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carbon composites as electrochemical supercapacitor electrodes
Solid State Ionics 152 – 153 (2002) 833 – 841 www.elsevier.com/locate/ssi
Amorphous V2O5/carbon composites as electrochemical supercapacitor electrodes T. Kudo a,*, Y. Ikeda a, T. Watanabe a, M. Hibino a, M. Miyayama a, H. Abe b, K. Kajita b a
Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan b Battery R&D Laboratory, Hitachi Maxell, Ltd., 1-1-88 Ushitora, Ibaraki, Osaka 567-8567, Japan Accepted 20 March 2002
Abstract High rate intercalation electrode performance of V2O5 gel/carbon composites has been demonstrated. A V2O5 sol was prepared by a reaction of metallic vanadium with a hydrogen peroxide solution. Acetylene black powder was added into the sol with acetone to yield a homogeneous suspension. A composite of amorphous V2O5 and carbon was loaded on a macroporous nickel current collector, and heat-treated at 120 jC to obtain a sample electrode. Electrochemical measurements were performed in some organic electrolytes like LiClO4/PC or LiPF6/g-butyrolactone (g-BL) at room temperature. It was confirmed that a composite electrode with the V2O5/carbon ratio of 0.7 in weight showed 54% of the ideal capacity, 360 mA h/g (4.2 – 2.0 V) based on V2O5, even at a very high rate discharge at 150 C or 54 A/g V2O5. The diffusion length of this host – guest system was ˜ =1012 cm2/s. The estimated as 30 – 50 nm by means of a simulation of the discharge curves using a diffusion model assuming D reversibility was also satisfactory and no capacity loss was observed after thousands of times of discharge/charge cycles between 4.2 and 3.0 V at the rate of 20 C. A prototype electrode was fabricated by coating a thin layer of the composite on an Al sheet current collector using an applicator. It showed the capacity of 40 mA h/g electrode at a current density as high as 30 mA/cm2. D 2002 Elsevier Science B.V. All rights reserved. PACS: 84.60.Dn Keywords: Amorphous vanadium oxide; Vanadium oxide sol; Lithium insertion electrode; Supercapacitor
1. Introduction Fuel cells and secondary batteries have a considerably high specific energy (W h/kg), but their specific power (W/kg) is relatively small. Thus, electric vehicles propelled by them need a complementary or auxiliary power source capable of supplying high power during the vehicle’s acceleration or climbing. Such a power source can also be used to quickly sink energy in the deceleration stage. Supercapacitors have attracted *
Corresponding author. Tel./fax: +81-3-5728-6498. E-mail address: [email protected] (T. Kudo).
much attention, because they can cope with highly time-dependent power demand. The term ‘‘supercapacitor’’ usually means a device utilizing the storage of charge in the electric double layer between an electrolyte and an electrochemically inert electrode like the high surface area active carbon. Such devices, often called electrical double layer capacitors (EDLC), have already been practically used primarily for semiconductor memory backup purposes. Typical specific energy and power of EDLCs are in the order of 1 W h/kg and 1 kW/kg, respectively. However, these specific values are too small for being a complementary energy source – sink for electric vehicles. In particular,
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 3 8 3 - 1
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the specific energy must be improved by a factor of more than 10. Some efforts have been made to improve the EDLC device itself. It has been reported that the specific capacity of 140 F/g is attained by use a carbon aerogel material with the specific surface area as large as 2000 m2/g [1]. However, this capacity is not high enough, because it is equivalent to about 4 W h/kg, if the capacitor voltage window is 1 V. Such an approach seems to have reached a limit. Another approach is an attempt to improve the specific power of a secondary battery like lithium ion cells based on the faradaic processes of an intercalation or host –guest system. The specific energy of those batteries is usually 100 –1000 W h/kg, far larger than that required for the present complementary power source purpose, though their specific power is in the order of 0.1 kW/kg, being 1/10 of EDLCs. It is realistic to think that their specific power or kinetic performance can be improved at a reasonable sacrifice of the specific energy, though the durability to discharge/charge cycles must be also taken into account. Recently, some high rate intercalation electrodes based on the host – guest reaction with lithium have been investigated along such an idea. Passerini et al. [2] and Parent et al. [3] have proposed the use of vanadium oxide aerogels derived from sol –gel precursors by supercritical drying. Those V2O5 aerogels can act as high rate and high capacity hosts for lithium, because they have a very high surface area and the diffusion distance that must be penetrated by lithium is very short. A composite formed with a nickel fiber network substrate has been demonstrated to serve as a high rate positive electrode for ultracapacitor applications. More recently, the performance of Li– Mn – O aerogels derived from a similar sol – gel process has also been reported [4]. Previously, the present authors obtained a sol by a direct reaction of metallic vanadium with hydrogen peroxide and found that V2O5 xerogels derived from this sol showed good performance as a reversible insertion host for lithium [5,6]. The chemical diffusion coefficient of lithium, as determined by a chronoamperometric technique using a thin film of the gel, is relatively large (1011 – 1012 cm2/s) [7]. However, these materials themselves are not suitable for high rate use, because their specific surface area is not high enough. It seems that, as is often observed in conventional drying processes, removal of pore water causes
collapse of the solid network. Moreover, the electronic conductivity of the material is considerably low. We thus attempted to deposit nanoscale layers or particles of a V2O5 gel on a high surface area carbon material like acetylene black using our sol precursor. In this paper, we report preparation and fundamental performances of a V2O5/carbon composite thus obtained. As we are constructing a prototype supercapacitor device utilizing such a composite as a positive electrode, preliminary results of discharge/charge performance of scaled up electrodes are also described. In addition, from a practical point of view, this type of composite electrodes for supercapacitor applications is advantageous, because they can be produced without costly supercritical drying processes.
