Studies of Li3V2(PO4)3

Dec 13, 2007 - scanning electron microscopy, transmission electron microscopy, cyclic voltammetry, alternating current impedance, and charge-discharge...
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J. Phys. Chem. C 2008, 112, 308-312

Studies of Li3V2(PO4)3 Additives for the LiFePO4-Based Li Ion Batteries Lina Wang,† Zhengchun Li,† Hongjie Xu,† and Keli Zhang*,†,‡ College of Chemistry and Molecular Sciences and Centre of Nanoscience and Nanotechnology Research, Wuhan UniVersity, Wuhan, 430072, People’s Republic of China ReceiVed: July 24, 2007; In Final Form: October 17, 2007

Compounds of LiFePO4-based materials with the addition of Li3V2(PO4)3 were simply prepared by a rheological phase reaction method. Differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, cyclic voltammetry, alternating current impedance, and charge-discharge cycles were used to evaluate the LiFePO4-Li3V2(PO4)3 compound powders. Phase analysis showed that no new phase formed in these compounds. The electronic conductivity and electrochemical performance of the LiFePO4-based compounds were improved by the addition of Li3V2(PO4)3 composites. High rate discharge capacity was developed for LiFePO4-Li3V2(PO4)3 as cathode material.

Introduction For their great potential to be used as power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs), currently, lithium ion batteries have captured a large share of the rechargeable battery market. The cathode material, which is a crucial factor of performance and safety of Li ion batteries, has the greatest potential for improvement. In addition to low cost, high capacity, better stability, and environmental friendliness, a high operation voltage is required for cathode active materials. As a result of the lightweight metallic lithium anode, the rocking-chair battery must then use high-voltage cathode materials in order to increase its energy density and compensate for the increase in anode weight and potential. These factors are further emphasized with the decreasing of Co resources. Also, current commercially available LiCoO2 cathode material suffers from a relatively small capacity, because only half of the Li ions in LiCoO2 can be extracted reversibly.1 In an intense search for alternative materials, there has been considerable interest in orthophosphates LiMPO4 (M ) Fe, Mn, Co, Ni), which crystallize with the olivine-like structure, and polyphosphates Li3M2(PO4)3 (M ) Fe, V) isostructural with Nasicon-type frameworks. Among these phosphates, LiFePO4 has emerged as the most promising.2-6 However, low electronic conductivity and low lithium diffusivity hinder the LiFePO4 olivine compound from commercial utilization. Many efforts including metal doping,7,8 coating with the electronically conductive materials like carbon, metal, and metal oxide,9-11 and optimization of particles with suitable preparation procedures have been made to improve the performance of LiFePO4 cathode materials. Recently, some study has also been given to the monoclinic lithium vanadium phosphate, R-Li3V2(PO4)3, the substitution of the large polyanion, instead of the smaller O2ions, helping to stabilize the structure in an open 3D framework and allowing a faster ion migration.12,13 Moreover, Li3V2(PO4)3 exhibits four redox plateaus around 3.62, 3.68, 4.08, and 4.55 V. It can extract/insert two Li ions reversibly between 3.0 and * To whom correspondence should be addressed. E-mail: klzhang@ whu.edu.cn. † College of Chemistry and Molecular Sciences, Wuhan University. ‡ Centre of Nanoscience and Nanotechnology Research, Wuhan University.

4.3 V based on the V3+/V4+ redox couple with an average operation voltage of 3.8 V; three Li ions may be completely extracted when charged to 4.8 V with a mean intercalation voltage up to 4.0 V.14,15 In this work, we attempted to prepare a series of compounds based on LiFePO4 cathode material, with which Li3V2(PO4)3 formed together as an additive during the heating process by a simple rheological phase reaction method under the assumption that the following reactions occurred

2nLiOH‚H2O + 2nFePO4‚4H2O + HO (C2H4O)nH f 2nLiFePO4 + 2nC + (13n + 1)H2O (1) 3LiOH‚H2O + V2O5 + 3NH4H2PO4 + HO(C2H4O)nH f Li3V2(PO4)3 + 2nC + 3NH3 v + (n - 2)H2 v + (n + 12)H2O (2) Nanosized LiFePO4-Li3V2(PO4)3 (labeled as LFP-LVP) compounds were obtained. Except to enhance the conductivity and the mean operation voltage, a high capacity was also expected for the LiFePO4-based Li ion batteries. Experimental Section LiFePO4-based compounds were prepared using polyethylene glycol (PEG, mean molecular weight of 10 000; the amounts of PEG were equal for all samples) as the reductive agent and carbon source by a rheological phase reaction (labeled as RPR), which has been reported by our group as an efficient method to prepare LiFePO4-C.16,17 LiOH‚H2O, FePO4‚4H2O, and PEG powders were used as the starting materials for LiFePO4-C. The molar ratio of Li+:Fe3+ is 1:1. LiOH‚H2O, V2O5, NH4H2PO4, and PEG powders were used as the starting materials for Li3V2(PO4)3-C. The molar ratio of Li+:V5+ is 3:2. Stoichiometric amounts of LiOH‚H2O, FePO4‚4H2O, V2O5, NH4H2PO4, and PEG powders were used as the starting materials for LFPLVP compounds. A general procedure of the experiment is described as follows. Starting materials were mixed for 10 min, and then an appropriate amount of deionized water was added to get a rheological body. Finally, the resulting precursors were heated in Ar flow for 6 h to get the powders of LiFePO4-based

