COMPOSITE CARBON-CARBON PARTICLES

Abstract

Carbon particles are coated with a water-soluble carbon residue material by oxidizing the carbon residue in water and forming a solid coating on the particles. The coated particles may be heated to graphite forming temperatures to prepare the coated particles for use an anode powder for a battery.

Claims

1. A process for producing core and shell particles where both the core and the shell are carbon-based materials that are precursors for graphitic battery anode powder, where the process comprises: a) dissolving a water-soluble carbon precursor in water to form a liquid coating solution; b) dispersing calcined petroleum coke in water to form a dispersion; c) depositing the water-soluble carbon precursor as a coating on the calcined petroleum coke by oxidizing the water-soluble carbon precursor to form a solidified coating; and d) separating the coated particles from the dispersion.

2. The process according to claim 1 wherein the step of oxidizing the water-soluble carbon precursor by oxidation comprises adding a liquid oxidant.

3. The process according to claim 1 where the water-soluble carbon precursor comprises lignin.

4. The process according to claim 3 where the step of oxidizing the water-soluble carbon precursor by oxidation comprises adding a nitric oxide.

5. The process according to claim 1 where the dispersion includes a wetting agent.

6. The process according to claim 5 where the wetting agent includes acetone.

7. The process according to claim 5 where the wetting agent includes N-Methyl-2-pyrrolidone.

8. The process according to claim 1 where calcined petroleum coke is calcined to a temperature above 1000 C.

9. The process according to claim 8 where calcined petroleum coke is calcined to a temperature above 1200 C.

10. The process according to claim 9 where calcined petroleum coke is calcined to a temperature about 1350 C.

11. The process according to claim 1 further including the step of combining the solution with the dispersion and further where the step of oxidizing the water-soluble carbon precursor by oxidation comprises adding a liquid oxidant to the solution prior to the step of combining the solution to the dispersion.

12. A process for producing graphitic anode powder comprising core and shell particles where both the core and the shell are carbon-based materials where the anode powder is suitable for use in the anode of a battery, where the process comprises: a) dissolving a water-soluble carbon precursor in water to form a liquid coating solution; b) dispersing calcined petroleum coke in water to form a dispersion; c) depositing the water-soluble carbon precursor as a coating on the calcined petroleum coke by oxidizing the water-soluble carbon precursor to form a solidified coating; d) separating the coated particles from the dispersion; e) washing the coated particles; and f) heating the coated particles in an oxygen free environment to convert the carbon precursor materials in the core and shell to graphite structures.

13. The process according to claim 12 wherein the step of oxidizing the water-soluble carbon precursor by oxidation comprises adding a liquid oxidant.

14. The process according to claim 12 where the water-soluble carbon precursor comprises lignin.

15. The process according to claim 14 where the step of oxidizing the water-soluble carbon precursor by oxidation comprises adding a nitric oxide.

16. The process according to claim 12 where the dispersion includes a wetting agent.

17. The process according to claim 16 where the wetting agent includes acetone.

18. The process according to claim 16 where the wetting agent includes N-Methyl-2-pyrrolidone.

19. The process according to claim 12 where calcined petroleum coke is calcined to a temperature above 1000 C.

20. The process according to claim 12 further including the step of combining the solution with the dispersion and further where the step of oxidizing the water-soluble carbon precursor by oxidation comprises adding a liquid oxidant to the solution prior to the step of combining the solution to the dispersion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which:

[0012] FIG. 1 is a photograph of a poorly coated carbon particle where the underlying graphite structure is readily visible; and

[0013] FIG. 2 is a photograph of a well-coated particle suitable for use as anode in a lithium ion battery where the coating is smooth and continuous to protect the underlying graphite structure from the electrolyte while permit ions to pass through in and out of the graphite storage inside the particle.

DETAILED DESCRIPTION

[0014] Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow.

