I ; A Invited Talk, for presentation at the Advanced Non-Aqueous Battery R&D Workshop

to be held in Hunt Valley, MD on October 15-17, 1997. 1

'.z NOVEL CARBONACEOUS MATERIALS USED (id/?- 9 '7 / 7 ~ AS ANODES IN LITHJSJM ION CELLS

G. Sandi, R. E. Winans and K. A. Carrado Chemistry Division, Argonne National Laboratory, Argonne, IL 60439

(630) 252-1903/(630) 252-9288 fax/gsandi@anl.gov (630) 252-7479/(630) 252-9288 fax/rewinans@anl.gov (630) 252-7968/(630) 252-9288 fax/kcarrado@anl.gov

OBJECTIVE

The objective of this work is to synthesize disordered carbons used as anodes in lithium ion batteries, where the porosity and surface area are controlled. Both parameters are critical since the ineversible capacity obtained in the first cycle seems to be associated with the surface area (an exfoliation mechanism occurs in which the exposed surface area continues to increase) (1).

APPROACH

Our approach is to use pillared clays (PILCs) as inorganic templates. These modified clays have aluminum oxide supports between the layers that help to prevent the collapse of the layers upon heat treatment. Five organic compounds are used as carbon precursors: pyrene, styrene, trioxane/ pyrene, ethylene and propylene. Carbon from pyrene as the precursor is produced by a mechanism described by Sandi et al. (2 ) in which the alumina pillars in the clay should act as acid sites to promote condensation similar to the Scholl reaction (3). Carbon from styrene and trioxanel pyrene are produced by the incorporation of liquid monomer in the PILC, followed by a low temperature polymerization reaction. Carbon from ethylene and propylene are synthesized by a mechanism similar to that described by Tomita et al. (3) in which the gaseous hydrocarbon is deposited in the PILC layers and subsequently pyrolyzed. After elimination of the inorganic matrix. the resulting layered carbons show holes due to the pillaring AI,, cluster unit where lithium diffusion should be enhanced.

Figure 1 shoLvs the voltage-capacity profiles for the second cycle of lithium carbon cells made from the different precursor. X plateau near 0.7 V in charge u.as obtained. with a magnitude that

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0 200 400 600 800 1000 1200 1400 1600 1800 Capacity, mAh/g

Figure I . Voltage-Capacity profiles for the second cycle of lithiudcarbon cells. Data has been offset in the Y axis for clarity.

increases as propylene> styrene> pyrene> trioxane/pyrene. Most of the capacity is delivered between 1 and 0.1 V, thus avoiding the possibility of safety problems associated with the lithium metal deposition close to 0 V.

Table I summarizes the specific capacity, irreversible capacity in the first cycle and the standard deviation associated with the capacity upon cycling. The irreversible capacity is defined as the difference in capacity between the first and second discharge. Pyrene exhibited the lowest irreversible capacity and capacity fade. However, the performance of these cells in terms of delivered capacity is much higher than graphite (370 mAh/g at 200-300 mV vs. Li) and some other alternative materials currently under study. While some hysteresis occurs in the discharge- charge voltage profiles, these carbon electrodes do deliver very high (675-794 mAh/,o) and stable capacities (>50 cycles). Electrodes prepared from the trioxane precursor showed the largest

The submitted manuscript has been authored by a contractor of the U S Government under contract No W-31-109-ENG 38 Accordingly the U S Government retams a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution or allow others to do so for U S Government purposes

D"IC QTJALRT it%YZZmD 21

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liabili- ty or responsibility for the accuracy, completeness, or usefulness of any information, appa- ratus, product, or process disdased, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessar- ily state or reflect those of the United States Government or any agency thereof.

hysteresis effect. We believe that oxygen on the surface is the main cause of this undesirable phenomenon in lithium ion batteries (5 ) .

Table I. Effect of different carbon precursors on the performance of LVcarbon coin cells.

Sample Capacity, S D Irr. cap.

Pyrene 720 60 62

mAh-k N g d g

Styrene

Propylene

730

794

45

91

177

180

Trioxanel 675 75 165 pyrene

Based on the results from X-ray diffraction, small- angle neutron (SAVS) and X-ray (SAXS) scattering, and elemental analysis, a model for a disordered carbon produced with pillared clays as templates has been developed. The carbon sheets within the crystallites appear to have holes due to the pillaring units, and there are no dangling ends on the periphery of these holes. Multiple layers of carbon are formed between the inorganic layers.

FUTURE DIRECTIONS

The research will continue to focus on the synthesis of novel materials using appropriate natural or synthetic clays. One of the clays that is planned to be used is called sepiolite. This clay has the general structure Si,,Mg,O,,(OH>, (OH,), 6H,O. It is a clay that does not have to be pillared since it contains a channeled structure. Another potential class of organic-inorganic templates from which to derive designed carbons are the synthetic polymer-clays made in our laboratories for composite applications (6). These materials are created from a clay sol gel that contains, in addition to the synthetic clay precursor (silica and magnesium hydroxide sols), a water-soluble polymer. These materials can be carbonized as is to explore the resulting carbons.. Alternatively, the polymer “glue” holding the silicate layers together can be removed to leave behind a pure mesoporous silicate network to use as the templates. The second study concerns the mechanistic investigation of ion transport by using

nuclear magnetic resonance. Klingler et al. (7) have developed a technique called rotating frame imaging. Structural identification of the intermediates in the diffusion layer, intercalation of composite electrodes. and more accurate calculations of .transference numbers and diffusion coefficients, c b be made with this new technique. In a new facilities initiative at the Advanced Photon Source, we are building a time resolvedanomalous small angle X-ray scattering instrument in the BESSRC CAT. In a third study, we plan to examine carbons as they are being charged with Li, in situ. Because of the intense source of X-rays provided by the undulator, we will be able to do time resolved experiments. After the first charge of Li, a significant amount of capacity is lost. This experiment will provide information on the changes in structure of the carbons down to the length scale of 6 A. In addition, larger features will be observed out to 1000 A. The changes with cycling will be followed.

ACKNOWLEDGMENTS

This work was performed under the auspices of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under contract number W-3 1-109-ENG-38.

REFERENCES

1. B. M. Way, and Ulrich von Sacken, Abstract 830, p. 1020, The Electrochemical Society Extended Abstracts, Vol96-2, San Antonio, TX, October 6-1 1, 1996.

2 G. Sandi, R. E. Winans, and K. A. Carrado, J. Electrochem. Soc., 143, L95 (1996).

3. A. Balaban, and T. Nenitzescu, in Friedef- Crafrs Chemist?. G. Olah, Editor, vol. 2, p.979, Wiley, New York (1973).

4. T. Kyotani, L. F. Tsai, and A. Tomita, Chem. of Materials, 8,2109 (1996).

5. G. Sandi, K. Song, K. A. Carrado, and R. E. Winans, submitted to Carbon, July, 1997.

6. K. A. Carrado, P. Thiyagarajan, and D. L. Elder, in Synthesis of Porous Materials, 34. L. Occelli, and H. Kessler, Editors, p. 55 1, Marcel Dekker, New York (1996).

7. K. Woek, R. E. Gerald. R. J. Klingler, and J. W. Rathke. Journal of Magnetic Resonance Series, 131,74 (1996).

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