In recent, due to the mounting exigency for developing green and sustainable energy storage for enormous applications, until now tremendous research efforts have been devoted to electrochemical devices such as flexible, portable electronic appliances and grid-scale long durable energy storage systems [1-5]. In this current alleviation of the global situation, it is not obligatory to shift completely to renewable energy sources and electric-powered vehicles [6]. Till date, the progress toward these energy storage technologies tends to utilize both renewable energy and electric vehicles with the high power density and fast recharge capability are still immature and require further development [7-9]. The majorly utilized substitutes for commercial usage of lithium and sodium-ion batteries (LIBs and SIBs) are still complicated since they have superior cycle performance and higher specific energy, which open up in a wide range of applications [10]. These major concerns over their safety, limited recycling infrastructure, restricted Li resources, and environmental impact are being deployed in real-life applications [11,12]. Comparatively, the water-based aqueous electrolytes endow rechargeable batteries with low cost and superior safety precautions. In this regard, the rechargeable aqueous zinc-ion battery offers a high-security and low-cost pathway on futuristic energy storage devices owing to the merits of decreased toxicity level and inimitable global abundance of earth sources of zinc [4,13,14]. However, these challenging roles remain constant in the manipulative preparation of cathodes since it has comparatively large radii of intercalated hydrated Zn²⁺ in robust electrostatic potency between its divalent Zn²⁺ and its host structures [3,15,16].

The diversity of V-O polyhedron structure leads to many types of V-based oxides. In addition, with the change in V valence state (V^y; y = 2⁺, 3⁺, 4⁺, 5⁺), the structure of the V-O polyhedron also changes simultaneously, resulting to an assortment of vanadium oxides with diverse compositions and structural skeletons [17]. During earlier investigations on vanadium materials for ZIBs are majorly fixated on vanadium oxides such as orthorhombic V₂O₅ [18], bilayered V₂O₅·nH₂O [19], V₂O₅@V₂C [5], VO₂ [20], V₂O_x@V₂CT_x [21], K_0.25V₂O₅ [22], Na₆V₁₀O₂₈ [23], Zn-V₂O₅ [24], V₆O₁₃·nH₂O [25], V₂O₃@AC [26] and V₅O₁₂·6H₂O [27], endure its structure with repeated implantation of Zn²⁺ ions. Similarly, the rise of vanadium-based Transition Metal Dichalcogenides (TMDs) with diverse crystal structures and also rare distinctive physical and chemical properties exhibit better electrochemistry due to the vanadium's multi-valence state and large ion transfer channel [28-33]. They mainly attribute the electrochemical energy storage performance to diverged vanadium oxidation states with low operating voltage and because of their suitable open-framework structures [34-36]. Very recently, researchers showed attention more to bimetallic compounds as a newer cathode material because of their advantages besides mono-metallic compounds. In particular, the spinel structure binary metal oxides (M_xV_mO_n, M = metal cations) have been triggered more via investigating their electrochemistry it is found to be the most promising cathode materials with benefits such as widened potential window, more active sites, superior conductivity, and improved cycling stability. Recently, Liu et al. firstly demonstrates about this typical spinel ZnV₂O₄ by a significant electro activation process during the initial electrochemical sequences the occurrence of vanadium's mixed-valence state acts as a stochiometric conductive frame which enhances their stable high capacity and excellent cyclic stability for aqueous ZIBs [37]. Li et al. have reported porous like ZnV₂O₄ particles coated with N-doped carbon (ZnV₂O₄/C—N composite) synthesized via pyrolysis technique which showcases a specific discharge capacity of 301 mAh/g at 300 mA/g current density [38]. Furtherly, Tang et al. recommended that a new novel urchin-like magnesium vanadate could provide an enlarged specific surface area with induced more active sites, shortens the conduction channel of the host electrolyte ions, and affordable structural stability that helps for better electrochemical performance on aqueous zinc-ion batteries (AZIBs) [39]. Noticeably, it is observed that the binary metal vanadate has a drawback on structural degradation with the influence of reactant solvent in it further leading to low cycling stability [40]. From the recent research, it is evidenced that the development of interest in metal vanadium sulfides (MVS) as an electrode with more sophisticated electrical conductivity than all other oxide counterparts of it, because of much inferior optical bandgap in sulphides than oxides and more potentially viable in energy storage materials [41-43].

