C composite cathode materials for lithium ion batteries

C composite cathode materials for lithium ion batteries

Electrochimica Acta 87 (2013) 224–229 Contents lists available at SciVerse ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/loca...

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Electrochimica Acta 87 (2013) 224–229

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

xLi3 V2 (PO4 )3 ·LiVPO4 F/C composite cathode materials for lithium ion batteries Jiexi Wang, Zhixing Wang ∗ , Xinhai Li, Huajun Guo, Xianwen Wu, Xiaoping Zhang, Wei Xiao School of Metallurgical Science and Engineering, Central South University, Changsha, 410083, China

a r t i c l e

i n f o

Article history: Received 14 May 2012 Received in revised form 7 September 2012 Accepted 7 September 2012 Available online 17 September 2012 Keywords: Lithium ion battery Cathode material Composite cathode Li3 V2 (PO4 )3 –LiVPO4 F/C

a b s t r a c t xLi3 V2 (PO4 )3 ·LiVPO4 F/C composite is synthesized by mechanically activated chemical reduction. The value of x is designed to be 2.0 but in fact is 2.7 in the prepared sample because of volatilization of HF or VF3 . The as-prepared powders with the average primary size of 0.5–1 ␮m are coated with amorphous carbon layer. Structural analysis of the composite indicates that two phases of monoclinic Li3 V2 (PO4 )3 and triclinic LiVPO4 F coexist. The composite as cathode for LIB shows excellent electrochemical performances. In the potential range of 3.0–4.4 V, it delivers initial discharge capacity of 139, 137, 131, and 123 mAh g−1 at 0.1, 1, 5 and 10 C, respectively. And 97.4% of initial capacity is retained even after 300 cycles at 10 C. The cyclic voltammetry (CV) and AC impedance analysis indicate that the prepared composite separately shows good ionic diffusivity and low resistance. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Lithium transition-metal phosphates have been widely regarded as promising positive electrodes for their low cost, high Li+ intercalation/deintercalation reversibility and excellent thermal stability [1–3]. Among them, NASICION-related structure ␣-Li3 V2 (PO4 )3 exhibits a theoretical capacity of 197 mAh g−1 between 3.0 and 4.8 V, and can extract/insert two lithium ions reversibly based on the V3+ /V4+ redox between 3.0 and 4.3 V versus Li/Li+ electrode [4]. But the low electronic conductivity of Li3 V2 (PO4 )3 has hindered the development of its practical use [5,6]. Many efforts have been made to improve the performance of Li3 V2 (PO4 )3 , including doping elements in V site or Li site [7–10], coating with electronically conductive materials [11–15] and optimization of particles [16]. According to recent reports, composites of two or more pure substances can improve electrochemical performance of materials [17,18]. Triclinic structure lithium vanadium fluorophosphate with chemical formula of LiVPO4 F has been reported by Barker et al. [19–21]. Owing to the impact of structural fluorine on the inductive effect of the PO4 3− polyanion, V3+ /V4+ redox couple in LiVPO4 F is located at a high potential of about 4.2 V, which is about 0.3 V higher than the average potential for the same transition (V3+ /V4+ ) in the lithium vanadium phosphate, Li3 V2 (PO4 )3 [19]. In addition, LiVPO4 F exhibits higher electronic conductivity than traditional phosphates such as LiFePO4 , LiMnPO4 and LiVOPO4 [20].

∗ Corresponding author. Tel.: +86 731 88836633; fax: +86 731 88836633. E-mail addresses: [email protected], [email protected] (Z. Wang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.09.014

