- Email: [email protected]
titanium carbide nanocomposite film anode for Li-ion batteries
Thin Solid Films 517 (2009) 4767–4771
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Electrochemical investigation on silicon/titanium carbide nanocomposite film anode for Li-ion batteries Z.Y. Zeng, J.P. Tu ⁎, X.H. Huang, X.L. Wang, J.Y. Xiang Department of Materials Science and Engineering, State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Available online 10 March 2009 Keywords: Nanosized Si Titanium carbide Nanocomposite film Lithium ion battery
a b s t r a c t A Si/TiC nanocomposite film was synthesized by a surface sol–gel method in combination with a following heat-treatment process. The electrochemical properties of the film anode for lithium ion batteries were investigated by galvanostatic charge–discharge tests, cyclic voltammetry (CV) and electrochemical impedance spectrum (EIS). Because of the homogeneous distribution of Si active particles in TiC matrix, the Si/TiC composite showed reversible lithium storage capacities of about 1000 and 1300 mAh g− 1 at 160 and 80 mA g− 1 even after 80 cycles, respectively. Using two-parallel diffusion path model, the reactive mechanisms of Li with Si/TiC composite film were interpreted. The chemical diffusion coefficients of the Si/ TiC nanocomposite film at different electrode potentials were also discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently there have been intensive research activities in lithium ion batteries due to their high energy density. The developments of lithium ion batteries are based on lithiation and delithiation mechanism of host materials, which is one of the most important areas in modern electrochemistry. Silicon, with its high theoretical capacity of 4200 mAh g− 1, abound resource, is one of the most attractive candidates for lithium ion batteries. However, the cycling performance of silicon is poor, owing to its severe volume expansion and shrinkage during the insertion and extraction of lithium ions, which results in pulverization of Si particles and eventually loss of Li+ storage ability [1]. Generally, the characterization of the lithiation process of Si and Si-based composite has been mainly performed using galvanostatic charging, coupled, in some cases, with in-situ XRD (X-ray diffraction) technique [2]. Strong correlation between the results obtained by both techniques has created a reliable basis for identification of the Li–Si alloy compounds. Nevertheless, the problem of the cycling stability of Sibased compounds, which attracts much attention in connection with the properties of the films formed on the surface of Si particles when these electrodes are polarized to low potentials in Li salt solutions, cannot be fully characterized by using these two routine techniques. The need for optimization of lithium ion batteries requires precise information on the mass and charge transport processes during lithiation, such as the solid state difusion
of lithium ions. Thus, some kinetic studies on the lithium intercalation reaction have also been carried out using several electrochemical techniques such as the potentiostatic intermittent titration technique (PITT) [3], cyclic voltammetry (CV) [4], and electrochemical impedance spectrum (EIS) [5,6]. Among the above-mentioned electrochemical techniques, EIS and CV are powerful tools to analyze the electrode reactions. By using a sol–gel method in combination with a following heat-treatment process, a Si/TiC nanocomposite film, in which Si nanoparticles were dispersed homogeneously into the TiC particles, was synthesized, and its primary electrochemical properties were investigated previously [7]. In this experiment, only the Si/TiC composite with mole ratio Si:TiC = 4 was investigated. This specimen selection was based on our previous result that it had higher specific capacity and good cyclability. That to say, we should get high proportion of Si active materials to get high specific capacity, at the same time, there must be enough TiC matrix to buffer the expansion of active Si materials, thus get good reversibility. So, this specific ratio is the compromise between specific capacity and reversibility. Because of its good charge/discharge cyclability as anode material for lithium ion batteries, in this present work, simultaneous measurements including cyclic voltammetry (CV) and electrochemical impedance (EIS) characterization of the Si/TiC composite film were investigated. The detail reactive information on the phase transition process obtained from the simultaneous EIS and CV measurements and analysis of the film electrode were also discussed. 2. Experimental
⁎ Corresponding author. Tel.: +86 571 87952573; fax: +86 571 87952856. E-mail address: [email protected] (J.P. Tu). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.007
Nanosized Si powder with particle sizes of 30−50 nm was purchased from SinoSi, China, which were prepared by laser driven
4768
Z.Y. Zeng et al. / Thin Solid Films 517 (2009) 4767–4771
