English

• Due to the increasing resource consumption and environmental pollution, it is urgent to develop clean and safe energy storage devices [1,2]. In recent years, aqueous zinc ion batteries (AZIBs) with features of abundant resource, low cost, high safety, and environmental friendliness, have shown a great potential to substitute commercial lithium ion batteries in the field of large-scale energy storage [3-5]. However, the practical application of AZIBs has been hindered by the lack of high-performance cathode materials [6]. As a result, several kinds of cathodes including Mn-based oxides [7,8], V-based compounds [9,10], Prussian blue analogues [11], polyanionic compounds [12], and sulphides [13], have been explored as cathodes for AZIBs. It is worth noting that most of these cathode materials do not perform well enough in terms of cycle performance, rate capability, and/or discharge capacity [14,15]. Particularly, open-framework layered-type V-based materials consisting of zinc vanadate [16], sodium vanadate [17], potassium vanadate [18], and ammonium vanadate [19], have been extensively studied on account of their tunable interlayer spacing and multistep redox reactions. Among them, the family of ammonium vanadates has been considered as promising cathodes for AZIBs due to the inherent merits of light weight resulting in higher gravimetric capacity, and robust hydrogen bonds (N—H…O) between NH₄⁺ and V-O layer contributing to better structural stability. Furthermore, the reversible insertion/extraction of Zn²⁺ (0.76 Å) with a smaller ionic radius than that of NH₄⁺ (1.43 Å) supporting the V-O layers can be achievable during the charge-discharge process, enabling the fast diffusion of Zn²⁺ ions.

  Currently, the hydrothermal method as an effective and facile approach has been primarily implemented to prepare ammonium vanadates [20], displaying distinct advantages of low reaction temperature, easily controllable morphology, and exemption from further calcination. In spite of some virtues brought by the hydrothermal technique using water as the solvent, the resultant products yet suffer from a time-consuming process and large particle size owing to the small viscosity, high fluidity, and large interface tension of water which can lead to difficult control of reactant diffusion in different directions [21]. To tackle these issues, several studies have shown that adding viscous organic-solvent ethylene glycol (EG) [22-24] into an aqueous solution is favorable to increasing the viscosity of the mixed solution, lowering the solubility of precursors, and controlling the nucleation and growth of grain to obtain the desired particle size distribution, morphology and electrochemical performance as well as shortened reaction time. For example, the prepared uniform rice grain-like LiFePO₄/C composites after introducing EG during the synthetic process could exhibit an initial discharge capacity of 155 mAh/g at a current density of 0.1 C as well as maintain 91.6% capacity retention after 100 cycles [21]. In addition, Guo and cooperators have revealed that EG adsorbing on the specific crystalline plane could drive the crystal growth in a definite direction, ultimately endowing the resulting LiFePO₄ nanoplates with the special morphology for facilitating the diffusion of lithium ions [25]. Accordingly, utilizing EG as a viscous organic solvent to construct the anticipant ammonium vanadate cathodes for AZIBs may be an ingenious and feasible route to address such aforementioned shortcomings.

  More importantly, EG containing multi‑hydroxyl groups can also act as a nontoxic, pollution-free, and low-cost reducing agent to obtain the specific crystalline phase under the conditions of high temperature and saturated vapor pressure in a sealed reactor. Inspired by these intriguing features of EG, it can be rationally inferred that the species of vanadium ions with multiple changeable valence states in ammonium vanadate compounds can be modulated by EG during the hydrothermal reaction, thus obtaining various ammonium vanadates. Moreover, for all we know, there have been few reports on the synthesis of ammonium vanadate compounds by EG regulating.

  Thereby, in this work, for the first time, we synthesized various ammonium vanadate compounds with different morphologies and crystalline structures by an EG-regulated facile hydrothermal route combining polyol reduction strategy with the solvent effect of EG. The influences of EG addition amount on the synthesis of ammonium vanadates were systematically discussed. Compared with other products, the synthesized rod-like NH₄V₄O₁₀ cathode material can deliver a relatively high specific capacity of up to 427.5 mAh/g at a current density of 0.1 A/g and achieve excellent cycling stability over 5000 cycles even at 10 A/g. The investigation of the electrochemical mechanism gives further insight into the optimal performances based on the stable layered structure and larger (001) interplanar spacing. Such a facile and cost-effective strategy could light the practical application of AZIBs.

