Phase Engineering of Mo-V Oxides for Zinc-Ion Batteries

With the ever-increasing demands of grid-scale energy storage, aqueous zinc-ion batteries (ZIBs) have garnered increasing attention around the world. However, limited Zn2+ host materials have hindered the commercialization of ZIBs. Hence, Mo-V oxides with different phase structures (orth-, tri-, and tetra-MoVO) were precisely constructed to develop phase-dependent Mo-V oxide cathodes for Zn2+ storage in ZIBs. The open frameworks and varied tunnel structures formed a favorable alternative for achieving suitable Zn2+ diffusion kinetics. With optimized phase engineering, a high specic capacity of approximately 400 mAh g−1 and the excellent cyclic stability of 1000 cycles were achieved with orth-MoVO as the cathode. The large amount of six- and seven-member rings in the orth-MoVO phase, which allow for alternative Zn2+ insertion, played a vital role in hosting Zn2+ ions reversibly. The proposed phase engineering strategy provides a new approach toward cathode design in ZIBs.


Introduction
Nowadays, the energy crisis and environmental pollution are the most critical global problems. [1][2][3][4][5] Although numerous clean and renewable energy sources have been successfully investigated, these energy sources are severely limited by time and space. [6][7][8][9][10][11] Therefore, an e cient large-scale energy storage system is necessitated. [12][13][14][15][16] Aqueous zinc-ion batteries (ZIBs) are ideal candidates, owing to their low-cost and environmental e ciency. [17][18][19][20] Zinc is abundant on earth, and Zn metal is easy to exploit, transfer, and store. 21,22 Additionally, it exhibits high theoretical capacities of 819 mAh g −1 and 5851 mA h mL −1 . [23][24][25][26] The redox potential for Zn/Zn 2+ is as low as −0.76 V versus the standard hydrogen electrode, when the overpotential for the hydrogen evolution reaction is very high. [27][28][29][30] The two-electron transfer reaction renders high energy density feasible for ZIBs. [31][32][33][34] Moreover, the neutral electrolyte provides high conductivity, alleviates the zinc loss, and eliminates the environmental impact. Regardless of these advantages, developing suitable cathodes in ZIBs is still an enormous challenge. [35][36][37][38][39] In fact, most cathode materials often undergo a structural collapse and severe degradation. 37,40 Therefore, innovations with regard to new cathode materials with high-energy density, which are capable of preventing structural collapse and degradation, are urgently needed. 41 Mo-V-based oxides with a solid open framework, tunable phase, tunnel structures, and improved conductivity owing to the intervalence electron transfer between Mo and V, are potential candidates for Zn 2+ storage in ZIBs. 42,43 Phase engineering is a promising strategy for structural modulation in developing advanced cathode materials. 44 The crystal phase with a de ned arrangement and organization of atoms and/or building blocks signi cantly in uences the physical and chemical properties. 45 Thus, subtle structural changes may generate different properties. For instance, the conductivities and structural stability are closely associated with the atomic arrangements. 46 The organization of building blocks dictates the thermodynamic and kinetic processes, when the adsorption and activation of reactant molecules signi cantly depend on the atomic con gurations. These phase engineering guidelines contribute toward the designing and synthesizing of the desired phase-dependent cathodes for ZIBs. 47 Moreover, successful phase engineering approaches may allow the further investigation of the charge/discharge mechanism and structure-activity relationship for ne-tuning the electrochemical performance of ZIBs. 48,49 In this study, Mo-V oxides with diverse phase structures (orth-, tri-, and tetra-MoVO) were obtained, which comprised corner-sharing [MO 6 ] octahedra and pentagonal [(M)M 5 O 27 ] to construct three-, four-, ve-, six-, and seven-member rings. The open framework and varied phase structures were favorable to the Zn 2+ diffusion kinetics. Consequently, high speci c capacity (400 mAh g −1 ) and excellent cyclic stability of 1000 cycles were achieved with orth-MoVO as the cathode. The Zn 2+ insertion/extraction process was analysed in detail to elucidate the energy storage mechanism. Compared with tri-and tetra-MoVO, a large amount of six-and seven-member rings in orth-MoVO plays a vital role in hosting the Zn 2+ reversibly.
