**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+ [1–5]. 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 [6–10]. 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 [13–17]. 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 [13,14,18–21]. 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 [1,24–26].

**Citation:** Guo, K.; Cheng, W.; Liu, H.; She, W.; Wan, Y.; Wang, H.; Li, H.; Li, Z.; Zhong, X.; Ouyang, J.; et al. Sn-Doped Hydrated V2O5 Cathode Material with Enhanced Rate and Cycling Properties for Zinc-Ion Batteries. *Crystals* **2022**, *12*, 1617. https://doi.org/10.3390/ cryst12111617

Academic Editor: Sergio Brutti

Received: 7 October 2022 Accepted: 8 November 2022 Published: 11 November 2022

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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<sup>+</sup> and Ca2+) into V2O5 not only strongly modifies its crystal structure, but it also plays an important role in its electrochemical kinetics [28–30]. 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**