  Firstly, a series of ammonium vanadates, named as EG-0, EG-1, EG-1.5, EG-2.5 and EG-15, were synthesized using a modified hydrothermal approach by adding different amounts of EG (0, 1, 1.5, 2.5 and 15 mL, respectively), as described in Fig. 1a and Experimental section in Supporting information. Interestingly, the crystalline phases of obtained products are sharply distinguishable, indicating the significant role of reduction ability of EG in the hydrothermal reaction. Observation from XRD patterns (Fig. 1b) clearly displays that pure NH₄V₄O₁₀ could be synthesized while adding a proper amount (~1 mL) of EG. By contrast, the hydrothermal product without adding EG could be synthesized by simply converting V₂O₅ and NH₃⋅H₂O into pure (NH₄)₂V₆O₁₆. Moreover, it can be found that the valence state of V has also been changed. As depicted in the XPS spectra of V 2p (Fig. 1c), these peaks appearing at about 517.4 eV (V 2p_3/2) and 524.7 eV (V 2p_1/2) for (NH₄)₂V₆O₁₆ are attributed to V⁵⁺ [26,27]. However, the deconvoluted lower-energy bands of V⁴⁺ are ever-strengthening along with the increase of EG addition, indicating that more V⁵⁺ could be reduced to V⁴⁺by EG, in line with the XRD results which show the growing intensity of characteristic peaks belonged to (NH₄)₂V₄O₉ with the increase of EG from 1.5 to 2.5 mL. Specially, with the addition of sufficient EG (15 mL), sole crystallized (NH₄)₂V₄O₉ can be formed owing to the complete reduction of V⁵⁺ ions into V⁴⁺ ions by EG additive (Figs. 1b and c). Additionally, it is worth noting that the small amounts of V⁵⁺ in EG-15 can be ascribed to the V⁴⁺ oxidation when exposed to air [28].

  Figure 1

  

  Figure 1.  (a) Synthetic procedure of ammonium vanadates. (b) XRD patterns and (c) V 2p XPS spectra of various samples.

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  Importantly, NH₄V₄O₁₀ composed of VO₆ octahedra has exclusive single-connected oxygen atoms on the V-O layers which could afford strong interactions with NH₄⁺ ions acting as strong “pillars” to strengthen the structure stability [29]. Nevertheless, (NH₄)₂V₆O₁₆ consisting of VO₅ square pyramids and VO₆ octahedra [30] as well as (NH₄)₂V₄O₉ built up of VO₄ tetrahedra and VO₅ square pyramids [31] own the undesired tri-connected oxygen atoms in the layered structure, which presents weak interactions with NH₄⁺ ions to result in an unstable structure. Furthermore, NH₄V₄O₁₀ is endowed with larger (001) interplanar spacing of 9.6 Å than those of (NH₄)₂V₆O₁₆ (7.8 Å) and (NH₄)₂V₄O₉ (8.9 Å), being conducive to fast Zn²⁺ diffusion and vast Zn²⁺ storage sites for achievements of superior rate capability and high discharge capacity.

  The morphology and microstructure characteristics of various samples were investigated by SEM and TEM observations. As exhibited in Figs. 2a-g and Fig. S1 (Supporting information), EG-0 has a dendritic structure (Fig. 2a), while EG-1 shows a defined rod-like structure with a diameter of about 120–160 nm (Figs. 2b, f and g), demonstrating the evident impact of EG addition in terms of preferential growth of ammonium vanadate compounds. The lamellar structure could be evolved apparently as increasing EG (Figs. 2c and d). In particular, the EG-15 sample shows a thorough lamellar morphology (Fig. 2e). The morphology evolution mechanism could be reasonably elucidated that EG as a viscous organic solvent cannot only regulate the nucleation and growth of grains, as well as ameliorate the size and homogeneity of crystals [32], but also increase the solution viscosity, decrease the precursor solubility and ion diffusion rate in the solution [33]. Thus, excessive EG would seriously slow the axial growth rate, resulting in the dendritic structure evolving into a lamellar structure. The substantial gaps originating from rod stacking of EG-1 not only facilitate electrolyte penetration to increase the contact area between electrolyte and NH₄V₄O₁₀ electrode, but also provide abundant active sites for zinc ion storage [34,35]. Furthermore, HRTEM images (Fig. 2h) show that the lattice spacing of 0.348 nm matches well with the (110) facet of monoclinic NH₄V₄O₁₀. In addition, EDS results (Fig. 2i) display that N, V, and O elements are uniformly distributed throughout the whole rod of EG-1.

  Figure 2

  

  Figure 2.  SEM images of (a) EG-0, (b) EG-1, (c) EG-1.5, (d) EG-2.5, and (e) EG-15. (f) SEM, (g) TEM, (h) HRTEM images, and (i) elemental mappings of EG-1.

