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C composites with controllable impurity phases
Electrochimica Acta 62 (2012) 416–423
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Electrochemical performance in Na-incorporated nonstoichiometric LiFePO4 /C composites with controllable impurity phases Zhao-Hui Wang a , Li-Xia Yuan a , Jun Ma b,∗ , Long Qie a , Lu-Lu Zhang a , Yun-Hui Huang a,∗∗ a State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China b School of Automotive Studies, Tongji University, Shanghai 201804, China
a r t i c l e
i n f o
Article history: Received 25 September 2011 Received in revised form 21 November 2011 Accepted 13 December 2011 Available online 23 December 2011 Keywords: Lithium-ion batteries Lithium iron phosphate Sodium-incorporation Off-stoichiometry Electrochemical performance
a b s t r a c t Nonstoichiometry and Na incorporation are designed to attain controllable impurity phases Li4 P2 O7 and Li3 PO4 in LiFePO4 . The effects of Li4 P2 O7 and Li3 PO4 on structure and electrochemical performance have been investigated. Both Li4 P2 O7 and Li3 PO4 impurities are observed in (Fe, P)-deficient LiFe0.9 P0.95 O4−ı . With Na+ incorporation, Li4 P2 O7 phase disappears while Li3 PO4 content increases. Proved by X-ray photoelectron spectroscopy, nonstoichiometry and Na-incorporation do not change the chemical state of Fe(II). Our experiments indicate that Li4 P2 O7 and Li3 PO4 show different effects on the electrochemical performance. Li4 P2 O7 leads to degradation of cyclability, whereas a small amount of Li3 PO4 is beneficial for the improvement in capacity and rate capability. The 1% Na-doped LiFe0.9 P0.95 O4−ı composite exhibits the best electrochemical performance. We ascribe the improvement to the structural stabilization caused by the existence of Li3 PO4 . © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Olivine LiFePO4 has been extensively investigated as a promising cathode material for lithium-ion batteries due to its high capacity, low cost, nontoxicity and environmental friendliness [1]. The key barrier to LiFePO4 commercialization is its low electronic conductivity and sluggish Li+ diffusion across the LiFePO4 /FePO4 interface [2,3]. Attempts have been devoted to optimizing the material for superior electrochemical performance, such as modifying the surface of LiFePO4 with conductive nanostructures [4–7] and doping alien ions [8–11] in the lattice, etc. Apart from the improvement in electrochemical property for LiFePO4 by carbon coating and alien-ion doping, there is also a growing need to identify the effects of some impurities in LiFePO4 [12]. Some of the impurities benefit the electrochemical performance while some may poison the performance seriously [3]. In general, control of impurity phases is especially important for the electrochemical properties in LiFePO4 . Kang and Ceder [13] recently discovered that non-stoichiometry of the reactants in preparation of LiFePO4 resulted in Li4 P2 O7 phase. Hu et al. [14] synthesized LiFePO4 from nonstoichiometric starting materials containing excess iron source
∗ Corresponding author. ∗∗ Corresponding author. Tel.: +86 27 87558241; fax: +86 27 87558241. E-mail addresses: jun [email protected] (J. Ma), [email protected] (Y.-H. Huang). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.12.055
with a ratio of 2:1 to phosphorus in order to avoid or suppress formation of insulating Li3 PO4 phase, and found that conductive Fe2 P phase formed at high temperatures. Yu et al. [12] observed that Li3 PO4 impurity was formed in LiFePO4 grown via hydrothermal route if excess Li source was added. Kadoma et al. [15] obtained Fe2 P and Li3 PO4 impurities along with LiFePO4 by varying sintering temperature and the reductive amount. Therefore, by creating offstoichiometry or changing reaction temperature and atmosphere, impurity phases can be expectably controlled in LiFePO4 . However, it is still difficult to selectively control the impurity phases. Besides off-stoichiometry and sintering condition, we believe that alien-ion doping should influence formation of impurity phases. Few researches have paid attention to the doping effect on the impurities in LiFePO4 . In this work, Na doping and nonstoichiometry were simultaneously employed to selectively control the impurity phases in LiFePO4 . With deficient Fe and P sources, the obtained nominal LiFe0.9 P0.95 O4−ı /C sample consists of well-recognized LiFePO4 and impurity phases like Li4 P2 O7 and Li3 PO4 . It is found that Na doping is effective to suppress formation of Li4 P2 O7 impurity. The effects of Li4 P2 O7 and Li3 PO4 on the electrochemical performance of LiFePO4 have been investigated.
2. Experimental Nominal LiNax Fe0.9 P0.95 O4−ı /C (x = 0, 0.01, 0.02) and stoichiometric LiFePO4 were synthesized via a solid-state reaction.
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According to the desired stoichiometry, Li2 CO3 , FeC2 O4 ·2H2 O and NH4 H2 PO4 were mixed with dopant source NaNO3 in ethanol and ball-milled for 12 h. The mixed slurry was then dried at 80 ◦ C in the oven in air for another 12 h. After being dried, the mixture was calcined at 350 ◦ C for 6 h under N2 atmosphere. The yielded precursor powder was reground together with 12 wt% glucose. Glucose served as a carbon source. The final product was obtained by sintering the mixture at 700 ◦ C for 10 h under high pure N2 . Carbon contents were measured by an Elementar Vario MICRO Cube (Elementar, Germany). Elemental compositions (Li, Fe, Na, P) of the samples were determined by inductively coupled plasma optical emission spectrometer (ICP-OES, PerkinElmer Instruments, Optima 4300DV). The phase was checked by X-ray powder diffraction (XRD) with the Rigaku D/MAX-RB diffractometer using filtered Cu K␣ radiation within the scanning angle, 2, ranged from 10◦ to 90◦ in a step of 0.02◦ . Structural analysis was carried out by Rietveld refinement using RIETAN-2000. The chemical valence states of Fe, Na were examined by using X-ray photoelectron spectroscopy (XPS, PerkinElmer, PHI 5600) with a monochromatic Al X-ray source (1486.6 eV). The data were calibrated by adventitious C 1s peak with a fixed value of 284.4 eV and analyzed using XPSPEAK41 processing software. The morphology was observed with scanning electron microscope (SEM, Sirion 200, Holland) equipped with an energy dispersive X-ray detector (EDX). Electrochemical tests were carried out with CR2032 coin cells. The cathode film was fabricated from a mixture of active material, C-black, and polytetrafluoroethylene (PTFE) (75:20:5 wt%) with an area of 0.5 cm2 and a total mass of 5–7 mg. Metallic lithium foil was used as anode electrode; the electrolyte was 1 mol L−1 LiPF6 in a 1:1 solvent mixture of ethylene carbonate and dimethyl carbonate (EC/DMC). A thin sheet of microporous polyethylene (Celgard 2400) served as the separator. The cells were assembled in an argonfilled glove box. Cyclic voltammetry (CV) was measured by an electrochemical working station (PARSTAT 2273, Princeton Applied Research, US) at a scan rate of 0.1 mV s−1 between 2.5 and 4.2 V. Electrochemical impedance spectroscopy (EIS) was conducted also on the PARSTAT 2273 with a potential amplitude of 5 mV in the frequency range from 104 to 10−2 Hz. Charge/discharge tests were performed between 2.5 and 4.2 V with a Land CT2001 battery tester (Wuhan Land Electronic Co. Ltd., China) at 25 ◦ C at various rates (0.1–5 C).