[0015] The desire for attractive anode powder is micron sized particles comprising a graphite core with an unaligned smooth and continuous graphite coating. The formation of such particles does not have to start with naturally occurring graphite or synthetic graphite that is to be coated, but rather a carbon precursor core with a carbon precursor coating. The precursors should be sufficiently different that when they are subject to graphite conditions (very high temperatures) the formation of the graphite crystals occurs distinct in one another although they are contiguous.

[0016] Premium petroleum coke is excellent precursor material in that it has really high carbon content and excludes contaminants that evolve in the graphitization process that disrupt the formation of extensive graphite sheets in the core portion of the coated particles.

[0017] For the coating, there are many choices, but the process of coating is not so easy. However, one class of carbon precursor material has been studied. Lignin is term describing complex organic polymers that are found in the cell walls of many plants, making them rigid and woody. Chemically, they are described as cross-linked phenolic polymers. Most lignins are generally insoluble in water, but alkali lignin is known to be water-soluble. Being an organic material, it comprises a significant proportion of carbon although it includes oxygen, hydrogen and other heteroatoms. Being water-soluble, alkali lignin is amenable to being applied as a coating on solid particles via a precipitation process.

[0018] Since many pitches that are candidate coating materials are known to melt and foam easily when heated, the processes must be carefully and precisely designed and controlled. Foaming of the coating renders it unsuitable for battery powder.

[0019] The process for coating carbon precursor material in an aqueous process includes dissolving the coating material in water and getting the premium petroleum coke material to form a dispersion in the water. It is preferred to stir the coke into water although sometimes the coke is resistant to wetting, some wetting-aiding agents may be used. Typical wetting agent are organic compounds that are soluble in both aqueous and non-aqueous liquids such as any detergents, surfactants, solvent acetone or NMP (N-methyl-2-pyrrolidone), etc. The alkali lignin may be dissolved into this dispersion but is preferably dissolved in water first before being combined with the coke whether by stirring the coke into the dissolved lignin or the dissolved lignin combined with the dispersion of coke in water.

[0020] The alkali lignin is precipitated on to the coke particles by oxidation by the addition of a liquid oxidant. The preferred oxidant is nitric acid, HNO.sub.3, but other oxidant liquids may also suffice. It is important to get a good coating on to the surface of the core coke particle as much of the coating will be lost in the graphitization process as the heteroatoms will be driven off to leave only carbon that forms the graphite crystals. Referring to FIG. 1, the coating on the core particle is thin and insufficient to protect the graphite in the core particle. The alkali lignin oxidized on the particle using nitric acid provides a nice continuous coating as shown in FIG. 2 which is smooth and complete.

[0021] What is really attractive is the oxidation process is not simply a cross linking process but provides stable coating that is not amenable to agglomeration. Agglomeration is a real problem when it occurs as it will eventually be broken and leave ragged edges that themselves become vulnerable to the degradation process that the coating is intended to protect the underlying graphite from.

[0022] One aspect of the invention that seems to make the process more robust is to pre-calcine the coke particle to a fairly high temperature prior to coating. Coke particles that were pre-calcined to 950 C. were not as satisfactory as those calcined to 1350 C. It is supposed that coke that is pre-calcined to a temperature between 950 C. and 2000 C. will be fully satisfactory such as above 1000 C. or above 1200 C. or more preferably 1400 C. The coating bonds better to the pre-calcined coke and shrinks with the underlying coke particle while enduring subsequent heat treatments.

[0023] It is very attractive that this is an aqueous process in that although not all of the alkali lignin precipitates out of the water solution, the solution may be re-used with added lignin to be deposited in a subsequent step of adding the liquid oxidant without concern for recovering or disposing of the water like one would have with a solvent deposition process.

[0024] The now coated particle may be washed and dried and then heated in an oxygen free or inert environment up to graphitic temperatures such as above 2900 C. Along the way to such high temperatures, which may be administered at relatively rapid progression without worry about melting the carbon precursor as would normally be a concern with thermoplastic coatings.

