# Conversion of Li2FeSbO5 to the Fe(III)/Fe(V) Phase LiFeSbO5 via Topochemical Lithium Extraction

Xabier Martínez de Irujo-Labalde, Alex Scrimshire, Paul A. Bingham,
Emmanuelle Suard, Michael A. Hayward

► To cite this version:

Xabier Martínez de Irujo-Labalde, Alex Scrimshire, Paul A. Bingham, Emmanuelle Suard, Michael A. Hayward. Conversion of Li2FeSbO5 to the Fe(III)/Fe(V) Phase LiFeSbO5 via Topochemical Lithium Extraction. Chemistry of Materials, 2022, 34 (5), pp.2468-2475. 10.1021/acs.chemmater.2c00156. hal-05330194

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# Conversion of Li₂FeSbO₅ to the Fe(III)/Fe(V) Phase LiFeSbO₅ via Topochemical Lithium Extraction

*Published as part of the Virtual Special Issue “John Goodenough at 100”.*

Xabier Martínez de Irujo-Labalde, Alex Scrimshire, Paul A. Bingham, Emmanuelle Suard, and Michael A. Hayward*

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Cite This: *Chem. Mater.* 2022, 34, 2468–2475

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## ACCESS

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Supporting Information

**ABSTRACT:** Reaction between Na₂FeSbO₅ and LiNO₃ at 300 °C yields the metastable phase Li₂FeSbO₅ which is isostructural with the sodium “parent” phase (space group *Pbna*, *a* = 15.138(1) Å, *b* = 5.1440(3) Å, *c* = 10.0936(6) Å) consisting of an alternating stack of Li₂Fe₂O₅ and Li₂Sb₂O₅ sheets containing tetrahedral coordinated Fe³⁺ and octahedrally coordinated Sb⁵⁺, respectively. Further reaction between Li₂FeSbO₅ with NO₂BF₄ in acetonitrile at room temperature yields LiFeSbO₅, which adopts an orthorhombic structure (space group *Pbn2*₁, *a* = 14.2943(4) Å, *b* = 5.2771(1) Å, *c* = 9.5610(3) Å) in which the LiFeO₅ layers have shifted on lithium extraction, resulting in an octahedral coordination for the iron cations. ⁵⁷Fe Mössbauer data indicate that the nominal Fe⁴⁺ cations present in LiFeSbO₅ have disproportionated into a 1:1 combination of Fe³⁺ and Fe⁵⁺ centers which are ordered within the LiFeSbO₅ structural framework. It is widely observed that Fe⁴⁺ centers tend to be unstable in delithiated Li–Fe–X–O phases currently proposed as lithium-ion battery cathode materials, so the apparent stability of highly oxidized Fe⁵⁺ centers in LiFeSbO₅ is notable, suggesting cathode materials based on oxidizing Fe³⁺ could be possible. However, in this instance, the structural change which occurs on delithiation of Li₂FeSbO₅ prevents electrochemical cycling of this material.

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## INTRODUCTION

Rechargeable lithium-ion batteries have become the power source of choice for a wide variety of technologies from personal electronic devices to electric vehicles. However, the majority of cathode materials used in high-voltage batteries rely on rare, expensive, and toxic elements, such as cobalt, nickel, and manganese, for their energy storage performance.¹⁻³ If lithium-ion batteries are to be utilized widely in transport applications or as energy stores for renewable power generation, the use of these elements will need to be minimized.⁴,⁵

In principle, utilizing iron in lithium-ion battery cathode materials looks like an attractive prospect, due to the high abundance, low cost, and low toxicity of iron compounds.⁶ However, while it has been possible to make use of the FeII/III couple in cathodes such as LiFePO₄,⁷ utilizing the FeIII/IV couple in high-voltage cathodes with an enduring high capacity has proved challenging.

For example, LiFeO₂ can be prepared with a number of different crystal structures,⁸,⁹ but none of these materials exhibit good, long-term electrochemical performance. The most stable form at high temperature, α-LiFeO₂,¹⁰,¹¹ adopts a disordered rock salt structure that exhibits slow lithium intercalation/deintercalation kinetics unless prepared in nanoparticulate form¹²,¹³—a feature shared by cation-ordered γ-

