Polypyrrole Wrapped V2O5 Nanowires Composite for Advanced Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (ZIBs) have obtained increasing attention owing to the high safety, material abundance, and environmental benignity. However, the development of cathode materials with high capacity and stable cyclability is still a challenge. Herein, the polypyrrole (PPy)-wrapped V2O5 nanowire (V2O5/PPy) composite was synthesized by a surface-initiated polymerization strategy, ascribing to the redox reaction between V2O5 and pyrrole. The introduction of PPy on the surface of V2O5 nanowires not only enhanced the electronic conductivity of the active materials but also reduced the V2O5 dissolution. As a result, the V2O5/PPy composite cathode exhibits a high specific capacity of 466 mAh g–1 at 0.1 A g–1 and a superior cycling stability with 95% capacity retention after 1000 cycles at a high current density of 5 A g–1. The superior electrochemical performance is ascribed to the large ratio of capacitive contribution (92% at 1 mV s–1) and a fast Zn2+ diffusion rate. This work presents a simple method for fabricating V2O5/PPy composite toward advanced ZIBs.


INTRODUCTION
The ever-increasing energy consumption, and limited fossil fuels, necessitates effective utilization of renewable energy resources . For that purpose, large-scale efficient energy storage systems are desired (Shao et al., 2021). Although lithium-ion battery has found widespread applicability, it suffers from safety issues caused by flammable organic electrolytes as well as the availability of Li source Lu et al., 2020). Aqueous zinc-ion batteries (ZIBs) are regarded as a suitable alternative for scalable energy storage systems, due to the usage of zinc metal anode which, apart from high abundance and environmental friendliness, has a large theoretical capacity (820 mAh g −1 ) and a low redox potential [−0.76 V vs. SHE Wang et al., 2020b;Zhang et al., 2019a). Furthermore, the possibility of an aqueous electrolyte endows an intrinsic non-flammability and high ionic conductivity (Wang et al., 2020a). However, corresponding ZIB cathode materials with high capacity and stable cyclability need to be further explored (Zhang et al., 2019b).
Manganese oxides (Khamsanga et al., 2019;, Prussian blue analogs Zampardi and La Mantia, 2020), vanadium-based compounds , and some organic materials (Wang et al., 2020c) have been investigated as cathode materials for aqueous ZIBs. Among those, vanadium-based materials, particularly vanadium oxides, are very attractive because of the advantage of high theoretical capacities due to multiple oxidation states of vanadium. Unfortunately, the electrochemical performance of vanadium oxides in ZIBs is hindered by their poor electronic conductivity and noticeable solubility in the electrolyte . To address these issues, various strategies have been applied, such as using pre-insertion materials (V 2 O 5 ·H 2 O) , integration with carbon materials (Yan et al., 2018), as well as electrolyte modifications (Wan et al., 2018). Another viable approach is to incorporate conducting polymers along with V 2 O 5 (Du et al., 2020). Polypyrrole (PPy) is a widely used conductive polymer, and V 2 O 5 /PPy composites have been shown to exhibit enhanced performance in supercapacitors and LIBs (Wang J.G. et al., 2018). Therefore, with regard to aqueous ZIBs, an effective PPy coating can aid in enhancing the electronic conductivity of V 2 O 5 as well as help to reduce the solubility in the electrolyte.
Herein, V 2 O 5 nanowires were synthesized by a facile hydrothermal method, and a surface-initiated polymerization method was utilized to fabricate a PPy-wrapped V 2 O 5 nanowire composite. V 2 O 5 served as the initiator to induce the polymerization reaction of pyrrole monomer at room temperature due to the strong oxidizing property of V 5+ . Benefiting from the improved electronic conductivity and restricted V 2 O 5 dissolution due to the PPy layer, V 2 O 5 /PPy cathode delivered a higher specific capacity and rate performance in comparison to the pristine V 2 O 5 nanowire cathode. Therefore, the V 2 O 5 /PPy composite is a promising high-performance cathode material for aqueous ZIBs toward large-scale energy storage applications.

EXPERIMENTAL SECTION
V 2 O 5 nanowires were synthesized by a facile hydrothermal method according to previously reported literature (Wang J.G. et al., 2018). 200 mg of obtained V 2 O 5 nanowires was dispersed into deionized water. Then, pyrrole (0.1 ml) dissolved in DMF (4 ml) was slowly added to the above V 2 O 5 nanowire suspended solution and stirred for 24 h. The obtained V 2 O 5 /PPy was washed carefully and dried in a vacuum oven.
More detailed synthesis and characterization processes are available in electronic Supplementary Information.

