WO1997046314A1

WO1997046314A1 - Carbon electrode material for electrochemical cells and method of making same

Info

Publication number: WO1997046314A1
Application number: PCT/US1997/009775
Authority: WO
Inventors: Jinshan Zhang; Frank R. Denton
Original assignee: Motorola Inc.
Priority date: 1996-06-07
Filing date: 1997-06-05
Publication date: 1997-12-11

Abstract

A method (50) for preparing an amorphous carbon material for use as an electrode (20) in an electrochemical cell (10). The amorphous carbon material is fabricated from inexpensive abundant, renewable sources, in a pyrolysis/carbonization process. The pyrolysis/carbonization process may be carried out in the presence of an acid. The resulting material may then be reheated in the presence of a lithium containing compound so as to improve the characteristics thereof.

Description

CARBON ELECTRODE MATERIAL FOR ELECTROCHEMICAL CELLS AND METHOD OF MAKING 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 synthesizing said electrodes and electrode materials.

Background of the Invention

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 electronic technology that the limiting factor to portability of a given device is the size and the 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 market place. Lead acid batteries are preferred for applications in which ruggedness and durability are required and hence have been the choice of automotive and heavy 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 offer the promise of better capacity, better power density, longer cycle life, and lower weight, 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. Lithium polymer batteries are also receiving considerable attention, although they have not yet reached the market. 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 material 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, however, most of the attention has been focused on the transition metal oxide electrode.

Activated carbon electrode materials are routinely prepared by using difunctional monomers as polymer precursors. Examples of such precursors include resins of furfural alcohol, phenyl, formaldehyde, acetone, furfuryl or furfuryl alcohol-phenyl copolymers. Other precursors include polyacrylonitrile, and rayon polymers, as disclosed in Jenkins, et al, 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 low capacity.

More recently, multi-functional 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, for example, U.S. Patent Application Serial No.

08/534,427, filed September 27, 1995 in the name of Zhang, et al, and assigned to Motorola, Inc., and 08/561,641 filed November 22, 1995 in the name of Zhang, et al and assigned to Motorola, Inc., the disclosures of which are incorporated herein by reference. While the materials disclosed in the foregoing U.S. patent applications have demonstrated excellent characteristics for purposes of electrochemical cells, they are the result of synthetic processing, and hence and are not easily renewable. Moreover, those materials have less than ideal yield and less than optimal molecular weight. Accordingly, there exists a need for improved carbon materials for use in electrochemical cell applications. The improved carbon materials should be fabricated from relatively inexpensive, readily available and renewable precursor materials. The precursor materials should also have a relatively high char-yield and a moderate to high molecular weight so as to yield an amorphous carbon material with the most desirable characteristics. Brief Description of the Drawings

FIG. 1 is a schematic representation of an electrochemical cell including an electrode fabricated of a carbon material, in accordance with the instant invention; FIG. 2 is a flowchart illustrating the steps for preparing a carbon material in accordance with the instant invention;

FIG. 3 is an x-ray diffraction analysis for a carbon material fabricated in accordance with the instant invention;

FIG. 4 is a charge and discharge curves for an electrochemical cell including an electrode fabricated of a carbon material in accordance with the instant invention;

FIG 5 is a x-ray diffraction analysis of a second example of a carbon material, in accordance with the instant invention;

FIG. 6 is a charge and discharge curve for an electrochemical cell made with carbon material in accordance with the instant invention,

FIG. 7 is a charge and discharge curve for a third electrochemical cell made with a carbon material in accordance with the instant invention;

FIG. 8 is a charge and discharge curve for a fourth electrochemical cell made with a carbon material in accordance with the instant invention, FIG. 9 is a charge and discharge curve for a fifth electrochemical cell made with a carbon material in accordance with the instant invention;

FIG. 10 is a chart illustrating discharge capacity versus cycle life for an electrochemical cell made with a carbon material in accordance with the instant invention; and FIG. 11 is a charge and discharge curve for a sixth electrochemical cell made with a carbon 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 a carbon electrode material fabricated in accordance with the instant invention. The electrochemical 4 cell 10 includes a positive electrode or cathode 20, a negative electrode or anode 30 and an electrolyte 40 disposed therebetween. The cell negative electrode 30 is fabricated of an amorphous carbon material such as that described in greater detail hereinbelow. The positive electrode 20 of the cell 10 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, copending 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, nonaqueous, 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 a carbon material for use as an electrode in an electrochemical cell such as that shown in FIG. 1. The instant invention also teaches a method of making said material. The carbon materials are substantially amorphous, though may be partially or completely crystalline, or may be amorphous but possessing crystalline inclusions if desired. They may further include an amount of one or more modifier materials. The exact nature of the modifiers is dependent upon the specific application contemplated.

