Binder-free V₂O₅/CNT paper electrode for high rate performance zinc ion battery

Abstract

Organic compounds, such as polyvinylidene fluoride (PVDF), have been widely used as a binder in battery electrode preparations. While such an approach does not have a significant impact on the performance of the batteries that utilize low valence ions, such as the Li ion battery (LIB), the diffusion of high valence ions (such as Zn²⁺) will be severely impaired. This will be especially pronounced if the polymeric binder contains highly electronegative atoms, such as fluorine. The high charge density ions, such as Zn²⁺, tend to adsorb onto these electronegative atoms, thus the mobility of these ions across the material is inevitably affected. As such, it becomes highly necessary to consider the binder-free electrode architecture when designing a high rate performing and cycling-stable zinc ion battery (ZIB) cathode. Herein, this work demonstrates an improved Zn ion battery by adopting a freestanding electrode. The obtained V₂O₅/CNT paper electrode delivers a specific capacity of 312 mA h g⁻¹, while achieving a respectable 75% retention in capacity after increasing the current density by 10-fold. Furthermore, excellent cycling stability is recorded with 81% capacity retention after 2000 cycles at 1.0 A g⁻¹. Thus, this work clearly demonstrated that the freestanding electrode is a promising approach for high valence ion batteries.

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1. Introduction

Energy storage devices, such as lithium ion batteries (LIBs), have become an essential part that contributes toward technological advancements in modern society.^1–6 In this technological revolution, energy storage devices are not only expected to last longer, they are also expected to be fully charged within a short time period. Such a shifting paradigm indicates that the rate performances of these energy storage devices are gradually capturing increasing attention and growing scrutiny.^7–12 As one of the most widely studied materials in LIB, V₂O₅ has gained considerable attention due to its ease of fabrication, high capacity, earth abundance, and low cost.^13–16 While V₂O₅ has generally shown promising electrochemical performance as an LIB electrode, V₂O₅ possesses low ionic conductivity, which inevitably leads to a poor rate performance and less-than-ideal cycling stability. Such a challenge is easily mitigated with the incorporation of carbonaceous materials to enhance the overall conductivity of the composite. Even though improvements can be observed via the V₂O₅/carbon composite, these materials are usually in powder form and require the addition of foreign additives, such as polymeric binders, during electrode preparation.^17–20 This addition of an organic binder not only complicates the electrode preparation process, but also clogs the transportation channels, which can severely affect the rate capability and cycling stability. However, it should be noted that in the case of LIBs, such channel blockages are not very pronounced due to the easy diffusion of the low valence Li⁺ ions. Such a situation would be further aggravated when these diffusing ions have higher charge densities than Li⁺ and hence, this consideration must be revitalized for alternative high valence ion batteries.

With the recent elevated interest in V₂O₅ as a zinc ion battery (ZIB) cathode, the magnitude of this sluggish diffusion across the material with an organic binder is greatly increased. Even though the Zn²⁺ diameter is close to that of Li⁺, it possesses a higher charge density that can pose a polarizing effect on the material during the diffusion process. Due to the high charge density, Zn²⁺ tends to be adsorbed to the C–F bonds in the polarizing polymeric binder. This consequently reduces the mobility of Zn²⁺ and ultimately leads to a poor rate performance and large initial irreversible capacity loss. Thus, it becomes highly necessary to avoid the use of polymeric binders to ensure minimal disruption to the mobility of Zn²⁺ during the charging/discharging process. In order to achieve this, interest in a freestanding electrode should be renewed, especially for high valence ion batteries such as ZIB. A typical strategy is to introduce a carbonaceous scaffold that can offer structural integrity for the metal oxides, while simultaneously contributing to the overall conductivity of the electrode. As such, one of the favorite carbon monoliths is graphene paper. However, due to its layer-by-layer arrangement, the extent of electrolyte infiltration becomes a challenge as the electrolyte can only enter the material in the planar direction. In order to address such a limitation, carbon nanotube (CNT) paper would hence become an appealing scaffold material for V₂O₅ due to its random stacking arrangement, which offers better electrolyte infiltration from all directions.

Herein, this work reports a significant electrochemical performance enhancement (especially the rate performance and cycling stability) by adopting a freestanding V₂O₅/CNT composite paper (VCP) as the ZIB cathode. As a polymer binder is not used in the electrode preparation, the V₂O₅/CNT paper delivers a specific capacity of 312 mA h g⁻¹, while achieving a respectable 75% retention in capacity after increasing the current density by 10-fold. It is interesting to note that its counterpart with an organic binder demonstrated a poor rate performance of 36% at the same current density. Furthermore, our binder-free composite papers exhibit superior cycling stability, as compared to the counterpart with an organic binder. This work demonstrates a significant progress of the binder-free electrode in achieving excellent electrochemical performance and its potential for high valence ion batteries.

