- Email: [email protected]
Stability, electrochemical behaviors and electronic structures of iron hydroxyl-phosphate
Materials Chemistry and Physics 123 (2010) 28–34
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Stability, electrochemical behaviors and electronic structures of iron hydroxyl-phosphate Zhongli Wang a , Shaorui Sun a , Fan Li a , Ge Chen a , Dingguo Xia a,∗ , Ting Zhao b,c , Wangsheng Chu b,c , Ziyu Wu b,c,∗∗ a b c
College of Environmental and Energy Engineering, Beijing University of Technology, Pingleyuan 100, Chaoyang District, Beijing 100022, PR China Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China University of Science and Technology of China, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 1 July 2009 Received in revised form 2 February 2010 Accepted 27 February 2010 Keywords: Thermogravimetric analysis (TGA) Phase transitions ab initio calculations Electronic structure
a b s t r a c t Iron hydroxyl-phosphate with a uniform spherical particle size of around 1 m, a compound of the type Fe2−y y (PO4 )(OH)3−3y (H2 O)3y−2 (where represents a vacancy), has been synthesized by hydrothermal methods. The particles are composed of spheres of diameter <100 nm. The compound exhibits good electrochemical performance, with reversible capacities of around 150 mAh g−1 and 120 mAh g−1 at current densities of 170 mA g−1 and 680 mA g−1 , respectively. The stability of crystal structure of this material was studied by TGA and XRD which show that the material remains stable at least up to the temperature 200 ◦ C. Investigation of the electronic structure of the iron hydroxyl-phosphate by GGA + U calculation has indicated that it has a better electronic conductivity than LiFePO4 . © 2010 Published by Elsevier B.V.
1. Introduction LiFePO4 , as the cathode material for lithium-ion batteries, has a theoretical capacity of 170 mAh g−1 , combined with a lithium intercalation potential of 3.45 V. It exhibits particularly excellent structural stability [1]. Researchers have paid a great deal of attention to this material owing to its suitability as a cathode material for power batteries [2–7]. Some researchers have attempted to synthesize LiFePO4 doped with cations to improve its dynamic performance by increasing the number of carriers [8–10]. There have been few reports concerning the preparation of LiFePO4 doped with anions. Replacement of [PO4 ]3− with [(PO4 )(OH)]4− allows the generation of a new family of host lattice structures and compositions, as it alters the charge balance and the dimensionality of the structure [11–16]. Whittingham and co-workers [17] reported the synthesis and the electrochemical and magnetic properties of the tetragonal phase of the ferric end member Fe2−y y (PO4 )(OH)3−3y (H2 O)3y−2 (y = 2/3 and y = 4/5). The charge/discharge mechanism of the tetragonal phase Fe1.19 (PO4 )F0.11 (OH)0.46 (H2 O)0.43 was characterized by Richardson and co-workers [18] using the potentiostatic intermittent titration test (PITT).
∗ Corresponding author. Tel.: +86 10 67396158; fax: +86 10 67396158. ∗∗ Corresponding author. E-mail addresses: [email protected] (D. Xia), [email protected] (Z. Wu). 0254-0584/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2010.02.071
In this work, we have synthesized an iron hydroxyl-phosphate by hydrothermal methods, and have focused on the thermal stability and the electronic structure. The crystal structures and electrochemical performances of those samples, which were obtained by sintering the products synthesized by hydrothermal methods in air at 200 ◦ C, 400 ◦ C and 600 ◦ C, were characterized. The band gap and the energy barrier for the diffusion of lithium-ions in the tetragonal iron hydroxyl-phosphate have been estimated by ab initio calculations.
2. Experimental The iron hydroxyl-phosphate was prepared by a hydrothermal method in an autoclaved stainless steel reactor. The starting materials were Fe2 (SO4 )3 (a.c.) and H3 PO4 (a.c.). In a typical synthesis, the molar ratio of Fe:P was 1:1, and the concentration of Fe ions was 0.01 mol L−1 . The solution was initially adjusted to pH 0.9 by adding LiOH. 120 mL resulting solution was transferred to a 200 mL stainless steel autoclave, which was sealed and heated to 200 ◦ C for 2 h. The green precipitate obtained was washed with distilled water and then dried at 120 ◦ C for 5 h in a vacuum oven. XRD measurements on the obtained materials were carried out using a D8 Advance X-ray diffractometer with Cu-K␣ radiation at a scan speed of 10 s/step and 0.02◦ /step at room temperature. The particle morphology of the samples was observed using a scanning electron microscope (SEM) (JEOL, JSM 6400). Electrodes were prepared by pressing a mixture of the active material, acetylene black, and binder (PTFE) in a weight ratio of 70:20:10. Li metal was used as the counter electrode. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The cells were assembled in an argon-filled dry box. The electrochemical properties of the cells were measured at current densities of 1 C (170 mA g−1 ) or 4 C (680 mA g−1 ) with a charge–discharge voltage limit of 2.0–4.2 V.
Z. Wang et al. / Materials Chemistry and Physics 123 (2010) 28–34
29
Fig. 1. Rietveld refinement plot for Fe1.39 PO4 (OH) powder based on the tetragonal I41/amd model (experimental, pink line; calculated, red line; difference, gray line; reflections, vertical lines).
3. Results and discussion 3.1. Hydrothermal synthesis, XRD, SEM, and electrochemical performance The ratio of Fe to P in the product prepared by the hydrothermal method, as determined by ICP-AES, is 1.39:1. The sample was labeled as Fe1.39 PO4 (OH) based on the general formula Fe2−y y (PO4 )(OH)3−3y (H2 O)3y−2 . The X-ray diffraction pattern of Fe1.39 PO4 (OH) is shown in Fig. 1. The Rietveld refinement was carried out by Topas3. Two types of unit cell configuration and two space groups were considered as the starting models: a tetragonal lattice with I41/amd symmetry (Mg1.33 [SO4 (OH)0.66 (H2 O)0.33 ], ICSD 26545) [17,19] and a monoclinic lattice with C12/c1 symmetry (Fe4 (PO4 )3 (OH)3 , ICSD 72726) [20], respectively. I41/amd symmetry with a tetragonal lattice cell with a = b = 5.242(1) Å, c = 12.995(3) Å and C12/c1 symmetry with a monoclinic lattice cell with a = 19.555(2) Å, b = 7.376(1) Å, c = 7.429(1) Å, ˇ = 102.26(1)◦ were adopted, however, the C12/c1 model gave a rather large value for the reliability factor of structural model RB (=10.569%). The result of Rietveld refinement based on the I41/amd model was shown in Fig. 1 and Table 1. Cell parameters, atomic positions, isotropic thermal parameters, occupation factors for iron, scale factor, and profile parameters were refined along with up to five background parameters, with Fe replacing Mg and P replacing S. The R-factors of the Rietveld refinement are RB = 2.507%, Rp = 8.26%, and Rwp = 11.24%. Thus, the product Fe1.39 PO4 (OH) could be mainly
Table 1 Crystallographic date and atomic coordinates for Fe1.39 PO4 (OH). a (Å)
c (Å)
V (Å3 )
RB
Rp
Rwp
5.1929(2)
12.9396(6)
348.93(3)
2.507%
8.26%
11.24%
Atom
x
y
z
Occ.
