Investigation of Electronic Conductivity and Occupancy Sites of Mo

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J. Phys. Chem. C 2008, 112, 17450–17455

Investigation of Electronic Conductivity and Occupancy Sites of Mo Doped into LiFePO4 by ab Initio Calculation and X-ray Absorption Spectroscopy Zhongli Wang,† Shaorui Sun,† Dingguo Xia,*,† Wangsheng Chu,‡ Shuo Zhang,‡ and Ziyu Wu*,‡ College of EnVironmental and Energy Engineering, Beijing UniVersity of Technology, Beijing 100022, People’s Republic of China, and Institute of High Energy Physics, Chinese Academy of Science, Beijing 100049, People’s Republic of China ReceiVed: February 20, 2008; ReVised Manuscript ReceiVed: July 2, 2008

Mo-doped LiFePO4 composite was synthesized by the solid state method, using (NH4)6Mo7O24 · 4H2O as the starting doping material. The electrochemical properties of Mo-doped LiFePO4 were measured. The electronic structures of Mo-doped LiFePO4 were studied by ab initio calculations. With respect to the density of states (DOS) of the LiFePO4, in which no electronic states are at the Fermi level, the Mo doping is predicted largely impacting the conductivity. Experimental data of the electronic conductivity and of the electrochemical performance of Mo-doped LiFePO4 synthesized by a solid state reaction confirm the results of the calculations. To characterize the doped LiFePO4 structure and the type and the symmetry of the Mo sites, X-ray absorption spectroscopy (XAS) experiments were performed. 1. Introduction Lithium iron phosphate (LiFePO4) is a relatively new system that is being extensively investigated as the main component of the active cathode of a new generation of lithium-ion batteries introduced about a decade ago. The main obstacle of this promising material toward battery application is due to its low intrinsic electronic conductivity.1,2 To reach a higher capacity of LiFePO4 at ambient temperature, two different approaches have been investigated. The first was the introduction of carbon coating that led to a room temperature capacity up to 160 mAh g-1 (quite close to the theoretical value of 170 mAh g-1),3-11 although the presence of carbon in LiFePO4 decreases the energy density.12 The second method is associated with metal ion doping. Chung et al.13 showed how doping of supervalent cations such as Nb replacing Li+ increases the electronic conductivity of the pure LiFePO4 phase more than 108 times, e.g., >10-2 S cm-1 at room temperature. Recently Zhang14 et al. reported the preparation of a Modoped LiFePO4 coated with carbon via a solution method. It is well-known that carbon coating is an effective method for improving electronic conductivity although it is not clear if Mo doping plays a role in the conductivity enhancement. Moreover, it is still an open question as to where the doped cations go and no clear evidence exists on Li14-16 and Fe sites.17 The occupancy site of the doping cation is typically inferred on the base of the cation size, but experimental methods such as X-ray absorption spectroscopy (XAS) may be used to identify and characterize the site of a doping ion. In this paper, we used XAS to investigate the Mo-doped LiFePO4 system, synthesized by solid state reaction using (NH4)6Mo7O24 · 4H2O as the starting doping material in a flow of N2. The electrochemical performance of the Mo-doped LiFePO4 was measured and the Mo K-edge XAS spectra were collected and analyzed to identify the Mo site. In addition, the electronic structure of Mo-doped LiFePO4 was investigated by * Corresponding author. E-mail: [email protected] and [email protected]. † Beijing University of Technology. ‡ Chinese Academy of Science.

Figure 1. XRD patterns of LiFePO4 and Mo-doped LiFePO4.

first-principle calculations and, to evaluate the results, the electronic conductivity was also measured. 2. Experimental Section The Mo-doped LiFePO4 was prepared by mixing Fe(C2O4) · 2H2O, NH4H2PO4, LiOH, and (NH4)6Mo7O24 · 4H2O in the molar ratio (0.99:1:1:0.01). The stoichiometric amount of precursors was thoroughly mixed together in acetone by ball-milling. After being dried, the mixture was heated at 350 °C for 6 h and 700 °C for 12 h in a flow of N2 gas. The cathode was made by dispersing 75 wt % active material, 15 wt % acetylene black, and 10 wt % polytetrafluoroethylene (PTFE) in isopropanol. The electrochemical performance was evaluated by using coin cells, using a lithium metal anode and 1 M LiPF6 in a solution of ethylene carbonate/dimethyl carbonate. Room temperature cycling tests were carried out between 2.0 and 4.2 V with a current density of 17 and 170 mA g-1, corresponding to a 0.1 and 1 C rate, respectively.