In our work, in consideration of the advantage of Li3 V2 (PO4 )3 and LiVPO4 F, we presented the synthesis of xLi3 V2 (PO4 )3 –LiVPO4 F/C composite coated with carbon via mechanically activated chemical reduction followed by annealing. To the best of our knowledge, no investigation about Li3 V2 (PO4 )3 –LiVPO4 F composite has been reported so far. 2. Experimental xLi3 V2 (PO4 )3 –LiVPO4 F/C was synthesized by mechanically activated chemical reduction followed by annealing. The synthetic procedures can be briefly described as follows: (1) mixing raw materials of 0.1433 g LiF (A.R., 98.5%), 1.4276 g LiOH·H2 O (A.R., 96%), 5.0000 g V2 O5 (A.R., 99%), 4.4270 g NH4 H2 PO4 (A.R., 99%), 8.2760 g H2 C2 O4 ·2H2 O (A.R., 99.5%, 20% excessive) by magnetic stirring using alcohol as dispersing agent; (2) ball milling for 4 h and drying in the oven at 80 ◦ C for 12 h; (3) annealing 300 ◦ C for 2 h and then at 750 ◦ C for 12 h under argon atmosphere. The weight of raw materials can be adjusted to keep mole ratio of Li3 V2 (PO4 )3 to LiVPO4 F being 2:1. The powder X-ray diffraction (XRD, Rint-2000, Rigaku, Japan) using Cu K␣ radiation was employed to identify the crystalline phase of the synthesized material. The morphology of the composite was measured by scanning electron microscope (JEOL, JSM-5612LV) with an accelerating voltage of 20 kV, and by transmission electron microscope (Tecnai G12, 200 kV). Elemental carbon content in the sample was determined by C-S analysis equipment (Eltar, Germany). Inductively coupled plasma-atomic emission spectrometry (ICP-AES, IRIS intrepid XSP, Thermo Electron Corporation) was employed

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in prepared sample may due to the volatilization of F as the form of VF3 or HF at high temperature, which occurs on the basis of the following equations: 6LiVPO4 F(s) + 3H2 O(g) → 2Li3 V2 (PO4 )3 (s) + V2 O3 (s) + 6HF(g) (1) 3LiVPO4 F(s) → VF3 (g) + Li3 V2 (PO4 )3 (s)

Fig. 1. Multiphase refinement for powder XRD patterns of the prepared composite.

to determine the mass contents of V and F in prepared samples. The electrochemical characterizations were evaluated via a standard CR2025 coin-type cell composed of the cathode, metallic lithium anode, a commercial polypropylene micro-porous separator, and 1 mol L−1 LiPF6 in EC, EMC and DMC (1:1:1 v/v/v). For positive electrode fabrication, the prepared powders were mixed with 10% acetylene black and 10% polyvinylidene fluoride in Nmethyl pyrrolidinone until homogeneous slurry was obtained. And then, the blended slurries were cast onto an aluminum current collector, followed by drying at 120 ◦ C for 12 h in the air. Then it was cut into rounded pieces with an area of 1.54 cm2 . The active material on each piece weighs about 1.6 mg. The assembly of the cells was carried out in a dry Ar-filled glove box. Electrochemical tests were carried out with an automatic galvanostatic charge–discharge unit (NEWARE battery circler) between 3.0 V and 4.4 V versus Li/Li+ electrode at room temperature. A constant current–constant voltage condition was employed for charge process and a galvanostatic condition was conducted for discharge process. 3. Results and discussion Fig. 1 shows the structure characterization of the prepared material, which was carried out with powder X-ray diffraction. The Rietveld refinement is used to fit the XRD data. Two phases of Li3 V2 (PO4 )3 and LiVPO4 F are detected, without other undesired phases such as LiF or V2 O3 . Table 1 shows the refined cell parameters of the Li3 V2 (PO4 )3 and LiVPO4 F in the prepared sample, which are very close to that reported in the other papers [11,20,22–24]. It indicates that the prepared composite has independent well crystallized phases of triclinic LiVPO4 F and monoclinic Li3 V2 (PO4 )3 . The mass content of Li3 V2 (PO4 )3 and LiVPO4 F determined by multiphase refinement is 86.3 ± 1.4 wt.% and 13.7 ± 0.9 wt.%, respectively. And C-S analysis shows the mass content of carbon is about 1.4%, but there are no peaks corresponding to carbon, indicating the carbon is amorphous. ICP-AES analysis was used to determine the contents of V and F of prepared sample, and the results are shown in Table 2. The calculated mass ratio of Li3 V2 (PO4 )3 to LiVPO4 F compares well with that determined by Rietveld method. The mole ratio of V/F is 6.4:1 and the calculated mole ratio of Li3 V2 (PO4 )3 to LiVPO4 F is 2.7:1, which is higher than that in raw materials. The decrease of LiVPO4 F

