Fe‐Ion Bolted VOPO <sub>4</sub> ∙2H <sub>2</sub> O as an Aqueous Fe‐Ion Battery Electrode

batteries, many of which are primary batteries; [1] new aqueous batteries often employ neutral or mildly acidic electrolytes that exhibit a wider electrochemical stability window. [9,10] Recently, the field has been inundated with reports of progress made on zinc metal anode that has many attributes. [11][12][13][14][15][16] Nevertheless, batteries that employ an iron metal anode have been left out despite iron's high specific capacity of 960 mAh/g and exceedingly low price of $60 per tonne in contrast to $2600 for zinc. 17 Moreover, Fe possesses a higher M 2+ /M redox potential of -0.44 V (vs SHE, hereafter) compared to -0.76 V for Zn 2+ /Zn, which promises no or a lower extent of hydrogen evolution reaction (HER) depending on the pH value of the electrolyte. [17,18] The history of iron batteries dates back to the NiFe battery, which was patented by in 1901 and commercialized soon afterward by the Edison Storage Battery Company. [19] In the NiFe battery, hydroxide ions serve as the charge carrier, instead of Fe 2+ , for the operation of both cathode and anode. [20,21,22] Despite the advantages, Fe-ion batteries remain underexplored, particularly with fewer Fe 2+ -hosting cathode materials reported than the Zn 2+ -hosting cathodes, e.g., MnO 2 , V 2 O 5 , VS x , Prussian blue, and polyphosphates. [23][24][25] Recently, our group reported the performance of "insoluble" Prussian blue (IPB) (Fe 3+ [Fe 2+ (CN) 6 ] 0.73 □ 0.27 3.6H 2 O) that delivers a stable specific capacity of 60 mAh/g and an average potential of 0.75 V versus Fe 2+ /Fe. 17 Studies were extended to a few other materials for Fe 2+ storage, which, unfortunately, encountered issues, where the capacity of V 2 O 5 fades quickly, MnO 2 shows a large overpotential (~0.5 V), and both FePO 4 and sulfur exhibit low average discharge potentials. [17,26] Sundara et al.

reported a non-aqueous rechargeable Fe battery that comprises a V 2 O 5 cathode and 1 M hydrated Fe(ClO 4 ) 2 in tetraethylene glycol dimethylether (TEGDME) as an electrolyte. [27] In this non-aqueous electrolyte, the V 2 O 5 cathode suffers a larger overpotential and lower reversibility compared to its performance in the aqueous electrolytes.

Herein, we report the promising performance of VOPO 4 •2H 2 O as a cathode for Fe-ion batteries, which delivers a specific capacity of 100 mAh/g at an average potential of ~0.6 V vs Fe 2+ /Fe. Most importantly, VOPO 4 •2H 2 O does not suffer fast capacity fading in the Fe 2+ electrolytes by retaining 68% of its capacity over 800 cycles. Nevertheless, when hosting Zn 2+ , VOPO 4 •2H 2 O suffers rapid dissolution and capacity fading in aqueous electrolytes, which is a known challenge. [13,28,29] To avoid electrode dissolution, Wang and Xu et al. used an acetonitrile-based electrolyte that has 1% of water. [12] Srinivasan et al. strengthened the structure of VOPO 4 •2H 2 O by the pre-insertion of polypyrrole, which, unfortunately, still cannot stabilize the capacity of VOPO 4 •2H 2 O in pure aqueous electrolytes, albeit this approach is more effective in a hybrid aqueous/non-aqueous electrolyte. [13] In this study, we found that VOPO 4 •2H 2 O favors the Fe 2+ storage, where its structure is stabilized by the initial spontaneous incorporation and oxidation of Fe 2+ at open circuit voltage (OCV), and the trapped Fe 3+ in the structure bolts the structure of VOPO 4 •2H 2 O for reversible Fe 2+ hosting. VOPO 4 •2H 2 O. (b) XRD pattern. (c) The schematic structure. (d) FTIR spectrum. (e) TGA curve. (f) XPS spectrum.