LiFeO₂,¹⁴,¹⁵ which is the most stable polymorph at room temperature.¹⁰ By utilizing low-temperature synthesis approaches, metastable forms of LiFeO₂ can be prepared such as the “corrugated layer” phase synthesized by reaction of γ-FeOOH and LiOH,⁹,¹⁶ or the “O3”-LiFeO₂ or t-LiFeO₂ polymorphs prepared via Li-for-Na cation exchange from α-NaFeO₂ and β-NaFeO₂, respectively.¹⁷,¹⁸ These low-temperature forms of LiFeO₂ can show appreciable electrochemical activity; however, detailed analysis reveals that during the first lithium deintercalation cycle these materials, along with α-LiFeO₂ and γ-LiFeO₂, are converted to the spinel LiFe₅O₈,¹¹,¹⁸⁻²¹ with subsequent electrochemical activity most likely due to cycling between LiFe₅O₈ and Li₃Fe₅O₈.²² Furthermore, postcycling analysis casts doubt on the stability of Fe⁴⁺ in the Li–Fe–O system, with either rapid loss of oxygen or reaction with the electrolyte occurring in tandem with the initial removal of lithium.¹⁸,²¹,²³

Received: January 17, 2022

Revised: February 16, 2022

Published: February 24, 2022

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Similar instabilities of the Fe⁴⁺ oxidation state have been observed in other Li–Fe–X–O systems, with anion–redox processes observed on lithium extraction.²⁴,²⁵ For example, the extraction of a single lithium from Li₂FeSiO₄ is associated with the oxidation of Fe²⁺ to Fe³⁺. However, removal of a second lithium is accompanied by the formation of “ligand holes” in the O-2p bands, although this does not appear to lead to oxygen release.²⁶ Conversely, lithium extraction from Li₄FeSbO₆ leads to oxidation of both Fe³⁺ and oxygen, with the former being reversible but the anion oxidation being apparently irreversible.²⁷,²⁸

Here we describe the synthesis of another Li–Fe–Sb–O phase, Li₂FeSbO₅, via cation exchange from Na₂FeSbO₅.²⁹,³⁰ Oxidative lithium extraction from Li₂FeSbO₅ occurs via oxidation of Fe³⁺; however, in this instance the Fe⁴⁺ cations nominally present in LiFeSbO₅ disproportionate into a 1:1 mixture of Fe³⁺ and Fe⁵⁺.

## EXPERIMENTAL SECTION

**Synthesis.** Polycrystalline samples of Na₂FeSbO₅ were synthesized by a high-temperature ceramic method. Suitable ratios of Fe₂O₃ (Alfa Aesar, 99.995%), Sb₂O₃ (Alfa Aesar, 99.999%), and a 5% excess of Na₂CO₃ (Alfa Aesar, 99.95%) were ground together using an agate pestle and mortar. These mixtures were placed an alumina crucibles and heated in air at 600 °C for 12 h. The powders were reground, pressed into 13 mm pellets, and then heated at 1050 °C for four periods of 12 h in air with intermediate grindings. X-ray powder diffraction data collected from samples of Na₂FeSbO₅ prepared in this way yielded lattice parameters of (a = 15.7202(1) Å, b = 5.3250(1) Å, c = 10.8950(1) Å) consistent with previous reports.²⁹,³⁰

Li-for-Na cation exchange of Na₂FeSbO₅ was achieved by heating samples at 300 °C with 10 molar equiv of LiNO₃ (Alfa Aesar, 99%) for 3 days. The resulting material was then washed with distilled water to remove the NaNO₃ and excess LiNO₃ and then dried for 12 h at 140 °C in air.

Attempts to oxidatively remove Li from Li₂FeSbO₅ were performed using NO₂BF₄, a reagent with very strong oxidizing character (5.1 V vs Li/Li⁺).³¹ A total of 200 mg of Li₂FeSbO₅ was suspended in a solution of 1 g of NO₂BF₄ (Sigma-Aldrich, 95%) in 10 mL of acetonitrile (Merck, 99.8%). The suspension was stirred under N₂ for 2 days at room temperature. The material was then filtered and washed repeatedly with clean acetonitrile under an inert atmosphere before being dried under vacuum.

Reintercalation of lithium into Li₂-xFeSbO₅ was attempted by stirring samples in acetonitrile with LiI at 50 °C for 2 days. There was no iodine formed in this process, indicating the deintercalation reaction is not readily reversible.

**Characterization.** Reaction progress and initial structural characterization was performed using laboratory X-ray powder diffraction (PXRD) data collected using a PANalytical X'pert diffractometer incorporating an X'celerator position-sensitive detector (monochromatic Cu Kα₁ radiation). High-resolution synchrotron X-ray powder diffraction (SXRD) data were collected using the I11 instrument at the Diamond Light Source Ltd. Diffraction patterns were collected using Si-calibrated X-rays with an approximate wavelength of 0.825 Å from samples, sealed in 0.3 mm diameter borosilicate glass capillaries. Neutron powder diffraction (NPD) data were collected using the D2B diffractometer (λ = 1.594 Å) at the ILL neutron source, from samples contained within vanadium cans. Rietveld refinement of powder diffraction data was performed using the TOPAS Academic (V6).³²

