Crystal Structural Changes and Charge ... - ACS Publications

Apr 22, 2015 - Juliette Billaud , Christopher Eames , Nuria Tapia-Ruiz , Matthew R. Roberts , Andrew J. Naylor , A. Robert Armstrong , M. Saiful Islam...
2 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/JPCC

Crystal Structural Changes and Charge Compensation Mechanism during Two Lithium Extraction/Insertion between Li2FeSiO4 and FeSiO4 Titus Masese,† Cédric Tassel,‡,§ Yuki Orikasa,*,† Yukinori Koyama,∥ Hajime Arai,∥ Naoaki Hayashi,⊥ Jungeun Kim,# Takuya Mori,† Kentaro Yamamoto,† Yoji Kobayashi,‡ Hiroshi Kageyama,‡ Zempachi Ogumi,∥ and Yoshiharu Uchimoto† †

Graduate School of Human and Environmental Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Graduate School of Engineering, Kyoto University, Katsura-cho, Nishikyo-ku, Kyoto 615-8510, Japan § The Hakubi Center for Advanced Research, Kyoto University, Yoshida-ushinomiya-cho, Sakyo-ku, Kyoto 606-8302, Japan ∥ Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan ⊥ Institute for Integrated Cell-Material Sciences, Kyoto University, Yoshida-ushinomiya-cho, Sakyo-ku, Kyoto 606-8501, Japan # SPring-8/JASRI, 1-1-1 Kouto, Sayo-cho, Sayo-ku, Hyogo 679-5198, Japan ‡

S Supporting Information *

ABSTRACT: Li2FeSiO4 is a promising cathode material for lithium ion batteries because of its theoretically high capacity if two lithium ions can be extracted/inserted per formula unit; however, the extraction/insertion of two lithium ions from Li2FeSiO4 remains a challenge. Herein, we successfully synthesized carbon-coated Li2FeSiO4 nanoparticles which exhibit a capacity commensurate to a reversible two-lithium extraction/insertion at elevated temperature. This study investigates the mechanism underlying a two lithium ion extraction/insertion in Li2FeSiO4 using synchrotron X-ray absorption spectroscopy and X-ray diffraction. Our results reveal that the contribution of the Fe-3d band is dominant for the first lithium extraction process from Li2FeSiO4 to LiFeSiO4. During the second lithium extraction process from LiFeSiO4 to FeSiO4, however, ligand holes are formed in the O-2p band rather than further oxidation of Fe3+. Structural analyses further reveal a phase transformation between Li2FeSiO4 and LiFeSiO4, while a single-phase behavior is observed for Li2−xFeSiO4 (1.0 ≤ x ≤ 2.0). Together with a tentatively refined crystal structure of the FeSiO4 phase (x = 2.0), we discuss the charge compensation mechanism during two lithium extraction/insertion in Li2FeSiO4. sites.11 The crystal structure of Li2FeSiO4 changes from the initial monoclinic (P21/n) phase to a stable orthorhombic (Pmn21) phase upon cycling.12 Note that the P21/n is the simplified space group of the lower symmetry P21 space group, originally reported by Nishimura et al.13 Further, the charge compensation associated with charging and discharging is made via the redox process of Fe, as monitored by both the X-ray absorption spectra at Fe K-edge and Mössbauer spectroscopy.6,14 While the crystal structures, redox potentials and electronic band structures of Li2FeSiO4, LiFeSiO4, and FeSiO4 have been investigated using DFT calculations,1,15,16 experimental investigations of the precise structural changes, particularly those occurring upon the more-than-one lithium extraction/insertion process, have not been well understood yet, which is pivotal for the development of Li2FeSiO4 as well as related iron-based polyanion compounds as high-capacity cathode materials.

