Patent Application
20080044656
15 In the subject invention, a “graphite particle” means a particle with a graphite structure that can be a graphitized mesocarbon microbead, a natural graphite particle, or a synthesized graphite particle. A “covering” means a partial or entire coverage. In other words, the phrase “covering the particle” means that the entire or partial surface of the particle is covered.
The carbonaceous composite particle can be formed by mixing the graphite particle and an ACS-forming material so that the ACS material coats the graphite particle. Thereafter, a heat treatment to the coated particle transforms the layer of the ACS-forming material into an ACS to provide the composite particle. Any materials that can provide an ACS with heat treatment can be used as the ACS-forming material. Preferably, the ACS-forming material is selected from a group consisting of phenol resin, furan resin, polyvinyl alcohol resin, polystyrene resin, polyimide resin, epoxy resin, cellulose resin, and a combination thereof.
The mixing proportions of the graphite particle and the ACS-forming material are not critical to the subject invention, as long as the ACS-forming material is in a mixing amount to partially or entirely cover the surface of the graphite particle. Generally, under the same amount of coverage, if the particle size of the graphite particle is smaller, the amount of the ACS-forming material is higher. If the particle size of the graphite particle is no more than 200 μm, the amount of the ACS-forming material ranges from 1 to 70 wt %, preferably from 10 to 60 wt %, based on the total weight of the graphite particle and the ACS-forming material.
Optionally, the ACS-forming material is dissolved in a solvent to obtain a solution. The solution is then mixed with the graphite particle to coat the ACS-forming material on the particle surface. If the selected ACS-forming material per se has fluidity, the material can be directly mixed with the graphite particle. For example, if a phenol resin is used as the ACS-forming material, the phenol resin can be directly mixed with the graphite particle. To achieve a uniform coverage, the mixing step can be conducted by stirring for a period ranging from 1 to 60 minutes.
Afterwards, the mixture comprising the ACS-forming material and the graphite particle is heated to transform the ACS-forming material into an ACS. At least two heat treatment steps are conducted in said heat treatment to accomplish the transformation. The first heat treatment removes the solvent which is optionally used in the mixing step and allows the ACS-forming material to crosslink and provides a crosslinked structure. The second heat treatment allows the crosslinked structure to further transform into the desired ACS.
The first heat treatment is conducted under a temperature that is not higher than the pyrolysis temperature of the ACS-forming material. Generally, the temperature should not be higher than 300□. Preferably, the first heat treatment is conducted in two stages. Particularly, the first stage is conducted at a temperature ranging from 40 to 120□ to remove the solvent which is optionally used in the mixing step and to cure the ACS-forming material to allow it to crosslink so as to provide a crosslinked structure. Optionally, the first stage can be conducted under vacuum for a period ranging from 5 minutes to 100 hours to enhance the removal of the solvent. Then, a second stage of stabilization is conducted at a temperature ranging from 150 to 300□ to allow the crosslinking of the ACS-forming material to be sufficiently conducted. Preferably, the stabilization is conducted under a temperature ranging from 180 to 250□ under an oxygen-containing atmosphere (e.g., air) for a period ranging from 5 minutes to 240 hours to sufficiently accomplish the crosslinking of the ACS-forming material.
The second heat treatment is conducted at a temperature higher than 400□ under an oxygen deficient atmosphere to carbonize the crosslinked structure from the ACS-forming material to form a layer of ACS on the surface of the graphite particle. Preferably, the carbonation is conducted at a temperature ranging from 500 to 1500□ with inert gas (e.g., nitrogen gas, helium gas, and/or argon gas) for a period ranging from 5 minutes to 10 hours. More preferably, the carbonation is conducted at a temperature ranging from 500 to 1000□.
Because the crosslinking reaction of the ACS-forming material allows the mixture to form a block product in the aforementioned heat treatment, the discrete form of the graphite particle may not exist. Therefore, a crushing treatment should be conducted before and/or after the second heat treatment step to reduce the product size so as to provide a product in the desired form.
Any common means for reducing particle size (e.g., mechanical crushing, bead milling, and grinding) can be used in the subject invention. For example, a crushing treatment can be conducted after the second heat treatment step. Or, prior to the second heat treatment step, the crosslinked product that has been subjected to the first heat treatment is first crushed to reduce its size and then subjected to carbonization in the second heat treatment. Preferably, the crushing treatment is conducted before and after the second heat treatment step to enhance the efficiency of the second heat treatment and provide the carbonaceous composite particle with a desired size. For example, a mechanical crushing treatment can be conducted before the second heat treatment to reduce the size of the block product obtained from the first heat treatment to facilitate the second heat treatment. Then, bead milling is conducted after the second heat treatment step to further reduce the product size so as to provide the desired particulate products.
