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

: Reaction between Na 2 FeSbO 5 and LiNO 3 at 300 °C yields the metastable phase Li 2 FeSbO 5 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 Li 2 Fe 2 O 5 and Li 2 Sb 2 O 5 sheets containing tetrahedral coordinated Fe 3+ and octahedrally coordinated Sb 5+ respectively. Further reaction between Li 2 FeSbO 5 with NO 2 BF 4 in acetonitrile at room temperature yields LiFeSbO 5 which adopts an orthorhombic structure (space group Pbn 2 1 , a = 14.2943(4) Å , b = 5.2771(1) Å , c = 9.5610(3) Å) in which the LiFeO 5 layers have shifted on lithium extraction resulting in an octahedral coordination for the iron cations. 57 Fe Mössbauer data indicate that the nominal Fe 4+ cations present in LiFeSbO 5 have disproportionated into a 1:1 combination of Fe 3+ and Fe 5+ centers which are ordered within the LiFeSbO 5 structural framework. It is widely observed that Fe 4+ centers tend to be unstable in de-lithiated Li-Fe-X-O phases currently proposed as lithium-ion


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.2][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 principle, utilizing iron in lithium-ion battery cathodes materials looks like an attractive prospect, due to the high abundance, low cost, and low toxicity of iron compounds. 6However, while it has been possible to make use of the Fe II/III couple in cathodes such as LiFePO4, 7 utilizing the Fe III/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 which exhibits slow lithium intercalation/deintercalation kinetics unless prepared in nanoparticulate form 12, 13 -a feature shared by cation-ordered γ-LiFeO2, 14,15 which is the most stable polymorph at room temperature. 10By 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, 18These lowtemperature 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][19][20][21] with subsequent electrochemical activity most likely due to cycling between LiFe5O8 and Li3Fe5O8. 22Furthermore, post-cycling analysis casts doubt on the stability of Fe 4+ 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 Slar instabilities of the Fe 4+ oxidation state have been observed in other Li-Fe-X-O systems, with anion-redox process observed on lithium extraction. 24,25 or example, the extraction of a single lithium from Li2FeSiO4 is associated with the oxidation of Fe 2+ to Fe 3+ .However, removal of a second lithium is accompanied by the formation of 'ligand holes' in the O-2p bands, although this does not ap-pear to lead to oxygen release. 26Conversely, lithium extraction from Li4FeSbO6 leads to oxidation of both Fe 3+ and oxygen, with the former being reversible, but the anion oxidation being apparently irreversible. 27,28 e we describe the synthesis of another Li-Fe-Sb-O phase, Li2FeSbO5, via cation exchange from Na2FeSbO5. 29,30 Oidative lithium extraction from Li2FeSbO5 occurs via oxidation of Fe 3+ , however in this instance the Fe 4+ cations nominally present in LiFeSbO5 disproportionate into a 1:1 mixture of Fe 3+ and Fe 5+ .