2. Experimental

In a typical synthesis procedure, the V₂O₅/CNT paper electrode was fabricated via a vacuum filtration route. The CNTs were purchased from time/nano and the V₂O₅ powder was purchased from Sigma. First, 1 g Triton X-100 was added to 100 mL of deionized water. V₂O₅ powder and CNTs were subsequently dispersed under magnetic ultrasonication for 1 h in air with a mass ratio of 7 : 3. Then, the as-obtained solution was filtered through the membrane filter. After the film was dried, it was easily peeled off from the filtration paper. The mass loading was about 7 mg cm⁻².

Morphology and structure characterization

The powder XRD pattern was measured using a powder diffractometer (Bruker D8 Advanced Diffractometer System) with a Cu Kα (1.5418 Å) source. The component content was determined using thermogravimetric analysis (TGA, DMSE SDTQ600). The morphology and size of the samples were determined using scanning electron microscopy (SEM, ZEISS SEM Supra 40 (5 kV)) and energy dispersive X-ray spectroscopy (EDS). The micro-structure was observed using a three-dimensional white light interference surface topography instrument. Transmission electron microscopy (TEM) was done on a JEOL-3010 (300 kV acceleration voltage) microscope. TEM samples were prepared by dripping the sample solutions onto a copper grid. The surface composition was analyzed by XPS using a Kratos Analytical Axis UltraDLD UHV spectrometer with a monochromatized Al Kα X-ray source (1486.6 eV), scanning a spot size of 700 μm × 300 μm.

Electrochemical measurements

All electrochemical tests were performed at room temperature. CV and galvanostatic charge/discharge measurements were conducted using an electrochemical system (Bio-logic VMP 3). Zinc foil and filter paper were used as the anode and separator, respectively, and 1 M ZnSO₄ was employed as the electrolyte. A CR2025-type coin cell was assembled under ambient conditions to evaluate the electrochemical performance. Three different working electrodes were prepared. The V₂O₅ electrode was prepared by mixing the purchased V₂O₅ powder with carbon black and polyvinylidene fluoride in a ratio of 7 : 2 : 1 with N-methyl-2-pyrrolidone. The mixture was hand-ground for at least 20 min to obtain a slurry. The slurry was later coated onto carbon paper, which served as a current collector, and then heated at 80 °C overnight for further use. The CNT/V₂O₅ mixed powder was obtained in the same way. The ratio of CNT/V₂O₅ was 7 : 3. Then, the powder was mixed by ultrasonic dispersing technology in DI water. After drying at 80 °C in the atmosphere, the mixed powder was collected and used as the electrochemical active substance. Then, it was mixed with carbon black and polyvinylidene fluoride in a ratio of 7 : 2 : 1. The V₂O₅/CNT paper electrode was directly used as the working electrode. For both CV and charge/discharge of the full cell test, the measurement voltage was controlled in the range of 0.2–1.7 V for the aqueous electrolyte test. The current densities of 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0 A g⁻¹ were used for the charge/discharge measurement. The capacity, energy density, and power density were calculated based on the mass of the active materials from the cathode.

3. Results and discussion

A facile vacuum filtration method was employed to fabricate the V₂O₅/CNT paper (VCP) electrode, as presented in Fig. 1a. Two more samples pasted on carbon paper were prepared as control: (1) V₂O₅ powder (VP) mixed with carbon black and binder and (2) CNT/V₂O₅ mixture powder (CVMP) mixed with carbon black and binder. Fig. 1b presented the X-ray diffraction (XRD) patterns of these three different electrodes. All Bragg peaks of the XRD patterns of the three samples showed the shcherbinaite reflections, which belonged to an orthorhombic structure of V₂O₅ with space group Pmmm (indexed the standard PDF card 41-1426).²¹ To determine the amount ratio of V₂O₅ to CNT in VCP, thermogravimetric analysis was performed under ambient conditions. As shown in Fig. 1c, two pronounced degradation steps were identified. The first degradation step at 100 °C represented the evaporation of the water molecules in VCP, and the second step corresponded to the decomposition of the CNTs. Based on the TGA result, the weight percentage of the electrochemical active material V₂O₅ in VCP was ca. 67%.

Fig. 1 (a) Schematic illustration of the preparation process of the V₂O₅/CNT paper; (b) the XRD patterns of V₂O₅ powder, CNT/V₂O₅ mixed powder and V₂O₅/CNT paper; (c) the thermogravimetric analysis (TGA) curves of the V₂O₅/CNT paper.

