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. 2020 Apr 20;5(17):10115–10122. doi: 10.1021/acsomega.0c00728

ϵ-FeOOH: A Novel Negative Electrode Material for Li- and Na-Ion Batteries

Kazuhiko Mukai †,*, Ikuya Yamada ‡
PMCID: PMC7203964  PMID: 32391499

Abstract

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The demand for eco-friendly materials for secondary batteries has stimulated the exploration of a wide variety of Fe oxides, but their potential as electrode materials remains unknown. In this contribution, ϵ-FeOOH was synthesized using a high-pressure/high-temperature method and examined for the first time in nonaqueous Li and Na cells. Under a pressure of 8 GPa, α-FeOOH transformed into ϵ-FeOOH at 400 °C and then decomposed into α-Fe2O3 and H2O above 500 °C. Here, FeO6 octahedra form [2 × 1] tunnels in α-FeOOH or [1 × 1] tunnels in ϵ-FeOOH. The ϵ-FeOOH/Li cell exhibited a rechargeable capacity (Qrecha) of ∼700 mA h·g–1 at 0.02–3.0 V, whereas the ϵ-FeOOH/Na cell indicated a Qrecha of less than 30 mA h·g–1 at 0.02–2.7 V. The discharge and charge profiles of ϵ-FeOOH and α-FeOOH were similar, but the rate capability of ϵ-FeOOH was superior to that of α-FeOOH.

Introduction

With the growing market for secondary batteries, the abundance of resources for these batteries and their eco-friendliness are becoming crucial issues for building a sustainable society. Electrode materials consisting of Fe oxides are a possible solution because Fe has the fourth highest Clark number and low toxicity. Since the commercialization of lithium-ion batteries (LIBs), various Fe oxides such as FeOOH,1−11 LiFeO2,12−15 Fe2O3,6,16−22 and Fe3O46,18,23−25 have been proposed. Among these Fe oxides, FeOOH has especially attracted attention as a negative electrode material for LIBs1−4,6,8,9,11 or as a catalyst for Li–O2 batteries.5,10 Furthermore, FeOOH has been utilized as a precursor to synthesize Fe2O3 and Fe3O4 powders, exhibiting interesting particle morphologies.6,17,24,25

FeOOH includes several polymorphs, as illustrated in Figure 1.26−29 For example, α-FeOOH (goethite) crystallizes into an orthorhombic structure with the Pnma space group, building [2 × 1] tunnels (Figure 1a).26 LIBs with α-FeOOH nanorods with a length of 100–200 nm and a diameter of ∼40 nm indicated a rechargeable capacity (Qrecha) of ∼600 mA h·g–1 at 0.01–3.0 V versus Li+/Li.4 Meanwhile, β-FeOOH (akaganeite) possesses a tetragonal structure with the I4/m space group, forming two types of [2 × 2] and [1 × 1] tunnels (Figure 1b).27 The chemical formula of β-FeOOH is specifically represented as FeO1–x(OH)1+xClx, where Cl– ions lie at the center to maintain the [2 × 2] tunnels.9,27 The electrochemical properties of β-FeOOH strongly depend on its particle morphologies and particle sizes.2,3,7 Yu et al. reported that the Qrecha of LIBs with nano-sized β-FeOOH reached ∼1400 mA h·g–1.7 However, the mechanism underlying this large Qrecha is a debatable subject because the maximum theoretical capacity (Qtheo) of FeOOH is limited to 746.1 mA h·g–1 by the following reactions

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for the initial decomposition reaction and

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for the subsequent charge and discharge reactions. As shown in Figure 1c, γ-FeOOH (lepidocrocite) exhibits a zigzag layer structure with the Cmcm space group28 and has thus been employed as a precursor to synthesize FeOCl1 or FeOOLi12 via ion exchange reactions. As a positive electrode material, FeOOLi resulted in a Qrecha of ∼90 mA h·g–1.12 The crystal structure of δ-FeOOH depends on the material origins or preparation methods, and thus, several structural models have been proposed.29−31 The average structure of δ-FeOOH is usually interpreted to be hexagonal with the P3̅m1 space group, as shown in Figure 1d.

Figure 1.