2. Experimental 2.1. Preparation of V2O5 sol A V2O5 sol was prepared basically according to our previous report [5]. Metallic vanadium powder (1 g for example) was dissolved completely in an icecooled 30% H2O2 (100 ml), to yield a bright reddish brown solution. It was quickly filtrated to remove a small amount of insoluble impurities and the supernatant was kept at room temperature. A vigorous evolution of oxygen generated by decomposition of excess H2O2 occurred in 10– 15 min. At the same time, the solution turned to a dark brown sol with a considerable viscosity. This sol was stable in storage for more than 1 year. According to observation using a light-scattering technique [8], colloidal particles in the sol seems to be prolate rod-like. They were estimated to be 5.0 Am in length and 0.060 Am in thickness at a concentration of 10 g/l. The size increased logarithmically with V2O5 concentration. The shape might become more prolate with their growth. 2.2. Preparation of composite electrodes An as-prepared sol (V2O5 content of which was about 30 g/l) was diluted with pure water and acetone at a volume fraction of 1:1:1. Then, carbon powder was added to the sol while stirring it vigorously at a desired carbon/V2O5 ratio in the range of 10 –400% in weight. For most cases, carbon was acetylene black (nominal
T. Kudo et al. / Solid State Ionics 152 – 153 (2002) 833–841
specific surface area: 61 m2/g, supplied by Tokyo Denkikagaku Kogyo). For comparison, a BP-20 active carbon material (nominal specific area: 1980 m2/g, Kurare Chemical) was also used. After stirring for 1 h, a homogeneous suspension of a V2O5/carbon composite was obtained. Addition of acetone as a surfactant was essential for obtaining a homogeneous suspension. A macroporous nickel-foamed metal (Sumitomo Denka) with an average pore diameter of 20 Am, as a current collector substrate, was soaked in a suspension thus formed. Then, it was removed from the suspension, dried in a desiccator for 12 h and heat-treated at 120 jC for 5 h to load amorphous V2O5/carbon composites on the substrate. Mass loading of the composite was typically 2 mg/cm2 substrate, in which 0.5 –1.5 mg of V2O5 was contained depending on the carbon/V2O5 ratio of the suspension. The vanadium content was determined on the inductively coupled plasma spectroscopy (ICPS) technique. The morphology of the composites was observed with a scanning electron microscope. 2.3. Electrochemical measurements A substrate loaded with the composite was cut into a 1010-mm2 square and used as a sample electrode. Measurements were performed in a 1 M LiClO4/ propylene carbonate electrolyte (unless otherwise mentioned) using a sealed three-electrode cell equipped with lithium counter and reference electrodes. To minimize the effect of the IR drop associated with the electrolyte resistance, the tip of a capillary in connection to the reference electrode was placed as close as possible to the sample electrode. The cutoff voltages for galvanostatic discharges/charges were mostly set at 4.2 and 2.0 V. 2.4. Engineering tests of prototype electrodes See Section 3.5.
3. Results 3.1. SEM observation Scanning electron micrographs of carbon (acetylene black) and some V2O5/carbon composites are
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compared in Fig. 1. The particles of acetylene black are roughly spherical with a diameter of about 100 nm. The calculated specific surface area upon this diameter is 30 m2/g, which roughly agrees the BET surface area. Thus, the particles have a smooth surface without nanoscale pore structures. Composite particles at the carbon/V2O5 ratio of 1.4 are shown in Fig. 1c. It seems that carbon particles are covered with a very thin layer of V2O5 gel. In a micrograph taken with a carbon-lean composite (carbon/V2O5 ratio=0.1), flatter masses assumed as isolated V2O5 gels are visible (Fig. 1b). 3.2. OCV of V2O5 gel Fig. 2 shows the OCV (equilibrium potential) curve of 120 jC-treated V2O5 gel as a function of lithium composition measured by a coulometric titration technique under potentiostatic conditions (PITT). The sample electrode was a mechanical mixture of the gel and 50% (in weight) carbon. The gel accommodates about 2.4 Li per V2O5 unit between the potentials of 4.0 and 2.0 V (vs. Li). The shape of the curve suggests that there are two distinct lithium sites in the gel. The voltammogram of the compound also showed two peaks in the relevant voltage range, confirming such a situation. Shapes of the voltammogram as well as the OCV curve, as a whole, resemble those of reported V2O5 gels [9]. 3.3. Discharge performance For a typical example, galvanostatic discharge curves recorded for a composite at carbon/V2O5=1.4 under various current densities or C rates are shown in Fig. 3. The C rates are based on the equilibrium or thermodynamic capacity between 4.2 and 2.0 V, which is 360 mA h/g V2O5 or 2.4 in terms of x (LixV2O5), as shown in Fig. 2. Thus, 1 C is equivalent to 0.36 A/g V2O5. Curves taken with current densities in the range of 1 –6 C were almost the same. Therefore, we can think that the curve for 6 C represents the OCV curve for the composite. It very much resembles the OCV curve in Fig. 2 in shape. The capacities in the relevant voltage range are also almost the same. Note that the fine structure of the curve shape is more clearly shown by the 6-C curve of the composite electrode.
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Fig. 2. The OCV of V2O5 gel as a function of x in terms of LixV2O5.
Although the capacity falls with increasing the current density, more than 50% of the ideal or thermodynamic capacity is obtainable even at a very high rate such as 150 C (i.e., 54 A/g V2O5). On the other hand, the capacity of a mechanical mixture of the present V2O5 gel and acetylene black (1:1 in weight) fell very rapidly with an increase of the discharge rate. In Fig. 4, available capacities with these two electrode materials are plotted against logarithmic current density. Comparing them at the same capacity level, the composite electrode can be discharged at a factor of 100 with higher current density. This indicates that the diffusion length of lithium in the composite is far shorter than that in the mechanically mixed counterpart. For the latter, a significant ohmic voltage drop was also observed. One curve in Fig. 4 is the plot for a composite at carbon/V2O5=0.2, indicating that the performance depends very much on the carbon content in the composite. As shown in Fig. 5, the galvanostatic capacity obtainable between 4.2 and 2.0 V is sharply increased with the carbon/V2O5 ratio in a carbon-lean region, but it levels off at a certain carbon content. The onset of leveling off shifts to the higher carbon content side as the applied current density is increased. It is thus necessary to optimize the carbon content depending on the usage of devices, because waste carbon only lowers the specific power or energy. Fig. 1. SEM photographs of carbon (acetylene black) and V2O5/ carbon composites. (a) Acetylene black, (b) composite (carbon/ V2O5 ratio=0.1 in weight) and (c) composite (carbon/V2O5=1.4).
