Sn-Doped Hydrated V₂O₅ Cathode Material with Enhanced Rate and Cycling Properties for Zinc-Ion Batteries

Abstract

Water molecules and cations with mono, binary, and triple valences have been intercalated into V₂O₅ to significantly improve its electrochemical properties as a cathode material of zinc-ion batteries. Sn as a tetravalent element is supposed to interact aggressively with the V₂O₅ layer and have a significant impact on the electrochemical performance of V₂O₅. However, it has been rarely investigated as a pre-intercalated ion in previous works. Hence, it is intriguing and beneficial to develop water molecules and Sn co-doped V₂O₅ for zinc-ion batteries. Herein, Sn-doped hydrated V₂O₅ nanosheets were prepared by a one-step hydrothermal synthesis, and they demonstrated that they had a high specific capacity of 374 mAh/g at 100 mA/g. Meanwhile, they also showed an exceptional rate capability with 301 mAh/g even at a large current density of 10 A/g, while it was only 40 mAh/g for the pristine hydrated V₂O₅, and an excellent cycling life (87.2% after 2500 cycles at 5 A/g), which was far more than the 25% of the pure hydrated V₂O₅. The dramatic improvement of the rate and cycling performance is mainly attributed to the faster charge transfer kinetics and the enhanced crystalline framework. The remarkable electrochemical performance makes the Sn-doped hydrate V₂O₅ a potential cathode material for zinc-ion batteries.

1. Introduction

Aqueous zinc-ion batteries (ZIBs), as one of the candidates for next-generation rechargeable batteries, have attracted tremendous interest because their zinc metal anodes have some unique features, including a high theoretical capacity (819 mA h/g), a low redox potential (−0.76 V vs. SHE), a small radius (0.74 Å), and the two-electron reaction of Zn/Zn²⁺ [1,2,3,4,5]. Unfortunately, the relatively large radius (4.3 Å) of the hydrated Zn²⁺ ion in an aqueous electrolyte and a strong electrostatic interaction with the cathode host both add to a high energy barrier for its intercalation/deintercalation in the cathode materials, resulting in sluggish electrochemical kinetics, serious electrochemical polarization, as well as unsatisfied cycling and rate performances [6,7,8,9,10]. Therefore, it is crucial to design and develop suitable cathode materials for constructing high-performance ZIBs [3,11,12].

A variety of cathode materials have been investigated, such as manganese-based oxides, Prussian blue analogs, conducting polymers, and vanadium-based oxides, over the past few years [13,14,15,16,17]. Among these cathode materials, the vanadium-based oxides have been widely studied for ZIBs because of their multivalence, open skeleton structure, and high theoretical capacities [13,14,18,19,20,21]. Vanadium pentoxide (V₂O₅) is one of the promising materials due to its high theoretical capacity and layered structure with it having a large interspace [1,3,22,23]. However, V₂O₅ usually displays a low conductivity, a poor ion diffusion coefficient, a long activation process, and an unsatisfying cyclic stability [1,24,25,26].

Generally, the nanostructures with high specific surface areas and short ion diffusion paths are conducive to good Zn²⁺ diffusion rates and rapid electrochemical kinetics [27]. Additionally, recent research suggests that the pre-insertion of water molecules or foreign metal ions (e.g., Na⁺ and Ca²⁺) into V₂O₅ not only strongly modifies its crystal structure, but it also plays an important role in its electrochemical kinetics [28,29,30]. The water molecules intercalated in the V₂O₅ interlayers pillar its layered structure and effectively function like a “lubricant” to facilitate fast Zn²⁺ transport, significantly improving the rate and cycle performance of V₂O₅ [31].