10.1021/jp0758014 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/13/2007

Studies of Li3V2(PO4)3 Additives

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Figure 1. XRD patterns of LiFePO4-based compounds with different amounts of Li3V2(PO4)3 additives.

compounds directly. The temperature for the reaction condition was described and discussed in the text. Thermal analysis measurement of thermogravimetry and different thermal analysis (DSC/TG) was carried out under flowing Ar between ambient temperature and 800 °C with a heating rate of 20 °C min-1 on a Netzsch STA4499C thermal analyzer. X-ray diffraction (XRD) profiles of the samples were carried out on a Shimadzu XRD-6000 diffractometer using Cu KR1 radiation. The morphology and microstructure was observed using scanning electron microscopy (SEM) with a Hitachi FEG scanning electron microscope and high-resolution transmission electron microscope (HRTEM) with JEOL JEM 2010 FEF. The particle size distribution was determined by the optical particle size analyzer (Mastersizer 2000, England). The amount of C, N, and H was determined by an element analyzer (FLASH 112SERIES, Italy). The charge/discharge tests were carried out using a simulated test cell, which consisted of a working electrode and a lithium foil electrode separated by a Celgard 2400 microporous membrane. The working electrode was prepared by mixing the LiFePO4-based cathode material with 15% acetylene black and 5% polytetrafluoroethylene binder. A 1 M solution of LiClO4 dissolved in ethylene carbonate/dimethyl carbonate (1:1, v/v) is used as the electrolyte. The cells were charged-discharged between 2.0 and 4.4 V at room temperature. The cathode performance was investigated in terms of charge-discharge curves and cycle efficiency. The charge and discharge rates were equal under a given current density, and all capacity is calculated based on the LiFePO4-C or LFP-LVP/C materials, not merely the active materials. Results and Discussion To study the effect of different amounts of Li3V2(PO4)3 additives on the properties of LiFePO4-based compounds, a series of such powders with various Li3V2(PO4)3 additions (mol ratios) were synthesized at 700 °C. The XRD patterns of these samples are shown in Figure 1. Pure LiFePO4 and pure Li3V2(PO4)3 can be prepared without mixing their precursors. For the LiFePO4-based samples obtained from heating the precursors with vanadium additions, the synthesized powders are composed of LiFePO4 and Li3V2(PO4)3 phases without a new impurity phase, and the relative peak intensity of LFP: LVP increases when the ratio of Li3V2(PO4)3 additives decreases. The cathode performance of LiFePO4 and LFP-LVP was examined by charge-discharge test, as shown in Figure 2; the specific capacity of LiFePO4-based compounds increases with increasing of the Li3V2(PO4)3 additives till the ratio of LFP: LVP decreases to 8:1, and then the specific capacity decreases with further increasing of Li3V2(PO4)3 additives. A highest

Figure 2. Charge-discharge curves of LiFePO4-based compounds in the second cycle (1 C rate).

Figure 3. TG-DSC curves of precursor mixture of LFP-LVP compound.

capacity of 151 mAh g-1 was obtained, and the chargedischarge efficiency was 100% for the sample of LFP-LVP compound with a mol ratio of 8:1 at the rate of 1 C (170 mA g-1); thus, to give further investigation, the LFP-LVP compounds index the samples with equal mol ratios of 8:1 for LFP: LVP. The carbon contents of these samples were similar, and the average carbon content is 8% through element analysis. Figure 3 shows TG-DSC curves of the precursor mixture of the LFP-LVP compound. The TG curve shows four weight loss main stages. The first one attributed to the release of little physical adsorbed water within the range from ambient temperature to around 120 °C can be observed in the TG curve, corresponding with an obvious endothermic peak in the DSC curve. The weak weight loss from 120 to 310 °C in the TG curve should be due to the loss of crystal water of LiOH‚H2O and the release of NH3 for NH4H2PO4. The steep weight loss, which occurs between about 310-500 °C in the TG curve can be ascribed to the decomposition of reactants, with carbonization of PEG and formation of the LiFePO4-based compound. An endothermic peak at 408 °C and an exothermic peak around 475 °C corresponding with the TG curve can be found in the DSC curve. Finally, the slow and gradual weight loss above 500 °C can be associated with the oxidation of carbon for trace air in Ar flow, which produces carbon monoxide or carbon dioxide. According to this result, we choose 600, 700, and 800 °C as heating temperatures to evaluate the effect of synthesis temperature on the performance of the LFP-LVP compounds.