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Fig. 3. Discharge curves of a V2O5/carbon composite (carbon/ V2O5=1.4) observed at various C rates (1 C: 0.36 A/g V2O5).
We prepared similar composite materials using high surface area active carbon (BP-20) instead of acetylene black. They only showed poorer performance. This indicates that sub-nanopores in active carbon is not effective, probably because the present vanadium oxide sol would not penetrate through such pores. Even though pores would be filled with V2O5 gels, most part of the gels would not be electrochemically accessible. 3.4. Discharge/charge cycles Fig. 6 shows some results of discharge/charge cycle tests performed at 20-C rate (7.2 A/g V2O5) in
Fig. 4. Capacity vs. current density curves of composite as compared with a mechanical mixture of V2O5 gel and acetylene black. (a) Composite (carbon/V 2 O 5 =1.4), (b) composite (carbon/ V2O5=0.2) and (c) mechanical mixture (carbon/V2O5=0.5).
Fig. 5. Capacity (4.2 – 2.0 V) obtained at various C rates as a function of carbon/V2O5 ratio (IR drop of electrolyte is included).
LiClO4/PC or LiPF6/g-butyrolactone (g-BL) electrolyte. Cycle performance depends very much on the depth of cycle (DOD) and electrolyte solutions used. The capacity normalized by that of the initial cycle falls rapidly when cycles are repeated in LiClO4/PC between 4.2 and 2.0 V. Although the same electrolyte is used, the fall of capacity is much slower if the cutoff voltage range is narrowed (4.2 –3.0 V). In this case, about 70% of the initial capacity is maintained at 1700 times the cycle. In the test with g-BL in the same voltage range, no fall in capacity was observed for more than 2000 cycles.
Fig. 6. Results of discharge/charge cycle tests of composite (carbon/ V2O5=1.4) at 20-C rate. Cutoff voltages and electrolyte are: (a) 4.2 – 2.0 V, LiClO4/PC, (b) 4.2 – 3.0 V, LiClO4/PC and (c) 4.2 – 3.0 V, LiPF6/g-BL.
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3.5. Performance of prototype electrodes Prototype scaled up electrodes were also fabricated to evaluate performance of the composites in a more practical form. A thin film of the composite was formed on an aluminum thin foil by coating a similar suspension as that in the above using an applicator. Then, it was pressed at 3 tons/cm2 and heat-treated at 120 jC for 5 h. The final thickness and density of the composite film were 16 Am and 1.4 g/cm3, respectively. The loading per unit area was 2.3 mg/cm2 in which 0.77 mg of V2O5 was contained (V2O5/carbon=1:2). It is noteworthy that films of the composite can be successfully fabricated without a binder like poly(tetrafluoroethylene) dispersion. Performance of a 3838 mm2 electrode thus formed was tested by constructing a two-electrode cell with a lithium foil counter electrode. In this case, a 1 M LiPF6/g-BL (gbutyrolactone) was used for an electrolyte, because this electrolyte system has been found to improve the cycle performance, as shown in the above. Moreover, it has been found that this electrolyte also suits a nitride anode in the Li –Co – N system. We are planning to construct a supercapacitor device using the present composite as a positive electrode and such a nitride as a negative electrode. The charge and discharge curves measured under 0.5 mA/cm2 (about 2-C rate) at the second cycle are shown in Fig. 7. The specific capacity on the abscissa is based on the weight of the composite (i.e., carbon+V2O5). The shape of the discharge curve reproduces the 6-C curve (=OCV curve) very well in Fig. 2.
Fig. 7. Discharge and charge curves of a prototype composite electrode at a small current density (0.5 mA/cm2 or 2-C rate).
Fig. 8. Voltammogram recorded for a prototype composite electrode. Sweep rate: (a) 0.035 and (b) 7.0 V/min.
The specific capacity per V2O5 instead of ‘‘per mass of composite’’ is also the same (360 mA h/g). The current efficiency associated with the charge/dis-
Fig. 9. Discharge curves of a prototype composite electrode at various current densities. (a) 10 mA/cm2 (37-C rate), (b) 20 mA/cm2 (74 C) and (c) 30 mA/cm2 (110 C).
T. Kudo et al. / Solid State Ionics 152 – 153 (2002) 833–841
charge process is almost 100%, indicating good reversibility. Voltammograms taken for this prototype electrode with two different sweep rates are shown in Fig. 8. In a slow sweep (0.035 V/min) diagram, there are two shoulders (near 3 V) and one prominent peak (2.5 V) in the reducing side. They correspond to oxidizing current peak or shoulder at 2.7 and 3.2 V, respectively. Even at a very fast sweep rate (7 V/min), those peaks are observable, though the peak potentials are somewhat shifted. The facts confirm that insertion and extraction of lithium with this electrode are very fast and highly reversible. Fig. 9 shows discharge curves taken under some constant current densities. Even at a current density as high as 30 mA/cm2 (about 110-C rate), this prototype electrode shows a relatively large capacity of 40 mA h/g electrode or 120 mA h/g V2O5 between 4.0 and 1.5 V. On those values, specific energy and power based on mass of the composite electrode are calculated to be 80 W h/kg and 26 kW/kg, respectively, assuming that an average working voltage of a device is 2.0 V. However, the capacity of the composite electrode falls rapidly with repeated cycles if the DOD is deep, as shown in Fig. 6. Taking this into account, the practical specific energy would be much lower, i.e., 15 – 20 W h/kg. Improvement of the cyclability at deep cycles is needed.