1. INTRODUCTION Li2FeSiO4 is a promising cathode material for lithium ion batteries.1−3 The presence of two Li atoms per one polyanion unit, in principle, engenders a multielectron charge transfer (Fe2+/Fe 4+ redox couple). The theoretical capacity of Li2FeSiO4 is, thus, 331 mAhg−1, which is approximately a manifold of the values in commercialized cathode materials, such as LiCoO2 and LiFePO4. However, the extracted capacity of Li2FeSiO4 was limited to one lithium extraction/insertion until recently, when nanostructuring allowed a more-than-one lithium reaction.4−6 To enhance two lithium extraction/ insertion in Li2FeSiO4, it is crucial to investigate how the crystal structure, local environment, and valence change during the charge/discharge processes. The structural evolution during the charge and discharge process in Li2FeSiO4 is intriguing, as Li2FeSiO4 exhibits rich polymorphism depending on the synthesis protocols.7−10 For the one lithium extraction/insertion process between Li2FeSiO4 and LiFeSiO4, the as-prepared Li2FeSiO4 undergoes irreversible phase transformation predominantly during the first charge, which entails significant cationic mixing of some Fe and Li © XXXX American Chemical Society

Received: January 13, 2015 Revised: April 21, 2015

A

DOI: 10.1021/acs.jpcc.5b00362 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

For XAS measurements at Fe K-edge, (dis)charged Li2−xFeSiO4 powders were mixed with boron nitride powder (Koujundo chemical) and pressed into pellets. The pellets were sealed in laminated packets in the argon-filled glovebox. XAS spectra were obtained at the beamline BL14B2 in SPring-8. The measurements were performed in transmission mode at room temperature. Treatment of the raw X-ray absorption data was performed with the Athena package. Quantitative analysis of Fe K-edge EXAFS spectra were performed with the Rigaku REX 2000 program package. O K-edge XAS spectra were measured at BL-2 of the SR center at Ritsumeikan University. The spectra were collected in fluorescence yield (FY) mode.

Recently, Lv et al. analyzed the structural changes underlying a more-than-one lithium ion extraction/insertion by using Xray diffraction (XRD) and X-ray absorption spectroscopy (XAS).17 They suggested that, upon the further extraction of lithium ions from LiFeSiO4 to Li0.5FeSiO4, the oxidation state of iron changes from Fe3+ to Fe4+ and the transformation is of a biphasic nature. However, a direct experimental evidence of the crystal and electronic structural changes occurring particularly between LiFeSiO4 and FeSiO4, based on spectroscopic and diffraction measurements, is not provided. There still are unclear points in the mechanism during two-lithium ion extraction/insertion. In this study, a two-lithium ion extraction/ insertion capacity in Li2FeSiO4 is achieved by using carboncoated nanoparticles, while operating at high temperature. To elucidate the Li extraction/insertion mechanism, which includes the electronic structure and crystal structure changes, we performed XAS at Fe K- and O K-edges and XRD in Li2−xFeSiO4.

3. RESULTS AND DISCUSSION All the reflections of the XRD patterns of the pristine (asprepared) Li2FeSiO4 were fully indexed to a monoclinic structure (space group P21/n). The pattern was refined using the Rietveld method (Figure S1). The refined lattice parameters (Table S1) are consistent with the previously reported values.12,19 Elemental analysis of as-prepared Li2FeSiO4 by ICP spectroscopy reveals lithium atomic content of about 1.97 relative to Fe, indicating that there is entirely no lithium deficiency in the samples prepared. An approximate crystallite size of 50 nm has been exhibited by Li2FeSiO4/C synthesized through the study as indicated by the TEM micrographs (Figure S2b), which coincides very well with the theoretical average crystallite size (44 nm) deduced from XRD studies. Further inspection using TEM and, more specifically, from the corresponding selected-area electron diffraction (SAED) pattern (Figure S2b) on a crystallite chosen arbitrarily reveal an array of pseudohexagonal symmetry dots that could be indexed using the monoclinic Li2FeSiO4 settings as the [141] zone axis. Note that this does not mean that crystallites oriented in other diffraction zone axis were not observed. Highresolution TEM image (Figure S2a) further indicates the presence of an amorphous carbon-coating on the Li2FeSiO4 nanoparticle. Figure 1 shows the (dis)charge voltage profiles of the pristine Li2FeSiO4/C nanoparticles operated at 55 °C. A capacity