saline gas reaction. The Si/TiC composite film was synthesized by a surface sol–gel process as described in a previous work [7]. After calcined at 700 °C under carbonous ambience, the Si/TiC nanocomposite film on the Cu substrate was obtained. In this work, the composite film contained 65.2 wt.% Si, i.e., the mole ratio of Si and TiC was 4:1. The thickness of the composite film was about 500 nm, as determined by an Alpha-step 200 profilometry. The morphology and structure of the composite film were observed by a scanning electron microscopy (SEM, SIRION JY/ T010-1996). The film electrodes were dried in vacuum at 95 °C for 12 h. Test cells were assembled in an argon-filled glove box using Li foil as counter electrode, polypropylene film as separator, and an electrolyte of 1 M LiPF6 in a 50:50 (w/w) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). All cells were tested at a constant current density of 160 and 80 mA g− 1 with the cut-off voltages from 0.02 V to 3 V (vs. Li/Li+). Cyclic voltammetry (CV) measurements were tested at a scan rate of 0.1 mV s− 1 between 0 and 3.5 V. The electrochemical impedance spectroscopy (EIS) was measured by applying an ac voltage of 5 mV over the frequency range from 10 kHz to 10 mHz. 3. Results and discussion Fig. 1a shows the cycling performances of the Si/TiC nanocomposite film anode with cut-off voltages from 0.02 to 3 V (vs. Li/Li+) at current densities of 160 and 80 mA g− 1. In the case of high
current density (160 mA g− 1), the first discharge (lithiation) capacity is 1564.3 mAh g− 1, but its charge (delithiation) capacity is only 936.3 mAh g− 1, that to say, its first coulombic efficiency is 59.9%. Whereas, in case of low current density (80 mA g− 1), the first discharge and charge capacity are 2039.7 and 1249.6 mAh g− 1, respectively, i.e. the first coulombic efficiency is 61.3%, which is a little higher than that tested at high current density. The reason, may be at low current density case, the bloat/contract effect caused by the intercalation/deintercalation of lithium ions from the composite is slow than that of high current density, thus the speed of accumulating lithium ions in the composite is also slow, so the phase transition or chemical reaction is slow, which guarantee the structure stability of the composite and attain a little higher initial coulombic efficiency. From the data shown in Fig. 1a, it can be seen that both the coulombic efficiencies for these two cases of the first few cycles are not high enough but increasing gradually, indicating that it needs several cycles to establish electrochemically stabled electrode for Li+ insertion/extraction. From 8th to 80th cycle, both the discharge/ charge capacities at current densities of 160 and 80 mA g− 1 keep a steady level of 1000 and 1300 mAh g− 1, respectively. The coulombic efficiencies at high current density achieve as high as 97% and upwards, wherever at low current density they achieve a magnitude of 95% or so. That to say, when the charge/discharge current density increases, the reversible capacities decrease but the coulombic efficiencies increase. Considering the lower initial coulombic efficiency, the increase of coulombic efficiency in the following charge/discharge cycles is reasonable for the higher current density case. Such promoted cyclability is attributed to the effect of less active TiC [7]. Besides, there is an interesting phenomenon that should be emphasized. In case of low current density, the charge capacity begins to surpass the discharge capacity since the 60th cycle, i.e. the coulombic efficiencies exceed 100%. This phenomenon can be ascribed to the inserted Li ions during initial 60 cycling tests that were not fully de-inserted timely, deintercalated or removed after 60th cycles. At a high current density, the same phenomenon also existed, but not obvious as that at a low current density. That means low current density case can accumulate more lithium ions because of its slow diffusion or reaction process before 60th cycle, when the extra accumulation of lithium ions in the compsite achieve a maximum level, it will release these extra ions and cause the over 100% coulombic efficiency effect. Fig. 1b shows the first galvanostatic charge/discharge profiles of the Si/TiC nanocomposite film at 160 and 80 mA g− 1. In the discharge state, there are two discharge plateaus centered at 1.25 and 0.05 V for both current densities. In the charge state, the charge plateaus that attributed to the reaction of Li with Si for both the cases are centered at 0.5 V. However, the potential differences between the charge and discharge plateaus, which correspond to the oxidation and reduction of Li with the active composite anode, respectively, are less at the low current density than those at the high current density, indicating a lower polarization when the current density is reduced. Fig. 2 compares the cyclic voltammograms of the Si/TiC nanocomposite film between 0 and 3.5 V (vs. Li/Li+) at scan rates of 0.1 and 0.05 mV s− 1. Five cathodic peaks (Li insertion state) centered at 2.42, 1.58, 0.81, 0.18 and 0.01 V, and three anodic peaks (Li extraction state) centered at 0.35, 0.53 and 2.48 V are observed for both the scan rates. Two cathodic peaks at 0.18 and 0.01 V and two anodic peaks at 0.35 and 0.53 V are attributed to the reversible reaction of Li with Si, as indicated in the following reaction: þ
−
Lix Si X xLi þe þSi Fig. 1. (a) The charge/discharge capacity and coulombic efficiency of Si/TiC composite film as function of the cycle number at current densities of 160 and 80 mA g− 1, (b) The first galvanostatic charge/discharge profiles of Si/TiC composite film at current densities of 160 and 80 mA g− 1.
