KR20020064309A - Composite carbon electrode for rechargeable lithium-based batteries - Google Patents

Abstract

The present invention relates to a composition suitable for coating a connector in a lithium ion battery comprising carbon fibers and electrically conductive particles coating the carbon fibers. The electrically conductive particles have an average particle size of less than about 40 nm. Preferably, the average particle size is about 30 nm and the specific surface area is greater than about 1200 m 2 / g. Electrodes prepared using the compositions of the present invention have significantly improved electrical conductivity, intracell capacity and regenerative properties, which are generally believed to be due to improved bonding properties that confer overall improved electrical conductivity.

Description

Composite Carbon Electrode for Rechargeable Lithium-based Battery {COMPOSITE CARBON ELECTRODE FOR RECHARGEABLE LITHIUM-BASED BATTERIES}

Lithium-based batteries made as rechargeable lithium ion batteries generally comprise electrodes that may be formed, in part, of a composition comprising carbon fibers. Generally, carbon fibers are formed in mesophase pitch and contain sperm of graphitized carbon. This composition often contains a conductive coating additive, such as carbon black, to promote electrical conductivity to improve the performance of the electrode.

The amount of carbon black included in the electrode composition used in the lithium ion battery is generally about 5 weight percent of the composition. In general, the average particle size of the carbon black used is from about 50 nm to about 100 nm. The specific surface area of the particles in carbon black is generally about 80 m 2 / g or less. The surface area of the carbon black is generally limited to suppress electrolyte decomposition that may occur in the battery.

There are several disadvantages to using carbon black with a relatively small surface area. One of these drawbacks is that the amount of carbon black that must be used is generally relatively large in order to significantly improve the electrical conductivity of the carbon fiber containing electrode. If the amount of carbon black is relatively large, the in-cell capacity is lowered due to the fact that the corresponding amount of the corresponding active material, that is, the graphite carbon fiber is small. Further, even if electrolyte decomposition is suppressed by using carbon black having a relatively small surface area, the electrolyte nevertheless decomposes in any way. Reducing the amount of carbon black to increase the intracellular capacity reduces the rate capability of the electrode. It is also generally known that using carbon black with a large surface area produces electrodes with low first cycle efficiency.

Accordingly, what is needed is a composition suitable for use in an electrode of a battery that overcomes or minimizes the above problems. There is also a particular need for compositions suitable for use in anodes of lithium ion batteries that overcome or minimize the above problems.

The present invention relates to a composition comprising carbon fibers and electrically conductive particles coating the carbon fibers.

1 is a scanning electron micrograph of graphitized carbon fibers coated with carbon black having a large surface area relative to the composition of the present invention.

2 is a scanning electron micrograph of graphitized carbon fibers coated with carbon black having a small surface area relative to the composition of the comparative example.

The present invention relates to a composition comprising carbon fibers and electrically conductive particles coating the carbon fibers. The electrically conductive particles have an average particle size of less than about 40 nm. In a preferred embodiment, the electrically conductive particles have an average particle size of about 30 nm. In a particularly preferred embodiment, the surface area of the electrically conductive particles is at least about 1200 m 2 / g and the average particle size is at most 30 nm. More preferably, the surface area of the electrically conductive particles is at least 1500 m 2 / g. The electrically conductive particles can be, for example, carbon black, metal particles, metal oxide particles or any combination of these particles.

The present invention has several advantages. For example, when using conductive particles having a relatively large surface area, it is contemplated that a correspondingly large number of electronic conductive pathways will be created, thus increasing the electrical conductivity at the electrode as a whole. In addition, the use of conductive particles having a relatively large surface area results in a relatively larger surface area of the graphitized carbon fibers being brought into contact, and thus for lithium ions during insertion into and extraction from the graphitized carbon fibers. The reaction site can be larger. In addition, regions in the graphitized carbon fibers that may be isolated through defect structures from synthesis are more likely to be contacted, and thus lithium ions are more easily inserted. It has also been found that the coating of the composition allows for better adhesion to the collector substrate. Thus, the interface between the coating and the collector is less resistant and the mechanical stability of the electrode is improved. For example, a lithium ion battery comprising a composition of the present invention may have a larger specific capacity because of its high insertion capacity, a larger intracellular capacity due to a larger material loading, and because of its electrical conductivity and greater surface contact. The speed performance may be better, and may have better reproducibility, which is considered to be because of the wide range of protection for protecting the mechanical stability and the surface of the graphitized carbon fiber particles from decomposition. In addition, because of the advantages obtained by using conductive particles having a relatively large specific surface area, the battery anode can be configured to have a significantly reduced weight percent of such conductive particles and a correspondingly increased amount of carbon fibers.