Both Mo and V form various polyoxometalates, which may have acted as the building blocks of supramolecular compounds and constituted mixed polyoxometalates, thus resulting in a stabilized molecular architecture and exhibiting unique physical and chemical properties. [50][51][52][53][54][55] The phases and bulk frameworks could be tuned by controlling the arrangement and organization of the Mo-and V-blocks.
Thus, the modi ed metal-oxygen bond, pore size, and a nity in the molecules may have contributed toward achieving versatile performance. Therefore, it is possible to employ and engineer Mo-V oxides to reversibly host Zn 2+ by adjusting the building blocks and designing suitable phase and tunnel structures.  Figure S1, the ultra-visible-near infrared (UV-vis-NIR) diffuse re ectance spectra of the orth-, tri-, and tetra-MoVO exhibit an apparent absorption in the visible light region, owing to the electron transfer between Mo and V. The unique band structure indicates improved conductivity, which is favorable to the electrochemical process.

Results And Discussion
To estimate the Zn 2+ storage ability of the as-prepared Mo-V oxides, the CR2016-type cell was assembled with Mo-V oxides as cathodes and Zn foil discs as anodes. The electrolyte was 2 M of aqueous Zn(CF 3 SO 3 ) 2 solution. Figure 2 shows the cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) curves of the orth-, tri-, and tetra-MoVO cathodes. Obviously, the orth-MoVO exhibited a Zn 2+ insertion potential of approximately 0.55 V and a multistep extraction potential of approximately 0.55, 0.7, and 0.8 V (Figure 2a and 2d). In the subsequent 2 nd and 3 rd cycles, the Zn 2+ insertion processes became facile and the CV curves exhibited good reversible properties. In Figure 2b, the tri-MoVO exhibited similar CV as the orth-MoVO and an additional Zn 2+ insertion potential at 0.3 V. However, the tetra-MoVO exhibited one pair of redox peaks (Figure 2c), which suffered from severe irreversibility in the subsequent 2 nd and 3 rd cycles. As shown in Figure 2d-f, the orth-MoVO had a speci c capacity of approximately 400 mAh g −1 at 0.1 A g −1 owing to the large amount of six-and seven-member ring tunnels, while the tri-and tetra-MoVO delivered less than 200 mAh g −1 . Additionally, the speci c capacity of the orth-MoVO was approximately without attenuation during the rst three cycles. Hence, the four-and ve-member ring tunnels of the tetra-MoVO contribute toward slow diffusion kinetics, which lead to a lower speci c capacity. The poor reversibility of the tetra-MoVO was caused by the large charge repulsion force of Zn 2+ and the narrow tunnels. Thus, the orth-MoVO is a more suitable cathode for ZIBs. The galvanostatic intermittent titration technique (GITT) measurements shows that the Zn 2+ diffusion coe cient for the orth-MoVO range from 10 −8 to 10 −9 cm 2 s −1 order of magnitude, revealing the fast ion migration.
