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Influence of micropore structure on Li-storage capacity in hard carbon spherules
Solid State Ionics 176 (2005) 1151 – 1159 www.elsevier.com/locate/ssi
Influence of micropore structure on Li-storage capacity in hard carbon spherules Jin Hu, Hong Li, Xuejie HuangT Nanoscale Physics and Devices Laboratory, Institute of Physics, Chinese Academy of Sciences, Beijing, 100080, China Received 17 September 2004; received in revised form 31 January 2005; accepted 2 February 2005
Abstract Hard carbon spherules (HCS) were synthesized by a normal hydrothermal method (HCS1) and a microemulsion-mediated (reverse micelles) hydrothermal method (HCS2). The results from nitrogen adsorption isotherm at 77 K and carbon dioxide adsorption isotherm at 273 K showed that the micropores of HCS2 are smaller than that of HCS1. HCS2 with smaller micropores shows higher emf value and larger Li-storage capacity around 0 V while HCS1 with larger micropores exhibits better kinetic performance at high current density. It is found that micropore structure plays a key role in the thermodynamic and kinetic behaviors of Li-ion insertion and extraction process. D 2005 Elsevier B.V. All rights reserved. Keywords: Hard carbon spherules; Anode; Micropore; Micromulsion; Hydrothermal
1. Introduction Carbon materials have been applied as anode materials for lithium ion batteries due to their low cost and relative high lithium storage capacity. The theoretical specific capacity of the graphite-type materials (372 mAh/g) is still not high enough to satisfy the rapid development of the electric devices. Hard carbon has been attracted much attention due to its relative higher Li-ion storage capacity [1–5]. It is known that the micropores in hard carbon act effectively as breservoirQ for Li-ion storage [6]. But the influence of the micropore structure on the electrochemical performance of hard carbon materials is still not clear. Our laboratory has synthesized a kind of hard carbon spherules (HCS) material based on hydrothermal technique [7,8]. Besides the attractive use as anode material for lithium ion batteries, it also shows potential applications in other fields, such as template, molecular sieve carbon and the support of Pt catalyst for direct methanol fuel cells [9]. Water-in-oil microemulsions (or reverse micelles) method has been used to synthesize ultrafine or microT Corresponding author. Tel.: +86 10 82648073; fax: +86 10 82649046. E-mail address: [email protected] (X. Huang). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.02.002
pores powder, especially nanosized materials with narrow particle size distribution [10–14]. In this paper, HCS were prepared by a normal hydrothermal method and a microemulsion-mediated hydrothermal method. They show different micropore structure, which leads to different electrochemical performance. The influence of the micropore structure on the Li-storage capacity in HCS are investigated.
2. Experimental 2.1. Composition of the microemulsion system A microemulsion is a thermodynamically stable, optically isotropic, and transparent solution of two immiscible liquids (water and oil) stabilized by an interfacial film of surfactant [14,15]. For a water-in-oil microemulsion, the aqueous phase is dispersed as microdroplets surrounded with a monolayer of surfactant molecules in the continuous hydrocarbon phase. The composition of the microemulsions is shown in Table 1, which is similar to that described by Chhabra et al. [15,16]. TritonX-100 was used as the surfactant, n-hexanol
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Table 1 Composition of the microemulsion system used for hydrothermal synthesis Aqueous phase Surfactant Cosurfactant Oil phase
Component
Volume (mL)
Sugar solution (20 wt.%) Triton-100 n-Hexanol Cyclohexane
16 20 12 32
as the cosurfactant and cyclohexane as the continuous oil phase. The solution of sugar (20 wt.%) was employed as the dispersed aqueous phase. Transparent microemulsion was prepared by dispersing the aqueous phase into the Triton X-100/n-hexanol/cyclohexane mixture through intense stirring. All of the reagents (A.R.) are commercial products (purchased from Aldrich). 2.2. Preparation of the HCS The HCS was prepared by a normal hydrothermal method [7,8] and a microemulsion-mediated (reverse micelles) hydrothermal method through a two-step procedure: hydrothermal at a low temperature and carbonized at a high temperature. For the normal hydrothermal method, 80 mL sugar solution (20 wt.%) without any surfactant additives was filled into a 100 mL autoclave and heated at 190 8C for 5 h. The obtained black powder was washed by distilled water and dried at 100 8C for 24 h. Then the black powder was further pyrolyzed in a tube furnace at 1000 8C in an argon atmosphere. The HCS prepared by normal hydrothermal method was named HCS1. While for the microemulsion-mediated hydrothermal method, the 80 mL transparent feedstock (mentioned in Section 2.1) was filled into the same PTFE-lined stainless autoclave and conducted at 190 8C for 5 h. The obtained black powder was rinsed with ethanol and distilled water to remove the oil and surfactant, respectively and then underwent the same treatment as that of HCS1. The HCS synthesized by microemulsion-mediated hydrothermal method was named HCS2. 2.3. Characterization of the HCS The morphology of the sample was investigated by a scanning electron microscope (XL30 S-FEG, FEI). The structure of the sample was characterized by X-ray diffraction (Rigaku B/max-2400 X-ray diffractometer equipped with Cuna radiation). The small angle XRD data was collected in the 2h ranges from 1.58 to 108 with a step size of 0.058 at 1 s/step. The nitrogen and carbon dioxide adsorption/de-adsorption isotherms were performed at 77 and 273 K, respectively, on a NOVA 2000e surface area and pore size analyzer. The BET surface area (S BET) was deduced from the isotherm analysis in the relative pressure (p/p 0) range of 0.04– 0.20. The total pore volume (V p) was calculated from the amount adsorbed at a relative pressure of 0.99. The