3. Results and discussion Determined by ICP, the compositions of LiFePO4 , LiFe0.9 P0.95 O4−ı and Na-doped LiFe0.9 P0.95 O4−ı are close to the expected nominal chemical formula. Carbon contents are about 3 wt% for all the samples, which implies that the influence of carbon content on the electrochemical performance for the prepared samples can be ignored [16]. Fig. 1a shows their XRD patterns. For all the samples, the main diffraction peaks match well with the standard olivine LiFePO4 structure (JCPDS 40-1499) indexed by orthorhombic Pnma. The calculated lattice parameters obtained by Rietveld refinement are listed in Table 1. The refined lattice parameters of LiFePO4 are almost equal to those reported in the literatures [17,18]. For the Na+ -incorporated samples, the lattice parameters change slightly with Na content, which may be because the Na doping level is very low. It is difficult to determine from the lattice parameters whether Na+ ions are doped into the olivine structure. However, we can see from Fig. 1b that Na incorporation obviously affects the formation of impurity phases. With deficient Fe and P sources, the LiFe0.9 P0.95 O4−ı /C sample consists of well-recognized LiFePO4 and impurity phases including Li4 P2 O7 and Li3 PO4 . Similar phenomenon was observed in the case where
417
Fig. 1. (a) XRD patterns for LiFePO4 -based samples, and (b) partial magnified patterns with intensities in logarithmic scale.
excess Li source was used [13,19]. Interestingly, with Na incorporation, the peak corresponding to Li4 P2 O7 phase disappears and Li3 PO4 diffractions remain. As Na content increases, the intensities of Li3 PO4 diffraction peaks are enhanced, demonstrating that Na incorporation facilitates the formation of Li3 PO4 impurity. Relative intensity ratio (RIR) method is used to calculate the contents of the impurities [20]. For LiFe0.9 P0.95 O4−ı /C, the contents of Li4 P2 O7 and Li3 PO4 are around 3.1 wt% and 0.82 wt%, respectively. For Naincorporated LiFe0.9 P0.95 O4−ı /C, the Li3 PO4 content is ca. 4.4 wt% for 1% Na-doped LiFe0.9 P0.95 O4−ı /C and 7.3 wt% for 2% Na-doped LiFe0.9 P0.95 O4−ı /C. Fig. 2 shows SEM images of LiFePO4 and Na-doped offstoichiometric LiFePO4 samples. It can be seen that all nonstoichiometric samples show similar morphologies and wide particle-size distribution. In addition, Na+ incorporation does not change the morphology. From these results, it is clear that an appreciable amount of impurity phase is in situ formed along with LiFePO4 , which is well consistent with the previous reports [13,14,16]. The EDX result of 1% Na-doped LiFe0.9 P0.95 O4−ı /C in Fig. 3 indicates that the particles in the selected region include P, Fe, O, C components and a trace of Na, which is in accordance with the nominal formula. The chemical composition of 1% Na-doped LiFe0.9 P0.95 O4−ı /C by the EDX analysis in Fig. 3 shows a P/Fe ratio of 1.1:1. Analyzed by combining EDX and XRD, it can be concluded that the synthesis based on designed nonstoichiometry with Fe and P deficiency as well as Na-incorporation can effectively create impurity like Li3 PO4 in LiFePO4 . For further insight into phase separation
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Table 1 Lattice parameters obtained from XRD Rietveld refinement for LiFePO4 -based samples. Sample
LiFePO4 /C
LiFe0.9 P0.95 O4−ı /C
1% Na-doped LiFe0.9 P0.95 O4−ı /C
2% Na-doped LiFe0.9 P0.95 O4−ı /C
Lattice constant (Å) a (Å) b (Å) c (Å) V (Å3 )
10.3247(5) 6.0072(1) 4.6929(9) 291.07
10.3257(1) 6.0055(8) 4.6901(1) 290.84
10.3255(3) 6.0067(1) 4.6916 (9) 290.99
10.3237(1) 6.0056(9) 4.6920(4) 290.91
Reliability factors (%) Rwp Rp
8.03 5.67
9.20 6.30
8.84 5.96
10.06 6.70
Fig. 2. SEM images for (a) LiFePO4 , (b) LiFe0.9 P0.95 O4−ı , (c) 1% Na-doped LiFe0.9 P0.95 O4−ı , and (d) 2% Na-doped LiFe0.9 P0.95 O4−ı .
of Li3 PO4 from LiFePO4 , EDX analysis was conducted at different regions in the SEM image for 1% Na-doped LiFe0.9 P0.95 O4−ı /C sample (Fig. 4). It is found that the Fe/P ratio is higher in the central part than in the edge of the particles. However, it is difficult to
determine the real distribution of Fe and P in the LiFePO4 particles [21,22]. Nevertheless, the XRD, SEM and point elemental analysis confirm that the impurities are actually formed in the LiFePO4 phase.
Fig. 3. SEM image and the corresponding EDX spectrum for 1% Na-doped LiFe0.9 P0.95 O4−ı .
