WO1998004009A1

WO1998004009A1 - High volumetric capacity electrodes and electrochemical cells using same

Info

Publication number: WO1998004009A1
Application number: PCT/US1997/010933
Authority: WO
Inventors: Zhenhua Mao; Dong Chen; Jinshan Zhang; Anaba A. Anani
Original assignee: Motorola Inc.
Priority date: 1996-07-19
Filing date: 1997-07-16
Publication date: 1998-01-29

Abstract

An electrode material for a lithium rechargeable cell (10) in which the negative electrode (30) is fabricated of a combination of first and second carbon materials. The first carbon material is a substantially amorphous carbon material which is preferably the pyrolytic decomposition product of a lignin precursor material. The second carbon material used in the negative electrode material is preferably also a carbon material, though one characterized by a greater density than the amorphous carbon. In this regard, the second carbon material is preferably a graphite carbon material. The presence of graphite in the overall electrode composition increases electro density, acts as a binder in the compression on the electrode materials onto a current carrying substrate and as a lubricant in the fabrication process.

Description

HIGH VOLUMETRIC CAPACITY ELECTRODES AND ELECTROCHEMICAL CELLS USING SAME

Technical Field This invention relates in general to the field of electrodes and electrode materials for electrochemical cells and in particular to methods of fabricating said electrodes and electrode materials.

Background As electronic devices increasingly become portable, advances must be made in energy storage systems to enable such portability. Indeed, it is often the case with current electronics technology that the limiting factor to portability of a given device is the size and weight of the associated energy storage device. A small energy storage device such as a battery may be fabricated for a given electrical device but at the cost of energy capacity.

Conversely, a long lasting energy source can be built but it is often too large or too bulky to be comfortably portable. The result is that the energy source is either too heavy or does not last long enough for a particular user's application. Numerous different battery systems have been proposed for use over the years. Early rechargeable battery systems included lead acid and nickel cadmium (Nicad), each of which have enjoyed considerable success in the marketplace. Lead acid batteries were preferred for applications in which ruggedness and durability were required, and hence have become the choice of automotive and industrial settings. Conversely, Nicad batteries have been preferred for smaller portable applications. More recently, nickel metal hydride systems (NiMH) have found increasing acceptance for both large and small applications.

Notwithstanding the success of the foregoing battery systems, other new batteries are appearing on the horizon, which batteries offer the promise of better capacity, better power density, longer cycle life, and lower weight, all as compared with the current state of the art. The first such system to reach the market is the lithium ion battery which is already finding its way into numerous consumer products such as cellular telephones, and portable computers to name a few. Lithium polymer battery systems are also receiving considerable attention, although as yet they have not reached the marketplace. Lithium batteries in general include a positive electrode fabricated of, for example, a transition metal oxide material, and a negative electrode fabricated of an activated carbon such as graphite or petroleum coke. New materials for both electrodes have been investigated intensely because of the high potential for improved energy density. To date, most of the attention has been focused on the transition metal oxide electrode.

Activated carbon materials are routinely prepared by using difunctional monomers as polymer precursors. Examples of such precursors include resins of furfuryl alcohol, phenol, formaldehyde, acetone, or furfuryl alcohol-phenol copolymers. Other precursors include polyacrylonitrile, and rayon polymers, both as described in a publication to Jenkins, et al entitled "Polymeric Carbons-Carbon Fiber, Glass and Char., Cambridge University Press, Cambridge, England (1976). Materials which result from these processes are typically characterized by relatively low yields as well as high cost and/or low capacity.

More recently, multifunctional organic monomers and highly aromatic polyesters with aliphatic spacers have produced excellent carbons for use in lithium rechargeable electrochemical cells. Specifically, such materials are disclosed in detail in for example, U.S. Patent Application Serial No. 08/534,427 filed September 27, 1995 in the names of Zhang, et al and assigned to Motorola, Inc.; U.S. Patent Application Serial No. 08/561,641 filed November 22, 1995, in the name of Zhang, et al, and assigned to Motorola, Inc.; and assigned to Motorola, Inc., the disclosures of each of which are incorporated herein by reference. The carbon materials resulting from these processes are essentially amorphous, and hence are physically "hard" and less easily compressible into electrodes. Conversely, other prior art carbons were relatively "soft" and easily compressible into electrodes.

