MXPA98002415A - Carbon electrode materials for electrochemical cells and method to prepare mys - Google Patents

Abstract

A method for preparing an amorphous carbon material to be used as an electrode, such as the anode of an electrochemical cell. Amorphous carbon is manufactured in a single stage heating process from the multifunctional organic monomers. The electrodes thus manufactured can be incorporated into the electrochemical cells (10) as the anode (20) of the same electrodes.

Description

ELECTROCHEMICAL CELL ELECTRODE MATERIALS AND METHOD FOR DEVELOPING THE SAME TECHNICAL FIELD This invention relates in general to the field of electrodes and materials for electrochemical cells, and in particular to methods for synthesizing said electrodes and materials. Background of the Invention As electronic devices and other electrical devices become increasingly portable, advances must be made in energy storage systems to allow such portability. In fact, it is often the case with current electronic technology that the limiting factor of transportability of a given device is the size and weight of the associated energy storage device. Obviously, a small energy storage device can be manufactured for a given electrical device, but at the expense of energy capacity. Conversely, a long-term power source can be built, but it is then too large to be comfortably portable. The result is that the energy source is either too bulky, too heavy or it is not durable enough. The main energy storage device used for portable electronics is the electrochemical battery cell, and less frequently, the electrochemical capacitor. Over the years, numerous different battery systems have been proposed for use. Previous rechargeable battery systems included lead acid, and nickel-cadmium (Nicad), each of which has enjoyed considerable success in the market. Lead acid batteries, due to their strength and durability, have been the batteries of choice in heavy industrial and automotive applications. In contrast, Nicads have been preferred for smaller or portable applications. More recently, nickel metal hydride (NiMH) systems have found increasing acceptance for both large and small applications. Despite the success of the battery systems mentioned above, other new batteries appear on the horizon, which offer the promise of better capacity, better energy density, and longer life cycle compared to 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 consumer products. Lithium polymer batteries also receive considerable attention, although they still do not reach the market. Lithium batteries generally include a positive electrode made of a transition metal oxide material, and a negative electrode made of an activated carbon material such as graphite or petroleum coke. New materials have been intensively investigated for both electrodes due to their high potential gravimetric energy density. To date, however, most attention has been focused on the transition metal oxide electrode. The importance of carbon-based materials in electrochemical systems in general can not be underestimated. In energy storage and power generation applications, carbon-based materials are vigorously sought after as a component of the active material. The electrodes of the energy cell, and the catalysts also make use of the carbon-based materials as active ingredients for the various chemical reactions. These carbonaceous or carbon-based materials are prepared routinely by using the difunctional monomers as polymer precursors. Examples of such precursors include furfuryl alcohol resins, phenol, formaldehyde, acetone-furfural, or alcohol-phenol-furfural copolymer. Other precursors include polyacrylonitrile and rayon polymers, as discussed in Jenkins, and Cois, Polymeric Carbons-Carbon Fiber, Gl ass and Char (Carbon Fibers-Polymer Coals, Glass and Coal), Cambridge University Press, Cambridge, England (1976 ). These precursors are subjected to a process of healing and carbonization, usually very slowly, and at temperatures of up to 2,000 ° C. Two main stages are included in these processes: (1) synthesis of polymer precursors from difunctional monomers through wet chemistry; and (2) pyrolysis of the precursors. The method typically results in a relatively low total output due to the two process steps. For example, conventional polyacrylonitrile processing typically produces only about 10% of a usable carbonaceous material. In addition, many impurities can be incorporated into the carbonaceous material, effecting the electrochemical properties in a noxious manner. According to the above, there is a need for an amorphous carbon material, improved for use in electrochemical and other applications. The material should be easily processed in a high, simple production