Instead of the multi-functional or difunctional monomers or chars known in the prior art, the instant invention, uses lignin as a starting material. Lignin is the by-product of paper and pulp industry, and, as will be described in greater detail hereinbelow, can yield an amorphous carbon material with capacities in excess of 500 milliampere hours per gram

(mAh/g), and a yield in excess of 50%. Since lignin is generated at a rate of over 50 million metric tons a year, using lignin as a starting material to make an amorphous carbon electrode for rechargeable electrochemical cells provides a tremendous economic advantage. Moreover, lignin is a renewable source: Its existence in the biosphere is estimated to be 3x10* 1 metric tons with an annual biosynthetic rate of 2x10 0 tons per year. There are several types of lignin, which are defined by relatively small variations in the chemical structure. The chief distinctions between lignins are: hard wood lignin versus soft wood lignin; the type of chemical pulping used to remove the lignin from raw wood; and subsequent chemical modifications. The type of lignin described for use herein is a byproduct of relatively soft wood, specifically Southern Yellow Pine. The chemical pulping process used to isolate the lignin is known as the "Kraft process". The Kraft process uses aqueous mixtures of sodium sulfide and sodium hydroxide to separate the cellulosic fibers from the lignin material. The degree of oxidation and/or degradation of the obtained lignins varies with the choice of the pulping process. Indeed, lignin exhibits slow, spontaneous oxidation and degradation even upon prolonged exposure to air. However, lignin products from the various pulping methods are substantially similar for purposes of the pyrolysis process described herein.

Other common pulping processing include: the "green liquor" process usually used for the so-called hardwoods, and which comprises treatment of the product with aqueous sodium carbonate and sodium sulfide; the acid sulfite process employing an aqueous sulfite salt of calcium, magnesium, sodium, or ammonium; mechanical and thermal mechanical pulping; and organo-solv pulping wherein an organic solvent is substituted for some or all of the water used in the aqueous methods.

The lignin used herein is subjected to a subsequent modification. Specifically, following the pulping process, the liquor comprising the aqueous lignin dispersion is spray-dried to obtain a powder, as is common in the industry. The resulting lignin has a molecular weight of approximately 1000.

The lignins described herein include 3 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. The biogenesis of lignins proceeds through the Shikimic acid pathway; thus, the same or similar hydroxyphenyl propenyl intermediates found in lignin are also present in other products which are the result of that acid pathway. Examples of such materials include flavanoids, suberin, betalains, coumarins, sporopollenins, and certain amino acids such as tyrosine, tryptophan and phenylalanine. Further, the oxyphenylpropyl backbone units characteristic of the cinnamyl alcohols described hereinabove, can also be found in the first stages of decomposition of most carbon based botanical matter. Specifically, the humic substances such as humic acid, folic acid, and humin as well as cerogens, may be useful. In addition, chemical degradation products of lignin, such as hibberts ketones, also have this useful structure. Accordingly, while the preferred material, described herein are the result of acid pyrolysis of lignin, it is to be understood that the invention is not so limited. Rather, the invention disclosed herein relates to amorphous carbonaceous materials which are the result of acid pyrolysis of products of the Shikimic acid pathway, and in particular, substances possessing an oxyphenyl propyloxy backbone unit.

