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Synthesis and characterization of carbon-coated Li3V2(PO4)3 cathode materials with different carbon sources
Electrochimica Acta 54 (2009) 3374–3380
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Synthesis and characterization of carbon-coated Li3 V2 (PO4 )3 cathode materials with different carbon sources X.H. Rui, C. Li, C.H. Chen ∗ Key Laboratory of Materials for Energy Conversion of Academia Sinica, Department of Materials Science and Engineering, University of Science and Technology of China, Anhui Hefei 230026, China
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Article history: Received 2 December 2008 Received in revised form 6 January 2009 Accepted 7 January 2009 Available online 15 January 2009 Keywords: Lithium vanadium phosphates Lithium ion batteries Monoclinic Solid-state reaction Carbon coating
a b s t r a c t The carbon-coated monoclinic Li3 V2 (PO4 )3 (LVP) cathode materials were synthesized by a solid-state reaction process under the same conditions using citric acid, glucose, PVDF and starch, respectively, as both reduction agents and carbon coating sources. The carbon coating can enhance the conductivity of the composite materials and hinder the growth of Li3 V2 (PO4 )3 particles. Their structures and physicochemical properties were investigated using X-ray diffraction (XRD), thermogravimetric (TG), scanning electron microscopy (SEM) and electrochemical methods. In the voltage region of 3.0–4.3 V, the electrochemical cycling of these LVP/C electrodes all presents good rate capability and excellent cycle stability. It is found that the citric acid-derived LVP owns the largest reversible capacity of 118 mAh g−1 with no capacity fading during 100 cycles at the rate of 0.2C, and the PVDF-derived LVP possesses a capacity of 95 mAh g−1 even at the rate of 5C. While in the voltage region of 3.0–4.8 V, all samples exhibit a slightly poorer cycle performance with the capacity retention of about 86% after 50 cycles at the rate of 0.2C. The reasons for electrochemical performance of the carbon coated Li3 V2 (PO4 )3 composites are also discussed. The solidstate reaction is feasible for the preparation of the carbon coated Li3 V2 (PO4 )3 composites which can offer favorable properties for commercial applications. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries are currently considered to be the most efficient rechargeable energy storage systems for the increasing demands from portable electronic products, electrical vehicles (EVs), hybrid electrical vehicles (HEVs), etc. Recently, lithium transition metal phosphates such as LiMPO4 (M = Fe, Co, Ni, Mn) [1–8], Li3 M2 (PO4 )3 (M = Fe, V, Ti) [9–19] and LiVPO4 F [20] have been proposed as highly promising cathode materials for secondary lithium batteries. This is because that they possess competitive energy densities and better thermal stabilities than the oxidative unstable lithium transition metal oxides such as layer-structured LiCoO2 and LiNiO2 , spinel-type LiMn2 O4 and their substitutional derivative compounds [21–24]. Moreover, the phosphate-based materials exhibit higher redox potentials than their metal oxides counterparts through an inductive effect generated by the poly-anion, which lowers the energy of the transition metal redox couples [1,25,26]. Among these compounds, lithium vanadium phosphate has attracted considerable interest as the cathode material. Li3 V2 (PO4 )3
∗ Corresponding author. Tel.: +86 551 3606971; fax: +86 551 3601592. E-mail address: [email protected] (C.H. Chen). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.01.011
(LVP) crystallizes in two different frameworks: the rhombohedral (NASICON) phase [27] and the thermodynamically more stable monoclinic phase, which is isotypic with the ␣-Li3 Fe2 (PO4 )3 [12]. The monoclinic Li3 V2 (PO4 )3 (space group P21 /n) structure comprises a three-dimensional network of metal octahedral and phosphate tetrahedral sharing oxygen vertices, where all three Li are mobile. It can extract and insert two lithium ions reversibly between 3.0 and 4.3 V based on the V3+ /V4+ redox couple. When charged to 4.8 V, all three lithium ions can be completely extracted, corresponding to a theoretical capacity of 197 mAh g−1 . It is believed that ␣-Li3 V2 (PO4 )3 undergoes a complex series of twophase transitions during lithium extraction and a solid solution regime on lithium reinsertion [12]. However, Li3 V2 (PO4 )3 has poor conductivity, which restricts its practical application for lithium ion batteries. Fortunately, the problem can be largely solved by coating the Li3 V2 (PO4 )3 particles with a thin carbon layer [13,14,16–19]. The carbon coating is usually realized by introducing an organic precursor in the starting materials, such as citric acid [16,17], sucrose [18], humic acid [19] and phenolic resin [14]. The organic precursors can be converted into electronically conductive carbon through pyrolysis processes at high temperatures under inert atmospheres. The carbon may coat on the LVP particles to form a conducting LVP/C composite electrode material. On the other hand, the oxidation state of vanadium in the vanadium-precursors such as V2 O5 or NH4 VO3 is usually +5. The carbon pyrolyzed from the