method. Brief Description of the Drawings Figure 1 is a schematic representation of an electrochemical cell including an electrode made of an amorphous carbon material, according to the present invention; Figure 2 is a schematic representation of an energy cell including an electrode made of an amorphous carbon material, according to the present invention; Fig. 3 is a flow chart illustrating the steps for preparing an amorphous carbon material according to the present invention; Figure 4 is a diagram illustrating thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) for 5-hydroxyisophthalic acid; Figure 5 is a diagram illustrating thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) for a-resorcyclic acid; Figure 6 is a loading and unloading curve for a material prepared at 600 ° C according to the present invention; Figure 7 is a series of loading and unloading curves for a material prepared at 700 ° C according to the present invention; Figure 8 is a series of loading and unloading curves for a material prepared at 1100 ° C in accordance with the present invention. Detailed Description of the Preferred Modality Although the specification concludes with the claims defining the characteristics of the invention that are considered as novelty, it is believed that the invention will be better understood from the consideration of the following description in conjunction with the figures of the design, in which the numerals of similar references are carried. Referring now to Figure 1, there is illustrated a schematic representation of an electrochemical cell 10 such as a battery or an electrochemical capacitor, and including an amorphous or carbon based carbon electrode material made in accordance with the present invention. invention. The electrochemical cell includes a positive electrode or cathode 20, a negative electrode or anode 30 and an electrolyte 40 placed therebetween. The cell negative electrode 30 is made of an amorphous carbon or carbon based material such as that described in more detail below. The positive electrode 20 of the cell 10 can be made of a lithium transition metal oxide as is well known in the art. Alternatively, the positive electrode material can be made of a material such as that described in the co-pending patent application, commonly assigned series no. 08 / 464,444 filed on June 5, 1995, in the name of Mao and Cois, and entitled "Positive Electrode Materials for Rechargeable Electrochemical Cells and Method of Making Same" ("Materials of the Positive Electrode for Rechargeable Electrochemical Cells and Method for Making the Same "), the exhibition of which is incorporated herein for reference. The electrolyte 40 placed between the electrodes can be any of the electrolytes known in the art including, for example, LiC104 in propylene carbonate, or polyethylene oxide impregnated with a lithiated salt. The electrolyte 40 can also act as a separator between the positive and negative electrodes. The electrolyte can also be aqueous, non-aqueous, solid state, gel, or some combination thereof. Referring now to figure 2, there is illustrated a schematic representation of an energy cell including an amorphous carbon electrode according to the present invention. The energy cell 50 includes electrodes, first and second, 52, 54, of which at least one is made of the present material. The operation of the energy cell is similar to that of a battery, except that one or both reagents are not permanently contained as in the electrochemical cell. Preferably the reagents are fed into the energy cell through an external source when energy is desired. The fuels are usually gaseous or liquid (compared to the metal anodes generally used in batteries), and the oxidant is oxygen or air. The electrode material of the energetic cells is inert in that it is not consumed during the reaction of the cell, but has the catalytic properties, which improve the oxidation of the electrode of the active materials of the energetic cells. A reaction of the typical energetic cell is illustrated by the hydrogen / oxygen energy cell, whose reactions are illustrated in Figure 2. In such a device, hydrogen is oxidized at the anode, electrocatalyzed by platinum or platinum alloys, while at the cathode, oxygen is again reduced with platinum or platinum alloys as an electrocatalyst. Platinum or platinum alloys are typically captured in a carbon matrix. In this way, the electrodes 52, 54 of the energy cell can be manufactured from an amorphous carbon according to the invention. The typical uses of energy cells are found in applications that require electrical power for long periods of time, such as in space flights, as a substitute for moderate power motor generators, and for the convenience of leveling the load. .