Referring now to FIG. 2, there is illustrated therein a flowchart of the steps necessary to fabricate an amorphous carbon material in accordance with the instant invention. The flowchart 50, at Box 52 illustrates the first step of the fabrication process and specifically the step of providing the lignin material. The lignin material provided at Box 52 is similar to that described hereinabove, or alternatively, may be any other of the other materials described herein. Illustrated at Box 54 is the optional step of mixing the lignin material with an acid catalyst. It has been found that when the lignin material is heated in the presence of an acid catalyst, the reaction of the lignin is more complete and results in an improved yield of the final product. Hence, the carbon material may be formed with an acid present. Examples of acids preferred include acids selected from the group consisting of acetic acid, boric acid, phosphoric acid, p-toluene sulfonic acid, 4-amino benzoic acid, trifluoroacetic acid and combinations thereof. The acid may be present in amounts between 1 and 25 wt%. While preparation of the amorphous carbon material is preferably carried out in the presence of an acid as described, such materials may be fabricated without the acid.

Thereafter as is illustrated in Box 56, the acid catalyst and the lignin material are thoroughly mixed as by a blender, a ball mill, or a jar mill. Once thoroughly mixed, the lignin/ acid mixture is ready for the pyrolysis or carbonization process.

The heating temperatures used in the pyrolysis/carbonization process can be determined by Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) which have generally indicated that the pyrolysis/carbonization process should be a two-temperature one-step heating process.

The mixture of lignin and acid catalyst is placed in a furnace or reactor which is held at temperature Ti or essentially room temperature. Thereafter, the furnace is ramped from room temperature up to a temperature T2 which is generally between 100 and 150°C, and preferably about 120°C. This step is illustrated at Box 58 of the flowchart. Alternatively, as is illustrated in Box 59, the reactor may be preheated to temperature T2. After placing the mixture of lignin and acid in the oven, and reaching temperature level T2, the atmosphere inside the reactor is purged with an inert gas so as to yield an inert atmosphere as illustrated by Box 60. Examples of an inert atmosphere used in connection herewith include argon, nitrogen, CO2, and helium. In a preferred embodiment, the inert atmosphere is argon. After flooding the reactor in an inert atmosphere, the temperature inside the reactor is ramped from the T2 level to a T3 level. The T3 temperature is typically between about 300°C and 400°C with the preferred temperature being approximately 350°C. The rate at which the temperature is ramped from T2 level to the T3 level is typically between 0.1 and 5°C per minute and preferably 0.5°C per minute. This step is illustrated in Box 62 of the flowchart. The mixture is held at the T3 temperature for a period of time of between three and twelve hours and preferably approximately six hours. This steps is illustrated at Box 64 of the flowchart.

Referring now to Box 66, there is illustrated therein the step of ramping the temperature of the oven from the T3 temperature level to the T4 temperature level. The T4 temperature level is preferably between approximately 800°C and 1200°C and most preferably about 1000 -1020°C The temperature is ramped from the T3 level to the T4 level at a rate of approximately 1-5°C per minute and preferably 2.5°C per minute. Thereafter, and as is illustrated in Box 68, the mixture is held at the T4 level in an inert atmosphere, for a period of time of between 0.5 and 5 hours and preferably about 1 hour. Thereafter and as is illustrated in Box 70, the reactor or furnace is turned off and the mixture is allowed to cool to room temperature. Cooling typically occurs in 4-6 hours time.

The pyrolysis/carbonization process described hereinabove with respect to FIG. 2 is a two-temperature, one-step process in which the lignin and acid mixture is heated at a relatively low temperature which results in the condensation of the lignin material driving off water vapor and other undesired aliphatics. As noted above, the lignin material is characterized by relatively high char yield and moderate to high molecular weight. This results in less aliphatic materials to be driven off as well as less boil off and more rapid aromatization. Thereafter, upon further heating as at the T4 level, the condensed lignin product decomposes and forms carbon-and is hypothesized, forms a variety of carbon-carbon bonds and ether bonds between the phenyl rings in the lignin precursor materials. As noted above, the lignin material is phenolic resulting in high reactivity in cross bonding of the phenyl rings.