As another choice, metal ions incorporated between the V₂O₅ layers may covert the crystal structure of V₂O₅ to a more stable tunnel framework or enlarge the interlayer spacing and strongly bond to the apical oxygens of the V₂O₅ layers to maintain the structural stability of V₂O₅, depending on radiuses and charges of the metal ions [7,32]. Cations with mono, binary, and triple valences, such as Na, K, Mg, Ca, Zn, Mn, and Fe, have already been studied [10,11,28,33]. Generally, multivalent metal ions with a higher charge density and stronger electrostatic interaction than those of the monovalent cations are beneficial to build a stronger bond with the vanadium oxide layers, resulting in a better structural stability and cycling performance [5]. Meanwhile, the strong electrostatic interaction between the V₂O₅ host and the foreign cations shields the interaction between the oxygen atom and Zn²⁺ and thus, it reduces the energy barrier of the Zn²⁺ diffusion inside V₂O₅, which is conducive to a better rate performance [27]. However, the charge numbers of the doped metal ions are no more than three. What will happen to the zinc-ion storage capability if alien ions with a charge number of more than three are hybridized with V₂O₅ is unsure. The element tin, which is commonly in a tetravalent state with ion Sn⁴⁺ and with a charge number of four, is believed to interact more strongly with the V₂O₅ layer than other previously reported elements do, and it will have a significant impact on the electrochemical performance of V₂O₅ if it is doping V₂O₅. However, it has been rarely investigated as a pre-intercalated ion in previous works. Hence, it is interesting and worthwhile to develop Sn-doped V₂O₅ cathodes for ZIBs and to clarify the role of the doped Sn element on the zinc-ion storage capability.

Herein, Sn-doped hydrated V₂O₅ was synthesized in a one-step hydrothermal method to realize a cathode material with a superior Zn-storage performance by a hydrothermal reaction. Compared with the pristine V₂O₅, the obtained SnVO displays larger interlayer spacing, a greatly improved rate performance, and a superior cycling stability. SnVO delivers a high reversible specific capacity of 374 mAh/g at a current density of 100 mA/g, retains 320 mAh/g at 5000 mA/g, and maintains 87.5% of its initial capacity after cycling at 2 A/g for 2000 times. The facile synthesis route and its significant electrochemical performance enhancement suggest that Sn doping is an effective strategy and Sn-doped hydrated V₂O₅ is a prospective cathode materials for zinc-ion batteries.

2. Experimental Section

2.1. Preparation of Sn Doped V₂O₅·nH₂O

Two mmol of V₂O₅ (Sinopharm Chemical, Shanghai, China) and 0.2 mmol of SnCl₄·5H₂O (Adamas-beta, Shanghai, China) were dissolved in a mixture of 20 mL of deionized water and 10 mL H₂O₂ (30 wt%, Sinopharm Chemical, Shanghai, China) by magnetic stirring at room temperature. Then, the solution was poured into the autoclave reactor and heated at 200 °C for 48 h. The resulting reactants were vacuum filtered, thoroughly washed with deionized water, and then, they were dried at 80 °C for 6 h to obtain a dark red, dry gel, which was labeled as SnVOH. For the comparison, a sample was synthesized following the same procedure without the addition of SnCl₄·5H₂O, and it was labeled as VOH.

2.2. Materials Characterizations

The crystalline structure of the samples was identified using an X’Pert 3 diffractometer (PANalytical, Almelo, The Netherlands) at the range from 5 to 80°. The morphologies and structure of the samples were investigated using a field emission scanning electron microscope (FESEM, HITACHI SU8200, Tokyo, Japan) equipped with an energy-dispersive X-ray (EDX) detector and transmission electron microscopy (TEM, FEI Talos S-FEG). X-ray photoelectron spectroscopy (XPS, PHI QUANTERA-II SXM) was used to identify the valence state of the samples.

2.3. Electrochemical Measurements

The obtained SnVOH or VOH were mixed with a carbon nanotube (CNT) solution (10 mg/mL, XFNANO, Nanjing, China) and then, it was filtered using a cellulose film (0.22 μm, Shanghai Xinya, Shanghai, China) to form freestanding composite films, for which the mass ratios of SnVOH or VOH were 70%. Additionally, the as-prepared composite films were cut into small pieces and used directly as the cathode to assemble the CR2025 button cells in the air with metallic zinc foil (100 μm thick) as the anode, 3 M zinc trifluoromethane sulfonate (Zn(CF₃SO₃)₂) solution as the electrolyte, and glass fiber as the separator. Cycle voltammetry (CV) was performed on the button cells using an electrochemical workstation (CHI 660E, CH Instruments, Inc., Bee Cave, TX, USA) within the potential range of 0.2–1.7 V vs. Zn/Zn²⁺. The electrochemical impedance spectra (EIS) were acquired with the same electrochemical workstation over the frequency range of 0.01–10⁵ Hz at a voltage of 5 mV. Additionally, galvanostatic charge–discharge (GCD) tests and galvanostatic intermittent titration technique (GITT) tests were carried out using a NEWARE 4000 system (Neware Technology Limited, Shenzhen, China) at room temperature. For the GITT test, the coin cell was charging/discharging for 20 min at 0.1 A/g with a 120 min relaxation duration. The solid diffusion coefficient was calculated according to the equation below:

$$\mathrm{D} =\frac{4L^2}{\pi \tau }{\left(\frac{∆E_s}{∆E_t}\right)}^2$$

(1)

where $t$, $\tau$, and $∆E_s$ represent the duration of the current pulse (s), the relaxation time (s), and the steady-state voltage change (V) that was induced by the current pulse, respectively [34]. $∆E_t$ is the voltage change (V) during the galvanostatic current pulse after eliminating the IR drop. $L$ is the ion diffusion length (cm) of the electrode, which was equal to the thickness of the composite film electrode (40.2 and 41.7 μm for the VOH and SnVOH composite film, respectively). The specific capacity, the energy density, and the power density were calculated based on the mass of active materials from the cathode.

3. Results and Discussion

The VOH and SnVOH were both synthesized via a facile hydrothermal reaction, as shown in Figure S1. The compositions of the as-prepared samples were characterized by XRD and XPS. No peaks related to tin metal or tin oxides were detected, and there are only six discrete peaks in the XRD pattern for both VOH and SnVOH, corresponding to the (00n) planes of a hydrated V₂O₅ phase (JCPDS NO. 40–1297) (Figure 1a), which is similar to previous research [35]. Additionally, there are slight blue shifts which occurred at the characteristic peaks of VOH after doping the Sn element, suggesting that the (00n) interlayer spacing of VOH was expanded through the Sn doping. The expanded interlayer spacing is commonly induced by the volume change after introducing doping ions to the interlayers of the V-O bilayers in the hydrated V₂O₅ [1,3]. According to the Bragg Equation in the Supporting Information file, the calculated interplanar distances of the (001) plane are d₀₀₁ = 14.91 and 14.81 Å for SnVOH and VOH, respectively. The small differences in the interplanar distance of two samples could be ascribed to a combination of two factors. On one hand, the intercalated Sn⁴⁺ can lead to an expanded interlayer spacing. On the other hand, the strong attraction between the intercalated Sn⁴⁺ ions and the adjacent V-O bilayers tends to bring the V-O bilayers close to or narrow to the interlayer spacing. These two factors counteract each other and they induce a small interplanar distance change [7,32]. The schematic crystalline structure of SnVOH is illustrated in Figure S2. In the XPS spectrum of SnVOH (Figure 1b), two pairs of characteristic peaks can be found in the V 2p spectra; the stronger peaks at 516.9 and 524.4 eV are ascribed to V⁵⁺, and the weaker peaks at 516.0 and 522.8 eV are from V⁴⁺, respectively [36]. The weak peaks from V⁴⁺ indicate a very small amount of V⁴⁺, which may be caused by the doping of Sn⁴⁺. The two O1s XPS peaks are deconvoluted into (V-O) lattice oxygen at 530.4 eV and a hydroxyl (V-OH) of defective oxygen at 532.1 eV, respectively. The peaks that can be observed in the Sn 3d spectrum belong to Sn⁴⁺ (Figure 1c), confirming the successful doping of the Sn element in the SnVOH [37].

SEM and TEM were performed to observe the morphology and structure of SnVOH. The SEM image (Figure 1d) shows that the SnVOH sample is composed of nanosheets with lateral sizes of tens of micrometers, and the VOH sample has a similar morphology with an even element distribution (Figure S3). The diagram in Figure 1e shows the uniform distribution of the V, O, and Sn elements. Additionally, the EDS elemental analysis result demonstrates that the molar ratio of the Sn element in the SnVOH accounts for 3.85%, as shown in Table S1. The TEM image of SnVOH in Figure 1f confirms the nanosheet morphology and demonstrates that the thickness of the nanosheets is in the range of 10–20 nanometers. The distribution of the elements Sn, V, and O in the SnVOH nanosheet was characterized by the EDS. The above analysis results suggest the Sn-doped V₂O₅ has been successfully synthesized.