10.1021/jp801497z CCC: $40.75  2008 American Chemical Society Published on Web 10/09/2008

Synthesis of Mo-Doped LiFePO4 Composite

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TABLE 1: Results of Rietveld Refinement of LiFePO4 and Mo-Doped LiFePO4 sample

a (Å)

b (Å)

c (Å)

vol (Å3)

RB

Rp

Rwp

GOF

LiFePO4 Mo-doped LiFePO4

10.3309 10.3312

6.0095 6.0101

4.6954 4.6973

291.5 291.7

1.77 2.94

3.95 5.61

5.53 8.14

1.50 2.18

The Mo K-edge X-ray absorption spectrum was measured in transmission mode at room temperature by using a Si (111) double crystal monochromator at the X-ray absorption station (Beam line 1W1B) of the Beijing Synchrotron Radiation Facility (BSRF). The storage ring was working at the typical energy of 2.5 GeV with the electron current typically decreasing from 250 to 150 mA in about 10 h. The incident and output beam intensities were monitored and recorded with use of two ionization chambers. The first ionization chamber was supplied with a continuous flow of 25% argon-doped nitrogen and the second one with pure argon gas. 3. Results and Discussion Figure 1 shows the X-ray diffraction patterns of the samples. The samples preserve a single ordered olivine structure indexed by the orthorhombic space group Pnmb. No impurity phases are detected in the XRD patterns. The structural parameters obtained from X-ray Rietveld refinement of the materials are listed in Table 1. We can see that the cell volume of Mo-doped sample is 291.7 Å3, which is similar with that of pure phase. The content of Mo in the sample obtained by ICP-AES is 0.35% by weight. Figure 2 shows the results of the characterization of the discharge capacity at room temperature for Mo-doped LiFePO4 and pure LiFePO4. The Mo-doped LiFePO4 sample exhibits a better cycling performance with an initial specific capacity of 161 mAh g-1 at a 0.1 C rate, near the theoretical limit of 170 mAh g-1, with negligible fading. In particular, at the higher rate (1 C), the Mo-doped LiFePO4 shows a remarkable power capability of ∼140 mAh g-1, which is 50 mAh g-1 more than that of pure LiFePO4 powders. X-ray absorption spectroscopy is a technique sensitive to short-range order and has been used already to study the local crystalline structure of LiFePO4.18-21 However, around the site of the doped ions, the results remain controversial. According to the mechanism proposed by Chung et al.,13 doped ions could occupy the Li site leading to the coexistence of Fe2+ and Fe3+ in a single phase, improving the crystal electronic conductivity. Wang et al.22 investigated the Ti occupancy site in a

Figure 2. The cycling curves of the LiFePO4 and Mo-doped LiFePO4.

LiTi0.01Fe0.99PO4 sample using XANES at the Fe K-edge. According to the increase of the white line intensity at the Fe K-edge in Ti-doped LiTi0.01Fe0.99PO4 in contrast to what had been observed in LiFePO4, they suggested that Ti replaces Fe. In Figure 3, we compare the experimental EXAFS data of Mo-doped LiFePO4 at the Mo K-edge with the theoretical EXAFS curves for the three possible sites of Mo. EXAFS spectra were calculated by Feff8,23 the code developed at Washington University, using three models and the parameters obtained from the results of the XRD pattern’s refinement. Both Fe and Li sites return similar EXAFS signals in reasonable agreement with experiment. Moreover, both Li and Fe occupy the center site of an octahedral cage with similar ion-O bond lengths. Although EXAFS allows exclusion of the P site position, due to the complexity of the olivine structure it is difficult to distinguish between the two remaining possibilities by just comparing EXAFS signals. To determine the Mo site, theoretical XANES spectra were calculated by Feff823 considering replacement of both Fe and Li atoms by Mo. Data were compared with the experimental XANES spectrum of the Mo-doped LiFePO4 at the Mo K-edge (Figure 4). Simulations show that both XANES spectra display a single main peak but they are separated in energy by about 8 eV, a shift almost equal to the energy separation of the two main peaks observed in the experimental data. Therefore, XANES suggests that Mo occupies both Fe (feature b) and Li sites (features a and c) in the Mo-doped LiFePO4 system. Moreover, both simulations give similar single-scattering (SS) features (see feature d) in good agreement with the experimental XANES. Data support a concurrent replacement of Mo ions at Fe and Li sites although data do not allow the determination of the ratio between the occupancy due to the limited accuracy of the XANES simulations and the limited energy resolution of the experimental spectra. First principle computations have been shown to be useful methods to predict many of the properties of electrode materials