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organic precursors should also act as a reductant in the synthesis of LVP, in which the oxidation state of V is +3. Thus, the synthesis is in fact a carbon thermal reduction method. In this paper, the carbon coated Li3 V2 (PO4 )3 (LVP/C) composites have been synthesized by the carbon thermal reduction method using four different organic precursors, i.e. citric acid, glucose, poly(vinylidene difluoride) (PVDF) and starch, respectively. Therefore, four LVP/C are compared systematically under the same heat treatment and characterization conditions. By this way, appropriate carbon sources for high energy LVP and high power LVP can be identified. 2. Experimental The monoclinic LVP/C composites were synthesized by a carbon thermal reduction method, using the solid-state reaction of Li2 CO3 , V2 O5 , NH4 H2 PO4 as well as four different carbon sources (citric acid, glucose, PVDF and starch). The reactants had the same molar ratio Li:V:P:C = 3:2:3:4.8. They were dispersed into acetone and then ball milled for several hours. The obtained mixtures were dried in an oven at 70 ◦ C to evaporate acetone. Then they were initially heated to 350 ◦ C in N2 atmosphere for 5 h to expel H2 O and NH3 . The precursors were then ground and sintered at 750 ◦ C for 16 h under flowing N2 to yield the LVP/C composites. Structural analyses of the samples were performed by the Xray diffraction (XRD) using a rotating anode X-ray diffractometer (MXPAHF, Cu K␣ radiation). The diffraction patterns were recorded at room temperature in the 2 range from 10◦ to 60◦ . In order to measure the residual carbon content of LVP/C composites, thermogravimetric analysis (TGA) of them was conducted in air at a heating rate of 10 ◦ C/min using a thermal analyzer (Shimadzu DT50). The morphology of the powders was observed under a scanning electron microscope (SEM, Hitachi X-650). The Li3 V2 (PO4 )3 electrodes were prepared by mixing 80 wt% active materials, 10 wt% carbon black and 10 wt% PVDF in NMP to ensure homogeneity. The resulting slurry was coated on aluminum foil, dried at 70 ◦ C. The coin-cells (CR2032 size) were assembled in an argon-filled glove box with the LVP/C as cathode, Li metal as anode and 1 M LiPF6 in EC:DEC (1:1, w/w) as the electrolyte. The cells were tested on a multi-channel battery test system (NEWARE BTS-610) with galvanostatic charge and discharge in the voltage range of 3.0–4.3 V and 3.0–4.8 V. The cyclic voltammogram (CV) and AC impedance measurements were performed with a CHI 604B electrochemical workstation. The intermittent current interruption method was also introduced to measure to the direct current (DC) resistance. During the second cycle, the current was interrupted intermittently for 1 min to record the voltage change (U) before and after interruption. Thus, the DC resistance of a certain state-of-charge (SOC) or depthof-discharge (DOD) can be calculated as U/I.
Fig. 1. X-ray diffraction patterns of LVP/C composites synthesized with (a) citric acid, (b) glucose, (c) PVDF and (d) starch.
Table 1 Lattice parameters of the samples. Samples
a (Å)
b (Å)
c (Å)
ˇ (◦ )
Cell volume (Å3 )
Citric acid Glucose PVDF Starch
8.570(7) 8.605(6) 8.556(5) 8.594(3)
8.577(3) 8.521(5) 8.569(6) 8.536(8)
11.946(7) 11.970(8) 11.967(3) 11.956(4)
90.245(0) 89.867(7) 90.565(6) 89.909(7)
878.23(6) 877.85(3) 877.46(8) 877.21(3)
usually higher sintering temperature (800–900 ◦ C). Obviously, the residual carbon content (see Fig. 2 below) has little effect on the lattice parameters (Table 1). It is necessary to know the residual carbon content of LVP/C composites due to its effect on the properties. Here, the residual carbon content was accurately measured through their thermogravimetric (TG) curves measured in air, which is shown in Fig. 2. We can clearly see that the LVP/C composites initially undergo a weight loss process, which is attributed to the burnout of the residual carbon. The following weight gain step is due to the decomposition of Li3 V2 (PO4 )3 , corresponding to the oxidation of V3+ in air. According to the weight loss step on the TG curves, the carbon content in the LVP/C composites is 1.33%, 13.27%, 12.68% and 10.46% for the samples from the carbon sources citric acid, glucose, PVDF and starch, respectively. Obviously, the carbon content in the composite synthesized with citric acid is much lower than the other three samples. This is because that the existence of car-
3. Results and discussion The X-ray diffraction patterns of the LVP/C composites synthesized with different carbon sources are shown in Fig. 1. It can be seen that the experiments with all four carbon sources have produced a single phase of Li3 V2 (PO4 )3 with monoclinic structure. No carbonrelated diffraction peak is detected, indicating that the residual carbon is amorphous. In addition, the lattice parameters were calculated by the Unit Cell software using the least-square method, which is shown in Table 1. The cell volume of all four samples is close to each other within experimental errors although it is a little smaller than that reported in the literature [12,19]. This may be due to the low sintering temperature (750 ◦ C) compared with the
Fig. 2. TG curves of LVP/C composites synthesized with different carbon sources.
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Fig. 3. SEM images of the LVP/C composites synthesized with (a) citric acid, (b) glucose, (c) PVDF and (d) starch.