In accordance with the present invention, there is provided a method for synthesizing an amorphous carbon or carbon based material to be used as an electrode in an electrochemical device such as a battery or a capacitor, an energy cell, or a catalyst. The carbon-based materials are substantially amorphous, although they may be partially or completely crystalline or include crystalline inclusions if desired, and may include an amount of one or more modifiers. The exact nature of the modifiers depends on the specific request contemplated. Instead of the dysfunctional monomer precursors used in the prior art, the present invention uses multifunctional organic monomers, each having at least three functional groups of two kinds. More specifically, the multifunctional organic monomers have the general formula of: wherein Ri, R2 and R3 are each a functional group, and all are selected from the group consisting of carboxylic acids of eight carbons or less, carboxylic esters of eight carbons or less, alcohols of eight carbons or less, carboxylic anhydrides of eight carbons or less, amines, and combinations thereof, and wherein at least one of Ri, R2 and R3 is different than the others. In a preferred embodiment, at least one functional group is a carboxylic ester. It should also be noted that in the manufacturing process of the materials described below, the functional groups that differ may in fact react with each other. In a preferred embodiment, the multifunctional organic monomer is selected from the group consisting of 5-hydroxyisophthalic acid, 5-aminoisophthalic acid, α-resorcyclic acid, β-resorcyclic acid, d-resorcyclic acid, gentisic acid, protocatechucic acid, and combinations of the same. In another particularly preferred embodiment, the multifunctional organic monomer is a-resorcinic acid. Although the preferred multifunctional organic monomers are cited above, it should be noted that the present invention is not so limited. In fact, many other equally advantageous organic monomers can be used. With respect to the manufacture of the carbon materials, it has been found that when the organic monomer is heated in the presence of an acid, the reaction of the monomer is more complete, and results in an improved yield of the final product. Therefore, amorphous carbon material can be formed with an acid present. Examples of preferred acids include acids selected from the group consisting of acetic acid, boric acid, phosphoric acid, p-toluenesulfonic acid, 4-amino benzoic acid, trifluoroacetic acid, and combinations thereof. It is hypothesized that the acids are acting as catalysts in the ester condensation reaction of the organic monomer. The acid may be present in amounts between 1 and 25% by weight percent. Although the preparation of the material is preferably carried out in the presence of an acid as described, such materials can be manufactured without the acid, the result being lower total yields of the final product. In preparing the amorphous material, it is contemplated that the monomer be heated, in conjunction with the acid catalyst, in an inert environment. Preferred inert environments include, for example, nitrogen, argon, and / or helium. The materials are heated to temperatures sufficient to induce a solid state carbonization of the multifunctional monomers. This process is similar in nature to a sublimation process, and occurs at temperatures less than about 1200 ° C, and preferably at about 600 ° C. The method of the present invention incorporates the stage of polymerization and carbonization of the materials in a single process, in the solid state. The multifunctional monomers described above are polymerized at lower temperatures. Once polymerized, the multifunctional monomers form a hyperbranched polymer, which subsequently carbonizes at slightly higher temperatures to form an amorphous carbon material. Since multifunctional organic monomers generally contain the elements of carbon, hydrogen, oxygen, and nitrogen in varying combinations, the carbonization process refers to the fact that the organic precursor decomposes, the enclosing compounds that include carbon-oxygen, carbon- hydrogen, hydrogen-oxygen, nitrogen-hydrogen, and other similar compounds. The remaining carbon atoms condense into flat structures that terminate predominantly with an edge of hydrogen atoms, the amount of hydrogen atoms depends on the temperature of the initial part of the carbonization process. The polymerization / carbonization step one of the multifunctional monomer can be understood from the following diagram, which illustrates the reaction mechanism for the polymerization / carbonization. The reaction involves an initial state, an intermediate state, and the final product. In the initial state, the multifunctional monomer, for example a-resorcinic acid, is heated to relatively lower temperatures, which results in condensation of the monomer and dispersion of the water vapor. This phase of the reaction is illustrated by the following formula: Hyperbranched Polymer In the additional heating, the resulting hyperbranched