ð1Þ
[8] The other voltammetric peaks, such as the cathodic peaks at 2.42 and 1.58 V and the anodic peaks at 2.48 V, are similar to the reactions
Z.Y. Zeng et al. / Thin Solid Films 517 (2009) 4767–4771
4769
Fig. 2. Cycle voltammetry of Si/TiC composite film electrode between 0 and 3.5 V (vs. Li/Li+) at a scan rate of 0.1 and 0.05 mV s− 1.
of Li with spinel Li[Li1/3Ti5/3]O4, and little anatase, which occur as follows: h i h i þ − Li þ e þLi Li1=3 Ti5=3 O4 X Li2 Li1=3 Ti5=3 O4
ð2Þ
[9] þ
−
Li þe þTiO2 ðanatase; sÞ X Lix TiO2 ðsÞ
ð3Þ
[10] For the anodic/cathodic peaks located at around 2.5 V, it cannot be made sure whether it is attributed to the reaction of Li with TiC or may be attributed to the decomposition of the Li2O [11]. According to Poizot's research [12], the transition metal oxides can decrease the binding energy of Li2O tremendously. As a result, Li2O should be easily decomposed. Besides, there is a cathodic peak at 0.82 V yet, which may be attributed to the reaction of Li+ with not well crystallized titanium-based compound [13]. From Fig. 2, it can be seen that the CV peaks that represent the peak current Icv locate at the same potentials at different current densities, but the area that CV curve encircled for the slow scan rate (0.05 mV s− 1) is larger than that for the quick scan rate (0.1 mV s− 1), which is in good agreement with the charge/ discharge capacities shown in Fig. 1. Fig. 3a and b represent the magnified SEM micrographs of the cycled Si/TiC composite electrode at current densities of 160 and 80 mA g− 1. There appears to be little change in the morphology of the particles before and after cycling, and both the tested composites seem to be more compact than the fresh material [7]. Also, the tested film is porous as can be seen in Fig. 3, these pores can provide enough spaces for the expansion/contraction of Li–Si alloys, which prevent the exfoliation and the contact loss of the active Si. On the other hand, lithium ions in electrolyte can facilely get into these pores to reach the surface of active Si. Thus, the transfer distance of lithium ion is abbreviated. These compact structures show good structural stability of the nanocomposite film anode, which guarantee excellent electrochemical performances. To investigate in more detail the electrochemical behavior of the Si/TiC composite film, impedance spectrum measurements were conducted. Fig. 4 shows a typical Nyquist plot of the Si/TiC composite film. A squashed semicircle with an approximate unit slope appears in the Nyquist plot for the porous electrode in the limit of very high frequencies, which is due to the distributed character of the impedance at finite κ and σ [14]. From the high-frequency semicircle (HFS) at the characteristic frequency of 463 Hz, the value of the
Fig. 3. SEM micrographs of the Si/TiC nanocomposite film: galvanostatic tested anodes at current densities of (a) 160 mA g− 1; and (b) 80 mA g− 1.
double-layer capacitance can be obtained as Cdl = 40.4 µF, considering the surface area of the current collector is 1 cm2. The distributed character of the impedance for a porous electrode acquires pronounced typical feature in the middle-frequency domain. This impedance deviates completely from a semi-infinite Warburg-type behavior, as can be seen from the characteristic arc shown in Fig. 4. When the frequency becomes very low, the impedance shows a capacitive component, which is a sloping line, and it will become very steep at the lowest frequencies (the millihertz region). Using two-
Fig. 4. Typical Nyquist plot of the Si/TiC composite film obtained with the electrode area of 1 cm2, and frequency range from 10 kHz to 10 mHz.
4770
Z.Y. Zeng et al. / Thin Solid Films 517 (2009) 4767–4771
Fig. 5. (a) Equivalent circuit analogue for a two-parallel diffusion path model accounting for the ion migration through the surface layer covering each particle (high-frequency Rsl║Csl semicircle, with Rsl and Csl standing for the resistance of the migration and capacity of the layer, respectively), double-layer charging and slow interfacial charge transfer (middlefrequency Rct║Cdl semicircle, with Rct and Cdl designating the related charge-transfer resistance and a double-layer capacitance) and a finite-diffusion Warburg element, FSW. Indices 1 and 2 refer to paths (branches) 1 and 2, respectively; (b) Comparison between the experimental impedance spectra and that calculated using two-parallel diffusion paths model.