Fig. 2a presents a typical scanning electron microscopy (SEM) image of the VCP. Under the SEM observation, the VCP electrode exhibited a 3-dimensional porous structure that comprised a continuous carbon nanotube network and V₂O₅ particles. Fig. 2(b–e) show the cross-sectional image of VCP from energy dispersive X-ray spectra (EDX). The thickness of the paper electrode was about 200 μm and the EDX results indicated a uniform distribution of V and C elements across the entire cross-section, which confirmed an even distribution of V₂O₅ in the CNT conductive network. Fig. 2f is an image of the three-dimensional white light interference surface topography. The appearance of a large number of blue areas confirmed the existence of 3D channels, which were beneficial to the zinc ion transportation. Transmission electron microscopy (TEM) was employed to further study the morphology of the paper electrode. Based on Fig. 2g, the presence of the CNTs and V₂O₅ particles further confirmed the successful preparation of a freestanding V₂O₅/CNT paper. The CNT and single-crystalline V₂O₅ (according to the SAED pattern in Fig. 2h) were entangled together to form a continuous electron conducting network.

Fig. 2 Structural characterization. (a–e) SEM images and EDX mapping of V₂O₅/CNT paper; (f) the image of the three-dimensional white light interference surface topography; (g) TEM image and (h) SAED pattern of the corresponding V₂O₅/CNT paper.

To compare the electrode performance with/without a binder, electrochemical testing was conducted for the pristine V₂O₅ powder (VP), the CNT/V₂O₅ mixed powder (CVMP) and freestanding V₂O₅/CNT composite paper (VCP). The CV curves of these three electrodes are presented in Fig. 3a. Under the same scan rate of 0.2 mV s⁻¹, the integrated area of the CV curves for VCP was larger than those of the two control electrodes, suggesting a larger capacity. The cyclic voltammetry (CV) curves of the VCP electrode conducted from 0.2 mV s⁻¹ to 1 mV s⁻¹ were recorded and are presented in Fig. S1a.† Two reduction peaks (around 0.4–0.5 V and 0.8–0.9 V) and two oxidation peaks (around 0.9–1.0 V and 1.2–1.3 V) were observed, which were caused by the zinc ion intercalation/deintercalation during the discharge/charge processes. The galvanostatic charge/discharge (GCD) measurements of VCP were performed in the range of 0.2–1.7 V (as shown in Fig. 3b). An average specific discharge capacity of 312 mA h g⁻¹ was recorded at a current density of 1 A g⁻¹. Even at a higher current density of 10 A g⁻¹, a good specific capacity of 232 mA h g⁻¹ was achieved. The areal and volumetric capacities at various current densities are also shown in Fig. S2a.† An areal capacity of 2.2 mA h cm⁻² and a volumetric capacity of 109.2 mA h cm⁻³ can be obtained at a current density of 1 A g⁻¹. The corresponding energy/power density is presented in Fig. 3c and the areal/volumetric energy/power density are also given in Fig. S2b.† The VCP electrode exhibited an energy density of 891 W h kg⁻¹ at a power density of 278 W kg⁻¹ (Fig. 3c). A maximum power density of 8871 W kg⁻¹ could be reached. The excellent rate capability was further reflected in the Ragone plots by comparison with some recent reports (see ESI Table 1†).^22–29 The rate performances of the VP, CVMP and VCP are shown in Fig. 3d. The VCP exhibited superior rate capability with average specific discharge capacities of 312, 303, 281, 253 and 235 mA h g⁻¹ at current densities of 1, 2, 4, 8 and 10 A g⁻¹, respectively. Recently, some studies also demonstrated that structural water could function as a charge screening media in the redox reactions. The H₂O-solvated Zn²⁺ possesses a largely reduced effective charge and thus, reduced electrostatic interactions with the V₂O₅ framework, effectively promoting its diffusion.²⁴ Based on Fig. 3d, the capacity retention of these three electrodes were calculated and is shown in Fig. 3e. For the VCP electrode, the capacity retention is 75% (at 10 A g⁻¹) of the initial value (at 1 A g⁻¹), while only 42% retention was calculated for CVP and a poor 36% retention for VP. When the current density was decreased from 10 A g⁻¹ to 1 A g⁻¹, the capacity of VCP was fully recovered, which was superior to the 52% and 79% capacity recovery for VP and CVMP, respectively. It was believed that the capacity loss in VP and CVMP was due to the presence of the organic binder, which generated a strong electrostatic attraction with the highly positively charged zinc ions, and thus these electrostatically “glued” zinc ions could not be completely extracted. The capacity fading of electrodes with the polymeric binder could be clearly seen by comparing the discharge capacity of the second and third cycles (Fig. S3†). On the other hand, the capacity loss of the VCP electrode was negligible. To demonstrate the high reversibility of VCP, all three electrodes were cycled at a low current density of 1.0 A g⁻¹ to evaluate the long-term cycling stability (as shown in Fig. 3f). After 2000 cycles, the VCP electrode retained 81% of its initial capacity with ca. 99% coulombic efficiency for all the cycles, suggesting an excellent cycling life. However, the other two powder electrodes demonstrated 80% capacity retention under 250 cycles. The structure and morphology of the V₂O₅/CNT paper after a long-term cycling process was also an important factor toward the stable electrochemical properties of the Zn ion batteries. After 2000 cycles, compared with the original XRD result (Fig. S4a†), similar peaks were revealed without an obvious change. The morphology of the V₂O₅ blocks in the CNT network could still be clearly distinguished in Fig. S4b.†