In concern to the above-mentioned issues, we developed a highly standardized novel cathode material by accomplishing a high rate and stable electrochemical performance for ZIBs. The innovative design of a bimetallic sulfide-based zinc-ion battery was fabricated by employing a cauliflower-like nano-ZnV₂S₄ as a cathode. It delivered an admirable specific discharge capacity (348.2 mAh/g during 0.5 A/g), and the interface engineering of the following MVS nanostructure contributes to good cyclic stability (~89.2% retention after 1000 cycles@4 A/g). This better electrochemical involvement of sulfide electrodes, when compared to other oxide electrodes, is mainly attributed to two major reasons such as truncated electronegativity and the enormous particle size of sulfur that builds tightly bonded structure and improved ion transfer assets.

Synthesis of ZnV₂S₄: In a distinctive synthesis process, 2 mmol of ammonium metavanadate (NH₄VO₃) was added into a mixture of 50 ml of ethylene glycol and 20 mL of deionized water. After a few minutes of sonication, 1 mmol/L of zinc nitrate (Zn(NO₃)₂) was added to the above solution and dynamically stirred until it attains a homogeneous solution. To this, 3 mmol/L of oxalic acid [H₂C₂O₄] was added as a chelating agent. Subsequently, 50 mL of 30% H₂O₂ and 70% HNO₃ were added dropwise into the mixture solution. This final solution mixture was then transferred and maintained at 180 ℃ for 12 h in 100 mL of Teflon-lined autoclave, and then chilled to room temperature to get a black-colored ZnV₂(OH)₈. The precipitates were collected and washed repeatedly with deionized water followed by absolute ethanol, finally dried in a vacuum furnace at moderate room temperature. Further, the initially prepared ZnV₂(OH)₈ sample and 0.1 mol Na₂S were diluted in 70 mL of DI water and engaged in an autoclave and heated at 120 ℃ for 8 h. In this process, ZnV₂(OH)₈ reacts with sulfide ions and replaces the OH⁻ functional group to form cauliflower-like nano-ZnV₂S₄. All the samples were repeatedly washed with de-ionized water and kept drying for 12 h at 80 ℃ for further usage.

The XRD analysis (Empyrean, Malvern Panalytical) from the range 5°–80° in the presence of nickel-filtered Cu-Kα (λ = 1.54 Å) radiation at an increment of 0.05° was utilized to examine the phase purity of the prepared material. The (Renishaw RM 2000) confocal micro-Raman spectrometer instrument was used to record its bandgap spectra by passing a 10 mW Ar⁺ laser under 516 nm. X-ray photoelectron spectroscopy (XPS) (Model: PHI 5000 VersaProbe III) is favored to analyze the surface elements in monochromatized Al-Kα. The morphology of nano-ZnV₂S₄ was analyzed using FESEM (CARL ZEISS, Model: Sigma with Jemini column, USA). The morphology of nano-ZnV₂S₄ and its elemental compositions were confirmed by TEM (JEM-2010F) and energy-dispersive X-ray spectroscopy (EDAX), respectively.

To examine the electrochemical proficiency of the as-prepared ZnV₂S₄ sample, the coin-cell of CR2032 was built with ZnV₂S₄ as the cathode, Zn sheet (0.62 mm with purity phase > 99.99%, Alfa Aesar) as anode (where E = −0.76 V vs. SHE), Waterman filter paper as the separator and 2 mol/L ZnSO₄ of acidic phase was used as water-based electrolyte. The working electrode consists of ZnV₂S₄, acetylene black, and poly(tetrafluoroethylene) as the tightly bonded binder in N-methyl-2-pyrrolidone (NMP) with a standardized weight ratio of 70:20:10 to prepare a homogeneous slurry. This slurry paste was applied onto the graphite sheet and kept under a vacuum furnace for 12 h at 90 ℃ to decay the solvent present in it. After cooling, the slurry-coated film was made sliced into circular disks of 15 mm. The mass loading of each cathode disk is maintained under 0.8–1.4 mg/cm² with an approximate thickness of ~4 µm.