parallel diffusion path model [15], the distributed character of the Si/ TiC composite film can be simplified as a porous electrode in which there are two different spherical particle size distributions (radius Rs,1 and Rs,2). Combining the impedance models of Meyers et al. [14] and Levi et al. [15], an equivalent circuit of two-parallel diffusion paths is presented in Fig. 5a. The corresponding impedance equations are presented previously [15,16]. From the impedance fitting processes achieved by us, this two-parallel diffusion path model fitted profoundly well with the experimental data of the impedances for the Si/TiC electrode, as shown in Fig. 5b. Based on the two-parallel diffusion path model, when the semiinfinite diffusion conditions are applied, the variation of Warburg slope σ with the open-circuit cell voltage (or the stage of charge) E is 1=2 σ = ðdE=dXÞVm = nFA 2DÞ
particles in the following layer, then the total surface area of the active porous electrode composite is 12.56 cm2. The chemical diffusion coefficients at different potentials are calculated using Eq. (4) and presented in Fig. 6. From the data shown in this figure, the value of DLi is on the order of 10− 12 cm2 s− 1. In comparison with the log D vs E dependence and the corresponding CV peak, it can be seen that the minima of D is in accordance with the maxima of the CV peaks. For example, there is a cathodic peak ranging from 0.78 to 0.83 V and
ð4Þ
where Vm is the molar volume of the Si/TiC composite (12.04 cm3 mol− 1), the dE/dX is the gradient of open-circuit potential vs. the composition x, n is the number of electrons transferred and F is the Faraday constant, and A is the active surface area. As discussed above, there are some characteristic sizes which have intensive distribution in the nanocomposite film. To simplify the calculation model, we select two characteristic radii of the spherical particles, Rs,i, 25 nm and 100 nm, respectively. Assuming the radii of the particles at the same layer are identical, then the composite film with geometric surface area of 1 cm2 and thickness of 500 nm have two layers with the large particles and two layers with the small particles, and particles in the upper layer are located exactly above the
Fig. 6. Variation of diffusion coefficient with the open-circuit potential for the Si/TiC composite film calculated from Eq. (4).
Z.Y. Zeng et al. / Thin Solid Films 517 (2009) 4767–4771
centered at 0.81 V, then the chemical diffusion coefficient measured at 0.81 V will be lower than that measured at any other potentials at 0.78−0.83 V. As for this interpretation, it has been previously shown by strict thermodynamic base that the minima in D (vs E) corresponds approximately to equal molar amounts of the coexisting phases, whereas D approaches maximal values for the pure phases (e.g., X = 0, X = 1). In other words, the minima in D and the peak currents of the slow scan rate CV curves for these electrodes, appear at the same E values, which correspond to the midpoint of the specific phase transition. This shape of the D vs E curves may simply be a result of the phase transition which characterizes the Li–Si/TiC composite intercalation process. Comparing the chemical diffusion coefficients for the current densities of 160 and 80 mA g− 1 at each potential, it can be seen that the D value calculated at the current density of 80 mA g− 1 is a little higher than that calculated at 160 mA g− 1. This phenomenon should be arisen from the lower polarization of the composite anode at 80 mA g− 1, which made the insertion process more accessible and the insertion depth increased. 4. Conclusions The Si/TiC nanocomposite film was prepared by a surface sol–gel process. Because of the homogeneous distribution of Si active particles in TiC matrix, the Si/TiC composite film showed reversible lithium storage capacities of about 1000 and 1300 mAh g− 1 at current densities of 160 and 80 mA g− 1 even after 80 cycles, respectively. Using two-parallel diffusion path model, the reactive mechanism of Li
4771
with Si/TiC composite was interpreted. By investigating the chemical diffusion coefficient of the Si/TiC composite at different electrode potentials, it could be concluded that, the minima of DLi and the maxima peak currents of the slow scan rate CV curves, appeared at the same E values, which was in accordance with the midpoint of the specific phase transition. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
J.O. Besenhard, J. Yang, M. Winter, J. Power Sources 68 (1997) 87. T. Ohzuku, Y. Iwakoshi, K.J. Sawai, J. Electrochem. Soc. 140 (1993) 2490. M.D. Levi, E.A. Levi, D. Aurbach, J. Electroanal. Chem. 421 (1997) 89. D.Q. Shi, J.P. Tu, Y.F. Yuan, H.M. Wu, Y. Li, X.B. Zhao, Electrochem. Commun. 8 (2006) 1610. Z.Y. Zeng, J.P. Tu, X.L. Wang, X.B. Zhao, Electrochem. Solid-State Lett. 11 (2008) A105. M.D. Levi, D. Aurbach, J. Phys. Chem. B 101 (1997) 4630. Z.Y. Zeng, J.P. Tu, Y.Z. Yang, J.Y. Xiang, X.H. Huang, F. Mao, M. Ma, Electrochim. Acta 53 (2008) 2724. G.X. Wang, J.H. Ahn, J. Yao, Electrochem. Commun. 6 (2004) 689. L. Kavan, J. Prochazka, T.M. Spitler, M. Kalbac, M. Zukalova, T. Drezen, M. Gratzel, J. Electrochem. Soc. 150 (2003) A1000. L. Kavan, M. Gratzel, J. Rathousky, A. Zukal, J. Electrochem. Soc. 143 (1996) 394. Y.M. Kang, K.T. Kim, J.H. Kim, H.S. Kim, P.S. Lee, J.Y. Lee, H.K. Liu, S.X. Dou, J. Power Sources 133 (2004) 252. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496. C.M. Shen, X.G. Zhang, Y.K. Zhou, H.L. Li, Mater. Chem. Phys. 78 (2002) 437. J.P. Meyer, M. Doyle, R.M. Darling, J. Newman, J. Electrochem. Soc. 147 (2000) 2930. M.D. Levi, D. Aurbach, J. Phys. Chem. B 108 (2004) 11693. Z.Y. Zeng, J.P. Tu, X.L. Wang, X.B. Zhao, J. Electroanal. Chem. 616 (2008) 7.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Electrochemical investigation on silicon/titanium carbide nanocomposite film anode for Li-ion batteries Z.Y. Zeng, J.P. Tu ⁎, X.H. Huang, X.L. Wang, J.Y. Xiang Department of Materials Science and Engineering, State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Available online 10 March 2009 Keywords: Nanosized Si Titanium carbide Nanocomposite film Lithium ion battery