In addition, the small particulate carbon black used in the present invention generally allows carbon fibers to have higher specific capacities in lithium ion batteries as compared to the types that do not use carbon black. Preferably, this increase is in harmony with the high rate performance. The intracell capacity is the total capacity calculated based on the total weight of the formulation, which depends on the relative weight of the carbon fibers, polymer and carbon black of the coated electrode. Intracell capacity is defined as follows.

ICC = (SC) (CF)

Where ICC is the intracellular capacity (mAh / g), SC is the specific capacity (mAh / g), and CF is the relative amount of carbon fiber in the formulation (the weight of carbon fiber plus the weight of carbon fiber, polymer, carbon black) Divided by).

However, the more carbon coating the electrode coating contains, the lower the first cycle efficiency will be. Since the first cycle efficiency affects the total capacity of the lithium-based electrochemical cell, it is advantageous that the first cycle efficiency is high. The lower the primary cycle efficiency, the greater the amount of cathode material that must be added to the battery when the cathode initially supplies lithium. If the primary cycle efficiency is 100%, the capacity of the anode can be balanced by the exact added capacity of the cathode, ie no excess cathode material needs to be added. If the primary cycle efficiency is lower than 100%, excess cathode material must be added to the positive side of the battery. In subsequent cycles, the efficiency is generally close to 100%. Low first cycle efficiency is due to side reactions such as the formation of a lithium containing interface on the electrode surface. In practice, the first cycle efficiency should be at least 85%. In general, carbon black having a large surface area has a lower first cycle efficiency per reference weight than carbon black having a small surface area. This disadvantage is solved by using less carbon black in the present invention. Since the amount of carbon black can be reduced, the first cycle efficiency for the preferred formulation will be at least 85%. (The examples described below show that this efficiency can be achieved.)

The above features and other details of the invention will be described in more detail with reference to the accompanying drawings, which will be pointed out in the claims. Certain embodiments of the present invention illustrate, but do not limit, the present invention. The basic features of the invention can be used in various embodiments without departing from the scope of the invention.

The composition of the present invention comprises carbon fibers and electrically conductive particles coating the carbon fibers.

Carbon fibers are suitable for use in the compositions used to coat anode connectors in lithium ion batteries. In one embodiment, the carbon fiber is graphitized carbon fiber. In a preferred embodiment, the carbon fibers are graphitized carbon fibers formed from mesophase pitch. Particularly preferred carbon fibers are those formed from solvated mesophase pitches. An example of a preferred carbon fiber is Petoca M5161 (registered trademark) carbon fiber. Examples of other suitable carbon fibers are described in US Pat. No. 5,766,523 (Conoco Inc., June 16, 1998), the contents of which are incorporated herein by reference. According to the manufacturer's instructions, the Petoca M5161 (registered trademark) has the following characteristics.

Surface Area: 1.3 ㎡ / g

Typical average fiber diameter: 10 μm

Typical average fiber length: 30 μm

In addition, Petoca M5161 fibers have the following average dimensional distribution as measured by the light scattering method.

10% of the fiber is less than 8.4 μm

50% of the fiber is less than 18 μm

90% of the fibers are less than 57 μm.

Suitable carbon fibers have an average length of about 20 to about 70 μm and an average diameter of about 5 to about 20 μm. For compositions comprising carbon fibers, electrically conductive particles and polymeric binders, the amount of carbon fibers is from about 85 to about 98 weight percent. Preferably, the carbon fibers present are about 90 to about 97 weight percent in the composition.

Suitable electrically conductive particles include, for example, carbon black, metal particles and electrically conductive metal oxide particles. In one embodiment, the electrically conductive particles are carbon blacks having an average particle size of less than about 40 nm. In a preferred embodiment, the electrically conductive particles are carbon black with an average particle size of about 10-20 nm and a specific surface area of at least about 1500 m 2 / g. Particularly preferred electrically conductive particle materials are large surface area materials such as Cabot Carbon Black Black Pearls 2000. Cabot carbon black Black Pearls 2000 has an average particle size of 12 nm and a surface area of 1500 m 2 / g. Generally, the amount of conductive particles present in the composition is about 0.3 to about 8 weight percent.