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Fig. 4. EDX data at different regions in the SEM image for 1% Na-doped LiFe0.9 P0.95 O4−ı sample. Red cross indicates the regions where the EDX measurement was taken. A represents the central part of the particle; B and C are the edges of the particle. The measurement was repeated at different regions. The given EDX data in the tables are the average ones for three parallel measurements at different regions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 5a gives the XPS spectra within a wide range of binding energy. With the reference binding energy at 284.6 eV for the C 1s peak, the spectra of Li 1s, P 2p and O 1s are observed at 56.1, 132.6 and 530.9 eV, respectively. The peak at 1071.4 eV is ascribed to Na 1s. Fig. 5b presents Na 1s XPS spectra for 1% and 2% Na-doped LiFe0.9 P0.95 O4−ı /C samples. As can be seen, the intensity of Na 1s spectrum of 2% Na-doped sample is almost two times stronger than that of 1% Na-doped LiFe0.9 P0.95 O4−ı /C, indicating that Na+ ions exist in the samples. In order to examine the effects of Na-incorporation and nonstoichiometry on the oxidation state of Fe ions, the Fe 2p XPS spectra are investigated. As shown in Fig. 6, all the samples have two doublet peaks at 710.3 and 723.6 eV corresponding to Fe 2p1/2 and 2p3/2 , which match perfectly with those in LiFePO4 [23]. Compared with the pristine sample, the binding energies of the main peaks for the Na-incorporated samples do not change, indicating that no observable change occurs in the chemical valence of Fe (II). In general, doping with alien atoms may induce structural distortion, which, however, can be balanced by the formation of mixed phases caused by off-stoichiometry. Such an influence of off-stoichiometry has been reported by Kang and Ceder [13] and Axmann et al. [24]. Electrochemical performance was investigated to see the influence of impurity in LiFePO4 on Li insertion/extraction behavior. Figs. 7 and 8 compare the charge/discharge profiles at 0.1 C and discharge capacities at various C rates for different cathodes. All samples clearly show a flat and wide dischage voltage plateau at
∼3.4 V. Even in the case of the (Fe, P)-deficient cathodes, e.g. 2% Na-doped sample containing some Li3 PO4 impurity, the flat and wide voltage plateau still appears, similar to that of ideal stoichiometric LiFePO4 . This demonstrates that the charge/discharge process for the (Fe, P)-deficient cathode corresponds to the two-phase reaction between LiFePO4 and FePO4 . The initial discharge capacities at 0.1 C are 149, 152, 154 and 148 mAh g−1 for LiFePO4 , LiFe0.9 P0.95 O4−ı /C, 1% and 2% Na-doped LiFe0.9 P0.95 O4−ı /C, respectively. The Na-incorporated (Fe, P)-deficient cathodes show enhanced capacity as compared with pristine LiFePO4 . From Fig. 8, we can see that 1% Na-doped LiFe0.9 P0.95 O4−ı /C exhibits the best rate capability; its discharge capacity is 130 mAh g−1 at 2 C and 111 mAh g−1 at 5 C. We ascribe the difference in capacity and rate capability among the LiFePO4 /C-based electrodes to Li4 P2 O7 and Li3 PO4 impurities caused by nonstoichiometry and Naincorporation. It was reported that Li4 P2 O7 and Li3 PO4 impurities in LiFePO4 could facilitate fast charging and discharging by providing a guest ion-conductive surface [7,13,25]. However, if only a small amount of Li3 PO4 exists at the surface of particles, it may act as an inert mass to reduce the electrochemical properties of LiFePO4 [12,14,24]. Therefore, the content of Li3 PO4 is essential for the electrochemical performance. In our case, the 1% Nadoped LiFe0.9 P0.95 O4−ı /C sample containing 4.4 wt% Li3 PO4 shows the highest capacity and best rate capability; LiFe0.9 P0.95 O4−ı /C containing 3.1 wt.% Li4 P2 O7 and 0.82 wt% Li3 PO4 also present good performance; whereas the performance for 2% Na-doped LiFe0.9 P0.95 O4−ı /C containing 7.3 wt% Li3 PO4 is the poorest.
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Fig. 7. Initial charge/discharge curves for LiFePO4 -based composite electrodes at 0.1 C.
Apparently, if Li3 PO4 content is too high, it may act as an inert mass to prevent the transfer of electrons and Li+ ions and hence to degrade the electrochemical performance [24]. In order to clarify the effects of Li4 P2 O7 and Li3 PO4 impurities, we have carefully compared the electrochemical properties of LiFe0.9 P0.95 O4−ı /C and 1% Na-doped LiFe0.9 P0.95 O4−ı /C. The cell potential as a function of specific capacity at 0.1–5 C rates is depicted in Fig. 9. The capacity and rate capability of 1% Na-doped LiFe0.9 P0.95 O4−ı /C are both better than those of LiFe0.9 P0.95 O4−ı /C. It
was reported that the Li4 P2 O7 phase could lead to the improvement in rate capability [7,13]. LiFe0.9 P0.95 O4−ı /C contains Li4 P2 O7 impurity but 1% Na-doped LiFe0.9 P0.95 O4−ı /C does not contain detectable Li4 P2 O7 . The superior performance for LiNa0.01 Fe0.9 (PO4 )0.95 /C should be contributed to the existence of Li3 PO4 impurity. Fig. 10 shows the charge/discharge curves of the 1st, 10th, 50th, 100th, 150th and 200th cycles for LiFe0.9 P0.95 O4−ı /C and 1% Na-doped LiFe0.9 P0.95 O4−ı /C composite electrodes at 1 C. It can be found that the Li4 P2 O7 -involved sample shows higher polarization and lower capacity retention than the Li3 PO4 -involved sample. Fig. 11 presents their cycling performances at 1 C. Excellent cyclability is attained in the Li3 PO4 -involved 1% Na-doped LiFe0.9 P0.95 O4−ı /C composite cathode. It still delivers a discharge capacity of 142 mAh g−1 after 200 cycles. The capacity loss over 200 cycles is only 1.3% for 1% Na-doped LiFe0.9 P0.95 O4−ı /C, whereas the loss is as high as 14.6% for LiFe0.9 P0.95 O4−ı /C. The existence of Li4 P2 O7 impurity results in degradation of cyclability, which has been also observed in LiFePO4 by other groups [18]. Since Li3 PO4 is inactive, it can not contribute additional capacity to LiFePO4 [24]. Structural information is attained by Rietveld refinement for comparison between LiFe0.9 P0.95 O4−ı /C and 1% Nadoped LiFe0.9 P0.95 O4−ı /C. The interatomic distances between Atom 1 (Li, Fe, P) and Atom 2 (O1, O2, O3) are listed in Table 3. The average ˚ and that of bond distance of Li–O varies from 2.1462 to 2.1494 A, ˚ The former is elongated with P O varies from 1.5467 to 1.5370 A. Na incorporation, which indicates that the extraction of Li ions from the lattice of 1% Na-doped LiFe0.9 P0.95 O4−ı /C becomes easier due to
Fig. 6. XPS core levels of Fe 2p for LiFePO4 -based samples.
Fig. 8. Cycling performances for LiFePO4 -based composite electrodes at various rates.
Fig. 5. (a) XPS spectrum of 1% Na-doped LiFe0.9 P0.95 O4−ı ; (b) Na 1s XPS spectra of Na-doped LiFe0.9 P0.95 O4−ı samples.
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Fig. 9. Charge/discharge curves of the first cycle measured at various rates in the range of 2.5–4.2 V at 25 ◦ C for LiFe0.9 P0.95 O4−ı and 1% Na-doped LiFe0.9 P0.95 O4−ı cathodes.
the weakness of interaction between Li and O ions. Meanwhile, the length of the P O bond after Na incorporation decreases by 0.56%, 0.32% and 0.32% for P O1, P O2 and P O3, respectively, demonstrating that the framework of LiFePO4 becomes more stable [9,11]. Therefore, we argue that the better cycle performance for the 1% Na-doped LiFe0.9 P0.95 O4−ı /C composite is mainly due to the more stable structure caused by the balance of the formation of impurity Li3 PO4 . Fig. 12 displays the CV profiles for LiFePO4 -based composites in the first cycle obtained at a scan rate of 0.1 mV s−1 . All curves show a pair of redox peaks corresponding to the oxidation and reduction of Fe2+ /Fe3+ ions in LiFePO4 . The difference between oxidation and reduction peak potential for Fe3+ /Fe2+ redox couple, E, is displayed in Table 2. The 1% Na-doped LiFe0.9 P0.95 O4−ı /C sample shows the lowest potential interval with E = 0.227 V between anodic and cathodic peaks. Since the E is determined by the potential polarization of the active material during charge and discharge process, lower E demonstrates that the lithium insertion into the 1% Na-doped LiFe0.9 P0.95 O4−ı /C composite behaves more likely as a Nernst system.
