Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES
Article
Increased conductivity in the metastable intermediate in LixFePO4 electrode Jiechen Lu, Gosuke Oyama, Shin-ichi Nishimura, and Atsuo Yamada Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04508 • Publication Date (Web): 01 Feb 2016 Downloaded from http://pubs.acs.org on February 4, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
Increased conductivity in the metastable intermediate in LixFePO4 electrode Jiechen Lu, Gosuke Oyama, Shin-ichi Nishimura, and Atsuo Yamada*. Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan ABSTRACT: With increasing concerns about energy and environmental issues, lithium ion batteries are now penetrating into large-scale applications such as electric vehicles. As an electrode reaction process, it is generally believed that twophase reaction with structural rearrangement and large lattice mismatch impedes high-rate capability. However, LixFePO4, with its two-phase reaction between LiFePO4 and FePO4, exhibits an exceptional high-rate performance. In this paper, after confirming the existence of a single-phase reaction even under moderate rates, we demonstrate an approximately 2 orders of magnitude increase of the conductivity for the quenched intermediate Li0.6FePO4. In addition to the widely accepted strain relaxation effect at the two-phase interface, the dramatically increased conductivity due to polaron/lithium carrier density increase in the intermediate phase should be highlighted as an important factor to accelerate the electrode reaction of olivine LixFePO4.
INTRODUCTION 1
Since the initial report by Goodenough and coworkers in 1997, LiFePO4 has attracted tremendous attention as a cathode material for lithium ion batteries due to its high specific theoretical energy density, low cost, nontoxicity, and superb thermal/chemical stability. Open-circuit voltage curves, neutron diffraction measurements2 and reaction entropy behavior indicate that lithium intercalation/deintercalation occurs through a two-phase transition between LiαFePO4 and Li1–βFePO4, forming a large miscibility gap at α < x < 1–β (α ≈ 0.05, 1–β ≈ 0.89). The two-phase interface forms on the (100) plane and moves in the a-direction by the cooperative lithium onedimensional diffusion in the b-direction during charging and discharging3–9. LiFePO4 was first mischaracterized as a low power material due to its insulating nature and later revealed to generate high power by optimizing the wiring of particles by carbon-coating7 and reducing the diffusion distance by nanosizing8,9. However, it is imprudent and one-sided to only attribute the exceptionally high-rate capability to these external electrode-scale optimizations. Many scientists have attempted to explore the underlying phase transition mechanisms enabling such an exceptional high-rate capability, and the Domino-cascade model10,11 is widely accepted. In the Domino-cascade model, an energetically unfavorable distorted interface perpendicular to the [100] direction between the LiαFePO4 and Li1–βFePO4 phases moves extremely fast as a wave through the entire crystal. This suitably explains the coexistence of the fully intercalated and fully
deintercalated individual particles observed after relaxation in ex situ X-ray diffraction and electron microscopy. Interestingly, in 2011, ab initio calculations12 predicted that an alternative single-phase pathway can be fulfilled at very low overpotential of approximately 10–20 mV. Once the overpotential is removed (same situation as the ex situ case), the single-phase nanoparticles will immediately relax to equilibrium individual LiFePO4 or FePO4 states, as is often observed in the ex situ case. Recently, using in situ X-ray diffraction on micrometersized LiFePO4 during the high rate charge/discharge process, Orikasa et al. traced the existence of the metastable intermediate phase Li0.6–0.75FePO413. Their successive study14 on nanoparticle LiFePO4 (60 nm) at a moderate current rate (1C, where nC is the current required to either charge or discharge fully in 1/n hours) indicates an extended solid solution region of the two end members LiαFePO4 and Li1–βFePO4, which is consistent with the size-dependent miscibility limits15,16. In another in situ study, the non-equilibrium single-phase transition behavior of nanoparticle LiFePO4 (186 nm) under high current rates (10 C) was reported17 and metastable solid solution phases are claimed to be formed at the twophase interface with a width of approximately 100 nm. It is accepted that the existence of the single-phase reaction process will promote the high rate capability due to the absence of sluggish nucleation and growth processes. In LixFePO4, the ground state phase separation and metastable solid solution are energetically competitive, and the solid solution phase is induced in a non-equilibrium electrochemical reaction. However, until