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  To explore the electrochemical properties of these ammonium vanadates as cathodes, aqueous Zn//ammonium vanadates cells were assembled. Fig. 3a shows the rate capabilities of five samples at various current densities from 0.1 A/g to 5 A/g. Due to the larger ion diffusion channel, EG-1 can deliver higher discharge capacities (427.5, 399.2, 362.1, 336.4, 307.2 and 260.6 mAh/g at 0.1, 0.2, 0.5, 1, 2 and 5 A/g, respectively) than the remaining four samples, comparable to or even better than other reported cathodes for AZIBs listed in Table S1 (Supporting information). Besides, the specific capacity of EG-1 with a more stable crystal structure could still recover to 390.2 mAh/g as the current density returns to 0.1 A/g, exhibiting a high recovery rate of 91.3%. The corresponding GCD curves in Fig. 3b also confirm the excellent electrochemical performance. In spite of capacity fading with the increase of current density, all the GCD curves still have a similar shape and possess multiple voltage plateaus, suggesting that the insertion/extraction process of Zn²⁺ concerns a highly reversible multi-step electrochemical reaction.

  Figure 3

  

  Figure 3.  (a) Rate performances, (b) charge/discharge curves for EG-1. (c) Cycling stability at 2 A/g. Cycling stability at current densities of (d) 5 A/g and (e) 10 A/g for EG-1.

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  What is more, Fig. 3c and Fig. S2 (Supporting information) exhibit the comparisons of cycling performances and corresponding GCD curves at the current densities from 0.1 A/g to 2 A/g. Visibly, the samples with adding EG reveal more excellent cycling stability than EG-0. Intriguingly, the initial discharge capacity of EG-1 can reach as high as 378.7 mAh/g at 0.5 A/g and still maintain a capacity of 361.2 mAh/g with a retention rate of 95.4% after 50 cycles, better than those of EG-1.5 and EG-2.5. Additionally, the maximum discharge capacity of EG-1 at 2 A/g reaches a higher 372.8 mAh/g than those of EG-0, EG-1.5, EG-2.5, and EG-15. Even after 500 cycles, EG-1 can still retain a discharge capacity of 340.2 mAh/g. The outstanding performances of EG-1 could be attributed to the existing gap structure of stacked rods affording sufficient solid/liquid contact as well as plentiful active sites. Moreover, it is found that EG-1 requires fewer than 40 cycles to reach the maximum specific capacity compared with 55 cycles for EG-1.5 and 60 cycles for EG-2.5, manifesting the faster activation process of EG-1 [36-38]. The fastest activation process for EG-1 can be ascribed to the favorable rod-like morphology affording abundant active sites and convenient diffusion channels for Zn²⁺ insertion/extraction, enabling the achievement of reduced polarization and enhanced kinetics. Excitingly, EG-1 benefiting the structure stability from its exclusive single-connected oxygen atoms on the V-O layers can also keep a capacity retention of 85.2% after 2000 cycles at 5 A/g, and even retains an ultra-high specific capacity of 191.4 mAh/g with 90.4% retention after undergoing 5000 loops at 10 A/g (Figs. 3d and e).

  To dig a deep insight into the evolution of crystal structure for EG-1 during the electrochemical process, the ex-situ XRD tests were performed at different charge/discharge voltages in the 2^nd cycle at 0.1 A/g (Fig. S3a in Supporting information). As shown in the corresponding XRD patterns (Figs. S3b and c in Supporting information), the diffraction peak at about 9.2° assigned to the (001) facet of NH₄V₄O₁₀ shows a significant swing due to the repeated insertion/extraction of Zn²⁺ between V-O layers [39], indicating the contraction/expansion of layer spacing which mainly arises from the variable electrostatic repulsion between O²⁻ ions in the adjacent layers [20,29,40]. The ex-situ XPS spectra of Zn 2p and O 1s for EG-1 at different states have been exhibited in Fig. S4 (Supporting information). When firstly discharged to 0.3 V and then charged to 1.6 V, the initial enhancement and subsequent attenuation of Zn²⁺ signal further confirm the Zn²⁺ insertion/extraction in V-O layers. The significantly strengthened peak of crystal water after discharging to 0.3 V implies that H₂O molecules are also intercalated into the V-O layers. Furthermore, HRTEM images of EG-1 after charging and discharging (Fig. S5 in Supporting information) also reveal that the corresponding interlayer spacing of the (110) crystal plane is 0.347 nm when discharged to 0.3 V, but achieves 0.352 nm while charged to 1.6 V. Hence, the structural evolution mechanism of NH₄V₄O₁₀ (Fig. S6 in Supporting information) can be proposed that Zn²⁺ and H₂O would enter into V-O layers to form Zn_mNH₄V₄O₁₀⋅xH₂O and Zn₃(OH)₂V₂O₇⋅2H₂O (JCPDS No. 50–0570) along with the gradual voltage drop, whereas strip out during the gradual increase of voltage accompanying with Zn₃(OH)₂V₂O₇⋅2H₂O disappearing to obtain Zn_nNH₄V₄O₁₀⋅yH₂O [16,29,41]. During the discharging process, the presence of Zn₃(OH)₂V₂O₇⋅2H₂O byproduct is probably ascribed to the side reactions of V-O layers with Zn²⁺ ions, co-intercalated H₂O molecules and OH⁻groups in aqueous electrolyte [42].