Results and Discussion

The layered VOPO 4 •2H 2 O was synthesized via a well-established reflux method, with details provided in the experimental section. [30] Figure 1a shows the SEM image of the assynthesized material, which comprises aggregations of flakes with particle sizes ranging from ca. 1 to 10 µm. The X-ray diffraction (XRD) pattern (Figure 1b) indicates a phase-pure product, where the peaks can be indexed to VOPO 4 •2H 2 O (PDF# 36-1472) with the tetragonal P4/n space group. [31] Figure 1c schematically shows the crystal structure, where the VO 6 octahedra and PO 4 tetrahedra share corners, thus constructing a layered structure. The Fourier transform infrared (FTIR) spectrum confirms the presence of lattice water in the structure (Figure 1d). The peak at 3580 cm -1 and the hump around 3100 cm -1 are assigned to the water's stretching modes, 𝜈 "# . The one at the higher wavenumber corresponds to water that is not fully hydrogen bonded, and the one at the lower wavenumber can be assigned to the lattice water molecules in the VOPO 4 structure. The peak at 1580 cm -1 corresponds to the bending mode of water, 𝜈 #"# , which is much red-shifted. The mass loss of VOPO 4 •2H 2 O by 18.2 wt.% in the thermogravimetric analysis (TGA) suggests the presence of two lattice water molecules per formula of VOPO 4 (Figure 1e), and two endothermic peaks indicate two types of water with different chemical environments, where one type as surface water evaporates at a relatively low temperature, and the other type as the lattice water molecules evaporates at a higher temperature due to the interaction with the electrode lattice. The Fourier-transform infrared (FTIR) spectrum reveals the vibrations of a polyanionic network, where the signal at 680 cm -1 is assigned to the V-O-V extension vibration, the peak at 950 cm -1 corresponds to the V=O stretching motion, and the peak at 1080 cm -1 pertains to the P-O stretching of the PO 4 3-groups. [30] X-ray photoelectron spectroscopy (XPS) results suggest that the average vanadium valence state is +4.7 (Figure 1f). However, XPS is a surface technology; thus, the oxidation state lower than +5 could be attributed to the surface defects of oxygen and phosphate ions. The full XPS spectrum is provided in Figure S1, which confirms the existence of other elements such as phosphorous and oxygen. C = 100 mA/g). (b) CV curves at a scan rate of 0.1 mV/s. (c) GCD potential profiles at different C-rates. (d) Calculated diffusion coefficients as a function of potentials from the GITT results. (e) EIS spectra at different state of charge (SOC). (f) Cycling performance for Zn storage (discharge capacity) and Fe storage (discharge capacity) with Coulombic efficiency (CE) of Fe 2+ storage at 1 C. The electrochemical properties of VOPO 4 •2H 2 O were evaluated by using 1 M aqueous FeSO 4 (pH value of 5) as the electrolyte and metallic Fe powder as the counter electrode. It

should be noted that the study on Fe metal anode in this electrolyte has been reported by our group previously. [17] As shown in Figure 2a, the galvanostatic charge-discharge (GCD) cycling of (de)insertion of Fe 2+ in VOPO 4 •2H 2 O displays largely sloping potential profiles, which is different from the results of Zn 2+ storage in VOPO 4 •2H 2 O. [5,13] The first discharge exhibits a specific capacity of 75 mAh/g, the following charge delivers a capacity of 100 mAh/g, and the second discharge exhibits a capacity of 98 mAh/g. For mildly acidic aqueous electrolytes (pH value of 5 for 1 M FeSO 4 here), it is necessary to determine whether the storage of protons contributes much to the observed capacity. From a control experiment of resolved CV peaks and the quasi-rectangular shape of the CV curves liken the behavior here to that of pseudocapacitive electrode materials. [32][33][34][35] The kinetics of the Fe 2+ storage in the VOPO 4 •2H 2 O electrode can be evaluated by calculating the b-values of the peak currents using the following equation:

where a is a coefficient that is simplified from i= k 1 v+k 2 v 1/2 . An ideal capacitive behavior would give rise to a b value of 1, and an ideal diffusion-controlled process would lead to a b value of 0.5. [36,37] Figure S4 show that the cathodic and anodic current maxima exhibit b values of 0.79 and 0.88, respectively, which tells a good extent of the capacitive behavior.