[0025] The heating process may be accomplished in a two-step process where the heteroatoms are driven off in a process described as carbonization. The carbon precursors are thereby rendered to carbon. This is done experimentally in a nitrogen environment and the graphitization process is accomplished separately in an argon environment.

[0026] The development of this process was verified in the following experimental procedure comprising i) dissolving the coating carbon precursor and dispersing coke particles in aqueous Coating Solution and at the same time diluting or dissolving the reacting agent in another aqueous Reacting Solution; ii) mixing the two solutions so that the carbon precursor and reacting agent react form solid precipitate on coke particles, iii) separating the resulting solids from the solution by filtration, iv) washing the solids with water to remove residual solution and drying the powder, and v) carbonizing the resulting solids to convert the coating precursor into carbon by heating in an inert environment. Six examples were prepared as shown in Table 1 below.

TABLE-US-00001 TABLE 1 Coating Solution Reacting solution Sample W.sub.c Lignin H.sub.2O (CH.sub.3).sub.2CO 70% HNO.sub.3 30% H.sub.2O.sub.2 Number (g) (g) (g) (g) (g) (g) 1 10.21 3.07 29 5 2.13 2 10.10 1.09 38 g 3.0 2.83 Filtrate 3 10.58 1.55 41 g 3.0 1.69 Filtrate 4 10.06 2.73 25 7.0 2.07 2.02 5 10.07 2.91 41 6.0 2.08 2.18 6 20.44 4.63 42 6.3 2.27

[0027] The coated particles are characterized as shown in Table 2 below:

TABLE-US-00002 TABLE 2 Results Sample Number .sub.cp (wt %) .sub.c (wt %) 1 4.4 1.6 2 6.6 3.7 3 4.7 0.9 4 8.3 3.4 5 9.0 3.8 6 7.4 3.4

[0028] In Table 2, the percentage of the solid lignin is reported for the resulting powder (.sub.cp) along with the percentage of the lignin carbon in the carbonized sample (.sub.c).

[0029] For the six samples above, a cell coin test was performed as explained below with the results in Table 3 following that.

[0030] A coin cell (CR2032 size) consists of lithium metal, a separator, the electrode, and electrolyte between the electrodes. The separator included a thin sheet of porous polypropylene (Cellguard 2300) on the Li side and a glass fiber filter (Whatman GF/B). The electrodes (1.5 cm diameter disks) were prepared with standard solvent casting method with polyvinyl difluoride (PVDF) as binder; the solid composition was 92 wt % active material, 2% carbon black, and 6 wt % PVDF. The material loading (active only) was about 10 mg/cm.sup.2. The electrolyte was 1 M LiPF6 in the solvent mixture (40 v % ethylene carbonate, 30 v % dimethyl carbonates and 30 v % diethyl carbonate). Each cell was cycled five times under the conditions below: charging at constant current of 1 mA till the cell voltage reached zero volt and then at zero volts for one hour and subsequently discharging at constant current of 1 mA till the cell voltage reached two volts. The electrical charges accumulated during each charging and discharging cycles were used to calculate the specific capacity based on the total active electrode material in each cell. The tests were conducted with an electrochemical tester (Arbin LBT21084) at ambient temperature (22 C.).

TABLE-US-00003 TABLE 3 Initial Initial Lignin Carbon discharge coulombic coating coating capacity efficiency Sample level (%) level (%) (mAh/g) (%) 1 4.4 1.6 342.1 96.0 2 6.6 3.7 335.6 94.0 3 4.7 0.9 335.6 95.1 4 8.3 3.4 335.0 91.7 5 9.0 3.8 333.6 95.5 6 9.7 4.4 338.0 92.8

[0031] These are fully satisfactory attributes for high performance batteries and with this simpler and easier process with fewer ancillary concerns like recovering spent solvent or significant loss of product due to chipping and breaking, it is likely that the cost for anode material could be reduced for battery manufacturers.

[0032] In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as an additional embodiments of the present invention.

[0033] Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.