average pore size and pore size distribution calculated from the N2 adsorption isotherm based on Horvath– Kawazoe (HK, slit like) model by QuantrachromeR software, while those calculated from the CO2 adsorption isotherm based on Density Functional Theory (DFT, slit like) model. Before measurement, samples were vacuumized for 12 h at 623 K under the pressure of 10 3 torr. 2.4. Electrochemical measurements of the HCS The electrochemical properties of the HCS samples were investigated by using a Li/HCS cell. The composite electrode was prepared by mixing HCS (85 wt.%), acetylene black (5 wt.%) and polyvinylidene fluoride (PVDF) as binder (10 wt.%) in N-methyl pyrrolidone. The slurry of active material (0.215 g) was spread onto a copper foil (64 cm2). After that, the sheet was dried under vacuum at 120 8C for 12 h and cut into small electrode sheets (0.64 cm2). The weight difference is about 5% for each electrode sheet with the same area. Li foil was used as counter electrode and 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume) as the electrolyte. Test cells were assembled in an argon-filled glove box. Galvanostatic charge–discharge tests conducted on a LandR multichannel galvanostat/potentiostat instrument. In the open-circuit voltage (OCV) tests, the test cells were discharged to 0 V and the open-circuit voltage was measured by a SolartronR7050 digital voltmeter. For each experiment, the test cell had been rested for 24 h in order to approach the equilibrium state. All of the electrochemical measurements were carried out at constant temperature (25F0.2 8C).
3. Results and discussion 3.1. Morphology and structure of HCS Fig. 1 compares the morphologies of these two kinds of HCS. The HCS1 particles (Fig. 1a) are severely aggregated with average spherule diameter about 4–8 Am. In contrast, the HCS2 particles show perfect spherule shape and narrow particle size distribution (Fig. 1b). The average sphere diameter of the HCS2 sample is about 2–4 Am. It was noticed that the aggregation of the HCS1 occurs when the concentration of the sugar solution is over 5%. As for HCS2, severe aggregation appears only when the concentration of the sugar solution in water phase is higher than 45%. This may be benefited from the microemulsionmediated hydrothermal method [10–14]. However, the mechanism of preventing agglomeration is still not clear considering the microemulsion structure may not be kept during hydrothermal process. XRD patterns of the HCS1 and HCS2 are compared in Fig. 2. They show typical disordered-carbon structure [7,8].
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(PSD) of these two HCS are not uniform. But it cannot affirm that the pore size of HCS2 is smaller than that of HCS1 only based on the results of small angle XRD. Further investigation of the micropore structure needs the establishment of adsorption isotherm of a probe molecule at a subcritical temperature and very low relative pressure; and then applies a suitable theoretical model to interpret the data in terms of pore size. 3.2. The micropore structure of the HCS
Fig. 1. SEM images of (a) HCS1 and (b) HCS2.
Meanwhile, their diffraction intensities are very strong at low Bragg angles. The btailQ in the low Bragg angle region indicates a porous feature of these materials [17]. The small angle XRD patterns (1.58 to 68) are also given in Fig. 2. The peaks in small angle region should be attributed to the micropores in HCS samples based on the Bragg law. These two samples show multi-peaks patterns and their peak-width is broad, which indicates that the pore size distributions
The micropore structure of the carbon materials can be investigated by the adsorption/desorption isotherms based on SPE or BET model. The SPE model is a more accurate assessment for high and super-high surface area active carbon (N2630 m2 g 1) [18,19]. As for the BET model, standard N2 adsorption/desorption isotherm method at 77 K is suitable to analysis the micropores whose radiuses are larger than 3.5 2 [20]. While the CO2 adsorption/desorption isotherms at 273 K, the range of analysis is extended to ultramicropore (pores radius larger than 1.7 2) based on DFT model because at elevated temperature and under higher absolute pressure, CO2 molecules can access ultramicropores more easily than N2 at 77 K in spite of the fact that molecular critical dimensions of both gases are similar [21–23]. The complementary of nitrogen and carbon dioxide adsorption method to analyze materials with both micropores and ultramicropores materials has been recommended [24,25]. The nitrogen and carbon dioxide adsorption/desorption isotherm curves and the corresponding pore size distribution (PSD) of the HCS1 and HCS2 are compared in Figs. 3 and 4, respectively. Both the N2 and CO2 adsorption isotherm of these two kinds of HCS are typical of type I in the Brunauer,
Fig. 2. XRD patterns of (a) HCS1 and (b) HCS2. (Inset is small angle XRD patterns).