Figure 1

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Crystal structures of (a) α-FeOOH (goethite), (b) β-FeOOH (akaganeite), (c) γ-FeOOH (lepidocrocite), (d) δ-FeOOH (feroxyhyte), and (e) ϵ-FeOOH.

In contrast to α-, β-, γ-, and δ-FeOOH, another isomorph, namely, ϵ-FeOOH, is obtained at high pressures above 5 GPa and temperatures above 200 °C,32,33 and it crystallizes into an orthorhombic structure with the Pmn21 space group. As shown in Figure 1e, ϵ-FeOOH forms [1 × 1] tunnels through edge-sharing FeO6 octahedra. The crystal structure of ϵ-FeOOH can be regarded as a distorted rutile structure, and it is similar to those of InOOH, β-CrOOH (guyanaite), and GaOOH.34−36 In the field of geochemistry, ϵ-FeOOH and even higher-pressure phases have received a great deal of attention,37,38 but to our best knowledge, the electrochemical properties of ϵ-FeOOH remain unknown.

According to the true density, that is, the density determined via powder X-ray diffraction (XRD) measurements (dXRD), dXRD of ϵ-FeOOH (≃4.44 g·cm–3)32,33 is greater than that of α-FeOOH (≃ 4.18 g·cm–3),26 β-FeOOH (≃ 3.58 g·cm–3),27 or γ-FeOOH (≃4.00 g·cm–3).28 This allows increasing the volumetric energy density of LIBs, which is an important performance metric. Moreover, determining the electrochemical properties of ϵ-FeOOH provides an in-depth understanding of the relationship between crystal structures and electrochemical properties, which is essential to designing future eco-friendly electrode materials. We thus synthesized ϵ-FeOOH using a high-pressure/high-temperature (HP/HT) method and examined its electrochemical properties in nonaqueous Li cells. In addition, we investigated the electrochemical properties in nonaqueous Na cells because Na is more abundant than Li. To our best of our knowledge, this is the first time this electrode material has been demonstrated in LIBs and sodium-ion batteries (NIBs).

Results and Discussion

Figure 2a shows the XRD pattern of the pristine α-FeOOH sample. The crystal structure of α-FeOOH is assigned to the orthorhombic structure with the Pnma space group. According to the Rietveld analysis, the lattice parameters were determined to be ao = 9.9772(5) Å, bo = 3.0278(2) Å, and co = 4.6131(3) Å, which are slightly larger than those previously reported for α-FeOOH (ao = 9.9134 Å, bo = 3.0128 Å, and co = 4.5800 Å).26 The Rietveld analysis results and structural parameters are summarized in Figure S1 and Table S1.

Figure 2.

Figure 2

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XRD patterns of (a) α-FeOOH (pristine) and (b)–(i) HP-/HT-treated samples in order of increasing treatment temperature from top to bottom. Red and blue stars indicate diffraction lines from ϵ-FeOOH and Fe2O3, respectively.

Figure 2b–i shows the XRD patterns of the HP-/HT-treated samples in order of increasing treatment temperature. The applied pressure was 8 GPa for all the samples. The HP(100 °C), HP(200 °C), and HP(300 °C) samples maintained the α-FeOOH-type structure, although additional diffraction lines indicated by red stars (★) were observed in the HP(300 °C) sample. Here, we denote these samples by HP(XXX °C), where XXX represents the heating temperature. These additional diffraction lines originate from the ϵ-FeOOH phase, as described later. The lattice parameters of the α-FeOOH phase decrease with the increasing heating temperature; for instance, the parameters of HP(100 °C) decrease from ao = 9.9771(11) Å to ao = 9.9615(5) Å, from bo = 3.0260(4) Å to bo = 3.0224(1), and from co = 4.6112(6) Å to co = 4.5968(3) Å for HP(300 °C). The XRD patterns of the HP(400 °C) and HP(500 °C) samples can almost be indexed to a single phase with an ϵ-FeOOH-type structure and the Pmn21 space group. The lattice parameters of HP(400 °C) were determined to be ao = 3.0056(1) Å, bo = 4.4620(1) Å, and co = 4.9585(1) Å, while those of HP(500 °C) were ao = 3.0039(1) Å, bo = 4.4590(1) Å, and co = 4.9554(1) Å. The Rietveld analysis results and structural parameters are summarized in Figure S2 and Table 1. The slightly small lattice volume (Vo) of HP(500 °C) is probably due to a high crystallinity of the sample. The diffraction line indicated by a blue star (★) is caused by the α-Fe2O3 phase (hematite) with the R3̅c space group, but the weight fraction of the α-Fe2O3 phase in HP(400 °C) [or HP(500 °C)] is limited to 0.5% according to the Rietveld analysis.