ACS Paragon Plus Environment 1
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
now, there has been no discussion or experimental report of the transport properties of the metastable solid solution phase due to the following extreme technical difficulties. First, the solid solution phase LixFePO4 is metastable during electrochemical process and disappears within a few seconds after relaxation17,18. We overcame this limitation by quenching LixFePO4 (x = 0.6) at 350 °C to room temperature. This quenched phase remained stable for a couple of weeks, which enabled sufficient time to measure the intrinsic conductivity. Second, conductive carbon may form during sintering at high temperature from possible carbon sources such as polyolefin worn from jars, organic solvent used in milling and carboncontaining precursors (e.g., oxalate FeC2O4·2H2O). Finally, impurities such as Li3PO4, Li4P2O7 and FexP resulting from off-stoichiometric mixing or the carbon reduction effect above 800 °C will largely increase the apparent conductivity of LiFePO4 by several orders of magnitude19– 21 . The scattering of the LiFePO4 conductivity data resulting from the above extrinsic effects has troubled and confused scientists for a long time. In our present study, pure carbon-free LiFePO4 and FePO4 were prepared using carbon-free precursors and controlling sintering parameters, and the intrinsic conductivity of quenched single phase LixFePO4 (x = 0.6) was measured for the first time. EXPERIMENTAL SECTION Carbon-containing olivine LiFePO4 samples for in situ X-ray diffraction and conductivity test were synthesized by a conventional solid-state reaction. Li2CO3 (Wako, 99+%), FeC2O4·2H2O (Kojundo, 99+%) and (NH4)2HPO4 (Wako, 99+%) were stoichiometrically weighed and thoroughly mixed by high-energy ball milling for 6 h with acetone. This mixture was heated at 600 °C for 6 h under an engineering-grade Ar gas flow. Carbon-free LiFePO4 powder for the conductivity test was carefully prepared to prevent contamination with conductive carbon and impurities. A stoichiometric mixture of Fe2O3 (Wako, 99.9+%), Li3PO4 (Kanto, 99+%) and NH4H2PO4 (Wako, 99+%) was ball milled in stainless containers and sintered in flowing 5%H2/Ar at 600 °C for 6 h. Heterosite FePO4 was prepared by oxidizing pristine LiFePO4 with a slight excess of NO2BF4 in acetonitrile for 12 h, followed by centrifugation and several washing steps with fresh acetonitrile to remove the reduction product LiBF4. Then, the sample was dried at 100 °C in a vacuum glass tube oven for 12 h. In situ X-ray diffraction measurements of the nonequilibrium phase transformation were conducted using synchrotron radiation from the short gap undulator on beam line 3A at the Photon Factory in the High Energy Accelerator Research Organization (KEK). The photon flux of the beam was 1012 photons/s, and the energy of the synchrotron was set to 10 keV (1.24 Å). Diffraction data (Exposure time; 10 seconds) were gathered every 40 seconds with symmetrical reflection geometry by a 2D detector (PILATUS-100K, DECTRIS). In situ X-ray diffraction (XRD) measurements were performed with a
Page 2 of 7
commercial two-electrode cell (Battery cell attachment, Rigaku). LiFePO4/C composite electrode including 75 wt% LiFePO4, 15wt % carbon black (Ketjen Black, Lion Corp.), and 10 wt% polytetrafluoroethylene binder were well-mixed into a film and used as the cathode. The cathode film was assembled into a cell with a Li metal foil as the anode, a polypropylene film as the separator and 1 mol dm-3 LiPF6 dissolved in ethylene carbonate (EC) / diethyl carbonate (DEC) (3 : 7 vol%.) as the electrolyte. A 6 μm aluminum foil was placed between the beryllium window and the cathode to avoid unfavorable oxidation of the window. Carbon content was measured by a CE440 Elemental Analyzer (Exeter Analytical, Inc) with a minimum limit of 0.01%. Powder X-ray diffraction (XRD) patterns of olivine samples for conductivity measurement were measured by a Rigaku RINT-TTR III power diffractometer with Cu Kα radiation operating at 50 kV/300 mA. Structural parameters were obtained by Rietveld refinement using the software Topas version 3. Conductivity measurements were made with a Solartron AC impedance analyze. The powder samples of LiFePO4, FePO4 and the mixture 0.6LiFePO4/0.4FePO4 were pelletized by isostatic pressuring at 300 MPa. The pellets of the LiFePO4 and FePO4 were heated at 350 °C for 12 h in an inert Ar atmosphere. To make a quenched single-phase Li0.6FePO4, the pellet of mixture 0.6LiFePO4/0.4FePO4 was sealed in evacuated glass tube and sintered at 350 °C for 12 h, followed by a quenching process into cool water. After sintering, all of the three samples were polished to flatness on both sides. Typical samples