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Figure 4. XRD patterns of the LFP-LVP powders heat treated at different temperatures. The arrowhead signs signify peaks of Li3V2(PO4)3. Figure 6. The second charge-discharge curves of LFP-LVP compounds prepared at different temperatures with 1 C rate.

Figure 7. SEM (a) and TEM (b) images of as-prepared LFP-LVP samples. Figure 5. Particle size distribution of LFP-LVP powders synthesized at different temperatures.

The XRD patterns of the LFP-LVP powders heat-treated at different temperatures are shown in Figure 4. All main peaks can be indexed as crystallized LiFePO4 phase with an ordered olivine structure and a space group of Pnma. The peaks gradually sharpen with the increasing of temperature, indicating an increase of crystallinity may occur. The peaks attributable to crystallized Li3V2(PO4)3 phase (as the “arrowhead” indexed) also become more and more obvious with increasing of temperature. However, the relative intensity of LFP:LVP peaks is similar, which should be due to fact that the ratio of the Li3V2(PO4)3 additions is equal for the three samples. To find the relationship between synthesis temperature and particle size, we ran a particle size distribution analysis on the 600, 700, and 800 °C samples, as shown in Figure 5. The average particle size is 0.352, 0.207, and 0.427 µm for the above three samples, respectively. The value at 50% cumulative population (d50%) represents the average particle size. However, the smallest average particle size is attributed to the sample synthesized at 700 instead of 600 °C. The reason can be attributed to the fact that too small particles induce more agglomerated particles of the 600 °C sample. The particle distribution has two regions for all samples at different temperatures. The population peaks around the smaller particle size are LFP-LVP/C powders, and the other population peaks at the larger particle size region can be attributed to agglomerated particles that were not dispersed perfectly. The particle morphology, particle size, and particle size distribution of cathode materials are of great importance to the performance of the battery. The particle size distribution of the 700 °C sample is more uniform than other samples because the main particle distribution of 700 °C powders is in a nanosized region, which affects its electrochemical performance, as shown in Figure 6; it can be seen that the sample synthesized at 700 °C yields the

highest specific capacity. As a result of that, we chose this sample to give further investigation in the following tests. Figure 7a is the morphology of the sample observed on SEM; some of the small particles congregated together to form some large secondary particles. TEM investigation was also conducted to examine the LFP-LVP/C particles; as shown in Figure 7b, you can see that some LFP-LVP grains less than 70 nm disperse in the carbon webs, which would inhibit the crystal growth of LFP-LVP during heat treatment and provide good electronic contact between the active particles. However, the agglomeration of several grains to from a particle, as the TEM shows, which is consistent with the results obtained from the particle size distribution analysis.17 To determine the properties of the material, further electrochemical properties were studied. Cyclic voltammetry (CV) measurements were performed on LFP-LVP and LiFePO4 electrodes at the scanning rate of 2 mV s-1 between 2.0 and 4.4 V, as shown in Figure 8. The lithium ions were extracted from the LixFePO4 or LixV2(PO4)3 structures during an anodic sweep and then inserted into the structure reversibly when scanning from 4.4 to 2.0 V. Redox couple peaks at 3.9 and 3.0 V were ascribed to the Fe2+/Fe3+ redox potential for LiFePO4 electrode. As for the LFP-LVP electrode, one V3+/V4+ potential, corresponding to the first Li+ extracted from LixV2(PO4)3 structure, was overlapped with the oxidation peaks of Fe2+/ Fe3+ for polarization. However, the polarization of the Fe2+/ Fe3 redox potential for the LFP-LVP electrode is improved compared with LiFePO4 electrode, implying that an improvement of conductivity for this LiFePO4-based compound. To provide more information for the improved electrochemical property, alternating current (AC) impedance measurements are performed on LFP-LVP and LiFePO4 electrodes at the same charge and discharge state (Figure 9). All spectra have a semicircle and a straight line in the high- and low-frequency

Studies of Li3V2(PO4)3 Additives

Figure 8. CV of LFP-LVP and LiFePO4 electrodes at a scanning rate of 2 mV s-1.