polymer decomposes and forms carbon-carbon bonds between the phenyl rings of the initial monomers. As the temperature increases to, for example, 500 ° -700 ° C, the six carbon phenyl rings begin to break and form a stratified carbon network. The formation of hyperbranched carbon polymers in the first stage of the process results in the movement of the monomer molecules physically closest to one another, thus facilitating carbonization in the second stage of the process. This also accounts, at least partially, for improved performances compared to the prior art. In addition, and as described above, when the reaction is carried out in the presence of an acid, the acid catalyzes the reduction reaction of the ester and therefore causes an improved yield of the final product. The second stage of the process can be better understood from the reaction illustrated in the following formula: Hyperbranched Polymer Referring now to Figure 3, there is illustrated a flow chart 100 which describes the steps for preparing the amorphous carbon material described above. The first stage illustrated in Figure 2 is shown in block 102, and comprises the step of selecting an appropriate multifunctional organic monomer as described above. Then, as illustrated in block 104, it is the step of selecting the treatment temperature ranges for the solid state carbonization process for the selected monomer. More particularly, the performance of the amorphous carbon material from a particular multifunctional monomer will largely depend on the thermal regime, to which the monomer is attached. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) each provide an excellent medium, by which predetermines the temperature regime of the processing. The results generally indicate that the solid state carbonization process must be a one stage, two temperature heating process, as described below. In this way and referring now to Figure 4, an analysis of DSC 106 and TGA 108 for 5-hydroxyisophthalic acid shows a large endothermic reaction at about 303 ° C, which also marks the start of a weight loss, the which sum approximately 20% of the total mass of the monomer, after the peak. The peak is the result of the condensation reaction of the carboxylic acid group and the alcohol groups. Therefore, to correspond to the reaction described above, the first temperature plateau should be at this point, that is, 300 for 5-hydroxyisophthalic acid. Similarly, and referring now to Figure 5, an analysis of DSC 110 and TGA 112 for a-resorcyclic acid shows an endothermic or peak transition, but one of which occurs at a much lower temperature, i.e. ° C. After the transition, the monomer experiences a substantial weight loss up to about 367 ° C (about 36%). From this, it can be concluded that the first temperature plateau should be at approximately 240 ° C to condense the functional groups. A similar test is conducted on other potential monomers to determine the optimal heating rate for that particular material. Returning to Figure 3, the next step in the manufacturing process of the flow chart 100 is illustrated in block 114, and comprises the step of mixing the multifunctional organic monomer with an acid selected from the group consisting of the acids described above. The two materials should be thoroughly mixed, and they should also be dried, as in a drying oven, before subjecting the mixture to the solid state carbonization process. It should be noted that the acid catalyst provides an improved yield of the final product, but it is not necessary to carry out the reaction. In addition, as noted above, it is believed that the acid catalyzes the ester condensation reaction. Therefore, if the initial multifunctional monomers do not contain esters, the acid may not be required. The next step illustrated in Figure 2 is the solid state carbonization process 116, which may comprise a multi-stage heating regime. As illustrated in FIG. 3, step 116 actually comprises four stages illustrated by blocks 118, 120, 122 and 124. Each stage in the carbonization process will depend on the TGA and DSC test described above. Generally, however, the step illustrated by block 118 comprises the step of heating the dried monomer / acid mixture to a first temperature at a predetermined rate of X ° C / minute. Once the desired temperature is reached, the mixture is maintained at that temperature for a predetermined period of time, as illustrated in block 120. Then, the material is heated to a second, typically higher, temperature at a rate of X ° C / minute, as illustrated in block 122. Once the second desired temperature is reached, the mixture is maintained at that temperature for a predetermined period of time, as illustrated in block 116. After complete the carbonization in the solid state, the resulting amorphous carbon material slowly cools as illustrated in block 126. The cooling should be at an appropriate rate to ensure that the material retains substantially its amorphous character. The present invention can be better understood from the examples provided below. EXAMPLES Example