To evaluate the zinc-ion storage performance, VOH and SnVOH were mixed with the CNT solution and then, they filtered to form free-standing composite films, which were cut into small pieces and used directly as the cathode. The VOH and SnVOH nanosheets were both evenly mixed with the CNTs (Figure S4a,c), and the composite films were layered at a thickness of 40.2 and 41.7 μm, respectively (Figure S4b,d). The coin cells were assembled using the VOH and SnVOH-based cathodes and tested in the voltage range of 0.2–1.7 V vs. Zn/Zn²⁺. There are two pairs of redox peaks in the CV profile of the VOH-based cathode, as shown in Figure S5a. Similarly, the CV profile of SnVOH in Figure 2a shows two oxidation peaks at 0.75 and 1.11 V along with two reduction peaks at 0.37 and 0.75 V, which correspond to the multi-step extraction/insertion of the Zn²⁺ ion in the framework of SnVOH, respectively [37,38]. The redox reactions are consistent with the plateaus of the initial three galvanostatic charge and the discharge curves of SnVOH at 0.1 A/g in Figure 2b. The GCD profiles of SnVOH in Figure 2c show the maximum discharge capacity is 387 mAh/g at a current density of 0.1 A/g. Additionally, its capacity retention is impressive, at 77.8%, when the current density increases from 0.1 to 10 A/g. The pristine VOH shows an average discharge capacity of 395 mAh/g at 0.1 A/g (Figure 2d), which is close to that of SnVOH. However, its rate performance is unsatisfactory by possessing a dramatically lower retention of 12.6% when the current density increased to 10 A/g. The average discharge capacities of 387, 365, 349, 348, 345, 330, 321, 309, and 301 mAh/g were recorded for SnVOH at the current densities of 0.1, 0.3, 0.5, 0.8, 1, 3, 5, 8, and 10 A/g, respectively (Figure 2d). SnVOH had higher capacities than VOH did at the current densities over 0.5 A/g, and the capacity advantage is larger at a higher current density. The significant improvement in the rate performance of SnVOH can be accounted for by the highly facilitated Zn²⁺ diffusion in the VOH framework after incorporating the Sn element. Not only is there a dramatically enhanced rate performance, but SnVOH also demonstrates an improved electrochemical stability at both the small and large current densities, as shown in Figure 2e,f, respectively. During the cycling test at 10 A/g for 500 cycles, the capacity of SnVOH goes through a slow increasing process during the initial 100 cycles and then, it remains stable in the following cycles, while the capacity of VOH decays rapidly in the first 10 cycles, then, it slowly increases in the following 200 cycles, and stays steady in the last 300 cycles. The capacity retentions for SnVOH and VOH are 135.4% and 76.1% after the cycling at 10 A/g for 500 times, respectively (Figure 2e). During the cycling at 5 A/g for 2500 cycles, the VOH cathode undergoes fast capacity fading, leading to a final capacity retention of 24%, which is much lower than the 71% of the SnVOH cathode in the same condition (Figure 2f). In addition, the rate and cycling performance of the Sn-doped hydrated V₂O₅ is superior to the results from the recent research on hydrated V₂O₅, as shown in Table S2. The rapid capacity decay of VOH may be mainly attributed to the unstable layered structure and the dissolution of VOH during repeated insertion/extraction of zinc ions. Additionally, the enhanced cycling performance of SnVOH reveals the outstanding structural stabilizer function of the doped Sn ions.