Beq (Å2 )
P Fe O O
0 0 0 0
0.75 0 0.5010(5) 0.25
0.125 0.5 0.1917(2) 0.375
1 0.694(2) 1 1
0.76(6) 1.00(6) 2.44(9) 0.53(1)
indexed to the tetragonal and a few additional peaks at around 12.8◦ , 23.7◦ , 24.5◦ , 25.9◦ and 26.2◦ could be indexed to the monoclinic phase Fe4 (PO4 )3 (OH)3 compounds with the space group C12/c1 [20]. The morphology of the sample is shown in Fig. 2, where very uniform spherical particles can be seen. The average particle size is around 1 m. These particles are composed of spheres of diameter <100 nm. The theoretical capacity of Fe1.39 PO4 (OH), assuming that all of the ferric ions can be reduced to the ferrous form, is 188 mAh g−1 . The electrochemical insertion and removal of lithium at a current density of 170 mA g−1 is shown in Fig. 3(a). The reversible capacities of Fe1.39 PO4 (OH) are around 150 mAh g−1 and 120 mAh g−1 at current densities of 170 mA g−1 and 680 mA g−1 , respectively. Therefore, good cycling ability was found for this compound (Fig. 3(b)). 3.2. Thermal analysis, heat treatment, XRD, and electrochemical performance TGA of the compound Fe1.39 PO4 (OH) was carried out in oxygen, a heating rate of 5 ◦ C min−1 . The thermal behavior of Fe1.39 PO4 (OH) is shown in Fig. 4. There is one weight loss at each temperature which are 200 ◦ C, 400 ◦ C and 600 ◦ C, respectively. The weight losses are 1%, 3.2% and 8%, respectively. According to TGA, three samples were obtained by sintering the sample Fe1.39 PO4 (OH) in air at 200 ◦ C, 400 ◦ C, 600 ◦ C for 5 h with a heating rate of 5 ◦ C min−1 . Fig. 5 shows the XRD patterns of the samples sintered at 200 ◦ C, 400 ◦ C, 600 ◦ C and untreated (RT). Compared with the sample without heating treat, the sample heated at 200 ◦ C remains the same crystal structure, which shows that the weight loss is associated with surface-adsorbed water. It indicates that this material has sufficient thermal stability for use in a lithium-ion battery. The sample heated at 400 ◦ C shows the monoclinic structure with the space group C12/c1 (ICSD 72726) [20]. So it indicates that the material has a phase transition between the tetragonal and the monoclinic when the heating temperature is up to 400 ◦ C. When the temperature is more higher, at 600 ◦ C, the material of monoclinic phase decomposes into the hexagonal FePO4 (JCPDS 01-084-0876) [21]
30
Z. Wang et al. / Materials Chemistry and Physics 123 (2010) 28–34
Fig. 2. SEM of Fe1.39 PO4 (OH) (a) 7000× and (b) 50,000×.
and the tetragonal Fe25 (PO4 )14 (OH)24 (JCPDS 01-089-1671) [22]. In Fig. 5, the peaks labeled as “*” can be indexed to the hexagonal FePO4 and the peaks labeled as “+” can be indexed to the tetragonal Fe25 (PO4 )14 (OH)24 . The electrochemical behaviors of the three samples were carried out and the results were shown in Fig. 6. Fig. 6(a) shows the charge and discharge curves of the second cycle at 1 C (170 mAh g−1 ) for the heating treated samples. The sample sinter at 200 ◦ C delivers a 120 mAh g−1 discharge capacity and has a discharge curve which is similar to that of untreated sample. The discharge capacity of the sample sintered at 400 ◦ C is 66 mAh g−1 and its plat voltage is lower than that of the untreated sample. The sample sintered at 600 ◦ C gives the more less discharge capacity 37 mAh g−1 , and more lower plat voltage which is below 2.5 V. Fig. 6(b) shows the cycling test of the three samples. It can be seen that all the discharge capacities of the three samples have a increasing trend after several cycles of the lithium-ions insertion/extraction. After the high temperature treatment, these materials may undergo the collapse of the crystal structure which will be arranged well after several cycles of lithium-ions migration. The discharge capacity of the sample sintered at 200 ◦ C is even up to 160 mAh g−1 after 50 cycles of charge and discharge. The other two samples deliver lower discharge capacity, around 100 mAh g−1 and 70 mAh g−1 , even though they also have the increasing trend and have a higher capacity after 50 cycles. Thus, this material should not be treated at a temperature lower than 400 ◦ C.
3.3. ab initio calculations First-principle computations have been shown to be useful methods for predicting many of the properties of electrode materials in rechargeable Li-ion batteries [23–30]. Here, we present calculations for the tetragonal phase iron hydroxyl-phosphates carried out using the Vienna ab initio simulation program (VASP), a density functional theory (DFT) code with a plane wave. Electron–ion interaction has been described using the projectoraugmented wave method, and the exchange-correlation energy of electrons has been described in the framework of the generalized gradient approximation (GGA) parameterized by Perdew, Burke, and Ernzerhof (PBE) [31]. A suitable energy cut-off for the plane waves was determined to be 400 eV. In all calculations, the Fe and P atoms were fixed to maintain the structure framework, and the O and H atoms were allowed to fully relax until the forces acting on them became less than 0.01 eV Å−1 . In order to obtain the accurate electronic structure of this material, we applied the GGA + U method (Ueff = 4.3) [24] to treat the systems, which included geometry optimization and electronic structure calculation. The structure files were developed from Mg1.33 [SO4 (OH)0.66 (H2 O)0.33 ] (ICSD 26545) [19]. According to the formula Fe2−y y (PO4 )(OH)3−3y (H2 O)3y−2 (where represents a vacancy), it cannot be constructed a valid model with limited atoms based on Fe1.39 PO4 (OH) because it is not easy to be performed with a model over 2 hundreds atoms, especially transition
Z. Wang et al. / Materials Chemistry and Physics 123 (2010) 28–34
31
Fig. 5. XRD patterns of Fe1.39 PO4 (OH) before and after sintered.