(2)

The loss of VF3 at high temperature has been reported by Barker [25]. But in our synthetic process, the decomposition of H2 C2 O4 and NH4 H2 PO4 produces H2 O(g), which will induce the formation of HF(g). the white residue on the wall of silica tube after heat treatment was analyzed by XRD, which shows that the residue indexes with phase-pure (NH4 )2 SiF6 . We can conclude that the residue is derived from the reaction between NH3 , HF and SiO2 . Fig. 2 shows the morphology of the prepared sample. Fine particles with the average primary particle size of 0.5–1 ␮m can be observed in Fig. 2(a). The particles are well crystallized and slightly agglomerated. Moreover, it can be seen in the TEM image shown in Fig. 2(b) that the prepared powders are coated with carbon. A typical HRTEM image of the prepared sample is provided here for further investigation on the morphology and microstructure of the sample, as shown in Fig. 2(c) and (d). In Fig. 2(c), HRTEM image and the corresponding fast Fourier transform (FFT) image prove that the particles are composed of a crystallized core and an amorphous carbon layer with a thickness of about 5 nm around the core. In Fig. 2(d), grain boundary of crystalline Li3 V2 (PO4 )3 and LiVPO4 F can be seen clearly, which indicates that the Li3 V2 (PO4 )3 and LiVPO4 F crystals inset into each other well in the prepared composite. Fig. 3 shows the surface composition of the prepared sample. From Fig. 3(a) and (b), we can be informed that fluorine distributes in the prepared sample homogeneously, indicating a uniform insertion between Li3 V2 (PO4 )3 and LiVPO4 F, rather than a simple macroscopical mixing. From Fig. 3(a) and (c), we can get the information that the amorphous carbon evenly distributes on the particle surface. The charge/discharge performance of the crystalline the prepared xLi3 V2 (PO4 )3 –LiVPO4 F/C was evaluated as cathode in the Li-half cell configuration with cut-off potentials of 3.0 and 4.4 V at room temperature, and results are shown in Fig. 4. The initial discharge capacity is about 139 mAh g−1 at 0.1 C (1 C rate equals to 150 mA g−1 ). The as-prepared composite also performs well at high charge–discharge rates, delivering 137, 135, 131 and 123 mAh g−1 at 1, 2, 5 and 10 C, respectively. Fig. 4(b) shows that the cell retains 98.9%, 99.5%, 99.5% and 97.3% of its initial discharge capacity after 50, 100, 100 and 300 cycles at 1, 2, 5 and 10 C, respectively. Furthermore, after 300 cycles at 10 C, only a slight capacity fading at 1 C is obtained. The electrochemical performance of the prepared composite (LVP–LVPF) is enhanced obviously compared with LiVPO4 F/C (LVPF) and Li3 V2 (PO4 )3 /C (LVP) synthesized in similar method [26–28], as seen in Table 3. The improved electrochemical performance of the prepared composite may be attributed to the synergistic effect of Li3 V2 (PO4 )3 and LiVPO4 F. The impact of structural fluorine on the inductive effect of the PO4 3− polyanion even expands to Li3 V2 (PO4 )3 lattice because of F doping, and it can also improve electronic transmission performance of the material during charge/discharge process. Besides, the addition of fluorine can also prevent electrode from being attacked by HF found in electrolyte, and therefore bring about good cycle performance [29–31]. On the other hand, as the fast ionic conductor, Li3 V2 (PO4 )3 possesses good Li+ intercalation/deintercalation reversibility, so the prepared composite possesses excellent rate capability. The thin carbon film coating may also contribute to the high conductivity among microparticles, reducing electronic transmission obstacle [32].

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Fig. 2. (a) SEM, (b) TEM, (c), (d) HRTEM images together with the inset FFT of the prepared composite.