Figure 9. Nyquist plots of LFP-LVP and LiFePO4 electrodes at 4.4 V in the frequency range of 10 mHz to 100 kHz.

regions. The high-frequency semicircle is attributed to the charge-transfer resistance of electrochemical reaction and the line to the diffusion-controlled Warburg impedance.18,19 The diameter of the semicircle for the LiFePO4 material is larger than that for the LFP-LVP compound. It indicates that the charge-transfer resistance of LFP-LVP is much lower than that of the LiFePO4 material. So we can conclude that the material becomes more conductive with Li3V2(PO4)3 additives, which is also confirmed by the cell performance from Figure 10. It is clearly seen that the capacity of the LFP-LVP compound is higher than that of the LiFePO4 as cathode materials. The cathode materials were cycled with a rate of 1 C at room temperature. Rate capability is one of the important electrochemical characteristics of a lithium secondary battery required for power storage application. In the present study, rate capabilities are investigated at higher current densities for the LFP-LVP compound cathode material (Figure 11). For each cycling, the charge and discharge processes are performed at the same current densities between 2.0 and 4.4 V at room temperature. One can see that the capacities were stably retained up to 100 cycles, and almost no obvious capacity loss can be observed. An initial capacity of 129 and 123 mAh g-1 was obtained, and then the capacity increased to 134 and 124 mAh g-1 at the 20th cycle and remained so for 130 and 117 mAh g-1 up to 100 cycles for 3 and 5 C rates, respectively. What is more, the material shows excellent charge-discharge efficiency. For

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Figure 10. Cell performance of LFP-LVP and LiFePO4 electrodes with 1 C rate in the voltage of 2.0-4.4 V.

Figure 11. Cell performance of LFP-LVP electrodes at high rates. Insert is charge-discharge curves of LFP-LVP in the 20th cycle.

example, the charge-discharge efficiency is almost 100% at both rates in the 20th cycle, as the insert shows in Figure 11. Conclusions LiFePO4-based cathode materials with Li3V2(PO4)3 additives were synthesized using Fe(III) and V(V) as the raw material with PEG as the carbon source and reductive agent by a simple RPR method. The addition of Li3V2(PO4)3 does not induce new impurity phase except the peaks of LiFePO4 and Li3V2(PO4)3 present in the XRD patterns. The LFP-LVP compound with a mole ratio of 8:1 for LiFePO4:Li3V2(PO4)3 exhibits the highest discharge capacity. CV measurements show that the Li3V2(PO4)3 addition improves the polarization of the LFP-LVP compound. AC impedance measurements show that Li3V2(PO4)3 addition decreases the charge-transfer resistance of the LiFePO4-based cathode material. Excellent high-rate performance was also achieved with the LFP-LVP compound as cathode material. This should be due to the fact that Li3V2(PO4)3 is more electronically conductive than LiFePO4 for its open Nascion-type structure. Also the increase of the number of voltage plateaus and the mean intercalation voltage should be advantageous for improving of cell performance. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20071026). The

312 J. Phys. Chem. C, Vol. 112, No. 1, 2008 authors are grateful to the Centre of Nanoscience and Nanotechnology Research of Wuhan University for their experimental assistance. References and Notes (1) Wang, H.; Jang, Y.; Huang, B.; Sadoway, D. R.; Chang, Y. J. Electrochem. Soc. 1999, 146, 473. (2) Andersson, A. S.; Thomas, J. O. J. Power Sources 2001, 97-98, 498. (3) MacNeil, D. D.; Lu, Z. H.; Chen, Z. H.; Dahn, J. R. J. Power Sources 2002, 108, 8. (4) Yamada, A.; Chung, S.-C.; Hinokuma, K. J. Electrochem. Soc. 2001, 148, A224. (5) Xie, H. M.; Wang, R. S.; Ying, J. R.; Zhang, L. Y.; Jalbout, A. F.; Yu, H. Y.; Yang, G. L.; Pan, X. M.; Su, Z. M. AdV. Mater. 2006, 18, 2609. (6) Ellis, B.; Perry, L. K.; Ryan, D. H.; Nazar, L. F. J. Am. Chem. Soc. 2006, 128, 11416. (7) Wang, G. X.; Bewlay, S. L.; Konstantinov, K.; Liu, H. K.; Dou, S. X.; Ahn, J.-H. Electrochim. Acta 2004, 50, 443. (8) Chung, S.-Y.; Bloking, J. T.; Chiang, Y.-M. Nat. Mater. 2002, 1, 123.

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