⁵⁷Fe Mössbauer spectroscopy measurements utilized acrylic absorber discs with a sample area of 1.767 cm², which were loaded to present 2.16 × 10⁻³ g cm⁻² of Fe and achieve a Mössbauer thickness of 1. Samples were homogeneously mixed with graphite to achieve this level of loading. The 14.4 keV γ-rays were supplied by the cascade decay of 25 mCi ⁵⁷Co in a Rh matrix source, oscillated at constant acceleration by a SeeCo W304 drive unit, and detected using

a SeeCo 45431 Kr proportional counter operating with 1.745 kV bias voltage applied to the cathode. All measurements were calibrated relative to α-Fe foil. Spectral data were fitted using the Recoil software package,³³ using Lorentzian line shapes. Thermogravimetric measurements were performed by heating powder samples at a rate of 5 °C min⁻¹ under flowing air, using a Mettler-Toledo MX1 thermogravimetric microbalance, and then cooling to 25 °C. DC magnetization data were collected using a Quantum Design MPMS SQUID magnetometer from samples contained in gelatin capsules.

## RESULTS

**Structural Characterization of Li₂FeSbO₅.** Direct synthesis of Li₂FeSbO₅ from Li₂CO₃, Fe₂O₃, and Sb₂O₃ was not possible, with the reaction between these reagents resulting in mixtures of LiSbO₃ and LiFe₁⁻ₓSbₓO₂. However, reaction between Na₂FeSbO₅ and LiNO₃, as described above, yields a crystalline material. SXRD data collected from this material can be indexed using an orthorhombic unit cell (a = 15.138(1) Å, b = 5.1440(3) Å, c = 10.0936(6) Å) with extinction conditions consistent with space group Pbna and with diffraction peak intensities similar to the Na₂FeSbO₅ parent phase (Figure 1),

<figure><img src="image_9.png" /><figcaption>**Figure 1.** SXRD data collected at room temperature from Na₂FeSbO₅ (top) and Li₂FeSbO₅ (middle) and Li₂-xFeSbO₅ (bottom).</figcaption></figure>

suggesting a simple Li-for-Na cation exchange has occurred. The widths of the diffraction peaks of Li₂FeSbO₅ are broader than those of Na₂FeSbO₅ (Figure 1) consistent with a smaller particle size/reduced crystallinity in the cation exchanged material. A model based on the reported structure of Na₂FeSbO₅ (space group Pbna),²⁹ but with the Na cations replaced by Li, was refined against NPD data collected from the cation-exchanged material at room temperature, to achieve a good fit to the data, as shown in Figure 2. Refinement of the Li-site occupancies did not result in deviations from unity, within error. Given the strong neutron scattering contrast between Li (−1.90 fm) and Na (3.63 fm),³⁴ this suggests complete cation exchange has occurred, and the exchanged phase has a composition of Li₂FeSbO₅, within the sensitivity of our measurements. Details of the refined model of Li₂FeSbO₅ are given in Table 1, with selected bond lengths in Table S1 in the Supporting Information.

**Structural and Compositional Characterization of Chemically Delithiated Li₂FeSbO₅.** Reaction between Li₂FeSbO₅ and NO₂BF₄, as described above, yields a further crystalline material. SXRD data collected from this material can be indexed using an orthorhombic unit cell with dimensions
<figure><img src="image_10.png" /><figcaption>**Figure 2.** Observed, calculated, and difference plots from the structural refinement of $Li_2FeSbO_5$ against NPD data collected at room temperature.</figcaption></figure>