I 10.0 grams (g) of a-resorcyclic acid were subjected to a solid state carbonization process, in an inert environment, according to the following program: 1) heating the monomer from an ambient temperature to 220 ° C at a speed of 1 ° C / minute; 2) keep the material at that temperature for 8 hours; 3) heat the material from 220 ° C to 500 ° C at a rate of 1 ° C / minute; and 4) keep the material at that temperature for 24 hours. The resulting amorphous carbon material weighed 5.20 g, indicating a yield of 52%. X-ray diffraction analysis indicates that the material was amorphous. Example II 5.0 grams (g) of α-resorcyclic acid were subjected to a solid state carbonization process, in an inert environment, according to the following program: 1) heating the monomer from an ambient temperature to 220 ° C to a speed of 1 ° C / minute; 2) keep the material at that temperature for 8 hours; 3) heat the material from 220 ° C to 900 ° C at a rate of 1 ° C / minute; and 4) keep the material at that temperature for 24 hours. The resulting amorphous carbon material weighed 1.88 g, indicating a yield of 37.6%. X-ray diffraction analysis indicates that the material was amorphous. Example III They were completely mixed in a glass flask . 0 grams (g) of a-resorcinic acid, 0.5 g of phosphoric acid and 5.0 g of deionized water. The mixture is then dried in an inert environment, producing a mixture weighing 5.5 g. This mixture was subjected to a carbonization process in solid state, in an inert environment, according to the following program: 1) heat the monomer from an ambient temperature to 220 ° C at a rate of 1 ° C / minute; 2) keep the material at that temperature for 8 hours; 3) heat the material from 220 ° C to 500 ° C at a rate of 1 ° C / minute; and 4) keep the material at that temperature for 24 hours. The resulting material weighed 2.81 g, indicating a yield of 56.2% of the original monomer. The resulting product was an amorphous carbon material. Example IV 5.0 grams (g) of a-resorcyclic acid, 0.5 g of p-toluenesulfonic acid and 5.0 g of deionized water were completely mixed in a glass flask. The mixture is then dried in an inert environment, producing a mixture weighing 5.5 g. This mixture was subjected to a carbonization process in solid state, in an inert environment, according to the following program: 1) heat the monomer from an ambient temperature to 220 ° C at a rate of 1 ° C / minute; 2) keep the material at that temperature for 8 hours; 3) heat the material from 220 ° C to 500 ° C at a rate of 1 ° C / minute; and 4) keep the material at that temperature for 24 hours. The resulting material weighed 3.02 g, indicating a yield of 60.4% of the original monomer. The resulting product was an amorphous carbon material. Example V 5.0 grams (g) of 5-hydroxyisophthalic acid were subjected to a solid state carbonization process, in an inert environment, according to the following schedule: 1) heating the monomer from an ambient temperature to 310 ° C to a speed of 1 ° C / minute; 2) keep the material at that temperature for 8 hours; 3) heat the material from 310 ° C to 500 ° C at a rate of 1 ° C / minute; and 4) keep the material at that temperature for 24 hours. The resulting material weighed 1.70 g, indicating a yield of 34.0%, and was an amorphous carbon material. Example VI 5.0 grams (g) of a-resorcinic acid was subjected to a solid state carbonization process, in an inert environment, according to the following program: (1) heating the monomer from an ambient temperature to 220 ° C at a speed of 1 ° C / minute; (2) keep the material at that temperature for 8 hours; (3) heat the material from 220 ° C to 600 ° C at a rate of 1 ° C / minute; and (4) keep the material at that temperature for 24 hours. The resulting material weighed 2.24 g, indicating a yield of 44.8%. X-ray diffraction indicates that the carbon material is amorphous. Referring now to Figure 6, there is illustrated a loading and unloading curve for an amorphous carbon material prepared according to this Example VI. The amorphous carbon material shows good intercalation capacity of lithium and more specifically as shown in figure 6, the load capacity is 1260 mAh / g and the discharge capacity is above 600 mAh / g. This indicates that the material could have excellent characteristics in a lithium-type electrochemical cell. Example VII 5.0 grams (g) of a-resorcinic acid was subjected to a solid state carbonization process, in an inert environment, according to the following program m: (1) heating the monomer from an ambient temperature to 220 ° C C at a speed of 1 ° C / minute; (2) keep the material at that temperature for 8 hours; (3) heat the material from 220 ° C to 700 ° C at a rate of 1 ° C / minute; and (4) keep the material at that temperature for 24 hours. The resulting material weighed 2.06 g, indicating a yield of 41.2%. X-ray diffraction indicates that the carbon material is amorphous. Referring now to Figure 7, the loading and unloading curves for the first ten cycles of an amorphous carbon material prepared according to this example are