All the electrochemical assessments were carried out by using a Biologic electrochemical workstation (Model: VSP, (France)). The cyclic voltammetry curves were decoded in the fixated potential window of 0.3–1.7 V at numerous scan rates of 0.25, 0.5, 1, 2, 5 mV/s. GCD (Galvanostatic charge-discharge) curves were recorded at different current densities of 0.5, 1, 2, 4 and 8 A/g in the potential range of 0.3–1.7 V. the electrochemical impedance measurements were recorded in an average frequency range of 0.01 Hz–100 kHz in open circuit potential with 5 mV of AC amplitude.

Hydrothermal synthesis is a majorly utilized simple technique for the preparation of nanostructures by establishing unique morphologies. In this study, we employ binary-step hydrothermal synthesis for forming this novel cauliflower-like nano-ZnV₂S₄ by a self-assembly process [37,39,44-49]. Here, the formation mechanism of cauliflower-like nano-ZnV₂S₄ arrays from ZnV₂(OH)₈ nanoparticles can be explained based on ion-exchange reaction as illustrated in Fig. 1 [40]. The reaction mechanism for the preparation of cauliflower like nano-ZnV₂S₄ is mentioned below.

The growth mechanism of cauliflower-like nano-ZnV₂S₄ might be involved in a multistep reaction process. Primarily, from Eq. 1 during the initial step of the hydrothermal heat treatment at 180 ℃, the NH₄VO₃ is reduced to 2VO₂ through the inclusion of oxalic acid as a reducing agent. Meanwhile, during step two Zn(NO₃)₂ is readily decomposed to its preformed state as ZnO nanoclusters as per Eq. 2. These nanoclusters are then self-restacked into nanoparticles to shrink their surface tension on the dispersed nanoclusters to form a by-product ZnV₂(OH)₈ nanoparticles are generated from the intermediates 2VO₂ and ZnO. During the final step of hydrothermal heat treatment, from Eqs. 3 and 4, the ZnV₂(OH)₈ reacts with sodium sulfide at 120 ℃ and replaced the OH⁻ functional group with S²⁻ to form cauliflower-like nano-ZnV₂S₄.

The FE-SEM images (Fig. 2a) of nano-ZnV₂S₄ show that the metal nanoparticles tend to aggregate themselves and eventually form a randomly arranged disordered nanostructure, which is composed of agglomerates of pseudo-spherical nanoparticles. Such integrated nanoparticles formed from the self-assembly of many nanoparticles are decorated like unique cauliflower-like structures with an average size of ~300 nm. This empowers the penetration and helps in the ultrafast transportation of electrolyte ions during the reaction process. Further, The TEM image (Fig. 2b) confirms the interconnected network structure of cauliflower-like nano-ZnV₂S₄ by exhibiting a globular shaped morphology and good dispersity. The SAED pattern (Fig. 2c) shows a typical building block labelled by the white dotted line circles reveal with a lattice fringe d-spacing of 0.138, 0.254, 0.214 and 0.146 nm corresponding to the (101), (112), (114) and (103) planes, respectively. The HR-TEM profile of Fig. 2d discloses that lattice fringe of 0.254 nm is indexed to (−112) plane matched suitably to nano-ZnV₂S₄ with good crystallinity. The lattice fringe of the energy-dispersive spectrum analysis (Fig. 2e) demonstrates the presence of S, V and Zn along with the weight percentage of 56.6%, 28.8% and 14.9%, respectively, it is also additionally consistent in our following XRD and XPS findings.