<table><caption>**Table 1.** Parameters from the Structural Refinement of $Li_2FeSbO_5$ against NPD Data Collected at Room Temperature</caption><thead><tr><th>atom</th><th>site</th><th>x</th><th>y</th><th>z</th><th>B<sub>eq</sub> (Å<sup>2</sup>)</th></tr></thead><tbody><tr><td>Li(1)</td><td>4c</td><td>0.120(2)</td><td>0.75</td><td>0.5</td><td>0.73(1)</td></tr><tr><td>Li(2)</td><td>4c</td><td>0.462(2)</td><td>0.25</td><td>0.5</td><td>0.81(4)</td></tr><tr><td>Li(3)</td><td>8d</td><td>0.334(1)</td><td>0.333(3)</td><td>0.761(2)</td><td>0.96(5)</td></tr><tr><td>Sb(1)</td><td>4c</td><td>0.235(1)</td><td>0.25</td><td>0.5</td><td>0.83(2)</td></tr><tr><td>Sb(2)</td><td>4c</td><td>0.341(1)</td><td>0.75</td><td>0.5</td><td>0.86(2)</td></tr><tr><td>Fe(1)</td><td>8d</td><td>0.947(4)</td><td>0.752(1)</td><td>0.2926(5)</td><td>0.33(7)</td></tr><tr><td>O(1)</td><td>8d</td><td>0.1458(7)</td><td>0.093(2)</td><td>0.606(1)</td><td>0.94(2)</td></tr><tr><td>O(2)</td><td>8d</td><td>0.3316(8)</td><td>0.051(2)</td><td>0.599(1)</td><td>0.44(2)</td></tr><tr><td>O(3)</td><td>8d</td><td>0.4431(8)</td><td>0.867(2)</td><td>0.3890(9)</td><td>1.03(2)</td></tr><tr><td>O(4)</td><td>8d</td><td>0.2498(7)</td><td>0.933(2)</td><td>0.388(1)</td><td>0.55(2)</td></tr><tr><td>O(5)</td><td>8d</td><td>0.0548(7)</td><td>0.925(2)</td><td>0.3422(7)</td><td>0.75(2)</td></tr></tbody></table><p>Li<sub>2</sub>FeSbO<sub>5</sub>, space group Pbna (No. 60)</p><p>a = 15.138(1) Å, b = 5.1440(3) Å, c = 10.0936(6) Å</p><p>volume = 786.01(8) Å<sup>3</sup></p><p>formula weight = 271.48 g mol<sup>-1</sup>, Z = 8</p><p>radiation source: neutron, λ = 1.594 Å</p><p>temperature: 298 K</p><p>R<sub>p</sub> = 3.38%, R<sub>wp</sub> = 4.30%, R<sub>Bragg</sub> = 1.19</p>

similar to those of the parent $Li_2FeSbO_5$ phase (a = 14.2943(4) Å, b = 5.2771(1) Å, c = 9.5610(3) Å).

The chemical composition of the delithiated phase was determined by thermogravimetric analysis. A sample of $Li_{2-x}FeSbO_5$ was heated in air to 1000 °C and observed to lose 8.31% of its mass. PXRD data collected from the product of this process indicated the $Li_{2-x}FeSbO_5$ had decomposed to a mixture of $FeSbO_4$, $LiSbO_3$, and $LiFe_2SbO_6$. Combining these observations, we determined the initial composition of the delithiated phase was $Li_{0.99(5)}FeSbO_5$, as described in detail in the Supporting Information. This phase will henceforth be referred to as $LiFeSbO_5$.

Close inspection of the SXRD data collected from $LiFeSbO_5$ reveals the presence of the [110] and [310] reflections (Figure 1) which violate the *hk0*: h = 2n extinction condition of the Pbna space group used to describe the structure of $Li_2FeSbO_5$. Specifically the conditions associated with the *a*-glide are violated, which suggests a symmetry lowering to either Pbnm (No. 62) or Pbn21 (No. 33).

A series of structural models were constructed in space groups Pbnm and Pbn21, with the assistance of the ISODISTORT software package.³⁵,³⁶ Lithium positions were

omitted from these models, as these ions do not contribute significantly to the X-ray diffraction data due to their small X-ray scattering power. A structural model constructed in space group Pbn21 which conserved the positions of the octahedrally coordinated Sb⁵⁺ cations from the $Li_2FeSbO_5$ parent phase but located the Fe cations in octahedral sites fit the data well. Initially the model was constrained to force the two crystallographically distinct Sb sites to be identical. However, this constraint was relaxed in later refinement cycles, leading to an improvement in the fit to the data (Figure 3). Full details of the refined structure of $LiFeSbO_5$ are given in Table 2, with selected bond lengths in Table S2 in the Supporting Information.

<figure><img src="image_11.png" /><figcaption>**Figure 3.** Observed, calculated, and difference plots from the structural refinement of $LiFeSbO_5$ against SXRD data collected at room temperature.</figcaption></figure>

⁵⁷Fe Mössbauer Spectroscopy. A ⁵⁷Fe Mössbauer spectrum collected from $Li_2FeSbO_5$ at room temperature can