A third component is a polymeric binder used to bond carbon fibers and electrically conductive particles to each other to impart mechanical stability to the electrode. Examples of suitable polymer components of the composition are polyvinylidene fluoride (PVDF), polyimide (PI) and polyvinylidene difluoride-hexafluoropropylene (PVDF-HFP). Preferably, the polymer component is PVDF, available for example from Kureha Chemical Industry Company, Ltd., Solvay, Elf Atochem. The amount of polymer in the composition is generally about 1 to about 10 weight percent in the composition.

The method of making the composition comprises combining and mixing the carbon fiber component, the electrically conductive particles and the polymeric binder in a container. Suitable solvents can be used to dissolve the polymer during mixing. In one embodiment, the solvent is 1-methyl-2-pyrrolidinone (NMP). Mixing can be facilitated by suitable mechanical aids such as steel balls. Mixing of the components is continued until a homogeneous composition is obtained.

The homogeneous mixture or slurry obtained is coated on a suitable connector material such as copper foil. Generally, the connector is a copper foil having a thickness of about 10 μm to about 25 μm. In a preferred embodiment, the copper foil is about 20 μm thick. The slurry may be coated using a doctor blade to form a coating having, for example, a thickness of about 152 μm to about 508 μm. Preferably, the thickness is about 254 μm. The coated connector is then heated to a suitable temperature, such as from about 110 ° C. to about 175 ° C., for a time sufficient to evaporate any solvent. In one embodiment, the coated electrode is heated to about 150 ° C for about 20 minutes. After the heat treatment, the electrode will generally have a total thickness of about 50 μm to about 150 μm. In a preferred embodiment, the anode has a total thickness of about 100 μm.

The electrode is then cut into strips of suitable size. The strip is then pressed onto the roll press at room temperature at the appropriate pressure. For example, suitable pressures are from about 250 kg / cm 2 to about 1500 kg / cm 2 for rolls of 5.08 cm (2 inches) in diameter. In a preferred embodiment, the strip is pressed on a roll press at room temperature at an apparent pressure of about 1000 kg / cm 2.

After compression, the electrodes generally have a thickness of about 30 to about 100 μm. In a preferred embodiment, the collector is about 80 μm thick after compaction.

In a preferred embodiment, the pressed electrode is then dried under suitable conditions to remove residual solvent and moisture. In a preferred embodiment, the electrode is dried in vacuo at about 80 ° C. for about 16 hours.

The electrode can be used as a negative electrode in a lithium ion battery.

The present invention will be described in more detail with reference to the following examples. All parts and percentages are by weight unless otherwise indicated.

Yes

Example 1

Petoca M5161 (registered trademark) 47 grams of carbon fiber (active material), Cabot carbon black Black Pearls (registered trademark) 2000 (conductive additive) 0.5 grams, 15 weight percent PVDF / NMP solution 31.25 grams and NMP 40 grams 50 steel balls ( φ = 1/4 inch) in a 250 mL vessel. The mixture was mixed for 40 minutes by paint shaking.

Using a doctor blade, the slurry was coated on a copper foil (thickness about 20 μm) so that the thickness in wet state was 250 μm. The coated electrode was heated at 150 ° C. for 20 minutes. The general thickness of the electrode, including the copper current collector, was 100 μm.

The electrode was cut out to a 3.5 x 4 cm 2 strip. This strip was pressed on a roll press at room temperature at an apparent pressure of 1000 kg / cm 2. After compression, the thickness of the electrode, including the current collector, was 80 μm. The crimped electrode was cut out to a disc of about 0.75 cm in diameter. The weight of the active material at the disk electrode was 15 mg. The dry disk electrode was prepared by drying the electrode in vacuum at 80 ° C. for 16 hours.

Lithium foil (Aldrich) as cathode, dry disk electrode as anode, glass fiber disk as separator, EC / DMC (1: 1) -LiPF6 1M (EM Industries) as an electrolyte Standard CR 2520 type available from US Pat. All work was done with water and oxygen levels of less than about 1 ppm in a glove box filled with argon.

Coin cells were regenerated using C / 5 and C / 2 currents in the voltage range of 0.000 to 2.000 V.

Example 2

All procedures were similar to Example 1 above, but the conductive additive was Cabot carbon black Vulcan XC 72R® having a surface area of 254 m 2 / g and a particle size of 30 nm.

Comparative Example A

All procedures were similar to the above examples, but the conductive additive was a Chevron 100% pressed acetylene black C-100 (registered trademark), a material having a relatively small surface area (surface area 80 m 2 / g, particle size 42 nm).

