C cathode material for lithium-ion batteries

Journal of Alloys and Compounds 536 (2012) 132–137

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Freeze-drying synthesis of Li3V2(PO4)3/C cathode material for lithium-ion batteries Y.Q. Qiao, X.L. Wang ⇑, Y.J. Mai, X.H. Xia, J. Zhang, C.D. Gu, J.P. Tu ⇑ State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

a r t i c l e

i n f o

Article history: Received 16 February 2012 Received in revised form 24 April 2012 Accepted 27 April 2012 Available online 8 May 2012 Keywords: Lithium vanadium phosphate Freeze-drying Polystyrene spheres Lithium ion battery

a b s t r a c t Li3V2(PO4)3/C cathode material was synthesized by using a freeze-drying method followed by carbonthermal reduction. This as-prepared material has a uniform particle size distribution and a well carbon coating on the surface of Li3V2(PO4)3 particles. The Li3V2(PO4)3/C exhibits good electrochemical performance and cycling stability. Between 3.0 and 4.3 V, the composite delivered a reversible capacity of 125.2 mAh g 1 at a charge–discharge rate of 1.48 C (1 C = 133 mA g 1) and without obviously capacity fading after 100 cycles. Even at 14.8 C and 29.6 C rates, it can still deliver discharge capacities of 105.6 mAh g 1 and 93.3 mAh g 1, and the discharge capacities of 84.5 and 60.5 mAh g 1 are sustained after 500 cycles, respectively. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Lithium-ion batteries (LIBs) have been extensively investigated for numerous applications such as portable electronic devices, plug-in hybrid electric vehicles (PHEVs), electric vehicles (EVs) and energy storage systems [1–4]. LiFePO4 is one of the most promising candidates for cathode materials of LIBs, which shows appealing features such as high theoretical capacity (170 mAh g 1), low cost, high safety and environmental benignity [5–7]. In recent years, another lithium metal phosphate, monoclinic Li3V2(PO4)3 is also considered to be one of the most promising cathode materials due to its good ion mobility, high theoretical capacity and relatively high operate voltage [8–14]. However, Li3V2(PO4)3 has intrinsic low electronic conductivity as LiFePO4, which greatly adverse impacts the electrochemical performance and utilization of Li3V2(PO4)3 and LiFePO4 [15–18]. Up to now, it is generally believed that nanocrystallization of the compound combined with carbon coating is probably the most effective way to overcome conductivity limitations of those two materials. Nanostructured materials show favorable properties such as improved kinetics and activity due to the short diffusion lengths of Li+ and large contact area between material and electrolyte. In the case of carbon coating, it not only suppressed particle growth but also enhanced the conductivity through improved contacts between the particles and reduction of the polarization. In this regard, several preparation methods have been employed to synthesize nanostructured Li3V2(PO4)3/C materials [9,19–24]. For instance, Huang et al. fabricated Li3V2(PO4)3/C with nanoscale ⇑ Corresponding authors. Tel.: +86 571 87952856; fax: +86 571 7952573. E-mail addresses: [email protected] (X.L. Wang), [email protected] (J.P. Tu). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.118

dimensions on the order of 100 nm by sol–gel method, which achieved almost full capacity with excellent rate capability and cycling stability [9]. Ren et al. [19] and Li et al. [20] also employed sol–gel method to prepare Li3V2(PO4)3/C nanocomposites, and they found that the as-prepared composites exhibited enhanced discharge capacities and cyclic ability which could be ascribed to the reduced particle size and the conductive carbon coating. Recently, Zheng et al. [21] synthesized a nanocrystalline Li3V2(PO4)3 by calcining amorphous Li3V2(PO4)3 obtained by chemical reduction and lithiation of V2O5 using oxalic acid as a reducer. This pure Li3V2(PO4)3 could exhibit a stable discharge capacity of 130.08 mAh g 1 at 0.1 C (14 mA g 1). More recently, Pan et al. [22] prepared a nanostructured Li3V2(PO4)3/C composite by incorporating the precursor solution into a highly mesoporous carbon with an expanded pore structure, which delivered a discharge capacity of 83 mAh g 1 at a high rate of 32 C (1 C = 140 mAh g 1). Freeze-drying, a new and simple method has been widely used in preparing cathode materials with fine particle size such as LiMn2O4 [25], LiCoO2 [26], LiNi0.5Mn0.5O2 [27], Li1+xV3O8 [28] and LiFePO4 [29,30]. The main advantage of this cryochemical route is the homogeneous precursor after freeze-drying that needs only moderate conditions of calcining temperature and time, thus providing the possibility to achieve higher homogeneity and smaller particle size of the final product. However, to the best of our knowledge, this method is rarely used to prepare Li3V2(PO4)3/C materials. In this work, we report the synthesis of a Li3V2(PO4)3/C composite by freeze-drying method. Considering the practical use, two lithium ions are usually extracted and inserted in the range of 3.0– 4.3 V, corresponding to a theoretical capacity of 133 mAh g 1 [9,11,22]. Thus, in this present work, two lithium ions were allowed to participate in redox reactions during the charge/discharge

