered the highest recorded discharge capacity (285 mA h g⁻¹) among the available zinc-supplied cathodes.^30,31 However, the reversible capacity of Zn₃V₃O₈ is still unsatisfactory when compared with that of non-Zn²⁺ vanadium oxide cathodes (>300 mA h g⁻¹).³² Therefore, higher capacity Zn-supplied cathode materials need to be discovered for constructing high-performance zinc-metal free AZIBs (ZFABs).

Considering there are two trivalent vanadium (V³⁺) and one tetravalent vanadium (V⁴⁺) in Zn₃V₃O₈, the existence of V⁴⁺ may limit the ability to supply zinc ions in the conversion reaction. Hence in this paper, Mo⁴⁺ is integrated into Zn₃V₃O₈ to replace V⁴⁺ in the spinel structure, and a novel Zn-supplied cathode material Zn₃V₂MoO₈ can be obtained via a facile sol–gel process. Benefiting from the muti-electron reaction of molybdenum (Mo³⁺/Mo⁴⁺ ↔ Mo⁵⁺/Mo⁶⁺), the ability to supply zinc ions is enhanced for Zn₃V₂MoO₈; thus the reversible capacity is 33.7% higher than that of Zn₃V₃O₈, which has created a new record (360.3 mA h g⁻¹) among existing Zn-supplied cathodes. Furthermore, the electrochemical behavior and the reaction mechanism of the Zn₃V₂MoO₈ cathode are explored. Finally, 9,10-anthraquinone (9,10AQ) is chosen as a zinc-metal free anode, and a Zn₃V₂MoO₈||9,10AQ battery has been built as a novel zinc-metal free AZIB to verify the Zn-supplied capacity of Zn₃V₂MoO₈.

2. Experimental section

2.1. Preparation of carbon-coated Zn₃V₂MoO₈

Carbon-coated Zn₃V₂MoO₈ powder was synthesized by using a facile sol–gel method. In a typical procedure, 1.126 g of zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), 0.400 g of ammonium vanadate (NH₄VO₃), 0.718 g of citric acid monohydrate (C₆H₈O₇·H₂O) and 0.246 g of molybdenum trioxide (MoO₃) were dissolved in 40 mL of deionized water and stirred at 80 °C until excess water was removed and a sol was acquired. Subsequently, the sol was transferred into an air oven and kept at 70 °C for 12 h to obtain a gel. Then, the gel was ground for 30 min, and transferred to a quartz tube furnace under a mixed gas flow of Ar/H₂ (5% in volume). Finally, after respectively annealing at 700 °C, 750 °C and 800 °C for 9 h at a heating rate of 2 °C min⁻¹, the resulting black powder of carbon-coated Zn₃V₂MoO₈ could be obtained.

2.2. Structural characterizations

X-ray diffraction (XRD) was performed with a Bruker D8 Advance X-ray diffractometer with an area detector, using Cu Kα radiation (λ = 1.5418 Å) and a step-scan mode (scanning step of 0.02°) was adopted with a scanning speed of 0.3 s per step for structural refinement and 0.1 s per step for phase identification. The Rietveld structural refinement of Zn₃V₂MoO₈ was carried out using the GSAS program (Larson & Von Dreele) via the EXPGUI interface (Brian Toby).³³ The particle size and morphology of the carbon-coated Zn₃V₂MoO₈ composite were characterized with a Gemini SEM 500 scanning electron microscope (SEM) from ZEISS, and a Talos F200X transmission electron microscope (TEM) from Thermo Scientific, respectively. The X-ray photoelectron spectroscopy (XPS) spectrum was obtained with a Thermo Scientific K-alpha XPS instrument with a 100 μm spot size. Raman spectra and carbon content were acquired using a laser confocal microscopic Raman spectrometer (HJY LabRAM Aramis) and elemental analyzer (C, H, N, S, Vario EL cube), respectively. The carbon content of the product sintered at 750 °C was 5.48 wt%, and the specific capacity was calculated based on the active material (Zn₃V₂MoO₈) alone.