have a diameter of approximately 8 mm and a thickness of approximately 3 mm. Gold metal was then sputtered on both sides to serve as an electronic electrode, blocking the ionic exchange. Impedance measurements were performed within the temperature range of 30 to 350 °C in Ar atmosphere by a Solartron 1296 Dielectric Interface combined with a 1260 Impedance/Gain-Phase Analyzer in the frequency range from 106 to 10-2 Hz. RESULTS AND DISCUSSION We first attempted to confirm the solid solution formation under non-equilibrium electrochemical condition (Pulse mode with a large applied overpotential) by in situ synchrotron X-ray diffraction measurements. After the assembly of the LiFePO4 cells, a chargedischarge test was conducted for two cycles to confirm their reversible capacity. All the LiFePO4 samples showed reversible capacities close to the theoretical value (ca. 170 mAh/g) with a potential plateau at 3.43 V v.s. Li/Li+, ensuring the quality of the sample with a negligible amount of anti-site defects and impurities. Then, the cells were subjected to in situ synchrotron X-ray diffraction measurement. A 500 mV potential step (3.2 V to 3.7 V) was applied across the equilibrium two-phase potential (3.43 V) and time resolved in-situ X-ray diffraction data was collected, as shown in Fig. 1 (a). Magnified views of the (011), (211) and (020) reflections in Fig. 1 (b) and Fig. 1 (c) clearly indicate the continuous peak shifts. A simple
ACS Paragon Plus Environment 2
Page 3 of 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
Figure 1. Phase evolution of LixFePO4 cathode under a large external potential step (3.2 V to 3.7 V). a) Time-resolved synchrotron in situ X-ray diffraction profiles of LixFePO4 b) Magnified view of (011) reflection profile. c) Magnified view of (211) and (020) reflections.
deconvolution analysis based on the two phase separation model was applied to the (200) peak in the 2θ range of 13° to 15° but this analysis failed, as demonstrated in supplemental Fig. S1 (b). Figure S1 (c) and (d) show one possible scenario for the residue of two peaks, accompanied by an intermediate phase formed as a diffuse interface to accommodate the lattice misfit between the two end members22. Consistent with several recent results17,18 measured in high-rate (dis)charging processes (> 20C), our in situ X-ray diffraction measurement under pulse mode with a large overpotential, confirm that the electrode reaction of LiFePO4 deviates from the equilibrium two-phase reaction and involves a non-equilibrium single-phase reaction. In the whole process of our pulse mode (time scale 1 hour), we can distinctly trace the intermediate phase, which suggests that the single-phase transformation may also occur at moderate rate (1C) during (dis)charging processes. Compared with the two-phase reaction with sluggish nucleation and growth processes, a single-phase reaction with solid solution LixFePO4 avoids major structural rearrangement and large volume change, which can be essential for the high-rate capability of olivine LiFePO4. However, another possible advantage is the increased carrier conductivity by the mixed Fe3+-Fe2+ valence states and the introduction of a significant amount of lithium defects. In the following experiments, we demonstrate the intrinsic transport properties of the intermediate metastable phase LixFePO4 (x = 0.6) and compare them with those of the two end members FePO4 and LiFePO4. The purity of the samples was carefully controlled as described in the experimental section to exclude extrinsic contributions.
Figure 2. Structure and X-ray diffraction patterns (Cu Kα) of prepared olivine samples. a) Carbon-free LiFePO4. b) Carbon-free FePO4. c) Carbon-free 0.6LiFePO4/0.4FePO4 mixture and d) Quenched single phase Li0.6FePO4.
Figures 2 (a) and (b) show the X-ray diffraction patterns of carbon-free LiFePO4 and FePO4. Based on the Rietveld refinement, they are pure without any impurities such as Fe2P, Li3PO4, and Fe2P2O7. To ensure the effectiveness of our use of non carbon-containing precursors in the synthesis of carbon-free samples, residual carbon content was analyzed by CHN analysis. Both the LiFePO4 and FePO4 contain no residual carbon (below the detection limit of the instrument, 0.01 wt%). However, under the same preparation conditions, LiFePO4 using conventional carbon-containing precursors such as Li2CO3 and FeC2O4·2H2O shows 0.24 wt% carbon content (Table S1). Such a small amount of residual carbon not only changes the sample color from white to gray but also results in a 1
ACS Paragon Plus Environment
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 7
Figure 3. Color and conductivity of LiFePO4 samples. Temperature-dependent conductivity of carbon-free LiFePO4 (< 0.01 wt%) and carbon-containing LiFePO4 (0.24 wt%). Insets show the color of both samples.