A non-graphitized mesocarbon microbead also can be used as the raw material for the preparation of the carbonaceous composite particle of the subject invention. In this case, the second heat treatment can just be conducted at a temperature above 1500□, under which the mesocarbon microbead can be transformed into a graphitized mesocarbon microbead and the crosslinked structure from the ACS-forming material can be transformed into the ACS. Preferably, the second heat treatment is conducted in two stages: a first stage of carbonization at a temperature ranging from 500 to 1500□ and a second stage of graphitization at a temperature above 1500□. It is preferred for the carbonization to be conducted under an inert gas, such as nitrogen gas, helium gas, and/or argon gas at a temperature ranging from 500 to 1000□ for a period ranging from 5 minutes to 10 hours. It also preferred for the graphitization to be conducted under an inert gas, such as helium gas and/or argon gas at a temperature that is higher than 1500 and not higher than 3000□ (more preferably, at a temperature ranging from 2000 to 3000□) for a period ranging from 0.1 second to 240 hours.
The carbonaceous composite particle of the subject invention can be used in the manufacturing of electrodes required for the secondary cell in order to provide a secondary cell, especially a lithium-ion secondary cell. Therefore, the subject invention also provides a secondary cell which comprises a first electrode, a second electrode; and a electrolytic solution arranged between the first electrode and the second electrode. The first electrode contains a plurality of carbonaceous composite particles, wherein the carbonaceous composite particles can be the same or different from each other and each composite particle comprises a graphite particle and a layer of amorphous carbon structure covering the graphite particle. The graphite particle is selected from a group consisting of a graphitized mesocarbon microbead, a natural graphite particle and a synthesized graphite particle. The size of the composite particle is preferably no more than 200 μm, more preferably no more than 100 μm, and most preferably no more than 40 μm.
In the lithium-ion secondary cell according to the subject invention, the first electrode comprising a plurality of carbonaceous composite particles is used as the cathode and the second electrode is used as the anode. The anode can be made of any materials suitable for manufacturing an anode for a lithium-ion secondary cell. For example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and/or lithium manganese oxide (LiMn2O4) can be used to provide the anode.
Any electrolytic solutions useful in lithium-ion secondary cells can be used in the lithium-ion secondary cell according to the subject invention. For example, the electrolytic solution of the lithium-ion secondary cell of the subject invention may utilize a lithium salt selected from the following group (but not limited to) as a solute: LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, and a combination thereof. The electrolytic solution may also utilize a solvent selected from the following group (but not limited to): ethylene carbonate, propylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and a combination thereof.
Preferably, in the lithium-ion secondary cell of the subject invention, LiPF6 is used as the solute and a mixture formulated by ethylene carbonate/propylene carbonate/ethyl methyl carbonate/dimethyl carbonate in a volumetric ratio of 2.5-3.5/1/3.5-4.5/1.5-2.5 is used as the solvent, in order to provide a desired electrolyte solution.
The subject invention is further illustrated by the following embodiments. The true density of the carbonaceous composite particle produced by each example, the lithium-ion secondary cell for analysis, and the test of the secondary cell are respectively described as follows:
Equipment: Accupyc 1330 Pycnometwr True Densimeter (manufactured by Micromeritics GmbH)
Test method: The dried sample was put in the container of the true densimeter and weighted. The high pressure helium gas was introduced into the true densimeter. After an equilibrium status was achieved, the ideal gas equation (PV=nRT) was used to calculate the sample volume. Then, the average value of the sample density was obtained as the true density of the sample.
An adhesive solid comprising the carbonaceous composite particles of the subject invention and polyvinylidene fluoride (“PVDF”) in a weight ratio of 9:1 was mixed with the solvent N-methylpyrrolidinone (“NMP”) with a solid/liquid ratio of 1:0.8 to form a slurry. The mixing sequence was as follow: 0.45% of oxalic acid based on the total weight of the slurry was first mixed with NMP, followed by mixing with PVDF for about 1.5 hours, so as to form the slurry. Oxalic acid was used to prevent the dissociation of fluoride ions in PVDF.
Afterwards, the above slurry was mixed with carbonaceous composite particles for about 1.5 hours to obtain a carbonaceous slurry. The carbonaceous slurry was coated on a copper foil by using a blade, and then the foil was put in a fuming hood and illuminated with an IR light to evaporate the solvent NMP. Thereafter, the coated copper foil was dried in a vacuum oven at a temperature of 80□ for 4 hours, followed by calendering the foil for densification to produce a densely compressed product. The densely compressed product was pressed to form a circle carbon electrode plate with a diameter of 1.2 cm, which was used as a cathode of the lithium-ion secondary cell.
1.15 moles of LiPF6 were dissolved in a mixture of ethylene carbonate/propylene carbonate/ethyl methyl carbonate/dimethyl carbonate in a volumetric ratio of 3/1/4/2 to prepare the electrolytic solution.
As shown in
Equipment: Arbin BT2400 (manufactured by Arbin Instrument Inc., U.S.A.)
