Hydrated vanadium pentoxide/reduced graphene oxide-polyvinyl alcohol (V₂O₅⋅nH₂O/rGO-PVA) film as a binder-free electrode for solid-state Zn-ion batteries

Highlights

• The binder-free V₂O₅⋅nH₂O/rGO-PVA film is synthesized and proposed as a novel batteries cathode material.

• V₂O₅⋅nH₂O/rGO-PVA film displays the excellent capacity of 481 mAh g⁻¹ at 0.1 A g⁻¹.

• The achieved performances are superior to most cathode materials reported for solid-state zinc-ion batteris.

Abstract

Recently, solid-state aqueous Zn-ion batteries (ZIBs) have become the most promising wearable storage systems to replace Li-ion batteries owing to their advantages such as high safety and environmental friendliness. However, the critical challenge is to develop stable and robust cathode materials for excellent Zn²⁺ storage. In this study, a hydrated vanadium pentoxide/reduced graphene oxide-polyvinyl alcohol (V₂O₅⋅nH₂O/rGO-PVA, abbreviated as VOH/rGO-P) film was synthesized as a binder-free cathode for solid-state aqueous ZIBs. The addition of PVA not only increases the layer spacing of VOH and forms a strong hydrogen bond network with GO and water molecules, but also enhances the mechanical properties of the film. As a binder-free cathode for solid-state aqueous ZIBs, the VOH/rGO-P film attains excellent electrochemical capacity as high as 481 mAh∙g⁻¹ at 0.1 A∙g⁻¹. Such a high specific capacity indicates that the VOH/rGO-P film shows great potential for the next generation solid-state aqueous ZIBs.

Introduction

The rapid progress of science and technology has been accompanied by critical damage to the environment, due to which researchers have increasingly focused on the electrochemical performance and environmental protection of energy storage equipment [1], [2], [3], [4], [5], [6], [7], [8]. Wearable electronic equipment has a large market with numerous commercial opportunities for personal health devices and portable electronic equipment [9], [10], [11], [12]. However, the real and flexible energy storage equipment has not been developed thus far. This has significantly hindered the development of next-generation flexible electronic equipment [11], [13], [14]. The most widely studied energy storage systems (ESSs) in wearable devices include supercapacitors and batteries. Among these, Li-ion batteries are the most extensively used owing to their scalable production, large market, and high energy density [15], [16], [17], [18], [19]. Recently, for lower cost and higher safety, researchers have started focusing on other metal-ion batteries that mainly comprise some alkali metal ions and alkaline earth metal ions with similar structures to Li ions, such as Na ions [20], Ca ions [21], and Zn ions [22], [23], [24], [25]. Zn-ion batteries (ZIBs) have become one of the most promising substitutes for Li-ion batteries owing to their ultrahigh theoretical specific capacity of zinc anode (820 mAh∙g⁻¹), low cost, and simple manufacturing process [26], [27], [28]. In addition, the most important concern for the electronic equipment is ensuring safety and environmental protection. Therefore, organic electrolytes are unsuitable for use in the fabrication of flexible batteries [29], [30], [31]. Compared to non-aqueous electrolytes in traditional Li-ion, Na-ion, and K-ion batteries, ZIBs require a mild aqueous gel electrolyte, which improves the stability, ensures a high probability of safety, protects the environment, increases the ion conductivity by 100–1000 times, and reduces the cost and manufacturing complexity [13], [26]. Therefore, solid-state ZIBs can be applied in wearable electronic devices owing to their high safety and electrochemical performance.

Among the most decisive factors for the electrochemical performance of batteries, the electrode material plays an important role in the performance of the batteries, including specific capacity, energy density, and cycle performance [32], [33], [34]. In terms of the reaction mechanism of batteries, the best cathode material for batteries is one with a porous morphology or layered structure, such as manganese oxides [35], Prussian blue [36], and vanadium oxides [37], by which the electrolyte ions can easily intercalate and deintercalate between the electrolyte and electrode [23]. One of the most representative layered substances is a variety of vanadium oxides [37]. Owing to their natural layered structures and excellent electrochemical performance, V₂O₅ and its derivatives are widely used as cathode materials for ZIBs and other coin cells [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48]. However, the application of vanadium oxides for solid-state ZIBs is still in its infancy. Xi Dai et al. assembled a freestanding reduced graphene oxide (rGO)/VO₂ composite film with Zn foil into soft-packaged ZIBs and obtained a high energy density of 65 Wh∙kg⁻¹ at a power density of 7.8 kW∙kg⁻¹ [33]. Hongmei Wang et al. used V₂O₅ and Zn₂V₂O₅-based hybrid fibers as flexible ZIBs cathode materials and achieved high capacities of 162 mAh∙g⁻¹ and 409 mAh∙g⁻¹ at 8 A∙g⁻¹, respectively [49]. Recently, an oxygen-deficient vanadium oxide cathode was utilized by Meng Liao et al., and a capacity of about 400 mAh∙g⁻¹ was confirmed [50]. Although the possibility and functionality of solid-state ZIBs have been proven, it is still necessary to select electrode materials with appropriate properties and study their corresponding energy storage mechanisms [13].

In this study, we attempted to fabricate a binder-free VOH/rGO-P film using three substances (NH₄VO₃, GO, and PVA) via hydrothermal synthesis and vacuum filtration. Various vanadium oxides, such as V₂O₅, suffer from poor cycling performance and slow charging and discharging processes, whereas carbon materials can always ameliorate this situation [51], [52], [53]. As an easy-to-obtain and safe carbon source, GO is the most popular one in electrochemistry [54], [55], [56]. However, GO is easily aggregated and the previous report demonstrates that PVA can address this issue [57]. As expected, the VOH/rGO-P film was obtained through a simple hydrothermal synthesis and suction filtration. When it was used as the free-standing cathode applied to solid-state aqueous ZIBs, the Zn//VOH/rGO-P showed excellent electrochemical performance.