order of magnitude increase in the conductivity, as indicated in Fig. 3. Therefore, the use of carbon-free LiFePO4 and FePO4 samples is essential for the reliable measurement of the intrinsic conductivity. Figures 2 (c) and (d) are the X-ray diffraction patterns of the simple mixture 0.6LiFePO4/0.4FePO4 and quenched Li0.6FePO4, respectively, with their Rietveld refinements. The mixed two phases of LiFePO4 and FePO4 merge into the single phase Li0.6FePO4, the structure of which is characterized by a monoclinic superlattice with three times the volume of the unit cell. Detailed structural aspects with the charge stripe ordering of the metastable phase are reported elsewhere23. The color of the quenched Li0.6FePO4 is dark green, induced by intervalence charge-transfer transition24 in the Fe2+-Fe3+ mixed valence states, whereas LiFePO4 and FePO4 are white and pale yellow, respectively, which are typical colors of the wide-gap insulators, as shown in Fig. 4 (b). Figure S2 shows images comparing freshly quenched Li0.6FePO4 and the same sample stored in air for one week. The intermediate composition x = 0.6 is the eutectoid point in the phase diagram,25 which provides the metastable quenched Li0.6FePO4 phase at room temperature. We found that the quenched sample targeted for LixFePO4 (x = 0.6) is stable for several weeks, consistent with other experimental reports26–28. Quenched LixFePO4 phases with x value far from the eutectoid point 0.6 (e.g. 0.48 and 0.74) were proved to separate into two end members quickly at room temperature.29
Figure 4. Increased conductivity of the intermediate Li0.6FePO4 phase. a) Representative nyquist plots of carbonfree LiFePO4, carbon-free FePO4 and quenched Li0.6FePO4 samples at 340 K. b) Temperature-dependent conductivity and sample color of carbon-free LiFePO4, carbon-free FePO4 and quenched Li0.6FePO4. Inset compares the temperaturedependent conductivity of the quenched sample Li0.6FePO4 and mixture 0.6LiFePO4/0.4FePO4 during the heating process.
In mixed conductors, both the electronic conductivity and the ionic one contribute to the electrical conductivity in parallel, and more or less correlate each other. The carrier transport mechanism of olivine LiFePO4 has been
extensively investigated by theoretical calculations30,31 and several experimental techniques such as temperaturedependent Mössbauer spectroscopy,32–34 X-ray nuclear
ACS Paragon Plus Environment
Page 5 of 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
resonant forward scattering35 and Nuclear Magnetic Resonance Spectroscopy.29 These studies consistently concluded that Li ions and polarons at Fe sites are strongly coupled each other in the LixFePO4. The strong binding of Li ion to the localized polarons increases the activation barrier of charge migration from ca. 215 meV to 715 meV.30 It is also worth noticing that the coupled motion of Li ion and electron is very complex and the mixed conductivity cannot be simply derived from independent electronic and ionic conductivities by meanfield approximation.36,37 In our research, we use impedance spectroscopy to measure the mixed conductivity of coupled motion, which serves as an essential and direct parameter for evaluating the transport capability. Figure 4 (a) shows representative Nyquist plots of carbon-free LiFePO4, carbon-free FePO4, and quenched Li0.6FePO4 samples measured at 340 K in the frequency range from 106 to 10-2 Hz. Of particular interest is that the resistance of the quenched single-phase Li0.6FePO4 was approximately 2 orders of magnitude smaller than those of the two end-members (LiFePO4 and FePO4) at 340 K. The temperature-dependent conductivity of the three samples (LiFePO4, FePO4, and single phase Li0.6FePO4) in the range of 30 to 350 °C was measured, as shown in Fig. 4 (b). All of them obey the linear relation of log σ versus 1/T, following the Arrhenius equation σ = σ0 exp(-E/RT). The slope for the intermediate phase and