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Fig. 1. Schematic illustration of synthetic process of Li3V2(PO4)3/C composite.

2. Experimental

Weight (%)

Li3V2(PO4)3/C was synthesized by a freeze-drying method. All chemicals used in our work were analytical grade. First, NH4VO3 and LiOH
H2O in a stoichiometric ratio of 3:2 were dissolved in deionized water under magnetic stirring at room temperature until a transparent solution was formed. Second, a stoichiometric amount of H3PO4 (85%, solution) was added dropwise to the above transparent solution, then a brownish-red solution was obtained. The resulting brownish-red solution was transferred into a refrigerator and chilled until the solution was transformed fully into an ice cake. Finally, the obtained ice cake was freeze-dried at 53 °C for 48 h in a FD-1A-50 vacuum freeze dryer. The precursor was ground with an appropriate quantity of polystyrene (PS) spheres as the carbon source using a mortar and pestle for 20 min, and the mixture was calcined at 750 °C for 8 h under Ar flow to get the Li3V2(PO4)3/C composite. DSC-TGA analysis was measured on a SDT Q600 apparatus in the temperature ranging from 25 to 900 °C at a heating rate of

10 °C min 1 under an Ar flow of 120 mL min 1. The morphology and structure of the as-synthesized powder were characterized using field emission scanning electron microscopy (FESEM, FEI SIRION), X-ray diffraction (XRD, RIGAKU D/Max-2550 with Cu Ka radiation) and high-resolution transmission electron microscopy (TEM, Tecnai G2 F30 S-Twin). The residual carbon content of the powder was determined by means of an automatic elemental analyzer (EA, Flash EA1112). Raman scattering spectroscopy (LABRAM HR-800) was recorded at room temperature with the wave number shift among 4000–100 cm 1 in ultraviolet laser excitation line of 325 nm. Electrochemical performances of the as-synthesized materials were investigated using CR2025 coin-type cell. A metallic lithium foil served as the anode electrode. The cathode consisted of 85 wt.% active material, 10 wt.% acetylene black and 5 wt.% polyvinylidene fluoride (PVDF) on aluminum foil. 1 M LiPF6 in ethylene carbonate (EC)-diethyl carbonate (DEC) (1:1 in volume) as the electrolyte, and a polypropylene micro-porous film (Cellgard 2300) as the separator. The cells were assembled in a glove box filled with high-purity argon. The charge–discharge tests were conducted on LAND battery program-control test system (Wuhan, China) between 3.0 and 4.3 V (vs. Li/Li+) by applying from 1.48 to 29.6 C

110

0.1

100

0.0 -0.1

90 DSC

80

-0.2

TGA

-0.3

70

-0.4

60 50

-0.5 0

200

400

600

800

Temperature Difference (°C/mg)

process. The electrochemical tests show that the Li3V2(PO4)3/C material obtained by freeze-drying method exhibits high-rate performance and good cycling stability.

-0.6

Temperature (°C)

Fig. 2. SEM image of PS spheres.

Fig. 3. DSC-TGA curves of the Li3V2(PO4)3/C precursor recorded from room temperature to 900 °C at a heating rate of 10 °C min 1 under an Ar flow of 120 ml min 1.

Y.Q. Qiao et al. / Journal of Alloys and Compounds 536 (2012) 132–137

10

(121)

20

30

(-152)

(420) (-233)

40

(-403)

(220) (-211) (-301) (310) (-311) (-204)

(-122)

Observed Calculated Difference hkl, Li3V2 (PO4 )3

(-113)

(002)

(-201)

(-111)

(120)

Intensity (a.u.)

