Improved cycling performance of polypyrrole coated potassium trivanadate as an anode for aqueous rechargeable lithium batteries

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Abstract

The energy storage properties of layered metal vanadate, especially alkali metal vanadates have been extensively studied. Metal vanadates have a more robust electrochemical output in contrast with pristine vanadium oxides. However, the detailed processes underlying the efficiency contrast of vanadates and vanadium oxides have rarely been studied. Herein a facile hydrothermal and low-temperature polymerization method was introduced to synthesize KV3O8 and KV3O8@PPy nanowire bundles as anode material for an aqueous rechargeable Lithium batteries. The nanowires are composed of KV3O8·0.59H2O calculated using thermal gravimetric analysis (TGA). Successfully synthesized layered vanadium based KV3O8 0.59H2O (KVO) and KV3O8 0.59H2O@PPy (KVO@PPy) nanowires and investigated the source of the improved electrochemical efficiency of PPy coated potassium vanadates compared to pristine KVO using crystal structure analysis and electrochemical tests. We demonstrated increase in electrochemical stability for KVO@PPy caused by synergistic effect of K+ in vanadate nanowires and PPy coating. In KVO the oxygen atoms have close contact with the K ions, and the stable K+ serve as “pillars” between interlayers to shield the layered structures from collapse during the charge/discharge phase, while the PPy reduces charge transfer resistance. This research adds helps us design better electrode materials to be used as an anode material for ARLB using alkali metal vanadate.

Introduction

Lithium batteries (LIBs) have been one of the primary power sources in recent years, including grid-scale applications, due to their high energy capacity and long-term reliability. The use of non-aqueous flammable alkyl carbonate electrolytes in LIBs poses safety issues [1], [2], [3], [4]. New rechargeable batteries with an aqueous electrolyte may be able to fix this problem since the use of water as a solvent may ultimately result in more excellent safety over non-aqueous electrolytes. Aqueous rechargeable lithium batteries have a similar mechanism but suffer from poor electrochemical performance due to a lack of optimal-performing anode materials [5]. In that case, vanadium's flexibility in terms of valence states (from 3 to 5), coordination number (4 to 6), and coordination polyhedral forms (tetrahedron to octahedron with intermediate square pyramid) make for a wide range of structural forms. Vanadium oxides and bronzes have aroused interest due to their intriguing electrochemical behavior against lithium intercalation besides their structural versatility. The type and extent of systemic changes are understood to influence electrochemical behavior [6].

Nanostructured vanadium oxides and their derivative compounds have been widely proposed as electrode materials for Lithium batteries due to their high capacity and low cost [7], [8], [9], [10]. Much interest is centered around lithium tri-vanadate (LiV3O8), the most prominent member of this family [11], [12]. However, the electrochemical performance of LiV3O8 is highly dependent on preparation methods that influence its morphology, particle size and crystallinity [13]. This inspires researchers to use other cations, such as Na+ or K+ to replace Li+ or NH4+ to form new anode materials for Lithium batteries [14], [15], [16]. Early efforts have emphasized the significance of pillar effect throughout electrochemical reaction to stable the structures of electrode material in alkali vanadates such as AVxOy (A = Li+, Na+, and K+) [17], [18], [19] as well as their derivatives [19], [20], [21]. Hu et al. presented Li+ intercalation into Na2V6O16.1.63H2O for aqueous batteries [21]. Alfaruqi et al. demonstrated the viability of Li+ into vanadium structure and at 0.1C, delivering 275 mAh g−1 of discharge capacity [17]. Further, they expanded their research to include K2V6O16 2.7H2O. During the electrochemical reaction, the alkali ions and water molecules remained immobile, which improved structural integrity throughout continuous lithiation/delithiation. In our recent work, we introduced a new anode material of ammonium trivanadate. The one-step facile hydrothermal reaction was used for synthesis, which showed a maximum discharge capacity of 187.6 mAh g−1 [22]. It was found that the NH4+ group in NH4V3O8 could not be extracted from the host materials and possibly acted as a pillar to stabilize the crystal structure [12], [22]. While the potassium trivanadate compounds synthesis methods are widely used as an electrode for aqueous batteries [15], [23], [24]. However, they suffer from low or no capacity without modification. For example, Kim, Hee Jae, et al. [25] fabricated KV3O8 and used it as cathode for Aqueous Zinc-ion battery (AZIB) without any modification and electrode material displayed a very meager discharge capacity of 17 mAh g−1 at the first cycle for the Zn-ion battery using a simple approach of ball milling, improving the capacity by 249 mAh g−1. Menav et al. [24] employed a high-temperature solid-state reaction to prepare KV3O8. The electrode exhibited only an initial discharge capacity of less than 100 mAh g−1 and poor cycling performance. Much more work should be done for this kind of material.