Fig. 3. (a) The curves of the first three charge and discharge cycles at 1 C (170 mAh g−1 ) for Fe1.39 PO4 (OH). (b) The cycling test of Fe1.39 PO4 (OH) at 1 C (170 mA g−1 ) and 4 C (680 mA g−1 ).
Fig. 4. TGA of Fe1.39 PO4 (OH) in oxygen, 5 ◦ C min−1 .
Fig. 6. (a) The charge and discharge curves of the second cycle at 1 C (170 mAh g−1 ) for the heat treated samples. (b) The cycling test of the heat treated samples at 1 C (170 mA g−1 ) and 4 C (680 mA g−1 ).
32
Z. Wang et al. / Materials Chemistry and Physics 123 (2010) 28–34
Fig. 7. Total DOS of (a) Fe5 (PO4 )4 (OH)3 ·H2 O, (b) Li5 Fe5 (PO4 )4 (OH)3 ·H2 O using GGA + U. The Fermi level is aligned to zero.
metal atoms. Thus a structure model with Fe:P = 1.25:1 was used in this work. The model Fe5 (PO4 )4 (OH)3 ·H2 O was determined by the general formula Fe2−y y (PO4 )(OH)3−3y (H2 O)3y−2 . Calculations for constructed iron hydroxyl-phosphates (Fe5 (PO4 )4 (OH)3 ·H2 O) were implemented in one unit cell, in which three Fe atoms and eleven H atoms were omitted while preserving the space framework. In Fig. 7(a) and (b) shows DOS of Fe5 (PO4 )4 (OH)3 ·H2 O respectively. The DOS of and Li5 Fe5 (PO4 )4 (OH)3 ·H2 O, Fe5 (PO4 )4 (OH)3 ·H2 O may be divided into two main parts. The first part, labeled I in Fig. 6(a), extends from −10 eV to 0 eV and is mainly the contribution of P-p, O-p, and Fe-d states; the second part (from 1.4 eV to 4 eV), labeled II in Fig. 7(a), is mainly due to the Fe-d states in the minority spin direction. The DOS of Li5 Fe5 (PO4 )4 (OH)3 ·H2 O was calculated on the basis of a unit cell in which five Li-ions randomly occupy the same count of 8c sites as in the Fe5 (PO4 )4 (OH)3 ·H2 O structure. After the insertion of lithium, it is obvious that the DOS of part II shifts to lower energy, and that a large amount of the DOS lies at the Fermi level. The narrow band composed of localized Fe-d orbitals (part II shown in Fig. 7(a) and (b)) becomes more dislocated, which means that the electronic conductivity of the material increases with the insertion of lithium-ions.
Polyhedral views of Li5 Fe5 (PO4 )4 (OH)3 ·H2 O along the [1 0 0], [0 1 0], and [0 0 1] directions are shown in Fig. 8(a), (b) and (c), respectively. Tunnels are formed by the face-sharing FeO6 octahedral of two neighboring FeO6 chains, which are interconnected by two columns of PO4 tetrahedral in the XY-plane. The tunnels along [1 0 0] and [0 1 0] are alternated along [0 0 1]. It can be seen that the lithium-ions are blocked by the FeO6 octahedral along the [0 0 1] direction and that there are similar tunnels for the lithium-ions moving along the [1 0 0] and [0 1 0] directions. Energy profiles can be mapped out by calculating the energy of the migrating Li-ion along the diffusion path. In this way, the position of highest potential energy can be identified, from which the migration energy is derived; this approach has been successfully used in numerous previous studies on oxide ion and cation migration in complex oxides [32–34]. It produces a wave-like trajectory for long-range migration along the [0 1 0] direction, as illustrated in Fig. 8(d). The diffusion path lies out of the XY-plane and the magnitude of this deviation at the saddle point is about 0.1 Å and 0.07 Å from the linear paths along the [1 0 0] and [0 0 1] directions, respectively. The energy profile for the curved path is shown in Fig. 9, which indicates a 1.3 eV energy barrier to limit the diffusion of lithium-ions in Fe5 (PO4 )4 (OH)3 ·H2 O according to the results calculated by GGA + U.
Z. Wang et al. / Materials Chemistry and Physics 123 (2010) 28–34
33
Fig. 8. Polyhedral views of Li5 Fe5 (PO4 )4 (OH)3 ·H2 O along (a) [1 0 0], (b) [0 1 0], and (c) [0 0 1]. (d) Curved trajectories for Li-ion migration between sites along the [0 1 0] direction; the diffusion path lies out of the XY-plane. The red octahedra represent FeO6 and the blue tetrahedra represent PO4 . The pink spheres represent Li atoms, the red are Fe, the blue are P, the yellow are O, and the green are H. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Compared with FePO4 and LiFePO4 , which have large band gaps (1.9 eV and 3.7 eV, respectively) and poor electron conductivities [20], Fe5 (PO4 )4 (OH)3 ·H2 O has a 1.2 eV band gap and Li5 Fe5 (PO4 )4 (OH)3 ·H2 O has not a band gap, which implies that these materials have good electronic conductivities. In the Fe5 (PO4 )4 (OH)3 ·H2 O/Li5 Fe5 (PO4 )4 (OH)3 ·H2 O framework, a face is shared by two neighboring FeO6 octahedral, whereas in the FePO4 /LiFePO4 framework only one vertex is shared by two neighboring FeO6 octahedral. Thus, electron hopping between the neighboring Fe3+ /Fe2+ pair in Fe5 (PO4 )4 (OH)3 ·H2 O/Li5 Fe5 (PO4 )4 (OH)3 ·H2 O can occur more easily than in FePO4 /LiFePO4 . What’s more, 37.5% 8d sites are unoccupied and 62.5% are occupied by Fe atoms in Fe5 (PO4 )4 (OH)3 ·H2 O/Li5 Fe5 (PO4 )4 (OH)3 ·H2 O. For the lithium-ions battery materials as a semiconductor, defects of vacancy are
Table 2 The structural and experimental properties of Fe1.39 PO4 (OH)/Fe5 (PO4 )4 (OH)3 ·H2 O and LiFePO4 .