Fig. 10. Charge/discharge curves of the 1st, 10th, 50th, 100th, 150th and 200th cycles at 1 C for (a) LiFe0.9 P0.95 O4−ı /C and (b) 1% Na-doped LiFe0.9 P0.95 O4−ı /C.
Electrochemical impedance spectroscopy was carried out to shed more light on the effects of Na+ -doping and off-stoichiometry. The impedance was recorded on the coin cells at fully discharge state after one cycle. The corresponding Nyquist plots of the spectra are presented in Fig. 13. All spectra have a depressed semicircle in the high frequency region and an inclined line in the low frequency
Table 2 Anodic peak potential locations (Ep,a ) and corresponding cathodic ones (Ep,c ) and potential differences (E) of Fe3+ /Fe2+ redox couple for the CV curve in Fig. 12. Sample
Ep,a (V)
Ep,c (V)
E (V)
LiFePO4 /C LiFe0.9 P0.95 O4−ı /C 1% Na-doped LiFe0.9 P0.95 O4−ı /C 2% Na-doped LiFe0.9 P0.95 O4−ı /C
3.575 3.564 3.555 3.602
3.298 3.319 3.328 3.307
0.277 0.245 0.227 0.295
Fig. 11. Specific discharge capacity versus cycle number for LiFe0.9 P0.95 O4−ı /C and 1% Na-doped LiFe0.9 P0.95 O4−ı /C electrodes at 1 C rate.
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Table 3 Interatomic distances in LiFe0.9 P0.95 O4−ı and 1% Na-doped LiFe0.9 P0.95 O4−ı obtained by Rietveld refinement. LiFe0.9 P0.95 O4−ı /C
1% Na-doped LiFe0.9 P0.95 O4−ı /C
Atom 1
Atom 2
Counts
d1,2 (Å)
Atom 1
Atom 2
Li
O1 O2 O3
2× 2× 2×
2.0893(2) 2.1632(9) 2.1862(5)
Li
O1 O2 O3
2× 2× 2×
2.0929(7) 2.1685(7) 2.1870(5)
Fe
O3 O2 O1 O3
2× 1× 1× 2×
2.0670(2) 2.2183(5) 2.0805(9) 2.2432(3)
Fe
O3 O2 O1 O3
2× 1× 1× 2×
2.0739(9) 2.2140(0) 2.0970(5) 2.2399(4)
P
O1 O2 O3
1× 1× 2×
1.5619(9) 1.5233(5) 1.5451(2)
P
O1 O2 O3
1× 1× 2×
1.5532(2) 1.5173(3) 1.5407(7)
Fig. 12. CV curves for LiFePO4 -based composite electrodes.
region. The intercept at the Zreal axis at high frequencies refers to Rs , which includes resistance of electrolyte solution and electric contact resistance. The semicircle in the high frequency range corresponds to the charge transfer resistance (Rct ) of electrochemical reaction; and the sloping line in the lower frequency represents Li+ -ion diffusion resistance within the cathode, namely the warburg impedance [11]. As shown in Fig. 13, the Rct values of pristine LiFePO4 , LiFe0.9 P0.95 O4−ı /C, 1% and 2% Na-doped LiFe0.9 P0.95 O4−ı /C, are 228, 167, 126 and 345 , respectively. It is believed that the decrease in charge transfer resistance is beneficial to the kinetic
Counts
d1,2 (Å)
behavior during charge/discharge process, and hence leads to the improvement of electrochemical performance [26]. Therefore, it is expected that the 1% Na-doped LiFe0.9 P0.95 O4−ı /C can provide an excellent kinetic behavior, which is consistent with the CV measurement in Fig. 12. It has been reported that Li3 PO4 modification in LiCoO2 and LiMn2 O4 can decrease charge transfer resistance, enhance electrode kinetics, and hence improve cycling performance and rate capability [27,28]. In our case, the Li3 PO4 -involved 1% Na-doped LiFe0.9 P0.95 O4−ı /C sample exhibits fast electrode kinetics, low charge-transfer resistance and structural stability, which is why it shows the best electrochemical performance. Kang and Ceder [13] reported that a Li4 P2 O7 -contained LiFe0.9 P0.95 O4−ı obtained by creation of an Fe/P deficiency exhibited an ultrafast charging rate capability. However, the Li4 P2 O7 -involved LiFe0.9 P0.95 O4−ı /C in our case shows high polarization and low capacity retention. This may be because of the difference in particle size, composition and electrode processing.
4. Conclusions Impurity phases can be effectively controlled by offstoichiometry and Na-incorporation in LiFePO4 . By creation of Fe and P deficiency, LiFe0.9 (PO4 )0.95 /C consists of well-recognized LiFePO4 accompanied with impurity phases Li4 P2 O7 and Li3 PO4 . Li4 P2 O7 impurity can be suppressed by Na incorporation. The Li4 P2 O7 and Li3 PO4 impurities show different effects on electrochemical performance. Existence of Li4 P2 O7 and a small amount of Li3 PO4 can improve the capacity and rate capability. However, high content of Li3 PO4 makes the electrochemical properties poor. The Li4 P2 O7 -involved LiFe0.9 P0.95 O4−ı /C cathode shows high polarization and low capacity retention. The Li3 PO4 -involved 1% Na-doped LiFe0.9 P0.95 O4−ı /C exhibits the best capacity, rate capability and cyclability, which is ascribed to its fast kinetics and low charge-transfer resistance. Our results indicate that small amounts of cationic doping and off-stoichiometry are useful to control the impurity phases and hence to optimize the electrochemical performance for the olivine phosphate cathode materials.
Acknowledgments
Fig. 13. EIS spectra for the LiFePO4 -based composite electrodes within the frequency range of 10 kHz to 100 mHz.
