V2O5 Nanospheres with Mixed Vanadium Valences as High Electrochemically Active Aqueous Zinc-Ion Battery Cathode
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Hollow V4+-V2O5 nanospheres are prepared by a novel and simple method using VOOH as the precursor.
V4+-V2O5 with mixed vanadium valences is firstly constructed as an electrochemically active cathode for aqueous zinc-ion batteries.
The V4+-V2O5 cathode exhibits a prominent cycling performance up to 1000 cycles and an excellent rate capability.
KeywordsV2O5 Mixed valences Hollow sphere Long-cycle-life Aqueous zinc-ion battery
Although significant achievements have been made for the high energy density and long-cycle-life lithium-ion batteries practical applications, the limited lithium supply, high cost, and low safety impede their further development in large-scale energy storage [1, 2, 3, 4], motivating us to find an alternative battery chemistry. As a result of the multiple electrons involved in redox reactions, aqueous multivalent ion battery systems possess higher energy density compared with battery systems and supercapacitors based on conventional aqueous alkali metal cations (e.g., Li+ and Na+) [5, 6, 7, 8]. Recently, aqueous zinc-ion batteries (ZIBs) have captured much attention due to their low cost, high safety, and environmental friendliness [9, 10, 11]. Furthermore, the use of non-toxic and safe aqueous electrolytes with high ionic conductivity makes the aqueous ZIBs a promising battery chemistry for grid-scale applications . So far, many cathode materials have been developed for aqueous ZIBs, but they still suffer from poor electrochemical performance, such as drastic capacity fading and inferior rate capability for manganese-based cathodes [13, 14, 15, 16, 17, 18, 19] and low capacity for Prussian blue analogs [20, 21].
Recently, vanadium-based compounds are receiving intensive research interest as cathode materials for aqueous ZIBs, owing to the multiple oxidation states of vanadium and its abundant supply [10, 22, 23, 24]. Since Nazar’s group developed Zn0.25V2O5nH2O as a cathode with a high energy density of 250 Wh kg−1 and good cycle stability up to 1000 cycles for aqueous ZIBs , a series of compounds such as Ca0.25V2O5·nH2O , K0.25V2O5 , Na2V6O16.1.63H2O [28, 29, 30], Na0.33V2O5 , NH4V4O10 , and MgxV2O5·nH2O  have been explored. In fact, vanadium is in a mixed valence state in these materials due to the insertion of guest ions. However, the introduction of guest ions may increase the molar mass and decrease the specific capacity to some extent. The pure phase of V2O5 has been demonstrated with poor performance as a cathode for aqueous ZIBs, due to its poor electronic and ionic conductivities [22, 34].
Inspired by the reported mixed valence states of vanadium oxides with enhanced electrochemical performance for energy application [35, 36, 37], we have, for the first time, prepared V4+-V2O5 hollow nanospheres by a novel synthetic method for application in zinc-ion storage cathodes. It is worth noting that V4+-V2O5 possesses higher electrochemical activity, lower polarization, faster ion transport, and better electrical conductivity than V2O5. As expected, V4+-V2O5 exhibits superior electrochemical performances as a cathode for aqueous ZIBs, with high capacity, excellent rate capability, and long-term cyclic life up to 1000 cycles. Moreover, the presented ZIB system using 2 M ZnSO4 aqueous solution as electrolyte is cost-effective and its electrochemical properties are excellent, which makes it practical for large-scale applications.
2 Experimental Section
2.1 Materials Synthesis
VOOH is synthesized based on the method reported by Xie’ s group . Firstly, 2 mmol NH4VO3 was dissolved into a beaker containing 45 mL of deionized water and was stirred vigorously for 10 min. Secondly, 5 mL of 1 M HCl solution was injected into the beaker until the turbid liquid turned into a yellow transparent solution, at a rate of 1 mL per minute. Thirdly, 5 mL of N2H4·3H2O, employed as a strong reducing agent, were added to the previously prepared solution while stirring continuously for 30 min. Then, the obtained V(OH)2NH2 brown turbid fluid was transferred to a Teflon-lined stainless-steel autoclave and kept in an electrical oven at 120 °C for 8 h. The precursor VOOH was prepared via suction filtration and was dried at 50 °C in vacuum. V4+-V2O5 was obtained by annealing the precursor in air atmosphere for 6 h at 250 °C with a heating rate of 2 °C min−1. V2O5 with pure pentavalent vanadium can be obtained at temperatures above 300 °C.