Figure 3. Comparison of the experimental EXAFS oscillations at the Mo K-edge in Mo-doped LiFePO4 and EXAFS simulations for the three different sites.

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Figure 4. Comparison of the experimental Mo K-XANES spectrum (black) of Mo-doped LiFePO4 and simulations for Mo replacing Fe and Li ions.

in rechargeable Li-ion batteries.24-29 In this paper, we present calculations carried out using the Vienna ab initio simulation program (VASP), a density functional theory (DFT) code with a plane wave. Electron-ion interaction was described by using the projector augmented wave method and the exchangecorrelation energy of electrons was described in the framework of the generalized gradient approximation (GGA) with the

Wang et al. functional parametrization of PBE.30 The spin-polarization and magnetism of Fe were also considered. A suitable energy cutoff for the plane waves was determined to be 400 eV. Moreover, in all calculations, atoms were allowed to fully relax until the forces acting on them became less than 0.01 eV/Å. To obtain the more accurate electronic structure of LiFePO4, we also applied the GGA+U method (U ) 4.3)25,31 to treat the systems including the geometry optimization and electronic structure calculation. Calculations for LiMoxFe1-xPO4 (x ) 1/32) and Li1-xMoxFePO4 (x ) 1/32) were implemented in a 2 × 2 × 2 super cell, in which one Fe atom or one Li atom was respectively replaced by a Mo atom, and only the γ-point is sampled in the first Brillouin zone. Figure 5 shows total and partial density of states (DOS) for pure LiFePO4. Pure LiFePO4 (Figure 5a) is a semiconductor with a 0.3 eV gap according to this GGA calculation. This is similar to the results which were reported previously.12 The DOS of pure LiFePO4 could be divided into three main parts. The first part extends from -10 to -7.3 eV and is mainly the contribution of P-p and O-p states; the second part from -7.3 to -3.5 eV is mainly that of O-p states; and the third part from -3.5 to 2.5 eV is the combination of Fe-d and O-p states. The narrow band just below EF, which is labeled as I in Figure 5a, can be mainly assigned to the dz2 of Fe in the minority spin direction (sayV) while that above EF, labeled II in Figure 5b, is composed of dxy, dyz, dx2-y2, and dzx of Fe in the minority spin direction.

Figure 5. (a) Total DOS of pure LiFePO4, (b) partial DOS of Fe-3d in pure LiFePO4 with GGA, (c) total DOS of pure LiFePO4, and (d) partial DOS of Fe-3d in pure LiFePO4 with GGA+U. The Fermi level is set as a reference.

Synthesis of Mo-Doped LiFePO4 Composite

J. Phys. Chem. C, Vol. 112, No. 44, 2008 17453

Figure 6. (a) Total DOS of LiMo1/31Fe31/32PO4, (b) partial DOS of Mo-4d in LiMo1/31Fe31/32PO4, (c) partial DOS of Fe-3d in LiMo1/32Fe31/32PO4 with GGA, (d) total DOS of LiMo1/31Fe31/32PO4, (e) partial DOS of Mo-4d in LiMo1/31Fe31/32PO4, and (f) partial DOS of Fe-3d in LiMo1/32Fe31/32PO4 with GGA+U. The Fermi level is set as a reference.