This work was supported by the NSFC (grant nos. 50825203 and 21175050), the MOST of China (grant nos. 2010DFB70090 and 2009AA03Z225), and the PCSIRT (Program for Changjiang Scholars and Innovative Research Team in University). In addition, the authors thank Analytical and Testing Center of Huazhong University of Science and Technology for XRD and SEM measurements, and South-Central University for Nationalities for XPS measurement.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical performance in Na-incorporated nonstoichiometric LiFePO4 /C composites with controllable impurity phases Zhao-Hui Wang a , Li-Xia Yuan a , Jun Ma b,∗ , Long Qie a , Lu-Lu Zhang a , Yun-Hui Huang a,∗∗ a State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China b School of Automotive Studies, Tongji University, Shanghai 201804, China
a r t i c l e
i n f o
Article history: Received 25 September 2011 Received in revised form 21 November 2011 Accepted 13 December 2011 Available online 23 December 2011 Keywords: Lithium-ion batteries Lithium iron phosphate Sodium-incorporation Off-stoichiometry Electrochemical performance
a b s t r a c t Nonstoichiometry and Na incorporation are designed to attain controllable impurity phases Li4 P2 O7 and Li3 PO4 in LiFePO4 . The effects of Li4 P2 O7 and Li3 PO4 on structure and electrochemical performance have been investigated. Both Li4 P2 O7 and Li3 PO4 impurities are observed in (Fe, P)-deficient LiFe0.9 P0.95 O4−ı . With Na+ incorporation, Li4 P2 O7 phase disappears while Li3 PO4 content increases. Proved by X-ray photoelectron spectroscopy, nonstoichiometry and Na-incorporation do not change the chemical state of Fe(II). Our experiments indicate that Li4 P2 O7 and Li3 PO4 show different effects on the electrochemical performance. Li4 P2 O7 leads to degradation of cyclability, whereas a small amount of Li3 PO4 is beneficial for the improvement in capacity and rate capability. The 1% Na-doped LiFe0.9 P0.95 O4−ı composite exhibits the best electrochemical performance. We ascribe the improvement to the structural stabilization caused by the existence of Li3 PO4 . © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Olivine LiFePO4 has been extensively investigated as a promising cathode material for lithium-ion batteries due to its high capacity, low cost, nontoxicity and environmental friendliness [1]. The key barrier to LiFePO4 commercialization is its low electronic conductivity and sluggish Li+ diffusion across the LiFePO4 /FePO4 interface [2,3]. Attempts have been devoted to optimizing the material for superior electrochemical performance, such as modifying the surface of LiFePO4 with conductive nanostructures [4–7] and doping alien ions [8–11] in the lattice, etc. Apart from the improvement in electrochemical property for LiFePO4 by carbon coating and alien-ion doping, there is also a growing need to identify the effects of some impurities in LiFePO4 [12]. Some of the impurities benefit the electrochemical performance while some may poison the performance seriously [3]. In general, control of impurity phases is especially important for the electrochemical properties in LiFePO4 . Kang and Ceder [13] recently discovered that non-stoichiometry of the reactants in preparation of LiFePO4 resulted in Li4 P2 O7 phase. Hu et al. [14] synthesized LiFePO4 from nonstoichiometric starting materials containing excess iron source
∗ Corresponding author. ∗∗ Corresponding author. Tel.: +86 27 87558241; fax: +86 27 87558241. E-mail addresses: jun [email protected] (J. Ma), [email protected] (Y.-H. Huang). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.12.055
with a ratio of 2:1 to phosphorus in order to avoid or suppress formation of insulating Li3 PO4 phase, and found that conductive Fe2 P phase formed at high temperatures. Yu et al. [12] observed that Li3 PO4 impurity was formed in LiFePO4 grown via hydrothermal route if excess Li source was added. Kadoma et al. [15] obtained Fe2 P and Li3 PO4 impurities along with LiFePO4 by varying sintering temperature and the reductive amount. Therefore, by creating offstoichiometry or changing reaction temperature and atmosphere, impurity phases can be expectably controlled in LiFePO4 . However, it is still difficult to selectively control the impurity phases. Besides off-stoichiometry and sintering condition, we believe that alien-ion doping should influence formation of impurity phases. Few researches have paid attention to the doping effect on the impurities in LiFePO4 . In this work, Na doping and nonstoichiometry were simultaneously employed to selectively control the impurity phases in LiFePO4 . With deficient Fe and P sources, the obtained nominal LiFe0.9 P0.95 O4−ı /C sample consists of well-recognized LiFePO4 and impurity phases like Li4 P2 O7 and Li3 PO4 . It is found that Na doping is effective to suppress formation of Li4 P2 O7 impurity. The effects of Li4 P2 O7 and Li3 PO4 on the electrochemical performance of LiFePO4 have been investigated.
2. Experimental Nominal LiNax Fe0.9 P0.95 O4−ı /C (x = 0, 0.01, 0.02) and stoichiometric LiFePO4 were synthesized via a solid-state reaction.
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According to the desired stoichiometry, Li2 CO3 , FeC2 O4 ·2H2 O and NH4 H2 PO4 were mixed with dopant source NaNO3 in ethanol and ball-milled for 12 h. The mixed slurry was then dried at 80 ◦ C in the oven in air for another 12 h. After being dried, the mixture was calcined at 350 ◦ C for 6 h under N2 atmosphere. The yielded precursor powder was reground together with 12 wt% glucose. Glucose served as a carbon source. The final product was obtained by sintering the mixture at 700 ◦ C for 10 h under high pure N2 . Carbon contents were measured by an Elementar Vario MICRO Cube (Elementar, Germany). Elemental compositions (Li, Fe, Na, P) of the samples were determined by inductively coupled plasma optical emission spectrometer (ICP-OES, PerkinElmer Instruments, Optima 4300DV). The phase was checked by X-ray powder diffraction (XRD) with the Rigaku D/MAX-RB diffractometer using filtered Cu K␣ radiation within the scanning angle, 2, ranged from 10◦ to 90◦ in a step of 0.02◦ . Structural analysis was carried out by Rietveld refinement using RIETAN-2000. The chemical valence states of Fe, Na were examined by using X-ray photoelectron spectroscopy (XPS, PerkinElmer, PHI 5600) with a monochromatic Al X-ray source (1486.6 eV). The data were calibrated by adventitious C 1s peak with a fixed value of 284.4 eV and analyzed using XPSPEAK41 processing software. The morphology was observed with scanning electron microscope (SEM, Sirion 200, Holland) equipped with an energy dispersive X-ray detector (EDX). Electrochemical tests were carried out with CR2032 coin cells. The cathode film was fabricated from a mixture of active material, C-black, and polytetrafluoroethylene (PTFE) (75:20:5 wt%) with an area of 0.5 cm2 and a total mass of 5–7 mg. Metallic lithium foil was used as anode electrode; the electrolyte was 1 mol L−1 LiPF6 in a 1:1 solvent mixture of ethylene carbonate and dimethyl carbonate (EC/DMC). A thin sheet of microporous polyethylene (Celgard 2400) served as the separator. The cells were assembled in an argonfilled glove box. Cyclic voltammetry (CV) was measured by an electrochemical working station (PARSTAT 2273, Princeton Applied Research, US) at a scan rate of 0.1 mV s−1 between 2.5 and 4.2 V. Electrochemical impedance spectroscopy (EIS) was conducted also on the PARSTAT 2273 with a potential amplitude of 5 mV in the frequency range from 104 to 10−2 Hz. Charge/discharge tests were performed between 2.5 and 4.2 V with a Land CT2001 battery tester (Wuhan Land Electronic Co. Ltd., China) at 25 ◦ C at various rates (0.1–5 C).
