ELECTROCHEMICAL SOCIETY LETTERS Polyacene as an Anode in Lithium Ion Batteries Sheng Shui Zhang and Qing Guo Liu* Laboratory of Solid State Ionics, University of Science and Technology Beijing, Beijing 100083, China

Lei Ling Yang Beijing Research Institute of Chemical industry, Beijing 10003, China

ABSTRACT Polyacene was prepared from a phenol-formaldehyde resin by pyrolysis under an inert atmosphere, whose morphological structure was shown to be amorphous and porous. The electrochemical properties of Li/polyacenecells using an 1M LiCIO4solution of ethylene carbonate (EC) and propylene carbonate (PC) as electrolyte were studied by means of constant current cycle and cyclic vottammetry. This cell was found to have a coulometric efficiency of more than 95% and a discharge plateau at around 0.4 V. The results shows that polyacene possesses reversibility for the intercalation or de-intercalation of lithium ions in the structure due to its larger surface area (ca. 1500 m2/g) and wider lamellar distance (0.3912 nm). In the voltage range between 2.0 and 4.0 V, a rocking chair battery of the type, (-)polyacene/LiCIO4 + PC + EC/polyaniline(+), can be reversibly cycled many times with a capacity of 35 mAh/g. Lithium batteries are of current interest as high-energy secondary power sources. These batteries, however, suffer from a limited rechargeability due mainly to the poor cyclability of lithium metal caused by the unavoidable passivation because of its reaction with nonaqueous electrolytes. As a result, the internal resistance of the battery increases and the cyclic efficiency rapidly declines. A possible solution to these problems could be to design a so-called "rocking chair battery" by replacing the lithium electrode with lithium-intercalating compounds. ~'2 In recent years carbonbased materials have been extensively studied as anodes in rocking chair batteries. 3'4 In this work we prepared a disordered polyacene and studied the intercalation or de-intercalation of lithium in it using Li/C-cells.

Experimental Polyacene was prepared as follows: ~ Under an argon atmosphere phenol-formaldehyde resin was pyrolyzed in the presence of an appropriate amount of zinc chloride at 1100~ for 36 h. The resultant powder was then washed with hot 1M hydrochloric acid and distilled water until no zinc ions were detected in the eluate. It was then dried, and finally ground into powder. The product was a black powder with a particle size of less than 200 mesh, a surface area of ca. 1500 m2/g and a conductivity of 12 S/cm at room temperature. Elementa! analysis showed: C, 96.40%; H, 0.61%; O, 0.31%, equivalent to a C/H ratio of 12.5 X-ray powder diffraction patterns were done using a D/Max-VB x-ray diffractometer. Eigenstate polyanine was chemically synthesized according to MacDiarmid's method. 6 Li/polyacene cells were assembled as described elsewhere. 3'4 The cathodes were made by mixing polyacene powder with a 10 weight percent (w/o) of polytetrafluoroethylene emulsion to form a uniform slurry, which was then pressed into a film with a thickness of ca. 200 i~m. A 1M LiCIO4 solution in a 1:1 (by weight) mixture of propylene carbonate (PC) and ethylene carbonate (EC) was used as the electrolyte. The cells were cycled at a constant current density of 0.6 mNcm 2 with a lower voltage limit of 0.02 V using a computer-controlling recorder. Cyclic voltammetry was carried out using a PAR model 173 potentiostat/galvanostat and a two-electrode system. For the purpose of comparison, a cell using petroleum coke, which has been treated at 1300~ for 24 h under an inert atmosphere, was also studied. All experiments were carried out at 15~

fore estimated to be 0.3912 nm, greater than that of pure graphite (0.3354 nm). This structure is favorable for the diffusion of lithium ions in the lamellar space of polyacene. As suggested by Ucno and co-workers, 5 the pyrolysis of phenolformaldehyde resins may take place in three steps: inter-molecular dehydration, cyclization, and intra-molecular dehydration. In the last step the further carbonation and the formation of aromatic backbones occur, yielding the desired polyacene. Polyacene is verified to be an amorphous and porous structure by scanning electron microscopy. This fact may be associated with the incomplete carbonation confirmed by elemental analysis since the residual hydrogen atoms will impede the regular arrangement of carbon backbone during pyrolysis, and accordingly a disordered structure is formed. On the other hand, the presence of zinc chloride is highly favorable for enhancing surface area. This may be because the zinc chloride promotes the inter-molecular dehydration, a step that determines the backbone structure of the products, and hence disorders the regularity of the aromatic backbones.