2.3. Cell assembly and electrochemical testing

For the electrochemical tests, all of the batteries were evaluated using CR 2032 coin-type cells assembled in air. The working electrode was fabricated by rolling the active material (carbon-coated Zn₃V₂MoO₈), conductive agent (Super P), and polyvinylidene fluoride (PVDF) in a weight ratio of 7:2:1 scattered in N-methylpyrrolidone (NMP). The mixture was coated onto Ti foil with a thickness of ∼150 μm and then heated at 90 °C in a vacuum oven overnight to remove NMP. The negative electrodes for the ZFABs were prepared by the same method, but the difference is that the active substances were replaced with 9,10-anthraquinone (9,10AQ) and brass, respectively. The typical loading of active material on each electrode is ∼0.8 mg. For the Zn₃V₂MoO₈||Zn batteries, saturated Zn(CF₃SO₃)₂ (∼2.8 M) aqueous solution, filter paper and zinc foil were used as the electrolyte, separator and counter electrode, respectively. The assembly of the ZFABs was the same as the above method, except that the anodes were replaced by 9,10AQ and brass, respectively.

The charge–discharge behaviors of the Zn₃V₂MoO₈||Zn batteries within the potential range of 0.2–1.7 V (vs. Zn²⁺/Zn) under a variety of current densities were tested on the LAND battery testing system. The Zn₃V₂MoO₈||9,10AQ and Zn₃V₂MoO₈||brass batteries were tested within the potential range of 0.01–1.4 V. To estimate the ionic diffusion coefficients at different electrochemical equilibrium voltages, the galvanostatic intermittent titration technique (GITT) mode was operated on the LAND battery testing system. To further characterize the electrochemical behavior of Zn₃V₂MoO₈, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to test the Zn₃V₂MoO₈||Zn coin cells on an AUTOLAB PG-STAT302N (Metrohm, Netherlands) within the voltage range of 0.2–1.7 V (vs. Zn²⁺/Zn) at 0.2 mV s⁻¹ and 100 kHz to 0.01 Hz, respectively.

3. Results and discussion

The single phase of Zn₃V₂MoO₈ can be prepared via a facile sol–gel method and then a high-temperature sintering reaction, which has low preparation requirements and is easily extended to mass production. When the sintering temperature reached 700 °C, the Zn₃V₂MoO₈ phase as the isologue of ZnV₂O₄ (ICSD-28963) had appeared as the major phase of the reaction product. The pure phase could be obtained at a sintering temperature of 750 °C, which was decomposed to V₂O₃, V₁₄O₆ and ZnMoO₃ at 800 °C (Fig. 1a). Hence, all of the following measurements and analyses of Zn₃V₂MoO₈ were conducted on the sample obtained at the sintering temperature of 750 °C. As shown in Fig. 1b, the Rietveld structural refinement of fresh Zn₃V₂MoO₈ was carried out using the GSAS program with the initial model of ZnV₂O₄, in which half of vanadium in the Wyckoff site of 8b was substituted by equal amounts of zinc and molybdenum. The relatively low weighted residual factor (R_wp = 3.39%) of the structural refinement suggests that a high-purity phase of Zn₃V₂MoO₈ had been obtained, and the refinement result is highly reliable. The lattice parameters and atomic coordinates of Zn₃V₂MoO₈ from the structural refinement are shown in Table 1. The Zn₃V₂MoO₈ compound is crystallized in a cubic syngony (Fdm space group) with cell parameters of a = b = c = 8.4070(6) Å, which are larger than those of Zn₃V₃O₈ (a = b = c = 8.402 Å),³⁰ which is due to the greater effective ionic radius of Mo⁴⁺ (0.79 Å) than that of V⁴⁺ (0.72 Å).³⁴ In a unit cell of Zn₃V₂MoO₈, both V and Mo atoms occupy the same Wyckoff sites (8b), while Zn and O atoms occupy 8b/16c and 32e sites, respectively. Fig. 1c demonstrates the crystal structure diagrams of Zn₃V₂MoO₈ drawn according to the refinement results. Like Zn₃V₃O₈, Zn₃V₂MoO₈ has a three-dimensional spinel structure composed of V(Zn2,Mo)O₆ octahedra, and Zn1 atoms are distributed in the tetrahedral interstices. The main difference between Zn₃V₂MoO₈ and Zn₃V₃O₈ is that the V⁴⁺ in Zn₃V₃O₈ are completely replaced by Mo⁴⁺ for Zn₃V₂MoO₈, and hence the Mo⁴⁺ can further contribute two electrons for Zn²⁺ extraction, that is increasing the redox pairs of Mo⁴⁺ ↔ Mo⁶⁺ in the spinel structure, which is beneficial to the zinc supply and capacity enhancement of the cathode material. Fig. 1d presents the Raman spectrum of the carbon-coated Zn₃V₂MoO₈ composite. Two characteristic peaks located at 1328 and 1588 cm⁻¹, indicating the disordered carbon (D band) and graphitic carbon (G band), respectively, show that the Zn₃V₂MoO₈ particles are covered with a hybrid carbon (I_D/I_G = 2.83) layer carbonized from the citric acid.