Test method: Charge-discharge cyclic tests were conducted according to the following steps:
Graphitized mesocarbon microbeads (with a particle size ranging from 15 μm and 30 μm, manufactured by China Steel Chemical Corporation, No. MGP) were mixed with phenol resin (available from Chang Chun Corporation, No. PF650), and then sufficiently stirred for 15 minutes to obtain a mixture. The amount of the phenol resin is 40% of the total weight of the mixture.
The mixture was placed in a vacuum oven and cured under an atmosphere with the following heating curve: heating from 35□ to 70□ within 40 minutes and holding at 70□ for 90 minutes; heating from 70□ to 75□ within 30 minutes and holding at 75□ for 2 hours; heating from 75□ to 80□ within 20 minutes and holding at 80□ for 1 hour; and cooling from 80□ to 35□ within 1 hour. The above curing treatment allowed the phenol resin to crosslink so that the mixture formed a block.
The block was placed in a high temperature furnace for stabilization under air through the following heating curve: heating from 35□ to 230□ at a heating rate of 0.5 □/min and holding at 230□ for one hour; and cooling to room temperature at a cooling rate of 2 □/min. The purpose of the stabilization was to allow a sufficient crosslinking reaction in the phenol resin. Stabilization promotes the subsequent carbonization and graphitization so as to form a stable amorphous carbon structure with a good structure.
Thereafter, under the protection of nitrogen gas, the block was subjected to a heat treatment as follows: heating to 600□ at a heating rate of 0.5 □/min and holding at 600□ for 10 minutes, and cooling to room temperature at a cooling rate of 2 □/min. In this stage, the polymer structure of the phenol resin was transformed into a fragile glassy carbon structure.
Then, under nitrogen gas, the block was subjected to the following heat treatment: heating from the room temperature to 1000□ at a heating rate of 1 □/min and holding for 30 minutes. In this stage, the glassy carbon structure of the coating structure on the graphitized mesocarbon microbead was transformed into an amorphous carbon structure.
Next, the block composite was crushed into small blocks, and then small blocks were graphitized under the protection of an inert gas as follows: heating from the room temperature to 2500□ at a heating rate of 20 □/min, holding at 2500□ for 5 minutes, and then cooling to the room temperature at a cooling rate of 20 □/min.
Then, the graphitized product was bead milled and screened to obtain carbonaceous composite particles with a size ranging from 10 μm to 100 μm. The composite particle comprised the graphitized mesocarbon microbead and a layer of the amorphous carbon structure covering the microbead.
The physical property of the carbonaceous composite particles and the results of the charge-discharge test of the lithium-ion secondary cell provided thereby are listed in Table 1.
The same raw materials and procedures as in Example 1 were used, except that the amount of the phenol resin was 45% of the total weight of the mixture. The physical property of the carbonaceous composite particles and the results of the charge-discharge test of the lithium-ion secondary cell are listed in Table 1.
The same raw materials and procedures as in Example 1 were used, except that the graphitization was not conducted. The physical property of the carbonaceous composite particles and the results of the charge-discharge test of the lithium-ion secondary cell are listed in Table 1.
The same heat treatments as in Example 1 were used, except that mesocarbon microbeads with an average particle size of 24 μm (manufactured by China Steel Chemical Corporation, No. GCSMB (UH-01-07)) were used as the raw material and the amount of the phenol resin was 33 wt % of the total weight of the mixture.
Moreover, for manufacturing a lithium-ion secondary cell in this example, the electrolytic solution was provided by dissolving 1 mole of LiPF6 into a solvent formulated by ethylene carbonate/diethyl carbonate/propylene carbonate in a volumetric ratio of 3/5/2.
The physical property of the obtained carbonaceous composite particles and the results of the charge-discharge test of the lithium-ion secondary cell prepared thereby are listed in Table 1.
The same raw materials and procedures as in Example 1 were used, except that no resin was used. The physical property of the obtained graphitized particles and the results of the charge-discharge test of the cell are listed in Table 1.
A 100 percent resin (manufactured by Chang Chun Corporation, No. PF650) was used as the raw material. The other processes and test conditions are the same as those in Example 1. The physical property of the obtained graphitized resin particles and the result of the charge-discharge test of the cell are listed in Table 1.
It can be known from Table 1, that as compared with conventional graphitized mesocarbon microbeads and graphitized resin particles, the carbonaceous composite particles according to the subject invention provide lithium-ion secondary cells with a better electrical property combination. Furthermore, the subject invention can attain a higher charge capacity and a lower irreversible capacity so as to provide a secondary cell with a longer cell life.
The above examples are used to illustrate the embodiments of the subject invention only, so as to state the technical features of the subject invention but not to limit the scope of the subject invention. Any changes or equal arrangements which can be easily accomplished by persons skilled in the technical features are within the scope claimed by the subject invention. The scope of protection of the subject invention should be based on the following claims as appended.
| Number | Date | Country | Kind |
|---|---|---|---|
| 095130063 | Aug 2006 | TW | national |