a b s t r a c t A Si/TiC nanocomposite film was synthesized by a surface sol–gel method in combination with a following heat-treatment process. The electrochemical properties of the film anode for lithium ion batteries were investigated by galvanostatic charge–discharge tests, cyclic voltammetry (CV) and electrochemical impedance spectrum (EIS). Because of the homogeneous distribution of Si active particles in TiC matrix, the Si/TiC composite showed reversible lithium storage capacities of about 1000 and 1300 mAh g− 1 at 160 and 80 mA g− 1 even after 80 cycles, respectively. Using two-parallel diffusion path model, the reactive mechanisms of Li with Si/TiC composite film were interpreted. The chemical diffusion coefficients of the Si/ TiC nanocomposite film at different electrode potentials were also discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently there have been intensive research activities in lithium ion batteries due to their high energy density. The developments of lithium ion batteries are based on lithiation and delithiation mechanism of host materials, which is one of the most important areas in modern electrochemistry. Silicon, with its high theoretical capacity of 4200 mAh g− 1, abound resource, is one of the most attractive candidates for lithium ion batteries. However, the cycling performance of silicon is poor, owing to its severe volume expansion and shrinkage during the insertion and extraction of lithium ions, which results in pulverization of Si particles and eventually loss of Li+ storage ability [1]. Generally, the characterization of the lithiation process of Si and Si-based composite has been mainly performed using galvanostatic charging, coupled, in some cases, with in-situ XRD (X-ray diffraction) technique [2]. Strong correlation between the results obtained by both techniques has created a reliable basis for identification of the Li–Si alloy compounds. Nevertheless, the problem of the cycling stability of Sibased compounds, which attracts much attention in connection with the properties of the films formed on the surface of Si particles when these electrodes are polarized to low potentials in Li salt solutions, cannot be fully characterized by using these two routine techniques. The need for optimization of lithium ion batteries requires precise information on the mass and charge transport processes during lithiation, such as the solid state difusion
of lithium ions. Thus, some kinetic studies on the lithium intercalation reaction have also been carried out using several electrochemical techniques such as the potentiostatic intermittent titration technique (PITT) [3], cyclic voltammetry (CV) [4], and electrochemical impedance spectrum (EIS) [5,6]. Among the above-mentioned electrochemical techniques, EIS and CV are powerful tools to analyze the electrode reactions. By using a sol–gel method in combination with a following heat-treatment process, a Si/TiC nanocomposite film, in which Si nanoparticles were dispersed homogeneously into the TiC particles, was synthesized, and its primary electrochemical properties were investigated previously [7]. In this experiment, only the Si/TiC composite with mole ratio Si:TiC = 4 was investigated. This specimen selection was based on our previous result that it had higher specific capacity and good cyclability. That to say, we should get high proportion of Si active materials to get high specific capacity, at the same time, there must be enough TiC matrix to buffer the expansion of active Si materials, thus get good reversibility. So, this specific ratio is the compromise between specific capacity and reversibility. Because of its good charge/discharge cyclability as anode material for lithium ion batteries, in this present work, simultaneous measurements including cyclic voltammetry (CV) and electrochemical impedance (EIS) characterization of the Si/TiC composite film were investigated. The detail reactive information on the phase transition process obtained from the simultaneous EIS and CV measurements and analysis of the film electrode were also discussed. 2. Experimental
⁎ Corresponding author. Tel.: +86 571 87952573; fax: +86 571 87952856. E-mail address: [email protected] (J.P. Tu). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.007
Nanosized Si powder with particle sizes of 30−50 nm was purchased from SinoSi, China, which were prepared by laser driven
4768
Z.Y. Zeng et al. / Thin Solid Films 517 (2009) 4767–4771