2. EXPERIMENTAL METHODS Carbon-coated Li2FeSiO4 (Li2FeSiO4/C) nanoparticles were synthesized by the conventional solid-state reaction. Stoichiometric amounts of SiO2 (99.9%, Kanto Chemical), FeC2O4· 2H2O (99%, Junsei Chemical), and Li2CO3 (99%, Wako pure chemical) powders were weighed in a molar ratio of 1:1:1. Carbon (acetylene black), at 10 wt%, was added prior to mixing. These precursors were mixed in a planetary ball mill with ethanol at 400 rpm for 6 h. The mixture was dried and calcined at 800 °C for 6 h with a fixed Ar flux. The obtained powder was then transferred to an argon-filled glovebox because of air sensitivity.18 Based on elemental carbon analysis, the carbon content in the pristine Li2FeSiO4/C composite was 10.12 wt% in line with the nominal composition expected during the preparation of the composite (i.e., 10 wt%). Electrodes were prepared from the basic active material (Li2FeSiO4), to which carbon (acetylene black) was added at a weight ratio of 8:1 and ball-milled at 400 rpm for 30 min. Polytetrafluoroethylene (PTFE) binder was added to obtain a final weight ratio of 8:1:1. Coin-type electrodes were cut out and dried at room temperature in vacuo. Two-electrode coin cells were prepared by using metallic lithium as a counter electrode. The electrolyte used was a 1 M solution of LiClO4 in propylene carbonate (PC; all received from Kishida chemical). The cells were cycled at different rates within the capacity ranges of two Li+ per Fe at 55 °C. Note that the charge and discharge cutoff voltage was limited to 4.8 and 1.3 V, respectively, in order to avoid undesirable reaction with the electrolyte. The specific capacity was calculated based on the mass of pure Li2FeSiO4 in the Li2FeSiO4/C composite. The atomic ratio of lithium in pristine Li2FeSiO4 and the fully charged FeSiO4 electrodes were more precisely determined by inductively coupled plasma (ICP) spectroscopy (AA240, Agilent Technologies, U.S.A.). Synchrotron XRD patterns of Li2FeSiO4 were collected at the beamline BL02B2, SPring-8, equipped with a large Debye− Scherrer camera. The wavelength of the incident X-ray beam was 0.50005 Å. Rietveld refinement was carried out with the program JANA2006. Charged and discharged Li2−xFeSiO4 electrodes at C/50 rate were removed from the cell in the glovebox, carefully washed with PC (Kishida chemical) and dried. The dried Li2−xFeSiO4 powders were placed in a glass capillary, which was then sealed with resin in the argon-filled glovebox.

Figure 1. Charge and discharge voltage profiles of Li2FeSiO4/C at a current density corresponding to C/50 rate at 55 °C. Low cutoff voltage range was set at 1.3 V.

corresponding to more-than-one lithium extraction/insertion is attained, based on the electrochemical response. The initial charge profile is different from the second charge profile, which has been ascribed to structural rearrangements.3 The voltage observed at the initial charge is higher than the reported value which was about 3.1 V.11 For the discharge process, a sloping profile is also observed in this study, although the previous B

DOI: 10.1021/acs.jpcc.5b00362 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C report observe a flat potential profile centered at 2.8 V. These differences are presumably derived from polarization effect. The open circuit potential shows plateau at 3.1 and 2.8 V during the initial charge and discharge process, respectively (shown in Figure S3). Differential capacity plots (shown in Figure S4) also reveal two redox peaks during the initial lithium ion extraction process, reminiscent of Fe2+/Fe3+ and Fe3+/Fe4+ redox couples. The higher voltage profile in the initial charge at 55 °C has also been reported by Rangappa et al.6 The initial discharge capacity is about 330 mAhg−1, which is commensurate to the twolithium insertion in Li2−xFeSiO4. The operation at elevated temperature enhances the reversible extraction/insertion of more-than-one lithium ion in Li2FeSiO4. As described before, a more-than-one lithium ion extraction/insertion has been reported in Li2FeSiO4 nanosheets and nanoparticles.4,6 Such tuning of the morphology is useful to improve the reactivity. We are aware that electrochemical measurements were conducted at elevated temperatures and at high voltage range, which might cause parasitic reactions stemming from the electrolyte without the contribution of the active materials. However, cyclic voltammograms of the electrolyte (shown in Figure S5) indicate that the electrolyte we used to perform the electrochemical tests is stable within the set voltage ranges, and thus, the obtained charge and discharge capacity emanates solely from the reaction of active materials. The initial lithium extraction/insertion process in Li2FeSiO4 was investigated by XAS. Figure 2a,b shows the Fe K-edge Xray absorption near edge structure (XANES) during the initial charging and discharging processes, where absorption edge shifts toward higher and lower energies, respectively. In general, the near K-edge structure in the Fe-3d metal compounds is