Crystal FeO6 Vacancies Band gap Discharge capacity
Fe1.39 PO4 (OH)/Fe5 (PO4 )4 (OH)3 ·H2 O
LiFePO4
Tetragonal Face sharing More than 30% 1.2 eV 160 mAh g−1
Orthogonal Vertex sharing 0% 3.7 eV [23,24] 143 mAh g−1 [35]*
* The material of LiFePO4 was synthesized by hydrothermal method and the charge and discharge capacity is at 0.1 C (17 mA g−1 ).
advantageous to improve their electronic conductivity [9]. To compare the differences of the two materials, the structural and experimental properties of Fe1.39 PO4 (OH) and LiFePO4 are listed in Table 2. Fe1.39 PO4 (OH) delivered 160 mAh g−1 at the rate of 170 mA g−1 , higher than those of LiFePO4 synthesized in our previous works [23,35]. The capacities are 143 mAh g−1 at 170 mA g−1 that of LiFePO4 synthesized by hydrothermal method (shown in Table 2) [35] and about 140 mAh g−1 at 170 mA g−1 that of LiFePO4 synthesized by solid state reaction [23]. Compared with LiFePO4 , Fe1.39 PO4 (OH) shows a better rate capability. This is consistent with the band gap by ab initio calculations. 4. Conclusion
Fig. 9. Energy profile for Li migration along the [0 1 0] direction for the path between adjacent Li sites.
Compounds of the type Fe2−y y (PO4 )(OH)3−3y (H2 O)3y−2 may be very easily and inexpensively synthesized by hydrothermal methods using ferric salts and phosphoric acid. The compound Fe1.39 PO4 (OH) described in this paper shows interesting properties of reversible intercalation/extraction of lithium-ions, corresponding to the reduction–oxidation of Fe3+ /Fe2+ at about 2.75 V vs. lithium, and a high capacity of over 120 mAh g−1 at a current density of 680 mA g−1 . It can be seen that it remains the structure stable at 200 ◦ C from the results of heat treatment. According to the results of
34
Z. Wang et al. / Materials Chemistry and Physics 123 (2010) 28–34
GGA + U calculations, Fe5 (PO4 )4 (OH)3 ·H2 O has a higher electronic conductivity than LiFePO4 . Thus, Fe2−y y (PO4 )(OH)3−3y (H2 O)3y−2 should be a promising cathode material for use in lithium-ion batteries. Acknowledgements This work was partly supported by the Beijing Natural Science Foundation (Grant No. 207001) and the Funding Project for Academic Human Resources Development in Institutions of Higher Learning Under the Jurisdiction of Beijing Municipality. References
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
[1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188–1194. [2] C. Delacourt, P. Poizot, J.-M. Tarascon, C. Masquelier, Nat. Mater. 4 (2005) 254. [3] T. Maxisch, G. Ceder, Phys. Rev. B 73 (2006) 174112. [4] F. Zhou, C.A. Marianetti, M. Cococcioni, D. Morgan, G. Ceder, Phys. Rev. B 69 (2004), 201101(R). [5] A. Yamada, H. Koizumi, N. Sonoyama, R. Kanno, Electrochem. Solid State Lett. 8 (2005) A409. [6] P. Gibot, C.-C. Montse, L. Laffont, C. Masquelier, Nat. Mater. 7 (2008) 741. [7] X. Xia, Z.X. Wang, L.Q. Chen, Electrochem. Commun. 10 (2008) 1442–1444. [8] W. Guan, C. Yan, M. Yan, Z.Y. Jiang, J. Solid State Electrochem. 11 (2007) 457–462. [9] S.-Y. Chung, J.T. Bloking, Y.-M. Chiang, Nat. Mater. 1 (2002) 123. [10] N. Meethong, H.-Y. Shadow, S.A. Speakman, W.C. Carter, Y.M. Chiang, Adv. Funct. Mater. 17 (2007) 1115–1123. [11] P.B. Moore, Am. Miner. 55 (1970) 135.
[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
L. Fanfani, P.F. Zanazzi, Acta Crystallogr. 22 (1967) 173. P.B. Moore, A.R. Kampf, Z. Kristall. 201 (1992) 263. P.B. Moore, A.R. Kampf, A.J. Irving, Am. Miner. 59 (1974) 900. J.L. Jambor, J.E. Dutrizac, Neues Jahrb. Miner. Monatsh. 159 (1988) 51. M.A. Gheith, Am. Miner. 38 (1953) 612. Y. Song, P.Y. Zavalij, N.A. Chernova, M.S. Whittingham, Chem. Mater. 17 (2005) 139–1147. M. Dolle, S. Patoux, T.J. Richardson, J. Power Sources 144 (2005) 208–213. K.D. Keefer, M.F. Hochella, B.H.W.S. Jong, Acta Crystallogr. B 37 (1981) 1003–1006. M. Malaman, M. Ijjaali, G. Venturini, C. Gleitzer, J.L. Soubeyroux, Eur. J. Solid State Inorg. Chem. 28 (1991) 519–531. A. Goiffon, J.-C. Dumas, E. Philippot, Rev. Chim. Miner. 23 (1986) 99. E.N. Matvienko, O.V. Yakubovich, M.A. Simonov, N.V. Belov, Zh. Strukt. Khim. 22 (1981) 121. Z. Wang, S. Sun, D. Xia, W. Chu, S. Zhang, Z. Wu, J. Phys. Chem. C 44 (2008) 17450–17455. K. Zaghib, A. Mauger, J.B. Goodenough, F. Gendron, C.M. Julien, Chem. Mater. 19 (2007) 3740. F. Zhou, M. Cococcioni, K. Kang, G. Ceder, Electrochem. Commun. 6 (2004) 1144. F. Zhou, K. Kang, T. Maxisch, G. Ceder, D. Morgan, Solid State Commun. 132 (2004) 181. S.Q. Shi, L.J. Liu, C.Y. Ouyang, D.S. Wang, Z.Y. Wang, L.Q. Chen, X.J. Huang, Phys. Rev. B 68 (2003) 195108. D.B. Laks, C.G. Van de Walle, G.F. Neumark, S.T. Pantelides, Phys. Rev. Lett. 66 (1991) 648. S.B. Zhang, J.E. Northrup, Phys. Rev. Lett. 67 (1991) 2339. A. Deb, U. Bergmann, E.J. Cairns, S.P. Cramer, J. Phys. Chem. B 108 (2004) 7046. J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. M.S. Islam, D.J. Driscoll, C.A. Fisher, P.R. Slater, Chem. Mater. 17 (2005) 5085–5092. M.S. Islam, J.R. Tolchard, P.R. Slater, Chem. Commun. (2003) 1486. J.R. Tolchard, P.R. Slater, M.S. Islam, J. Mater. Chem. 13 (2003) 1956. Z.L. Wang, S.R. Sun, D.G. Xia, J. Power Sources 184 (2008) 633–636.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Stability, electrochemical behaviors and electronic structures of iron hydroxyl-phosphate Zhongli Wang a , Shaorui Sun a , Fan Li a , Ge Chen a , Dingguo Xia a,∗ , Ting Zhao b,c , Wangsheng Chu b,c , Ziyu Wu b,c,∗∗ a b c
College of Environmental and Energy Engineering, Beijing University of Technology, Pingleyuan 100, Chaoyang District, Beijing 100022, PR China Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China University of Science and Technology of China, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 1 July 2009 Received in revised form 2 February 2010 Accepted 27 February 2010 Keywords: Thermogravimetric analysis (TGA) Phase transitions ab initio calculations Electronic structure
a b s t r a c t Iron hydroxyl-phosphate with a uniform spherical particle size of around 1 m, a compound of the type Fe2−y y (PO4 )(OH)3−3y (H2 O)3y−2 (where represents a vacancy), has been synthesized by hydrothermal methods. The particles are composed of spheres of diameter <100 nm. The compound exhibits good electrochemical performance, with reversible capacities of around 150 mAh g−1 and 120 mAh g−1 at current densities of 170 mA g−1 and 680 mA g−1 , respectively. The stability of crystal structure of this material was studied by TGA and XRD which show that the material remains stable at least up to the temperature 200 ◦ C. Investigation of the electronic structure of the iron hydroxyl-phosphate by GGA + U calculation has indicated that it has a better electronic conductivity than LiFePO4 . © 2010 Published by Elsevier B.V.