3. Results and discussion Determined by ICP, the compositions of LiFePO4 , LiFe0.9 P0.95 O4−ı and Na-doped LiFe0.9 P0.95 O4−ı are close to the expected nominal chemical formula. Carbon contents are about 3 wt% for all the samples, which implies that the influence of carbon content on the electrochemical performance for the prepared samples can be ignored [16]. Fig. 1a shows their XRD patterns. For all the samples, the main diffraction peaks match well with the standard olivine LiFePO4 structure (JCPDS 40-1499) indexed by orthorhombic Pnma. The calculated lattice parameters obtained by Rietveld refinement are listed in Table 1. The refined lattice parameters of LiFePO4 are almost equal to those reported in the literatures [17,18]. For the Na+ -incorporated samples, the lattice parameters change slightly with Na content, which may be because the Na doping level is very low. It is difficult to determine from the lattice parameters whether Na+ ions are doped into the olivine structure. However, we can see from Fig. 1b that Na incorporation obviously affects the formation of impurity phases. With deficient Fe and P sources, the LiFe0.9 P0.95 O4−ı /C sample consists of well-recognized LiFePO4 and impurity phases including Li4 P2 O7 and Li3 PO4 . Similar phenomenon was observed in the case where
417
Fig. 1. (a) XRD patterns for LiFePO4 -based samples, and (b) partial magnified patterns with intensities in logarithmic scale.
excess Li source was used [13,19]. Interestingly, with Na incorporation, the peak corresponding to Li4 P2 O7 phase disappears and Li3 PO4 diffractions remain. As Na content increases, the intensities of Li3 PO4 diffraction peaks are enhanced, demonstrating that Na incorporation facilitates the formation of Li3 PO4 impurity. Relative intensity ratio (RIR) method is used to calculate the contents of the impurities [20]. For LiFe0.9 P0.95 O4−ı /C, the contents of Li4 P2 O7 and Li3 PO4 are around 3.1 wt% and 0.82 wt%, respectively. For Naincorporated LiFe0.9 P0.95 O4−ı /C, the Li3 PO4 content is ca. 4.4 wt% for 1% Na-doped LiFe0.9 P0.95 O4−ı /C and 7.3 wt% for 2% Na-doped LiFe0.9 P0.95 O4−ı /C. Fig. 2 shows SEM images of LiFePO4 and Na-doped offstoichiometric LiFePO4 samples. It can be seen that all nonstoichiometric samples show similar morphologies and wide particle-size distribution. In addition, Na+ incorporation does not change the morphology. From these results, it is clear that an appreciable amount of impurity phase is in situ formed along with LiFePO4 , which is well consistent with the previous reports [13,14,16]. The EDX result of 1% Na-doped LiFe0.9 P0.95 O4−ı /C in Fig. 3 indicates that the particles in the selected region include P, Fe, O, C components and a trace of Na, which is in accordance with the nominal formula. The chemical composition of 1% Na-doped LiFe0.9 P0.95 O4−ı /C by the EDX analysis in Fig. 3 shows a P/Fe ratio of 1.1:1. Analyzed by combining EDX and XRD, it can be concluded that the synthesis based on designed nonstoichiometry with Fe and P deficiency as well as Na-incorporation can effectively create impurity like Li3 PO4 in LiFePO4 . For further insight into phase separation
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Table 1 Lattice parameters obtained from XRD Rietveld refinement for LiFePO4 -based samples. Sample
LiFePO4 /C
LiFe0.9 P0.95 O4−ı /C
1% Na-doped LiFe0.9 P0.95 O4−ı /C
2% Na-doped LiFe0.9 P0.95 O4−ı /C
Lattice constant (Å) a (Å) b (Å) c (Å) V (Å3 )
10.3247(5) 6.0072(1) 4.6929(9) 291.07
10.3257(1) 6.0055(8) 4.6901(1) 290.84
10.3255(3) 6.0067(1) 4.6916 (9) 290.99
10.3237(1) 6.0056(9) 4.6920(4) 290.91
Reliability factors (%) Rwp Rp
8.03 5.67
9.20 6.30
8.84 5.96
10.06 6.70
Fig. 2. SEM images for (a) LiFePO4 , (b) LiFe0.9 P0.95 O4−ı , (c) 1% Na-doped LiFe0.9 P0.95 O4−ı , and (d) 2% Na-doped LiFe0.9 P0.95 O4−ı .
of Li3 PO4 from LiFePO4 , EDX analysis was conducted at different regions in the SEM image for 1% Na-doped LiFe0.9 P0.95 O4−ı /C sample (Fig. 4). It is found that the Fe/P ratio is higher in the central part than in the edge of the particles. However, it is difficult to
determine the real distribution of Fe and P in the LiFePO4 particles [21,22]. Nevertheless, the XRD, SEM and point elemental analysis confirm that the impurities are actually formed in the LiFePO4 phase.
Fig. 3. SEM image and the corresponding EDX spectrum for 1% Na-doped LiFe0.9 P0.95 O4−ı .