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Fig. 2 presents the typical morphology and element distribution of Zn₃V₂MoO₈ particles photographed using SEM and TEM under different magnifications. At a glance, the sample is composed of irregular agglomerated particles of 2–10 μm (Fig. 2a), which is a typical phenomenon of a high-temperature sintering reaction and beneficial to improving the compaction density of the electrode material. Taking a closer look (Fig. 2b), the secondary particle is constituted of homogeneous nanoparticles of ∼200 nm in diameter, which can increase its contact area with the electrolyte and is conducive to its electrochemical kinetics in the battery. Moreover, there is a group of very obvious lattice fringes in the high-resolution TEM image (Fig. 2c), with an interplanar spacing of 0.254 nm, corresponding to the (311) crystal plane of Zn₃V₂MoO₈. The element mapping of a representative Zn₃V₂MoO₈ particle shown in Fig. 2d illustrates that the Zn, V, Mo, O and C elements are uniformly distributed throughout the particle without obvious element segregation, indicating the success of the material synthesis.

Fig. 3 shows the electrochemical performance of Zn₃V₂MoO₈ as a cathode material for AZIBs, in which zinc metal and saturated Zn(CF₃SO₃)₂ aqueous solution were used as anode and electrolyte, respectively. As a Zn-supplied cathode of Zn₃V₂MoO₈, all electrochemical tests began with a charging process to extract the Zn ions in the cathode. The representative galvanostatic charge–discharge curves of the Zn₃V₂MoO₈||Zn battery at a current density of 100 mA g⁻¹ are shown in Fig. 3a. It can be seen that there is a long voltage plateau at ∼1.35 V vs. Zn²⁺/Zn during the initial charge, which is obviously different from the charge behavior of the subsequent cycles, indicating that an electrochemically induced phase transformation of Zn₃V₂MoO₈ occurs during the maiden Zn-ion extraction. The extraction of Zn ions and the oxidation of V and Mo can be observed from the XPS results (Fig. S1†). After that, the charge–discharge behavior and Zn-storage mechanism of Zn₃V₂MoO₈ in the subsequent cycles tend to be stable and consistent, which can also be mutually confirmed with the results of CV measurement and ex situ XRD manifestation. The phenomenon of phase transformation of Zn₃V₂MoO₈ during the initial charge is similar to the case of Zn₃V₃O₈.³⁰ Significantly, through the incorporation of molybdenum, the specific capacity of Zn₃V₂MoO₈ is 26.4% higher than that of Zn₃V₃O₈, reaching 360.3 mA h g⁻¹ at 100 mA g⁻¹, which is a new record for a Zn-supplied cathode material. Fig. 3b presents the CV profiles of the initial three cycles of the Zn₃V₂MoO₈||Zn battery. The oxidation peak (1.48 V vs. Zn²⁺/Zn) of the first voltammetric cycle is completely different from those (1.05 and 0.73 V vs. Zn²⁺/Zn) of the following two cycles, which is attributed to the electrochemical phase transformation of Zn₃V₂MoO₈. The redox peaks of the subsequent two cycles tend to be consistent, and two pairs of redox peaks are located at 1.05/0.92 V and 0.73/0.55 V vs. Zn²⁺/Zn, respectively, indicating that the process of Zn-ion extraction and intercalation is a multi-stage electrochemical reaction.