While the materials disclosed in the foregoing U.S. patent applications have demonstrated excellent characteristics for use in electrochemical cells, it was found that when such carbons were used to make carbon electrodes via conventional solvent casting processes, the resulting electrodes have much lower densities than prior art electrodes made of "soft" carbon, for example, graphite. The density of an electrode has a significant effect upon the volumetric capacity of a cell. For example, in a AA size cell, a decrease of ten percent in carbon electrode density may result in a decrease of 6% in the overall cell capacity. The high capacity carbon which results from the patent applications described above would hence lose its advantage over other carbons, particularly graphite, if electrode fabricated therefrom cannot be made as dense as the prior art electrodes.

Accordingly, there exists a need to increase the density of carbon electrodes fabricated from the carbon materials described in the foregoing patent applications so as to achieve densities similar to that of the prior art. Furthermore, there exists a need to increase the density of electrodes fabricated of "hard" active materials so as to achieve electrode densities representative of higher performance electrodes. The improved carbon materials should be fabricated from relatively inexpensive and available precursor materials. Moreover, the increased density of the material should not be accompanied by unacceptable decreases in the capacity of the carbon materials when used as the electrode in an electrochemical cell.

Brief Description of the Drawings FIG. 1 is a schematic representation of an electrochemical cell, including an electrode fabricated of an active electrode material and a lubricant material, in accordance with the instant invention.

Detailed Description of the Preferred Embodiment While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward. Referring now to FIG. 1, there is illustrated therein a schematic representation of an electrochemical cell 10, such as a battery or an electrochemical capacitor, and including an electrode fabricated of an active electrode material such as carbon, and including a lubricant material for increasing the density of an electrode fabricated therefrom in accordance with the instant invention. The electrochemical cell 10 includes a positive electrode or cathode 20, and negative electrode or anode 30, and an electrolyte 40 disposed therebetween. The cell negative electrode or anode 30 is fabricated of an active electrode material capable of reversible charge storage such as the carbon materials described in the aforementioned patent applications, as well as a lubricant material such as is described in greater detail hereinbelow. The positive electrode 20 of the cell 10, in the case of a lithium based cell may be fabricated from a lithiated transition metal oxide such as is well known in the art. Alternatively, the positive electrode material may be fabricated of a material such as that described in commonly assigned, co-pending U.S. Patent Application Serial No. 08/464,440 filed June 5, 1995 in the name of Mao, et al and entitled "Positive Electrode Materials for Rechargeable Electrochemical Cells and Method of Making Same," the disclosure of which is incorporated herein by reference.

The electrolyte 40 disposed between the electrodes may be any of the electrolytes known in the art including, for example, LiClθ4 in propylene carbonate or a polyethylene oxide impregnated with a lithiated salt. The electrolyte 40 may also act as a separator between the positive and negative electrodes. The electrolyte may be aqueous, non-aqueous, solid state, gel or some combination thereof. Alternatively, the electrolyte material may be fabricated in a manner such as that described in commonly assigned copending Patent Application Serial No. 08/518,732 filed August 24, 1995 in the name of Oliver, et al and entitled "Blended Polymer Gel Electrodes" the disclosure of which is incorporated herein by reference.

In accordance with the instant invention, there is provided an electrode material capable of reversibly intercalating and deintercalating lithium ions and adapted for use as an electrode in an electrochemical cell such as that shown in FIG. 1. The electrode material comprises an active electrode material and a lubricant material. The active electrode material is preferably a carbon material, and is formed by the pyrolitic decomposition of a lignin material, which results in a substantially amorphous carbon material characterized by a reversible lithium capacity greater than approximately 500 milliamperes hours per gram (mAh/g) and a density of up to 1.6 grams per cubic centimeters (g/cc^). The carbon materials which result from the pyrolitic condensation of lignin materials are preferably amorphous though may be partially or completely crystalline. As the materials which result from this process are amorphous and hence relatively hard, it is difficult to pack them into a highly dense electrode structure.