The material resulting from the process illustrated in FIG. 2, may also be subjected to a secondary treatment as follows: the secondary treatment comprises the steps of comminuting the resulting amorphous carbon material to a particle size of approximately less than 100 microns, and preferably between 5 and 50 microns. Comminution may be carried out via conventional grinding techniques as are well known to those of ordinary skill in the art, examples of which include hammer milling, jet milling, ball milling, and others. Thereafter, the comminuted amorphous carbon material is mixed with a lithium containing component or lithium salt consisting of LiNθ3, Li3Pθ4, LiOH, Li2Sθ4, Li2Cθ3, lithium acetate, and combinations thereof. One preferred lithium containing compound with which the comminuted amorphous carbon material is mixed is LiNθ3. Thereafter, the mixture of the comminuted amorphous carbon material and the lithium containing compound is subjected to a heat treatment process. This heat treatment process comprises heating the mixture at temperatures between 500°C and 1200°C and preferably between 600°C and 800°C. The period of time for this heat treatment is approximately between 8 and 20 hours, with 12 hours being preferred.

The material resulting in the process illustrated hereinabove with respect to FIG. 2 is a substantially amorphous material which is characterized by a d- spacing of the (002) peak of between 3.8 angstroms and 4.2 angstroms. The true density of this material is on the order of approximately <1.6g/cm3 up to approximately 2.2g/ατ The capacity of the material disclosed herein is typically in excess of approximately 480 mAh/g and is indeed typically above 500 mAh/g.

The instant invention may be understood by the examples provided below.

EXAMPLE Example I

4.0g of a lignin material synthesized by the Kraft process described herein was well mixed with 0.4g ^-toluene sulfonic acid. The lignin material was purchased from Westvaco Corp and is known as Indulin AT. The mixture was placed in a ceramic crucible and pyrolyzed/ carbonized in a tube furnace under an argon atmosphere according to the following heating program: From room temperature to 700°C, at 2°C per minute; From 700°C to 1030°C at 10°C/per minute. Hold at 1030°C at 1 hour; and, Cool to room temperature. 1.9g of carbon was collected following this process. X-ray diffraction analysis of the carbon material resulting from this Example I is illustrated in FIG. 3 and indicates that the d-spacing of the (002) peak centered at approximately 4.03 angstroms. The reversible lithium intercalation capacity of the material is illustrated in FIG. 4 and demonstrates that the material capacity is approximately 500 mAh/g.

Example II

40.0 g of lignin produced by the Kraft process described above was well mixed with 4.0g of p-toluene sulfonic acid. The lignin was purchased from Westvaco Corp. and is known as Indulin AT. The mixture was placed in a ceramic crucible and was pyrolyzed/ carbonized in a tube furnace under an argon atmosphere according to the following heating program: From room temperature to 700°C, at 2°C per minute; From 700°C to 1050°C at a rate of 10°C per minute; Hold at 1050°C for 1 hour; and Cool to room temperature. 19.8g of carbon material was collected following this process. X-ray diffraction analysis of the carbon material is illustrated at FIG. 5 and indicates that the d-spacing of the (002) peak is centered at approximately 3.95 angstroms. The reversible lithium intercalation capacity of the material is illustrated in FIG. 6, and is shown to be 510 mAh/g.

Example III

4.0g of lignin material synthesized by the Kraft process described herein was mixed with 0.4 g of p-toluene sulfonic acid. The lignin material was purchased from Westvaco Corp. and is known as Indulin AT. The mixture was placed in a ceramic crucible and was pyrolyzed /carbonized in a tube furnace under an argon atmosphere according to the following heating program: 1. From room temperature to 700°C at a rate of 2°C per minute; 2. From 700°C to 800°C at a rate of 10°C per minute; 3. Hold at 800°C for 1 hour; 4. Cool to room temperature. 2.13g of carbon material was collected following this process The reversible lithium intercalation capacity of the material is illustrated in FIG. 7, and is shown to be 425 mAh/g. Example IV

4.0g of lignin material synthesized by the Kraft process described herein was mixed with 0.4 g of p-toluene sulfonic acid. The lignin material was purchased from Westvaco Corp. and is known as Indulin AT. The mixture was placed in a ceramic crucible and was pyrolyzed /carbonized in a tube furnace under an argon atmosphere according to the following heating program: 1. From room temperature to 700°C at a rate of 2°C per minute; 2. From 700°C to 900°C at a rate of 10°C per minute; 3. Hold at 900°C for 1 hour; 4. Cool to room temperature. 2.02g of carbon material was collected following this process. The reversible lithium intercalation capacity of the material is illustrated in FIG. 8, and is shown to be 465 mAh/g.