Fig. 3c demonstrates the short-run cycle performance of Zn₃V₂MoO₈. Under a low current density (200 mA g⁻¹), its peak discharge capacity is up to 336.7 mA h g⁻¹, and its capacity retention is 93% after 70 cycles. It can be seen that its specific capacity and cycle stability are significantly improved compared with Zn₃V₃O₈ (peak capacity of 285 mA h g⁻¹ at 150 mA g⁻¹).³⁰ Moreover, the charge–discharge rate ability of Zn₃V₂MoO₈ from 0.1 to 5 A g⁻¹ was also explored (Fig. 3d). The specific capacities are 348.6, 293.9, 264.7, 250.1, 218.4 and 192.6 mA h g⁻¹, corresponding to current densities of 0.1, 0.2, 0.5, 1, 2, and 5 A g⁻¹, respectively. When the current density returned from 5 A g⁻¹ to 0.5 A g⁻¹, the specific capacity was also fully restored to 268.8 mA h g⁻¹. The results above illustrate that the rate ability of Zn₃V₂MoO₈ is favorable, which is attributed to the conductive network constructed by the carbonization of citric acid. Fig. 3e depicts the long-run cycle performance of Zn₃V₂MoO₈ at a large current density of 10 A g⁻¹. The specific capacity reached its peak value (253.8 mA h g⁻¹) at about 11 cycles, then decreased to a stable value at 750 cycles, and had slight capacity fading (82% retention) in the 750–4500 cycles.

Vanadium-based electrodes often exhibit a prominent surface capacitance effect owing to their multiple redox reactions at the interface between electrode and electrolyte. To clarify the influence of capacitance effect and diffusion control on the battery capacity, the variable-speed CV (from 0.1 to 1 mV s⁻¹) of the Zn₃V₂MoO₈||Zn battery was measured and is shown in Fig. 4a. The peak current (i) of the battery response and the scan rate (v) of the CV test generally follow the power law i = av^b, where a is the coefficient that makes the equation balance, and b is in the range 0.5–1, which can embody the electrochemical kinetics of the cathode.³⁵ By linear fitting of the log(peak current) and log(scan rate) shown in Fig. 4b, we find that the b values during charge and discharge are 0.85 and 0.98, respectively. Therefore, capacitive control plays a dominant part in the capacity of Zn₃V₂MoO₈ in both charge and discharge processes. In addition, by distinguishing the response current under a fixed potential into the pseudo-capacitance effect and diffusion-controlled reaction, the proportions of these two reactions under different scanning speeds can be further quantified. The specific equation can be described as follows:

where, v is the sweep speed, k₁v is the contribution of the pseudo-capacitance effect, and is the contribution of the diffusion-controlled reaction.³⁶ By calculating the values of k₁ and k₂ from the variable-speed CV curves, we can evaluate the contribution of pseudo-capacitance and diffusion to current under a specific voltage. As shown in Fig. 4c, when the scanning rates were increased from 0.1 mV⁻¹ s to 1 mV s⁻¹, the corresponding pseudo-capacitance contribution ratios in the charge–discharge process were 56.2%, 59.1%, 64.8%, 69.8%, 76.3% and 79.5%, respectively. To further explore the diffusion kinetic behavior of zinc ions in the charge–discharge process of Zn₃V₂MoO₈, GITT was used in our study. According to Fig. 4d–f, the ionic diffusion coefficients contributed by both zinc ions and protons at different electrochemical equilibrium voltages are finally calculated to be ∼10⁻¹⁰ cm² s⁻¹, which are close to that of lithium ions in LiCoO₂ and at least two orders of magnitude higher than that of LiFePO₄, indicating that Zn₃V₂MoO₈ shows superior kinetic behavior.