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Fig. 3. Nitrogen adsorption (n,.) and desorption (5,o) isotherm (top) and the corresponding pore size distribution by HK model for (a) HCS1 and (b) HCS2 (bottom), respectively.
Deming, Deming and Teller (BDDT) classification, indicating that the HCS1 and HCS2 are typical microporous materials [26]. The BET surface area, micropore volume and average pore radius of HCS1 and HCS2 are listed in Table 2. All of the above parameters of HCS1 that calculated from N2 adsorption isotherm are higher than that of the HCS2. Furthermore, pores size distribution (PSD) calculated from the HK (slit like) model show an un-uniform micropore structure for both HCS in the range of 3.5–6.5 2. The PSD for HCS1 and HCS2 are centered at 4.2, 5.4 and 3.6, 5.4 2 respectively. This indicates that HCS2 has smaller micropores than that of the HCS1. Considering the detection limitation of the N2 adsorption isotherms method, in order to clarify this, the CO2 adsorption/desorption isotherms were done further. It is noted that the ratio of S BET (CO2)/S BET (N2) for HCS2 (1.33) is higher than that of the HCS1 (1.26). This indicates that HCS2 has more ultramicropore. The micropores can be characterized in the range of 0.00001–0.1 for p/ p 0 value [24]. The PSD curves of these two kinds of HCS samples are shown in Fig. 4 (bottom). It can be seen clearly from the PSD curves that HCS2 has smaller micropores compared to HCS1, which is in accordance with the results of the average pores size diameter (r a) listed in Table 2 and the results of N2 adsorption/desorption isotherms. In brief,
HCS1 and HCS2 have different micropore structures and the micropores of HCS2 is smaller than that of HCS1. 3.3. Electrochemical performances of HCS The voltage-capacity profiles of the Li/HCS cells show three parts (marked with I, II and III) at different potential regions (Fig. 5). It indicates the presence of at least three different Li+ storage sites in the HCS. At low-voltage region, the charge profile appears as a plateau (span I, 0– 0.09 V), then a sloped curve (span II, 0.09–1.2 V), and followed another short-sloped curve (span III, above 1.2 V). This is the typical electrochemical feature of hard carbon materials [3–5]. The HCS2 material has specific capacity of 387 mAh/g at the current density of 0.2 mA/cm2 when the cell was discharged to 0 mV (vs. Li/Li+), much higher than that of the HCS1 (230 mAh/g). The initial efficiency of the HCS2 is 71%, while that of the HCS1 is only 65% if the discharge cut-off voltage is 0 V. Fig. 6 shows the cycling performances of the HCS1 and HCS2 cycled between 0 and 2.0 V. Both HCS1 and HCS2 show good capacity retention at such voltage range. It should be mentioned that quite a large amount of Li can be stored in the micropores below 0 V but above the
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Fig. 4. Carbon dioxide adsorption isotherm at very low pressure (0.00001V p/p 0V0.1) (top) and the corresponding pore size distribution by DFT model for (a) HCS1 and (b) HCS2 (bottom), respectively.
electrodeposition voltage of lithium in the hard carbon [1]. Therefore, in order to achieve higher Li-storage capacity, hard carbon materials were usually discharged deeply just above the lithium electrodeposition voltage. The electrodeposition voltage of the lithium on both hard carbon sphere electrodes at such current density is about 22 mV (vs. Li/ Table 2 The BET surface area, micropore volume and average pore radius of the HCS1 and HCS2 Sample
S BET (m2/g)
V p (cm3/g)
r a(2)
HCS1 HCS2 HCS1 HCS2
288.3 248.1 364.0 330.2
0.158 0.152 0.262 0.264
4.3 3.7 4.65 3.89
(N2) (N2) (CO2) (CO2)
Li+). The initial charge capacity of HCS2 rises to 566 mAh/ g if the cell is discharged to 20 mV in comparison of 430 mAh/g for the HCS1. The initial efficiency of the HCS2 and HCS1 reached 83.2% and 72.3% at this discharge cut-off voltage, respectively. In order to investigate the origin of the increased Listorage capacity above 0 V in HCS2, the thermodynamic and kinetic experimental are performed. The open-circuit voltage (OCV) curves indicate the relationship between the Li-storage capacity and the thermodynamic equilibrium voltage (emf). The OCV curves in the plateau region at low-voltage (near 0 V) of HCS1 and HCS2 in the initial discharge process are shown in Fig. 7. In order to compare with each other, the capacities of them are normalized based on capacity at the lower potential region for each
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Fig. 5. The charge–discharge curves of the (a) HCS1 and (b) HCS2 that discharge to 0 and
other. It can be seen that the insertion voltage of HCS2 is higher than that of the HCS1 at the same percentage of capacity. This means that HCS2 has higher Li-ion insertion voltage than that of the HCS1. It has been supposed for hard carbon materials that the intercalation of Li-ion into the disordered graphite sheets take place at high voltage, while the insertion of lithium into micropores occur at lower voltage region near 0 V [1,27,28]. The OCV curves show a sloped feature instead of plateau. This indicates that the Li-storage in the micropores should be an adsorption processes or so-called under-potential deposition (UPD), since the voltage varies with the coverage of
20 mV.
the lithium on surface of the micropores [29–33]. The thermodynamic equilibrium voltage of the Li-storage reaction with the HCS can be calculated from the following equation: DadsorbG= nEF, assuming the formation energy of these two HCS can be ignored. The voltage of the Li-insertion into the micropores of the HCS in the plateau region at low-voltage depends on the adsorption energy of Li-ion within the micropores. The materials with higher adsorption energy should show higher Li-storage voltage. According to Fig. 7, HCS2 has higher voltage than that of the HCS1. It means that HCS2 shows higher Li-adsorption energy. Results from the N2 and CO2
Fig. 6. The cyclic performance of (a) HCS1 and (b) HCS2 (cycling between 0 and 2 V).