saline gas reaction. The Si/TiC composite film was synthesized by a surface sol–gel process as described in a previous work [7]. After calcined at 700 °C under carbonous ambience, the Si/TiC nanocomposite film on the Cu substrate was obtained. In this work, the composite film contained 65.2 wt.% Si, i.e., the mole ratio of Si and TiC was 4:1. The thickness of the composite film was about 500 nm, as determined by an Alpha-step 200 profilometry. The morphology and structure of the composite film were observed by a scanning electron microscopy (SEM, SIRION JY/ T010-1996). The film electrodes were dried in vacuum at 95 °C for 12 h. Test cells were assembled in an argon-filled glove box using Li foil as counter electrode, polypropylene film as separator, and an electrolyte of 1 M LiPF6 in a 50:50 (w/w) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). All cells were tested at a constant current density of 160 and 80 mA g− 1 with the cut-off voltages from 0.02 V to 3 V (vs. Li/Li+). Cyclic voltammetry (CV) measurements were tested at a scan rate of 0.1 mV s− 1 between 0 and 3.5 V. The electrochemical impedance spectroscopy (EIS) was measured by applying an ac voltage of 5 mV over the frequency range from 10 kHz to 10 mHz. 3. Results and discussion Fig. 1a shows the cycling performances of the Si/TiC nanocomposite film anode with cut-off voltages from 0.02 to 3 V (vs. Li/Li+) at current densities of 160 and 80 mA g− 1. In the case of high
current density (160 mA g− 1), the first discharge (lithiation) capacity is 1564.3 mAh g− 1, but its charge (delithiation) capacity is only 936.3 mAh g− 1, that to say, its first coulombic efficiency is 59.9%. Whereas, in case of low current density (80 mA g− 1), the first discharge and charge capacity are 2039.7 and 1249.6 mAh g− 1, respectively, i.e. the first coulombic efficiency is 61.3%, which is a little higher than that tested at high current density. The reason, may be at low current density case, the bloat/contract effect caused by the intercalation/deintercalation of lithium ions from the composite is slow than that of high current density, thus the speed of accumulating lithium ions in the composite is also slow, so the phase transition or chemical reaction is slow, which guarantee the structure stability of the composite and attain a little higher initial coulombic efficiency. From the data shown in Fig. 1a, it can be seen that both the coulombic efficiencies for these two cases of the first few cycles are not high enough but increasing gradually, indicating that it needs several cycles to establish electrochemically stabled electrode for Li+ insertion/extraction. From 8th to 80th cycle, both the discharge/ charge capacities at current densities of 160 and 80 mA g− 1 keep a steady level of 1000 and 1300 mAh g− 1, respectively. The coulombic efficiencies at high current density achieve as high as 97% and upwards, wherever at low current density they achieve a magnitude of 95% or so. That to say, when the charge/discharge current density increases, the reversible capacities decrease but the coulombic efficiencies increase. Considering the lower initial coulombic efficiency, the increase of coulombic efficiency in the following charge/discharge cycles is reasonable for the higher current density case. Such promoted cyclability is attributed to the effect of less active TiC [7]. Besides, there is an interesting phenomenon that should be emphasized. In case of low current density, the charge capacity begins to surpass the discharge capacity since the 60th cycle, i.e. the coulombic efficiencies exceed 100%. This phenomenon can be ascribed to the inserted Li ions during initial 60 cycling tests that were not fully de-inserted timely, deintercalated or removed after 60th cycles. At a high current density, the same phenomenon also existed, but not obvious as that at a low current density. That means low current density case can accumulate more lithium ions because of its slow diffusion or reaction process before 60th cycle, when the extra accumulation of lithium ions in the compsite achieve a maximum level, it will release these extra ions and cause the over 100% coulombic efficiency effect. Fig. 1b shows the first galvanostatic charge/discharge profiles of the Si/TiC nanocomposite film at 160 and 80 mA g− 1. In the discharge state, there are two discharge plateaus centered at 1.25 and 0.05 V for both current densities. In the charge state, the charge plateaus that attributed to the reaction of Li with Si for both the cases are centered at 0.5 V. However, the potential differences between the charge and discharge plateaus, which correspond to the oxidation and reduction of Li with the active composite anode, respectively, are less at the low current density than those at the high current density, indicating a lower polarization when the current density is reduced. Fig. 2 compares the cyclic voltammograms of the Si/TiC nanocomposite film between 0 and 3.5 V (vs. Li/Li+) at scan rates of 0.1 and 0.05 mV s− 1. Five cathodic peaks (Li insertion state) centered at 2.42, 1.58, 0.81, 0.18 and 0.01 V, and three anodic peaks (Li extraction state) centered at 0.35, 0.53 and 2.48 V are observed for both the scan rates. Two cathodic peaks at 0.18 and 0.01 V and two anodic peaks at 0.35 and 0.53 V are attributed to the reversible reaction of Li with Si, as indicated in the following reaction: þ
−
Lix Si X xLi þe þSi Fig. 1. (a) The charge/discharge capacity and coulombic efficiency of Si/TiC composite film as function of the cycle number at current densities of 160 and 80 mA g− 1, (b) The first galvanostatic charge/discharge profiles of Si/TiC composite film at current densities of 160 and 80 mA g− 1.