Example V

4.0g of lignin material synthesized by the Kraft process described herein was mixed with 0.4 g of p-toluene sulfonic acid. The lignin material was purchased from Westvaco Corp. and is known as Indulin AT. The mixture was placed in a ceramic crucible and was pyrolyzed /carbonized in a tube furnace under an argon atmosphere according to the following heating program: 1. From 120°C to 350°C at a rate of 0.5°C per minute; 2. Hold at 350°C for 6 hours; 3. From 350°C to 1100°C at a rate of 2.5°C per minute; 3. Hold at 1100°C for 1 hour; 4. Cool to room temperature. 1.90g of carbon material was collected following this process. The reversible lithium intercalation capacity of the material is illustrated in FIG. 9, and is shown to be 520 mAh/g. The rechargeability of the material is illustrated in FIG. 10, which illustrates that capacity remained substantially constant to 40 cycles. Example VI

4.0g of lignin material synthesized by the Kraft process described herein was mixed with 0.4 g of p-toluene sulfonic acid. The lignin material was purchased from Westvaco Corp. and is known as Indulin AT. The mixture was placed in a ceramic crucible and was pyrolyzed/ carbonized in a tube furnace under an argon atmosphere according to the following heating program: 1. From 120°C to 350°C at a rate of 0.5°C per minute; 2. From 350°C to 1200°C at a rate of 2.5°C per minute; 3. Hold at 1200°C for 1 hour; 4. Cool to room temperature. 1.87g of carbon material was collected following this process. The reversible lithium intercalation capacity of the material is illustrated in FIG. 11, and is shown to be 490 mAh/g.

Table 1

Carbon made from lignin with 10% of -toluenesulfonic acid

Entry Final Treatment Reversible Capacity Yield Based on Temperature (°C) (mAh/g) Lignin (%)

1 800 425 53.3

2 900 465 50.5

3 1030 500 47.5

4 1050 510 49.5

6 1100 520 47.5

7 1200 490 46.8

As may be appreciated from Table I hereinabove, a reversible capacity and yield of the material is inversely proportional with respect to increasing temperature. That is, as the final treatment increases up to approximately 1100°C, the reversible capacity of the material likewise increases up to a maximum of approximately 520 mAh/g. However, the yield based on the lignin goes down as the temperature increases from 800°C up to 1200°C. Accordingly, one may optimize both the yield and the reversible capacity of the material by selecting a preferred final treatment temperature.

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.

Claims

1. A method of fabricating an amorphous carbon material for use as an electrode in a rechargeable electrochemical cell, said method comprising the steps of: providing a lignin material; and subjecting said lignin material to a carbonization process in an inert environment.

2. A method as in claim 1, wherein said lignin material comprises a plurality of monomeric precursors, said precursors selected from the group of p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol and combinations thereof.

3. A method as in claim 1, wherein the step of subjecting said lignin material to a carbonization process comprises the further steps of heating said lignin to a first temperature to condense said lignin; and heating said condensed lignin material to a second temperature.

4. A method as in claim 3, wherein said first temperature is between 300°C and 400°C.

5. A method as in claim 3, wherein said second temperature is between 800°C and 1200°C

6. A method as in claim 1, wherein said inert environment is selected from the group of Ar, N, He, CO2, and combinations thereof.

7. A method as in claim 1, including the further step of mixing said lignin material with an acid.

8. A method as in claim 7, wherein said acid is selected from the group consisting of acetic acid, boric acid, phosphoric acid, p -toluene acid, 4- amino-benzoic acid, trifluoro acetic acid, and combinations thereof.

9. A method as in claim 1, including the further step of heating said amorphous carbon material in the presence of a lithium containing compound.

10. A method as in claim 9, wherein said lithium containing compound is selected from the group consisting of LiNθ3, Li3Pθ4, LiOH, Li2Sθ4, Li2Cθ3, lithium acetate, and combinations thereof.