interpreted by the transfer of core electrons into the 4p empty states. With an increase in the valence state of Fe, electrons are more strongly attracted to the Fe nucleus, resulting in the increase of the absorption edge energy.20,21 Therefore, it is expected in our case that the valence state of the Fe ion in Li2FeSiO4 increases from +2 to +4 during charging, while it decreases conversely during discharging. The increased energy shift in the absorption edge observed from Li2FeSiO4 to LiFeSiO4 indicates that the electronic structure change triggered by the change in valence state (from +2 to +3) is dominated by the outermost orbital of the Fe-3d band. To determine the Fe oxidation states during the extraction process, comparison with the Fe reference compounds, the pristine Li2FeSiO4, and the delithiated LiFeSiO4 samples were conducted and verified to be in the Fe2+ and Fe3+, respectively (see Supporting Information, Figure S6). In contrast, the difference in edge shift between LiFeSiO4 and FeSiO4 is much smaller. This small shift on the second lithium extraction has previously been reported by others, without any explanations.18 One may attribute the small edge shift to extrinsic side reactions such as electrolyte decomposition, which we have confirmed not to occur within the set voltage ranges and temperature (Figure S5). Local structural changes during electrochemical reaction in Li2FeSiO4 validate a more-thanone lithium ion extraction/insertion. Figure 3a,b shows the Fourier transform from EXAFS oscillation spectra of Li2FeSiO4, LiFeSiO4, and FeSiO4 during charging and discharging processes. The weighted EXAFS oscillations are furnished in Figure S7. The first main peak corresponds to the mode relevant to the Fe−O shell. The Fe−O bond length in FeSiO4

Figure 2. Normalized Fe K-edge XANES spectra of Li2−xFeSiO4 electrode as a function of x during (a) initial charging and (b) initial discharging processes at 55 °C.

Figure 3. Fe K-edge Fourier transform magnitudes EXAFS spectra of Li2−xFeSiO4 samples during (a) initial charging and (b) initial discharging processes at 55 °C. C

DOI: 10.1021/acs.jpcc.5b00362 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C is clearly shorter than that in LiFeSiO4. The reduced Fe−O distance in FeSiO4 (vs LiFeSiO4) appears to indicate the oxidation of iron to +4. O K-edge XAS measurements reveal significant contribution of the O-2p band to the redox mechanism. XANES spectra at the O K-edge of Li2−xFeSiO4 (x = 0, 1, and 2) during Li+ extraction are shown in Figure 4. Note that the XANES

Figure 5. Rietveld refinement pattern of the synchrotron XRD data for the highly delithiated phase designated as FeSiO4.