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Fig. 7. The open-circuit voltage (OCV) curves of (a) HCS1 and (b) HCS2 in the initial discharge processes.
adsorption/desorption isotherms have proved that the micropores of HCS2 is smaller than that of HCS1. That is to say, smaller micropores leads to higher adsorption energy. This is plausible since similar conclusion has been drawn in the case of carbon nanotube based on semiempirical calculation [34]. It was reported that the adsorption energy decreased with increase of the tube radius. Therefore, we believe that HCS2 with smaller micropores processes higher adsorption energy for Li-ion and leads to higher emf value, which is beneficial for achieving higher Li-storage capacity above 0 V.
The kinetic performances of the HCS1 and HCS2 are compared in Fig. 8 and the relationship of the initial charge capacities at different voltage ranges vs. current density are shown in Fig. 9. It can be seen that the capacities at higher voltage (span II plus span III) for both HCS1 and HCS2 almost do not vary with the current density. But the capacity at lower voltage (spans I) for both HCS1 and HCS2 is influenced strongly by the current density. This indicates that Li-storage capacity near 0 V (span I) shows poor kinetic performance. Relatively speaking, HCS1 has better rate performance
Fig. 8. The initial charge curves of (a) HCS1 and (b) HCS2 at different current densities. A: 0.8 mA/cm2, B: 0.6 mA/cm2, C: 0.4 mA/cm2, D: 0.2 mA/cm2, E: 0.16 mA/cm2, F: 0.10 mA/cm2, G: 0.06 mA/cm2, H: 0.02 mA/cm2.
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Fig. 9. The initial charge capacity at different voltage ranges vs. current densities. (a) HCS2, 0.09–2.00 V, (b) HCS1, 0.09–2.00 V, (c) HCS2, 0.00–0.09V, (d) HCS1, 0.00–0.09 V.
for high current density (more than 0.2 mA/cm2), while HCS2 shows better rate performance for low current density (less than 0.2 mA/cm2). HCS1 has larger micropores compared to HCS2. This may indicate that in view of the kinetics, larger micropores shows lower energy barrier for Li-ion transport under high current density. When the current density is low, the Li-storage capacity is determined by the thermodynamic factor, so HCS2 has higher capacity. One phenomenon for HCS1 which should be noticed is that a sloped plateau with the capacity of 43 mAh/g (marked with arrow) appears in the voltage range from 0.9 to 1.2 V in the initial charge curve (Fig. 5a, in span II region) when the discharge cut-off voltage is 20 mV. It does not appear if the discharge cut-off voltage rise to 0 V. This implies that part of the lithium adsorbed into the micropores of HCS1 at lower voltage but desorbed at higher voltage with large voltage hysteresis. In addition, this sloped plateau also appears when the current density is very low (Fig. 8a H). These phenomena indicate that such a specific Li-ion adsoption/desorption is thermodynamic feasible but kinetic difficult. It may be related to the reaction of Li with suspending bonds or hydrogenterminated surface groups [35,36]. However, this phenomenon is not observed for HCS2. Such difference may be attributed to the lower ratio of H/C (0.078) for HCS2 than that for HCS1 (0.091). Li-storage in high voltage region (span II and span III) is related to the intercalation of Li-ion in disordered graphite layer [1]. The results in Fig. 9 show that this part of Listorage exhibit excellent rate performance while Li-storage in micropores shows relative poor kinetic behavior. In view of potential application, hard carbon materials could be used
for high power-density batteries by limiting the voltage range to permit only the Li-intercalation reaction. As for high energy density batteries, further optimizing its micropore structure and working at relative low current density should be considered.
4. Conclusion Hard carbon spherules (HCS) were synthesized by normal hydrothermal method (HCS1) and microemulsionmediated (reverse micelles) hydrothermal method (HCS2). Results of nitrogen adsorption isotherm at 77 K and carbon dioxide adsorption isotherm at 273 K show that the micropore of HCS2 is smaller than that of the HCS1. HCS2 has Li-storage capacity of 387 mAh/g above 0 V, much higher than that of the HCS1. The improvement of electrochemical performance can be attributed to the thermodynamic and kinetic factors that influenced by the micropore structure of HCS. Smaller micropores leads to higher thermodynamic Li-storage voltage (emf) and it is beneficial for achieving higher Liinsertion capacity around 0 V. Larger micropores shows better kinetic performance at high current density. This work indicates that utilization of the hard carbon for different purposes should design materials with different micropore structure.