boxyl in the citric acid molecule can decompose to form CO2 instead of contributing to carbon residual during the heating process of synthesis. Fig. 3 shows the SEM images of the LVP/C composites. The particles of the sample synthesized with citric acid (Fig. 3a) have a granular shape with an average of ∼1 m, some of which are interconnected to form a porous structure. When the carbon source changes to glucose (Fig. 3b), the average particle size of the LVP/C powder is also about 1 m, but some large particles exist. As seen from Fig. 3c, when PVDF is chosen as the carbon source, the morphology is very different from the other three samples. Many nano-sized Li3 V2 (PO4 )3 particles are embedded in a continuous carbon network, and some particles with hollow ball and debrislike shapes are adhered to the surface of that. From Fig. 3d, we can clearly see that when starch is selected as the carbon source, the morphology is much similar to that of glucose; the average particle size is around 1 m and some large particles exist. This is easily understood because starch molecule, (C6 H10 O5 )n , is a polysaccharide carbohydrate consisting of a large number of glucose monosaccharide units joined together by glycosidic bonds. From the above we can see that the average particle size of all the samples was roughly comparable. Fig. 4 shows the initial charge-discharge curves and cycling performance of LVP/C samples at the 0.2C rate between 3.0 and 4.8 V. There are four plateaus in the charge curves around 3.62, 3.70, 4.10 and 4.56 V, which correspond to a sequence of phase tran-
sition processes between the single phases Lix V2 (PO4 )3 (x = 3.0, 2.5, 2.0, 1.0 and 0). In the charge process, the first lithium ion is extracted in two steps, 3.62 and 3.70 V, which is because of the presence of an ordered phase Li2.5 V2 (PO4 )3. Subsequently, the second lithium ion is extracted in one single plateau (4.10 V) to form Li1.0 V2 (PO4 )3 phase, which preserves the monoclinic symmetry of the lattice [12]. These three plateaus in total correspond to two lithium ions extraction associated with the V3+ /V4+ redox couple. Finally, the third lithium ion is removed (plateau at 4.56 V) to form V2 (PO4 )3 , which generates a mixed V4+ /V5+ state. As can be seen from Fig. 4a, the last plateau has a significant overvoltage by comparing the charge and discharge curves, manifesting the higher energetic needed in extracting the last lithium ion and the existence of irreversibility during charge–discharge processes. On the discharge curves, three plateaus around 3.54, 3.61, 4.01 V exist involving two steps, corresponding to reversible reinsertion of all three lithium ions. The first step consists of the reinsertion of two lithium ions, which exhibits an electrochemical signature of solid solution behavior [12]. Following two electrochemical plateaus corresponding to the reinsertion of the last lithium ion is the second step, exhibiting the characteristic of two-phase behavior, Li2 V2 (PO4 )3 → Li2.5 V2 (PO4 )3 → Li3 V2 (PO4 )3 . As seen in Fig. 4a, LVP/C composites have an initial discharge capacity of 166.7, 158.8, 152.9 and 152.2 mAh g−1 with carbon sources of citric acid, glucose, PVDF and starch, which Coulombic efficiency is 97.5%, 90.2%, 91.7% and 93.7%, respectively. Of all
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Fig. 4. The initial charge-discharge curves (a) and cycling performance (b) of the LVP/C samples at the 0.2C rate between 3.0 and 4.8 V.
the four samples, the initial discharge capacity is the highest for the citric acid-sample, the second for the glucose-sample, and the capacity of the other two samples is nearly same. This trend is kept during a long-term cycling (Fig. 4b). After 50 cycles, the discharge capacity reaches 144.7, 135.7, 130.0 and 130.8 mAh g−1 , respectively, is indicating that their capacity retention is 86.8%, 85.5%, 85.0% and 85.9% of the initial discharge capacity. We believe that the morphology and specific surface area of particles have the notable effect on its electrochemical performance. In this paper, from XRD and SEM analysis, we have found that the particle size of all four samples is very close to each other, indicating that their specific surface is roughly the same. Thus, why the citric acid-sample has the highest discharge capacity is because of its lowest residual carbon content, meaning that it owns the most active material LVP. It is also because that when the discharge current density (0.2C) is low, the residual carbon content (1.33–13.27%) has little effect on its cycle performance. Furthermore, all four samples have almost the same low capacity retention (about 86%), which may be due to the increasing side reactions and impedance during the high-voltage charge/discharge cycles. It should be pointed out that the specific capacity data shown in Fig. 4 are based on the mass of the LVP with the carbon that was coated on the particles. If considering the mass of LVP alone, the initial discharge capacity of the citric acid-, glucose, PVDF- and starch-samples is actually 169.0, 183.1, 175.1 and 170.0 mAh g−1 , respectively. Thus, the discharge capacity of the glucose-sample (183.1 mAh g−1 ) is very close to its theoretical capacity (197 mAh g−1 ). Also, the achievable capacity increases with the content of the residual carbon. For example, the glucose-sample
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Fig. 5. The rate performance (a) and AC impedance spectra (b) of the LVP/C composites measured at the voltage range of 3.0–4.8 V.
with the highest residual carbon exhibits the highest discharge capacity. Their rate performance is shown in Fig. 5a. We can obviously find that with increasing the rate of discharge current, the discharge capacity decreases rapidly. Of all four samples, the lowest capacity fading is PVDF-sample, the next is glucose-sample, and the capacity of other two samples attenuates very fast especially at the rate of 5C. To understand the difference of their rate performance, AC impedance measurements were performed (Fig. 5b). Firstly, the cell was cycled galvanostatically between 3.0 and 4.8 V for five cycles, which ensures the formation of stable SEI films and percolation of electrode particles by the electrolyte. Then, AC impedance was measured at the fully charged state. As shown in Fig. 5b, there are three regions, two semicircles and an inclined line. The high-frequency semicircle is attributed to the migration of Li+ ions through the SEI film, whereas, the high-middle frequency semicircle represents the charge–transfer process. The low frequency region of the inclined line corresponds to the diffusion of Li+ ions in the bulk of the electrode material. We can clearly see that the charge–transfer impedance for the PVDF-sample is much lower than the other three samples, which is well consistent with its good rate performance measured above. It is because that the PVDF-sample possesses a uniform carbon network with Li3 V2 (PO4 )3 particles imbedded (Fig. 3c), which can largely improve its electronic conductivity. From Fig. 4, we can find that the citric acid-sample has a poor rate performance and the largest charge–transfer impedance, which is attributed to its lowest residual carbon content (1.33%). Although glucose- and starch-samples own roughly same amount of the residual carbon content with PVDF-sample, their impedance
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Fig. 6. The initial charge–discharge curves (a), cycling (b) and rate (c) performance of the LVP/C samples at the 0.2C rate between 3.0 and 4.3 V.