trace amount of lithium (within the detection limit of the instruments) in LixFeSiO4, confirming that we have the formation of “FeSiO4” composition. Table 2 shows the refined lattice parameters of Li2−xFeSiO4 (x = 0, 1, and 2) during the initial charging and discharging. Other refined profiles and parameters are furnished in the Supporting Information (see Figures S9, S10, and S11). In the initial charging process, the monoclinic Li2FeSiO4 structure undergoes a transformation to the orthorhombic Pnma LiFeSiO4.27 A reconstructive polymorphic transformation occurs that involves the site interchange of half of the Fe ions moving into vacant Li sites concomitant with irreversible changes in the local structure of Fe atoms, as has been revealed in our previous work.27,28 The orthorhombic structure is maintained upon further lithium extraction to FeSiO4, judging from the similarity in the XRD patterns of FeSiO4 and the intermediate LiFeSiO4 phase. During the lithium insertion process from FeSiO4 to LiFeSiO4, this orthorhombic crystal structure is retained. Attempts to determine the precise orthorhombic space group of FeSiO4 were in vain using X-ray diffraction analyses. It is true that the Pnma S.G structure of delithiated FeSiO4 and Pmn21 S. G structure of relithiated Li2FeSiO4 show high distortions in the SiO4 tetrahedra. To address this, different orthorhombic structural models were used to refine the delithiated FeSiO4 phase; however, the orthorhombic Pnma space group proved to be the most suitable structural model in our refinements. Regardless of the exact space group, nevertheless, the crystal system change (monoclinic-to-orthorhombic transformation) holds. The unit cell volume expands from Li2FeSiO4 to FeSiO4, which is consistent with preliminary theoretical calculations.15 The orthorhombic Pmn21 structure of relithiated Li2FeSiO4 is reported to be the stable crystal structure of Li2FeSiO4, in subsequent cycles between Li2FeSiO4 and LiFeSiO4.12 Considering all the results, the charge compensation mechanism upon a more-than-one lithium extraction/insertion in Li2FeSiO4 is revealed. Figure 6 shows a schematic summary of the mechanism. During Li+ extraction/insertion between Li2FeSiO4 and LiFeSiO4, the contribution from the Fe-3d orbital is dominant (Fe2+ ↔ Fe3+). However, the O-2p orbital plays a pivotal role in oxidation/reduction between LiFeSiO4 and FeSiO4 by allowing holes at the ligands while maintaining the iron valence as +3. The initial monoclinic structure of Li2FeSiO4 transforms to the orthorhombic structure in LiFeSiO4.26 Between LiFeSiO4 and FeSiO4, the orthorhombic structure is maintained. During the lithium insertion process,

Figure 4. Normalized ex situ O K-edge XANES spectra of Li2−xFeSiO4 electrode as a function of x during initial Li+ extraction (charging) process.

measurement was conducted in fluorescence yield mode, which we confirmed to exhibit a high sensitivity (over 85%) of the Xray signals to the innate bulk properties of Li2FeSiO4. The intense absorption peak at about 530 eV in the pristine Li2FeSiO4 electrode is ascribed to the transition of the oxygen 1s electron to the (ligand) hole state in the oxygen 2p orbital level. The higher absorption peak at 530 eV corresponds to the highly hybridized state between O-2p and Fe-3d orbitals.22 The pre-edge peak increases significantly going from LiFeSiO4 to FeSiO4, which indicates that a large portion of the ligand holes located in the O-2p states compensate for the change in valence caused by lithium extraction/insertion between LiFeSiO4 and FeSiO4. In other words, Fe3+L (L denotes the ligand hole) is an appropriate description, instead of Fe4+. Such a ligand hole picture is seen in several perovskitetype iron oxides such as SrFeO3, CaFeO3, and LaCu3Fe4O12, which has been discussed in terms of the negative charge transfer.23−25 Recently, Tarascon et al. has demonstrated that the reversible lithium (de)intercalation of Li2(Ru,Sn)O3 involves not only metal redox process but also anion redox process (O2− → O22−) reversible redox processes, owing to the d−sp hybridization associated with a reductive coupling mechanism.26 Our work has also underlined the importance of considering the anion redox process, in addition to the metal redox process. Moreover, density functional theory (DFT) calculations (Figure S8) reveal shifts of the O-2p bands above the Fermi level upon delithiation process of Li2−xFeSiO4 (1 ≤ x ≤ 2), validating the contribution of oxygen to the charge compensation process upon further extraction of Li from LiFeSiO4 to FeSiO4. Crystal structural changes during the two-lithium extraction and insertion process conducted at 55 °C were investigated by XRD measurements. Figure 5 shows the Rietveld analysis of the XRD pattern of FeSiO4, with the refined atomic parameters in Table 1. The calculated pattern fits well with the observed pattern, resulting in low reliability factors. ICP measurements conducted on the fully delithiated phase indicate hardly any D

