Improvement of electrochemical and thermal stability of [email protected] batteries by depositing amorphous silicon film

Improvement of electrochemical and thermal stability of [email protected] batteries by depositing amorphous silicon film

Electrochimica Acta 56 (2011) 4937–4941 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elec...

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Electrochimica Acta 56 (2011) 4937–4941

Contents lists available at ScienceDirect

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

Improvement of electrochemical and thermal stability of LiFePO4 @C batteries by depositing amorphous silicon film Yingbin Lin ∗ , Yanmin Yang, Ying Lin, Baozhi Zeng, Guiying Zhao, Zhigao Huang ∗∗ Department of Physics, Fujian Normal University, Fuzhou 350007, PR China

a r t i c l e

i n f o

Article history: Received 24 January 2011 Received in revised form 29 March 2011 Accepted 29 March 2011 Available online 8 April 2011 Keywords: Lithium-ion batteries Cathode materials Phosphor-olivine Amorphous silicon

a b s t r a c t Amorphous silicon (␣-Si) films are deposited on LiFePO4 @C electrode by using vacuum thermal evaporation deposition technique and the effect of ␣-Si film on electrochemical performance of LiFePO4 @C cells is investigated systematically by the charge–discharge testing, cyclic voltammograms and AC impedance spectroscopy, respectively. The results reveal that the present of ␣-Si film on electrode surface could remarkably improve the electrochemical performance at high charge/discharge rate, especially at elevated temperature. This enhancement may be attributed to the amelioration of the electrochemical dynamics on the electrode/electrolyte interface resulting from the beneficial effects of ␣-Si film, which might significantly suppress the rise of both of the surface film resistance and charge transfer resistance.

1. Introduction In recent years, the olivine-type LiFePO4 has attracted intensive attention as a promising cathode material for the high-power Li-ion batteries. Compared with other cathode materials, LiFePO4 has the advantage of excellent thermal safety, low cost and environmental benign [1–4]. However, electrochemical properties of the LiFePO4 based Li-ion secondary batteries against high temperature are foreseen to be challenging requirement to electric-vehicles applications [5,6]. At elevated temperature, poor storage and cycling behaviors were generally observed in LiFePO4 -based Li-ion batteries with conventional organic electrolyte solvents due to the dissolution of Fe from the cathode side, which leads to the severe degradation of the electrode/electrolyte interface, promoted by too high temperature. To improve the high-temperature performance of LiFePO4 , several strategies have been implemented such as metal-oxide coating [7–9], electrolyte improvement [10,11] as well as current-collectors modification [12,13]. For instance, Chang et al. [9] have found that the TiO2 coating helps to reduce capacity fading at for LiFePO4 /Li cell at 55 ◦ C. Wu et al. [13] have revealed that the presence of a either Au or Cu layer on anode electrode as an ionsieve could also reduce capacity fading at elevated temperature. In contrast, amorphous silicon (␣-Si) not only acts as a protective layer of LiFePO4 in the electrolyte but also have some dangling bonds, which represent defects in the continuous random network and

∗ Corresponding author. Tel.: +86 591 2286 8132; fax: +86 591 2286 8132. ∗∗ Corresponding author. Tel.: +86 591 2286 7399; fax: +86 591 2286 7399. E-mail addresses: [email protected] (Y. Lin), [email protected] (Z. Huang). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.03.134

© 2011 Elsevier Ltd. All rights reserved.

may cause anomalous electric behaviors. Thus, ␣-Si seems to be more promising candidates acting as new electrode-modification materials. But to the best of our knowledge, there are scarce reports on the deposition of silicon on electrode surface for lithium-ion batteries and its effects on the electrochemical performance of LiFePO4 are unknown. In this paper, we report the effect of ␣-Si film deposition on the electrochemical performance of LiFePO4 /Li cell. The results unequivocally demonstrated ␣-Si film deposited on electrode does enhance the electrochemical performance, especially at elevated temperature. 2. Experimental 2.1. Preparation and characterization of LiFePO4 @C cathode material Amounts of CH3 COOLi·2H2 O (AR), FeC2 O4 ·2H2 O (AR) and NH4 H2 PO4 (AR) and C12 H22 O11 were mixed in a stoichiometric ratio (nLi :nFe :np = 1:1:1) and 7.0 wt.% carbon content. First, the mixtures were planetary milled for 24 h by wet ball-milling in the alcohol solution to decrease the particle size of the reactants and ensure homogeneous mixing and then dried at 60◦ C. Then, the obtained precursors were sintered in a tube furnace at 350 ◦ C for 3 h and subsequently at 700 ◦ C for 12 h in a flowing Ar atmosphere to obtain LiFePO4 coated with carbon (denoted as LiFePO4 @C). The crystal structures of the powder samples were characterized by X-ray powder diffraction (XRD) with monochromatized Cu K␣-radiation and by Raman spectroscopy using Renishaw spectrometer. Their morphologies were detected by using a scanning electronic microscopy (SEM).