However, a capacitive behavior cannot always be translated to faster rate capability, which is epitomized here. At 10 C, the electrode retains only 47% of its 1 C capacity (Figure 2c), which can be attributed to the bulk particle sizes of VOPO 4 •2H 2 O. Note that at 10 C rate, the IPB electrode retains 84% of its 1 C capacity for the Fe 2+ storage, where the particle sizes of the IPB electrode are below 100 nm. [17] As a comparison, the rate performance of To further examine the kinetics of Fe 2+ (de)intercalation, galvanostatic intermittent titration technique (GITT) was employed to calculate the diffusion coefficient over the charging process, which reveals the diffusivity ranging from 1E-13 ~1E-11 cm 2 /s, as shown in Figure 2d and Figure S6. Such diffusivity resembles that of VOPO 4 when storing Zn 2+ from aqueous electrolytes. [38] Interestingly, the diffusion coefficient continuously decreases with the increase of the cell potential. The slower kinetics at higher potentials is corroborated by the results of electrochemical impedance spectroscopy (EIS). As shown in Figure 2e, R ct gradually increases from 6.9 Ω (at 0 V) to 8.1 Ω (at 0.9 V).

The results imply that the initial removal of Fe 2+ renders further extraction of Fe 2+ more difficult; thus, we can deduce that the initial intercalation of Fe 2+ can facilitate the diffusion of forthcoming Fe 2+ intercalation. VOPO 4 •2H 2 O exhibits much more stable cycling performance in 1 M FeSO 4 than in 1 M ZnSO 4 (Figure 2f). VOPO 4 •2H 2 O electrode retains 81% of its initial capacity after 50 cycles at 1 C in 1 M FeSO 4 , whereas in the ZnSO 4 electrolyte, the specific capacity fades to 28% within 50 cycles. The VOPO 4 •2H 2 O as an electrode usually shows poor cycling performance for hosting multi-valence charge carriers, e.g., Mg 2+ , Ca 2+ , and Zn 2+ , because the (de)insertion of multivalent charge carriers causes the exfoliation of the layered structure

assembled by the weak van der Waals forces. [12,31,40] The stable cycling performance in FeSO 4 is surprising since one would expect similar capacity decaying performance for hosting Fe 2+ and Zn 2+ alike, given that they have the same charge (2+) and similar ionic radii (70 pm for Fe 2+ vs. 74 pm for Zn 2+ ). [17] Interestingly, soaking the VOPO (Figure 3d). Furthermore, after soaking, both V=O stretching (875.6 cm -1 ) and P-O stretching (1072.3 cm -1 ) of the VOPO 4 framework blue-shift from 875.6 cm -1 to 962.7 cm -1 and from 1072.3 cm -1 to 1143.9 cm -1 , respectively, suggesting the shortened bonds due to the removal of lattice water molecules.

Another piece of evidence of spontaneous Fe-ion insertion into the structure is that the OCV of the resting cells decreases, which indicates the reduction of the VOPO 4 •2H 2 O electrode. It can be described by the following chemical reaction:

where vanadium (+5) is reduced to a lower oxidation state, and Fe 2+ is oxidized to Fe 3+ . In order to compensate for the charge neutrality, Fe 3+ ought to be inserted into the lattice of at different SOC, which are color-coded corresponding to (a). (c) An enlarged view of the (001) peak. (d) Ex situ FTIR spectra at different SOC (dashed line: before soaking; solid line: after soaking). XPS spectra of electrodes for (e) V 2p 3/2 and (f) Fe 2p 3/2 at different SOC.