DOI: 10.1021/acs.jpcc.5b00362 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Table 1. Atomic Coordinates, Occupancies, And Atomic Displacement Parameters Obtained by Rietveld Refinement of Synchrotron X-ray Diffraction (SXRD) Data for FeSiO4 Indexed in the Orthorhombic Pnma Space Group with Lattice Constants: a = 10.3969(20) Å, b = 6.5618(16) Å, c = 5.0334(8) Å, V = 341.45 Å3a

a

atom

site

g

x

y

z

100 × Uiso

Fel Sil Ol O2 O3

8d 4c 4c 4c 8d

0.5 1.0 1.0 1.0 1.0

0.6604(3) 0.4180(5) 0.4071(11) 0.5640(8) 0.3489(8)

0.5158(7) 0.250 0.250 0.250 0.0675(7)

0.2316(13) 0.2903(12) 0.6154(14) 0.2445(42) 0.1799(14)

0.37(5) 0.22(8) 0.62(35) 0.68(31) 0.94(26)

Rwp = 1.69%; Rp = 1.08%; χ2 = 1.19.

Table 2. Refined Lattice Parameters for Li2−xFeSiO4 (x = 0, 1, and 2) During the Initial Charging and Discharging Processes Li2FeSiO4 (pristine) LiFeSiO4 (charged) FeSiO4 LiFeSiO4 (discharged) Li2Fe SiO4 (discharged)

lattice

space group

a (Å)

b (Å)

c (Å)

V (Å3)

Z

monoclinic orthorhombic orthorhombic orthorhombic orthorhombic

P 21/n Pnma Pnma Pnma Pmn 21

8.2433(4) 10.3476(12) 10.3969(20) 10.2993(20) 6.2961(19)

5.0226(1) 6.5746(8) 6.5618(16) 6.5906(14) 5.3348(15)

8.2373(3) 5.0202(9) 5.0334(8) 5.0137(9) 5.0077(12)

336.3 341.5 343.4 340.3 168.2

4 4 4 4 2

tion is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b00362.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-75-753-6850. Fax: +81-75-753-6850. E-mail: orikasa. [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partly supported by the Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING) project under the auspices of New Energy and Industrial Technology Department Organization (NEDO (Japan)). The synchrotron radiation experiments were performed at the BL02B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; Proposal Nos. 2012B1018, 2012A1022, and 2011B1029). T.M. acknowledges the Ito Foundation Scholarship and the Honjo International Scholarship Foundation.

Figure 6. Schematic summary of the two lithium ion extraction and insertion mechanism in Li2FeSiO4.

the orthorhombic structure and the Li/Fe cationic antisite ratio are maintained and relithiated Li2FeSiO4 adopts the orthorhombic phase.12

4. CONCLUSION In conclusion, the Li2FeSiO4 nanoparticles synthesized in this study exhibit a capacity commensurate with a two-lithium insertion/extraction with good cyclability. Our results confirm the earlier reports that the contribution of the Fe-3d band (i.e., oxidation of Fe2+ to Fe3+) is dominant for the one lithium extraction from Li2FeSiO4 to LiFeSiO4.14,17 Upon a more than one lithium extraction from LiFeSiO4 to FeSiO4, however, further oxidation of Fe3+ is not observed, and a ligand hole is formed in the O-2p band. Structural analyses further reveal a phase transformation between Li2FeSiO4 and LiFeSiO4, while a single-phase behavior is observed between LiFeSiO4 and FeSiO4. This study provides experimental evidence that the electrochemistry of Li2FeSiO4 utilizes the hole chemistry of oxygen to achieve high capacity.

■

■

REFERENCES

(1) Arroyo-de Dompablo, M. E.; Armand, M.; Tarascon, J. M.; Amador, U. On-Demand Design of Polyoxianionic Cathode Materials Based on Electronegativity Correlations: An Exploration of the Li2MSiO4 System (M = Fe, Mn, Co, Ni). Electrochem. Commun. 2006, 8, 1292−1298. (2) Muraliganth, T.; Stroukoff, K. R.; Manthiram, A. MicrowaveSolvothermal Synthesis of Nanostructured Li2MSiO4/C (M = Mn and Fe) Cathodes for Lithium-Ion Batteries. Chem. Mater. 2010, 22, 5754−5761. (3) Nyten, A.; Abouimrane, A.; Armand, M.; Gustafsson, T.; Thomas, J. O. Electrochemical Performance of Li2FeSiO4 as a New Li-Battery Cathode Material. Electrochem. Commun. 2005, 7, 156−160. (4) Chen, Z. X.; Qiu, S.; Cao, Y. L.; Qian, J. F.; Ai, X. P.; Xie, K.; Hong, X. B.; Yang, H. X. Hierarchial Porous Li2FeSiO4/C Composite with 2 Li Storage Capacity and Long Cycle Stability for Advanced LiIon Batteries. J. Mater. Chem. A 2013, 1, 4988−4992. (5) Gao, H. Y.; Hu, Z.; Zhang, K.; Cheng, F. Y.; Chen, J. Intergrown Li2FeSiO4·LiFePO4-C Nanocomposites as High-Capacity Cathode Materials for Lithium-Ion Batteries. Chem. Commun. 2013, 49, 3040− 3042.