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Fig. 4. EDX data at different regions in the SEM image for 1% Na-doped LiFe0.9 P0.95 O4−ı sample. Red cross indicates the regions where the EDX measurement was taken. A represents the central part of the particle; B and C are the edges of the particle. The measurement was repeated at different regions. The given EDX data in the tables are the average ones for three parallel measurements at different regions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 5a gives the XPS spectra within a wide range of binding energy. With the reference binding energy at 284.6 eV for the C 1s peak, the spectra of Li 1s, P 2p and O 1s are observed at 56.1, 132.6 and 530.9 eV, respectively. The peak at 1071.4 eV is ascribed to Na 1s. Fig. 5b presents Na 1s XPS spectra for 1% and 2% Na-doped LiFe0.9 P0.95 O4−ı /C samples. As can be seen, the intensity of Na 1s spectrum of 2% Na-doped sample is almost two times stronger than that of 1% Na-doped LiFe0.9 P0.95 O4−ı /C, indicating that Na+ ions exist in the samples. In order to examine the effects of Na-incorporation and nonstoichiometry on the oxidation state of Fe ions, the Fe 2p XPS spectra are investigated. As shown in Fig. 6, all the samples have two doublet peaks at 710.3 and 723.6 eV corresponding to Fe 2p1/2 and 2p3/2 , which match perfectly with those in LiFePO4 [23]. Compared with the pristine sample, the binding energies of the main peaks for the Na-incorporated samples do not change, indicating that no observable change occurs in the chemical valence of Fe (II). In general, doping with alien atoms may induce structural distortion, which, however, can be balanced by the formation of mixed phases caused by off-stoichiometry. Such an influence of off-stoichiometry has been reported by Kang and Ceder [13] and Axmann et al. [24]. Electrochemical performance was investigated to see the influence of impurity in LiFePO4 on Li insertion/extraction behavior. Figs. 7 and 8 compare the charge/discharge profiles at 0.1 C and discharge capacities at various C rates for different cathodes. All samples clearly show a flat and wide dischage voltage plateau at
∼3.4 V. Even in the case of the (Fe, P)-deficient cathodes, e.g. 2% Na-doped sample containing some Li3 PO4 impurity, the flat and wide voltage plateau still appears, similar to that of ideal stoichiometric LiFePO4 . This demonstrates that the charge/discharge process for the (Fe, P)-deficient cathode corresponds to the two-phase reaction between LiFePO4 and FePO4 . The initial discharge capacities at 0.1 C are 149, 152, 154 and 148 mAh g−1 for LiFePO4 , LiFe0.9 P0.95 O4−ı /C, 1% and 2% Na-doped LiFe0.9 P0.95 O4−ı /C, respectively. The Na-incorporated (Fe, P)-deficient cathodes show enhanced capacity as compared with pristine LiFePO4 . From Fig. 8, we can see that 1% Na-doped LiFe0.9 P0.95 O4−ı /C exhibits the best rate capability; its discharge capacity is 130 mAh g−1 at 2 C and 111 mAh g−1 at 5 C. We ascribe the difference in capacity and rate capability among the LiFePO4 /C-based electrodes to Li4 P2 O7 and Li3 PO4 impurities caused by nonstoichiometry and Naincorporation. It was reported that Li4 P2 O7 and Li3 PO4 impurities in LiFePO4 could facilitate fast charging and discharging by providing a guest ion-conductive surface [7,13,25]. However, if only a small amount of Li3 PO4 exists at the surface of particles, it may act as an inert mass to reduce the electrochemical properties of LiFePO4 [12,14,24]. Therefore, the content of Li3 PO4 is essential for the electrochemical performance. In our case, the 1% Nadoped LiFe0.9 P0.95 O4−ı /C sample containing 4.4 wt% Li3 PO4 shows the highest capacity and best rate capability; LiFe0.9 P0.95 O4−ı /C containing 3.1 wt.% Li4 P2 O7 and 0.82 wt% Li3 PO4 also present good performance; whereas the performance for 2% Na-doped LiFe0.9 P0.95 O4−ı /C containing 7.3 wt% Li3 PO4 is the poorest.
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Fig. 7. Initial charge/discharge curves for LiFePO4 -based composite electrodes at 0.1 C.
Apparently, if Li3 PO4 content is too high, it may act as an inert mass to prevent the transfer of electrons and Li+ ions and hence to degrade the electrochemical performance [24]. In order to clarify the effects of Li4 P2 O7 and Li3 PO4 impurities, we have carefully compared the electrochemical properties of LiFe0.9 P0.95 O4−ı /C and 1% Na-doped LiFe0.9 P0.95 O4−ı /C. The cell potential as a function of specific capacity at 0.1–5 C rates is depicted in Fig. 9. The capacity and rate capability of 1% Na-doped LiFe0.9 P0.95 O4−ı /C are both better than those of LiFe0.9 P0.95 O4−ı /C. It
was reported that the Li4 P2 O7 phase could lead to the improvement in rate capability [7,13]. LiFe0.9 P0.95 O4−ı /C contains Li4 P2 O7 impurity but 1% Na-doped LiFe0.9 P0.95 O4−ı /C does not contain detectable Li4 P2 O7 . The superior performance for LiNa0.01 Fe0.9 (PO4 )0.95 /C should be contributed to the existence of Li3 PO4 impurity. Fig. 10 shows the charge/discharge curves of the 1st, 10th, 50th, 100th, 150th and 200th cycles for LiFe0.9 P0.95 O4−ı /C and 1% Na-doped LiFe0.9 P0.95 O4−ı /C composite electrodes at 1 C. It can be found that the Li4 P2 O7 -involved sample shows higher polarization and lower capacity retention than the Li3 PO4 -involved sample. Fig. 11 presents their cycling performances at 1 C. Excellent cyclability is attained in the Li3 PO4 -involved 1% Na-doped LiFe0.9 P0.95 O4−ı /C composite cathode. It still delivers a discharge capacity of 142 mAh g−1 after 200 cycles. The capacity loss over 200 cycles is only 1.3% for 1% Na-doped LiFe0.9 P0.95 O4−ı /C, whereas the loss is as high as 14.6% for LiFe0.9 P0.95 O4−ı /C. The existence of Li4 P2 O7 impurity results in degradation of cyclability, which has been also observed in LiFePO4 by other groups [18]. Since Li3 PO4 is inactive, it can not contribute additional capacity to LiFePO4 [24]. Structural information is attained by Rietveld refinement for comparison between LiFe0.9 P0.95 O4−ı /C and 1% Nadoped LiFe0.9 P0.95 O4−ı /C. The interatomic distances between Atom 1 (Li, Fe, P) and Atom 2 (O1, O2, O3) are listed in Table 3. The average ˚ and that of bond distance of Li–O varies from 2.1462 to 2.1494 A, ˚ The former is elongated with P O varies from 1.5467 to 1.5370 A. Na incorporation, which indicates that the extraction of Li ions from the lattice of 1% Na-doped LiFe0.9 P0.95 O4−ı /C becomes easier due to
Fig. 6. XPS core levels of Fe 2p for LiFePO4 -based samples.
Fig. 8. Cycling performances for LiFePO4 -based composite electrodes at various rates.
Fig. 5. (a) XPS spectrum of 1% Na-doped LiFe0.9 P0.95 O4−ı ; (b) Na 1s XPS spectra of Na-doped LiFe0.9 P0.95 O4−ı samples.
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421
Fig. 9. Charge/discharge curves of the first cycle measured at various rates in the range of 2.5–4.2 V at 25 ◦ C for LiFe0.9 P0.95 O4−ı and 1% Na-doped LiFe0.9 P0.95 O4−ı cathodes.
the weakness of interaction between Li and O ions. Meanwhile, the length of the P O bond after Na incorporation decreases by 0.56%, 0.32% and 0.32% for P O1, P O2 and P O3, respectively, demonstrating that the framework of LiFePO4 becomes more stable [9,11]. Therefore, we argue that the better cycle performance for the 1% Na-doped LiFe0.9 P0.95 O4−ı /C composite is mainly due to the more stable structure caused by the balance of the formation of impurity Li3 PO4 . Fig. 12 displays the CV profiles for LiFePO4 -based composites in the first cycle obtained at a scan rate of 0.1 mV s−1 . All curves show a pair of redox peaks corresponding to the oxidation and reduction of Fe2+ /Fe3+ ions in LiFePO4 . The difference between oxidation and reduction peak potential for Fe3+ /Fe2+ redox couple, E, is displayed in Table 2. The 1% Na-doped LiFe0.9 P0.95 O4−ı /C sample shows the lowest potential interval with E = 0.227 V between anodic and cathodic peaks. Since the E is determined by the potential polarization of the active material during charge and discharge process, lower E demonstrates that the lithium insertion into the 1% Na-doped LiFe0.9 P0.95 O4−ı /C composite behaves more likely as a Nernst system.