Table 1 shows the capacity at various points during the regeneration of lithium ion cells using the carbon additives described above. As shown, specific capacity was best maintained over a large number of regenerations with a lithium ion battery using Black Pearls 2000 carbon black as the conductive additive material. In addition, as can be seen in Table 1, the examples using Black Pearls® 2000 carbon black had significantly improved specific capacity and speed performance.

Cycle count Primary 10th 30th Capacity at (C / 2) (C / 5) x 100 Specific capacity (mAh / g) Example 1 Black Pearls 2000 (registered trademark) 323 320 316 100 Example 2 Vulcan XC 72R (registered trademark) 309 298 264 98 Comparative Example AC-100 (registered trademark) 300 278 236 89

1 is a scanning electron micrograph of carbon fibers in a composition of the present invention using a carbon black material having a relatively large specific surface area. FIG. 2 is a scanning electron micrograph of carbon fiber in a composition of Comparative Example using relatively small specific surface area carbon black. As can be seen by comparing FIG. 1 and FIG. 2, the carbon fiber of FIG. 1 has a coating of carbon black distributed more uniformly than the carbon fiber shown in FIG. 2. While not wishing to adhere to any particular theory, it is believed that the improved distribution of carbon black shown in FIG. 1 improves the electrical conductivity and performance of lithium ion batteries using the compositions of the present invention. The improved distribution and the improved performance obtained are believed to be the result obtained by using a carbon black material having a surface area of at least 250 m 2 / g and a particle size of 40 nm or less.

Example 3

48.45 grams of graphitized carbon fiber (Conoco Inc. (active material) from solvated mesophase pitch, 0.3 grams of Cabot carbon black Black Pearls® 2000 (conductive additive), 8.33 grams of 15 weight percent PVDF / NMP solution and NMP 50 grams were mixed in a 250 ml vessel using 50 steel balls [φ = 0.64 cm (1/4 inch)]. The mixture was mixed for 40 minutes by paint shaking.

Using a doctor blade, the slurry was coated on a copper foil (thickness about 20 μm) so that the thickness in wet state was 250 μm. The coated electrode was heated at 130 ° C. for 30 minutes. The general thickness of the electrode, including the copper current collector, was 100 μm.

The electrode was cut out to a 3.5 x 4 cm 2 strip. The strip was pressed on a roll press at room temperature with an apparent pressure of 1000 kg / cm 2. After compression, the thickness of the electrode, including the current collector, was 80 μm. The crimped electrode was cut out to a disc of about 0.75 cm in diameter. The weight of the alternative active material at the disc electrode was 15 mg. The electrode was dried in vacuo at 80 ° C. for 16 h.

Coin cell (type CR 2520) using lithium foil (Aldrich) as cathode, dry disk electrode as anode, glass fiber disk as separator, EC / DMC (1: 1) -LiPF6 1M (EM Industries) as electrolyte Was prepared. All work was done with water and oxygen levels below about 1 ppm in an argon filled glove box.

Coin cells were regenerated using C / 5 and C / 2 currents in the voltage range of 0.000 to 2.000 V.

Comparative Example B

45.75 grams of graphitized carbon fiber (Conoco Inc. (active material) from solvated mesophase pitch, 2.23 grams of Chevron C-100® acetylene black (conductive additive), 13.33 grams of 15 weight percent PVDF / NMP solution and NMP 75 grams were mixed in a 250 ml vessel using 50 steel balls [φ = 0.64 cm (1/4 inch)]. The mixture was mixed for 40 minutes by paint shaking. All other procedures are the same as in the above example.

Table 2 summarizes the material and performance characteristics of Example 3 and Comparative Example B.

Example 3 Comparative Example B Formulation Active material Conoco Carbon Fiber 990585 Conoco Carbon Fiber 990585 Conductive additive Black Pearls (registered trademark) 2000 carbon black C-100 (registered trademark) acetylene black Inactive material: conductive material: PVDF binder 96.9: 0.6: 2.5 91.5: 4.5: 4 Primary Cycle Cell Capacity (mAh / g) 303 280 First Cycle Coulomb Efficiency (%) 90 90 18th cycle cell capacity (mAh / g) 294 284 36th cycle cell capacity (mAh / g) 292 283

Referring to Table 2, the optimal formulation was used for each sample. In general, a mutual balance between the coulomb effect and the dose is required. However, Table 2 shows that cells provided using the present invention can have the same coulomb efficiency as in the prior art, while achieving higher intracell capacity. As shown in Table 2, batteries made using the present invention have a higher total capacity.