The exact atomic ratio and electrochemical performances of the orth-MoVO was investigated in detail to clarify the Zn 2+ storage mechanism. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES), thermogrametric (TG) and in-situ XRD analysis revealed the molecular formula of the orth-MoVO can be de ned as Mo 2.55 VO 9.43 ( Figure S2 and S3). The speci c capacity gradually decreased from 400 mAh g −1 at 0.1 A g −1 to 170 mAh g −1 at 2 A g −1 (Figure 2g and 2h). And the orth-MoVO delivered a speci c capacity of 347 mAh g −1 at 0.1 A g −1 after 100 cycles (Figure 2i). Additionally, the speci c capacity reached 145 mAh g −1 at 2 A g −1 after 1000 cycles. However, the rate capabilities were quite poor for the tri-and tetra-MoVO, and both of them underwent severe capacity degradation at 0.1 A g −1 ( Figure  S4). The discharge plot was divided into two parts for the GCD curves. One part was a platform at 0.65 V, while the other part was an inclined line from 0.55 V to 0.2 V. Figure S6 shows the GCD plots at 0.1 A g −1 with Zn(CF 3 SO 3 ) 2 dissolved in water and acetonitrile, respectively. The 1 st discharge plot in water has a higher platform than that in acetonitrile, which indicates a more facile Zn 2+ diffusion process in water, compared with that in acetonitrile. The 2 nd discharge plots in water and acetonitrile are similar, delivering the same speci c capacity. This phenomenon indicates that water exerts a lubricant effect on the 1 st Zn 2+ insertion process. 59 However, the 2 nd discharge plot illustrates that the extraction of Zn 2+ is incomplete. The residual Zn 2+ may have acted as pillars for expanding and stabilizing the crystalline orth-MoVO structure, which facilitates the subsequent insertion process. In addition, the discharge plots for orth-MoVO rule out the intercalation of H + in aqueous electrolyte.
In-situ XRD was carried out to clarify the crystal structure evolution of the orth-MoVO during the Zn 2+ insertion/extraction process. Figure 3a shows the detailed pseudo-colouring XRD patterns evolution when the orth-MoVO undergoes a discharge/charge process ( Figure S7). The XRD pattern of the prepared cathode is similar to that of the pristine orth-MoVO. During the Zn 2+ insertion process, the diffraction peaks at 22.3° and 26.7° slightly moved toward a higher degree, while the peaks at 7. These results indicate that the lattice space of (020) increases, while that of (001) decreases during the Zn 2+ insertion process. During the Zn 2+ extraction process, all peaks approximately revert to the pristine state, which indicates excellent reversibility of the phase structure. In-situ Fourier transfer infrared spectroscopy (FT-IR) was conducted ( Figure S8). The peak at 901 cm −1 exhibited a redshift while the peak at 937 cm −1 exhibited a blue shift during the Zn 2+ insertion process. After the extraction of Zn 2+ , the two peaks merged at 925 cm −1 . These two peaks are considered to have been induced by the V-O and Mo-O vibrations, which indicate that Zn 2+ penetrates into the tunnels and coordinates with the O atoms. Additionally, a portion of Zn 2+ remains in tunnels after the extraction process, and acts as pillars for stabilizing the orth-MoVO. The extended X-ray absorption ne structure (EXAFS) ensues, which indicates bond variations at different discharge/charge stages. Figure S9 shows the K-edge FFT function spectra of Mo, V, and Zn. After a full discharge/charge process, the rst dominant peaks of Mo and V slightly increased, and there were approximately no changes for Zn. X-ray photoelectron spectroscopy (XPS) was performed to further demonstrate the changes of Mo, V, and C during the discharge/charge process ( Figure S10). This XPS results indicated that CF 3 SO 3 underwent an insertion/extraction process along with the Zn 2+ ions, which is consistent with the EDS results presented in Figure S11. The XPS and EDS analysis further revealed that the Zn 2+ extraction process was not completely reversible in the rst cycle. The TEM image of the orth-MoVO morphology under 0.2 V ( Figure S12b) shows a precipitation layer on the surface. This precipitation layer decomposed during the charging process ( Figure S12c). As shown in Figure S13, the voltage of the orth-MoVO cathode remained stable below 0.25 V after the discharge process. However, the voltage sharply jumped to about 0.6 V after dipping in HCl, which may be induced by the dissolution of the precipitation layer.