Table 1. Structural Parameters of HP(400 °C) and HP(500 °C) Samples Determined through the Rietveld Analysis.

sample atom Wyckoff position occupancy x y z Biso/Å2
HP(400 °C) Fe1 2a 1.0 0 0.220(1) 0.009(3) 0.3(1)
  O1 2a 1.0 0 0.002(1) 0.352(3) 0.1(1)
  O2 2a 1.0 0 0.498(1) 0.658(1) 0.1(1)
space group: Pmn21, Rwp = 3.51%, and S = 1.35
ao = 3.0056(1) Å, bo = 4.4620(1) Å, co = 4.9585(1) Å, and Vo = 66.498(1) Å3
HP(500 °C) Fe1 2a 1.0 0 0.220(1) 0.009(3) 0.3(1)
  O1 2a 1.0 0 0.004(1) 0.352(3) 0.1(1)
  O2 2a 1.0 0 0.499(1) 0.658(2) 0.1(1)
space group: Pmn21, Rwp = 3.38%, and S = 1.28
ao = 3.0039(1) Å, bo = 4.4590(1) Å, co = 4.9554(1) Å, and Vo = 66.374(1) Å3
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Further increasing the heating temperature produces α-Fe2O3, as evidenced by the XRD patterns of the HP(600 °C), HP(700 °C), and HP(800 °C) samples (Figure 2g–i). This process is represented by

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which involves a 10.13% weight loss during the reaction. According to the pressure–temperature phase diagram of FeOOH,32 α-FeOOH decomposed into α-Fe2O3 and H2O at pressures below 5 GPa and temperatures above 200 °C. Thus, the present study confirmed the stability of ϵ-FeOOH at 8 GPa and above 500 °C. Note that at ambient pressure, ϵ-FeOOH also decomposed into α-Fe2O3 and H2O, as demonstrated by the thermogravimetric (TG) curve shown in Figure S3. The observed weight loss (=10.5%) was consistent with the calculated weight loss described in eq 3.

Figure 3a–f shows the scanning electron microscopy (SEM) images of the pristine α-FeOOH, HP(100 °C), HP(200 °C), HP(400 °C), HP(500 °C), and HP(800 °C) samples. Field emission SEM (FE-SEM) images of only the pristine α-FeOOH and HP(500 °C) samples are shown in Figure 3g–i. Evidently, particles in the pristine α-FeOOH sample aggregate together, forming large secondary particles. As shown in Figure 3g,i, the primary particles exhibit a needle shape with dimensions of ∼100 × 500 nm. The HP(100 °C) sample indicates a spherical secondary particle with a radius of ∼10 μm, while spherical secondary particles are partially broken in the HP(200 °C) sample. The HP(400 °C) sample, which crystallized into the ϵ-FeOOH-type structure, shows a large, scaly-shaped secondary particle of more than 20 μm in diameter. In contrast, particles in HP(500 °C) are isolated from each other and exhibit a flat, smooth surface, as evidenced by the plate-shaped particles with a thickness of ∼200 nm and a lateral length of ∼1 μm in Figure 3i. Meanwhile, the HP(800 °C) sample exhibits nonuniform large particles of ∼50 μm in size.

Figure 3.

Figure 3

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SEM images of (a) α-FeOOH (pristine), (b) HP(100 °C), (c) HP(200 °C), (d) HP(400 °C), (e) HP(500 °C), and (f) HP(800 °C). According to XRD measurements, the major phase is α-FeOOH for HP(100 °C) and HP(200 °C), ϵ-FeOOH for HP(400 °C) and HP(500 °C), and Fe2O3 for HP(800 °C). FE-SEM images of α-FeOOH (pristine) at the (g) 1 μm scale and (h) 500 nm scale and (i) HP(500 °C) at the 1 μm scale.

