Structural and Electronic Changes in Li2FeP2O7 during

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Structural and Electronic Changes in Li2FeP2O7 during Electrochemical Cycling Andreas Blidberg,*,† Lennart Hag̈ gström,† Tore Ericsson,† Carl Tengstedt,‡ Torbjörn Gustafsson,† and Fredrik Björefors*,† †

Department of Chemistry − Ångström Laboratory, Uppsala University, Box 538, SE-75121 Uppsala, Sweden Scania CV AB, SE-15187 Södertälje, Sweden

‡

S Supporting Information *

more easily seen in the inset dx/dV plot. It has been suggested that these two voltage regions correspond to a preferential oxidation of different iron sites in Li2FeP2O7,12 since iron has two different coordination numbers in the crystal structure: FeO5 and FeO6. The aforementioned antisite defects are a mixed occupation of iron and lithium in the FeO5 site and a LiO5 site. Previous work by our group showed that preferential oxidation of iron sites can be identified by Mössbauer spectroscopy for the NASICON type Li3Fe2(PO4)313 and for tavorite LiFeSO4F,14 where iron is present in two different sixcoordinate sites. In the case of Li2FeP2O7 it is thus interesting to see if the different coordination around iron sites within the same crystal structure affects the cell voltage of the battery. In this study we used Mössbauer spectroscopy to investigate preferential oxidation of the different iron sites in Li2FeP2O7 upon electrochemical delithiation. Additionally, the evolution of Li−Fe antisite defects was studied by means of ex situ powder XRD. Thereby we aim to bring deeper understanding to how the local environment around iron sites affects the Fe3+/Fe2+ redox couple in Li-ion battery materials and the changes in crystal structure of Li2FeP2O7 upon cycling. Figure 2 shows the crystal structure of the pristine Li2FeP2O7 (see Supporting Information for details regarding Rietveld refinement). As previously mentioned, there are two different coordination numbers for iron, FeO5 and FeO6. These two units share an edge, forming a larger Fe2O9 unit, which in turn are linked together by corner sharing pyrophosphate units to form wavy layers in the bc-plane. The layers are linked together by pyrophosphate units along the a-axis, forming tunnels in the bc-plane where Li-ions can migrate during electrochemical cycling. There is an extensive mixing of the iron and lithium in the FeO5 site and a LiO5 site, so-called Li−Fe antisite defects, as indicated in Figure 2. The FeO5 site is only occupied to 2/3 by iron in the pristine material; the remaining 1/3 is filled by lithium. Consequently, the remaining 1/3 of the fivecoordinated iron occupies a LiO5 site. This is a feature of Li2MP2O7 for M = Fe1 and Co15,16and also for Mg-17 and Nidoped18 Li2FeP2O7, but not for M = Mn,19 where no Li−Mn mixing occurs. The Li2FeP2O7 used in this study was synthesized by the solid state method first presented by Nishimura et al.1 Powder

Li2FeP2O7 has recently been suggested as a cathode material in power optimized Li-ion batteries due to its low cost and multidimensional conduction network for Li-ions.1 Li2FeP2O7 has channels for Li-diffusion along the crystallographic b- and caxes, and atomistic simulations indicated a low Li-ion migration energy of 0.40 eV in the bc-plane.2,3 This is low compared to similar simulations of other iron-based cathode materials: e.g., 0.55 eV for LiFePO4,4 0.46 eV for tavorite LiFeSO4F,5 0.47 eV for Li2Fe(SO4)2,6 and 0.91 eV for Li2FeSiO4.7 The electrochemical rate performance of insertion cathode materials is to a large extent determined by their crystal structure. Already when Li2FeP2O7 was first reported in 2010 it was observed that the material undergoes a structural rearrangement during the first delithiation, resulting in a lower potential of the Fe3+/Fe2+ redox couple upon further electrochemical cycling.1 Similar results were obtained in this study; Figure 1 shows a higher voltage during the first charge. It was proposed that the structural rearrangement upon delithiation is a change in Li−Fe antisite defects,8 similar to what was observed for Li2FeSiO4,9 where such structural changes were investigated both experimentally10 and by DFT calculations.11 However, no experimental support has yet been available to a shift in Li−Fe antisite defects upon electrochemical cycling of Li2FeP2O7. The electrochemical cycling curve in Figure 1 also exhibits another interesting feature. There are two voltage regions during the first charge, one centered at 3.6 V and one at 3.8 V,

Received: February 4, 2015 Revised: May 20, 2015

Figure 1. Galvanostatic cycling of Li2FeP2O7 at C/50. The corresponding dx/dV plot is shown in the inset. © XXXX American Chemical Society