the two end members are almost identical due to the same transport mechanism (coupled polaron/ion hopping). The activation energy of LiFePO4 and FePO4 are estimated to be 620 meV, very close to previously reported data.38,39 Across the entire temperature range, the single-phase Li0.6FePO4 shows approximately 2 orders of magnitude superior conductivity. This increased conductivity of intermediate phase Li0.6FePO4 should be attributed to the increased carrier density resulting from mixed valence state of Fe2+/3+ and higher Li ion/vacancy concentration. To verify the increased conductivity of the single-phase Li0.6FePO4, we measured the temperature-dependent conductivity of the simple mixture 0.6LiFePO4/0.4FePO4 for the range of 30 to 350 °C during the heating process (inset of Fig. 4 (b)). At room temperature (30 °C), the conductivity of the mixture σmixture is approximately 2 orders of magnitude lower than that of the quenched single phase σsingle-phase Li0.6FePO4. During the heating process, σmixture approaches σsingle-phase and becomes identical above 300 °C. The origin of this phenomenon is the gradual phase merging from the mixture 0.6LiFePO4/0.4FePO4 to Li0.6FePO4 when the temperature increases above 200 °C.5,26 At high temperatures above 300 °C, the mixture 0.6LiFePO4/0.4FePO4 totally transforms into the solid solution phase Li0.6FePO4. The almost identical conductivity above 300 °C is for the identical Li0.6FePO4 solid solution phase. CONCLUSIONS In summary, the formation of the solid solution phases LixFePO4 was confirmed under strong non-equilibrium
conditions with a large external overpotential. A metastable solid solution phase LixFePO4 (x = 0.6) was isolated by the optimized quenching protocol and found to have approximately 2 orders of magnitude increased conductivity over the two end members of LiFePO4 and FePO4. Our research reveals that the single-phase transformation mechanism not only relaxes the interface strain energy but also improves the intrinsic charge transport, enabling the high-rate capability of olivine LiFePO4.
ASSOCIATED CONTENT Supporting Information. Carbon content analysis, fitting of in-situ XRD, stability of quenched phase and detailed bode plot of impedance data. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT Financial support from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) under the Grant in Aid for Scientific Research (A) No. 23235042 project is gratefully acknowledged. The synchrotron X-ray diffraction experiments were performed at KEK-PF under program No. 2011G683. The authors declare that they have no competing interests
REFERENCES 1. Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. Phospho-Olivines as Positive-Electrode Materials for Rechargeable Lithium Batteries. J. Electrochem. Soc. 1997, 144, 1188–1194. 2. Yamada, A.; Koizumi, H.; Nishimura, S. I.; Sonoyama, N.; Kanno, R.; Yonemura, M.; Nakamura, T.; Kobayashi, Y. Room-Temperature Miscibility Gap in LixFePO4. Nat. Mater. 2006, 5, 357–360. 3. Morgan, D.; Ven, A. Van der; Ceder, G. Li Conductivity in LiMPO4 (M = Mn, Fe, Co, Ni) Olivine Materials. Electrochem. Solid-State Lett. 2004, 7, 30-32. 4. Islam, M. S.; Driscoll, D. D. J.; Fisher, C. A J.; Slater, P. R. Atomic-Scale Investigation of Defects, Dopants, and Lithium Transport in the LiFePO4 Olivine-Type Battery Material. Chem. Mater. 2005, 17, 5085–5092. 5. Nishimura, S.; Kobayashi, G.; Ohoyama, K.; Kanno, R.; Yashima, M.; Yamada, A. Experimental Visualization of Lithium Diffusion in LixFePO4. Nat. Mater. 2008, 7, 707–711. 6. Laffont, L.; Delacourt, C.; Gibot, P.; Wu, M. Y.; Kooyman, P.; Masquelier, C.; Tarascon, J. M. Study of the LiFePO4/FePO4 Two-Phase System by High-Resolution Electron Energy Loss Spectroscopy. Chem. Mater. 2006, 18, 5520–5529. 7. Ravet, N.; Chouinard, Y.; Magnan, J. F.; Besner, S.; Gauthier, M.; Armand, M. Electroactivity of Natural and Synthetic Triphylite. J. Power Sources 2001, 97–98, 503–507.