ASSOCIATED CONTENT

S Supporting Information *

Details of the experimental condition, experimental results (TEM image, electrochemical data and XRD data for Li2−xFeSiO4 electrodes) are shown. The Supporting InformaE

DOI: 10.1021/acs.jpcc.5b00362 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C (6) Rangappa, D.; Murukanahally, K. D.; Tomai, T.; Unemoto, A.; Honma, I. Ultrathin Nanosheets of Li2MSiO4 (M = Fe, Mn) as HighCapacity Li-Ion Battery Electrode. Nano Lett. 2012, 12, 1146−1151. (7) Boulineau, A.; Sirisopanaporn, C.; Dominko, R.; Armstrong, A. R.; Bruce, P. G.; Masquelier, C. Polymorphism and Structural Defects in Li2FeSiO4. Dalton Trans. 2010, 39, 6310−6316. (8) Sirisopanaporn, C.; Boulineau, A.; Hanzel, D.; Dominko, R.; Budic, B.; Armstrong, A. R.; Bruce, P. G.; Masquelier, C. Crystal Structure of a New Polymorph of Li2FeSiO4. Inorg. Chem. 2010, 49, 7446−7451. (9) Sirisopanaporn, C.; Dominko, R.; Masquelier, C.; Armstrong, A. R.; Mali, G.; Bruce, P. G. Polymorphism in Li2(Fe,Mn)SiO4: A Combined Diffraction and NMR Study. J. Mater. Chem. 2011, 21, 17823−17831. (10) Yabuuchi, N.; Yamakawa, Y.; Yoshii, K.; Komaba, S. Lowtemperature Phase of Li2FeSiO4: Crystal Structure and a Preliminary Study of Electrochemical Behavior. Dalton Trans. 2011, 40, 1846− 1848. (11) Nyten, A.; Kamali, S.; Haggstrom, L.; Gustafsson, T.; Thomas, J. O. The Lithium Extraction/insertion Mechanism in Li2FeSiO4. J. Mater. Chem. 2006, 16, 2266−2272. (12) Armstrong, A. R.; Kuganathan, N.; Islam, M. S.; Bruce, P. G. Structure and Lithium Transport Pathways in Li2FeSiO4 Cathodes for Lithium Batteries. J. Am. Chem. Soc. 2011, 133, 13031−13035. (13) Nishimura, S. I.; Hayase, S.; Kanno, R.; Yashima, M.; Nakayama, N.; Yamada, A. Structure of Li2FeSiO4. J. Am. Chem. Soc. 2008, 130, 13212−13213. (14) Dominko, R.; Arčon, I.; Kodre, A.; Hanžel, D.; Gaberšcě k, M. In-Situ XAS Study on Li2MnSiO4 and Li2FeSiO4 Cathode Materials. J. Power Sources 2009, 189, 51−58. (15) Saracibar, A.; Van der Ven, A.; Arroyo-de Dompablo, M. E. Crystal structure, Energetics, And Electrochemistry of Li2FeSiO4 Polymorphs from First Principle Calculations. Chem. Mater. 2012, 24, 495−503. (16) Larsson, P.; Ahuja, R.; Liivat, A.; Thomas, J. O. Structural and Electrochemical Aspects of Mn Substitution into Li2FeSiO4 from DFT Calculations. Comput. Mater. Sci. 2010, 47, 678−684. (17) Lv, D. P.; Bai, J. Y.; Zhang, P.; Wu, S. Q.; Li, Y. X.; Wen, W.; Jiang, Z.; Mi, J. X.; Zhu, Z. Z.; Yang, Y. Understanding the High Capacity of Li2FeSiO4: In situ XRD/XANES Study Combined with First-Principles Calculations. Chem. Mater. 2013, 25, 2014−2020. (18) Deng, C.; Zhang, S.; Gao, Y.; Wu, B.; Ma, L.; Sun, Y. H.; Fu, B. L.; Wu, Q.; Liu, F. L. Regeneration and Characterization of AirExposed Li2FeSiO4. Electrochim. Acta 2011, 56, 7327−7333. (19) Sirisopanaporn, C.; Masquelier, C.; Bruce, P. G.; Armstrong, A. R.; Dominko, R. Dependence of Li2FeSiO4 Electrochemistry on Structure. J. Am. Chem. Soc. 2011, 133, 1263−1265. (20) Haas, O.; Deb, A.; Cairns, E. J.; Wokaun, A. Synchrotron X-ray Absorption Study of LiFePO4 Electrodes. J. Electrochem. Soc. 2005, 152, A191−A196. (21) Haas, O.; Vogt, U. F.; Soltmann, C.; Braun, A.; Yoon, W. S.; Yang, X. Q.; Graule, T. The Fe K-Edge X-ray Absorption Characteristics of La1−xSrxFeO3−δ Prepared by Solid State Reaction. Mater. Res. Bull. 2009, 44, 1397−1404. (22) Augustsson, A.; Zhuang, G. V.; Butorin, S. M.; Osorio-Guillén, J. M.; Dong, C. L.; Ahuja, R.; Chang, C. L.; Ross, P. N.; Nordgren, J.; Guo, J.-H. Electronic Structure of Phospho-Olivines LixFePO4 (x = 0, 1) from Soft-X-ray-Absorption and -Emission Spectroscopies. J. Chem. Phys. 2005, 123, 184717. (23) Bocquet, A. E.; Fujimori, A.; Mizokawa, T.; Saitoh, T.; Namatame, H.; Suga, S.; Kimizuka, N.; Takeda, Y.; Takano, M. Electronic Structure of SrFe4+O3 and Related Fe Perovskite Oxides. Phys. Rev. B 1992, 45, 1561−1570. (24) Akao, T.; Azuma, Y.; Usuda, M.; Nishihata, Y.; Mizuki, J.; Hamada, N.; Hayashi, N.; Terashima, T.; Takano, M. Charge-Ordered State in Single Crystalline CaFeO3 Thin Film Studied by X-ray Anomalous Diffraction. Phys. Rev. Lett. 2003, 91, 156405.