Cyclic voltammetry.--As also reported by Fong et al., 3 it is observed in the present work that the first cyclic voltammetry scan (CV) of the Li/C cells using polyacene or petroleum coke is partially irreversible. This is due to the decomposition of the electrolyte and the formation of a passivating film on the surface of the carbon. The fifth cyclic voitammograms of the cells illustrated in Fig. 2 show one pair of reversible redox peaks at 0.50/1.75 V for petroleum coke and 0.48/1.41 V for polyacene, reflecting a reversible intercalation/ de-intercalation of lithium ions. The CV plots of the latter cycles are

Results and Discussion Morphological structure of polyacene.--From x-ray diffraction patterns of polyacene (Fig. 1) only two broad peaks at 20 of around 240 and 44~ respectively, are observed, indicative of an amorphous structure. According to the standard x-ray diffraction pattern of pure graphite, it has been clear that a 2e of 20 - 24~is assigned to (002) band and 44~ to both (100) and (101) bands. On the basis of the data of (10) bands the melullar distance (dc) of polyacene is there* Electrochemical Society Active Member.

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Fig. 1. X-ray powder diffraction patterns of polyacene (a) and petroleum coke (b),

J. Electrochem. Soc., VoL 140, No. 7, July 1993 9 The Electrochemical Society, Inc.

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J. Electrochem. Soc., Vol. 140, No. 7, July 1993 9 The Electrochemical Society, Inc.

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Fig. 3. Typical discharge curves of Li/LiCIO4 + PC + EC/C cells at 15~ in the 1st (the upper two lines) and 25th cycles (the lower two lines), i~ = Ic = 0.6 mA/cm ~.

Potential ( V )

Fig. 2. Cyclic voltammograms of the Li/C cells at 15~ (a) polyacene, (b) petroleum coke. Scanning rate: 5 mV/s; electrode area: 0.75 cm 2. found to be reproducible. It has been observed that these two materials both demonstrate an excellent reversibility and can be cycled many times with negligible loss in capacity. An important difference is, however, noticed from their CV plots. The Li/polyacene cell always displays about five times higher peak currents than that using petroleum coke and relatively lower oxidation potentials (1.41 V) which corresponds to the de-intercalation of lithium ions from the carbon structure. This fact may be attributed to both the larger surface area and the wider mellular distance of polyacene. It is possible to enhance the discharge voltage of the rocking chair batteries, and hence increases the power density of the batteries, by using polyacene rather than other carbon materials. Working at a current density of 0.6 mA/cm2 between 0.02 and 0.40 V, the Li/polyacene cell has a capacity of 71 mAh/g and a coulometric efficiency of more than 95%. After cycling 75 times the capacity begins to drop due probably to the destruction of lithium anode, which is usually brought about by the reaction of lithium with electrolyte.

Charge~discharge characteristics.--The discharge curves of two Li/C cells are illustrated in Fig. 3, and a great difference is observed between them. During the first discharge, the voltage of the cell-1 (using petroleum coke) rapidly declines until the voltage reaches 0.02, whereas for cell-2 (using polyacene) the voltage first decreases rapidly and then more slowly after it has declined to 0.4 V. It is this low-voltage plateau that is desired for high-power batteries. Based on this plateau, the highest voltage for the charge of the cells is narrowed to 0.35 V. After 25 cycles of the two cells it is found that the cell-2 has a higher capacity than cell-l, correspondingly rocking chair batteries using polyacene-based anodes should have more capacity. In this

work a rocking chair battery composed of (-)polyacene/LiCIO4 + PC + EC/Eigenstate polyaniline(+) is assembled, and its charge/ discharge briefly studied. The newly assembled battery has an open voltage of ca. 0 V, and exhibited the characteristics of reversible batteries in the voltage range between 4.0 and 2.0 V. The battery could be cycled many times without loss of the capacity (35 mAh/g calculated according to the weight of active polyacene). The results show that the capacity may be increased considerably by varying the composition of the electrolyte and the polyaniline, suggesting that the polyacene described above is a promising candidate for use as an anode in rocking chair batteries. Further studies on polyacene-based rocking chair batteries are underway in our laboratory.

Acknowledgments Authors would like to thank Ms. W. H. Qiu and Dr. X. P. Qiu for their skillful technical assistance and Prof. Farrington for helpful discussions. Manuscript submitted Dec. 18, 1992; revised manuscript received March 16, 1993.

The University of Pennsylvania assisted in meeting the publication costs of this paper. REFERENCES 1. B. Dipietro, M. Patriarca, and B. Scrosati, J. Power Sources, 8, 289 (1982). 2. E. J. Plichta, W. K. Behl, D. Vujic, W. H. S. Chang, and D. M. Schleich, This Journal, 139, 1509 (1992). 3. R. Fong, U. von Sacken, and J. R. Dahn, ibid., 137, 2009 (1990). 4. A. K. Sleigh and U. von Sacken, Solid State Ionics, 57, 99 (1992). 5. H. Ucno, G. Ishii, K. Yoshino, K. Tanaka, T. Yamabe, and S. Yata, Synth. Met., 8, 515 (1987). 6. A. G. MacDiarmid, S. L. Mu, N. L. D. Somasiri, and W. Q. Wu, MoL Cryst. Liq. Cryst., 121,187 (1985).