The electrode material further comprise a lubricant material to provide the required compression attribute to the electrode structure. In a preferred embodiment, the lubricant is a second carbon material characterized by a density of at least 1.2 times greater than that of the first carbon material. The second carbon material is preferably a graphite or other activated carbon which has both a relatively high density as compared to the amorphous carbon resulting from the lignin decomposition, and is compressible along phases of its graphite sheet.

In a preferred embodiment of the instant invention, the first carbon material comprises between 60 and 90 wt% of the electrode material while the graphatic material comprises between 10 and 40 wt% of said electrode material. Moreover, in a particularly preferred embodiment, the graphatic second carbon material comprises between 20 and 35 wt% of the overall electrode material.

The first carbon material is fabricated by the pyrolitic decomposition of lignins as described in commonly assigned copending U.S. Patent

Application Serial No. 08/660,320 filed June 7, 1996 in the name of Zhang, et al, (case 412) and entitled "Amorphous Carbon Electrode Materials For Electrochemical Cells and Method of Making Same" the disclosure of which is incorporated herein by reference. Lignin is a by-product of the paper and pulp industry which can yield an amorphous carbon material with capacities in excess of 500 mAh/g, and a yield in excess of 50%. Lignin is generated at a rate of over 50 million metric tons a year, hence, using lignin as a starting material to make an amorphous carbon material for an electrode in accordance with the instant invention, provides a tremendous economic advantage. Moreover, lignin is a renewable resource and its existence in the biosphere is estimated to be 3xlθH metric tons with an annual biosynthetic rate of 2x10^0 tons per year.

Preferred lignins for use in the pyrolitic decomposition process as described in the aforementioned patent application, include three cinnamyl alcohols. These alcohols are the monomeric precursors of the lignin and include p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. The structural units of each of the three cinnamyl alcohols have oxyphenyl propyloxy skeletons and differ from each other only in the number of methoxyl groups attached to the benzene ring. In the polymeric structure of lignin, these structural units are held together by a variety of ether and carbon/carbon bonds. Amorphous carbon material is used as the active electrode material in the fabrication of a negative electrodes in accordance with the instant invention are characterized by a density on the order of up to approximately 1.6g/cm^, and a d-spacing of the (002) plane of approximately between 3.8 angstroms and 4.2 angstroms. In addition to the amorphous materials which result from the pyrolitic decomposition of lignin, the electrode materials of the instant invention further include a relatively soft material blended with the "hard" amorphous carbon. The addition of graphite as a lubricant is desirable since the density of graphite is relatively high and it is highly compressible.

Accordingly, blending operative amounts of a high density lubricant such as graphite, with the high capacity/low density carbon which results from the pyrolitic condensation of lignin results in a overall electrode material which has electrochemical properties better than either of the two materials by themselves. Since graphite is known to be an acceptable material for lithium ion intercalation and deintercalation, and its overall percentage in the mixture is relatively small, i.e., on the order of 10-40%, overall capacity of the mixture is not substantially compromised nor is it significantly lower than the carbon electrode by itself. However, electrode density is increased substantially thereby offsetting the density losses experienced by carbon alone. The lubricant has been described thus far only as graphite. However other materials, such as aluminum, cadmium, lead, palladium, silicon, polyvinylidene fluoride, and combinations thereof may be employed with equal success. Graphite also serves the function of an adhesive, thus reducing the content of binder material in the overall electrode, and serving to further increase the density of the carbon electrode.

Using a carbon material such as that described in the aforementioned U.S. Patent Application and having a reversible capacity of approximately 500 mAh/g and an initial charge efficiency of 70%, and commercially available graphite known as Lonza SFG-44, a series of tests were conducted comparing the total reversible capacity and the total charge capacity of varying amounts of amorphous carbon and graphite. The results are summarized below in Table 1.

Table 1. Capacities of composite carbon electrode

SFG-44 Carbon Total reversible Total charge Total irreversible % weight % weiqht capacity capacitv capacitv

0 100 500 714 214

1 0 90 480 680 200

20 80 460 646 1 86

30 70 440 612.5 172.5

40 60 420 578.5 158.5

50 50 400 544 144

1 00 0 300 375 75

Likewise, Table 2 below shows the maximum mass ratio of cathode to anode according to the calculated capacities. In this estimate, the cathode used has a capacity of 235 mAh/g. The cathode capacity in Table 2 is the total reversible capacity per unit weight of the cathode active material.