Fig. 10. Charge/discharge curves of the 1st, 10th, 50th, 100th, 150th and 200th cycles at 1 C for (a) LiFe0.9 P0.95 O4−ı /C and (b) 1% Na-doped LiFe0.9 P0.95 O4−ı /C.
Electrochemical impedance spectroscopy was carried out to shed more light on the effects of Na+ -doping and off-stoichiometry. The impedance was recorded on the coin cells at fully discharge state after one cycle. The corresponding Nyquist plots of the spectra are presented in Fig. 13. All spectra have a depressed semicircle in the high frequency region and an inclined line in the low frequency
Table 2 Anodic peak potential locations (Ep,a ) and corresponding cathodic ones (Ep,c ) and potential differences (E) of Fe3+ /Fe2+ redox couple for the CV curve in Fig. 12. Sample
Ep,a (V)
Ep,c (V)
E (V)
LiFePO4 /C LiFe0.9 P0.95 O4−ı /C 1% Na-doped LiFe0.9 P0.95 O4−ı /C 2% Na-doped LiFe0.9 P0.95 O4−ı /C
3.575 3.564 3.555 3.602
3.298 3.319 3.328 3.307
0.277 0.245 0.227 0.295
Fig. 11. Specific discharge capacity versus cycle number for LiFe0.9 P0.95 O4−ı /C and 1% Na-doped LiFe0.9 P0.95 O4−ı /C electrodes at 1 C rate.
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Table 3 Interatomic distances in LiFe0.9 P0.95 O4−ı and 1% Na-doped LiFe0.9 P0.95 O4−ı obtained by Rietveld refinement. LiFe0.9 P0.95 O4−ı /C
1% Na-doped LiFe0.9 P0.95 O4−ı /C
Atom 1
Atom 2
Counts
d1,2 (Å)
Atom 1
Atom 2
Li
O1 O2 O3
2× 2× 2×
2.0893(2) 2.1632(9) 2.1862(5)
Li
O1 O2 O3
2× 2× 2×
2.0929(7) 2.1685(7) 2.1870(5)
Fe
O3 O2 O1 O3
2× 1× 1× 2×
2.0670(2) 2.2183(5) 2.0805(9) 2.2432(3)
Fe
O3 O2 O1 O3
2× 1× 1× 2×
2.0739(9) 2.2140(0) 2.0970(5) 2.2399(4)
P
O1 O2 O3
1× 1× 2×
1.5619(9) 1.5233(5) 1.5451(2)
P
O1 O2 O3
1× 1× 2×
1.5532(2) 1.5173(3) 1.5407(7)
Fig. 12. CV curves for LiFePO4 -based composite electrodes.
region. The intercept at the Zreal axis at high frequencies refers to Rs , which includes resistance of electrolyte solution and electric contact resistance. The semicircle in the high frequency range corresponds to the charge transfer resistance (Rct ) of electrochemical reaction; and the sloping line in the lower frequency represents Li+ -ion diffusion resistance within the cathode, namely the warburg impedance [11]. As shown in Fig. 13, the Rct values of pristine LiFePO4 , LiFe0.9 P0.95 O4−ı /C, 1% and 2% Na-doped LiFe0.9 P0.95 O4−ı /C, are 228, 167, 126 and 345 , respectively. It is believed that the decrease in charge transfer resistance is beneficial to the kinetic
Counts
d1,2 (Å)
behavior during charge/discharge process, and hence leads to the improvement of electrochemical performance [26]. Therefore, it is expected that the 1% Na-doped LiFe0.9 P0.95 O4−ı /C can provide an excellent kinetic behavior, which is consistent with the CV measurement in Fig. 12. It has been reported that Li3 PO4 modification in LiCoO2 and LiMn2 O4 can decrease charge transfer resistance, enhance electrode kinetics, and hence improve cycling performance and rate capability [27,28]. In our case, the Li3 PO4 -involved 1% Na-doped LiFe0.9 P0.95 O4−ı /C sample exhibits fast electrode kinetics, low charge-transfer resistance and structural stability, which is why it shows the best electrochemical performance. Kang and Ceder [13] reported that a Li4 P2 O7 -contained LiFe0.9 P0.95 O4−ı obtained by creation of an Fe/P deficiency exhibited an ultrafast charging rate capability. However, the Li4 P2 O7 -involved LiFe0.9 P0.95 O4−ı /C in our case shows high polarization and low capacity retention. This may be because of the difference in particle size, composition and electrode processing.
4. Conclusions Impurity phases can be effectively controlled by offstoichiometry and Na-incorporation in LiFePO4 . By creation of Fe and P deficiency, LiFe0.9 (PO4 )0.95 /C consists of well-recognized LiFePO4 accompanied with impurity phases Li4 P2 O7 and Li3 PO4 . Li4 P2 O7 impurity can be suppressed by Na incorporation. The Li4 P2 O7 and Li3 PO4 impurities show different effects on electrochemical performance. Existence of Li4 P2 O7 and a small amount of Li3 PO4 can improve the capacity and rate capability. However, high content of Li3 PO4 makes the electrochemical properties poor. The Li4 P2 O7 -involved LiFe0.9 P0.95 O4−ı /C cathode shows high polarization and low capacity retention. The Li3 PO4 -involved 1% Na-doped LiFe0.9 P0.95 O4−ı /C exhibits the best capacity, rate capability and cyclability, which is ascribed to its fast kinetics and low charge-transfer resistance. Our results indicate that small amounts of cationic doping and off-stoichiometry are useful to control the impurity phases and hence to optimize the electrochemical performance for the olivine phosphate cathode materials.
Acknowledgments
Fig. 13. EIS spectra for the LiFePO4 -based composite electrodes within the frequency range of 10 kHz to 100 mHz.
This work was supported by the NSFC (grant nos. 50825203 and 21175050), the MOST of China (grant nos. 2010DFB70090 and 2009AA03Z225), and the PCSIRT (Program for Changjiang Scholars and Innovative Research Team in University). In addition, the authors thank Analytical and Testing Center of Huazhong University of Science and Technology for XRD and SEM measurements, and South-Central University for Nationalities for XPS measurement.
Z.-H. Wang et al. / Electrochimica Acta 62 (2012) 416–423
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