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DOI: 10.1021/acs.chemmater.5b00440 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

three doublets could be matched to the occupancy of iron sites in the Rietveld refinement of XRD data (see Tables S1 and S4 in Supporting Information). Thus, the outer doublet was identified as the FeO6 site and the two inner doublets were assigned to the FeO5 sites, shown in red and blue in Figure 3, respectively. Furuta et al.12 also assigned the outer doublet to FeO6 and used only one FeO5 site in the fitting procedure. After extracting 0.28 Li-ions per formula unit mainly the FeO6 doublet decreased in intensity, resulting in a better resolution for the Fe2+ doublets. Lithium extraction also caused the advent of two Fe3+ doublets, with the inner Fe3+ doublet having the strongest intensity (gray in Figure 3). XRD and Rietveld refinement support an initial preference for oxidation of the FeO6 site by showing a decrease in the average Fe−O bond distance from 2.17 to 2.09 Å, while the average bond distances of the FeO5 sites were not largely affected. Upon further delithiation to Li1.5FeP2O7 all three Fe2+ doublets decreased in intensity and the outer Fe3+ doublet (orange in Figure 3) started to increase faster than the inner one. The highest degree of electrochemical delithiation resulted in the composition Li1.13FeP2O7, where all three Fe2+ doublets decreased further, while the outer of the two Fe3+ doublets continued to increase faster than the inner one. These observations indicate an initial preferential oxidation of the FeO6 site, followed by a preference for oxidation of the FeO5 site at higher degrees of delithiation. Similar results could be obtained through a chemical delithiation with NO2BF4 as the oxidizing agent, as shown in the Supporting Information Table S4 and Figure S5. A thorough analysis of the Mössbauer parameters indicated an increased distortion of the iron polyhedra during delithiation. The quadrupole splitting (QS) for Fe2+ in FeO6 increased from about 2.52 mm/s at x ≈ 0 to about 2.77 mm/s at x ≈ 0.5. For Fe3+, both doublets had increased QS’s of about 0.05 mm/s from x ≈ 0.5 to x ≈ 0.9 (see Table S4 in Supporting Information). In Ingalls’ theory of QS, this can be explained by increased distortion around the iron atoms.22 The evolution of the oxidation of iron in the FeO5 and FeO6 sites is summarized in Figure 4. The populations of Fe2+ sites (blue) and Fe3+ sites (red) are plotted as a function of x in Li2−xFeP2O7, with polynomials fitted to the data as a guide to the eye. The curvatures of the polynomials show the initial preferential oxidation of the FeO6 site and the preference for oxidation of the FeO5 site at higher degrees of delithiation. Up to x ≈ 0.4, there is a preference for the FeO6 to be oxidized, as shown by the derivative of the fitted polynomials (see Figure S4 in Supporting Information). Considering the covalency of the Fe−O bond the five coordinated iron would be expected to oxidize prior to the six coordinated iron.23 With increased coordination number for iron, the polyhedra become more sterically crowded, resulting in a longer and more ionic Fe−O bond. Increased ionicity of the Fe−O bond normally results in a higher potential for the Fe3+/Fe2+ redox couple.24 Also, experimentally, the average Fe−O bond lengths in Li2FeP2O7 obtained from Rietveld refinement were 2.17 Å for FeO6 and 2.11 Å for FeO5. Thus, FeO5 can be considered to be more covalent, even disregarding the fact that the mixed occupancy of Fe and Li probably results in an apparently longer Fe−O bond for FeO5. Nevertheless, Mössbauer spectroscopy indicates that FeO6 is oxidized prior to FeO5 in Li2−xFeP2O7. In addition to the Mössbauer spectroscopy investigation, an ex situ XRD study was performed to investigate structural changes upon extraction and insertion of Li-ions in Li2FeP2O7. There is a discussion whether Li2FeP2O7 undergoes a two-

Figure 2. Crystallographic structure of Li2FeP2O7 viewed along the caxis. The three different local environments around iron are specifically indicated.

XRD showed that the sample purity was 95.9(4)%, with small amounts of Li4P2O7 and Fe2P2O7 impurities. The powder was mixed with conductive carbon additive (Super P, Erachem) and slowly electrochemically (de)lithiated in Swagelok cells to reach a quasi-equilibrium state before any analysis was performed. The (de)lithiated samples were analyzed with powder XRD and Mö ssbauer spectroscopy. The Fe2P2O7 impurity is not electrochemically active in the voltage window used for cycling the batteries,20 and consists of only ca. 4 mol % of the total iron content in the sample. Thus, its signal should remain at a constant and low level in the Mössbauer experiments, even though its doublets overlap with the doublets of Li2FeP2O7.21 A detailed experimental procedure, XRD patterns, Rietveld refinements, and Mössbauer fitting parameters are available in the Supporting Information. Figure 3 shows the Mössbauer spectra of Li2−xFeP2O7 at x = 0, x = 0.28, x = 0.5, and x = 0.87. The pristine sample gave no detectable Fe3+-signal, indicating favorable conditions during synthesis. Three Fe2+ doublets were fitted to the experimental data of the pristine sample, and the intensity ratio between the