<table><caption>**Table 2.** Parameters from the Structural Refinement of $LiFeSbO_5$ against SXRD Data Collected at Room Temperature<sup>a</sup></caption><thead><tr><th></th><th>site</th><th>x</th><th>y</th><th>z</th><th>B<sub>eq</sub> (Å<sup>2</sup>)</th></tr></thead><tbody><tr><td>Sb(1)</td><td>4a</td><td>0.4978(1)</td><td>0.1365(1)</td><td>0</td><td>0.22(2)</td></tr><tr><td>Sb(2)</td><td>4a</td><td>0.8932(1)</td><td>0.1364(1)</td><td>0.9942(1)</td><td>0.22(2)</td></tr><tr><td>Fe(1)</td><td>4a</td><td>0.6916(1)</td><td>0.1040(1)</td><td>0.7431(1)</td><td>0.28(1)</td></tr><tr><td>Fe(2)</td><td>4a</td><td>0.6955(1)</td><td>0.8657(1)</td><td>0.2538(1)</td><td>0.28(1)</td></tr><tr><td>O(1)</td><td>4a</td><td>0.0840(23)</td><td>0.7855(18)</td><td>0.8928(17)</td><td>0.48(2)</td></tr><tr><td>O(2)</td><td>4a</td><td>0.0994(8)</td><td>0.5779(17)</td><td>0.1522(13)</td><td>0.48(2)</td></tr><tr><td>O(3)</td><td>4a</td><td>0.9031(9)</td><td>0.7910(20)</td><td>0.8478(16)</td><td>0.48(2)</td></tr><tr><td>O(4)</td><td>4a</td><td>0.8993(10)</td><td>0.4508(17)</td><td>0.1165(18)</td><td>0.48(2)</td></tr><tr><td>O(5)</td><td>4a</td><td>0.7040(8)</td><td>0.5779(17)</td><td>0.1080(11)</td><td>0.48(2)</td></tr><tr><td>O(6)</td><td>4a</td><td>0.6889(9)</td><td>0.7855(18)</td><td>0.8463(11)</td><td>0.48(2)</td></tr><tr><td>O(7)</td><td>4a</td><td>0.5039(10)</td><td>0.4508(17)</td><td>0.1027(16)</td><td>0.48(2)</td></tr><tr><td>O(8)</td><td>4a</td><td>0.5077(9)</td><td>0.7910(20)</td><td>0.9079(14)</td><td>0.48(2)</td></tr><tr><td>O(9)</td><td>4a</td><td>0.2039(16)</td><td>0.0657(18)</td><td>0.1961(13)</td><td>0.48(2)</td></tr><tr><td>O(10)</td><td>4a</td><td>0.8057(14)</td><td>0.7080(20)</td><td>0.3965(13)</td><td>0.48(2)</td></tr></tbody></table><p>LiFeSbO<sub>5</sub>, space group Pbn2<sub>1</sub> (No. 33)</p><p>a = 14.2943(4) Å, b = 5.2771(1) Å, c = 9.5610(3) Å</p><p>volume = 721.21(4) Å<sup>3</sup></p><p>radiation source: synchrotron X-ray, λ = 0.8268 Å</p><p>temperature: 298 K</p><p>R<sub>p</sub> = 2.01, R<sub>wp</sub> = 1.40, R<sub>Bragg</sub> = 1.03</p>

<sup>a</sup>The lithium positions are omitted due to their small X-ray scattering power. The z-coordinate of Sb(1) is fixed at 0 to “anchor” the structure in this non-centrosymmetric space group.
be satisfactorily fit by two doublets as shown in Figure 4 and detailed in Table 3. The chemical shift (CS) and quadrupole

<figure><img src="image_12.png" /><figcaption>**Figure 4.** $^{57}$Fe Mössbauer spectra collected from $Li_{2}FeSbO_{5}$ (top) and $LiFeSbO_{5}$ (bottom) at room temperature.</figcaption></figure>

<table><caption>**Table 3.** Hyperfine Parameters Extracted from the Fits to $^{57}$Fe Mössbauer Spectra<sup>a</sup></caption><thead><tr><th></th><th>CS (mm/s)<br/>[±0.02]</th><th>Δ (mm/s)<br/>[±0.02]</th><th>HWHM<br/>(mm/s)<br/>[±0.02]</th><th>spectral area<br/>(%) [±2]</th></tr></thead><tbody><tr><td></td><td colspan="4" style="text-align: center;">Li<sub>2</sub>FeSbO<sub>5</sub></td></tr><tr><td>doublet 1</td><td>0.39</td><td>0.76</td><td>0.37</td><td>87</td></tr><tr><td>doublet 2</td><td>0.40</td><td>0.53</td><td>0.18</td><td>13</td></tr><tr><td></td><td colspan="4" style="text-align: center;">LiFeSbO<sub>5</sub></td></tr><tr><td>doublet 1</td><td>0.01</td><td>0.80</td><td>0.23</td><td>47</td></tr><tr><td>doublet 2</td><td>0.41</td><td>0.88</td><td>0.23</td><td>53</td></tr></tbody><tfoot><tr><td colspan="5"><sup>a</sup>CS values are stated relative to α-Fe.</td></tr></tfoot></table>

splitting (Δ) values of both doublets are consistent with tetrahedrally coordinated $Fe^{3+}$, and the spectrum of $Li_{2}FeSbO_{5}$ is very similar to that collected from $Na_{2}FeSbO_{5}$.²⁹ The requirement to use two doublets is attributed to a small amount of Li/Fe cation disorder in the phase, which appears to be introduced in the cation exchange reaction.

