*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:

$$\mathcal{D} = \frac{4L^2}{\pi \tau} \left(\frac{\Delta E\_s}{\Delta E\_t}\right)^2 \tag{1}$$

where *t*, *τ*, and Δ*Es* 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]. Δ*Et* 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].

**Figure 1.** Characterization of SnVOH. (**a**) XRD patterns of SnVOH sample; (**b**) XPS spectra of (**b**) O1s and V 2p and (**c**) Sn 3d; (**d**) SEM of SnVOH sample; (**e**) EDS mapping of SnVOH sample; (**f**) TEM of SnVOH sample.

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.

**Figure 2.** Electrochemical performance of VOH and SnVOH. (**a**) C–V curves at 0.1 mV/s in the voltage range of 0.2–1.7 V vs. Zn2+/Zn, (**b**) corresponding GCD curves, (**c**) representative galvanostatic charge–discharge curves at different current densities, (**d**) rate capacities at current densities between 0.1 to 10 A/g, (**e**) cycling performance at 10 A/g and (**f**) cycling performance at 5 A/g for the VOH and SnVOH sample.

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]:

$$I = a\nu^b \tag{2}$$

which can be transformed to

$$
\log(i) = b \log(\nu) + \log(a) \tag{3}
$$

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 (*k*1*ν*) and diffusion-induced part (*k*2*ν*1/2) which are described by the following equations [40]:

$$i = k\_1 \nu + k\_2 \nu^{1/2} \tag{4}$$

**Figure 3.** Kinetics study of VOH and SnVOH. (**a**) C–V curves of SnVOH electrode at different scan rates, (**b**) log (current) vs. log (scan rate) plots of four peaks in C–V curves during the cycles, and (**c**) capacity contribution ratios of battery type capacity and capacitance, (**d**) the discharge/charge curves in GITT measurement of SnVOH, (**e**) corresponding diffusivity coefficient of Zn2+ in discharge and charge processes of SnVOH samples, and (**f**) electrochemical impedance profiles recorded for the VOH and SnVOH electrodes before electrochemical tests and after cycling at 5 A/g for 2500 times.

or

$$k/\nu^{1/2} = k\_1 \nu^{1/2} + k\_2 \tag{5}$$

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.