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Fig. 10. A plane sheet diffusion model for simulation of discharge curves.
conductivity of Li+ is negligibly small compared with the electronic conductivity. The boundary conditions relevant for this case are @C e @C and ¼ 0, J ¼ ð¼ I=SÞ ¼ F D @y y¼0 @y y¼L ð1Þ where S is the area of the electrolyte/host interface, F e is the chemical diffusion is the Faraday constant, D coefficient of Li in the host and C(=C(t,y)) is the concentration of lithium. Solving the diffusion equation [10] under these boundary conditions with an initial condition
4. Discussion
Cðt ¼ 0,yÞ ¼ C0 ,
The present composite of carbon and V2O5 gel as a host for lithium exhibits very high intercalation rate, as shown in Fig. 3. This is probably because the characteristic lithium diffusion distance is short enough for intercalating (and de-intercalating) lithium at a high rate exceeding 100 C. According to SEM micrographs, very thin layers of the V2O5 gel are formed on the surface of carbon particles. The average thickness of such a gel layer can be estimated by simulating discharge curves (Fig. 3) using a simple diffusion model. Let us assume that a plane sheet of a host (V2O5 gel) with a uniform thickness of L is formed on a conductor (carbon) and the surface of the host faces to a Li+ electrolyte (Fig. 10). On galvanostatic discharge (under a current of I), lithium is intercalated from the surface ( y=0) and diffuses into the host sheet, if its
we have (
e Dt 3ðy LÞ2 L2 þ L2 6L2 ! n l e 2 X ð1Þ 2 2 Dt exp n p 2 2 L p n¼1 n2 npðy LÞ cos : L
JL Cðt,yÞ ¼ e FD
ð2Þ
ð3Þ
Putting y=0 in Eq. (3), the surface concentration Cs at time t is given as Cs ðtÞ ¼ Cðt,y ¼ 0Þ
! l C* 1 2 X expðn2 p2 nÞ nþ 2 ¼ , n 3 p n¼1 n2
ð4Þ
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The surface Li content xs is given as vmCs. Now, we can calculate the actual discharge voltages under various current densities as a function of x: V ðxÞ ¼ f ðxs Þ,
ð6Þ
where a function f(x) represents the equilibrium potential – composition curve. Ohmic polarizations are neglected. Results of calculation for some values of L are e and vm were set at shown in Fig. 11. Values of the D 12 2 1.010 cm /s and 100 cm3/mol, respectively, according to our previous measurement with a similar V2O5 gel [7]. We adopted the curve for 6 C in Fig. 3 as a quasi-equilibrium potential – composition curve f(x); it is approximated by f ðxÞ ¼ 0:105894x5 þ 0:739777x4 2:03367x3 þ 2:75492x2 2:26598x þ 3:60619:
ð7Þ
If we assume L=100 nm (Fig. 11a), high rate discharge curves are shifted to the lower site far more than experimental curves in Fig. 3, because the surface Li content xs quickly departs from the equilibrium concentration x, as shown in Fig. 12. On the other hand, curves for L=20 nm (Fig. 11c) stick too close to the equilibrium curve even at very high rate conditions. Each curve calculated with L=50 nm
Fig. 11. Simulated discharge curves at various current densities assuming some values of V2O5 gel thickness (L). (a) L=100 nm, (b) L=50 nm and (c) L=20 nm.
where C0 is set at zero, and e Dt Jt x ¼ : ð5Þ n ¼ 2 and C* ¼ L FL vm Here, x is the average Li content per mole of the host (i.e., Li/V2O5) and vm the molar volume of the host.
Fig. 12. Simulated surface concentration of lithium (xs) vs. average x in terms of LixV2O5 at various current densities (L=100 nm).
T. Kudo et al. / Solid State Ionics 152 – 153 (2002) 833–841
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shows a reasonable agreement with the experimental curves, as shown in Fig. 11b. It is therefore thought that the average thickness of the V2O5 gel layers is around 50 nm. This thickness (L=50 nm) means that the coverage of the V2O5 gel over the carbon surface is only about 10%, because the specific surface area of the present acetylene black is 61 m2/g and the carbon/ V2O5 ratio is 1.4 (in weight). Thus, techniques to improve the coverage are needed. If the total carbon surface is covered homogeneously, the thickness of the gel would be about 5 nm and performance of the composite electrode would be dramatically improved.
poses. We are now developing a cell capable of very high rate use utilizing the present composite as a cathode in combination with lithium cobalt nitride as a negative electrode material.
5. Conclusions
References
Composites of carbon and amorphous vanadium oxide have been prepared by a sol – gel technique using a sol formed by a reaction of metallic vanadium with hydrogen peroxide. Vanadium oxide in this composite form can intercalate and de-intercalate lithium at a very high rate such as 150 C at a relatively small sacrifice of capacity. We have demonstrated that a prototype electrode fabricated by coating the composite on an aluminum foil shows a capacity of 40 mA h/g composite at a high rate of 30 mA/g composite. These values correspond to the specific power and energy of 26 kW/kg and 80 W h/kg (based on the mass of composite), respectively, assuming the average working voltage of 2 V. Although the composite shows nice intercalation reversibility, discharge/ charge cycle performance must be improved, if it is used as an electrode for faradaic supercapacitor pur-
Acknowledgements The study was supported jointly by the Japan Society for the Promotion of Science (JSPS) and New Energy and Industrial Technology Development Organization (NEDO) as a Research Development Program of University –Industry Alliance—A matching Fund Approach.