is larger, possibly because of their uneven distribution of the carbon and the existence of some large particles (Fig. 3). Fig. 6 shows the initial charge-discharge curves, cycling and rate performances of the LVP/C samples at the 0.2C rate between 3.0 and 4.3 V. As shown in Fig. 6a, when the upper cut-off voltage drops to 4.3 V, the cell exhibits three charge plateaus and correspondingly three discharge plateaus, which is considered as the two-phase transition processes during the electrochemical reactions. The initial discharge capacity for samples with the carbon sources citric acid, glucose, PVDF and starch is 114.6, 104.5, 107.4 and 107.7 mAh g−1 , respectively. The excellent cycling performances of all four samples are displayed in Fig. 6b, where almost no capacity fading is measured for 100 cycles. As shown in Fig. 6c, the rate performances of samples are a little better than those when cycled in the voltage range of 3.0–4.8 V. We can find that the rate performance of the PVDF-sample is best, which only has a little capacity drop when the rate changes from 0.5C to 5C; the citric acid and glucose-samples are a little poorer, whereas, the capacity of the starch-sample decreases fast with increasing current. Fig. 7 shows CV curves for the citric acid-sample at 0.05 mV s−1 in the voltage range of 3.0–4.3 V and 3.0–4.8 V. In the voltage of 3.0–4.3 V, there are three oxidation peaks around 3.68, 3.74, 4.15 V and three reduction peaks around 3.52, 3.60, 3.97 V, which is in good agreement with the above initial charge–discharge curves (Fig. 6a). The symmetrical feature of the CV data (curve a in Fig. 7) demonstrates a good reversibility of the cycle processes between 3.0 and 4.3 V. From curve b in Fig. 7, four oxidation peaks and three reduction peaks appear in the voltage range of 3.0–4.8 V, which corresponds to the plateaus of the initial charge–discharge
curves (Fig. 3a) and verifies the electrochemical reactions mechanism during cycles. The fourth oxidation peak is located around 4.59 V, associated with the V4+ /V5+ redox couple. An inclining curve is measured in the initial discharge process (Fig. 3a) representing the solid solution behaviour, consistent with the low and wide reduction peak at 3.9 V on curve (b) in Fig. 7. The DC resistance of the carbon-coated-Li3 V2 (PO4 )3 /Li cells as a function of voltage (3.0–4.3 V) measured during the 2nd cycle with a simultaneous current interruption method is shown in Fig. 8. Obvi-
Fig. 7. CV curves for the citric acid-sample at 0.05 mV s−1 in the voltage range of 3.0–4.3 V (a) and 3.0–4.8 V (b).
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Fig. 9. The DC resistance of the citric acid-sample as a function of voltage (3.0–4.8 V) in the 2nd charge process measured with a simultaneous current interruption method. Inset is the 2nd discharge process.
with the DC resistance results in the voltage ranger of 3.0–4.3 V, we can conclude that the DC resistance cannot only be used as a auxiliary tool to study the reversibility and mechanism of the electrochemical reaction, but also can infer the degree of difficulty of extraction and re-insertion of lithium ions during various steps of the charge–discharge cycle. As shown in Fig. 9, we can find that the resistance of extraction of the third lithium ion is a little larger, meaning that higher energetics is needed, which can bring about irreversible capacity loss. 4. Conclusion Fig. 8. The DC resistance of the carbon-coated-Li3 V2 (PO4 )3 /Li cells as a function of voltage (3.0–4.3 V) measured with a simultaneous current interruption method. (a) In the 2nd charge process and (b) in the 2nd discharge process.
ously, we can see three minimum resistance values around 3.6, 3.7, 4.1 V during the charge process (Fig. 8a) as well as around 3.55, 3.65, 4.05 V during the discharge process (Fig. 8b), which agrees well with the three charge/discharge plateaus and the cyclic voltammograms in the voltage of 3.0–4.3 V. Between two neighboring minimums, the resistance is markedly higher, which is corresponding to a twophase transition process. Among all the cells, the one with the glucose-sample shows the highest minimum resistance values during both the charge and discharge cycles, which must be the reason why it has the poorest rate performance (Fig. 5a and Fig. 6c). As for the other three samples, the resistance values are close to each other, which is also roughly in line with their rate performances. This result seems contradictory with the content of residual carbon in these samples. Nevertheless, it is understandable if we consider both the content and structure of carbon coating. A uniform carbon coating layer and a higher carbon content should improve its electronic conductivity. On the other hand, a thick carbon layer may hinder the transport rate for lithium ions. To compromise the electronic and ionic conductivities, an optimal carbon-content should exist. For the citric acid-sample, although its residual carbon content is the lowest, the diffusion of Li+ could be improved largely. Hence, its resistance is close to other samples. Fig. 9 shows the DC resistance of the citric acid-sample as a function of voltage (3.0–4.8 V) during the 2nd cycle measured with a simultaneous current interruption method. Also, four minimum resistance values 235, 206, 168 and 267 around 3.6, 3.7, 4.1 and 4.55 V can be observed during the charge process, and in the discharge process there are three minimum resistance values 263, 214 and 194 around 3.56, 3.64 and 4.0 V, respectively. Combined
The carbon-coated Li3 V2 (PO4 )3 (LVP) cathode materials have been successfully synthesized by the carbon thermal reduction method using citric acid, glucose, PVDF and starch as different carbon sources at a low temperature of 750 ◦ C. X-ray diffraction results show a pure single phase with monoclinic structure (space group P21 /n) in these samples. Of all powder samples, LVP particles with a particle size of about 1 m are covered by a carbon layer, and the citric acid-sample has the lowest carbon content (1.33%). In the voltage region of 3.0–4.3 V, the LVP/C electrodes present a fine reversibility, good rate capability and excellent cycle stability with no capacity fading even after 100 cycles. While in the voltage region of 3.0–4.8 V, the extraction of the third lithium ion (V4+ /V5+ ) taking place at 4.6 V appears to be energetically unfavorable and, hence displays a large overvoltage. All samples in this wide voltage range exhibit a slightly poorer cycle performance with the capacity retention of about 86% after 50 cycles, and a not very good rate capability. Of all four samples cycled in the two voltage regions, the citric acid-sample owns a higher capacity at the rate of 0.2C during the charge–discharge cycle, and the PVDF-sample displays the best rate performance. Based on the SEM images, the AC impendence and DC resistance of LVP/C composites, the higher capacity and good rate performance are owing to the coexistence of the uniform small particles and the carbon network, which improves their electronic conductivity leading to a lower resistance. Combined with a good electrochemical performance and ease of synthesis, lithium vanadium phosphate can be a very promising cathode material for higher energy density and power demanding lithium-ion batteries. Acknowledgment This study was supported by National Science Foundation of China (grant no. 20471057).