(25) Chen, W.-T.; Saito, T.; Hayashi, N.; Takano, M.; Shimakawa, Y. Ligand-Hole Localization in Oxides with Unusual Valence Fe. Sci. Rep. 2012, 2, 449. (26) Sathiya, M.; Rousse, G.; Ramesha, K.; Laisa, C. P.; Vezin, H.; Sougrati, M. T.; Doublet, M. L.; Foix, D.; Gonbeau, D.; Walker, W.; et al. Reversible Anionic Redox Chemistry in High-Capacity LayeredOxide Electrodes. Nat. Mater. 2013, 12, 827−835. (27) Masese, T.; Orikasa, Y.; Tassel, C.; Kim, J.; Minato, T.; Arai, H.; Mori, T.; Yamamoto, K.; Kobayashi, Y.; Kageyama, H.; et al. Relationship between Phase Transition Involving Cationic Exchange and Charge-Discharge Rate in Li2FeSiO4. Chem. Mater. 2014, 26, 1380−1384. (28) Masese, T.; Orikasa, Y.; Mori, T.; Yamamoto, K.; Ina, T.; Minato, T.; Nakanishi, K.; Ohta, T.; Tassel, C.; Kobayashi, Y.; et al. Local Structural Change in Li2FeSiO4 Polyanion Cathode Material during Initial Cycling. Solid State Ionics 2014, 262, 110−114.

F

DOI: 10.1021/acs.jpcc.5b00362 J. Phys. Chem. C XXXX, XXX, XXX−XXX