The XRD technique is utilized to recognize its crystalline phase and the structural composition of cauliflower-like nano-ZnV₂S₄ as represented in Fig. 3a. The diffraction peaks which correspond to 2θ values of 17.43°, 34.86°, 44.6°, 52.03°, 63.52°, 66.02°, 72.8° and 75.10° are indexed and well-matched to the patters of (101), (112), (114), (103), (202), (008), (203) and (224) respectively. All the diffraction maxima could be indexed with the standard pattern of NiV₂S₄ (JCPDS No. 76–1058). In our case, Instead of Nickel peaks, the combination of ZnV₂S₄ is also well addressed by ZnS (JCPDS No. 79–2204). Further, it is asserted that the top layer of ZnV₂S₄ nanostructure contains some OH⁻, while its bulk is made fully nano-ZnV₂S₄. Therefore, it is competent to provide high pure ZnV₂S₄, and maintaining the particular ratio of reactant and solvent plays a key role to obtain a highly pure form of nano-ZnV₂S₄. Thus, XRD results confirm the successful substitution of ZnV₂(OH)₈ into cauliflower-like ZnV₂S₄ nanostructures. The nano-ZnV₂S₄ is further characterized by Raman spectroscopy (Fig. 3b). the prominent peaks detected at 259.75 cm⁻¹ and 379.4 cm⁻¹ are corresponds to the in-plane E_1g and A_1g vibration modes which strongly belongs to VS₄, correspondingly [50]. the band wave located at 467.2 cm⁻¹ can be attributed to the sulfur bond or their combination [51]. The Zn-S shows its characteristic phonon modes with high intensity at 797 cm⁻¹. the weak band waves of nano-ZnV₂S₄ located at 125.58 cm⁻¹ (stretching and bending vibrations of V–S bonds), 865.18 cm⁻¹ (V-S-V bond vibration), and 914 cm⁻¹ (vibration of V=S bonds) [52].

The nano-ZnV₂S₄ valance states and their chemical structure are analyzed by XPS analysis (Fig. 4a). The full spectrum survey indicates that synthesized cauliflower-like nano-ZnV₂S₄ contains V, Zn and S. All the values of the peaks are perfectly associated with V 2p, Zn 2p and S 2p, respectively. Through peak deconvolution of the V 2p spectrum (Fig. 4b), the occurrence of vanadium during mixed valance state of V⁴⁺ and V³⁺ is confirmed, which is more beneficial for the Zn-ion storage mechanism [53]. The spin-orbit distance between 2p_3/2 and 2p_1/2 peaks for V⁴⁺ and V³⁺ is measured to be 513.42, 517.46, 520.96 and 524.71 eV, which correspond to the previously reported values [5,54]. Fig. 4c Meanwhile the S 2p core level spectrum characteristic of the sulfide S²⁻ ligand shows a doublet with S 2p_2/3 and S 2p_1/2 binding energy of 160.58 and 164.15 eV [41]. As shown in Fig. 4d, it is clear that zinc ions are present in the bivalent state with two distinctive peaks at 1021.49 and 1043.7 eV, respectively [55].

To understand the electrochemical behavior of nano-ZnV₂S₄ cathode material for zinc ion battery was examined in an aqueous 2 mol/L ZnSO₄ electrolyte solution. The fabricated CR2032 coin cell is initially tested using cyclic voltammetry (CV) at 2 mV/s of scan rate in the fixed potential range of 0.3–1.7 V versus Zn²⁺/Zn. The resultant CV profile is substantially overlapped and highly reversible (Fig. 5a). It demonstrates the great reversibility of ZnV₂S₄. On both cathodic and anodic sweeps, two significant redox peaks are seen, corresponding to Zn²⁺ intercalation/de-intercalation of ZnV₂S₄. The cathodic/anodic sweep reflects the two pairs of redox couple at 0.823/0.49 and 1.17/0.75 V representing the V^4+/V³⁺ couple. This vanadium multiphase redox couple often produces electrochemical performance in multistep intercalation/de-intercalation with different charge carriers for bimetallic vanadium-based oxides and sulfide materials [56]. The electrochemical reaction kinetics of ZnV₂S₄ cathode are explored at various scan rates ranging from 0.25 mV/s to 5 mV/s (Fig. 5b). It indicates that the CV curves increasingly widen the peaks and increased the peaks current while a raise in scan speeds further. This suggests the predominant contribution by diffusion reaction and a considerable capacitive contribution.