A corresponding $^{57}$Fe Mössbauer spectrum collected from $LiFeSbO_{5}$ at room temperature can also be fit by two doublets, as shown in Figure 4 and detailed in Table 3. However, in this case the CS values of the two doublets are significantly different (0.01(2) mm/s and 0.41(2) mm/s), indicating the $Fe^{4+}$ cations in $LiFeSbO_{5}$ have disproportionated into a 1:1 mixture of $Fe^{3+}$ and $Fe^{5+}$,³⁷,³⁸ in line with the different bond valence sums (Fe +4.04; Fe +2.09) calculated for the two crystallographically distinct Fe sites in $LiFeSbO_{5}$ (Table S2) and the orange color of the material.

**Magnetic Characterization.** Zero-field cooled (ZFC) and field cooled (FC) magnetization data collected from $Li_{2}FeSbO_{5}$ as a function of temperature in an applied field of 100 Oe are shown in Figure 5. On cooling, the ZFC and FC

<figure><img src="image_13.png" /><figcaption>**Figure 5.** ZFC and FC data collected from $Li_{2}FeSbO_{5}$ as a function of temperature in an applied field of 100 Oe. Inset shows magnetization-field data collected at 300 and 5 K after cooling in an applied field of 50 000 Oe.</figcaption></figure>

data diverge weakly below $T = 275$ K and then much more strongly below $T = 75$ K, with the ZFC data exhibiting a maximum at 65 K. The ZFC data do not obey the Curie-Weiss law over any temperature range measured. Magnetization-field data collected at 300 K are linear and pass through the origin, consistent with simple paramagnetic behavior. However, magnetization-field data collected at 5 K after cooling in an applied field of 50 000 Oe are sigmoidal and shifted above the origin, suggesting the divergence between ZFC and FC data at $T = 75$ K is the freezing of a spin glass.

ZFC and FC magnetization data collected from $LiFeSbO_{5}$ in an applied field of 100 Oe (Figure 6) obey the Curie-Weiss law ($\chi = C/(T - \theta)$) in the range $150 < T (K) < 300$ to yield values of $C = 3.258(3) \text{ cm}^{3} \text{ K mol}^{-1}$ and $\theta = -1.92(3) \text{ K}$. This value of the Curie constant is in good agreement with that expected for a 1:1 mixture of $S = 5/2 Fe^{3+}$ and $S = 3/2 Fe^{5+}$ ($C_{\text{expected}} = 3.125 \text{ cm}^{3} \text{ K mol}^{-1}$) and is thus consistent with the disproportionation of $Fe^{4+}$ to $Fe^{3+}$ and $Fe^{5+}$ indicated by the $^{57}$Fe Mössbauer data. On cooling below $T = 150$ K the ZFC and FC data diverge weakly before exhibiting a maximum at $T = 10$ K. Magnetization-field data (Figure 6) are linear at 300 K and weakly sigmoidal but centered on the origin at 5 K, increasing almost linearly with increasing field to achieve a value of ~0.8 µB per formula unit in 5T. This combination of features is rather unusual and suggests an antiferromagnetic ground state for $LiFeSbO_{5}$ in small applied fields, which changes in an as-yet unidentified way as the applied field increases.

## DISCUSSION

The structure of $Na_{2}FeSbO_{5}$ can be considered as a layered intergrowth of $Na_{2}Sb_{2}O_{5}$ sheets (structurally reminiscent of $LiSbO_{3}$), stacked with $Na_{2}Fe_{2}O_{5}$ sheets (reminiscent of $\beta$-NaFeO$_{2}$), as shown in Figure 7. While the structures of both $LiSbO_{3}$ and $\beta$-NaFeO$_{2}$ are based on hexagonal close packed (hcp) arrays of oxide ions, the hcp stacking of the oxide-ion sheets is perturbed in $Na_{2}FeSbO_{5}$. The close packed sheets of oxide ions surrounding the $Na_{2}Sb_{2}O_{5}$ layers retain a close-packed arrangement (layers A and B in Figure 7). However, the sheets of oxide ions surrounding the $Na_{2}Fe_{2}O_{5}$ layers (layers B and B') are related by a displacement along the x-
<figure><img src="image_14.png" /><figcaption>**Figure 6.** (top) ZFC and FC data collected from $LiFeSbO_5$ as a function of temperature in an applied field of 100 Oe. Inset shows fit to the Curie–Weiss law in the range 300 < *T* (K) < 150. (bottom) Magnetization-field data collect at 300 and 5 K after cooling in an applied field of 50 000 Oe.</figcaption></figure>

axis, rather than having a close packed relationship. So the stacking sequence of the sheets of oxide ions is $A(Na_2Sb_2)B(Na_2Fe_2)B'(Na_2Sb_2)A'(Na_2Fe_2)A$ as shown in Figure 7.