[1] K. Sawai, T. Ohzuku, J. Electrochem. Soc. 144 (1997) 988. [2] S. Passerini, J.J. Ressler, D.B. Le, B.B. Owens, W.H. Smyrl, The Electrochemical Society Proceeding, vol. 95-29, The Electrochemical Society Inc., Pennington, NJ, USA, 1995, p. 86. [3] M.J. Parent, S. Pesserini, B.B. Owens, W.H. Smyrl, J. Electrochem. Soc. 146 (1999) 1346. [4] S. Passerini, F. Coustier, M. Giorgetti, W.H. Smyrl, Electrochem. Solid-State Lett. 2 (1999) 483. [5] M. Hibino, T. Kudo, Solid State Ionics 79 (1995) 239. [6] M. Ugaji, M. Hibino, T. Kudo, J. Electrochem. Soc. 142 (1995) 3664. [7] T. Kudo, M. Hibino, in: The Electrochemical Society Proceeding, vol. 95-22, The Electrochemical Society Inc., Pennington, NJ, USA, 1995, p. 31. [8] T. Watanabe, Y. Ikeda, T. Ono, M. Hibino, M. Hosoda, K. Sakai, T. Kudo, Solid State Ionics, accepted for publication. [9] R. Baddour, J.P. Peria-Ramos, R. Messina, J. Perichon, J. Electroanal. Chem. (1991) 81. [10] J. Crank, Mathematics of Diffusion, 2nd edn., Oxford Science Publications, Oxford, UK, 1975, p. 61.
Amorphous V2O5/carbon composites as electrochemical supercapacitor electrodes T. Kudo a,*, Y. Ikeda a, T. Watanabe a, M. Hibino a, M. Miyayama a, H. Abe b, K. Kajita b a
Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan b Battery R&D Laboratory, Hitachi Maxell, Ltd., 1-1-88 Ushitora, Ibaraki, Osaka 567-8567, Japan Accepted 20 March 2002
Abstract High rate intercalation electrode performance of V2O5 gel/carbon composites has been demonstrated. A V2O5 sol was prepared by a reaction of metallic vanadium with a hydrogen peroxide solution. Acetylene black powder was added into the sol with acetone to yield a homogeneous suspension. A composite of amorphous V2O5 and carbon was loaded on a macroporous nickel current collector, and heat-treated at 120 jC to obtain a sample electrode. Electrochemical measurements were performed in some organic electrolytes like LiClO4/PC or LiPF6/g-butyrolactone (g-BL) at room temperature. It was confirmed that a composite electrode with the V2O5/carbon ratio of 0.7 in weight showed 54% of the ideal capacity, 360 mA h/g (4.2 – 2.0 V) based on V2O5, even at a very high rate discharge at 150 C or 54 A/g V2O5. The diffusion length of this host – guest system was ˜ =1012 cm2/s. The estimated as 30 – 50 nm by means of a simulation of the discharge curves using a diffusion model assuming D reversibility was also satisfactory and no capacity loss was observed after thousands of times of discharge/charge cycles between 4.2 and 3.0 V at the rate of 20 C. A prototype electrode was fabricated by coating a thin layer of the composite on an Al sheet current collector using an applicator. It showed the capacity of 40 mA h/g electrode at a current density as high as 30 mA/cm2. D 2002 Elsevier Science B.V. All rights reserved. PACS: 84.60.Dn Keywords: Amorphous vanadium oxide; Vanadium oxide sol; Lithium insertion electrode; Supercapacitor
1. Introduction Fuel cells and secondary batteries have a considerably high specific energy (W h/kg), but their specific power (W/kg) is relatively small. Thus, electric vehicles propelled by them need a complementary or auxiliary power source capable of supplying high power during the vehicle’s acceleration or climbing. Such a power source can also be used to quickly sink energy in the deceleration stage. Supercapacitors have attracted *
Corresponding author. Tel./fax: +81-3-5728-6498. E-mail address: [email protected] (T. Kudo).
much attention, because they can cope with highly time-dependent power demand. The term ‘‘supercapacitor’’ usually means a device utilizing the storage of charge in the electric double layer between an electrolyte and an electrochemically inert electrode like the high surface area active carbon. Such devices, often called electrical double layer capacitors (EDLC), have already been practically used primarily for semiconductor memory backup purposes. Typical specific energy and power of EDLCs are in the order of 1 W h/kg and 1 kW/kg, respectively. However, these specific values are too small for being a complementary energy source – sink for electric vehicles. In particular,
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 3 8 3 - 1
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the specific energy must be improved by a factor of more than 10. Some efforts have been made to improve the EDLC device itself. It has been reported that the specific capacity of 140 F/g is attained by use a carbon aerogel material with the specific surface area as large as 2000 m2/g [1]. However, this capacity is not high enough, because it is equivalent to about 4 W h/kg, if the capacitor voltage window is 1 V. Such an approach seems to have reached a limit. Another approach is an attempt to improve the specific power of a secondary battery like lithium ion cells based on the faradaic processes of an intercalation or host –guest system. The specific energy of those batteries is usually 100 –1000 W h/kg, far larger than that required for the present complementary power source purpose, though their specific power is in the order of 0.1 kW/kg, being 1/10 of EDLCs. It is realistic to think that their specific power or kinetic performance can be improved at a reasonable sacrifice of the specific energy, though the durability to discharge/charge cycles must be also taken into account. Recently, some high rate intercalation electrodes based on the host – guest reaction with lithium have been investigated along such an idea. Passerini et al. [2] and Parent et al. [3] have proposed the use of vanadium oxide aerogels derived from sol –gel precursors by supercritical drying. Those V2O5 aerogels can act as high rate and high capacity hosts for lithium, because they have a very high surface area and the diffusion distance that must be penetrated by lithium is very short. A composite formed with a nickel fiber network substrate has been demonstrated to serve as a high rate positive electrode for ultracapacitor applications. More recently, the performance of Li– Mn – O aerogels derived from a similar sol – gel process has also been reported [4]. Previously, the present authors obtained a sol by a direct reaction of metallic vanadium with hydrogen peroxide and found that V2O5 xerogels derived from this sol showed good performance as a reversible insertion host for lithium [5,6]. The chemical diffusion coefficient of lithium, as determined by a chronoamperometric technique using a thin film of the gel, is relatively large (1011 – 1012 cm2/s) [7]. However, these materials themselves are not suitable for high rate use, because their specific surface area is not high enough. It seems that, as is often observed in conventional drying processes, removal of pore water causes
collapse of the solid network. Moreover, the electronic conductivity of the material is considerably low. We thus attempted to deposit nanoscale layers or particles of a V2O5 gel on a high surface area carbon material like acetylene black using our sol precursor. In this paper, we report preparation and fundamental performances of a V2O5/carbon composite thus obtained. As we are constructing a prototype supercapacitor device utilizing such a composite as a positive electrode, preliminary results of discharge/charge performance of scaled up electrodes are also described. In addition, from a practical point of view, this type of composite electrodes for supercapacitor applications is advantageous, because they can be produced without costly supercritical drying processes.