Table 2. Maximum mass ratio

SFG-44 Carbon Capacity Ire. Capacity C:A mass ratio Cathode wt% wt% (mAh/q) (mAh/q) capacity

0 1 00 500 214 3.04 164.5

10 90 480 200 2.9 165.5

20 80 460 1 86 2.75 167.3

30 70 440 172 2.6 169.2

40 60 420 158.5 2.46 170.5

When the percentage of graphite changes from 20% to 40%, the mass ratio of cathode to anode changes from 2.75 to 2.46. The capacity based on the cathode thus increases by only 1.7%. However, the amount of carbon necessary to match the cathode material increases by 10%. Therefore, it appears that using low graphite content and high mass ratio improves the volumetric energy density if the densities of low and high graphite content film are not significantly different.

Electrodes according to the instant invention were made of different compositions by fabricating the mixtures and pressing under 500 lbs. per square inch to a width of 2.5 inches. Densities of those electrodes were calculated according to the active material loading and coating thickness. Results are listed below in Table 3.

Table 3. Electrode active material density and composition

SFG-44 Amorphous PVDF (wt%) Electrode Rever. capacity (wt%) carbon (wt%) density (g/cc) mAh/q

94 6 1 .95 400

9.4 84.6 6 0.96 433

18.8 75.2 6 1 .10 474

23.5 70.5 6 1 .18 500

32.9 61.1 6 1 .19 482

94 6 0.81 382

92 8 0.92 425

89.16 10.84 1 .00 445

It maybe appreciated from Table 3 that by adding graphite and polyvinylidene fluoride (PVDF) lubricant, the electrode is more easily compressed, and reversible capacity increases significantly. This is believed to be due to the fact that graphite and PVDF acting both as lubricants and as binders help the alignment of carbon particles. The combination of this effect will increase the density of the electrode and active material loading. Thus, increases in volumetric capacity of the electrode are also realized. As may be further appreciated form Table 3, samples using between 20 and 35% graphite and 6% PVDF demonstrate the best reversible capacity.

While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.

What is claimed is:

Claims

1. An electrode material capable of reversibly intercalating and deintercalating lithium ion, said material comprising: an active electrode material formed by the pyrolytic decomposition of a lignin material and characterized by a reversible capacity of greater than approximately 500 mAh/g and a density of less than about 1.6 g/cc; and a second material characterized by a density at least 1.2 times greater than said carbon material, said second material being a lubricant for enhancing formation of an electrode and from said electrode material.

2. An electrode material as in claim 1 , wherein said active electrode material is substantially amorphous.

3. An electrode material as in claim 1 , wherein said active electrode material is carbon.

4- An electrode material as in claim 1 , wherein said active electrode material comprises 60-90 wt% of said electrode material-

5. An electrode material as in claim 1 , wherein said second material is a graphite material.

6. An electrode material as in claim i , wherein said second material comprises between 10 and 40 wt% of said electrode material.

7. An electrode material as in claim 1 , wherein said active electrode material is characterized by a d-spacing of the (002) plane of approximately between 3.8 angstroms and 4.2 angstroms.

8. An electrode material capable of reversibly intercalating and deintercalating lithium ions, said material comprising:

60-90 t% of an amoφhous carbon resulting from the pyrolytic decomposition of a lignin material, and 10-40 wt% of a lubricant material having a density of less than 1.6 g/cc.

9. An electrochemical cell comprising: a cathode formed of a lithium containing transition metal oxide as a cathode active material; a non-aqueous electrolyte; and an anode material comprising an amoφhous carbon material resulting from the pyrolytic decomposition of a lignin and a graphitic material.

10. An electrochemical cell as in claim 9, wherein said amoφhous carbon material has a reversible capacity greater than approximately 500 mAh/g and a density of less than about 1.0 g/cc.

11. An electrochemical cell as in claim 9, wherein said amoφhous carbon material comprises between 60 and 90 wt% of said anode material-

12- An electrochemical cell as in claim 9, wherein said graphite material is characterized by a density of at least 1.2 times greater than said amoφhous carbon.