Lithium-for-sodium cation exchange yields $Li_2FeSbO_5$, which is isostructural with $Na_2FeSbO_5$. Replacement of the

$Na^+$ cations with smaller $Li^+$ cations leads to an anisotropic contraction of the unit cell ($\Delta$ volume = -13.8%), which arises principally from a contraction along the z-axis ($\Delta a$ = -3.6%, $\Delta b$ = -3.5%, $\Delta c$ = -7.2%) and is accommodated by a flexing of the Sb–O–Fe and Fe–O–Fe bond angles, rather than a compression of the SbO$_6$ or FeO$_4$ units.

While the large contraction of the c lattice parameter can be directly attributed to the replacement of $Na^+$ with $Li^+$, the resulting torsion of the FeO$_4$ units leads to more complex consequences for the rest of the framework. Most notably there is a significant reduction in one of the separations between SbO$_6$ chains along the x-axis, which occurs at the expense of a slight increase in the other, as shown in Figure 8.

<figure><img src="image_15.png" /><figcaption>**Figure 8.** Comparison of the crystal structures of $Na_2FeSbO_5$ and $Li_2FeSbO_5$.</figcaption></figure>

Furthermore, the rotation of the FeO$_4$ units around the y-axis tightens the Fe–O–Fe bond angles, as shown in Figure 8, which is likely to be the cause of the reduction in the magnetic transition temperature of $Li_2FeSbO_5$ (75 K) compared to $Na_2FeSbO_5$ (104 K)$^{29}$ due to a weakening of superexchange interactions.

Deintercalation of one lithium per formula unit from $Li_2FeSbO_5$ leads to a large scale reorganization of the structure. As shown in Figure 9, the iron centers in $LiFeSbO_5$ are located in octahedral coordination sites which share edges to form zigzag chains which run parallel to the y-axis.

The close-packed sheets of oxide ions which surround the iron-containing layers are now stacked in a close packed

<figure><img src="image_16.png" /><figcaption>**Figure 7.** Structure of $Na_2FeSbO_5$ is an intergrowth of $Na_2Sb_2O_5$ layers (related to $LiSbO_3$) and $Na_2Fe_2O_5$ layers (related to $\beta$-NaFeO$_2$) with an A–B–B’–A’–A stacking sequence of close packed layers of oxide ions.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Figure 9.** Structure of LiFeSbO₅ is an intergrowth of LiSb₂O₅ layers (related to LiSbO₃) and LiFe₂O₅ layers consisting of edge-sharing FeO₆ units, with an A—B—A—B hexagonal sequence of close packed layers of oxide ions.</figcaption></figure>

manner, so the stacking sequence of LiFeSbO₅ is A(LiSb₂)-B(LiFe₂)A(LiSb₂)B(LiFe₂)A. This differs from the stacking sequence of Li₂FeSbO₅, revealing that on lithium deintercalation the Li₂-xSb₂O₅ blocks slide relative to each other to generate octahedral coordination sites for the iron centers, which are occupied with a minor rearrangement of the Fe centers, to generate the structure shown in Figure 9. Similar shifting of close packed blocks has been observed during intercalation of lithium into layered, cation-deficient perovskite oxides and other related reactions.³⁹ This large structural change observed on lithium removal is probably responsible for the apparent irreversibility of the chemical delithiation of Li₂FeSbO₅, as noted above.

Iron migration on charging/discharging has been observed in a number of other systems.<sup>11,18–20,26</sup> This behavior can be broadly attributed to the differing ionic radii and ligand-field stabilization energies of d<sup>6</sup> Fe<sup>2+</sup>, d<sup>5</sup> Fe<sup>3+</sup>, and d<sup>4</sup> Fe<sup>4+</sup>, leading to differing coordination preferences for the different oxidation states of iron.