Fig. 5c implies that log(i) vs. log(v) plot for nano-ZnV₂S₄. Thus, coordination between these peak currents (i) and the following sweep rate (ν) in the all-CV curve tested at various scan rates reflects the charge storage reaction of cauliflower-like ZnV₂S₄.

From Eqs. 5 and 6 [5], the intended b values of the corresponded peaks R1 and R2 were determined to be 0.89 and 0.92, respectively for discharge; and for the peaks O1 and O2, during charge, the b values are found to be 0.86 and 0.94, respectively. Thus, the ratio of capacitive and diffusion contribution of nano-ZnV₂S₄ at different sweep rates is shown in Fig. 5d. Surface-controlled capacitance progressively increases as the sweep rate increases, it designates that the electrode is more surface-controlled capacitance rather than diffusion-controlled capacitance. Further, it is also enduring the cauliflower-like ZnV₂S₄ to attain high-rate capability for the Zn²⁺ ion storage.

In elucidatory with the miscellaneous electrochemical behavior of ZIB, to evaluate the material's capability via half-cell configuration by using nano-ZnV₂S₄ as electrode material and a standardized anode as zinc plate. The galvanostatic charge/discharge profile comparison of Zn//ZnV₂S₄ at different current densities ranging from 0.5 A/g to 8 A/g is shown in Fig. 6a. Clear charge/discharge slopes and plateaus perfectly correspond to their redox mechanism on its subsequent CV curves, which is directly related to its unique morphology. The interconnecting dark spots inside each cauliflower-like nanostructured ZnV₂S₄ provide numerous spacer junctions that allow easy distribution of the electrolyte towards reducing the diffusion resistance of the electrolyte. Further, a prominent discharge specific capacity of 348.2 mAh/g during 0.5 A/g is attained due to their improved penetration of more Zn²⁺ ions into pores during the charging-discharging process.

In the charge/discharge process, the Zn²⁺ ions of the host electrolyte solution enclosures onto the cathode, and the Zn anode drops electrons to yield Zn²⁺ that diffuses back to the electrolyte to keep up the charge steadiness of the electrolyte. During its irreversible process, the movement of Zn²⁺ passes from the cathode towards the electrolyte, and the Zn²⁺ in the obtained electrolyte tends the electrons in friction to deposit on the Zn anode.

Furthermore, the cauliflower-like ZnV₂S₄-based AZIB upholds extreme durability of 70.6% specific capacity retention even after it is augmented till the 16-fold increase of its current density, it indicates a steady reaction with less ionic polarization due to their dominancy and interface engineering on the surface-based conveyance. The high remarkable rate capability is strongly attributed to the novel cauliflower-like structured ZnV₂S₄-based cathode material contributed to a better charge transfer rate and curtails the ion transfer pathway (Fig. 6b). It attains a linear specific discharge capacity of 348.2, 307.6, 284.71, 265.93 and 246.04 mAh/g at 0.5, 1, 2, 4 and 8 A/g, respectively. Overall, this superior rate capability of ZnV₂S₄ is compared with the previously reported bimetallic transition metal oxides/sulfides based AZIBs electrode (Table S1 in Supporting information).