Acknowledgements This work is supported by the National 863 key program (2004AA302G70).
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Influence of micropore structure on Li-storage capacity in hard carbon spherules Jin Hu, Hong Li, Xuejie HuangT Nanoscale Physics and Devices Laboratory, Institute of Physics, Chinese Academy of Sciences, Beijing, 100080, China Received 17 September 2004; received in revised form 31 January 2005; accepted 2 February 2005
Abstract Hard carbon spherules (HCS) were synthesized by a normal hydrothermal method (HCS1) and a microemulsion-mediated (reverse micelles) hydrothermal method (HCS2). The results from nitrogen adsorption isotherm at 77 K and carbon dioxide adsorption isotherm at 273 K showed that the micropores of HCS2 are smaller than that of HCS1. HCS2 with smaller micropores shows higher emf value and larger Li-storage capacity around 0 V while HCS1 with larger micropores exhibits better kinetic performance at high current density. It is found that micropore structure plays a key role in the thermodynamic and kinetic behaviors of Li-ion insertion and extraction process. D 2005 Elsevier B.V. All rights reserved. Keywords: Hard carbon spherules; Anode; Micropore; Micromulsion; Hydrothermal
1. Introduction Carbon materials have been applied as anode materials for lithium ion batteries due to their low cost and relative high lithium storage capacity. The theoretical specific capacity of the graphite-type materials (372 mAh/g) is still not high enough to satisfy the rapid development of the electric devices. Hard carbon has been attracted much attention due to its relative higher Li-ion storage capacity [1–5]. It is known that the micropores in hard carbon act effectively as breservoirQ for Li-ion storage [6]. But the influence of the micropore structure on the electrochemical performance of hard carbon materials is still not clear. Our laboratory has synthesized a kind of hard carbon spherules (HCS) material based on hydrothermal technique [7,8]. Besides the attractive use as anode material for lithium ion batteries, it also shows potential applications in other fields, such as template, molecular sieve carbon and the support of Pt catalyst for direct methanol fuel cells [9]. Water-in-oil microemulsions (or reverse micelles) method has been used to synthesize ultrafine or microT Corresponding author. Tel.: +86 10 82648073; fax: +86 10 82649046. E-mail address: [email protected] (X. Huang). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.02.002
pores powder, especially nanosized materials with narrow particle size distribution [10–14]. In this paper, HCS were prepared by a normal hydrothermal method and a microemulsion-mediated hydrothermal method. They show different micropore structure, which leads to different electrochemical performance. The influence of the micropore structure on the Li-storage capacity in HCS are investigated.
2. Experimental 2.1. Composition of the microemulsion system A microemulsion is a thermodynamically stable, optically isotropic, and transparent solution of two immiscible liquids (water and oil) stabilized by an interfacial film of surfactant [14,15]. For a water-in-oil microemulsion, the aqueous phase is dispersed as microdroplets surrounded with a monolayer of surfactant molecules in the continuous hydrocarbon phase. The composition of the microemulsions is shown in Table 1, which is similar to that described by Chhabra et al. [15,16]. TritonX-100 was used as the surfactant, n-hexanol
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Table 1 Composition of the microemulsion system used for hydrothermal synthesis Aqueous phase Surfactant Cosurfactant Oil phase
Component
Volume (mL)
Sugar solution (20 wt.%) Triton-100 n-Hexanol Cyclohexane
16 20 12 32
as the cosurfactant and cyclohexane as the continuous oil phase. The solution of sugar (20 wt.%) was employed as the dispersed aqueous phase. Transparent microemulsion was prepared by dispersing the aqueous phase into the Triton X-100/n-hexanol/cyclohexane mixture through intense stirring. All of the reagents (A.R.) are commercial products (purchased from Aldrich). 2.2. Preparation of the HCS The HCS was prepared by a normal hydrothermal method [7,8] and a microemulsion-mediated (reverse micelles) hydrothermal method through a two-step procedure: hydrothermal at a low temperature and carbonized at a high temperature. For the normal hydrothermal method, 80 mL sugar solution (20 wt.%) without any surfactant additives was filled into a 100 mL autoclave and heated at 190 8C for 5 h. The obtained black powder was washed by distilled water and dried at 100 8C for 24 h. Then the black powder was further pyrolyzed in a tube furnace at 1000 8C in an argon atmosphere. The HCS prepared by normal hydrothermal method was named HCS1. While for the microemulsion-mediated hydrothermal method, the 80 mL transparent feedstock (mentioned in Section 2.1) was filled into the same PTFE-lined stainless autoclave and conducted at 190 8C for 5 h. The obtained black powder was rinsed with ethanol and distilled water to remove the oil and surfactant, respectively and then underwent the same treatment as that of HCS1. The HCS synthesized by microemulsion-mediated hydrothermal method was named HCS2. 2.3. Characterization of the HCS The morphology of the sample was investigated by a scanning electron microscope (XL30 S-FEG, FEI). The structure of the sample was characterized by X-ray diffraction (Rigaku B/max-2400 X-ray diffractometer equipped with Cuna radiation). The small angle XRD data was collected in the 2h ranges from 1.58 to 108 with a step size of 0.058 at 1 s/step. The nitrogen and carbon dioxide adsorption/de-adsorption isotherms were performed at 77 and 273 K, respectively, on a NOVA 2000e surface area and pore size analyzer. The BET surface area (S BET) was deduced from the isotherm analysis in the relative pressure (p/p 0) range of 0.04– 0.20. The total pore volume (V p) was calculated from the amount adsorbed at a relative pressure of 0.99. The