Figure 3. Selected Mö ssbauer spectra at different degrees of delithiation of Li2FeP2O7. B


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Chemistry of Materials

chemical mechanism by Kim et al.,15 is also present after cycling. Since the relithiated Li2FeP2O7 has the same structure as the pristine material, albeit an increase in antisite defects, a two-phase electrochemical reaction mechanism between the lithiated and delithiated Li2FeP2O7 cannot be ruled out, despite the somewhat sloping open circuit voltage profile.8 In summary, Mössbauer spectroscopy showed a preferential oxidation of iron in the FeO6 site over iron in the FeO5 sites up to x ≈ 0.4 in Li2−xFeP2O7. Thereafter, at higher degrees of delithiation, a preference for oxidation of iron in the FeO5 sites was observed. This is the opposite to what is expected from a discussion on covalency, where the more covalent FeO5 should give a lower Fe3+/Fe2+ redox potential than FeO6. Ex situ powder XRD revealed that the amount of Li−Fe antisite defects increased from about 1/3 to 1/2 during electrochemical extraction and reinsertion of Li-ions in Li2FeP2O7. The Li−Fe antisite defects could be restored to 1/3 by annealing the cycled material, implying that a metastable state is formed upon electrochemical cycling. These findings provide additional insight into how the local environment around iron affects the electrochemical potential and how the amount of Li−Fe antisite defects is changed during electrochemical cycling in intercalation battery electrode materials.

Figure 4. Oxidation of iron in FeO6 and FeO5 sites at different degrees of electrochemical delithiation, as determined by Mössbauer spectroscopy. Polynomials are fitted to the data as a guide to the eye.

phase15 or a solid solution8 electrochemical mechanism. In the present study it was found that Rietveld refinement was possible with only one Li2−xFeP2O7 phase up to x ≈ 0.3, corresponding to the point where there is a kink in the galvanostatic charge curve (Figure 1). At higher degrees of delithiation the difference curves between observed and calculated intensities were fairly good when using only one Li2−xFeP2O7 phase, but very large distortion of iron and phosphate polyhedra was observed in the fitted model (see Figures S2 and S3 in Supporting Information). This could indicate a different structure of the delithiated Li2FeP2O7 or that a larger unit cell should be chosen, a super structure with slightly different atomic positions. The extensive Li−Fe antisite defects also point in the direction of a super structure, and a thorough crystallographic investigation, especially using single crystal XRD, would be beneficial to clarify these details. Rietveld refinement of the cycled Li2FeP2O7 showed that the amount of Li−Fe antisite defects had increased from about 1/3 to 1/2, which could be restored to 1/3 after annealing at 600 °C (see Tables S1 and S2 and Figure S1 in Supporting Information). The fact that the amount of defects was restored to the as-synthesized sample’s level suggests that the structure formed upon electrochemical cycling is metastable. No other structural transformations than the extent of Li−Fe antisite defects were identified, and the structure of the pristine Li2FeP2O7 with edge-sharing Fe polyhedra was preserved during the electrochemical cycling. The change in Li−Fe antisite defects could result in an increased energy of the relithiated state and an accompanied lowered cell voltage when cycling Li2FeP2O7 further in batteries, as suggested by Ye et al.17 They also reasoned that the lowered voltage could be due to a decreased energy of the delithiated state. Our findings suggest that despite the fact that all Bragg reflections can be indexed with the pristine structure model and the small difference in unit cell parameters given in previous studies,8 the delithiated Li2−xFeP2O7 seems to have a different crystal structure. Examples of materials with a twophase (de)lithiation mechanism and a small change in unit cell volume have previously been reported, e.g., the zero-strain insertion material Li4+xTi5O7 (0 < x < 3).25 To the best of our knowledge, no detailed structural models have been presented for the delithiated Li2−xFeP2O7. It is also worth noting that the strong intensity difference of the low-angle reflection between lithiated and delithiated Li2−xFeP2O7 (see Figure S2 in the Supporting Information), interpreted as a two-phase electro-

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ASSOCIATED CONTENT

* Supporting Information S

Detailed experimental procedure, XRD patterns, Rietveld refinements, additional crystal structure images, and Mössbauer hyperfine parameters. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b00440.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.B.). *E-mail: [email protected] (F.B.). Author Contributions

All authors have given approval to the final version of the manuscript. Funding

The work presented here was realized by support from the Swedish Foundation for Strategic Research (SSF) through the project From road to load − A Swedish lithium battery materials group. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge Mohammed Dahbi for inspiring discussions regarding structural changes and synthesis of battery electrode materials.

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ABBREVIATIONS QS, quadrupole splitting; V, voltage; x, lithium extracted in Li2−xFeP2O7; XRD, X-ray diffraction REFERENCES

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Structural and Electronic Changes in Li2FeP2O7 during

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