Atomic scale analysis of Zn2+ storage in robust tunnel frameworks

Realizing rapid and reversible Zn2+ storage at the cathode is imperative for the advancement of aqueous Zn-ion batteries (ZIBs), which offer an excellent option for large-scale electrochemical energy storage. However, owing to limitations of the structural stability of previously investigated frameworks, the Zn2+ storage processes remain unclear, thus hindering progress towards the above goal. Herein, we present the novel application of MoVTe oxide with an M1 phase (MVT-M1) as a potential cathode material for ZIBs. MVT-M1 features broad and robust tunnels that facilitate reversible Zn2+ insertion/extraction during cycling, as well as rich redox centers (Mo, V, and Te) to aid in charge redistribution, resulting in good performances in ZIBs. The exceptional resilience of MVT-M1 to high-energy electron beams allows for direct observation of Zn2+ insertion/extraction at the atomic scale within the tunnels for the first time using high-angle annular dark field scanning transmission electron microscopy; the storage location of zinc ions within the cathode is accurately determined layer by layer from the surface to the bulk phase by employing time-of-flight secondary ion mass spectrometry. Additionally, solvent molecules (H2O and methanol) are also found inside the tunnels along with Zn2+. Due to the broader heptagonal tunnels and Te ions in the hexagonal tunnels, MVT-M1 exhibits good cycling stability, outperforming MoVTe oxide with the M2 phase (no heptagonal tunnels) and MoV oxide with the M1 phase (no Te). These findings hold significant importance in advancing our understanding of the Zn2+ storage mechanism and enable the design of novel materials specifically optimized for efficient Zn2+ storage.

X-ray diffraction (XRD) patterns of the samples were collected using a Rigaku D/MAX-2500/PC with Cu Kα radiation (λ=1.54 Å at 40 kV and 200 mA). The data were recorded from 5º to 60º with an interval of 0.02º and a scan speed of 5º/min.

Microstructure.
The surface morphologies of the samples were captured with the FEI Quanta 200 F. Elemental mapping along with morphology were obtained by a scanning transmission electron microscope (STEM, JEM-ARM200F) equipped with an energy-dispersive X-ray spectrometer (EDS). The atomic tunnels, crystal structures and morphology were acquired by high angle annular dark field (HAADF) imaging in the scanning transmission electron microscopy (STEM) (Titan Themis ETEM G3).

Chemical Analysis.
The concentrations of the elements of interest were analyzed by an inductively coupled plasma optical emission spectrometer (PerkinElmer ICP-OES 7300DV) and X-ray fluorescence spectrometer (Zetium).

Surface chemistry.
The surface chemical compositions and oxidation states of the elements were analyzed using a Thermofisher Escalab 250 Xi+ spectrometer with Al Ka X-ray radiation (hν=1486.6eV). Prior to this analysis, the cycled electrodes were washed thoroughly with DI water to remove electrolyte residue and then dried in a glove box. All the binding energies were corrected by adventitious C 1s at 284.6 eV.

Battery cell assembly
Electrochemical tests were carried out in CR2032-type coin cells. To prepare a cathode, 60wt% active materials, 26wt% Super-P, and 14wt% polyvinylidene fluoride (PVDF) were thoroughly mixed and dispersed into N-methyl pyrrolidone (NMP). The high ratios of carbon and binder are just used to ensure insufficient conductivity and adhesive strength. The resultant slurry was then coated uniformly onto a 14 mm diameter stainless steel mesh (SSM), resulting in an ~ 1.2 mg cm -2 active mass loading, followed by drying at 100 ºC for 12 h. In a full ZIB cell, zinc foil was used as the anode, 2 M Zn(OTf)2 as the electrolyte, and glass microfiber filters (Whatman, Grade GF/A) as the separator.

Electrochemical testing
The CR2032-type coin cells for zinc ion batteries were assembled in air and tested using a LAND battery testing system (CT2001A) within a potential window of 0.2-1.6 V (vs Zn/Zn 2+ ).
The CR2032-type coin cells for lithium batteries were assembled in Ar-filled glovebox and tested using a LAND battery testing system (CT2001A) within a potential window of 1.0-3.5 V (vs Li/Li + ).
Cyclic voltammetry (CV) measurements were performed in a CR2032-type coin-cell using a VersaSTAT 3F electrochemical workstation. Linear sweep voltammogram (LSV) experiments were performed in a three-electrode configuration using a VersaSTAT 3F electrochemical workstation. A saturated calomel electrode (SCE) and Pt wire were used as reference and counter electrodes, respectively. The galvanostatic intermittent titration technique (GITT) was used to determine ionic diffusivity from a series of galvanostatic discharge pulses of 10 min at 50 mA g -1 , followed by a 1 h relaxation. The total ionic diffusion coefficient ( ) is calculated by 2 : where is the constant current pulse time; L corresponds to the ion diffusion length, which equals the thickness of the electrode; ∆Es is the change in steady-state voltage during a single-          Compared to MVT-M1 in the OCV state ( Figure S10a), many nanoflakes appear on the surface of MVT-M1 in the discharged state ( Figure S10b) and then mostly disappear in the charged state ( Figure S10c).         Note that the higher discharge capacity than charge capacity in the 1 st cycle indicates the inserted ions cannot be fully extracted when charge.   The ohmic resistances of the battery in the electrolytes using different solvents range from 1.9 to 6 ohm, and the current is about 0.3 mA. Therefore, the IR drop is lower than 2 mV, that is the ionic conductivities in the five electrolytes are sufficiently high.    The corresponding plots of log (peak current, i) vs log (scan rate, v) at cathodic and anodic peaks. (c-g) Capacitive contribution (grey portion) and diffusion contribution (white portion) of MVT-M1 at multiple scan rates from 0.2 to 1 mV s -1 .
Generally, the peak current (i) of a CV can be related to the scan rate (v) using an empirical power-law relationship to describe the combination of surface-controlled capacitive effects (i1 = k1v) and diffusion-controlled processes (i2 = k2v 1/2 ) 6 , where k1, k2, a, and b are variable parameters with b = 0.5 for a diffusion-controlled chargetransfer process and 1.0 for a surface-controlled capacitive process. Figure S27. Galvanostatic discharge-charge profiles at different current densities for the MVT-M1 cathode in ZIBs.  As VO2 and V2O3 with tunnel structures would be oxidized to layered ZnxV2O5·nH2O, the Ragone plot of ZnxV2O5·nH2O is provided to represent those for VO2 and V2O3 with tunnel structures. Figure S29 SEM images of MVT-M1 cathodes after 5000 cycles at 5 A g -1 .