RESULTS AND DISCUSSION
V 2 O 5 nanowires were synthesized by the hydrothermal method. The as-obtained V 2 O 5 nanowires show a diameter of approximately 15 nm with a cable-like nanostructure (Supplementary Figure S1A). The V 2 O 5 /PPy composites were prepared using a surface-initiated polymerization strategy, as shown in Figure 1A. Owing to the strong oxidizing property of V 2 O 5 , the pyrrole monomer can be polymerized with V 2 O 5 initiation, resulting in the surface coating of V 2 O 5 with PPy. The morphology of V 2 O 5 nanowires was well-maintained after PPy coating, indicating that the wrapping procedure has no significant influence on the V 2 O 5 morphology ( Figure 1B). The TEM image also confirms the nanowire morphology of the V 2 O 5 /PPy composite ( Figure 1D). EDS elemental mappings show the homogeneous distribution of C, O, V, and N throughout the entire V 2 O 5 /PPy composite, indicating the presence of PPy ( Figure 1C and Supplementary Figure S1B).
The XRD data of the V 2 O 5 nanowires mainly fit with the layered orthorhombic structure (JCPDS no. 40-1296), and typical (001) and (003) reflection peaks are present ( Figure 1E). A little amount of V 4 O 7 was also indexed and may be assigned to the reduction of P123. The interlayer distance is estimated to be 0.96 nm by Bragg's law from the (001) peak. This large distance is beneficial for Zn 2+ insertion/extraction during the electrochemical process. After the PPy coating, no significant change is observed in the XRD data, indicating that the layered structure was well maintained after the polymerization process. In order to confirm the PPy coating and identify the valence state of vanadium in the V 2 O 5 /PPy composite, XPS was carried out. Figure 1F shows the survey spectrum with the clear presence of N 1s and C 1s, confirming the polymeric coating (see also Supplementary Figure S2). Figure 1G shows the V 2p spectrum, with strong V 2p3/2 and V 2p1/2 peaks of V 5+ located at 517.6 eV and 525 eV, along with shoulder peaks at 516 eV and 523.8 eV, corresponding to V 4+ . The presence of a small amount of V 4+ (9.3 at.%) corresponds to the oxygen vacancies generated in the V 2 O 5 surface due to the redox reaction between V 2 O 5 and pyrrole. Previous studies on V 2 O 5 demonstrated that such vacancies enhance the electrochemical performance (Liao et al., 2020).
The electrochemical performance of pristine V 2 O 5 and V 2 O 5 /PPy composites is evaluated in aqueous ZIBs. Figure 2A presents the rate capability of the pristine V 2 O 5 cathode and V 2 O 5 /PPy composite cathode. The V 2 O 5 /PPy composite cathode delivers a high initial capacity of 466 mAh g −1 at 0.1 A g −1 , as compared to the V 2 O 5 nanowire electrodes (425 mAh g −1 ). Even at a very high current density of 5.0 A g −1 , the V 2 O 5 /PPy composite cathode still possesses a higher discharge capacity of 174 mAh g −1 than that observed for the V 2 O 5 nanowire cathode (142 mAh g −1 ). The results point to the better rate performance of V 2 O 5 /PPy composite in comparison to non-modified V 2 O 5 nanowire electrodes. The voltage-capacity plots for the V 2 O 5 /PPy composite at different current rates demonstrate that the redox plateaus are well maintained even at a high current density of 5.0 A g −1 (Figure 2B). In comparing to voltage-capacity plots for pristine V 2 O 5 , the overpotentials are slightly lower suggesting improved kinetics due to the higher electrical conductivity of the composite (Supplementary Figure S3).