illustrated therein. As can be seen from Figure 7, the material shows a capacity of approximately 400 mAh / g with a high degree of repeatability between each cycle, and little fading. Example VIII 5.0 grams (g) of a-resorcyclic acid were subjected to a solid state carbonization process, in an inert environment, according to the following program m: (1) heating the monomer from an ambient temperature to 220 ° C at a speed of 1 ° C / minute; (2) keep the material at that temperature for 8 hours; (3) heat the material from 220 ° C to 1100 ° C at a rate of 1 ° C / minute; and (4) keep the material at that temperature for 24 hours. The resulting material weighed 1.78 g, indicating a yield of 35.6%. X-ray diffraction indicates that the carbon material is amorphous. Referring now to Figure 8, there is illustrated the loading and unloading curves for the first ten cycles of an amorphous carbon material prepared according to this Example 8. As can be seen from Figure 8, the characteristics of Loading and unloading are extremely uniform between cycles, however, the capacity is only about 250 mAh / g. The results of each of the examples can be compared in order to make some determinations that take into account the method for preparing the amorphous materials according to the present invention. For example, both Examples I and II use a-resorcyclic acid as the starting material and both are heated from 0 to 220 C in an initial heating process. Then, the carbonization processes are different from each other in that the final heating temperature is 500 ° C in Example I against 900 ° C in Example II. From these two examples, it can be seen that the yield of the final product is higher at lower than high temperatures since the yield in Example I (500 °) was 52% versus 37% for Example II. Similarly, when comparing the results of Examples III and IV, one can once again appreciate some distinctions. Both Examples III and IV use a-resorcinic acid and a heating regime of 0-220 ° and 220-500 °, both as in Example I. However, in Examples 3 and 4, a-resorcyclic acid is heated in the presence of an acid; H3P04 in Example III and p-toluenesulfonic acid in Example IV. In each of Examples III and IV, the yield of the final product is improved above that of Example I. In Example III, the yield of the final product is 56%, while in Example IV (p-acid) toluenesulfonic), the yield of the final product is 60%. According to the above, one can appreciate that adding the step of providing an acid in the presence of the initial organic monomer material will improve the performance of the final product. As noted above, this is believed to be due to the fact that the acid catalyzes the ester condensation reactions, which will prevail when resorcyclic acid is used as an initial monomer. With respect to Example V, the amorphous carbon material is made from a different initial multifunctional monomer, ie, 5-hydroxyisophthalic acid. The material was finally heated to 500 ° C providing a 34% yield.

With respect to Examples VI, VII and VIII, the effect of the last temperature on the carbonization process can be easily appreciated. At lower temperatures, as for example in Example VI, the yield of the material is 44.8%, but it decreases in Examples VII and VIII. By comparing the results of Examples VI, VII, and VIII, with those of Example II, one can easily observe that high temperatures have a deleterious effect on the performance of the final product. Although the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited. Numerous modifications, changes, variations, substitutions and equivalents will occur for those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.

Claims (6)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and therefore the property described in the following claims is claimed as property. 1. A rechargeable electrochemical cell comprising: a cathode; an electrolyte; and an anode made of an amorphous carbon material resulting from the condensation and reduction of a multifunctional organic monomer having the structure: wherein Ri, R2 and R3 are all selected from the group consisting of carboxylic acids of eight carbons or less, carboxylic esters of eight carbons or less, alcohols of eight carbons or less, carboxylic anhydrides of eight carbons or less, amines, and combinations thereof, and wherein at least one of Rj, R2 and R3 is different from one another.
2. 2. A rechargeable electrochemical cell according to claim 1, characterized in that said multifunctional organic monomer is a-resorcyclic acid.
3. 3. A rechargeable electrochemical cell according to claim 1, characterized in that said cathode is a lithiated transition metal oxide.
4. 4. A rechargeable electrochemical cell according to claim 1, characterized in that said electrochemical cell is a battery.
5. 5. A rechargeable electrochemical cell according to claim 1, characterized in that said electrochemical cell is an energetic cell.
6. 6. A rechargeable electrochemical cell according to claim 1, characterized in that said electrochemical cell is an electrochemical capacitor.