2. Experimental 2.1. Preparation of V2O5 sol A V2O5 sol was prepared basically according to our previous report [5]. Metallic vanadium powder (1 g for example) was dissolved completely in an icecooled 30% H2O2 (100 ml), to yield a bright reddish brown solution. It was quickly filtrated to remove a small amount of insoluble impurities and the supernatant was kept at room temperature. A vigorous evolution of oxygen generated by decomposition of excess H2O2 occurred in 10– 15 min. At the same time, the solution turned to a dark brown sol with a considerable viscosity. This sol was stable in storage for more than 1 year. According to observation using a light-scattering technique [8], colloidal particles in the sol seems to be prolate rod-like. They were estimated to be 5.0 Am in length and 0.060 Am in thickness at a concentration of 10 g/l. The size increased logarithmically with V2O5 concentration. The shape might become more prolate with their growth. 2.2. Preparation of composite electrodes An as-prepared sol (V2O5 content of which was about 30 g/l) was diluted with pure water and acetone at a volume fraction of 1:1:1. Then, carbon powder was added to the sol while stirring it vigorously at a desired carbon/V2O5 ratio in the range of 10 –400% in weight. For most cases, carbon was acetylene black (nominal
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specific surface area: 61 m2/g, supplied by Tokyo Denkikagaku Kogyo). For comparison, a BP-20 active carbon material (nominal specific area: 1980 m2/g, Kurare Chemical) was also used. After stirring for 1 h, a homogeneous suspension of a V2O5/carbon composite was obtained. Addition of acetone as a surfactant was essential for obtaining a homogeneous suspension. A macroporous nickel-foamed metal (Sumitomo Denka) with an average pore diameter of 20 Am, as a current collector substrate, was soaked in a suspension thus formed. Then, it was removed from the suspension, dried in a desiccator for 12 h and heat-treated at 120 jC for 5 h to load amorphous V2O5/carbon composites on the substrate. Mass loading of the composite was typically 2 mg/cm2 substrate, in which 0.5 –1.5 mg of V2O5 was contained depending on the carbon/V2O5 ratio of the suspension. The vanadium content was determined on the inductively coupled plasma spectroscopy (ICPS) technique. The morphology of the composites was observed with a scanning electron microscope. 2.3. Electrochemical measurements A substrate loaded with the composite was cut into a 1010-mm2 square and used as a sample electrode. Measurements were performed in a 1 M LiClO4/ propylene carbonate electrolyte (unless otherwise mentioned) using a sealed three-electrode cell equipped with lithium counter and reference electrodes. To minimize the effect of the IR drop associated with the electrolyte resistance, the tip of a capillary in connection to the reference electrode was placed as close as possible to the sample electrode. The cutoff voltages for galvanostatic discharges/charges were mostly set at 4.2 and 2.0 V. 2.4. Engineering tests of prototype electrodes See Section 3.5.
3. Results 3.1. SEM observation Scanning electron micrographs of carbon (acetylene black) and some V2O5/carbon composites are
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compared in Fig. 1. The particles of acetylene black are roughly spherical with a diameter of about 100 nm. The calculated specific surface area upon this diameter is 30 m2/g, which roughly agrees the BET surface area. Thus, the particles have a smooth surface without nanoscale pore structures. Composite particles at the carbon/V2O5 ratio of 1.4 are shown in Fig. 1c. It seems that carbon particles are covered with a very thin layer of V2O5 gel. In a micrograph taken with a carbon-lean composite (carbon/V2O5 ratio=0.1), flatter masses assumed as isolated V2O5 gels are visible (Fig. 1b). 3.2. OCV of V2O5 gel Fig. 2 shows the OCV (equilibrium potential) curve of 120 jC-treated V2O5 gel as a function of lithium composition measured by a coulometric titration technique under potentiostatic conditions (PITT). The sample electrode was a mechanical mixture of the gel and 50% (in weight) carbon. The gel accommodates about 2.4 Li per V2O5 unit between the potentials of 4.0 and 2.0 V (vs. Li). The shape of the curve suggests that there are two distinct lithium sites in the gel. The voltammogram of the compound also showed two peaks in the relevant voltage range, confirming such a situation. Shapes of the voltammogram as well as the OCV curve, as a whole, resemble those of reported V2O5 gels [9]. 3.3. Discharge performance For a typical example, galvanostatic discharge curves recorded for a composite at carbon/V2O5=1.4 under various current densities or C rates are shown in Fig. 3. The C rates are based on the equilibrium or thermodynamic capacity between 4.2 and 2.0 V, which is 360 mA h/g V2O5 or 2.4 in terms of x (LixV2O5), as shown in Fig. 2. Thus, 1 C is equivalent to 0.36 A/g V2O5. Curves taken with current densities in the range of 1 –6 C were almost the same. Therefore, we can think that the curve for 6 C represents the OCV curve for the composite. It very much resembles the OCV curve in Fig. 2 in shape. The capacities in the relevant voltage range are also almost the same. Note that the fine structure of the curve shape is more clearly shown by the 6-C curve of the composite electrode.
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Fig. 2. The OCV of V2O5 gel as a function of x in terms of LixV2O5.