Based on the voltage profiles, the energy/power densities of the batteries were calculated and are shown in the Ragone plot ( Figure 2C). Impressively, the batteries base on the V 2 O 5 /PPy composite cathode display a high energy density of 235 Wh kg −1 at a power density of 56 W kg −1 and exhibit a relatively high energy density of 100 Wh kg −1 even at a high power density of 2335 W kg −1 . Moreover, the V 2 O 5 /PPy composite cathodes are highly competitive among the aqueous ZIBs based on the different cathodes: V 2 O 5 (Hu et al., 2017),    , Na 3 V 2 (PO 4 ) 3 (Li et al., 2016), heterogeneous vanadium oxide nanowire with V 2 O 5 ·nH 2 O shell and V 3 O 7 ·H 2 O core (h-VOW) , VS 2 (He et al., 2017b), Zn 3 V 2 O 7 (OH) 2 (Chao et al., 2018), and NaV 6 O 15 /V 2 O 5 (Lanlan et al., 2020). In addition, as shown in Figure 2D, the V 2 O 5 /PPy composite cathode exhibits a high initial capacity of 329 mAh g −1 at 1 A g −1 and a capacity retention of 94% after 100 cycles, which is much higher than that of pristine V 2 O 5 cathodes (234 mAh g −1 , 82%). Furthermore, long-term cycling performance of the cathodes was evaluated, because it is a key feature for practical applications. Even after 1000 cycles, the batteries based on the V 2 O 5 /PPy composite cathode show a reversible capacity of 174 mAh g −1 with a capacity retention of 95%. In contract, pristine V 2 O 5 cathodes exhibit a poor cycling stability, with a specific capacity of only 93 mAh g −1 after 1000 cycles corresponding to a capacity retention of 62% ( Figure 2E). The strong capacity fading observed for pristine V 2 O 5 cathode is most probably be a result of V 2 O 5 dissolution during cycling, which is minimized with the PPy coating for the V 2 O 5 /PPy composite. Moreover, such a high cycling stability for the V 2 O 5 /PPy composite is better compared to the recently reported literature on aqueous ZIBs with vanadium oxide-based cathodes ( Table 1). The high rate performance and stable long cycle life of the V 2 O 5 /PPy composite cathode are ascribed to the introduction of a conductive polymer PPy layer, which not only increases the electronic conductivity but also reduces the dissolution of V 2 O 5 in the electrolyte. The electrochemical kinetics of the V 2 O 5 /PPy composite cathode was further investigated to understand the impressive performance. Cyclic voltammetry (CV) was performed at various scan rates from 0.1 to 1.0 mV s −1 (Figure 3A). The CV curves show similar redox peaks in the voltage window of 0.3-1.6 V. The characteristic peaks appeared at 0.5/0.7 V as well as 0.8/1.0 V, reflecting the redox processes in V 2 O 5 that is consistent with reported literature (Yang Y. et al., 2018;Zhang et al., 2018). The capacity is regarded to be originated from two contributed parts: a surface-controlled capacitive part and a diffusion-induced part, as described in the literature (Ming et al., 2018): (1) In this equation, v is the scan rate, and a and b refer to adjustable parameters. The b values range from 0.5 to 1.0. Corresponding to b = 0.5, the observed capacity is fully diffusioninduced. When the capacity is completely determined by a surface-controlled capacitive part, the b value is close to 1.0. The peak currents at different scan rates are plotted and fitted with a linear function (Figure 3B). The b values are 0.51, 0.75, 0.61, and 0.64, which implies that the capacity of the V 2 O 5 /PPy composite cathode is simultaneously influenced by both the capacitive and diffusion processes. Furthermore, the capacity is divided as a capacitive-controlled part (k 1 v) and diffusion-induced part (k 2 v 1/2 ) as described by the following equations: The ratios of surface-controlled capacitive and diffusioninduced parts with various scan rates are displayed in Figure 3C. The surface-controlled capacitive contribution ratio increases from 57% (0.1 mV s −1 ) to 92% (1.0 mV s −1 ), indicating that the batteries possess fast charge-transfer kinetics. The kinetics of the V 2 O 5 /PPy composite cathode is further evaluated by galvanostatic intermittent titration technique (GITT). The profiles in GITT curves of V 2 O 5 /PPy electrode are well in coincidence with the galvanostatic charge-discharge profiles ( Figure 3D). The zinc-ion diffusion coefficient during discharging-charging procedures for V 2 O 5 /PPy is 3.03 × 10 −9 -1.46 × 10 −10 cm 2 S −1 (Figure 3E), which is comparable to that of the reported aqueous ZIBs based on the V 2 O 5 @CNT composite and porous V 2 O 5 nanofiber cathodes .

CONCLUSION
In this work, a surface-initiated polymerization strategy was utilized to synthesize PPy-wrapped V 2 O 5 nanowires. Owing to the strong oxidizing property of V 5+ , the polymerization of the pyrrole monomer could be initiated at room temperature. Due to the introduction of the conductive PPy layer, the V 2 O 5 /PPy cathode displayed a superior specific capacity and excellent cycling stability. The outstanding electrochemical properties are explained by the large ratio of a capacitive-controlled process (92% at 1 mV s −1 ) and a fast zinc ion diffusion coefficient. Considering the excellent electrochemical performance, coupled with the safe and simple operation process of aqueous ZIBs, the V 2 O 5 /PPy composite cathode holds great promise for practical grid-level storage applications.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.