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2.2. Deposition of ˛-Si film on LiFePO4 electrode The cathode electrodes for the electrochemical evaluation were prepared by homogeneously pasting a slurry containing 80 wt.% active material (LiFePO4 @C) powder, 10 wt.% super-P and 10 wt.% polyvinylidene fluoride (PVDF) dissolved in N-methylpyrolline (NMP) onto an aluminum foil and dried at 110 ◦ C for 12 h under vacuum. Amorphous silicon (␣-Si) films were deposited onto electrodes by vacuum thermal evaporation deposition at a base pressure of 3 × 10−3 Pa with the crystalline Si as the vaporization source. The electrode was pressed under 8 Mpa and punched into 12.5 mm diameter circular disks. 2.3. Cell fabrication and characterization Standard 2025 coin cells were assembled in an Ar-filled glove box, using lithium metal foil as negative electrode, Celgard 2300 microporous polyethylene membrane as the separator and 1 M LiPF6 in a mixture of ethyl carbonate (EC) and dimethyl carbonate (DMC) (1:1 in vol. ratio) as the electrolyte. Cells were charged and discharged versus Li+ /Li on a battery cycler (LAND, CT2001A, China). Cyclic voltammetry (CV) was measured at a scan rate of 0.1 mV/s ranging from 2.5 to 4.5 V versus Li+ /Li. Electrochemical impedance spectroscopy (EIS) was performed with a CHI660C electrochemical workstation (China) with applied 10 mV sinusoidal perturbation in a frequency range from 100 KHz to 10 mHz at room temperature. 3. Results and discussion

Fig. 2. SEM photograph of LiFePO4 @C powder containing 7% carbon.

on the LiFePO4 @C electrode was carried out with Raman spectroscopy is employed to investigate the structure of the silicon phase on the LiFePO4 @C electrode Renishaw Raman microscope using radiation 785 nm, shown in Fig. 4. The resulting Raman spectrum is dominated by the stretching and bending vibration modes of PO4 3+ ions FeOx groups [15–17]. The bands centered around 165 and 490 cm−1 are typical features of amorphous silicon vibration modes, associated with transverse acoustic (TA) and transverse

3.1. Material characterization

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The X-ray diffraction pattern of the LiFePO4 @C composite containing 7% carbon is given in Fig. 1. All the observed diffraction peaks can be well indexed in the orthorhombic system (space group Pnma) [14]. Within the sensitivity of measurement, no obvious evidence for the presence of second phase and well-defined peaks indicate the phase purity and degree of crystallinity. The typical SEM images of LiFePO4 @C shown in Fig. 2 indicate that the homogeneous particles size is in nanometer range. The sugar added in the reactants prevents the agglomeration of LiFePO4 to a certain extent during the calcinations. Moreover, the carbon coating is considered to protect LiFePO4 from oxidation, enhancing the electrochemical performance. Fig. 3 shows the surface morphologies of LiFePO4 @C and LiFePO4 @C/␣-Si film electrode and nano-scale spherical particles are observed on the surface of the LiFePO4 @C/␣Si film electrode. Investigation on the structure of the silicon phase

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2 / degree Fig. 1. XRD pattern of LiFePO4 @C composites containing 7% carbon.

Fig. 3. Surface morphologies of (a) LiFePO4 @C and (b) LiFePO4 @C/␣-Si film electrode.