Although the capacity falls with increasing the current density, more than 50% of the ideal or thermodynamic capacity is obtainable even at a very high rate such as 150 C (i.e., 54 A/g V2O5). On the other hand, the capacity of a mechanical mixture of the present V2O5 gel and acetylene black (1:1 in weight) fell very rapidly with an increase of the discharge rate. In Fig. 4, available capacities with these two electrode materials are plotted against logarithmic current density. Comparing them at the same capacity level, the composite electrode can be discharged at a factor of 100 with higher current density. This indicates that the diffusion length of lithium in the composite is far shorter than that in the mechanically mixed counterpart. For the latter, a significant ohmic voltage drop was also observed. One curve in Fig. 4 is the plot for a composite at carbon/V2O5=0.2, indicating that the performance depends very much on the carbon content in the composite. As shown in Fig. 5, the galvanostatic capacity obtainable between 4.2 and 2.0 V is sharply increased with the carbon/V2O5 ratio in a carbon-lean region, but it levels off at a certain carbon content. The onset of leveling off shifts to the higher carbon content side as the applied current density is increased. It is thus necessary to optimize the carbon content depending on the usage of devices, because waste carbon only lowers the specific power or energy. Fig. 1. SEM photographs of carbon (acetylene black) and V2O5/ carbon composites. (a) Acetylene black, (b) composite (carbon/ V2O5 ratio=0.1 in weight) and (c) composite (carbon/V2O5=1.4).
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Fig. 3. Discharge curves of a V2O5/carbon composite (carbon/ V2O5=1.4) observed at various C rates (1 C: 0.36 A/g V2O5).
We prepared similar composite materials using high surface area active carbon (BP-20) instead of acetylene black. They only showed poorer performance. This indicates that sub-nanopores in active carbon is not effective, probably because the present vanadium oxide sol would not penetrate through such pores. Even though pores would be filled with V2O5 gels, most part of the gels would not be electrochemically accessible. 3.4. Discharge/charge cycles Fig. 6 shows some results of discharge/charge cycle tests performed at 20-C rate (7.2 A/g V2O5) in
Fig. 4. Capacity vs. current density curves of composite as compared with a mechanical mixture of V2O5 gel and acetylene black. (a) Composite (carbon/V 2 O 5 =1.4), (b) composite (carbon/ V2O5=0.2) and (c) mechanical mixture (carbon/V2O5=0.5).
Fig. 5. Capacity (4.2 – 2.0 V) obtained at various C rates as a function of carbon/V2O5 ratio (IR drop of electrolyte is included).
LiClO4/PC or LiPF6/g-butyrolactone (g-BL) electrolyte. Cycle performance depends very much on the depth of cycle (DOD) and electrolyte solutions used. The capacity normalized by that of the initial cycle falls rapidly when cycles are repeated in LiClO4/PC between 4.2 and 2.0 V. Although the same electrolyte is used, the fall of capacity is much slower if the cutoff voltage range is narrowed (4.2 –3.0 V). In this case, about 70% of the initial capacity is maintained at 1700 times the cycle. In the test with g-BL in the same voltage range, no fall in capacity was observed for more than 2000 cycles.
Fig. 6. Results of discharge/charge cycle tests of composite (carbon/ V2O5=1.4) at 20-C rate. Cutoff voltages and electrolyte are: (a) 4.2 – 2.0 V, LiClO4/PC, (b) 4.2 – 3.0 V, LiClO4/PC and (c) 4.2 – 3.0 V, LiPF6/g-BL.
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3.5. Performance of prototype electrodes Prototype scaled up electrodes were also fabricated to evaluate performance of the composites in a more practical form. A thin film of the composite was formed on an aluminum thin foil by coating a similar suspension as that in the above using an applicator. Then, it was pressed at 3 tons/cm2 and heat-treated at 120 jC for 5 h. The final thickness and density of the composite film were 16 Am and 1.4 g/cm3, respectively. The loading per unit area was 2.3 mg/cm2 in which 0.77 mg of V2O5 was contained (V2O5/carbon=1:2). It is noteworthy that films of the composite can be successfully fabricated without a binder like poly(tetrafluoroethylene) dispersion. Performance of a 3838 mm2 electrode thus formed was tested by constructing a two-electrode cell with a lithium foil counter electrode. In this case, a 1 M LiPF6/g-BL (gbutyrolactone) was used for an electrolyte, because this electrolyte system has been found to improve the cycle performance, as shown in the above. Moreover, it has been found that this electrolyte also suits a nitride anode in the Li –Co – N system. We are planning to construct a supercapacitor device using the present composite as a positive electrode and such a nitride as a negative electrode. The charge and discharge curves measured under 0.5 mA/cm2 (about 2-C rate) at the second cycle are shown in Fig. 7. The specific capacity on the abscissa is based on the weight of the composite (i.e., carbon+V2O5). The shape of the discharge curve reproduces the 6-C curve (=OCV curve) very well in Fig. 2.
Fig. 7. Discharge and charge curves of a prototype composite electrode at a small current density (0.5 mA/cm2 or 2-C rate).
Fig. 8. Voltammogram recorded for a prototype composite electrode. Sweep rate: (a) 0.035 and (b) 7.0 V/min.
The specific capacity per V2O5 instead of ‘‘per mass of composite’’ is also the same (360 mA h/g). The current efficiency associated with the charge/dis-
Fig. 9. Discharge curves of a prototype composite electrode at various current densities. (a) 10 mA/cm2 (37-C rate), (b) 20 mA/cm2 (74 C) and (c) 30 mA/cm2 (110 C).
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charge process is almost 100%, indicating good reversibility. Voltammograms taken for this prototype electrode with two different sweep rates are shown in Fig. 8. In a slow sweep (0.035 V/min) diagram, there are two shoulders (near 3 V) and one prominent peak (2.5 V) in the reducing side. They correspond to oxidizing current peak or shoulder at 2.7 and 3.2 V, respectively. Even at a very fast sweep rate (7 V/min), those peaks are observable, though the peak potentials are somewhat shifted. The facts confirm that insertion and extraction of lithium with this electrode are very fast and highly reversible. Fig. 9 shows discharge curves taken under some constant current densities. Even at a current density as high as 30 mA/cm2 (about 110-C rate), this prototype electrode shows a relatively large capacity of 40 mA h/g electrode or 120 mA h/g V2O5 between 4.0 and 1.5 V. On those values, specific energy and power based on mass of the composite electrode are calculated to be 80 W h/kg and 26 kW/kg, respectively, assuming that an average working voltage of a device is 2.0 V. However, the capacity of the composite electrode falls rapidly with repeated cycles if the DOD is deep, as shown in Fig. 6. Taking this into account, the practical specific energy would be much lower, i.e., 15 – 20 W h/kg. Improvement of the cyclability at deep cycles is needed.