Subsequently, HR-TEM was employed to track the structural transfer of the orth-MoVO during the GCD process. The HR-TEM image of the orth-MoVO in Figure 3b and the corresponding FFT pattern in the [100] zone axis reveal a crystalline structure, which is in agreement with the results of scanning transmission electron microscopy (STEM, Figure S14). After discharged to 0.2 V, the insertion of Zn 2+ resulted in crystalline transformation (Figure 3c). The lattice space of (001) decreased from 0.408 to 0.371 nm, while the lattice space of (020) increased from 1.27 to 1.39 nm. This phenomenon is in agreement with the insitu XRD analysis results. The angle between (010) and (001) became 77.23° (Figure 3c), then returned to 88.8° after charging to 1.6 V (Figure 3d). Additionally, the lattice spaces of (001) and (020) underwent reversible changes during the charging process. Figure S15 shows the Rietveld-re ned XRD of the orth-MoVO at 0.2 V and the corresponding atomic structure in the [001] direction. It is proposed that Zn 2+ inserted into the six-and seven-member rings of the orth-MoVO. The Zn 2+ storage mechanism is illustrated in Figure 3e according to the abovementioned results. In the discharge process, Zn 2+ and CF 3 SO 3 entered into the orth-MoVO tunnels. Then, part of Zn 2+ and CF 3 SO 3 remained in the tunnels to stabilize the orth-MoVO after the 1 st charge process. Thereafter, the Zn 2+ ions can insert and extract reversibly from the orth-MoVO in the subsequent GCD processes.
With consideration to the most favorable path of Zn 2+ diffusion, the possible adsorption sites on the oxygen atoms of MoO 6 and/or VO 6 in seven-, six-and ve-member MO 6 units were considered. The density functional theory (DFT) calculation was carried out to theoretically evaluate the adsorption properties of Zn 2+ . The formation energies of Zn 2+ ion embedded into the three investigated systems are employed. The porous channel consisting of heptagonal and hexagonal MO 6 clusters (channel) were selected for the orth-and tri-MoVO, while pentagonal channel for tetra-MoVO, with the consideration to the most favorable path of Zn 2+ diffusion. Additionally, different possible adsorption sites on the oxygen atoms around the porous channel were considered, and the one with strongest adsorption was chosen as the most stable adsorption site. According to the calculation results in Figure 4a and S16, the Zn atoms is prone to entering the hepta-member channel with two Zn 2+ simultaneously in, which is consistent with the experimental estimations. The 11 diffusion pathways were considered to give the optimal model of Zn 2+ diffusion. In Figure 4b, Zn 2+ tends to alternately enter the hepta-and hexa-member channels of orth-MoVO, both of which allow for 2 Zn 2+ insertions. The system becomes unstable if the third Zn 2+ entered the hepta-and hexa-member channel. Consistent with the Figure S13, about one third of discharge capacity is ascribed to the 4 Zn 2+ insertion process, whereas the other two thirds discharge capacity is obtained from the precipitation reaction on the surface of the orth-MoVO. Figure 4c and S17 show the optimal atomic model after accommodating Zn 2+ for orth-MoVO. However, the situation is different for tri-MoVO. In Figure S18 and S19, the system is unstable for Zn 2+ insertion in hepta-or hexa-member channels. As shown in Figure S20, the only possible channel to accommodate Zn 2+ is the penta-member channel in tetra-MoVO. Considering the channel density and the structural stability, the orth-MoVO exhibits the maximal capacity accommodation and highest kinetic properties for ZIBs.

Conclusions
In summary, a series of Mo-V oxides with diverse phase structures were precisely constructed. The obtained orth-, tri-, and tetra-MoVO were composed of various tunnels, which provided alternative paths for Zn 2+ diffusion as cathodes in the ZIBs. Consequently, the orth-MoVO phase achieved the highest speci c capacity and best cyclic stability. The structural and elemental evolutions are also discussed in detail. According to the analyses, the outstanding Zn 2+ storage ability is attributed to the numerous sixand seven-member ring tunnels in the orth-MoVO phase. Additionally, the orthorhombic phase structure provides a stable framework for the alternative zinc insertion process, which results in a superior cyclic stability. Both the phase and tunnel structures are responsible for the outstanding electrochemical behaviors. In conclusion, the proposed phase engineering strategy provides a new approach for designing cathodes that can be effectively incorporated in ZIBs.