The long-term cycling performance of the Zn//ZnV₂S₄ battery is explored and shown in Fig. 6c. When the cycling performance begins at their initial stage, the electrode seems to be fully activated and tends to linear specific capacity drop occurs from 265 mAh/g very slowly and maintains the electrode steady with accomplishing 89.2% capacity retention for 4 A/g high current density even after 1000 cycles. Fig. 7a represents the Zinc storage mechanism in the ZnV₂S₄ electrode. It is evident by the diminished resistivity and enhanced reaction kinetics of ZnV₂S₄ and finally, it can facilitate effectual electron transportation and more diffusion pathways for the Zn²⁺ storage mechanism. In addition, the energy density of this battery is 361.09 Wh/kg at a power density of 288.6 W/kg. Those power and energy densities are counted and compared across state-of-the-art power sources (Fig. 7b), in which one can see that the ZnV₂S₄-based ZIB system delivers superior electrochemical performance [37-39]. Furtherly, fewer electronegative sulfur ions with more active bonds result in a strong adhesion between other metal ions. The larger size and polarizability of 3p orbital of sulfur is compared to 2p orbital of oxygen reduces the structural strain formed during ion insertion, because the stain developed in the bigger 3p orbital is lower, thereby causing better cycling stability [57]. Hence, the formation nature of scaffolding nanostructured framework is widely reported in several ion batteries and proved to be one of the essential methods in mitigating their volumetric energy changes during the charge/discharge cycle. These outcomes indicate a high retainability of nanostructure even after repeated long cycling stability as previously reported [58-64].

The electrochemical impedance spectra measurement (EIS) is accomplished to know further deeply to enhance the electrochemical performance of ZnV₂S₄ are projected in Fig. 8a. The Nyquist plots are likely similar to the well-compressed semicircle from the high-frequency range to the low-frequency range, a line tending at approximately 45° in the low-frequency range. The inset figure shows an equivalent circuit for the Nyquist plots of nano-ZnV₂S₄ and nano-ZnV₂(OH)₈, the compressed semicircle defines the charge transfer resistance (R_ct), and the inclined line is denoted as Warburg impedance (Z_W). In similar, R_s represent the ohmic resistance of the electrode, including the electrolyte and cell components. Where CPE and Z_w are the capacitance related to double layer and Warburg impedance, respectively [65]. Besides, a slope from the lower frequency range upsurges simultaneously shows enhanced ion diffusion kinetics. The electrode, ZnV₂S₄ shows the Rct value of 48.2 Ω with a significantly diminished charge-transfer resistance which is much lower than that of the ZnV₂(OH)₈ (63.7 Ω) electrode. Simultaneously, it indicates faster ion transfer during the electrode-electrolyte interaction. While the charge transfer resistance decreased, the kinetics of cauliflower-like nano-ZnV₂S₄ gets improved and thus empowers the penetration and fast conduct transport of electrolyte ions and support them for more diffusion pathways for zinc ions [66]. Fig. 8b shows an attempt to utilize the stored renewable solar energy in aqueous ZIBs has been prepared, from that the sustainable solar energy can be harvested for self-charging of the Zn//ZnV₂S₄ battery device, where the red lamp blinking indicates the charging process. In this, two aqueous ZIB devices are connected in series and can easily light a bright green lamp (3 W). Meanwhile, it also runs the commercial digital clock normally. The aqueous ZIB is an efficient alternative for commercially used sustainable energy storage devices that can look upon to practical energy demands making them more reliable in the portable/wearable electronic engineering field. Therefore, this work paves an innovative pathway for designing cauliflower-like nanostructured ZnV₂S₄ cathode material for aqueous ZIBs.

In conclusion, the cauliflower-like nanostructured ZnV₂S₄ spinel structure has been proven to store massive Zn²⁺ ions during the charging-discharging process and is reported as the first for aqueous ZIB. This Zn//ZnV₂S₄ cell exhibits a highly stable discharge capacity (348.2 mAh/g during 0.5 A/g), superior rate capability, and excellent long cycling stability (~89.2%) even after 1000 cycles@4 A/g. Additionally, a good structural framework is retained even after repeated implantation of Zn²⁺ which is attributed to the intact spinal structured ZnV₂S₄ and its stable form even after certain phase transformation. These results pave a new pathway for the preparation of low-valent bimetallic vanadium-based sulfide materials by a unique anion-exchange reaction mechanism with superior electrochemical performance for aqueous zinc-ion batteries.

The authors declare that they have no conflict of interest.

The authors acknowledge the funding for this project through the National Nature Science Foundations of China (No. 51873083), Jasmine Jiangsu Fellowship of Jiangsu Province (No. 180511800007), and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (No. SJCX20_1453).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.108076.