1. Introduction LiFePO4 , as the cathode material for lithium-ion batteries, has a theoretical capacity of 170 mAh g−1 , combined with a lithium intercalation potential of 3.45 V. It exhibits particularly excellent structural stability [1]. Researchers have paid a great deal of attention to this material owing to its suitability as a cathode material for power batteries [2–7]. Some researchers have attempted to synthesize LiFePO4 doped with cations to improve its dynamic performance by increasing the number of carriers [8–10]. There have been few reports concerning the preparation of LiFePO4 doped with anions. Replacement of [PO4 ]3− with [(PO4 )(OH)]4− allows the generation of a new family of host lattice structures and compositions, as it alters the charge balance and the dimensionality of the structure [11–16]. Whittingham and co-workers [17] reported the synthesis and the electrochemical and magnetic properties of the tetragonal phase of the ferric end member Fe2−y y (PO4 )(OH)3−3y (H2 O)3y−2 (y = 2/3 and y = 4/5). The charge/discharge mechanism of the tetragonal phase Fe1.19 (PO4 )F0.11 (OH)0.46 (H2 O)0.43 was characterized by Richardson and co-workers [18] using the potentiostatic intermittent titration test (PITT).
∗ Corresponding author. Tel.: +86 10 67396158; fax: +86 10 67396158. ∗∗ Corresponding author. E-mail addresses: [email protected] (D. Xia), [email protected] (Z. Wu). 0254-0584/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2010.02.071
In this work, we have synthesized an iron hydroxyl-phosphate by hydrothermal methods, and have focused on the thermal stability and the electronic structure. The crystal structures and electrochemical performances of those samples, which were obtained by sintering the products synthesized by hydrothermal methods in air at 200 ◦ C, 400 ◦ C and 600 ◦ C, were characterized. The band gap and the energy barrier for the diffusion of lithium-ions in the tetragonal iron hydroxyl-phosphate have been estimated by ab initio calculations.
2. Experimental The iron hydroxyl-phosphate was prepared by a hydrothermal method in an autoclaved stainless steel reactor. The starting materials were Fe2 (SO4 )3 (a.c.) and H3 PO4 (a.c.). In a typical synthesis, the molar ratio of Fe:P was 1:1, and the concentration of Fe ions was 0.01 mol L−1 . The solution was initially adjusted to pH 0.9 by adding LiOH. 120 mL resulting solution was transferred to a 200 mL stainless steel autoclave, which was sealed and heated to 200 ◦ C for 2 h. The green precipitate obtained was washed with distilled water and then dried at 120 ◦ C for 5 h in a vacuum oven. XRD measurements on the obtained materials were carried out using a D8 Advance X-ray diffractometer with Cu-K␣ radiation at a scan speed of 10 s/step and 0.02◦ /step at room temperature. The particle morphology of the samples was observed using a scanning electron microscope (SEM) (JEOL, JSM 6400). Electrodes were prepared by pressing a mixture of the active material, acetylene black, and binder (PTFE) in a weight ratio of 70:20:10. Li metal was used as the counter electrode. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The cells were assembled in an argon-filled dry box. The electrochemical properties of the cells were measured at current densities of 1 C (170 mA g−1 ) or 4 C (680 mA g−1 ) with a charge–discharge voltage limit of 2.0–4.2 V.
Z. Wang et al. / Materials Chemistry and Physics 123 (2010) 28–34
29
Fig. 1. Rietveld refinement plot for Fe1.39 PO4 (OH) powder based on the tetragonal I41/amd model (experimental, pink line; calculated, red line; difference, gray line; reflections, vertical lines).
3. Results and discussion 3.1. Hydrothermal synthesis, XRD, SEM, and electrochemical performance The ratio of Fe to P in the product prepared by the hydrothermal method, as determined by ICP-AES, is 1.39:1. The sample was labeled as Fe1.39 PO4 (OH) based on the general formula Fe2−y y (PO4 )(OH)3−3y (H2 O)3y−2 . The X-ray diffraction pattern of Fe1.39 PO4 (OH) is shown in Fig. 1. The Rietveld refinement was carried out by Topas3. Two types of unit cell configuration and two space groups were considered as the starting models: a tetragonal lattice with I41/amd symmetry (Mg1.33 [SO4 (OH)0.66 (H2 O)0.33 ], ICSD 26545) [17,19] and a monoclinic lattice with C12/c1 symmetry (Fe4 (PO4 )3 (OH)3 , ICSD 72726) [20], respectively. I41/amd symmetry with a tetragonal lattice cell with a = b = 5.242(1) Å, c = 12.995(3) Å and C12/c1 symmetry with a monoclinic lattice cell with a = 19.555(2) Å, b = 7.376(1) Å, c = 7.429(1) Å, ˇ = 102.26(1)◦ were adopted, however, the C12/c1 model gave a rather large value for the reliability factor of structural model RB (=10.569%). The result of Rietveld refinement based on the I41/amd model was shown in Fig. 1 and Table 1. Cell parameters, atomic positions, isotropic thermal parameters, occupation factors for iron, scale factor, and profile parameters were refined along with up to five background parameters, with Fe replacing Mg and P replacing S. The R-factors of the Rietveld refinement are RB = 2.507%, Rp = 8.26%, and Rwp = 11.24%. Thus, the product Fe1.39 PO4 (OH) could be mainly
Table 1 Crystallographic date and atomic coordinates for Fe1.39 PO4 (OH). a (Å)
c (Å)
V (Å3 )
RB
Rp
Rwp
5.1929(2)
12.9396(6)
348.93(3)
2.507%
8.26%
11.24%
Atom
x
y
z
Occ.