average pore size and pore size distribution calculated from the N2 adsorption isotherm based on Horvath– Kawazoe (HK, slit like) model by QuantrachromeR software, while those calculated from the CO2 adsorption isotherm based on Density Functional Theory (DFT, slit like) model. Before measurement, samples were vacuumized for 12 h at 623 K under the pressure of 10 3 torr. 2.4. Electrochemical measurements of the HCS The electrochemical properties of the HCS samples were investigated by using a Li/HCS cell. The composite electrode was prepared by mixing HCS (85 wt.%), acetylene black (5 wt.%) and polyvinylidene fluoride (PVDF) as binder (10 wt.%) in N-methyl pyrrolidone. The slurry of active material (0.215 g) was spread onto a copper foil (64 cm2). After that, the sheet was dried under vacuum at 120 8C for 12 h and cut into small electrode sheets (0.64 cm2). The weight difference is about 5% for each electrode sheet with the same area. Li foil was used as counter electrode and 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume) as the electrolyte. Test cells were assembled in an argon-filled glove box. Galvanostatic charge–discharge tests conducted on a LandR multichannel galvanostat/potentiostat instrument. In the open-circuit voltage (OCV) tests, the test cells were discharged to 0 V and the open-circuit voltage was measured by a SolartronR7050 digital voltmeter. For each experiment, the test cell had been rested for 24 h in order to approach the equilibrium state. All of the electrochemical measurements were carried out at constant temperature (25F0.2 8C).
3. Results and discussion 3.1. Morphology and structure of HCS Fig. 1 compares the morphologies of these two kinds of HCS. The HCS1 particles (Fig. 1a) are severely aggregated with average spherule diameter about 4–8 Am. In contrast, the HCS2 particles show perfect spherule shape and narrow particle size distribution (Fig. 1b). The average sphere diameter of the HCS2 sample is about 2–4 Am. It was noticed that the aggregation of the HCS1 occurs when the concentration of the sugar solution is over 5%. As for HCS2, severe aggregation appears only when the concentration of the sugar solution in water phase is higher than 45%. This may be benefited from the microemulsionmediated hydrothermal method [10–14]. However, the mechanism of preventing agglomeration is still not clear considering the microemulsion structure may not be kept during hydrothermal process. XRD patterns of the HCS1 and HCS2 are compared in Fig. 2. They show typical disordered-carbon structure [7,8].
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(PSD) of these two HCS are not uniform. But it cannot affirm that the pore size of HCS2 is smaller than that of HCS1 only based on the results of small angle XRD. Further investigation of the micropore structure needs the establishment of adsorption isotherm of a probe molecule at a subcritical temperature and very low relative pressure; and then applies a suitable theoretical model to interpret the data in terms of pore size. 3.2. The micropore structure of the HCS
Fig. 1. SEM images of (a) HCS1 and (b) HCS2.
Meanwhile, their diffraction intensities are very strong at low Bragg angles. The btailQ in the low Bragg angle region indicates a porous feature of these materials [17]. The small angle XRD patterns (1.58 to 68) are also given in Fig. 2. The peaks in small angle region should be attributed to the micropores in HCS samples based on the Bragg law. These two samples show multi-peaks patterns and their peak-width is broad, which indicates that the pore size distributions
The micropore structure of the carbon materials can be investigated by the adsorption/desorption isotherms based on SPE or BET model. The SPE model is a more accurate assessment for high and super-high surface area active carbon (N2630 m2 g 1) [18,19]. As for the BET model, standard N2 adsorption/desorption isotherm method at 77 K is suitable to analysis the micropores whose radiuses are larger than 3.5 2 [20]. While the CO2 adsorption/desorption isotherms at 273 K, the range of analysis is extended to ultramicropore (pores radius larger than 1.7 2) based on DFT model because at elevated temperature and under higher absolute pressure, CO2 molecules can access ultramicropores more easily than N2 at 77 K in spite of the fact that molecular critical dimensions of both gases are similar [21–23]. The complementary of nitrogen and carbon dioxide adsorption method to analyze materials with both micropores and ultramicropores materials has been recommended [24,25]. The nitrogen and carbon dioxide adsorption/desorption isotherm curves and the corresponding pore size distribution (PSD) of the HCS1 and HCS2 are compared in Figs. 3 and 4, respectively. Both the N2 and CO2 adsorption isotherm of these two kinds of HCS are typical of type I in the Brunauer,
Fig. 2. XRD patterns of (a) HCS1 and (b) HCS2. (Inset is small angle XRD patterns).
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Fig. 3. Nitrogen adsorption (n,.) and desorption (5,o) isotherm (top) and the corresponding pore size distribution by HK model for (a) HCS1 and (b) HCS2 (bottom), respectively.