Experimental
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 hours.The powders were reground, pressed into 13 mm pellets, and then heated at 1050 °C for 4 periods of 12 hours 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 for-Na cation exchange of Na2FeSbO5 was achieved by heating samples at 300 °C with 10 mole equivalents 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 hours at 140 °C in air.
Attempts to oxidatively remove Li from Li2FeSbO5 were performed using NO2BF4, a reagent with very strong oxidizing character (5.1V vs Li/Li + ). 31 200 mg of Li2FeSbO5 was suspended in a solution of 1g 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 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 two days.There was no iodine formed in this process, indicating the deinterclation reaction is not readily reversible.
Characterization.Reaction progress and initial structural characterization was performed using laboratory Xray 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 dif-fraction (NPD) data were collected using the D2B diffractometer (λ = 1.594Å) 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 57 e Mössbauer spectroscopy measurements utilized acrylic absorber discs with a sample area of 1.767 cm 2 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 57 Co 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. Sectral 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. D magnetization data were collected using a Quantum Design MPMS SQUID magnetometer from samples contained in gelatine 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), 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  The chemical composition of the delithiated phase was determined by thermogravimetric analysis.A sample of Li2-xFeSbO5 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 Li2-xFeSbO5 had decomposed to a mixture of FeSbO4, LiSbO3 and LiFe2SbO6.Combining these observations we determined the initial composition of the delithiated phase was Li0.99(5)FeSbO5, as described in detail in the supporting information.This phase will henceforth be referred to as LiFeSbO5.
Close inspection of the SXRD data collected from LiFeSbO5 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 Li2FeSbO5.Specifically the conditions associated with the a-glide are violated, which suggests a symmetry lowering to either Pbnm (# 62) or Pbn21 (#33).Table 1.Parameters from the structural refinement of Li2FeSbO5 against NPD data collected at room temperature.
A series of structural models were constructed in space groups Pbnm and Pbn21, with the assistance of the ISODISTORT software package. 35,36 ithium 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 Sb 5+ cations from the Li2FeSbO5 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 LiFeSbO5 are given in Table 2, with selected bond lengths in Table S2 in the supporting information.2. Parameters from the structural refinement of LiFeSbO5 against SXRD data collected at room temperature.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.
57 Fe Mössbauer Spectroscopy.A 57 Fe Mössbauer spectrum collected from Li2FeSbO5 at room temperature can be satisfactorily fit by two doublets as shown in Figure 4 and detailed in Table 3.The chemical shift (CS) and quadrupole splitting (Δ) values of both doublets are consistent with tetrahedrally coordinated Fe 3+ , and the spectrum of Li2FeSbO5 is very similar to that collected from Na2FeSbO5. 29The 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 LiFeSbO5 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 LiFeSbO5 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 LiFeSbO5 (Table S2) and the orange color of the material.
Magnetic Characterization.Zero-field cooled (ZFC) and field cooled (FC) magnetization data collected from Li2FeSbO5 as a function of temperature in an applied field of 100 Oe are shown in Figure 5. On cooling the ZFC and FC 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 LiFeSbO5 in an applied field of 100 Oe (Figure 6) obey the Curie-Weiss law (χ = C/(T-θ)) in the range 150 < T/K < 300 to yield values of C = 3.258(3) cm 3 K mol -1 and θ = -1.92(3)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+ (Cex-  (bottom) Magnetization-field data collect at 300 K and 5 K after cooling in an applied field of 50,000 Oe. in 5T.This combination of features is rather unusual, and suggests an antiferromagnetic ground state for LiFeSbO5 in small applied fields, which changes in an asyet unidentified way as the applied filed increases.