The ex situ XRD technique was applied to investigate the structural evolution of the Zn₃V₂MoO₈ cathode during the initial three charge–discharge cycles (Fig. 5a). In the primal XRD pattern, there are characteristic peaks at 30.0°, 35.4°, 43.0°, 56.9° and 62.5° corresponding to the Zn₃V₂MoO₈ phase. During the progress of the initial charge, the diffraction peaks of Zn₃V₂MoO₈ gradually weakened until they completely disappeared when charged to 1.7 V vs. Zn²⁺/Zn. Meanwhile, new diffraction peaks emerged at 29.8° and 35.1°, suggesting the formation of Zn_y(V,Mo)₂O_5−x·nH₂O, which shows the isomorphism of V₂O₅·nH₂O.³⁷ These two processes accompany each other, revealing the phase transition from Zn₃V₂MoO₈ to Zn_y(V,Mo)₂O_5−x·nH₂O during the first charge, which corresponds to the charging plateau at ∼1.35 V vs. Zn²⁺/Zn (Fig. 3a) and the oxidation peak in the initial CV curve (Fig. 3b). Fortunately, this electrochemically induced conversion reaction turns the spinel framework of Zn₃V₂MoO₈ into a layered structure in favor of Zn-ion storage. Zn₃V₂MoO₈ was almost not detected again in the subsequent charge–discharge cycles, while the peaks corresponding to Zn_y(V,Mo)₂O_5−x·nH₂O persisted and were only slightly shifted, which indicates that the conversion reaction (1.3–1.7 V vs. Zn²⁺/Zn) during the first charge is irreversible, which is also consistent with the above charge–discharge and CV curves in Fig. 3a and b. Fig. 5b and c are the HRTEM images of the cathodes at the first cycle after full charge and full discharge, respectively. Between them, the crystal plane spacing measured in Fig. 5b is 3.05 Å, which corresponds to the diffraction peak at 29.8° of Zn_y(V,Mo)₂O_5−x·nH₂O in Fig. 5a, while the crystal plane spacing in Fig. 5c is 4.97 Å, which corresponds to the (001) crystal plane of the H⁺/Zn²⁺ co-insertion product Zn_y+zH_w(V,Mo)₂O_5−x·nH₂O, which is the isomorphism of Zn_xV₂O₅·nH₂O.³⁷ According to the XPS result (Fig. S1†), the binding energy of oxygen increased during the initial charge, indicating that oxygen in Zn₃V₂MoO₈ has oxidized. Meanwhile, the intensity of the oxygen signal decreased, which further suggests the oxygen escaped from the spinel lattice into oxygen gas.

During the discharge of the first cycle, when the potential reached ∼0.8 V vs. Zn²⁺/Zn, new diffraction peaks gradually appeared at 12.5° and 33.0°, corresponding to the formation of Zn_x(OH)_y(CF₃SO₃)_2x−y·nH₂O,^38–40 which also reveal the surface change of the Zn₃V₂MoO₈ electrode during the discharge process (Fig. 5d and e). Compared with the original electrode (Fig. 5d), there is obviously a lamellar structure deposited on the electrode surface after full discharge (Fig. 5e). The morphology of the electrode surface at the 2^nd full discharge (Fig. S2†) is similar to the 1^st cycle. Since the hydroxyls in Zn_x(OH)_y(CF₃SO₃)_2x−y·nH₂O come from water decomposition in the electrolyte,^38–40 the same amount of protons as hydroxyls will be generated. During discharge and recharge, these protons can reversibly intercalate into and deintercalate from the cathode material. Therefore, the discharge capacity is contributed by the co-insertion of zinc ions and protons, and thus the final discharge product is Zn_y+zH_w(V,Mo)₂O_5−x·nH₂O. In the following two cycles, the diffraction peaks of Zn_x(OH)_y(CF₃SO₃)_2x−y·nH₂O gradually disappeared during the charge process, suggesting the decomposition of Zn_x(OH)_y(CF₃SO₃)_2x−y·nH₂O. At the same time, the diffraction peaks of Zn_y(V,Mo)₂O_5−x·nH₂O are only slightly shifted to the right. In the discharge process, the diffraction peaks of Zn_x(OH)_y(CF₃SO₃)_2x−y·nH₂O reappeared, and the diffraction peaks of Zn_y(V,Mo)₂O_5−x·nH₂O are also slightly shifted to the left. These phenomena show that after the first charge, the subsequent discharge and recharge processes tend to be the same: that is, the intercalation and deintercalation of zinc ions and protons in Zn_y(V,Mo)₂O_5−x·nH₂O, accompanied by the formation and decomposition of Zn_x(OH)_y(CF₃SO₃)_2x−y·nH₂O, respectively. Based on the above discussion, the reaction mechanism of the Zn₃V₂MoO₈||Zn battery during charge and discharge can be summarized as follows:

first charge in the cathode:

subsequent discharge–recharge in the cathode:

the reactions on the zinc-metal anode:

Zn ⇄ Zn2+ + 2e−

The use of a zinc-metal anode is accompanied by a serious dendrite problem, which has a negative impact on the cycle stability of AZIBs.⁴¹ To solve this problem, referring to the idea of LIBs, we can develop Zn-metal free aqueous batteries by using Zn₃V₂MoO₈ as a cathode in conjunction with a non-zinc-metal anode. Herein, brass and 9,10AQ were chosen as the anode and Zn₃V₂MoO₈ as a cathode to assemble a Zn-metal free aqueous battery (Fig. 6a). Between them, the Zn₃V₂MoO₈||brass battery can provide a discharge capacity of 83.1 mA h g⁻¹ at 100 mA g⁻¹, but only retains 50.2 mA h g⁻¹ after 1300 cycles (Fig. S3 and S4†), while the Zn₃V₂MoO₈||9,10AQ battery can provide higher specific capacity and better cycle stability. The better electrochemical performance of the Zn₃V₂MoO₈||9,10AQ battery is mainly due to the zinc storage mechanism of 9,10AQ based on highly reversible intercalation–deintercalation reactions,²² while that of brass is based on the deposition and stripping of zinc ions.⁴² As shown in Fig. 6b, the Zn₃V₂MoO₈||9,10AQ battery presents a high charge capacity of 143.3 mA h g⁻¹ and low discharge capacity of 61.8 mA h g⁻¹ in the first cycle, suggesting the loss of active zinc in the initial electrochemical activation stage of cathode and anode materials. In the subsequent discharge–recharge cycles, the profile of charge curve is totally different from the initial one, and the coulombic efficiency gradually improved. Finally, the Zn₃V₂MoO₈||9,10AQ battery presents tolerable cycle performance (Fig. 6c), which has a peak value of 116.6 mA h g⁻¹ and a stable capacity of 100.4 mA h g⁻¹ lasting for 200 cycles, and still exceeds 79.6 mA h g⁻¹ after 1100 cycles.

4. Conclusions

A novel mixed transition-metal spinel, Zn₃V₂MoO₈, has been synthesized via a sol–gel technique and proposed as a Zn-supplied cathode material for AZIBs. The structure and morphology of Zn₃V₂MoO₈ were first revealed using XRD, SEM, TEM, etc. According to the result of Rietveld refinement, both V and Mo occupy the 8a site of the spinel structure. Hence, utilizing the synergistic effect of vanadium and molybdenum in Zn₃V₂MoO₈, the ability to supply zinc of a spinel-type cathode for Zn-metal free aqueous batteries has been fully optimized. Zn₃V₂MoO₈ can provide a specific capacity of 360.3 mA h g⁻¹ at 100 mA g⁻¹, and it shows excellent cycle stability at both low and high current density. Concretely, its specific capacity reaches 336.7 mA h g⁻¹ at 200 mA g⁻¹, and remains 292.9 mA h g⁻¹ after 70 cycles. At 10 A g⁻¹, capacity retention is 82% over 700–4500 cycles. Compared with previous Zn-supplied cathode materials, Zn₃V₂MoO₈ is the state-of-the-art in zinc supply capacity and cycle stability. A series of characterizations (XRD, XPS, SEM and TEM) were conducted during the charge–discharge process, which reveals that Zn₃V₂MoO₈ undergoes a phase transition to Zn_y(V,Mo)₂O_5−x·nH₂O in the initial charge, and then protons and zinc ions intercalate/deintercalate concurrently into/from the new host during the subsequent cycles. To construct Zn-metal free aqueous batteries to avoid the zinc dendrite problem, two non-zinc materials (brass and 9,10AQ) were used as anodes to match the Zn₃V₂MoO₈ cathode. Thereby, the Zn₃V₂MoO₈||9,10AQ battery showed more satisfactory electrochemical performance, with a stable capacity of 100.4 mA h g⁻¹ lasting for 200 cycles, and remained 79.6 mA h g⁻¹ after 1100 cycles. These findings are expected to provide a feasible scheme for the promotion and commercialization of AZIBs.

Author contributions

Gang Wang: data curation, formal analysis, investigation, software, visualization, writing – original draft, writing – review & editing. Quan Kuang: conceptualization, funding acquisition, methodology, project administration, resources, supervision, validation, writing – original draft, writing – review & editing. Pan Jiang: investigation, software. Qinghua Fan: methodology, resources. Youzhong Dong: methodology, resources. Yanming Zhao: funding acquisition, methodology, supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study is funded by the grant (No. 52177213) of the National Natural Science Foundation of China (NSFC), and the foundation (No. 2017B030308005 and No. 2020A1414010346) supported through the Science and Technology Bureau of Guangdong Government. This study is also sponsored by the project (No. 2019622163008) of the Science and Technology Bureau of Dongguan Government, and supported by “the Fundamental Research Funds for the Central Universities” from the South China University of Technology.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nr00136a

This journal is © The Royal Society of Chemistry 2023