ACS Paragon Plus Environment
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
8. Yamada, a.; Chung, S. C.; Hinokuma, K. Optimized LiFePO4 for Lithium Battery Cathodes. J. Electrochem. Soc. 2001, 148, 224-229. 9. Huang, H.; Yin, S.-C.; Nazar, L. F. Approaching Theoretical Capacity of LiFePO4 at Room Temperature at High Rates. Electrochem. Solid-State Lett. 2001, 4, 170-172. 10. Chen, G. Y.; Song, X. Y.; Richardson, T. J. Electron Microscopy Study of the LiFePO4 to FePO4 Phase Transition. Electrochem. Solid State Lett. 2006, 9, 295–298. 11. Delmas, C.; Maccario, M.; Croguennec, L.; Cras, F. Le; Weill, F. Lithium Deintercalation in LiFePO4 Nanoparticles via a Domino-Cascade Model. Nat. Mater. 2008, 7, 665–671. 12. Malik, R.; Zhou, F.; Ceder, G. Kinetics of NonEquilibrium Lithium Incorporation in LiFePO4. Nat. Mater. 2011, 10, 587–590. 13. Orikasa, Y.; Maeda, T.; Koyama, Y.; Murayama, H.; Fukuda, K.; Tanida, H.; Arai, H.; Matsubara, E.; Uchimoto, Y.; Ogumi, Z. Direct Observation of a Metastable Crystal Phase of LixFePO4 under Electrochemical Phase Transition. J. Am. Chem. Soc. 2013, 135, 5497–5500. 14. Orikasa, Y.; Maeda, T.; Koyama, Y.; Murayama, H.; Fukuda, K.; Tanida, H.; Arai, H.; Matsubara, E.; Uchimoto, Y.; Ogumi, Z. Transient Phase Change in Two Phase Reaction between LiFePO4 and FePO4 under Battery Operation. Chem. Mater. 2013, 25, 1032–1039. 15. Kobayashi, G.; Nishimura, S. I.; Park, M.-S. S.; Kanno, R.; Yashima, M.; Ida, T.; Yamada, A. Isolation of Solid Solution Phases in Size-Controlled LixFePO4 at Room Temperature. Adv. Funct. Mater. 2009, 19, 395–403. 16. Wagemaker, M.; Singh, D. P.; Borghols, W. J. H.; Lafont, U.; Haverkate, L.; Peterson, V. K.; Mulder, F. M. Dynamic Solubility Limits in Nanosized Olivine LiFePO4. J. Am. Chem. Soc. 2011, 133, 10222–10228. 17. Liu, H.; Strobridge, F. C.; Borkiewicz, O. J.; Wiaderek, K. M.; Chapman, K. W.; Chupas, P. J.; Grey, C. P. Capturing Metastable Structures during High-Rate Cycling of LiFePO4 Nanoparticle Electrodes. Science. 2014, 344, 1252817–1252817. 18. Zhang, X.; Hulzen, M. van; Singh, D. P.; Brownrigg, A.; Wright, J. P.; Dijk, N. H. van; Wagemaker, M. Rate-Induced Solubility and Suppression of the First-Order Phase Transition in Olivine LiFePO4. Nano Lett. 2014, 14, 2279–2285. 19. Chung, S. Y.; Bloking, J. T.; Chiang, Y. M. Electronically Conductive Phospho-Olivines as Lithium Storage Electrodes. Nat. Mater. 2002, 1, 123–128. 20. Herle, P.; Ellis, B.; Coombs, N.; Nazar, L. NanoNetwork Electronic Conduction in Iron and Nickel Olivine Phosphates. Nat. Mater. 2004, 147–152. 21. Kang, B.; Ceder, G. Battery Materials for Ultrafast Charging and Discharging. Nature 2009, 458, 190–193. 22. Nakamura, A.; Furutsuki, S.; Nishimura, S.; Tohei, T.; Sato, Y.; Shibata, N.; Yamada, A.; Ikuhara, Y. Phase Boundary Structure of LixFePO4 Cathode Material Revealed by AtomicResolution Scanning Transmission Electron Microscopy. Chem. Mater. 2014, 26, 6178–6184. 23. Nishimura, S.; Natsu, R.; Yamada, A. Superstructure in the metastable intermediate-phase Li2/3FePO4 accelerating the lithium battery cathode reaction. Angew. Chemie Int. Ed. 2015, 54, 8939-8942. 24. Furutsuki, S.; Chung, S.-C.; Nishimura, S.; Kudo, Y.; Yamashita, K.; Yamada, A. Electrochromism of LixFePO4 Induced by Intervalence Charge Transfer Transition. J. Phys. Chem. C 2012, 116, 15259–15264. 25. Dodd, J. L.; Yazami, R.; Fultz, B. Phase Diagram of LixFePO4. Electrochem. Solid State Lett. 2006, 9, 151–155. 26. Delacourt, C.; Poizot, P.; Tarascon, J.-M. M.; Masquelier, C. The Existence of a Temperature-Driven Solid Solution in LixFePO4 for 0