As shown in XRD patterns (Figure 3c), the discharge process causes the structure to further contract along the [001] direction continuously, where the (001) interlayer d-spacing shrinks from 6.42 Å to 6.16 Å upon the full discharge. Upon charging, the structure reversibly expands back to the (001) d-spacing of the soaked electrode but not to that of the pristine dry electrode, suggesting good structural elasticity and the trapping of Fe 3+ inserted during soaking. Interestingly, the discharge process does not vary the positions of the peaks of P-O and V=O in the FTIR spectra, which suggests a lack of bonding covalency between the inserted Fe 2+ ions and the host framework. Ex situ XPS results reveal the valence changes of V and Fe during the GCD processes, as shown in Figure 3e, f. The first discharge process renders V 5+ nearly completely reduced to V 4+ and brings in much Fe 2+ . The following charge eliminates most of the Fe 2+ from the electrode; however, even after the electrode is fully charged, there still exists significant presence of V 4+ due to the trapping of Fe 3+ in the electrode.

To this point, a mechanism surfaces, as illustrated in Figure 4a.

takes in and traps Fe 3+ upon touching the electrolyte, which contracts the lattice and expels some of the lattice water. During the subsequent discharge process, the intercalation of Fe 2+ causes further structural contraction between the (001) layers. This structural contraction due to the electrochemical Fe 2+ insertion can be reversed in the following charge process.

However, this charge process cannot reverse the structural changes of the electrode caused by the electrode soaking, which suggests that the pre-intercalation of Fe 3+ is permanent, and these ions are trapped in the structure throughout cycling. and 1 M ZnSO 4 before (top) and after one week (bottom). We further examined the cycling stability of VOPO 4 •2H 2 O for Fe 2+ storage, Figure 4b shows the cycling performance of VOPO 4 •2H 2 O in the FeSO 4 electrolyte at 8 C, where the electrode exhibits stable cycling for 800 cycles with 65% capacity retention. The cycling results offer additional evidence for the mechanism of Fe-ion storage in VOPO 4 •2H 2 O, where the trapped Fe 3+ ions provide ionic bonding between the layers, thus bolting the structure and making it difficult to exfoliate. Chemical stability of VOPO 4 •2H 2 O powder in 1 M FeSO 4 and 1 M ZnSO 4 electrolytes can be differentiated by a soaking process. As shown in Figure 4c, after one week, VOPO 4 •2H 2 O (100 mg) is largely dissolved in 3 ml 1 M ZnSO 4 , rendering a solution dark red. In contrast, the VOPO 4 •2H 2 O powder remains as solid in 1 M FeSO 4 after soaking.

The mechanism of slow capacity fading for storing Fe 2+ is speculated to be the oxidation of Fe 2+ in the electrolyte as side reactions. As shown in Figure S8a, the separator collected after 300 cycles shows the color of brown precipitation, indicating the presence of Fe 3+ due to the oxidation of the electrolyte. The capacity of the cycled electrode is restored after the cycled electrode is rinsed and put into a new cell with the fresh FeSO 4 electrolyte (Figure S8b). Note that the Swagelok cells we used for the tests are not as hermetic as pouch cells or coin cells. Therefore, provided that the iron batteries are better sealed, the cycling life of such batteries can be significantly provided.

Fe-ion batteries with Fe 2+ as a charge carrier holds a unique position in aqueous batteries research due to the extremely low cost and abundance of Fe. However, this battery chemistry also has its own drawbacks compared to other aqueous batteries such as the sensitivity of the Fe 2+ electrolyte to air and the upper limit of the cathode's operation potential that should not be high enough to oxidize Fe 2+ . In addition, to date, the plating/stripping of Fe metal anode has yet not demonstrated a high CE comparable to that of Zn metal anode. To better illustrate the pros and cons of this emerging battery, Table S1 is provided in the Supporting Information, where the potentially developed Fe-ion batteries are compared with other aqueous storage batteries. The message is that Fe-ion batteries would potentially be competitive with other aqueous batteries by its low cost rather than by its energy density, provided that its long cycle life can be secured with highly reversible Fe metal anode.