  The merits originated from the interesting structural evolution of NH₄V₄O₁₀ during the charge/discharge process provoke the motivation for exploring its electrochemical properties as cathode material for AZIBs. Fig. 4a displays the CV curves of EG-1 within the voltage window from 0.3 V to 1.6 V at a scan rate of 0.1 mV/s, where the redox peaks near 1.12/0.96 and 0.56/0.43 V correspond to the redox reactions of V⁵⁺/V⁴⁺ and V⁴⁺/V³⁺ [43]. In addition, the redox peaks at 1.43/1.32 V represent a reversible electrochemical reaction for the formation and decomposition of Zn₃(OH)₂V₂O₇⋅2H₂O [44], which can be also confirmed by the emergence and vanish of Zn₃(OH)₂V₂O₇⋅2H₂O phase when the pristine EG-1 is firstly discharged to 0.3 V and then charged to 1.6 V (see the ex-situ XRD patterns in Fig. S3b). The reduction peak that appears at near 0.8 V is associated with the entry of H₂O into V-O layers [45]. It is noteworthy that the potential differences between each pair of redox peaks are small as well as the same-type peak areas are very close, demonstrating the excellent electrochemical stability. Moreover, all CV curves are highly overlapping except the first cycle, hinting at good reversibility of Zn²⁺ insertion/extraction [46]. In addition, several platforms exhibited in the GCD curves (Fig. S7a in Supporting information) can also verify the multiple insertion/deinsertion process of Zn²⁺.

  Figure 4

  

  Figure 4.  (a) CV curves at a scan rate of 0.1 mV/s, (b) CV curves at different scan rates, (c) plots of log(i) versus log(v), (d) capacitive contributions at different scan rates for EG-1. (e) Nyquist plots after 50 cycles (inset is the equivalent circuit model). (f) GITT curves and ion diffusion coefficients of EG-1.

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  To further investigate the Zn²⁺ion diffusion kinetics, CV tests were performed on all five samples at different scan rates from 0.1 mV/s to 1 mV/s. Observations from Fig. 4b and Figs. S8a, d, g and j (Supporting information) show the slight shift of the redox peaks due to polarization effect [47]. The similar shape of CV curves toward overall scan rates indicates the remarkable electrochemical stability for EG-1 during the insertion/extraction of Zn²⁺ [48]. Moreover, the zinc-storage behavior can be determined based on the measured current and scan rate via the equation: i = av^b, where i is the peak current, v represents the scan rate, as well as a and b are variable parameters. Generally, the b value ranges from 0.5 to 1.0, where b = 0.5 suggests a diffusion-controlled process while b = 1 represents a completely surface-controlled capacitive process. Depending on the formula: log(i) = blog(v) + log(a), the calculated b-values for EG-1 are 0.71, 1.01, 0.79 and 0.84, respectively (Fig. 4c). Thus, it illustrates that EG-1 has the dominated Zn²⁺ storage mechanism of capacitive behavior and displays an obvious advantage over other four samples (Figs. S8b, e, h and k in Supporting information). In addition, the capacitive contribution can be calculated by the equation: i = k₁v + k₂v^1/2, where k₁v means the surface capacitive contribution, and k₂v^1/2 represents the diffusion-controlled proportion. As shown in Fig. S7b (Supporting information), EG-1 has a capacitive contribution of up to 83.5% at a scan rate of 0.6 mV/s (light green shaded area). A more intuitive capacitance contribution is displayed in Fig. 4d, of which the capacitive contribution rate increases from 65.2 to 94.2% with increasing scan rate, much higher than those of EG-0, EG-1.5, EG-2.5 and EG-15 (Figs. S8c, f, i and m in Supporting information). To this end, the favorable capacitive behavior could endow EG-1 with excellent electrochemical properties, especially the high-rate capability [49], thereby greatly facilitating the reversible Zn²⁺ intercalation/deintercalation.