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Fig. 10. A plane sheet diffusion model for simulation of discharge curves.
conductivity of Li+ is negligibly small compared with the electronic conductivity. The boundary conditions relevant for this case are @C e @C and ¼ 0, J ¼ ð¼ I=SÞ ¼ F D @y y¼0 @y y¼L ð1Þ where S is the area of the electrolyte/host interface, F e is the chemical diffusion is the Faraday constant, D coefficient of Li in the host and C(=C(t,y)) is the concentration of lithium. Solving the diffusion equation [10] under these boundary conditions with an initial condition
4. Discussion
Cðt ¼ 0,yÞ ¼ C0 ,
The present composite of carbon and V2O5 gel as a host for lithium exhibits very high intercalation rate, as shown in Fig. 3. This is probably because the characteristic lithium diffusion distance is short enough for intercalating (and de-intercalating) lithium at a high rate exceeding 100 C. According to SEM micrographs, very thin layers of the V2O5 gel are formed on the surface of carbon particles. The average thickness of such a gel layer can be estimated by simulating discharge curves (Fig. 3) using a simple diffusion model. Let us assume that a plane sheet of a host (V2O5 gel) with a uniform thickness of L is formed on a conductor (carbon) and the surface of the host faces to a Li+ electrolyte (Fig. 10). On galvanostatic discharge (under a current of I), lithium is intercalated from the surface ( y=0) and diffuses into the host sheet, if its
we have (
e Dt 3ðy LÞ2 L2 þ L2 6L2 ! n l e 2 X ð1Þ 2 2 Dt exp n p 2 2 L p n¼1 n2 npðy LÞ cos : L
JL Cðt,yÞ ¼ e FD
ð2Þ
ð3Þ
Putting y=0 in Eq. (3), the surface concentration Cs at time t is given as Cs ðtÞ ¼ Cðt,y ¼ 0Þ
! l C* 1 2 X expðn2 p2 nÞ nþ 2 ¼ , n 3 p n¼1 n2
ð4Þ
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The surface Li content xs is given as vmCs. Now, we can calculate the actual discharge voltages under various current densities as a function of x: V ðxÞ ¼ f ðxs Þ,
ð6Þ
where a function f(x) represents the equilibrium potential – composition curve. Ohmic polarizations are neglected. Results of calculation for some values of L are e and vm were set at shown in Fig. 11. Values of the D 12 2 1.010 cm /s and 100 cm3/mol, respectively, according to our previous measurement with a similar V2O5 gel [7]. We adopted the curve for 6 C in Fig. 3 as a quasi-equilibrium potential – composition curve f(x); it is approximated by f ðxÞ ¼ 0:105894x5 þ 0:739777x4 2:03367x3 þ 2:75492x2 2:26598x þ 3:60619:
ð7Þ
If we assume L=100 nm (Fig. 11a), high rate discharge curves are shifted to the lower site far more than experimental curves in Fig. 3, because the surface Li content xs quickly departs from the equilibrium concentration x, as shown in Fig. 12. On the other hand, curves for L=20 nm (Fig. 11c) stick too close to the equilibrium curve even at very high rate conditions. Each curve calculated with L=50 nm
Fig. 11. Simulated discharge curves at various current densities assuming some values of V2O5 gel thickness (L). (a) L=100 nm, (b) L=50 nm and (c) L=20 nm.
where C0 is set at zero, and e Dt Jt x ¼ : ð5Þ n ¼ 2 and C* ¼ L FL vm Here, x is the average Li content per mole of the host (i.e., Li/V2O5) and vm the molar volume of the host.
Fig. 12. Simulated surface concentration of lithium (xs) vs. average x in terms of LixV2O5 at various current densities (L=100 nm).
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shows a reasonable agreement with the experimental curves, as shown in Fig. 11b. It is therefore thought that the average thickness of the V2O5 gel layers is around 50 nm. This thickness (L=50 nm) means that the coverage of the V2O5 gel over the carbon surface is only about 10%, because the specific surface area of the present acetylene black is 61 m2/g and the carbon/ V2O5 ratio is 1.4 (in weight). Thus, techniques to improve the coverage are needed. If the total carbon surface is covered homogeneously, the thickness of the gel would be about 5 nm and performance of the composite electrode would be dramatically improved.
poses. We are now developing a cell capable of very high rate use utilizing the present composite as a cathode in combination with lithium cobalt nitride as a negative electrode material.
5. Conclusions
References
Composites of carbon and amorphous vanadium oxide have been prepared by a sol – gel technique using a sol formed by a reaction of metallic vanadium with hydrogen peroxide. Vanadium oxide in this composite form can intercalate and de-intercalate lithium at a very high rate such as 150 C at a relatively small sacrifice of capacity. We have demonstrated that a prototype electrode fabricated by coating the composite on an aluminum foil shows a capacity of 40 mA h/g composite at a high rate of 30 mA/g composite. These values correspond to the specific power and energy of 26 kW/kg and 80 W h/kg (based on the mass of composite), respectively, assuming the average working voltage of 2 V. Although the composite shows nice intercalation reversibility, discharge/ charge cycle performance must be improved, if it is used as an electrode for faradaic supercapacitor pur-
Acknowledgements The study was supported jointly by the Japan Society for the Promotion of Science (JSPS) and New Energy and Industrial Technology Development Organization (NEDO) as a Research Development Program of University –Industry Alliance—A matching Fund Approach.
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