<sup>57</sup>Fe Mössbauer data, supported by magnetic data, indicate that the nominally Fe<sup>4+</sup> cations in LiFeSbO<sub>5</sub> disproportionate into a 1:1 ratio of Fe<sup>3+</sup> and Fe<sup>5+</sup>. Structural analysis utilizing bond valence sums,<sup>40,41</sup> detailed in Table S2, suggests that charge-disproportionated Fe<sup>3+</sup> and Fe<sup>5+</sup> centers order crystallographically into chains containing either exclusively Fe<sup>3+</sup> or Fe<sup>5+</sup>, which alternate along x- and z-axes as shown in Figure 9. This charge ordering pattern is rather unusual, as the majority of disproportionate Fe<sup>3+/5+</sup> systems adopt 3-dimensional “rock-salt” or 2-dimensional “checkerboard” arrangements of alternating Fe<sup>3+</sup> and Fe<sup>5+</sup> in order to minimize the electrostatic repulsions and lattice strain.<sup>42–44</sup> Nevertheless, the unusual charge ordering pattern present in LiFeSbO<sub>5</sub> seems to be very stable since charge disproportionate charges are found at room temperature. It is possible that the edge-sharing connectivity of the FeO<sub>6</sub> octahedra in LiFeSbO<sub>5</sub> (rather than the apex-linked connectivity more commonly observed in Fe<sup>3+/5+</sup> systems) could be responsible for the unusual ordering scheme or that the phase is the product of a topochemical reaction and is thus far from equilibrium, unlike the majority of reported Fe<sup>3+/5+</sup> oxide phases.

As noted above, Fe<sup>4+</sup> tends to be unstable in delithiated oxide phases,<sup>18,21,23</sup> so the observation of apparently stable Fe<sup>5+</sup> centers in LiFeSbO<sub>5</sub> is notable as it demonstrates high oxidation states of iron can be stable in delithiated oxides, offering the prospect of preparing cathode materials which utilize the oxidation of Fe<sup>3+</sup>. However, in this instance, the disproportionation of Fe<sup>4+</sup> to Fe<sup>3+</sup>/Fe<sup>5+</sup> appears to drive a structural reorganization on the delithiation of Li<sub>2</sub>FeSbO<sub>5</sub>, making the oxidation irreversible, highlighting a further barrier

to the development of high-voltage, high-capacity Fe-based cathode materials.

## ■ CONCLUSION

Facile Li-for-Na cation exchange readily converts Na₂FeSbO₅ to the metastable phase, Li₂FeSbO₅, with only a small relaxation of the structural framework. This Fe<sup>3+</sup> phase can be chemically oxidized via delithiation with NO₂BF₄ to form LiFeSbO₅. The delithiation of Li₂FeSbO₅ is accompanied by a minor structural rearrangement of the Li₂<sub>x</sub>FeSbO₅ sheets in the system, resulting in a change to the local iron coordination environment from tetrahedral in Li₂FeSbO₅ to octahedral in LiFeSbO₅. <sup>57</sup>Fe Mössbauer data and magnetization measurements indicate that the nominal Fe<sup>4+</sup> centers in LiFeSbO₅ have disproportionate into a 1:1 combination of Fe<sup>3+</sup> and Fe<sup>5+</sup>. The structural rearrangement that occurs on oxidation can be attributed to the favorability of locating d<sup>3</sup> Fe<sup>5+</sup> cations within octahedral rather than tetrahedral coordination sites and represents a further undesirable feature (in addition to the apparent instability of Fe<sup>4+</sup> in Li—Fe—X—O systems) which needs to be overcome if high-voltage, high-capacity, Fe-based cathode materials are to be developed.

## ■ ASSOCIATED CONTENT

### ● Supporting Information

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.chemmater.2c00156.

Complete description of the compositional analysis of LiFeSbO₅ and selected bond lengths from the refined structures of Li₂FeSbO₅ and LiFeSbO₅ (PDF)

## ■ AUTHOR INFORMATION

### Corresponding Author

Michael A. Hayward – Inorganic Chemistry Laboratory,
Department of Chemistry, University of Oxford, Oxford OX1
3QR, United Kingdom; The Faraday Institution, Didcot
OX11 0RA, United Kingdom; orcid.org/0000-0002-6248-2063;
Email: michael.hayward@chem.ox.ac.uk

### Authors

Xabier Martínez de Irujo-Labalde – Inorganic Chemistry
Laboratory, Department of Chemistry, University of Oxford,
Oxford OX1 3QR, United Kingdom; The Faraday
Institution, Didcot OX11 0RA, United Kingdom

Alex Scrimshire – Materials and Engineering Research
Institute, Sheffield Hallam University, Sheffield S1 1WB,
United Kingdom
**Paul A. Bingham** — Materials and Engineering Research Institute, Sheffield Hallam University, Sheffield S1 1WB, United Kingdom

**Emmanuelle Suard** — Institut Laue Langevin (ILL), Grenoble, 9 F-38042, France; [orcid.org/0000-0001-5966-5929](orcid.org/0000-0001-5966-5929)

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.chemmater.2c00156

## Author Contributions

The manuscript was written through contributions of all authors.

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

This work was supported by the Faraday Institution project FutureCat (Grant No. FIRG017). Experiments at the Diamond Light Source were performed as part of the Block Allocation Group award "Oxford Solid State Chemistry BAG to probe composition-structure-property relationships in solids" (CY25166).

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