Above, the drastic changes in particle morphologies and particle sizes were shown to correlate with the structural changes associated with the α-FeOOH → ϵ-FeOOH → α-Fe2O3 transformation. Based on the crystallographic relationship, the Pmn21 (ϵ-FeOOH) space group is a maximal nonisomorphic subgroup of the Pnma (α-FeOOH) space group.41 On the other hand, the R3̅c (α-Fe2O3) space group is not a minimal nonisomorphic supergroup of the Pmn21 space group. Hence, the α-FeOOH structure can change to ϵ-FeOOH by a simple translation of Fe and O atoms, while the structural change from ϵ-FeOOH to α-Fe2O3 should be accompanied by breaking and reforming the Fe–O bonds. These drastic changes in particle morphologies and particle sizes also appeared along with the structural changes in Li[Li1/3Ti5/3]O4, in which a spinel structure with the Fd3̅m space group altered into a columbite structure with the Pbcn space group under a pressure of 12 GPa and at temperatures above 600 °C.39

Figure 4 shows the discharge and charge curves of Li cells with the pristine α-FeOOH, HP(100 °C), HP(200 °C), HP(300 °C), HP(400 °C), HP(500 °C), HP(600 °C), and HP(800 °C) samples operated at a current of 0.1 mA. The cell voltage of the α-FeOOH/Li cell decreases to ∼1.0 V at the beginning of the discharge reaction and then remains constant at ∼0.8 V up to a discharge capacity (Qdis) of ∼800 mA h·g–1. However, the cell voltage in the charge reaction gradually increases to 3 V without a significant plateau. The Qdis value at the first cycle reached 1230.9 mA h·g–1, while the charge capacity (Qcha) at the first cycle was limited to 667.9 mA h·g–1. This large irreversible capacity can be attributed to the formation of LiOH and/or decomposition of a polytetrafluoroethylene (PTFE) binder that occurred during the discharge reaction in the first cycle.39,40 Indeed, the subsequent discharge curve indicates a plateau at ∼0.8 V with a Qdis of 697.6 mA h g–1, which is close to the Qcha of the first cycle.

Figure 4.

Figure 4

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Discharge and charge curves of Li cells with (a) α-FeOOH (pristine) and (b–h) HP-/HT-treated samples in order of increasing treatment temperature from top to bottom. The cells were operated at a current of 0.1 mA. The red lines in (a–h) are the discharge and charges curves at the first cycle. dQdis/dV (or dQcha/dV) curves of the (i) α-FeOOH (pristine), (j) HP(400 °C), and (k) HP(800 °C) samples. The red and black lines in (i–k) are the dQdis/dV (or dQcha/dV) curves at the first and second cycles, respectively.

The discharge and charge profiles of HP(100 °C) and α-FeOOH are similar, but the Qdis and Qcha values of HP(100 °C) are much lower than those of α-FeOOH. Specifically, HP(100 °C) exhibits Qdis = 293.6 mA h·g–1 at the second cycle and Qcha = 291.7 mA h·g–1 at the first cycle. This is probably because the aggregated large secondary particles in the HP(100 °C) sample hinder the insertion of Li+ ions and the conduction of electrons (see Figure 3b). With the increasing treatment temperature, the Qdis and Qcha values increase in the HP(300 °C), HP(400 °C), and HP(500 °C) samples but then decrease in the HP(600 °C) and HP(800 °C) samples.

Ignoring the Qdis and Qcha values, the discharge and charge profiles seem to be similar to each other. To clarify the differences between electrochemical properties of the samples, we performed electrochemical measurements over a voltage range of 1.0–3.0 V. Figure S4 shows the discharge and charge curves of Li cells with the pristine α-FeOOH, HP(400 °C), and HP(800 °C) samples. The Li cells with α-FeOOH and HP(400 °C) indicated a plateau at ∼1.6 V up to a Qdis of ∼150 mA h·g–1, whereas the cell voltage of HP(800 °C) rapidly dropped to 1.0 V. Figure 4i–k shows dQdis/dV (or dQcha/dV) curves of α-FeOOH, HP(400 °C), and HP(800 °C) obtained by using the discharge and charge curves at 0.02–3.0 and 1.0–3.0 V. For both α-FeOOH and HP(400 °C) samples, one or two reduction peaks are observed in the discharge curve at the first cycle. Moreover, the dQdis/dV curves at the second cycle are similar between the α-FeOOH and HP(400 °C) samples. According to electrochemical measurements on β-FeOOH,2,8,9 the cell voltage gradually decreased (increased) as the discharge (charge) reaction proceeded and exhibited no characteristic plateau down to 1.0 V. Therefore, the electrochemical properties of ϵ-FeOOH resemble those of α-FeOOH but differ from those of β-FeOOH. This finding indicates that [2 × 2] tunnels, which are only present in β-FeOOH, significantly influence the formation of the plateau at ∼1.6 V. Note that the electrochemical properties shown in Figure S4 are free from the contributions of the decomposition of the PTFE binder and the conversion reaction described in eq 2. In other words, Li+ ions are thought to be inserted into the FeOOH lattice.