Example 3

Stability test by Accelerated Rate Calorimetry (ARC)

The onset temperature of the exothermic reaction is a measure of safety in lithium-based battery systems. When the exothermic reaction begins, the temperature increases with the heat of reaction and increases at a rate defined as "self heating rate". The higher the magnetic heating rate, the faster the temperature increases. Exothermic reactions that proceed in an uncontrolled state are sometimes referred to as "thermal runaway" or "autocatalytic". For battery manufacturers, thermal runaway is important (especially at low temperatures) because the battery can actually explode in the hands of the user of the battery-powered facility. For safety reasons, it is advantageous for the starting temperature to be as high as possible and the self heating rate as low as possible.

To study this phenomenon, 0.5 grams of carbon fiber powder (solvated mesophase carbon fiber), 0.5 grams of Black Pearls® 2000 powder and 1.0 grams of PC-LiPF ₆ 1M electrolyte were placed in a titanium sample container. Stored in an argon filled glove box with less than 1 ppm oxygen and moisture. The filled titanium vessel was then placed on an ARC 2000 Acceleration Rate Calorimeter (Arthur D. Little) under argon atmosphere. The exotherm was recorded during the experiment starting at 50 ° C and ending at 350 ° C. The heating rate used was 5 ° C./min and the waiting time for each step was 17 min. The starting temperature and maximum magnetic heating rate of the exothermic reaction derived from the experiment are shown in Table 3 below.

For comparison, another experiment was conducted using 0.5 grams of C-100® powder instead of Black Pearls® 2000 powder. All other experimental conditions are as in the above examples. As shown in the table below, experiments with Black Pearls® 2000 powder had a higher starting temperature and are therefore considered to be a safer material for lithium battery systems.

Starting temperature Magnetic heating rate (° C / min) Black Pearls® 2000 Powder 170 54 C-100 (registered trademark) powder 135 10

Those skilled in the art to which the present invention pertains will be able to recognize and identify many equivalents to the specific embodiments of the invention specifically described herein, using experiments that are routine. . All such equivalents are within the scope of the claims.

Claims (23)

a) with carbon fiber, b) electrically conductive particles coating the carbon fiber Including; Wherein the electrically conductive particles have an average particle size of less than about 40 nm. The composition of claim 1, wherein the carbon fibers are graphite fibers. The composition of claim 1, wherein the average particle size of the electrically conductive particles is about 30 nm. The composition of claim 1, wherein the specific surface area of the electrically conductive particles is greater than about 1,500 m 2 / g. The composition of claim 1, wherein the specific surface area of the electrically conductive particles is about 1,200 m 2 / g to about 1,500 m 2 / g. The composition of claim 1, wherein the specific surface area of the electrically conductive particles is about 800 m 2 / g to about 1,500 m 2 / g. The composition of claim 1, wherein the specific surface area of the electrically conductive particles is about 250 m 2 / g to about 1,500 m 2 / g. The composition of claim 1, wherein the electrically conductive particles comprise carbon black. The composition of claim 1, wherein the electrically conductive particles comprise metal particles. The composition of claim 1, wherein the electrically conductive particles comprise metal oxide particles. The composition of claim 1, wherein the electrically conductive particles are present in the composition in an amount of about 0.5 weight percent to about 10 weight percent. The composition of claim 11, wherein the carbon fiber is present in the composition in an amount of about 85 weight percent to about 96 weight percent. The composition of claim 12 further comprising a polymer component. The composition of claim 13, wherein the polymer component is present in the composition in an amount from about 1 weight percent to about 10 weight percent. The composition of claim 13, wherein the polymer component comprises polyvinylidene difluoride. The composition of claim 13, wherein the polymer component comprises a polyimide. The composition of claim 13, wherein the polymer component comprises polyvinylidene difluoride-hexafluoropropylene. In a lithium based battery having an anode comprising a composition of carbon fiber and electrically conductive particles, the electrode composition having an average particle size of the electrically conductive particles is less than about 40 nm. The composition of claim 18, wherein the specific surface area of the electrically conductive particles is greater than about 1,500 m 2 / g. The composition of claim 18, wherein the specific surface area of the electrically conductive particles is about 250 m 2 / g to about 1500 m 2 / g. An anode comprising a composition consisting of carbon fibers and electrically conductive particles, An anode having an average particle size of the electrically conductive particles of less than about 40 nm. The composition of claim 21, wherein the specific surface area of the electrically conductive particles is greater than about 1,500 m 2 / g. The composition of claim 21, wherein the specific surface area of the electrically conductive particles is about 250 m 2 / g to about 1500 m 2 / g.