Discussion
The structure of Na2FeSbO5 can be considered as a layered intergrowth of Na2Sb2O5 sheets (structurally reminiscent of LiSbO3), stacked with Na2Fe2O5 sheets, (reminiscent of β-NaFeO2), as shown in Figure 7.While the structures of both LiSbO3 and β-NaFeO2 are based on hexagonal close packed (hcp) arrays of oxide ions, the hcp stacking of the oxide-ion sheets is perturbed in Na2FeSbO5.The close packed sheets of oxide ions surrounding the Na2Sb2O5 layers retain a close-packed arrangement (layers A and B in Figure 7).However, the sheets of oxide ions surrounding the Na2Fe2O5 layers (layers B and B') are related by a displacement along the x-axis, rather than having a close packed relationship.So the stacking sequence of the sheets of oxide ions is A(Na2Sb2)B(Na2Fe2)B'(Na2Sb2)A'(Na2Fe2)A as shown in Figure 7.
Lithium-for-sodium cation exchange yields Li2FeSbO5 which is isostructural with Na2FeSbO5.Replacement of the Na + cations with smaller Li + cations leads to an anisotropic contraction of the unit cell (∆volume = -13.8%)which arises principally from a contraction along the z-axis (∆a = -3.6%,∆b = -3.5%,∆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 SbO6 or FeO4 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 FeO4 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 SbO6 chains along the x-axis, which occurs at expense of a slight increase in the other, as shown in Figure 8. Furthermore the rotation of the FeO4 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 Li2FeSbO5 (75 K) compared to Na2FeSbO5 (104 K) 29 due to a weakening of superexchange interactions.Deintercalation of one lithium per formula unit from Li2FeSbO5 leads to a large scale reorganization of the structure.As shown in Figure 9, the iron centers in LiFeSbO5 are located in octahedral coordination sites which share edges to form zig-zag 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 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. 39This large structural change observed on lithium removal is probably responsible for the apparent irreversibility of the chemical delithiation of Li2FeSbO5, as noted above.
]26 This behavior can be broadly attributed to the differing ionic radii and ligand-field stabilization energies of d 6 Fe 2+ , d 5 Fe 3+ and d 4 Fe 4+ leading to differing coordination preferences for the different oxidation states of iron. 57Fe Mössbauer data, supported by magnetic data, indicate that the nominally Fe 4+ cations in LiFeSbO5 disproportionate into a 1:1 ratio of Fe 3+ and Fe 5+ .Structural analysis utilizing bond valence sums 40,41 , detailed in Table S2, suggests that charge-disproportionated Fe 3+ and Fe 5+ centers order crystallographically into chains containing either exclusively Fe 3+ or Fe 5+ which alternate along x-and z-axes as shown in Figure 9.3][44] Nevertheless, the unusual charge ordering pattern present in LiFeSbO5 seems to be very stable since charge disproportionated charges are found at room temperature.It is possible that the edge-sharing connectivity of the FeO6 octahedra in LiFeSbO5 (rather than the apexlinked connectivity more commonly observed in Fe 3+/5+ 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 3+/5+ oxide phases.
As noted above, Fe 4+ tends to be unstable in delitihated oxide phases, 18,21,23 so the observation of apparently stable Fe 5+ centers in LiFeSbO5 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 3+ .However, in this instance, the disproportionation of Fe 4+ to Fe 3+ /Fe 5+ appears to drive a structural reorganization on the delithiation of Li2FeSbO5 making the oxidation irreversible, highlighting a further barrier to the development of high-voltage, high-capacity Fe-base 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 3+ 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-xFeSbO5 sheets in the system, resulting in a change to the local iron coordination environment from tetrahedral in Li2FeSbO5 to octahedral in LiFeSbO5. 57Fe Mössbauer data and magnetization measurements indicate that the nominal Fe 4+ centers in LiFeSbO5 have disproportionated into a 1:1 combination of Fe 3+ and Fe 5+ .The structural rearrangement which occurs on oxidation can be attributed to the favorability of locating d 3 Fe 5+ cations within octahedral rather than tetrahedral coordination sites, and represents a further undesirable feature (in addition to the apparent instability of Fe 4+ 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
Complete description of the compositional analysis of LiFeSbO5.Selected bond lengths from the refined structures of Li2FeSbO5 and LiFeSbO5.

Figure 2 .
Figure 2. Observed, calculated and difference plots from the structural refinement of Li2FeSbO5 against NPD data collected at room temperature.crystalline material.SXRD data collected from this material can be indexed using an orthorhombic unit cell with dimensions similar to the parent Li2FeSbO5 phase (a = 14.2943(4)Å, b = 5.2771(1) Å, c = 9.5610(3) Å).

Figure 5 .
Figure 5. ZFC and FC data collected from Li2FeSbO5 as a function of temperature in an applied field of 100 Oe.Inset shows magnetization-field data collect at 300 K and 5 K after cooling in an applied field of 50,000 Oe.

Figure 6 .
Figure 6.(top) ZFC and FC data collected from LiFeSbO5 as a function of temperature in an applied field of 100 Oe.Inset shows fit to the Cure-Weiss law in the range 300 < T/K < 150.(bottom)Magnetization-field data collect at 300 K and 5 K after cooling in an applied field of 50,000 Oe.

Figure 7 .
Figure 7. Structure of Na2FeSbO5 is an intergrowth of Na2Sb2O5 layers (related to LiSbO3) and Na2Fe2O5 layers (related to β-NaFeO2) with an A-B-B'-A'-A stacking sequence of close packed layers of oxide ions.

Figure 8 .
Figure 8.A comparison of the crystal structures of Na2FeSbO5 and Li2FeSbO5.

Figure 9 .
Figure 9. Structure of LiFeSbO5 is an intergrowth of LiSb2O5 layers (related to LiSbO3) and LiFe2O5 layers consisting of edgesharing FeO6 units, with an A-B-A-B hexagonal sequence of close packed layers of oxide ions.

Table 3 .
Hyperfine parameters extracted from the fits to 57 Fe Mössbauer spectra.CS values are stated relative to α-Fe.