(020)

134

50

60

2θ (degree) Fig. 4. Refined XRD pattern of Li3V2(PO4)3/C composite.

rates at room temperature, respectively. For electrochemical impedance spectroscopy (EIS) measurements, the test cells were with the metallic lithium foil as both the reference and counter electrodes. EIS measurements were performed on CHI660C electrochemical workstation over a frequency range of 100 kHz to 10 mHz at the charge stage around 3.60 V after cycling by applying an AC signal of 5 mV. Cyclic voltammetry (CV) experiments were performed on this electrochemical workstation in the potential range of 3.0–4.3 V at a scan rate of 0.1 mV s 1. 3. Results and discussion Fig. 1 illustrates the preparation process of the nano-structured Li3V2(PO4)3/C material. Firstly, NH4VO3 and LiOH
H2O were dissolved in deionized water under magnetic stirring at room

temperature until a transparent solution was formed. Then a brownish-red solution can be obtained after a stoichiometric amount of H3PO4 (85%, solution) was added. Secondly, the resulting precursor solution was frozen in a refrigerator or using liquid nitrogen and the solution was transformed into an ice cake. In the freeze-drying step, the water was removed from the ice cake in a vacuum and low temperature condition (the sublimation of ice). In our previous works, it was found that the Li3V2(PO4)3/C and LiFePO4/C composites using PS or PS spheres as the carbon source exhibited good electrochemical performance due to the uniform distribution of carbon coating on the particles [31,32]. Therefore, we employed the PS spheres with diameter of 300 nm as the carbon source by grinding with the obtained precursor using a mortar and pestle, as shown in Fig. 2. It is clear that the PS spheres are monodispersed, having good fluidity that can fully mix with the obtained precursor. Finally, the final precursor was sintered at 750 °C for 8 h under Ar flow to get the Li3V2(PO4)3/C composite according to the DSC-TGA analysis (Fig. 3). The XRD patterns and Rietveld refinement of the Li3V2(PO4)3/C material are shown in Fig. 4. All diffraction peaks of the composite are indexed to the single-phase Li3V2(PO4)3 (ICSD No. 96962), which are in good agreement with the previous reports [9,33]. According to the results of structure refinement, the Li3V2(PO4)3 compound has lattice parameters with a = 8.6142(1) Å, b = 8.6028(5) Å, c = 12.0438(8) Å and b = 90.58836 (deg), which are very close to those data in previous reports [33–36]. In addition, there is no additional diffraction peak related carbon being observed due to its amorphous structure or the thickness of carbon layer on the particles is too thin [19,32]. However, according to the elemental analysis, the amount of residual carbon in the final composite is 4.21 wt.%. Obviously, the carbon presence does not influence the structure of Li3V2(PO4)3. The SEM image of Li3V2(PO4)3/C composite is shown in Fig. 5a. It is found that the as-prepared material shows a uniform particle size distribution and the particle size is much smaller than

Scattering Intensity (a.u.)

(d) -1

974.4 cm

-1

1051.3 cm -1 1140.2 cm

800

1000

1200

D

1400

G

1600

1800

2000

-1

Wavenumbers (cm ) Fig. 5. (a) SEM and (b) TEM images of Li3V2(PO4)3/C composite; (c) A HRTEM image showing nearly 8.6 nm-thick amorphous carbon layer on the surface of Li3V2(PO4)3 particle; (d) Raman scattering spectrum of Li3V2(PO4)3/C composite.

Y.Q. Qiao et al. / Journal of Alloys and Compounds 536 (2012) 132–137

+

Potential (V vs. Li/Li )

(a)

4.2 4.0

1.48 C 7.4 C 14.8 C 29.6 C

3.8 3.6 3.4 3.2 3.0

0

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60

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Discharge capacity (mAh/g)

(b) 150 1.48 C 14.8 C

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7.4 C 29.6 C

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(c)

0.6

Current (A/g)

0.4 0.2

1st 2nd 3rd

0.0 -0.2 -0.4 -0.6 3.0

3.3

3.6

3.9

4.2

Potential (V vs. Li/Li+) Fig. 6. (a) The initial charge–discharge curves, (b) cycling performance at 1.48 C, 7.4 C, 14.8 C and 29.6 C and (c) CV curves of the Li3V2(PO4)3/C composite synthesized by freeze-drying between 3.0 and 4.3 V. The charge and discharge current densities are the same correspondingly. CV scan rate: 0.1 mV s 1.