Fig. 3. SEM image (a) and corresponding EDS dot mappings of element C (b) and F (c).

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Table 1 Refined unit cell lattice parameters of the prepared composite. Phase

Li3 V2 (PO4 )3 LiVPO4 F

Lattice parameters a (nm)

b (nm)

c (nm)

˛

ˇ



Volume (nm3 )

0.8598(2) 0.5168(3)

0.8590(4) 0.5313(5)

1.2032(3) 0.7490(4)

– 66.827(6)

90.591(3) 67.044(4)

– 81.591(2)

0.8886 0.1741

Table 2 Compositions of V and F in raw material and synthesized sample. Sample

Mass content of V (%)

Mass content of F (%)

Mole ratio of V/F

Mole ratio of Li3 V2 (PO4 )3 /LiVPO4 F

Synthesized sample Raw material

25.36 25.80

1.48 1.92

6.4:1 5.0:1

2.7:1 2.0:1

Table 3 Comparison on electrochemical properties of LiVPO4 F/C, Li3 V2 (PO4 )3 /C and the as-prepared composite. Samples

Initial discharge capacity (mAh g−1 )

Rate capacity (mAh g−1 )

Cyclic performance (mAh g−1 )

LVP in Ref. [23] LVP in Ref. [25] LVPF in Ref. [24] LVPF in Ref. [24] LVP–LVPF in this work

130 at 0.1 C (3.2–4.3) 140 at 0.2 C (3.0–4.4) 143 at 0.5 C (3.0–4.4) 138 at 0.2 C (3.0–4.4) 138.5 at 0.1 C (3.0–4.4)

113 at 2 C 120 at 5 C 110 at 3 C 89 at 5 C 122.3 at 10 C

101 at 2 C after 100 cycles 109 at 5 C after 50 cycles – 82 at 5 C after 50 cycles 119.2 at 10 C after 300 cycles

–: No data in details.

Fig. 5 shows the cyclic voltammetry (CV) for the prepared sample at a scan rate of 0.05 mV s−1 between 3.0 and 4.5 V. Redox peaks for both Li3−x V2 (PO4 )3 (3.62, 3.70, 4.12 V for oxidation peaks correspond to 3.56, 3.63, 4.01 for reduction peaks) and Li1−x VPO4 F

Table 4 Effective charge-transfer number (ne ) and effective concentration of lithium ions (C0e ) corresponding to various states. Peaks

State 

A1/A1 A2/A2 A3/A3 A4/A4

0.73Li2.5 V2 (PO4 )3 ·0.27LiVPO4 F 0.73Li2.0 V2 (PO4 )3 ·0.27LiVPO4 F 0.73LiV2 (PO4 )3 ·0.27LiVPO4 F 0.73LiV2 (PO4 )3 ·0.27VPO4 F

C0e (mol cm−3 )a −2

1.9 × 10 1.5 × 10−2 9.8 × 10−3 5.4 × 10−3

ne b 0.365 0.365 0.730 0.270

a c0e = xc0−Li3−y V2 (PO4 )3 + (1 − x)c0−Li1−z VPO4 F . Where x represents mole ratio of LVP in xLi3 V2 (PO4 )3 ·(1−x)LiVPO4 F, so here x = 0.73. b ne = xnLi3−y V2 (PO4 )3 + (1 − x)nLi1−z VPO4 F . Where x represents mole ratio of LVP in xLi3 V2 (PO4 )3 ·(1−x)LiVPO4 F, so here x = 0.73.

(4.28, 4.34 V for oxidation peaks correspond to 4.17 V for reduction peak) with small polarization are observed in the curves, which are consistent with the initial charge–discharge study. For Li1−x VPO4 F, there are two peaks in deintercalation process, while only one peak can be seen in intercalation process (signed with orange parallelogram). It can be explained that the extraction from two different Li+ sites need different energy, but all Li+ cations tend to take over

Fig. 4. (a) Initial charge/discharge curves and (b) cycle performance of Li/xLi3 V2 (PO4 )3 –LiVPO4 F half cell at various rates.