Beq (Å2 )
P Fe O O
0 0 0 0
0.75 0 0.5010(5) 0.25
0.125 0.5 0.1917(2) 0.375
1 0.694(2) 1 1
0.76(6) 1.00(6) 2.44(9) 0.53(1)
indexed to the tetragonal and a few additional peaks at around 12.8◦ , 23.7◦ , 24.5◦ , 25.9◦ and 26.2◦ could be indexed to the monoclinic phase Fe4 (PO4 )3 (OH)3 compounds with the space group C12/c1 [20]. The morphology of the sample is shown in Fig. 2, where very uniform spherical particles can be seen. The average particle size is around 1 m. These particles are composed of spheres of diameter <100 nm. The theoretical capacity of Fe1.39 PO4 (OH), assuming that all of the ferric ions can be reduced to the ferrous form, is 188 mAh g−1 . The electrochemical insertion and removal of lithium at a current density of 170 mA g−1 is shown in Fig. 3(a). The reversible capacities of Fe1.39 PO4 (OH) are around 150 mAh g−1 and 120 mAh g−1 at current densities of 170 mA g−1 and 680 mA g−1 , respectively. Therefore, good cycling ability was found for this compound (Fig. 3(b)). 3.2. Thermal analysis, heat treatment, XRD, and electrochemical performance TGA of the compound Fe1.39 PO4 (OH) was carried out in oxygen, a heating rate of 5 ◦ C min−1 . The thermal behavior of Fe1.39 PO4 (OH) is shown in Fig. 4. There is one weight loss at each temperature which are 200 ◦ C, 400 ◦ C and 600 ◦ C, respectively. The weight losses are 1%, 3.2% and 8%, respectively. According to TGA, three samples were obtained by sintering the sample Fe1.39 PO4 (OH) in air at 200 ◦ C, 400 ◦ C, 600 ◦ C for 5 h with a heating rate of 5 ◦ C min−1 . Fig. 5 shows the XRD patterns of the samples sintered at 200 ◦ C, 400 ◦ C, 600 ◦ C and untreated (RT). Compared with the sample without heating treat, the sample heated at 200 ◦ C remains the same crystal structure, which shows that the weight loss is associated with surface-adsorbed water. It indicates that this material has sufficient thermal stability for use in a lithium-ion battery. The sample heated at 400 ◦ C shows the monoclinic structure with the space group C12/c1 (ICSD 72726) [20]. So it indicates that the material has a phase transition between the tetragonal and the monoclinic when the heating temperature is up to 400 ◦ C. When the temperature is more higher, at 600 ◦ C, the material of monoclinic phase decomposes into the hexagonal FePO4 (JCPDS 01-084-0876) [21]
30
Z. Wang et al. / Materials Chemistry and Physics 123 (2010) 28–34
Fig. 2. SEM of Fe1.39 PO4 (OH) (a) 7000× and (b) 50,000×.
and the tetragonal Fe25 (PO4 )14 (OH)24 (JCPDS 01-089-1671) [22]. In Fig. 5, the peaks labeled as “*” can be indexed to the hexagonal FePO4 and the peaks labeled as “+” can be indexed to the tetragonal Fe25 (PO4 )14 (OH)24 . The electrochemical behaviors of the three samples were carried out and the results were shown in Fig. 6. Fig. 6(a) shows the charge and discharge curves of the second cycle at 1 C (170 mAh g−1 ) for the heating treated samples. The sample sinter at 200 ◦ C delivers a 120 mAh g−1 discharge capacity and has a discharge curve which is similar to that of untreated sample. The discharge capacity of the sample sintered at 400 ◦ C is 66 mAh g−1 and its plat voltage is lower than that of the untreated sample. The sample sintered at 600 ◦ C gives the more less discharge capacity 37 mAh g−1 , and more lower plat voltage which is below 2.5 V. Fig. 6(b) shows the cycling test of the three samples. It can be seen that all the discharge capacities of the three samples have a increasing trend after several cycles of the lithium-ions insertion/extraction. After the high temperature treatment, these materials may undergo the collapse of the crystal structure which will be arranged well after several cycles of lithium-ions migration. The discharge capacity of the sample sintered at 200 ◦ C is even up to 160 mAh g−1 after 50 cycles of charge and discharge. The other two samples deliver lower discharge capacity, around 100 mAh g−1 and 70 mAh g−1 , even though they also have the increasing trend and have a higher capacity after 50 cycles. Thus, this material should not be treated at a temperature lower than 400 ◦ C.
3.3. ab initio calculations First-principle computations have been shown to be useful methods for predicting many of the properties of electrode materials in rechargeable Li-ion batteries [23–30]. Here, we present calculations for the tetragonal phase iron hydroxyl-phosphates carried out using the Vienna ab initio simulation program (VASP), a density functional theory (DFT) code with a plane wave. Electron–ion interaction has been described using the projectoraugmented wave method, and the exchange-correlation energy of electrons has been described in the framework of the generalized gradient approximation (GGA) parameterized by Perdew, Burke, and Ernzerhof (PBE) [31]. A suitable energy cut-off for the plane waves was determined to be 400 eV. In all calculations, the Fe and P atoms were fixed to maintain the structure framework, and the O and H atoms were allowed to fully relax until the forces acting on them became less than 0.01 eV Å−1 . In order to obtain the accurate electronic structure of this material, we applied the GGA + U method (Ueff = 4.3) [24] to treat the systems, which included geometry optimization and electronic structure calculation. The structure files were developed from Mg1.33 [SO4 (OH)0.66 (H2 O)0.33 ] (ICSD 26545) [19]. According to the formula Fe2−y y (PO4 )(OH)3−3y (H2 O)3y−2 (where represents a vacancy), it cannot be constructed a valid model with limited atoms based on Fe1.39 PO4 (OH) because it is not easy to be performed with a model over 2 hundreds atoms, especially transition
Z. Wang et al. / Materials Chemistry and Physics 123 (2010) 28–34
31
Fig. 5. XRD patterns of Fe1.39 PO4 (OH) before and after sintered.