Deming, Deming and Teller (BDDT) classification, indicating that the HCS1 and HCS2 are typical microporous materials [26]. The BET surface area, micropore volume and average pore radius of HCS1 and HCS2 are listed in Table 2. All of the above parameters of HCS1 that calculated from N2 adsorption isotherm are higher than that of the HCS2. Furthermore, pores size distribution (PSD) calculated from the HK (slit like) model show an un-uniform micropore structure for both HCS in the range of 3.5–6.5 2. The PSD for HCS1 and HCS2 are centered at 4.2, 5.4 and 3.6, 5.4 2 respectively. This indicates that HCS2 has smaller micropores than that of the HCS1. Considering the detection limitation of the N2 adsorption isotherms method, in order to clarify this, the CO2 adsorption/desorption isotherms were done further. It is noted that the ratio of S BET (CO2)/S BET (N2) for HCS2 (1.33) is higher than that of the HCS1 (1.26). This indicates that HCS2 has more ultramicropore. The micropores can be characterized in the range of 0.00001–0.1 for p/ p 0 value [24]. The PSD curves of these two kinds of HCS samples are shown in Fig. 4 (bottom). It can be seen clearly from the PSD curves that HCS2 has smaller micropores compared to HCS1, which is in accordance with the results of the average pores size diameter (r a) listed in Table 2 and the results of N2 adsorption/desorption isotherms. In brief,
HCS1 and HCS2 have different micropore structures and the micropores of HCS2 is smaller than that of HCS1. 3.3. Electrochemical performances of HCS The voltage-capacity profiles of the Li/HCS cells show three parts (marked with I, II and III) at different potential regions (Fig. 5). It indicates the presence of at least three different Li+ storage sites in the HCS. At low-voltage region, the charge profile appears as a plateau (span I, 0– 0.09 V), then a sloped curve (span II, 0.09–1.2 V), and followed another short-sloped curve (span III, above 1.2 V). This is the typical electrochemical feature of hard carbon materials [3–5]. The HCS2 material has specific capacity of 387 mAh/g at the current density of 0.2 mA/cm2 when the cell was discharged to 0 mV (vs. Li/Li+), much higher than that of the HCS1 (230 mAh/g). The initial efficiency of the HCS2 is 71%, while that of the HCS1 is only 65% if the discharge cut-off voltage is 0 V. Fig. 6 shows the cycling performances of the HCS1 and HCS2 cycled between 0 and 2.0 V. Both HCS1 and HCS2 show good capacity retention at such voltage range. It should be mentioned that quite a large amount of Li can be stored in the micropores below 0 V but above the
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Fig. 4. Carbon dioxide adsorption isotherm at very low pressure (0.00001V p/p 0V0.1) (top) and the corresponding pore size distribution by DFT model for (a) HCS1 and (b) HCS2 (bottom), respectively.
electrodeposition voltage of lithium in the hard carbon [1]. Therefore, in order to achieve higher Li-storage capacity, hard carbon materials were usually discharged deeply just above the lithium electrodeposition voltage. The electrodeposition voltage of the lithium on both hard carbon sphere electrodes at such current density is about 22 mV (vs. Li/ Table 2 The BET surface area, micropore volume and average pore radius of the HCS1 and HCS2 Sample
S BET (m2/g)
V p (cm3/g)
r a(2)
HCS1 HCS2 HCS1 HCS2
288.3 248.1 364.0 330.2
0.158 0.152 0.262 0.264
4.3 3.7 4.65 3.89
(N2) (N2) (CO2) (CO2)
Li+). The initial charge capacity of HCS2 rises to 566 mAh/ g if the cell is discharged to 20 mV in comparison of 430 mAh/g for the HCS1. The initial efficiency of the HCS2 and HCS1 reached 83.2% and 72.3% at this discharge cut-off voltage, respectively. In order to investigate the origin of the increased Listorage capacity above 0 V in HCS2, the thermodynamic and kinetic experimental are performed. The open-circuit voltage (OCV) curves indicate the relationship between the Li-storage capacity and the thermodynamic equilibrium voltage (emf). The OCV curves in the plateau region at low-voltage (near 0 V) of HCS1 and HCS2 in the initial discharge process are shown in Fig. 7. In order to compare with each other, the capacities of them are normalized based on capacity at the lower potential region for each
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Fig. 5. The charge–discharge curves of the (a) HCS1 and (b) HCS2 that discharge to 0 and
other. It can be seen that the insertion voltage of HCS2 is higher than that of the HCS1 at the same percentage of capacity. This means that HCS2 has higher Li-ion insertion voltage than that of the HCS1. It has been supposed for hard carbon materials that the intercalation of Li-ion into the disordered graphite sheets take place at high voltage, while the insertion of lithium into micropores occur at lower voltage region near 0 V [1,27,28]. The OCV curves show a sloped feature instead of plateau. This indicates that the Li-storage in the micropores should be an adsorption processes or so-called under-potential deposition (UPD), since the voltage varies with the coverage of
20 mV.
the lithium on surface of the micropores [29–33]. The thermodynamic equilibrium voltage of the Li-storage reaction with the HCS can be calculated from the following equation: DadsorbG= nEF, assuming the formation energy of these two HCS can be ignored. The voltage of the Li-insertion into the micropores of the HCS in the plateau region at low-voltage depends on the adsorption energy of Li-ion within the micropores. The materials with higher adsorption energy should show higher Li-storage voltage. According to Fig. 7, HCS2 has higher voltage than that of the HCS1. It means that HCS2 shows higher Li-adsorption energy. Results from the N2 and CO2
Fig. 6. The cyclic performance of (a) HCS1 and (b) HCS2 (cycling between 0 and 2 V).