Fig. 3 Electrochemical performance of V₂O₅ powder, CNT/V₂O₅ mixed powder and V₂O₅/CNT paper. (a) Cyclic voltammetry curves at a scan rate of 0.2 mV s⁻¹; (b) discharge/charge profiles of V₂O₅/CNT paper; (c) Ragone plots; (d) rate performances; (e) capacity retention and (f) cycling performances at a current density of 1.0 A g⁻¹.

An ex situ XRD technique was employed to explore the structural changes of VCP during the discharge/charge process (Fig. 4a). When the electrode was discharged to a lower voltage, the intensity of the characteristic peaks became weaker, which was ascribed to the intercalation of Zn²⁺ ions from the layered structure and resulted in lattice distortions. It could be seen that the location of the diffraction peaks corresponding to the (001), (101), and (110) lattice planes were consistent with those in the initial sample, indicating that the sample was still a layered structure of V₂O₅ after the charge–discharge process.³⁰ The valence state changes of vanadium are also presented in Fig. 4b. When discharged to 0.2 V, the V 2p_3/2 peaks were split into two peaks. The reduction in vanadium happened along with the insertion of Zn²⁺. Then, the vanadium was further oxidized back to its original state during the charging process.³¹ The SEM images under different potentials are shown in Fig. S5a to S5f.† The morphology of the V₂O₅/CNT paper remained the same after the discharging–charging process. In order to investigate the mechanical property of our freestanding film electrode, we have performed a simple mechanical fatigue test, as shown in Fig. S6.† After 100 bends, the freestanding film remained intact without flaking. Ultimately, we assembled a flexible zinc ion battery using VCP as the cathode, zinc foil as the anode, filter membrane as the separator and 1 M ZnSO₄ aqueous solution as the electrolyte (Fig. 4d and e). The digital watch and white LED were powered by two charged zinc batteries connected in series (as shown in Fig. 4f and inset of Fig. 4g). The unchanged shape of the CV curves under different bending angles also verified the stable performance of this flexible device (Fig. 4g). This paper electrode will open up new opportunities for advanced energy storage systems taking advantage of large mass-loading, flexibility, and long cycling ability.

Fig. 4 (a) and (b) The ex situ XRD patterns of V₂O₅/CNT paper under different potentials; (c) the ex situ XPS spectra of V 2p at the original and fully discharged/charged state. The digital photos of (d) V₂O₅/CNT paper; (e) Zn/V₂O₅/CNT paper battery; (f) the flexible device based on the paper electrode can easily power the digital watch; (g) cyclic voltammetry curves of the device at different bending states.

4. Conclusions

A binder-free, freestanding V₂O₅/CNT electrode was successfully prepared as a high rate performance and high cycling-stable zinc ion battery cathode. This work demonstrated a strategy that is beneficial to realize the high-performance multivalent metal battery (such as ZIB), by using a freestanding and binder-free electrode. Because an organic binder such as a polymer additive was not used in the electrode preparation, the V₂O₅/CNT paper delivered a specific capacity of 312 mA h g⁻¹ while achieving a respectable 75% retention in capacity after increasing the current density by 10-fold. Finally, the flexible device based on the paper electrode easily powered the digital watch and white LED even at different bending states. These achievements cast light on the design of more advanced, binder-free, flexible cathode materials for rechargeable zinc ion batteries.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Singapore MOE Tier 2 grant MOE-2018-T2-1-149, China Scholarship Council (201806120308), the National Natural Science Foundation of China (Grant No. 21273058, 21673064, 51802059 and 21503059), China postdoctoral science foundation (Grant No. 2017M621285 and 2018T110292), Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 2019040 and 2019041).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nr07458a

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