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Synthesis and characterization of carbon-coated Li3 V2 (PO4 )3 cathode materials with different carbon sources X.H. Rui, C. Li, C.H. Chen ∗ Key Laboratory of Materials for Energy Conversion of Academia Sinica, Department of Materials Science and Engineering, University of Science and Technology of China, Anhui Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 2 December 2008 Received in revised form 6 January 2009 Accepted 7 January 2009 Available online 15 January 2009 Keywords: Lithium vanadium phosphates Lithium ion batteries Monoclinic Solid-state reaction Carbon coating
a b s t r a c t The carbon-coated monoclinic Li3 V2 (PO4 )3 (LVP) cathode materials were synthesized by a solid-state reaction process under the same conditions using citric acid, glucose, PVDF and starch, respectively, as both reduction agents and carbon coating sources. The carbon coating can enhance the conductivity of the composite materials and hinder the growth of Li3 V2 (PO4 )3 particles. Their structures and physicochemical properties were investigated using X-ray diffraction (XRD), thermogravimetric (TG), scanning electron microscopy (SEM) and electrochemical methods. In the voltage region of 3.0–4.3 V, the electrochemical cycling of these LVP/C electrodes all presents good rate capability and excellent cycle stability. It is found that the citric acid-derived LVP owns the largest reversible capacity of 118 mAh g−1 with no capacity fading during 100 cycles at the rate of 0.2C, and the PVDF-derived LVP possesses a capacity of 95 mAh g−1 even at the rate of 5C. While in the voltage region of 3.0–4.8 V, all samples exhibit a slightly poorer cycle performance with the capacity retention of about 86% after 50 cycles at the rate of 0.2C. The reasons for electrochemical performance of the carbon coated Li3 V2 (PO4 )3 composites are also discussed. The solidstate reaction is feasible for the preparation of the carbon coated Li3 V2 (PO4 )3 composites which can offer favorable properties for commercial applications. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries are currently considered to be the most efficient rechargeable energy storage systems for the increasing demands from portable electronic products, electrical vehicles (EVs), hybrid electrical vehicles (HEVs), etc. Recently, lithium transition metal phosphates such as LiMPO4 (M = Fe, Co, Ni, Mn) [1–8], Li3 M2 (PO4 )3 (M = Fe, V, Ti) [9–19] and LiVPO4 F [20] have been proposed as highly promising cathode materials for secondary lithium batteries. This is because that they possess competitive energy densities and better thermal stabilities than the oxidative unstable lithium transition metal oxides such as layer-structured LiCoO2 and LiNiO2 , spinel-type LiMn2 O4 and their substitutional derivative compounds [21–24]. Moreover, the phosphate-based materials exhibit higher redox potentials than their metal oxides counterparts through an inductive effect generated by the poly-anion, which lowers the energy of the transition metal redox couples [1,25,26]. Among these compounds, lithium vanadium phosphate has attracted considerable interest as the cathode material. Li3 V2 (PO4 )3
∗ Corresponding author. Tel.: +86 551 3606971; fax: +86 551 3601592. E-mail address: [email protected] (C.H. Chen). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.01.011
(LVP) crystallizes in two different frameworks: the rhombohedral (NASICON) phase [27] and the thermodynamically more stable monoclinic phase, which is isotypic with the ␣-Li3 Fe2 (PO4 )3 [12]. The monoclinic Li3 V2 (PO4 )3 (space group P21 /n) structure comprises a three-dimensional network of metal octahedral and phosphate tetrahedral sharing oxygen vertices, where all three Li are mobile. It can extract and insert two lithium ions reversibly between 3.0 and 4.3 V based on the V3+ /V4+ redox couple. When charged to 4.8 V, all three lithium ions can be completely extracted, corresponding to a theoretical capacity of 197 mAh g−1 . It is believed that ␣-Li3 V2 (PO4 )3 undergoes a complex series of twophase transitions during lithium extraction and a solid solution regime on lithium reinsertion [12]. However, Li3 V2 (PO4 )3 has poor conductivity, which restricts its practical application for lithium ion batteries. Fortunately, the problem can be largely solved by coating the Li3 V2 (PO4 )3 particles with a thin carbon layer [13,14,16–19]. The carbon coating is usually realized by introducing an organic precursor in the starting materials, such as citric acid [16,17], sucrose [18], humic acid [19] and phenolic resin [14]. The organic precursors can be converted into electronically conductive carbon through pyrolysis processes at high temperatures under inert atmospheres. The carbon may coat on the LVP particles to form a conducting LVP/C composite electrode material. On the other hand, the oxidation state of vanadium in the vanadium-precursors such as V2 O5 or NH4 VO3 is usually +5. The carbon pyrolyzed from the
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organic precursors should also act as a reductant in the synthesis of LVP, in which the oxidation state of V is +3. Thus, the synthesis is in fact a carbon thermal reduction method. In this paper, the carbon coated Li3 V2 (PO4 )3 (LVP/C) composites have been synthesized by the carbon thermal reduction method using four different organic precursors, i.e. citric acid, glucose, poly(vinylidene difluoride) (PVDF) and starch, respectively. Therefore, four LVP/C are compared systematically under the same heat treatment and characterization conditions. By this way, appropriate carbon sources for high energy LVP and high power LVP can be identified. 2. Experimental The monoclinic LVP/C composites were synthesized by a carbon thermal reduction method, using the solid-state reaction of Li2 CO3 , V2 O5 , NH4 H2 PO4 as well as four different carbon sources (citric acid, glucose, PVDF and starch). The reactants had the same molar ratio Li:V:P:C = 3:2:3:4.8. They were dispersed into acetone and then ball milled for several hours. The obtained mixtures were dried in an oven at 70 ◦ C to evaporate acetone. Then they were initially heated to 350 ◦ C in N2 atmosphere for 5 h to expel H2 O and NH3 . The precursors were then ground and sintered at 750 ◦ C for 16 h under flowing N2 to yield the LVP/C composites. Structural analyses of the samples were performed by the Xray diffraction (XRD) using a rotating anode X-ray diffractometer (MXPAHF, Cu K␣ radiation). The diffraction patterns were recorded at room temperature in the 2 range from 10◦ to 60◦ . In order to measure the residual carbon content of LVP/C composites, thermogravimetric analysis (TGA) of them was conducted in air at a heating rate of 10 ◦ C/min using a thermal analyzer (Shimadzu DT50). The morphology of the powders was observed under a scanning electron microscope (SEM, Hitachi X-650). The Li3 V2 (PO4 )3 electrodes were prepared by mixing 80 wt% active materials, 10 wt% carbon black and 10 wt% PVDF in NMP to ensure homogeneity. The resulting slurry was coated on aluminum foil, dried at 70 ◦ C. The coin-cells (CR2032 size) were assembled in an argon-filled glove box with the LVP/C as cathode, Li metal as anode and 1 M LiPF6 in EC:DEC (1:1, w/w) as the electrolyte. The cells were tested on a multi-channel battery test system (NEWARE BTS-610) with galvanostatic charge and discharge in the voltage range of 3.0–4.3 V and 3.0–4.8 V. The cyclic voltammogram (CV) and AC impedance measurements were performed with a CHI 604B electrochemical workstation. The intermittent current interruption method was also introduced to measure to the direct current (DC) resistance. During the second cycle, the current was interrupted intermittently for 1 min to record the voltage change (U) before and after interruption. Thus, the DC resistance of a certain state-of-charge (SOC) or depthof-discharge (DOD) can be calculated as U/I.