Fig. 3. (a) The curves of the first three charge and discharge cycles at 1 C (170 mAh g−1 ) for Fe1.39 PO4 (OH). (b) The cycling test of Fe1.39 PO4 (OH) at 1 C (170 mA g−1 ) and 4 C (680 mA g−1 ).
Fig. 4. TGA of Fe1.39 PO4 (OH) in oxygen, 5 ◦ C min−1 .
Fig. 6. (a) The charge and discharge curves of the second cycle at 1 C (170 mAh g−1 ) for the heat treated samples. (b) The cycling test of the heat treated samples at 1 C (170 mA g−1 ) and 4 C (680 mA g−1 ).
32
Z. Wang et al. / Materials Chemistry and Physics 123 (2010) 28–34
Fig. 7. Total DOS of (a) Fe5 (PO4 )4 (OH)3 ·H2 O, (b) Li5 Fe5 (PO4 )4 (OH)3 ·H2 O using GGA + U. The Fermi level is aligned to zero.
metal atoms. Thus a structure model with Fe:P = 1.25:1 was used in this work. The model Fe5 (PO4 )4 (OH)3 ·H2 O was determined by the general formula Fe2−y y (PO4 )(OH)3−3y (H2 O)3y−2 . Calculations for constructed iron hydroxyl-phosphates (Fe5 (PO4 )4 (OH)3 ·H2 O) were implemented in one unit cell, in which three Fe atoms and eleven H atoms were omitted while preserving the space framework. In Fig. 7(a) and (b) shows DOS of Fe5 (PO4 )4 (OH)3 ·H2 O respectively. The DOS of and Li5 Fe5 (PO4 )4 (OH)3 ·H2 O, Fe5 (PO4 )4 (OH)3 ·H2 O may be divided into two main parts. The first part, labeled I in Fig. 6(a), extends from −10 eV to 0 eV and is mainly the contribution of P-p, O-p, and Fe-d states; the second part (from 1.4 eV to 4 eV), labeled II in Fig. 7(a), is mainly due to the Fe-d states in the minority spin direction. The DOS of Li5 Fe5 (PO4 )4 (OH)3 ·H2 O was calculated on the basis of a unit cell in which five Li-ions randomly occupy the same count of 8c sites as in the Fe5 (PO4 )4 (OH)3 ·H2 O structure. After the insertion of lithium, it is obvious that the DOS of part II shifts to lower energy, and that a large amount of the DOS lies at the Fermi level. The narrow band composed of localized Fe-d orbitals (part II shown in Fig. 7(a) and (b)) becomes more dislocated, which means that the electronic conductivity of the material increases with the insertion of lithium-ions.
Polyhedral views of Li5 Fe5 (PO4 )4 (OH)3 ·H2 O along the [1 0 0], [0 1 0], and [0 0 1] directions are shown in Fig. 8(a), (b) and (c), respectively. Tunnels are formed by the face-sharing FeO6 octahedral of two neighboring FeO6 chains, which are interconnected by two columns of PO4 tetrahedral in the XY-plane. The tunnels along [1 0 0] and [0 1 0] are alternated along [0 0 1]. It can be seen that the lithium-ions are blocked by the FeO6 octahedral along the [0 0 1] direction and that there are similar tunnels for the lithium-ions moving along the [1 0 0] and [0 1 0] directions. Energy profiles can be mapped out by calculating the energy of the migrating Li-ion along the diffusion path. In this way, the position of highest potential energy can be identified, from which the migration energy is derived; this approach has been successfully used in numerous previous studies on oxide ion and cation migration in complex oxides [32–34]. It produces a wave-like trajectory for long-range migration along the [0 1 0] direction, as illustrated in Fig. 8(d). The diffusion path lies out of the XY-plane and the magnitude of this deviation at the saddle point is about 0.1 Å and 0.07 Å from the linear paths along the [1 0 0] and [0 0 1] directions, respectively. The energy profile for the curved path is shown in Fig. 9, which indicates a 1.3 eV energy barrier to limit the diffusion of lithium-ions in Fe5 (PO4 )4 (OH)3 ·H2 O according to the results calculated by GGA + U.
Z. Wang et al. / Materials Chemistry and Physics 123 (2010) 28–34
33
Fig. 8. Polyhedral views of Li5 Fe5 (PO4 )4 (OH)3 ·H2 O along (a) [1 0 0], (b) [0 1 0], and (c) [0 0 1]. (d) Curved trajectories for Li-ion migration between sites along the [0 1 0] direction; the diffusion path lies out of the XY-plane. The red octahedra represent FeO6 and the blue tetrahedra represent PO4 . The pink spheres represent Li atoms, the red are Fe, the blue are P, the yellow are O, and the green are H. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Compared with FePO4 and LiFePO4 , which have large band gaps (1.9 eV and 3.7 eV, respectively) and poor electron conductivities [20], Fe5 (PO4 )4 (OH)3 ·H2 O has a 1.2 eV band gap and Li5 Fe5 (PO4 )4 (OH)3 ·H2 O has not a band gap, which implies that these materials have good electronic conductivities. In the Fe5 (PO4 )4 (OH)3 ·H2 O/Li5 Fe5 (PO4 )4 (OH)3 ·H2 O framework, a face is shared by two neighboring FeO6 octahedral, whereas in the FePO4 /LiFePO4 framework only one vertex is shared by two neighboring FeO6 octahedral. Thus, electron hopping between the neighboring Fe3+ /Fe2+ pair in Fe5 (PO4 )4 (OH)3 ·H2 O/Li5 Fe5 (PO4 )4 (OH)3 ·H2 O can occur more easily than in FePO4 /LiFePO4 . What’s more, 37.5% 8d sites are unoccupied and 62.5% are occupied by Fe atoms in Fe5 (PO4 )4 (OH)3 ·H2 O/Li5 Fe5 (PO4 )4 (OH)3 ·H2 O. For the lithium-ions battery materials as a semiconductor, defects of vacancy are
Table 2 The structural and experimental properties of Fe1.39 PO4 (OH)/Fe5 (PO4 )4 (OH)3 ·H2 O and LiFePO4 .