The band gap of pure LiFePO4 by GGA+U calculation (shown in Figure 5c) is 3.7 eV. Compared with the experimental values,32 the more precise band gap can be obtained by the calculation GGA+U. The value of the band gap is in good agreement with the result calculated by Zhou et al.25 Only electron states near the Fermi level play a key role in the electron transport processes, so that we focus our attention on the DOS around the Fermi energy. For the LiMo1/32Fe31/32PO4, both total and partial DOSs of Mo-d and Fe-d are compared with pure LiFePO4 in Figure 6a,b,c. Both shape and composition of the DOSs around the Fermi level are quite similar to that of pure LiFePO4. However, the Fermi level shifts from the gap

(between regions I and II in the pure systems) toward region II, addressing an injection of electrons to the bottom of the conduction band. The total and partial DOSs of LiMo1/32Fe31/32PO4 calculated by GGA+U are shown in Figure 6d,e,f. According to the results, Mo-d and Fe-d states both contribute to the electronic states on the Fermi level, which means that the Mo-doped LiFePO4 may have better electronic conductivity than the pure phase. For the case of Li31/32Mo1/32FePO4, the total and partial DOSs are presented in Figure 7. Figure 7a shows that the Fermi level is shifted from the gap in region II now containing two peaks. Peak I (Figure 7a) is due to spin-down Fe-dz2 states (Figure 7c)

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Wang et al.

Figure 7. (a) Total DOS for Li31/32Mo1/32FePO4., (b) partial DOS of Mo-4d in Li31/32Mo1/32FePO4., (c) partial DOS of Fe-3d in Li31/32Mo1/32FePO4. with GGA, (d) total DOS for Li31/32Mo1/32FePO4., (e) partial DOS of Mo-4d in Li31/32Mo1/32FePO4, and (f) partial DOS of Fe-3d in Li31/32Mo1/ 32FePO4. with GGA+U. The Fermi level is set as a reference.

and Mo-d states (Figure 7b). The DOS (Figure 7c) just below the Fermi level (from -0.1 to 0.0 eV) is mainly due to Fe-dxy while that up to 0.2 eV, just above the Fermi level, is mainly due to Fe-dx2-y2. The total and partial DOSs of Li31/32Mo1/32FePO4 with GGA+U calculation are presented in Figure 7d,e,f. It is clear that the Fermi level shifts out of the energy gap, on which the electronic states are mainly due to Mo-d and Fe-d states.

For the two models, e.g., Mo ions at the Li- and Fe-sites in the LiFePO4 system, the electronic structures around the Fermi level predicted by GGA and GGA+U calculations imply that the electronic conductivities should significantly increase with respect to the pure phase. To evaluate the ab initio calculations, electronic conductivity measurements for pure LiFePO4 and the Mo-doped LiFePO4 samples were carried by the two-point d.c. method. A Picoam-

Synthesis of Mo-Doped LiFePO4 Composite

J. Phys. Chem. C, Vol. 112, No. 44, 2008 17455 (Project No. 10125523 to Z.W.), and the Knowledge Innovation Program of the Chinese Academy of Sciences (KJCX2-SWN11, KJCX2-SW-H12-02). References and Notes

Figure 8. Comparison of the electron current for Mo-doped and pure LiFePO4 samples.

meter/Voltage Source (Keithley 6487) was used to record the currents. LiFePO4 powders (0.15 g) were pressed into a slice with a diameter of 13 mm and a thickness of 0.48 mm with a pressure of 20 Mp for 2 h. Figure 8 shows how the electronic conductivity of Mo-doped LiFePO4 increases by 5-8 times relative to the LiFePO4 in the temperature range up to 150 °C. This is consistent with the results of the ab initio calculation. 4. Conclusions Mo-doped LiFePO4 compound was successfully synthesized by the solid-state reaction method and exhibits excellent electrochemical performances both at lower and higher charge-discharge rate. Accurate XAS analysis allowed us to distinguish between different possible Mo replacements, and in particular, XANES spectra clearly address how Mo ions can replace both Fe and Li ions. Total and partial DOSs of the two model replacements in both Li31/32Mo1/32FePO4 and LiMo1/ 32Fe31/32PO4 have been performed and clearly show that Modoping plays an important role in the electronic structures and in the conductivity. Clear changes of the electronic structure of Mo-doped LiFePO4 occur due to the injection of electrons near the Fermi level. Conductivity experiments performed on powder pellets confirm the calculation results regarding the electronic states of both LiFePO4 and Mo-doped LiFePO4 samples. Acknowledgment. This work was partly supported by the Beijing Natural Science Foundation (Grant No.207001), the Funding Project for Academic Human Resources Development in Institutions of Higher Learning Under the Jurisdiction of Beijing Municipality, the National 973 Program of China (Grant No. 2002CB211807), the National Outstanding Youth Fund

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