Fig. 1. X-ray diffraction patterns of LVP/C composites synthesized with (a) citric acid, (b) glucose, (c) PVDF and (d) starch.
Table 1 Lattice parameters of the samples. Samples
a (Å)
b (Å)
c (Å)
ˇ (◦ )
Cell volume (Å3 )
Citric acid Glucose PVDF Starch
8.570(7) 8.605(6) 8.556(5) 8.594(3)
8.577(3) 8.521(5) 8.569(6) 8.536(8)
11.946(7) 11.970(8) 11.967(3) 11.956(4)
90.245(0) 89.867(7) 90.565(6) 89.909(7)
878.23(6) 877.85(3) 877.46(8) 877.21(3)
usually higher sintering temperature (800–900 ◦ C). Obviously, the residual carbon content (see Fig. 2 below) has little effect on the lattice parameters (Table 1). It is necessary to know the residual carbon content of LVP/C composites due to its effect on the properties. Here, the residual carbon content was accurately measured through their thermogravimetric (TG) curves measured in air, which is shown in Fig. 2. We can clearly see that the LVP/C composites initially undergo a weight loss process, which is attributed to the burnout of the residual carbon. The following weight gain step is due to the decomposition of Li3 V2 (PO4 )3 , corresponding to the oxidation of V3+ in air. According to the weight loss step on the TG curves, the carbon content in the LVP/C composites is 1.33%, 13.27%, 12.68% and 10.46% for the samples from the carbon sources citric acid, glucose, PVDF and starch, respectively. Obviously, the carbon content in the composite synthesized with citric acid is much lower than the other three samples. This is because that the existence of car-
3. Results and discussion The X-ray diffraction patterns of the LVP/C composites synthesized with different carbon sources are shown in Fig. 1. It can be seen that the experiments with all four carbon sources have produced a single phase of Li3 V2 (PO4 )3 with monoclinic structure. No carbonrelated diffraction peak is detected, indicating that the residual carbon is amorphous. In addition, the lattice parameters were calculated by the Unit Cell software using the least-square method, which is shown in Table 1. The cell volume of all four samples is close to each other within experimental errors although it is a little smaller than that reported in the literature [12,19]. This may be due to the low sintering temperature (750 ◦ C) compared with the
Fig. 2. TG curves of LVP/C composites synthesized with different carbon sources.
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Fig. 3. SEM images of the LVP/C composites synthesized with (a) citric acid, (b) glucose, (c) PVDF and (d) starch.
boxyl in the citric acid molecule can decompose to form CO2 instead of contributing to carbon residual during the heating process of synthesis. Fig. 3 shows the SEM images of the LVP/C composites. The particles of the sample synthesized with citric acid (Fig. 3a) have a granular shape with an average of ∼1 m, some of which are interconnected to form a porous structure. When the carbon source changes to glucose (Fig. 3b), the average particle size of the LVP/C powder is also about 1 m, but some large particles exist. As seen from Fig. 3c, when PVDF is chosen as the carbon source, the morphology is very different from the other three samples. Many nano-sized Li3 V2 (PO4 )3 particles are embedded in a continuous carbon network, and some particles with hollow ball and debrislike shapes are adhered to the surface of that. From Fig. 3d, we can clearly see that when starch is selected as the carbon source, the morphology is much similar to that of glucose; the average particle size is around 1 m and some large particles exist. This is easily understood because starch molecule, (C6 H10 O5 )n , is a polysaccharide carbohydrate consisting of a large number of glucose monosaccharide units joined together by glycosidic bonds. From the above we can see that the average particle size of all the samples was roughly comparable. Fig. 4 shows the initial charge-discharge curves and cycling performance of LVP/C samples at the 0.2C rate between 3.0 and 4.8 V. There are four plateaus in the charge curves around 3.62, 3.70, 4.10 and 4.56 V, which correspond to a sequence of phase tran-
sition processes between the single phases Lix V2 (PO4 )3 (x = 3.0, 2.5, 2.0, 1.0 and 0). In the charge process, the first lithium ion is extracted in two steps, 3.62 and 3.70 V, which is because of the presence of an ordered phase Li2.5 V2 (PO4 )3. Subsequently, the second lithium ion is extracted in one single plateau (4.10 V) to form Li1.0 V2 (PO4 )3 phase, which preserves the monoclinic symmetry of the lattice [12]. These three plateaus in total correspond to two lithium ions extraction associated with the V3+ /V4+ redox couple. Finally, the third lithium ion is removed (plateau at 4.56 V) to form V2 (PO4 )3 , which generates a mixed V4+ /V5+ state. As can be seen from Fig. 4a, the last plateau has a significant overvoltage by comparing the charge and discharge curves, manifesting the higher energetic needed in extracting the last lithium ion and the existence of irreversibility during charge–discharge processes. On the discharge curves, three plateaus around 3.54, 3.61, 4.01 V exist involving two steps, corresponding to reversible reinsertion of all three lithium ions. The first step consists of the reinsertion of two lithium ions, which exhibits an electrochemical signature of solid solution behavior [12]. Following two electrochemical plateaus corresponding to the reinsertion of the last lithium ion is the second step, exhibiting the characteristic of two-phase behavior, Li2 V2 (PO4 )3 → Li2.5 V2 (PO4 )3 → Li3 V2 (PO4 )3 . As seen in Fig. 4a, LVP/C composites have an initial discharge capacity of 166.7, 158.8, 152.9 and 152.2 mAh g−1 with carbon sources of citric acid, glucose, PVDF and starch, which Coulombic efficiency is 97.5%, 90.2%, 91.7% and 93.7%, respectively. Of all
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Fig. 4. The initial charge-discharge curves (a) and cycling performance (b) of the LVP/C samples at the 0.2C rate between 3.0 and 4.8 V.