2.2 Materials Characterization
A combined differential scanning calorimetry (DSC)/thermogravimetric analysis (TG) instrument (Netzsch STA449 C, Germany) was used to study the evolution of VOOH in air at a heating ramp rate of 10 °C min−1. The phase composition of the as-prepared compounds was analyzed by X-ray power diffraction (XRD) patterns detected with a Rigaku D/MAX-2500 diffractometer (Cu Kα). The phase transformation process was monitored by high-temperature dynamic XRD (Rigaku SmartLab, Cu Kα), with the temperature increasing from 50 to 350 °C at a heating rate of 10 °C min−1 and was kept warm for 10 min. The morphology features were obtained by scanning electron microscopy (SEM, Quanta FEG 250). Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and TEM/energy-dispersive spectroscopy (TEM–EDS) mapping were carried out on the transmission electron microscope (Tecnai G2 F20). X-ray photoelectron spectroscopy (XPS) measurements taken on a spectrometer (Escalab 250xi, Thermo Scientific) explain the valence state of the elements in the product.
2.3 Electrode Fabrication and Electrochemical Measurements
The electrochemical properties of the as-prepared compounds were tested via CR2016 coin cells. The cathode electrodes were fabricated by coating a stainless-steel wire mesh with ropy slurry and drying it in a vacuum oven at 80 °C for 12 h. The slurry was prepared by mixing active material (70 wt%), acetylene black (20 wt%), polyvinylidene fluoride binder (10 wt%), and N-methyl-2-pyrrolidone. A metal zinc plate was utilized as anode, and a 2 M ZnSO4 aqueous solution was employed as electrolyte. The mass loading of active materials on the electrode was approximately 1.5 mg cm−2. The electrochemical behavior was evaluated at voltages between 0.4 and 1.4 V. Galvanostatic charge/discharge tests were carried out with a multichannel battery testing system (LAND CT2001A). Cyclic voltammetry (CV) tests were carried out with the CHI 660e electrochemical station. Electrochemical impedance spectrometry (EIS) measurements were performed in the frequency range of 100 kHz to 10 mHz on a ZAHNER-IM6ex electrochemical workstation (Kronach, Germany). All the electrochemical measurements were carried out at a controlled room temperature of 28 °C.
3 Results and Discussion
The electrochemical performance of V4+-V2O5 and V2O5 as cathodes in aqueous ZIBs is evaluated. V4+-V2O5 and V2O5 exhibit an initial specific capacity of 262.1 and 249.6 mAh g−1 at a current density of 1 A g−1, respectively (Fig. 3d). After 80 cycles, the specific capacities of both samples decrease rapidly, while V4+-V2O5 displays a better cyclic stability. The reason for the rapid decrease in specific capacity in the initial cycles will be discussed later. The selected discharge/charge voltage profile of V4+-V2O5 has three platforms at approximately 1.1, 1.0, and 0.6 V (Fig. 3e), which indicate a much more obvious Zn2+ insertion/extraction behavior than in V2O5. The V4+-V2O5 also exhibits superior rate capability with the average specific discharge capacities of 188.7, 149.9, 143, 138.31, 133, and 124.93 mAh g−1 at current densities of 0.5, 1, 2, 5, 10, and 15 A g−1, respectively (Fig. 3f). However, the V2O5 cathode exhibits a low capacity of 87.5 mAh g−1 at 15 A g−1. Furthermore, the V4+-V2O5 cathode exhibits a long-term cycling performance at a high current density of 10 A g−1, at which a high specific capacity of 140 mAh g−1 can be maintained after 1000 cycles (Fig. 3g).