300 nm. The particles with a small uniform particle size and a large specific surface area will be possible to reduce Li-ion diffusion and electron transportation distance, thus improving the electrochemical performance of the electrode. TEM investigation further reveals that the as-prepared material consists of nanoscale particles (Fig. 5b), which agrees well with that revealed by SEM. As shown by the arrows in Fig. 5b, dissociative carbon can also

135

be observed. Fig. 5c shows the high resolution TEM (HRTEM) image of the Li3V2(PO4)3/C material. It is clear that the Li3V2(PO4)3 particles are coated with uniform carbon layer, forming a core/shell structure, and the layer thickness is about 8.6 nm. Furthermore, the graphite fringes are clearly seen in the carbon layer, indicating a high degree of graphitization of the residual carbon. It is well known that the residual carbon from pyrolysis of organics or polymers is able to impede the grain growth of Li3V2(PO4)3 and provide good grain-to-grain electronic contact, thus resulting in the enhancement of electrochemical properties [20]. Raman spectroscopy is a useful tool for characterizing the structure of residual carbon in the composite. Fig. 5d shows the Raman spectrum of the Li3V2(PO4)3/C in the range of 800–2000 cm 1. In the Raman spectrum, two intense broad bands at 1599.6 cm 1 and 1354.8 cm 1 are attributed to the graphite band (G-band) and the disorder-induced phonon mode (D-band), respectively. The ID/IG value of the Li3V2(PO4)3/C composite is about 0.86, suggesting that the graphite-like carbon in the residual carbon is about 50%, which is helpful for improving the electronic conductivity and electrochemical performance of Li3V2(PO4)3 [11,37,38]. In addition, there are three broad Raman bands at 1140.2, 1051.3 and 974.4 cm 1, which can be indexed to the vibrations of Li3V2(PO4)3 [39]. Fig. 6a shows the initial charge/discharge curves of the Li3V2(PO4)3/C at different rates between 3.0 and 4.3 V. At a low rate of 1.48 °C, it exhibits three charge plateaus around 3.60, 3.68 and 4.07 V and three corresponding discharge plateaus around 3.56, 3.65 and 4.05 V, which are identified as the two-phase transition processes; these regions correspond to three compositional regions of Li3 xV2(PO4)3, where x = 0.0–0.5, 0.5–1.0 and 1.0–2.0 [8–10]. However, with the increase of charge–discharge rate, these plateaus become shorter and the differences of the charge and discharge plateaus become larger gradually which can be attributed to the electrode polarization at high rates. A high initial discharge specific capacity of 125.2 mAh g 1 at 1.48 C was achieved, reaching 94.1% of its theoretical capacity (133 mAh g 1, between 3.0 and 4.3 V). At a charge–discharge rate of 7.4 C, the Li3V2(PO4)3/C electrode gives a discharge capacity of 114.9 mAh g 1. Even at 14.8 C and 29.6 C rates, it can still deliver discharge capacities of 105.6 mAh g 1 and 93.3 mAh g 1, respectively. It demonstrates that our Li3V2(PO4)3/C prepared by freeze-drying method performs better than the Li3V2(PO4)3/C synthesized by carbon-thermal reduction method using polystyrene as a carbon source, especially at high rates [32]. For instance, the Li3V2(PO4)3/C synthesized by carbon-thermal reduction method can deliver an initial discharge capacity of 105.2 mAh g 1 at 5 C, which is lower that our present work (114.9 mAh g 1 at 7.4 C). The discharge capacity of our Li3V2(PO4)3/C is also higher than that of prepared by freeze-drying method reported by Wang et al. [40] recently (97.9 mAh g 1 at 10 C compares to 105.6 mAh g 1 at 14.8 C in our work). The cycle performances of the Li3V2(PO4)3/C measured at room temperature between 3.0 and 4.3 V at different charge/discharge rates are shown in Fig. 6b. It can be found that the Li3V2(PO4)3/C exhibits a good capacity retention without obviously capacity fading after 100 cycles at a charge–discharge rate of 1.48 C. Even after 500 cycles, the discharge capacities of 100.2, 84.5 and 60.5 mAh g 1 can still be sustained at 7.4, 14.8 and 29.6 C rates, respectively. The cell retains 87.2%, 80.0% and 64.8% of its initial discharge capacity correspondingly, showing a good cycling performance. Its high capacity and excellent cycling stability should be attributed to the optimized particle size by using freeze-drying method and the well carbon coating on the particles from the pyrolysis of PS spheres, which are propitious to lithium extraction and insertion reactions. Fig. 6c shows the CV curves of the Li3V2(PO4)3/C composite at a scan rate of 0.1 mV s 1 in the potential range of 3.0–4.3 V. There are three anodic peaks around 3.64, 3.70 and 4.12 V and three cathodic peaks about 3.55, 3.63 and 4.02 V, which is in accordance