the four samples, the initial discharge capacity is the highest for the citric acid-sample, the second for the glucose-sample, and the capacity of the other two samples is nearly same. This trend is kept during a long-term cycling (Fig. 4b). After 50 cycles, the discharge capacity reaches 144.7, 135.7, 130.0 and 130.8 mAh g−1 , respectively, is indicating that their capacity retention is 86.8%, 85.5%, 85.0% and 85.9% of the initial discharge capacity. We believe that the morphology and specific surface area of particles have the notable effect on its electrochemical performance. In this paper, from XRD and SEM analysis, we have found that the particle size of all four samples is very close to each other, indicating that their specific surface is roughly the same. Thus, why the citric acid-sample has the highest discharge capacity is because of its lowest residual carbon content, meaning that it owns the most active material LVP. It is also because that when the discharge current density (0.2C) is low, the residual carbon content (1.33–13.27%) has little effect on its cycle performance. Furthermore, all four samples have almost the same low capacity retention (about 86%), which may be due to the increasing side reactions and impedance during the high-voltage charge/discharge cycles. It should be pointed out that the specific capacity data shown in Fig. 4 are based on the mass of the LVP with the carbon that was coated on the particles. If considering the mass of LVP alone, the initial discharge capacity of the citric acid-, glucose, PVDF- and starch-samples is actually 169.0, 183.1, 175.1 and 170.0 mAh g−1 , respectively. Thus, the discharge capacity of the glucose-sample (183.1 mAh g−1 ) is very close to its theoretical capacity (197 mAh g−1 ). Also, the achievable capacity increases with the content of the residual carbon. For example, the glucose-sample
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Fig. 5. The rate performance (a) and AC impedance spectra (b) of the LVP/C composites measured at the voltage range of 3.0–4.8 V.
with the highest residual carbon exhibits the highest discharge capacity. Their rate performance is shown in Fig. 5a. We can obviously find that with increasing the rate of discharge current, the discharge capacity decreases rapidly. Of all four samples, the lowest capacity fading is PVDF-sample, the next is glucose-sample, and the capacity of other two samples attenuates very fast especially at the rate of 5C. To understand the difference of their rate performance, AC impedance measurements were performed (Fig. 5b). Firstly, the cell was cycled galvanostatically between 3.0 and 4.8 V for five cycles, which ensures the formation of stable SEI films and percolation of electrode particles by the electrolyte. Then, AC impedance was measured at the fully charged state. As shown in Fig. 5b, there are three regions, two semicircles and an inclined line. The high-frequency semicircle is attributed to the migration of Li+ ions through the SEI film, whereas, the high-middle frequency semicircle represents the charge–transfer process. The low frequency region of the inclined line corresponds to the diffusion of Li+ ions in the bulk of the electrode material. We can clearly see that the charge–transfer impedance for the PVDF-sample is much lower than the other three samples, which is well consistent with its good rate performance measured above. It is because that the PVDF-sample possesses a uniform carbon network with Li3 V2 (PO4 )3 particles imbedded (Fig. 3c), which can largely improve its electronic conductivity. From Fig. 4, we can find that the citric acid-sample has a poor rate performance and the largest charge–transfer impedance, which is attributed to its lowest residual carbon content (1.33%). Although glucose- and starch-samples own roughly same amount of the residual carbon content with PVDF-sample, their impedance
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Fig. 6. The initial charge–discharge curves (a), cycling (b) and rate (c) performance of the LVP/C samples at the 0.2C rate between 3.0 and 4.3 V.