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Fig. 7. The open-circuit voltage (OCV) curves of (a) HCS1 and (b) HCS2 in the initial discharge processes.
adsorption/desorption isotherms have proved that the micropores of HCS2 is smaller than that of HCS1. That is to say, smaller micropores leads to higher adsorption energy. This is plausible since similar conclusion has been drawn in the case of carbon nanotube based on semiempirical calculation [34]. It was reported that the adsorption energy decreased with increase of the tube radius. Therefore, we believe that HCS2 with smaller micropores processes higher adsorption energy for Li-ion and leads to higher emf value, which is beneficial for achieving higher Li-storage capacity above 0 V.
The kinetic performances of the HCS1 and HCS2 are compared in Fig. 8 and the relationship of the initial charge capacities at different voltage ranges vs. current density are shown in Fig. 9. It can be seen that the capacities at higher voltage (span II plus span III) for both HCS1 and HCS2 almost do not vary with the current density. But the capacity at lower voltage (spans I) for both HCS1 and HCS2 is influenced strongly by the current density. This indicates that Li-storage capacity near 0 V (span I) shows poor kinetic performance. Relatively speaking, HCS1 has better rate performance
Fig. 8. The initial charge curves of (a) HCS1 and (b) HCS2 at different current densities. A: 0.8 mA/cm2, B: 0.6 mA/cm2, C: 0.4 mA/cm2, D: 0.2 mA/cm2, E: 0.16 mA/cm2, F: 0.10 mA/cm2, G: 0.06 mA/cm2, H: 0.02 mA/cm2.
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Fig. 9. The initial charge capacity at different voltage ranges vs. current densities. (a) HCS2, 0.09–2.00 V, (b) HCS1, 0.09–2.00 V, (c) HCS2, 0.00–0.09V, (d) HCS1, 0.00–0.09 V.
for high current density (more than 0.2 mA/cm2), while HCS2 shows better rate performance for low current density (less than 0.2 mA/cm2). HCS1 has larger micropores compared to HCS2. This may indicate that in view of the kinetics, larger micropores shows lower energy barrier for Li-ion transport under high current density. When the current density is low, the Li-storage capacity is determined by the thermodynamic factor, so HCS2 has higher capacity. One phenomenon for HCS1 which should be noticed is that a sloped plateau with the capacity of 43 mAh/g (marked with arrow) appears in the voltage range from 0.9 to 1.2 V in the initial charge curve (Fig. 5a, in span II region) when the discharge cut-off voltage is 20 mV. It does not appear if the discharge cut-off voltage rise to 0 V. This implies that part of the lithium adsorbed into the micropores of HCS1 at lower voltage but desorbed at higher voltage with large voltage hysteresis. In addition, this sloped plateau also appears when the current density is very low (Fig. 8a H). These phenomena indicate that such a specific Li-ion adsoption/desorption is thermodynamic feasible but kinetic difficult. It may be related to the reaction of Li with suspending bonds or hydrogenterminated surface groups [35,36]. However, this phenomenon is not observed for HCS2. Such difference may be attributed to the lower ratio of H/C (0.078) for HCS2 than that for HCS1 (0.091). Li-storage in high voltage region (span II and span III) is related to the intercalation of Li-ion in disordered graphite layer [1]. The results in Fig. 9 show that this part of Listorage exhibit excellent rate performance while Li-storage in micropores shows relative poor kinetic behavior. In view of potential application, hard carbon materials could be used
for high power-density batteries by limiting the voltage range to permit only the Li-intercalation reaction. As for high energy density batteries, further optimizing its micropore structure and working at relative low current density should be considered.
4. Conclusion Hard carbon spherules (HCS) were synthesized by normal hydrothermal method (HCS1) and microemulsionmediated (reverse micelles) hydrothermal method (HCS2). Results of nitrogen adsorption isotherm at 77 K and carbon dioxide adsorption isotherm at 273 K show that the micropore of HCS2 is smaller than that of the HCS1. HCS2 has Li-storage capacity of 387 mAh/g above 0 V, much higher than that of the HCS1. The improvement of electrochemical performance can be attributed to the thermodynamic and kinetic factors that influenced by the micropore structure of HCS. Smaller micropores leads to higher thermodynamic Li-storage voltage (emf) and it is beneficial for achieving higher Liinsertion capacity around 0 V. Larger micropores shows better kinetic performance at high current density. This work indicates that utilization of the hard carbon for different purposes should design materials with different micropore structure.
Acknowledgements This work is supported by the National 863 key program (2004AA302G70).
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