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Fig. 6. The typical cyclic voltammogram curves of the LiFePO4 @C and LiFePO4 @C/␣Si film at a scanning rate of 0.05 mV s−1 .

optic (TO) phonons, indicating the prepared Si films on LiFePO4 @C electrode have an amorphous structure [18,19]. The band around 630 cm−1 is associated with the 2LA second-order phonon Raman scattering and the TO + TA overtone [20].

bonded to the silicon dangling bonds in ␣-Si film [24]. As a result, the electrochemical performance is enhanced. Fig. 6 presents the typical cyclic voltammogram curves of the LiFePO4 @C and LiFePO4 @C/␣-Si film at a scanning rate of 0.1 mV/s. They both exhibit a pair of redox peaks around 3.4 V, which corresponds to the charge/discharge reaction of the Fe3+ /Fe2+ redox couple [25]. However, the peak profiles of LiFePO4 @C/␣-Si film are more symmetric. The voltage separation of LiFePO4 @C is 0.17 V, whereas that of LiFePO4 @C/␣-Si film is 0.14 V, which indicates the lower electrode polarization and high lithium ion diffusivity in LiFePO4 @C/␣-Si film composite [26]. As for cyclic voltammogram, the potential interval between anodic and cathodic peaks is an important parameter to value the electrochemical reaction reversibility. The well-defined peaks and smaller value of voltage separation indicate the improvement of electrode reaction reversibility by the deposition of ␣-Si film. To get insight into the reason behind the enhancement of electrochemical performance via depositing ␣-Si film, the stability of LiFePO4 @C and LiFePO4 @C/␣-Si film in the presence of electrolyte at elevated temperature was investigated. The cells of LiFePO4 @C Vs Li and LiFePO4 @C/␣-Si film Vs Li were thermal-annealed at 60 ◦ C for 2 days, denoted as Sample A and Sample B respectively. The cycling performances of Sample A and B are tested and the charge–discharge processes of the samples are taken for 10 cycles at 0.5 C, 1.0 2.0 C and 3.0 C, respectively. As shown in Fig. 7, the beneficial effect of the ␣-Si film in reducing capacity fading of LiFePO4 @C cell is clearly observed. For instance, the discharge

3.2. Electrochemical properties Fig. 5 presents the charge/discharge profile evolution of several cycles of the ␣-Si film coated and uncoated LiFePO4 @C at 0.2 C and 60 ◦ C in the voltage range of 2.5–3.9 V. Compared with LiFePO4 @C/␣-Si film, the charge/discharge capacity fade of LiFePO4 @C is found to be more pronounced with cycling. The Fe dissolution from the cathode active material in conventional LiPF6 electrolyte is generally considered as one major cause to the observed poor electrochemical performance at elevated temperature in LiFePO4 -based Li-ion batteries. The irreversible structure changes occurring at the surface of LiFePO4 do not favor the Li insertion–deinsertion process. The deposition of ␣-Si film on electrode surface could prevent efficiently direct contact between the cathode material and the electrolyte, improving the structural stability and so on. Similar results have been reported in other systems [9,21]. On the other hand, traces of HF in LiPF6 solution at elevated temperature is well known to be responsible for the iron dissolution, which is considered as an important reason of the possible surface charges on the active materials [22,23]. The amount of H+ is expected to suppressed during cycling by means of hydrogen

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Cycle Number Fig. 5. The discharge-capacity versus cycle number for LiFePO4 @C and LiFePO4 @C/␣-Si film at 0.2 C and 60 ◦ C in the voltage range of 2.5–3.9 V.

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Fig. 9. The EIS profiles of Sample A and Sample B after 40 cycles.

capacity of the Sample A at 0.5 C, 1.0 C, 2.0 and 3.0 C are 113 mAh/g, 95 mAh/g, 70 mAh/g and 41 mAh/g, while the corresponding capacity of the Sample B remains 135 mAh/g, 122 mAh/g, 109 mAh/g and 98 mAh/g, respectively. Fig. 8 presents the charge–discharge profiles of cells containing LiFePO4 @C and LiFePO4 @C/␣-Si film with increasing charge–discharge rate from 0.5 to 3 C between 2.5 and 4.2 V. At a charge–discharge of 0.5 C, both samples exhibit typical flat charge and discharge plateaus around 3.4 V, corresponding to the Fe2+ /Fe3+ redox reaction. While the plateau separation for Sample B (135 mV) is lower than that of Sample A (282 mV), which corresponds to the lower electrochemical polarization suggesting the faster decrease of conductivity for Sample A. Furthermore, the polarization of the Sample A enlarges faster with increasing charge–discharge rate than that of Sample B. The LiFePO4 @C/␣Si film electrode shows stable discharge plateau voltage of 3.15–3.23 V versus Li+ /Li at 3 C rate. Whereas the discharge voltage of LiFePO4 @C is only about 2.5 V versus Li+ /Li. These test results indicate that ␣-Si film on electrode greatly reduces the electrode resistance, which contributes to good cycle stability of the cells. The beneficial effect of ␣-Si film on thermal-storage stability at elevated temperature is investigated by electrochemical impedance spectroscopy measurement. Fig. 9 shows the roomtemperature AC impedance of Sample A and B after 40 cycles at the fully discharge state. The overall shape of Nyquist plot is composed of a depressed semicircle in high frequency region and an oblique straight line in low frequency region. The curve profile is typical of the electrochemical reaction associated with Li-ion intercalation. The resistance of the semi-circle can be considered as the total interfacial resistance consisting of the resistance of charge transfer at the solid-film interface and the resistance of Li+ migration through the SEI film [9]. It has been well established that the dis-