is larger, possibly because of their uneven distribution of the carbon and the existence of some large particles (Fig. 3). Fig. 6 shows the initial charge-discharge curves, cycling and rate performances of the LVP/C samples at the 0.2C rate between 3.0 and 4.3 V. As shown in Fig. 6a, when the upper cut-off voltage drops to 4.3 V, the cell exhibits three charge plateaus and correspondingly three discharge plateaus, which is considered as the two-phase transition processes during the electrochemical reactions. The initial discharge capacity for samples with the carbon sources citric acid, glucose, PVDF and starch is 114.6, 104.5, 107.4 and 107.7 mAh g−1 , respectively. The excellent cycling performances of all four samples are displayed in Fig. 6b, where almost no capacity fading is measured for 100 cycles. As shown in Fig. 6c, the rate performances of samples are a little better than those when cycled in the voltage range of 3.0–4.8 V. We can find that the rate performance of the PVDF-sample is best, which only has a little capacity drop when the rate changes from 0.5C to 5C; the citric acid and glucose-samples are a little poorer, whereas, the capacity of the starch-sample decreases fast with increasing current. Fig. 7 shows CV curves for the citric acid-sample at 0.05 mV s−1 in the voltage range of 3.0–4.3 V and 3.0–4.8 V. In the voltage of 3.0–4.3 V, there are three oxidation peaks around 3.68, 3.74, 4.15 V and three reduction peaks around 3.52, 3.60, 3.97 V, which is in good agreement with the above initial charge–discharge curves (Fig. 6a). The symmetrical feature of the CV data (curve a in Fig. 7) demonstrates a good reversibility of the cycle processes between 3.0 and 4.3 V. From curve b in Fig. 7, four oxidation peaks and three reduction peaks appear in the voltage range of 3.0–4.8 V, which corresponds to the plateaus of the initial charge–discharge
curves (Fig. 3a) and verifies the electrochemical reactions mechanism during cycles. The fourth oxidation peak is located around 4.59 V, associated with the V4+ /V5+ redox couple. An inclining curve is measured in the initial discharge process (Fig. 3a) representing the solid solution behaviour, consistent with the low and wide reduction peak at 3.9 V on curve (b) in Fig. 7. The DC resistance of the carbon-coated-Li3 V2 (PO4 )3 /Li cells as a function of voltage (3.0–4.3 V) measured during the 2nd cycle with a simultaneous current interruption method is shown in Fig. 8. Obvi-
Fig. 7. CV curves for the citric acid-sample at 0.05 mV s−1 in the voltage range of 3.0–4.3 V (a) and 3.0–4.8 V (b).
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Fig. 9. The DC resistance of the citric acid-sample as a function of voltage (3.0–4.8 V) in the 2nd charge process measured with a simultaneous current interruption method. Inset is the 2nd discharge process.
with the DC resistance results in the voltage ranger of 3.0–4.3 V, we can conclude that the DC resistance cannot only be used as a auxiliary tool to study the reversibility and mechanism of the electrochemical reaction, but also can infer the degree of difficulty of extraction and re-insertion of lithium ions during various steps of the charge–discharge cycle. As shown in Fig. 9, we can find that the resistance of extraction of the third lithium ion is a little larger, meaning that higher energetics is needed, which can bring about irreversible capacity loss. 4. Conclusion Fig. 8. The DC resistance of the carbon-coated-Li3 V2 (PO4 )3 /Li cells as a function of voltage (3.0–4.3 V) measured with a simultaneous current interruption method. (a) In the 2nd charge process and (b) in the 2nd discharge process.
ously, we can see three minimum resistance values around 3.6, 3.7, 4.1 V during the charge process (Fig. 8a) as well as around 3.55, 3.65, 4.05 V during the discharge process (Fig. 8b), which agrees well with the three charge/discharge plateaus and the cyclic voltammograms in the voltage of 3.0–4.3 V. Between two neighboring minimums, the resistance is markedly higher, which is corresponding to a twophase transition process. Among all the cells, the one with the glucose-sample shows the highest minimum resistance values during both the charge and discharge cycles, which must be the reason why it has the poorest rate performance (Fig. 5a and Fig. 6c). As for the other three samples, the resistance values are close to each other, which is also roughly in line with their rate performances. This result seems contradictory with the content of residual carbon in these samples. Nevertheless, it is understandable if we consider both the content and structure of carbon coating. A uniform carbon coating layer and a higher carbon content should improve its electronic conductivity. On the other hand, a thick carbon layer may hinder the transport rate for lithium ions. To compromise the electronic and ionic conductivities, an optimal carbon-content should exist. For the citric acid-sample, although its residual carbon content is the lowest, the diffusion of Li+ could be improved largely. Hence, its resistance is close to other samples. Fig. 9 shows the DC resistance of the citric acid-sample as a function of voltage (3.0–4.8 V) during the 2nd cycle measured with a simultaneous current interruption method. Also, four minimum resistance values 235, 206, 168 and 267 around 3.6, 3.7, 4.1 and 4.55 V can be observed during the charge process, and in the discharge process there are three minimum resistance values 263, 214 and 194 around 3.56, 3.64 and 4.0 V, respectively. Combined
The carbon-coated Li3 V2 (PO4 )3 (LVP) cathode materials have been successfully synthesized by the carbon thermal reduction method using citric acid, glucose, PVDF and starch as different carbon sources at a low temperature of 750 ◦ C. X-ray diffraction results show a pure single phase with monoclinic structure (space group P21 /n) in these samples. Of all powder samples, LVP particles with a particle size of about 1 m are covered by a carbon layer, and the citric acid-sample has the lowest carbon content (1.33%). In the voltage region of 3.0–4.3 V, the LVP/C electrodes present a fine reversibility, good rate capability and excellent cycle stability with no capacity fading even after 100 cycles. While in the voltage region of 3.0–4.8 V, the extraction of the third lithium ion (V4+ /V5+ ) taking place at 4.6 V appears to be energetically unfavorable and, hence displays a large overvoltage. All samples in this wide voltage range exhibit a slightly poorer cycle performance with the capacity retention of about 86% after 50 cycles, and a not very good rate capability. Of all four samples cycled in the two voltage regions, the citric acid-sample owns a higher capacity at the rate of 0.2C during the charge–discharge cycle, and the PVDF-sample displays the best rate performance. Based on the SEM images, the AC impendence and DC resistance of LVP/C composites, the higher capacity and good rate performance are owing to the coexistence of the uniform small particles and the carbon network, which improves their electronic conductivity leading to a lower resistance. Combined with a good electrochemical performance and ease of synthesis, lithium vanadium phosphate can be a very promising cathode material for higher energy density and power demanding lithium-ion batteries. Acknowledgment This study was supported by National Science Foundation of China (grant no. 20471057).
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