When the discharge cutoff voltage decreases to 0.02 V, the so-called conversion reaction described in eq 2 becomes dominant, and thus, the differences between the discharge and charge profiles of the samples vanish. Because the complete conversion reaction offers a Qtheo of 746.1 mA h·g–1, Fe ions with the monovalent state still exist in the sample based on the observed Qcha values. In general, the maximum Qrecha value of FeOOH polymorphs seems to remain in the range of 600–800 mA h ·g–1,4,6,11 except for nano-sized β-FeOOH.8 As described in the Introduction, a Qrecha of more than 1000 mA h·g–1 cannot be explained without considering other contributions such as the capacitance of carbon additives.

To further clarify the differences between the electrochemical properties of α-FeOOH and ϵ-FeOOH, extended cycle tests were performed at a current of 0.3 mA over 30 cycles. Figure 5a,b shows the discharge and charge curves of Li cells with the HP(300 °C) and HP(400 °C) samples, respectively. As shown in Figure 2d,e, the major phase of HP(300 °C) was α-FeOOH, whereas that of HP(400 °C) was ϵ-FeOOH. For HP(300 °C), the Qcha value at the first cycle decreases from 827.6 to 338.8 mA h·g–1 as the applied current increases from 0.1 to 0.3 mA. Meanwhile, for HP(400 °C), the Qcha at the first cycle remains at 592.8 mA h·g–1, which is 85.2% of the Qcha value at 0.1 mA. Hence, ϵ-FeOOH is found to have a better rate capability than α-FeOOH, although its discharge and charge profiles are similar to those of α-FeOOH. Actually, the rate capability shown in Figure S5 supported such consideration.

Figure 5.

Figure 5

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Extended cycle tests on the (a) HP(300 °C) and (b) HP(400 °C) samples at a current of 0.3 mA. For clarity, the discharge curve for the first cycle is not displayed. (c) Qcha as a function of cycle number. The blue lines in (b) are the discharge and charge curves after the 30th cycle operated at a current of 0.1 mA.

For both HP(300 °C) and HP(400 °C), Qchas and (Qdiss) rapidly decrease during the initial five cycles. At the 30th cycle, HP(300 °C) and HP(400 °C) retained 47.1 and 31.1% of their initial capacities, respectively, as shown in Figure 5c. The rapid decrease in Qcha (Qdis) during the initial five cycles was also reported for previous β-FeOOH compounds.4,6,11 To reveal the origin of the capacity fading, discharge and charge tests at a low current (0.1 mA) were conducted for HP(400 °C). As indicated by the blue lines in Figure 5b, the Qcha value recovered to 481.5 mA h·g–1 at 0.1 mA, suggesting that the capacity fading in HP(400 °C) originates from a kinetic factor such as the isolation of ϵ-FeOOH from the conductive carbon. Note that the Qdis value at 0.1 mA was limited to 332.9 mA h·g–1. This finding indicates that the charge reaction is slower than the discharge reaction, and thus, Li+ ions remain in the ϵ-FeOOH particles in the charge-end state. Further optimizing the particle size and electrode mixture could improve the cyclability of ϵ-FeOOH.