The high-rate performance and long-term stability of the SnVOH cathode are substantially controlled by the electrochemical kinetics, which were analyzed through the C–V curves at different scanning rates and during the GITT test, as shown in Figure 3. The C–V curves of SnVOH in Figure 3a maintain a similar shape with two pairs of charge and discharge peaks, while the reduction peaks and oxidation peaks shift to lower and higher voltages at increased scan rates, respectively. The peak current (i) and scan rate (ν) have a relationship that is defined by the following equations [39]:

I = aν^b

(2)

which can be transformed to

log(i) = b log(ν) + log(a)

(3)

where a and b are the variables. The b value ranges from 0.5 to 1.0, which indicates a different mechanism. A b value of one is indicative of the dominated contribution of the surface capacitance to the total capacity, and a b value of 0.5 exhibits diffusion-controlled charge storage [27]. In addition, the slope of the log (i) versus the log (ν) plot can be used to estimate the b value, as shown in Figure 3b,c. The b values of the peaks 1–4 for the SnVOH electrode are 0.97, 0.81, 0.93, and 0.89, (Figure 3b), which implies the SnVOH has considerable kinetics, and its charge storage mainly comes from surface capacitance, and it is slightly influenced by the diffusion process. By fitting the C–V curves at different scan rates in Figure S5b, the b values of the peaks 1–4 for the VOH electrode are 0.85, 0.86, 0.72, and 0.96 (Figure S5c), suggesting the capacity of the VOH cathode is also influenced by both the capacitive and diffusion processes, and it is dominated by the surface capacitive capacity. Furthermore, the capacity is divided as a capacitive-controlled part (k₁ν) and diffusion-induced part (k₂ν^1/2) which are described by the following equations [40]:

i = k₁ν + k₂ν^1/2

(4)

or

i/ν^1/2 = k₁ν^1/2 + k₂

(5)

The ratios of the surface-controlled capacitive and the diffusion-induced parts of SnVOH at various scan rates are displayed in Figure 3c. The surface-controlled capacitive contribution ratio increases from 75.7% (0.1 mV/s) to 83.4% (0.5 mV/s), indicating that the batteries possess fast charge-transfer kinetics. For VOH, the surface-controlled capacitive contribution ratio is 21.5% (0.1 mV/s), and this increases to 36.3% (0.5 mV/s), as presented in Figure S5d. The much higher surface-controlled capacitive contribution ratio for SnVOH implies that it has much faster kinetics when one is doping the Sn element, which may be ascribed to the effect of Sn⁴⁺ shielding the interaction between the oxygen atom and Zn²⁺ [41].

Then, the constant current intermittent titration technique (GITT) was used to determine the ion diffusion coefficient of Zn²⁺ in the VOH and SnVOH cathodes (Figures S5e and S3d). As demonstrated in Figure 3e, The diffusion coefficients of SnVOH are from 10^−8.9 to 10^−8.6/(cm² s) during the charge and discharge processes, respectively, which are higher than those of the VOH samples (10^−9.0 to 10^−8.8/(cm² s), in Figure S5f). The results confirm that Zn²⁺ migration in SnVOH is faster when it is compared to that in VOH. Thus, the much higher rate performance of SnVOH can be ascribed to a reduced interfacial impedance and an enhanced ion diffusion. Moreover, doping the Sn element largely reduces the charge transfer resistance from 300 to 100 Ω (Figure 3f), which is indicative of a reduced ion diffusion impedance between the interface of the cathode and electrolyte. After cycling at 5 A/g for 2500 times, the charge transfer resistance of SnVOH-based cell rises to about 580 Ω, which is still smaller than 880 Ω of VOH based cell. The results confirm that faster electrochemical kinetics can be achieved by doping the Sn element.

4. Conclusions

In summary, the Sn-doped hydrated V₂O₅ ZIB cathode materials were prepared by simple one-step hydrothermal synthesis. Compared to the undoped sample, the Sn-doped hydrated V₂O₅ demonstrates a significant enhancement in its rate performance and cyclic stability. SnVOH shows a high initial reversible capacity of 387 mAh/g at 0.1 A/g, an excellent rate capability with 301 mAh/g even at a large current density of 10 A/g, and its retains 87.5% of its initial capacity after the cycling at 2 A/g for 2000 times. The rate and cycling performance of Sn-doped hydrated V₂O₅ are superior to the results from recent research on hydrated V₂O₅. These great improvements can be due the smaller charge transfer resistance and the higher zinc diffusion coefficient that occur after the Sn doping. Therefore, this work reveals that Sn doping is an effective strategy to improve the zinc storage performance of hydrated V₂O₅, and the Sn-doped hydrated V₂O₅ is a promising cathode material candidate to construct ZIBs of a high specific capacity, an excellent rate performance, and a high durability.