Using Mesoporous Nanoarchitectures to Improve Battery Performance

Chapter 46

Using Mesoporous Nanoarchitectures to Improve Battery Performance Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 19, 2015 | http://pubs.acs.org Publication Date: December 14, 2004 | doi: 10.1021/bk-2005-0890.ch046

Bruce Dunn*, François Bonet, Liam Noailles, and Paul Tang Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, CA 90095-1595 *Corresponding author: email: [email protected]; fax: 1-310-206-7353

Aerogels are composed of a three-dimensional network of nanometer-sized solid particles surrounded by a continuous macroporous and mesoporous volume. The porosity provides both molecular accessibility and rapid mass transport and for these reasons, aerogels have been widely used in the heterogeneous catalytic materials field (1). However, electrochemical materials in general, and battery materials in particular, have yet to exploit this nanoarchitecture, despite the fact that one would expect such physical features to be desirable for electrochemical reactions (2). This paper reviews some of the interesting, and unexpected, electrochemical results obtained with vanadium oxide aerogels. These results underscore the benefits to be gained by creating battery materials with mesoporous architectures. Aerogels differ substantially from traditional lithium intercalation electrode materials. First, aerogels are nanocrystalline, if not totally amorphous materials. The solid phase in an aerogel is composed of a colloidal oxide network that is characteristically quite thin, usually on the order of 10 to 50 nm. One can expect that diffusion distances will be rather short compared to traditional electrode materials. The high surface area of aerogels is significant because it means that surface effects can be amplified. Thus, surface defects that may not be evident in bulk materials, can now become prominent in aerogels because of the drastic increase in surface area. Moreover, since oxides have inherently defective surfaces, the defect chemistry associated with high vacancy concentrations is likely to influence aerogel properties. Finally, the high © 2005 American Chemical Society

In Nanotechnology and the Environment; Karn, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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342 porosity and mesoscopic pore diameters of aerogel structures enable the electrolyte to penetrate the entire aerogel particle.

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 19, 2015 | http://pubs.acs.org Publication Date: December 14, 2004 | doi: 10.1021/bk-2005-0890.ch046

Electrochemical Properties of Vanadium Oxide Aerogels In this paper, we briefly describe two studies that show how the mesoporous architecture of vanadium oxide aerogels leads to unique electrochemical properties. Early investigations of the electrochemical properties of aerogels did not fully appreciate the importance of preserving aerogel morphology in the electrode structures required for determining electrochemical properties. That is, in traditional electrode structures, where one combines the aerogel with other components: carbon black conductor, polymer binder and solvent, the aerogel morphology could be compromised. If the wrong solvent is used, the aerogel particles tend to agglomerate during the processing of the traditional electrodes, which reduces their surface area and collapses the interconnected mesoporous network. Moreover the carbon black particles aggregate and may occlude the aerogel surface. For these reasons, the electrochemical responses reported in many earlier studies did not represent the electrochemical properties of a highsurface-area aerogel, but rather the behavior of an agglomerated aerogel system, which did not necessarily possess an interconnected mesoporous architecture. In our recent research, we have deliberately created electrode structures that preserve aerogel morphology. One approach has been to examine the fundamental electrochemical properties of the V 0 aerogel by using "stickycarbon" electrodes (3). The aerogel particles are not aggregated on the electrode and there is ample electron, ion and solvent transport to the solid phase of the V 0 . The voltammetric response for the aerogel immobilized on the stickycarbon electrode is shown in Figure 1 (4). In the LiC10 /propylene carbonate (PC) electrolyte, the features are broad and capacitive and the intercalation peaks appear superimposed upon the capacitive response. This capacitive response is substantially different from that which is obtained for the same V 0s aerogel when it is prepared in a conventional composite electrode structure (i.e., with carbon black as a conductive additive). In that case, the voltammetric response consists of faradaic peaks associated with the characteristic intercalation behavior for sol-gel-derived V 0 materials (4). The electrochemical responses for vanadium oxide aerogels can be described in terms of both a specific capacitance (Fg" ) and as lithium capacity (mAhg" ). Depending upon the nature of the drying process, the specific capacitance values range from 960 Fg" to over 2000 Fg" . The largest specific capacitance was obtained using cyclohexane as the drying solvent (2150 Fg" ), which corresponds to nearly 1500 μ F cm" . This magnitude of specific capacitance represents pseudocapacitive behavior (5), which is a very different charge-storage mechanism than that which occurs with traditional intercalation 2

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343 materials (2). At the same time, these materials also possess very high capacity for lithium. Values range from 400 mAhg" to over 600 mAhg" , which are many times larger than the 140 mAhg" range exhibited by the cathodes used in commercial lithium secondary batteries (6). 1

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Figure 1. Voltammograms for V 0$ aerogels using different electrodes: (a) Sticky carbon electrode, (b) Traditional electrode structure using carbon black as the conductive additive andPVDF as a binder. The arrows in (a) and (b) refer to the same voltages. The electrolyte is L1CIO4 (1 M) in propylene carbonate. (Reproducedfrom reference 4. Copyright 2000 Electrochemical Society.) 2