Here, we report KVO@PPy as an anode material with the Li+ storage ability for ARLB and reveal reversible electrochemical activity. We use KV3O8 based on research previously reported, having ∼7.7 Å of interlayer spacing and the further optimized KVO@PPy, obtained by wrapping polypyrrole on the nanowire bundles of KVO as an electrode for ARLIBs. Since the K+(1.38 Å) have an ionic radius that is greater than that of Li+ (0.76 Å) and Na+ (1.02 Å), the presence of K+ and water molecules simultaneously may offer enhanced interlayer distances for the insertion of Li+ ions into the structure. As in Li cells, KV3O8 is known for having a low capacity (80 mAh g−1) and a high polarization, which might be due to morphology and dissolution of active material in an aqueous electrolyte. This strategy of reduction in particle size has been applied to optimize the nanowire-like structure by coating it with a conductive layer of PPy as anode for ARLB. Dramatically KVO@PPy exhibits a higher reversible capacity of approximately 93.7 mAh g−1 at 1 A g−1, when compared to pristine KVO. The TGA analysis suggests 11.8% of the polymer content inferred from weight loss, X-ray diffraction analysis revealed that both compounds showed similar and unchanged structure with addition of PPy. However, electrochemical study and galvanostatic charge–discharge performance showed significant improvement in the performance of PPy coated electrode which is highlighted in the result and discussion section. The existence of K+ and water ions among interlayers of KV3O8 as well as a conductive polymer coating is responsible for this stable cycling behavior. Which sustain the structure by acting as pillars and the conductive coating layer offer no resistance for continuous insertion/extraction of Li+ ions.

Section snippets

Material synthesis

Preparation of KV3O8 0.59H2O: The starting materials, V2O5 and KOH were directly used without any further treatment. In deionized water, 0.909 g V2O5 and 0.281 g KOH were first dissolved in order, then the mixed solution was moved to a 100 mL Teflon lined stainless steel autoclave after 3 hours of proper stirring. The autoclave was closed and heated at 180 °C for 48 hours before cooling naturally to room temperature. The precipitates are extracted and washed three times with deionized water.

Result and discussion

Fig. 1 illustrates the synthesis KVO by the facile hydrothermal method. After achieving the nanowire, washing, and drying process is carried out. For the sake of PPy coating, a low-temperature polymerization scheme is utilized, where the nanowires are covered with polymer components. The detailed process description is given in the material synthesis section.

Fig. 2(a) shows the XRD pattern of KV3O8 0.59H2O and its PPy coated derivative, in the range of 10–70 2θ. Arbitrating that the

Conclusion

We purposefully prepared KVO and KVO@PPy nanowire bundles via a facile hydrothermal method and its low-temperature polymerization. The electrochemical efficiency of potassium vanadate nanowires coated with PPy is significantly better than that of pristine KVO. We infer that the electrostatic interaction among layered configuration and the K+ results in the KVO@PPy’s outstanding electrochemical stability based on crystal analysis, galvanostatic charge–discharge, EIS tests, XRD, and SEM. Apart

Notes

The authors declare no competing financial interest.

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.

Acknowledgement

The authors acknowledge the Xi’an Polytechnic University for Doctoral research start-up funding (Grant No.310/107020560) and National Natural Science Funds of China (51302214), Shaanxi Province Natural Science Fund (2019JZ-49), Xi’an Science and Technology Plan Project (201805034YD12CG18(5))

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