As shown in Fig. S4, the V2O5 samples obtained at different temperatures exhibit similar electrochemical properties, indicating that the crystallinity may have a slight impact on the electrochemical performance of the as-prepared samples. The SEM images of V4+-V2O5 and V2O5 (Fig. 1d and S3) show that both have spherical morphology and are composed of nanosheets with similar size and shape, so the effect of morphology on the electrochemical performance difference could be ignored. As a result, the improved electrochemical performance of V4+-V2O5 compared to V2O5 may be due to the mixed valence states. It is known that introducing mixed valences of metal ions in electrode materials has appreciable impacts on their electrochemical reactions [35, 36, 52]. The presence of such defects at the electrode interface could not only increase the effective contact area between electrode and electrolyte , but also behave like a protective coating layer to maintain the morphology stability of the electrode [36, 37].
The area ratio of the shaded region in Fig. 4b illustrates that the intercalation pseudocapacitance contribution ratio of V4+-V2O5 raises from 33.9% to 57.2% as the scan rate increases from 0.2 to 1 mV s−1.
The structural changes of V4+-V2O5 are further evaluated by the ex situ HRTEM images of the electrodes discharged to 0.4 V and charged to 1.4 V (Fig. 5b, c). The interplanar spacing of (001) is 0.45 nm at the discharged state of 0.4 V, while it is 0.43 nm at the charged state of 1.4 V. Compared with the original state, such data confirm the insertion/extraction behavior of Zn2+ ions in the V4+-V2O5 electrode. The interplanar spacing of 0.16 nm at 0.4 V matches well that of the Zn4SO4(OH)6·5H2O (PDF#39-0688) state. Figure S8 displays the SAED images of V4+-V2O5 at different states. According to the extinction law of orthogonal crystal systems, the (001) crystal faces cannot be seen in the SAED images. We found (002) crystal faces with half the interplanar spacing of the (001) crystal faces (Fig. S8a). When discharged to 0.4 V, several obscure rings were observed, which may be classified as the new phase of Zn4SO4(OH)6·5H2O (Fig. S8b). When charged to 1.4 V, the substance appears to be a single and amorphous mixture (Fig. S8c), which is consistent with the XRD result.
The rapid decrease in the specific capacity in the initial stage may be due to the fact that some zinc ions located at the “dead Zn2+ sites” cannot be extracted from the V2O5 lattice in the charge process [56, 57], which can be further revealed in the TEM-EDS mapping images of the electrode discharged/charged to 0.4 V/1.4 V (Fig. 5b, c). It is reported that the zinc ions that fail to exit from the host structure during the charging process may act as layer pillars, making the structure of V2O5 more stable . We also concentrated on the valence state changes of vanadium, as presented in Fig. 5d. When discharged to 0.4 V, the V 2p3/2 peaks separated into three peaks located at 517.3, 516.4, and 515.5 V, which correspond to V5+, V4+, and V3+, respectively, and the V 2p1/2 peaks located at 524.5, 523.6, and 522.6 V also correspond to V5+, V4+, and V3+, respectively, indicating the reduction in vanadium accomplished by the insertion of Zn2+. The vanadium is further oxidized during the charging process. Note that the portion of V4+ is higher than that of its original state, which may be due to the incomplete extraction of Zn2+. This phenomenon is consistent with the high-resolution Zn 2p XPS spectra (Fig. 5e).
In summary, we have successfully synthesized V4+-V2O5 and V2O5 hollow spheres with different oxidation states of vanadium, by controlling the sintering process of the VOOH precursor. With the CV, GITT, and EIS techniques, we demonstrated that V4+-V2O5 with mixed vanadium valences exhibits higher electrochemical activity, lower polarization, faster ion diffusion capability, and higher electrical conductivity than V2O5. As expected, the V4+-V2O5 cathode exhibits excellent Zn2+ storage performances. For instance, it can maintain a high specific capacity of 140 mAh g−1 after 1000 cycles at 10 A g−1 and presents outstanding rate capability. The extra tetravalent vanadium ions could increase the electronic and ionic conductivities. The results suggest that V4+-V2O5 is a promising cathode for aqueous ZIBs.
This work was supported by National Natural Science Foundation of China (Nos. 51802356, 51872334, and 51572299), Innovation-Driven Project of Central South University (No. 2018CX004).
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