ð1Þ
[8] The other voltammetric peaks, such as the cathodic peaks at 2.42 and 1.58 V and the anodic peaks at 2.48 V, are similar to the reactions
Z.Y. Zeng et al. / Thin Solid Films 517 (2009) 4767–4771
4769
Fig. 2. Cycle voltammetry of Si/TiC composite film electrode between 0 and 3.5 V (vs. Li/Li+) at a scan rate of 0.1 and 0.05 mV s− 1.
of Li with spinel Li[Li1/3Ti5/3]O4, and little anatase, which occur as follows: h i h i þ − Li þ e þLi Li1=3 Ti5=3 O4 X Li2 Li1=3 Ti5=3 O4
ð2Þ
[9] þ
−
Li þe þTiO2 ðanatase; sÞ X Lix TiO2 ðsÞ
ð3Þ
[10] For the anodic/cathodic peaks located at around 2.5 V, it cannot be made sure whether it is attributed to the reaction of Li with TiC or may be attributed to the decomposition of the Li2O [11]. According to Poizot's research [12], the transition metal oxides can decrease the binding energy of Li2O tremendously. As a result, Li2O should be easily decomposed. Besides, there is a cathodic peak at 0.82 V yet, which may be attributed to the reaction of Li+ with not well crystallized titanium-based compound [13]. From Fig. 2, it can be seen that the CV peaks that represent the peak current Icv locate at the same potentials at different current densities, but the area that CV curve encircled for the slow scan rate (0.05 mV s− 1) is larger than that for the quick scan rate (0.1 mV s− 1), which is in good agreement with the charge/ discharge capacities shown in Fig. 1. Fig. 3a and b represent the magnified SEM micrographs of the cycled Si/TiC composite electrode at current densities of 160 and 80 mA g− 1. There appears to be little change in the morphology of the particles before and after cycling, and both the tested composites seem to be more compact than the fresh material [7]. Also, the tested film is porous as can be seen in Fig. 3, these pores can provide enough spaces for the expansion/contraction of Li–Si alloys, which prevent the exfoliation and the contact loss of the active Si. On the other hand, lithium ions in electrolyte can facilely get into these pores to reach the surface of active Si. Thus, the transfer distance of lithium ion is abbreviated. These compact structures show good structural stability of the nanocomposite film anode, which guarantee excellent electrochemical performances. To investigate in more detail the electrochemical behavior of the Si/TiC composite film, impedance spectrum measurements were conducted. Fig. 4 shows a typical Nyquist plot of the Si/TiC composite film. A squashed semicircle with an approximate unit slope appears in the Nyquist plot for the porous electrode in the limit of very high frequencies, which is due to the distributed character of the impedance at finite κ and σ [14]. From the high-frequency semicircle (HFS) at the characteristic frequency of 463 Hz, the value of the
Fig. 3. SEM micrographs of the Si/TiC nanocomposite film: galvanostatic tested anodes at current densities of (a) 160 mA g− 1; and (b) 80 mA g− 1.
double-layer capacitance can be obtained as Cdl = 40.4 µF, considering the surface area of the current collector is 1 cm2. The distributed character of the impedance for a porous electrode acquires pronounced typical feature in the middle-frequency domain. This impedance deviates completely from a semi-infinite Warburg-type behavior, as can be seen from the characteristic arc shown in Fig. 4. When the frequency becomes very low, the impedance shows a capacitive component, which is a sloping line, and it will become very steep at the lowest frequencies (the millihertz region). Using two-
Fig. 4. Typical Nyquist plot of the Si/TiC composite film obtained with the electrode area of 1 cm2, and frequency range from 10 kHz to 10 mHz.
4770
Z.Y. Zeng et al. / Thin Solid Films 517 (2009) 4767–4771
Fig. 5. (a) Equivalent circuit analogue for a two-parallel diffusion path model accounting for the ion migration through the surface layer covering each particle (high-frequency Rsl║Csl semicircle, with Rsl and Csl standing for the resistance of the migration and capacity of the layer, respectively), double-layer charging and slow interfacial charge transfer (middlefrequency Rct║Cdl semicircle, with Rct and Cdl designating the related charge-transfer resistance and a double-layer capacitance) and a finite-diffusion Warburg element, FSW. Indices 1 and 2 refer to paths (branches) 1 and 2, respectively; (b) Comparison between the experimental impedance spectra and that calculated using two-parallel diffusion paths model.
parallel diffusion path model [15], the distributed character of the Si/ TiC composite film can be simplified as a porous electrode in which there are two different spherical particle size distributions (radius Rs,1 and Rs,2). Combining the impedance models of Meyers et al. [14] and Levi et al. [15], an equivalent circuit of two-parallel diffusion paths is presented in Fig. 5a. The corresponding impedance equations are presented previously [15,16]. From the impedance fitting processes achieved by us, this two-parallel diffusion path model fitted profoundly well with the experimental data of the impedances for the Si/TiC electrode, as shown in Fig. 5b. Based on the two-parallel diffusion path model, when the semiinfinite diffusion conditions are applied, the variation of Warburg slope σ with the open-circuit cell voltage (or the stage of charge) E is 1=2 σ = ðdE=dXÞVm = nFA 2DÞ
particles in the following layer, then the total surface area of the active porous electrode composite is 12.56 cm2. The chemical diffusion coefficients at different potentials are calculated using Eq. (4) and presented in Fig. 6. From the data shown in this figure, the value of DLi is on the order of 10− 12 cm2 s− 1. In comparison with the log D vs E dependence and the corresponding CV peak, it can be seen that the minima of D is in accordance with the maxima of the CV peaks. For example, there is a cathodic peak ranging from 0.78 to 0.83 V and
ð4Þ
where Vm is the molar volume of the Si/TiC composite (12.04 cm3 mol− 1), the dE/dX is the gradient of open-circuit potential vs. the composition x, n is the number of electrons transferred and F is the Faraday constant, and A is the active surface area. As discussed above, there are some characteristic sizes which have intensive distribution in the nanocomposite film. To simplify the calculation model, we select two characteristic radii of the spherical particles, Rs,i, 25 nm and 100 nm, respectively. Assuming the radii of the particles at the same layer are identical, then the composite film with geometric surface area of 1 cm2 and thickness of 500 nm have two layers with the large particles and two layers with the small particles, and particles in the upper layer are located exactly above the
Fig. 6. Variation of diffusion coefficient with the open-circuit potential for the Si/TiC composite film calculated from Eq. (4).