  Furthermore, electrochemical impedance spectroscopy (EIS) tests were applied to evaluate the electrode resistance of EG-1. As shown in Fig. 4e, EIS curves of all samples consist of a semicircle representing charge transfer resistance (R_ct) and a slope line referring to ion diffusion in the high and low frequency regions, respectively [50,51]. Evidently, the R_ct of EG-1 (53.3 Ω) is smaller than those of EG-0 (151.9 Ω), EG-1.5 (87.8 Ω), EG-2.5 (100.4 Ω), and EG-15 (80.7 Ω), indicating faster charge transfer properties. Meanwhile, Fig. S9 (Supporting information) also unfolds the fitting results for Z’-ω^−1/2 in the low frequency region, where the slope of EG-1 is lower than those of other samples, verifying the superior Zn²⁺diffusion ability for EG-1.

  Additionally, five samples were subjected to galvanostatic intermittent titration technique (GITT) measurements to further survey the corresponding Zn²⁺ diffusion coefficients (D_Zn²⁺) according to the equation [52]:

  (1)

  where M_B and m_B represent the molecular weight and mass of active material, respectively, V_M and S are the molar volume and surface area of active material, respectively. ΔE_s and ΔE_t stand for the change of steady-state voltage and total charge of cell voltage during the current pulse, respectively, and τ is the relaxation time. Fig. 4f shows the GITT curves and the corresponding D_Zn²⁺ under the test conditions in Fig. S10 (Supporting information). Accordingly, the calculated D_Zn²⁺ values of EG-1 during insertion and extraction have a relatively small difference and range from 1.48×10⁻⁹ cm²/s to 5.54×10⁻¹¹ cm²/s, higher than those of EG-0, EG-1.5, EG-2.5 and EG-15 ranging from 4.55×10⁻¹⁰ cm²/s to 7.7 × 10⁻¹² cm²/s, 3.71×10⁻¹⁰ cm²/s to 6.72×10⁻¹² cm²/s, 5.67×10⁻¹¹ cm²/s to 2.21×10⁻¹² cm²/s, and 3.61×10⁻¹⁰ cm²/s to 1.82×10⁻¹¹ cm²/s, respectively (Fig. S11 in Supporting information). Thereby, combing excellent capacitive contribution with lower R_ct, it could conclude that EG-1 can not only perform rapid ion diffusion, but also maintain high electrochemical stability during cycling.

  In summary, various ammonium vanadates have been successfully fabricated through a facile hydrothermal method by regulating the amount of EG. The reduction ability and solvent effect from EG make a crucial impact on the morphology, phase composition and electrochemical properties of the synthetic products. Significantly, NH₄V₄O₁₀ with adding 1 mL EG has been proven to be a desirable cathode material for AZIBs in view of its favorable open-framework layered structure with excellent stability and large (001) interplanar spacing as well as the peculiar rod-like morphology contributing to favorable zinc ion diffusion, thus exhibiting the high initial capacity (427.5 mAh/g at 0.1 A/g), superior rate behavior (discharge capacity of 260.6 mAh/g at 5 A/g), and long-periodic cycle life (90.4% capacity retention after 5000 cycles at 10 A/g). This work provides an important direction for rationally designing advanced cathode materials for AZIBs by tailoring the morphology and crystalline structure of vanadates based on EG-regulated polyol reduction strategy and solvent effect.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (No. 22005172), Natural Science Foundation of Sichuan Province (No. 2023NSFSC1124), Yunnan Fundamental Research Projects (No. 202201AU070151).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108572.

  
  
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  Figure 1  (a) Synthetic procedure of ammonium vanadates. (b) XRD patterns and (c) V 2p XPS spectra of various samples.

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  Figure 2  SEM images of (a) EG-0, (b) EG-1, (c) EG-1.5, (d) EG-2.5, and (e) EG-15. (f) SEM, (g) TEM, (h) HRTEM images, and (i) elemental mappings of EG-1.

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  Figure 3  (a) Rate performances, (b) charge/discharge curves for EG-1. (c) Cycling stability at 2 A/g. Cycling stability at current densities of (d) 5 A/g and (e) 10 A/g for EG-1.

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  Figure 4  (a) CV curves at a scan rate of 0.1 mV/s, (b) CV curves at different scan rates, (c) plots of log(i) versus log(v), (d) capacitive contributions at different scan rates for EG-1. (e) Nyquist plots after 50 cycles (inset is the equivalent circuit model). (f) GITT curves and ion diffusion coefficients of EG-1.

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