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(a)

80

30th 40th 20th 10th

-Z'' (Ω )

60

1st

40

50th

20 0

100 0

80

10

Cyc

60

20

le n u

40

30

mbe

r

20

40 50

0

(Ω Z'

)

(3.60 V). The shapes of the Nyquist plots for each cycle are similar. It is clear seen that plots are composed of a small intercept at high frequency, a semicircle at high to medium frequency and a linear part in the low frequency. The impedance spectra are fitted using the equivalent circuit model of Fig. 7b. The intercept impedance is almost same (2.5–5.5 X) after different cycles, which corresponds to the solution resistance (Rel). The depressed semicircle in the middle frequency reflects the charge-transfer resistance (Rct) and the double layer capacitance (CPE). The inclined straight line in the low frequency represents the Warburg impedance (Zw) which is attributed to the diffusion of Li+ ions within the Li3 xV2(PO4)3 particles [32,41–45]. The value of Rct is 71.52 X in the first cycle, decreasing to 39.65 X after 20 cycles (Fig. 7c). The significant decrease of Rct with the cycle number may result from the porous structure of the electrode and the nanosizing of Li3V2(PO4)3/C particles, indicating the electrode formation/surface modification [46]. After 20 cycles, the value of Rct increases slowly, and reaches only 43.35 X in the 50th cycle (Fig. 7c). The low increase of impedance during cycling indicates low polarization, affecting good cycling behavior. 4. Conclusions

(c)

In summary, nanostructured Li3V2(PO4)3 with well carbon coating has been successfully synthesized by a freeze-drying method. The freeze-drying method is a useful pretreatment process for the preparation of Li3V2(PO4)3/C with optimized particle size. As a cathode material for lithium-ion batteries, the Li3V2(PO4)3/C material delivered a reversible capacity of 125.2 mAh g 1 at 1.48 C between 3.0 and 4.3 V and without obviously capacity fading after 100 cycles. Even at a charge–discharge rate of 29.6 C, it can still deliver an initial discharge capacity of 93.3 mAh g 1 and be sustained 60.5 mAh g 1 after 500 cycles. The Li3V2(PO4)3/C shows high rate capability and excellent cycling stability, making it an attractive high-power cathode for lithium-ion batteries.

120 Rct

Rct (Ω)

90

60

Acknowledgements

30

0

This work is supported by Key Science and Technology Innovation Team of Zhejiang Province (2010R50013) and Fundamental Research Funds for the Central Universities (2011QNA4006).

0

10

20

30

40

50

Cycle number Fig. 7. (a) Three-dimensional Nyquist plots of Li3V2(PO4)3/C composite measured after cycling (around 3.60 V after each cycle); (b) equivalent circuit used for simulating the experimental impedance data; (c) variation of Rct with cycle number calculated from fitting the Nyquist plots for the Li3V2(PO4)3/C electrode.

with the charge/discharge curves depicted in Fig. 6a. The first two couples of anodic/cathodic peaks correspond to the extraction/ insertion of the first Li+ ion with two steps, as the Li2.5V2(PO4)3 phase appears in the oxidation/reduction processes [8,9]. The third couple of anodic/cathodic peaks correspond to the extraction/ insertion of the second Li+ ion via a single step, associated with the phase transition processes between Li2V2(PO4)3 and LiV2(PO4)3. Since only two Li+ ions are removed, all of the three current peaks are assigned to the V3+/V4+ redox couple. After 3 cycles, the redox peaks were little changed in their shapes and magnitudes. Thus, the symmetrical and well-defined oxidation and reduction peaks in the CV plots indicate that the Li3V2(PO4)3/C electrode has the high electrochemical reaction activity and reversibility. Fig. 7a shows the three-dimensional Nyquist plots of the Li3V2(PO4)3/C after different number of cycles at the state of charge

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