Fig. 5. CV curve at scan rate of 0.05 mV s−1 in the voltage range of 3.0–4.5 V.

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Table 5 The diffusion coefficients of lithium-ion in composite electrode calculated from CV based on the classical Randles Sevchik equation. State

0.73Li2.5 V2 (PO4 )3 ·0.27LiVPO4 F 0.73Li2.0 V2 (PO4 )3 ·0.27LiVPO4 F 0.73LiV2 (PO4 )3 ·0.27LiVPO4 F 0.73LiV2 (PO4 )3 ·0.27VPO4 F

Anodic oxidation process

Cathodic reduction process

Peak

Ds (cm2 s−1 )

Peak

Ds (cm2 s−1 )

A1 A2 A3 B1

8.6 × 10−10 3.1 × 10−9 1.2 × 10−9 5.7 × 10−9

A1 A2 A3 B1

1.4 × 10−9 1.7 × 10−9 1.3 × 10−9 1.8 × 10−9

only one of the Li+ sites during insertion process. Then the inserted Li+ cations will move from the occupied site to the other one automatically. Furthermore, cyclic voltammetry is widely used to obtain the apparent chemical coefficient of lithium ions. CV curves of prepared sample at scan rates of 0.05, 0.1, 0.2, 0.5, 1.0, 2.0 mV s−1 between 3.0 V and 4.5 V, combined with the relationship between the peak current (ip ) and the square root of scan rate (v1/2 ) are shown in Fig. 6(a) and (b). It can be observed from Fig. 6(a) that the anodic peaks shift to higher potential and the corresponding cathodic peaks turn to lower values, resulting in the increase of polarization and irreversibility with increasing scan rate. Also the heights of all peaks increase with increasing scan rate. For the semiinfinite and finite diffusion, the peak current is proportional to the

square root of the scan rate and can be expressed by the classical Randles Sevchik equation [33]: iP = 2.69 × 105 An3/2 C0 D1/2 v1/2

(3)

where ip is the peak current (A), A is the contact area between Li3 V2 (PO4 )3 /C and electrolyte (here 1.54 cm2 is used for simplicity), v is the potential scan rate (V s−1 ), D is the diffusion coefficient of lithium ion (cm2 s−1 ). Because the situation in this study is complex as the diffusion can likely be limited by the tortuous pass in the composite electrodes. We recommend the effective diffusion coefficients (Dse ) herein just for solid composite phase. n is the charge-transfer number, C0 is the concentration of lithium ions. Herein we used ne and C0e as effective one respectively, and the values are listed in Table 4. Based on Eq. (3) and the slope of ip versus v1/2 plots in Fig. 6(b), the effective diffusion coefficients of lithium ion in solid state composite are figured out and listed in Table 5. It can be seen that the values of Dse for the solid state composite are in the magnitude of 10−9 to 10−10 cm2 s−1 and close to each other, which indicates an improved ionic conductivity compared with that of Li3 V2 (PO4 )3 reported by others [33,34]. 4. Conclusions xLi3 V2 (PO4 )3 ·LiVPO4 F/C composite (x was designed 2.0 but 2.7 in the composite) was synthesized through mechanically activated chemical reduction for the first time. Two phases of Li3 V2 (PO4 )3 and LiVPO4 F coexist in the prepared powders and they inset into each other rather than simple mixing. The prepared composite exhibits excellent cycle performance and rate capability. The good electrochemical performance possessed by the synthesized sample is attributed to synergistic effect of Li3 V2 (PO4 )3 –LiVPO4 F composite, as well as carbon coating. Acknowledgement This study was supported by Major Special Plan of Science and Technology of Hunan Province, China (grant no. 2011FJ1005), Innovation Project of Academic Dissertation for Graduate Students of Central South University (no. 2011ssxt246). References

Fig. 6. Cyclic voltammetry of the prepared composite to derive lithium chemical diffusion coefficient. (a) The CV curves at various scan rates. (b) The line relation ship of the peak current (ip ) and square root of scan rate (1/2 ).

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