Z.Y. Zeng et al. / Thin Solid Films 517 (2009) 4767–4771
centered at 0.81 V, then the chemical diffusion coefficient measured at 0.81 V will be lower than that measured at any other potentials at 0.78−0.83 V. As for this interpretation, it has been previously shown by strict thermodynamic base that the minima in D (vs E) corresponds approximately to equal molar amounts of the coexisting phases, whereas D approaches maximal values for the pure phases (e.g., X = 0, X = 1). In other words, the minima in D and the peak currents of the slow scan rate CV curves for these electrodes, appear at the same E values, which correspond to the midpoint of the specific phase transition. This shape of the D vs E curves may simply be a result of the phase transition which characterizes the Li–Si/TiC composite intercalation process. Comparing the chemical diffusion coefficients for the current densities of 160 and 80 mA g− 1 at each potential, it can be seen that the D value calculated at the current density of 80 mA g− 1 is a little higher than that calculated at 160 mA g− 1. This phenomenon should be arisen from the lower polarization of the composite anode at 80 mA g− 1, which made the insertion process more accessible and the insertion depth increased. 4. Conclusions The Si/TiC nanocomposite film was prepared by a surface sol–gel process. Because of the homogeneous distribution of Si active particles in TiC matrix, the Si/TiC composite film showed reversible lithium storage capacities of about 1000 and 1300 mAh g− 1 at current densities of 160 and 80 mA g− 1 even after 80 cycles, respectively. Using two-parallel diffusion path model, the reactive mechanism of Li
4771
with Si/TiC composite was interpreted. By investigating the chemical diffusion coefficient of the Si/TiC composite at different electrode potentials, it could be concluded that, the minima of DLi and the maxima peak currents of the slow scan rate CV curves, appeared at the same E values, which was in accordance with the midpoint of the specific phase transition. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
J.O. Besenhard, J. Yang, M. Winter, J. Power Sources 68 (1997) 87. T. Ohzuku, Y. Iwakoshi, K.J. Sawai, J. Electrochem. Soc. 140 (1993) 2490. M.D. Levi, E.A. Levi, D. Aurbach, J. Electroanal. Chem. 421 (1997) 89. D.Q. Shi, J.P. Tu, Y.F. Yuan, H.M. Wu, Y. Li, X.B. Zhao, Electrochem. Commun. 8 (2006) 1610. Z.Y. Zeng, J.P. Tu, X.L. Wang, X.B. Zhao, Electrochem. Solid-State Lett. 11 (2008) A105. M.D. Levi, D. Aurbach, J. Phys. Chem. B 101 (1997) 4630. Z.Y. Zeng, J.P. Tu, Y.Z. Yang, J.Y. Xiang, X.H. Huang, F. Mao, M. Ma, Electrochim. Acta 53 (2008) 2724. G.X. Wang, J.H. Ahn, J. Yao, Electrochem. Commun. 6 (2004) 689. L. Kavan, J. Prochazka, T.M. Spitler, M. Kalbac, M. Zukalova, T. Drezen, M. Gratzel, J. Electrochem. Soc. 150 (2003) A1000. L. Kavan, M. Gratzel, J. Rathousky, A. Zukal, J. Electrochem. Soc. 143 (1996) 394. Y.M. Kang, K.T. Kim, J.H. Kim, H.S. Kim, P.S. Lee, J.Y. Lee, H.K. Liu, S.X. Dou, J. Power Sources 133 (2004) 252. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496. C.M. Shen, X.G. Zhang, Y.K. Zhou, H.L. Li, Mater. Chem. Phys. 78 (2002) 437. J.P. Meyer, M. Doyle, R.M. Darling, J. Newman, J. Electrochem. Soc. 147 (2000) 2930. M.D. Levi, D. Aurbach, J. Phys. Chem. B 108 (2004) 11693. Z.Y. Zeng, J.P. Tu, X.L. Wang, X.B. Zhao, J. Electroanal. Chem. 616 (2008) 7.