The sticky-carbon electrode studies show that the inherent electrochemical properties of the aerogel are different from those obtained using a traditional composite electrode. In this electrode, the entire particle is electrochemically accessible, with much shorter diffusion distances than those that occur in traditional electrodes that involve micron-scale dimensions for electron and ion transport. Under these conditions, the V2O5 aerogel displays a combination of battery-like ion incorporation and capacitor-like response. That is, the V 0 aerogels possess a high specific capacitance, with values comparable to those of supercapacitors, as well as a reversible capacity for lithium that is substantially greater than that of commercial intercalation materials. We have proposed that the origin of this behavior is associated with the high surface area of the aerogel, meaning that the aerogel morphology amplifies surface phenomena (2). Surface defects, such as cation vacancies, which are not prominent in bulk materials, are expected to be present in high concentrations because of the high surface area of the aerogel. Lithium ion access to such defect sites provides charge compensation for the vacancies and explains how higher lithium capacities are achieved in aerogels as compared to xerogels and crystalline V 0 (6). Recent 2

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344 research by Swider-Lyons et a l provides greater insight concerning the role of defect structure on lithium capacity (7). The second type of electrode structure that preserves aerogel morphology is based on using carbon nanotubes as the electronically conducting network in V 0 aerogel electrodes. Battery electrodes require the addition of an electronically conducting phase to overcome the low electronic conductivity of transition metal oxides and facilitate electrochemical reactions (6). Although carbon black is the typical additive, the unique morphology of vanadium oxide aerogels raises the question of whether carbon black is the best conductive phase for this system. In contrast to carbon black, which can aggregate and impede electrolyte access, carbon nanotubes possess a similar morphology and dimensional scale as the vanadium oxideribbonsthat compose the aerogel. This morphological resemblance suggested the prospect of fabricating a nanocomposite that exploits the high conductivity of SWNTs and does not block the high surface area of the aerogel. In view of the capacitor-like behavior exhibited by the aerogel using sticky carbon electrodes, it was anticipated that the V 0 -SWNT electrodes would have good performance at high discharge rates. We developed a synthetic approach whereby gelation of vanadium oxide occurred around the SWNTs (8). The resulting nanocomposites exhibit intimate contact, at the nanodimensional level, between the nanotubes and the vanadium oxide ribbons that compose the aerogel. This morphology leads to excellent charge transfer properties between the two phases as shown by galvanostatic studies carried out for different composite electrodes. The results (Figure 2) show that the V 0 -SWNT electrodes exhibit excellent electrochemical properties, particularly at high discharge rates (8). Above discharge rates of 2 C, it is evident that the specific capacities for the V 0 - S W N T electrodes are consistently higher than that of the traditional electrode even though the V 0 SWNT electrodes contain about half as much carbon (9 wt.% vs. 17 wt.%). The decrease in lithium capacity at high discharge rates is much less prominent with the V 0 - S W N T electrodes than with standard electrode structures. In summary, our studies show that aerogels possess a novel and very desirable microstructure for electrochemical systems. Diffusion lengths are short and the mesoporous morphology enables the electrolyte to penetrate the entire aerogel particle. Among the more significant properties exhibited by these materials are a substantial increase in lithium capacity at high discharge rates and fundamental electrochemical behavior which combines both battery and capacitor characteristics. It is believed that the unique properties of aerogels are influenced significantly by the presence of defects, which reach appreciable concentrations in high-surface-area aerogels and now dominate the electrochemical behavior.


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Acknowledgements The authors are grateful for the support of their research by the Office of Naval Research.

References 1.

Pajonk, G. M . Catalytic aerogels. Catal. Today 1997, 35, 319-337.

2.

Rolison, D. R.; Dunn, B. Electrically conductive oxide aerogels:

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3.

Long, J. W.; Swider, K. E.; Merzbacher, C. I.; Rolison, D. R. Voltammetric characterization of ruthenium oxide-based aerogels and other RuO solids: the nature of capacitance in nanostructured materials. Langmuir 1999, 15, 2

780-785 [correction: Langmuir 2003, 19, 2532].


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Dong, W.; Rolison, D. R.; Dunn, B. Electrochemical properties of high surface area vanadium oxide aerogels. Electrochem. Solid-State Lett. 2000, 3, 457-459. Conway, B. E. Transition from supercapacitor to battery behavior in electrochemical energy-storage. J. Electrochem. Soc. 1991, 138, 15391548. Winter, M . ; Besenhard, J. O.; Spahr, M . E.; Novák, P. Insertion electrode materials for rechargeable lithium batteries. Adv. Mater. 1998, 10, 725-763. Swider-Lyons, K. E.; Love, C. T.; Rolison, D. R. Improved lithium capacity of defective V O materials. Solid State Ionics 2002, 152-153, 99-104. Sakamoto, J. S.; Dunn, B. Vanadium oxide-carbon nanotube composite electrodes for use in secondary lithium batteries. J. Electrochem. Soc. 2002 149, A26-A30. 2

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Using Mesoporous Nanoarchitectures to Improve Battery Performance

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