solved Fe2+ is readily reduced at the interface and the iron metal acts as a catalyst in the formation and growth of an interfacial film at the interface. The formation of the solid electrolyte interface layer consumes Li ions and induces high surface resistance [27]. The results indicate that the ␣-Si film deposition significantly suppress the rise of both of the surface film resistance and charge transfer resistance. In addition, the formation of an oxide film at the silicon/electrolyte interface has to be considered in accounting for the enhancement of electrochemical performance with ␣-Si film deposition. With the present of O2 resulting from the degradation of the cathode materials [28], an oxide film of several monolayers is expected to form at the silicon/electrolyte interface, which acts as a barrier to the interaction between the LiFePO4 and the electrolyte. Meanwhile, some silicon atoms would enter into the oxide phase and be partially oxidized near the silicon/oxide interface region [29]. The partially oxidized silicon atoms have relatively high energy levels and are capable of injecting electrons into the conduction band and generating a current at the end of the oxide dissolution induced by HF [30].

4. Conclusions ␣-Si films were deposited on LiFePO4 @C electrode via vacuum vaporization method. Compared to LiFePO4 @C, the LiFePO4 @C/␣-Si film composite has a better rate performance and thermal stability. The ␣-Si films on LiFePO4 @C surface is considered to play a positive role in suppressing the dissolution of Fe2+ from the olivine material, which helps to reduce capacity fading at elevated temperature.

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Acknowledgements We gratefully acknowledge the financial support by the National Science Fund for Young Scholars (No. 11004032), Natural Science Foundation of China (No. 11074039) and Fujian Province Science Fund for Young Scholars (No. 2008F3039). References [1] P.G. Bruce, B. Scrosati, J.-M. Tarascon, Angew. Chem. Int. Ed. 47 (2008) 2930. [2] K. Zaghib, A. Mauger, J.B. Goodenough, F. Gendron, C.M. Julien, Chem. Mater. 19 (2007) 3740. [3] B. Kang, G. Ceder, Nature 458 (2009) 190. [4] B. Scrosati, J. Garche, J. Power Sources 195 (2010) 2419. [5] K. Amine, J. Liu, I. Belharouak, Electrochem. Commun. 7 (2005) 669. [6] J. Shim, R. Kostecki, T. Richardson, X. Song, K.A. Striebel, J. Power Sources 112 (2002) 222. [7] Y.D. Li, S.X. Zhao, C.W. Nan, B.H. Li, J. Alloys Compd. 509 (2011) 957. [8] Y. Liu, C.H. Mi, C.Z. Yuan, X.G. Zhang, J. Electroanal. Chem. 628 (2009) 73. [9] H.H. Chang, C.C. Chang, C.Y. Su, H.C. Wu, M.H. Yang, N.L. Wu, J. Power Sources 185 (2008) 466. [10] C.C. Chang, T.K. Chen, J. Power Sources 193 (2009) 834. [11] S.S. Zhang, K. Xu, T.R. Jow, J. Power Sources 159 (2006) 702. [12] H.C. Wu, H.C. Wu, E. Lee, N.L. Wu, Electrochem. Commun. 12 (2010) 488. [13] H.H. Chang, H.H. Wu, N.L. Wu, Electrochem. Commun. 10 (2008) 1823. [14] S.H. Luo, Z.L. Tang, J.B. Lu, Z.T. Zhang, Ceram. Int. 34 (2008) 1349.

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