Ex situ XRD measurements were performed on HP(400 °C) to clarify the stability of ϵ-FeOOH during the discharge reaction. Figure S6 shows the XRD patterns at discharged states of 1.2 V (D1), 1.0 V (D2), and 0.02 V (D3). The diffraction lines originated from the ϵ-FeOOH phase almost disappeared at D1, and three weak diffraction lines were observed at 2θ = 54.71, 81.21, and 105.54 ◦. The XRD pattern at D2 was similar to that at D1, but the intensities of the three diffraction lines became weak at D3. Although it is currently difficult to assign the above three diffraction lines, the cubic lattice parameter (ac) was estimated to be 4.205(1) Å assuming for a rock-salt (LiFe)O phase. This lattice parameter is comparable to ac (=4.162 Å) of α-LiFeO2 with a rock-salt structure.42

We then investigated the performance of ϵ-FeOOH as a negative electrode material for NIBs because β-FeOOH exhibited a Qrecha of ∼500 mA h·g–1.7Figure 6a shows the discharge and charge curves of a Na cell with the HP(400 °C) sample operated at a current of 0.1 mA. The discharge curve at the first cycle indicates a plateau at ∼0.8 V, while the charge curve at the first cycle shows a slight plateau at ∼1.8 V. This electrochemical characteristic is well understood by the dQdis/dV and dQcha/dV curves shown in Figure 6b. The Qdis and Qcha values at the first cycle were 289.6 and 96.8 mA h·g–1, respectively. The large irreversible capacity is attributed to the formation of LiOH and/or decomposition of the PTFE binder, as in the case for the Li cells shown in Figure 4a–h. The subsequent discharge and charge curves were featureless, and the voltage rapidly decreased (increased) with the proceeding discharge (charge) reaction. As a Qdis of less than ∼30 mA h·g–1 was observed in the subsequent discharge curves, ϵ-FeOOH was found to be an almost electrochemically inactive for NIBs, in contrast to β-FeOOH.

Figure 6.

Figure 6

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(a) Discharge and charge curves of the Na cell with the HP(400 °C) sample operated at a current of 0.1 mA and 25 °C. (b) Corresponding dQdis/dV (or dQcha/dV) curve at the first cycle.

Finally, we discuss the significance of ϵ-FeOOH as a negative electrode for LIBs. The ϵ-FeOOH/Li cell exhibited a Qrecha of ∼700 mA h·g–1 at 0.02–3.0 V, which was similar to previously reported results for α-FeOOH.4,6,11 However, because the dXRD of ϵ-FeOOH is greater than that of α-FeOOH, ϵ-FeOOH can increase the volumetric energy density of LIBs, which is more important than the gravimetric capacity because LIBs function in a limited volume. Nano-sized materials are disadvantaged in this regard, although they provide a large gravimetric capacity. Electrode materials with both a high capacity and a high dXRD, such as ϵ-FeOOH, are useful in actual LIBs. Moreover, from the structural point of view, ϵ-FeOOH has a unique electrochemical property. This is because TiO2 and SnO2 with a rutile structure were essentially electrochemical inactive and only nano-sized TiO2 and SnO2 particles indicated a Qrecha of ∼250 mA h·g–1.43−45 Because ϵ-FeOOH has the distorted rutile structure as shown in Figure 1e, further in situ and in operando analyses could reveal the relationship between the electrochemical properties and crystal structure.

Conclusions

An almost single phase of ϵ-FeOOH was synthesized from α-FeOOH using the HP/HT method and examined in nonaqueous Li and Na cells. The primary particles of the ϵ-FeOOH sample synthesized at 500 °C indicated a plate-like morphology with a thickness of ∼200 nm and a lateral length of ∼1 μm. This particle morphology was significantly different from that of α-FeOOH, in which secondary particles were formed by needle-shaped primary particles with dimensions of ∼100 × 500 nm. The ϵ-FeOOH/Li cell exhibited a Qrecha of 700 mA h·g–1 at 0.02–3.0 V, which was similar to that of α-FeOOH. However, the Qrecha of the α-FeOOH/Li cell rapidly decreased to ∼340 mA h·g–1 with the increasing applied current (0.3 mA). The Qcha values of ϵ-FeOOH at the charge currents of 0.6, 1.0, and 2.0 mA were greater than those of α-FeOOH. Hence, ϵ-FeOOH was found to provide a better rate capability than α-FeOOH. Furthermore, because the dXRD of ϵ-FeOOH is the highest among the FeOOH polymorphs, ϵ-FeOOH enables increasing the volumetric energy density of LIBs. Unfortunately, a Qrecha of less than 30 mA h·g–1 was observed in the ϵ-FeOOH/Na cell, in contrast to cells with β-FeOOH. The particle size and electrode mixture should be further optimized to improve the cyclability of ϵ-FeOOH.