Crystal FeO6 Vacancies Band gap Discharge capacity
Fe1.39 PO4 (OH)/Fe5 (PO4 )4 (OH)3 ·H2 O
LiFePO4
Tetragonal Face sharing More than 30% 1.2 eV 160 mAh g−1
Orthogonal Vertex sharing 0% 3.7 eV [23,24] 143 mAh g−1 [35]*
* The material of LiFePO4 was synthesized by hydrothermal method and the charge and discharge capacity is at 0.1 C (17 mA g−1 ).
advantageous to improve their electronic conductivity [9]. To compare the differences of the two materials, the structural and experimental properties of Fe1.39 PO4 (OH) and LiFePO4 are listed in Table 2. Fe1.39 PO4 (OH) delivered 160 mAh g−1 at the rate of 170 mA g−1 , higher than those of LiFePO4 synthesized in our previous works [23,35]. The capacities are 143 mAh g−1 at 170 mA g−1 that of LiFePO4 synthesized by hydrothermal method (shown in Table 2) [35] and about 140 mAh g−1 at 170 mA g−1 that of LiFePO4 synthesized by solid state reaction [23]. Compared with LiFePO4 , Fe1.39 PO4 (OH) shows a better rate capability. This is consistent with the band gap by ab initio calculations. 4. Conclusion
Fig. 9. Energy profile for Li migration along the [0 1 0] direction for the path between adjacent Li sites.
Compounds of the type Fe2−y y (PO4 )(OH)3−3y (H2 O)3y−2 may be very easily and inexpensively synthesized by hydrothermal methods using ferric salts and phosphoric acid. The compound Fe1.39 PO4 (OH) described in this paper shows interesting properties of reversible intercalation/extraction of lithium-ions, corresponding to the reduction–oxidation of Fe3+ /Fe2+ at about 2.75 V vs. lithium, and a high capacity of over 120 mAh g−1 at a current density of 680 mA g−1 . It can be seen that it remains the structure stable at 200 ◦ C from the results of heat treatment. According to the results of
34
Z. Wang et al. / Materials Chemistry and Physics 123 (2010) 28–34
GGA + U calculations, Fe5 (PO4 )4 (OH)3 ·H2 O has a higher electronic conductivity than LiFePO4 . Thus, Fe2−y y (PO4 )(OH)3−3y (H2 O)3y−2 should be a promising cathode material for use in lithium-ion batteries. Acknowledgements This work was partly supported by the Beijing Natural Science Foundation (Grant No. 207001) and the Funding Project for Academic Human Resources Development in Institutions of Higher Learning Under the Jurisdiction of Beijing Municipality. References
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
[1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188–1194. [2] C. Delacourt, P. Poizot, J.-M. Tarascon, C. Masquelier, Nat. Mater. 4 (2005) 254. [3] T. Maxisch, G. Ceder, Phys. Rev. B 73 (2006) 174112. [4] F. Zhou, C.A. Marianetti, M. Cococcioni, D. Morgan, G. Ceder, Phys. Rev. B 69 (2004), 201101(R). [5] A. Yamada, H. Koizumi, N. Sonoyama, R. Kanno, Electrochem. Solid State Lett. 8 (2005) A409. [6] P. Gibot, C.-C. Montse, L. Laffont, C. Masquelier, Nat. Mater. 7 (2008) 741. [7] X. Xia, Z.X. Wang, L.Q. Chen, Electrochem. Commun. 10 (2008) 1442–1444. [8] W. Guan, C. Yan, M. Yan, Z.Y. Jiang, J. Solid State Electrochem. 11 (2007) 457–462. [9] S.-Y. Chung, J.T. Bloking, Y.-M. Chiang, Nat. Mater. 1 (2002) 123. [10] N. Meethong, H.-Y. Shadow, S.A. Speakman, W.C. Carter, Y.M. Chiang, Adv. Funct. Mater. 17 (2007) 1115–1123. [11] P.B. Moore, Am. Miner. 55 (1970) 135.
[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
L. Fanfani, P.F. Zanazzi, Acta Crystallogr. 22 (1967) 173. P.B. Moore, A.R. Kampf, Z. Kristall. 201 (1992) 263. P.B. Moore, A.R. Kampf, A.J. Irving, Am. Miner. 59 (1974) 900. J.L. Jambor, J.E. Dutrizac, Neues Jahrb. Miner. Monatsh. 159 (1988) 51. M.A. Gheith, Am. Miner. 38 (1953) 612. Y. Song, P.Y. Zavalij, N.A. Chernova, M.S. Whittingham, Chem. Mater. 17 (2005) 139–1147. M. Dolle, S. Patoux, T.J. Richardson, J. Power Sources 144 (2005) 208–213. K.D. Keefer, M.F. Hochella, B.H.W.S. Jong, Acta Crystallogr. B 37 (1981) 1003–1006. M. Malaman, M. Ijjaali, G. Venturini, C. Gleitzer, J.L. Soubeyroux, Eur. J. Solid State Inorg. Chem. 28 (1991) 519–531. A. Goiffon, J.-C. Dumas, E. Philippot, Rev. Chim. Miner. 23 (1986) 99. E.N. Matvienko, O.V. Yakubovich, M.A. Simonov, N.V. Belov, Zh. Strukt. Khim. 22 (1981) 121. Z. Wang, S. Sun, D. Xia, W. Chu, S. Zhang, Z. Wu, J. Phys. Chem. C 44 (2008) 17450–17455. K. Zaghib, A. Mauger, J.B. Goodenough, F. Gendron, C.M. Julien, Chem. Mater. 19 (2007) 3740. F. Zhou, M. Cococcioni, K. Kang, G. Ceder, Electrochem. Commun. 6 (2004) 1144. F. Zhou, K. Kang, T. Maxisch, G. Ceder, D. Morgan, Solid State Commun. 132 (2004) 181. S.Q. Shi, L.J. Liu, C.Y. Ouyang, D.S. Wang, Z.Y. Wang, L.Q. Chen, X.J. Huang, Phys. Rev. B 68 (2003) 195108. D.B. Laks, C.G. Van de Walle, G.F. Neumark, S.T. Pantelides, Phys. Rev. Lett. 66 (1991) 648. S.B. Zhang, J.E. Northrup, Phys. Rev. Lett. 67 (1991) 2339. A. Deb, U. Bergmann, E.J. Cairns, S.P. Cramer, J. Phys. Chem. B 108 (2004) 7046. J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. M.S. Islam, D.J. Driscoll, C.A. Fisher, P.R. Slater, Chem. Mater. 17 (2005) 5085–5092. M.S. Islam, J.R. Tolchard, P.R. Slater, Chem. Commun. (2003) 1486. J.R. Tolchard, P.R. Slater, M.S. Islam, J. Mater. Chem. 13 (2003) 1956. Z.L. Wang, S.R. Sun, D.G. Xia, J. Power Sources 184 (2008) 633–636.