Experimental Section

Sample Preparation

First, ϵ-FeOOH was synthesized using the HP/HT method using the Walker-type equipment at the Osaka Prefecture University, as previously reported.39,40,46,47 Approximately 50 mg of α-FeOOH powder (Koujyundo Chemical Laboratory Co., Ltd.) was packed into a (Mg,Co)O pressure medium (Mino Ceramics, Ltd.) and then placed at the center consisting of tungsten carbide anvils with eight truncations (Fuji Die Co., Ltd.). After reaching a pressure of 8 GPa, each sample was heated at 100, 200, 300, 400, 500, 600, 700, or 800 °C for 30 min.

Characterization

The obtained samples were characterized via SEM (S-3600 N, Hitachi High-Technologies) and XRD measurements using a Fe Kα radiation source (D8 ADVANCE, Bruker AXS). Before SEM observations, approximately 1 mg of sample, which was attached onto the sample holder with a carbon tape, was coated with electrically conducting Au particles (IB-3, Eiko Co., Ltd.). We also employed a field-emission SEM system (SU8020, Hitachi High-Technologies) for several samples. For this observation, samples were coated with Os particles (HPC-1S, Vacuum Device Inc.). Rietveld analyses were conducted using RIEATN-FP software,48 and the crystal structures were drawn using VESTA software.49 TG/differential thermal analysis was conducted in air up to 1000 °C at a heating rate of 20 °C·min–1.

Electrochemical Measurements

Discharge and charge curves were recorded using the nonaqueous Li and Na cells. For the Li and Na cells, the electrolytes were 1 M LiPF6 and 1 M NaPF6 separately dissolved in ethylene carbonate (EC)/diethylene carbonate (DEC) (EC/DEC = 1/1 by volume, Kishida Chemical Co., Ltd), respectively. For both types of cells, a mixed electrode consisting of 70 wt % active material, 20 wt % conducting carbon (acetylene black, AB, HS-100, Denka Co., Ltd), and 10 wt % PTFE (Daikin Industries, Ltd.) was used as the working electrode (diameter = 10 mm), while Li or Na metal pressed onto a stainless steel plate (diameter = 19 mm) was used as the counter electrode. After being fabricated in an argon-filled glovebox, the Li cells were operated at a current of 0.1 mA, while the Na cell was operated at a current of 0.3 mA. The rate capability test was performed only for the Li cells at the charge currents of 0.3, 0.6, 1.0, and 2.0 mA. The discharge current was fixed to 0.1 mA. The current of 0.1 mA corresponds to a current density of ∼0.1 mA·cm–2. The voltage ranges were 0.02–3.0 or 1.0–3.0 V for the Li cells and 0.02–2.7 V for the Na cell. The environmental temperature of the cells was 25 °C.

Ex Situ XRD Measurements

We performed ex situ XRD measurements on HP(400 °C) to clarify the stability of ϵ-FeOOH. Three lithium cells were fabricated using the 1 M LiPF6/EC + DEC electrolyte, and then each lithium cell was discharged to 1.2 (D1), 1.0 (D2), or 0.02 (D3) V at a current of 0.1 mA. The discharge curves are shown in Figure 6a. The mixed electrode containing AB and PTFE was removed in an argon-filled glovebox and covered with a Kapton tape to avoid reactions with a moist air. Open-circuit voltages just before the ex situ XRD measurements were 1.771 V for D1, 1.573 V for D2, and 0.360 V for D3. XRD patterns were recorded using the Fe Kα radiation source.

Acknowledgments

The authors wish to thank Dr. Takeshi Morikawa, Dr. Tomiko M. Suzuki, and Takeshi Uyama of TCRDL for discussions about FeOOH polymorphs and their potential applications.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c00728.

  • Rietveld analysis results and structural parameters of the pristine α-FeOOH sample; Rietveld analysis results of HP(400 °C) and HP(500 °C); TG curve of HP(500 °C); discharge and charge curves at 1.0–3.0 V of α-FeOOH, HP(400 °C), and HP(800 °C); rate capability of HP(300 °C) (α-FeOOH) and HP(400 °C) (ϵ-FeOOH); and ex situ XRD patterns at several discharged states (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c00728_si_001.pdf (2MB, pdf)

References

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