This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application entitled METHOD FOR MANUFACTURING HIGH POWER ELECTRODE FOR LITHIUM SECONDARY BATTERY filed with the Korean Intellectual Property Office on 7 Jun. 2004, and there duly assigned Serial No. 2004-41258.
1. Field of the Invention
The present invention relates to high power electrodes for rechargeable lithium batteries and methods for manufacturing high power electrodes for lithium rechargeable batteries and, more particularly, to a method for manufacturing high power electrodes for lithium secondary batteries, which endows the batteries with an enhanced current discharge capacity.
2. Description of the Related Art
As appliances such as mobile phones and notebook computers become smaller and lighter, a higher performance battery is required. In particular, we have discovered that there is an urgent need for development of an electrode that may show excellent performance when discharging high current to an electrically powered appliance, tool or an electrically powered automobile. Currently however, it is difficult to develop a commercial version of a conventional high power electrode due to many problems such as the complexity of the processes which must be used for its manufacture, the increasing costs of materials used in its manufacture, the limited processing capability of contemporary manufacturing facilities, and the like.
As the need for higher performance by lithium secondary batteries becomes more acute, a new high power electrode that may overcome the limitations of existing electrodes for lithium secondary batteries should be developed without delay. Until now, the electrode for PLI (polymer Lithium ion) produced by Bellcore Co. in USA (i.e., Bell Communications Research Inc., Livingston, N.J.) has been a substantiallyunique high power electrode. Essentially, to make this electrode, the Bellcore Co. adds DBP (dibutyl phthalate) excessively, together with NMP (n-methyl pyrrolidone) which is capable of melting the PVDF (poly-vinylidene fluoride) that is used to make an electrode binder with the consistency of a slurry. DBP is then extracted from a solvent such as methanol and ether, so that micro-pores are formed in the electrode in order that an electrolyte may easily penetrate into the electrode via the pores.
This type of electrode manufactured by Bellcore Co. is expensive to manufacture, and concomitantly causes economic, environmental and logistical problems because DBP, which is environmentally classified as an environmental hormone, is used as a medium for forming the pores and the DBP should be extracted subsequently in a solvent such as either methanol or ether. In particular, because the current Fire Service Act prohibits processing of methanol in quantities greater than 200L, previous efforts to improve battery production and logistics that require mass production has been hindered by many obstacles.
The present invention is designed to solve the problems attendant to conventional electrode manufacturing methods, and therefore, it is an object of the present invention to provide a method for manufacturing a high power electrode for a lithium secondary battery.
It is another object to provide a method for enhancing the current discharge capacity of electrodes for lithium secondary batteries.
It is still another object to provide a method for treating electrodes for rechargable lithium batteries to create pores within the electrodes that accommodate free movement of an electrolyte via the pores.
It is yet another object to provide a method for inexpensively creating micro-pores within electrodes for rechargeable lithium batteries.
It is still yet another object to provide a method for manufacturing electrodes of lithium secondary batteries to permit an electrolyte to move freely into pores formed in the electrodes.
It is a further another object to provide a method for treating electrodes while manufacturing lithium batteries to enable an electrolyte to move freely into pores in the electrodes.
It is a still further object to provide a method that is capable of improving a high current discharge capacity of a battery by forming micro pores in the electrode for a lithium secondary battery in a cheap and easy way so that electrolyte may freely move into the pores while the battery is manufactured.
In order to accomplish these and other objects, the present invention provides a method for manufacturing a high power electrode for a lithium secondary battery by (a) preparing an EC (ethylene carbonate) solution by dissolving EC crystals in a suitable solution; (b) separately dissolving a binder in a suitable solution to make a binder solution, and then adding and mixing with the binder solution an active electrode material and an electrically conductive material of a desired composition; (c) adding a predetermined amount of the EC solution prepared in step (a) to the solution obtained in step (b) and stirring the combination sufficiently to make a slurry for use as an electrode binder that may be coated on an electrode; (d) coating a collector with the slurry and sufficiently drying the coated slurry at a predetermined temperature; and (e) forming a final electrode by compressing the dried electrode structure at a predetermined pressure after the coated slurry has been dried.
Before the slurry is coated on the collector in step (d), a step of degassing the slurry in a vacuum may preferably be included.
The range of temperatures which may be used in step (d) while drying the coated slurry is preferably kept in the range of between approximately 120° C. to approximately 140° C.
The range of pressures that may be used for the compression performed in step (e) is preferably kept in the range of approximately 500 kg/square centimeter to approximately 1500 kg/square centimeter.
A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:
a through 2d present a sequence of cross-sectional schematic views that illustrate process steps that may be taken during the manufacture of lithium ion secondary batteries constructed with electrodes manufactured according to the principles of the present invention;
a is a two-coordinate graph showing a rate capability of a battery constructed with a high power electrode manufactured according to the principles of the present invention;
b is a two-coordinate graph showing a rate capability of a battery using a conventionally manufactured electrode;
a is a two-coordinate graph showing a life cycle of a battery constructed with a high power electrode manufactured according to the principles of the present invention; and
b is a two-coordinate graph showing a life cycle of a battery using a conventionally manufactured electrode.
Hereinafter, the present invention will be described in more detail by referring to these accompanying drawings.
Referring to
If the EC solution is prepared, a binder is dissolved in a suitable solvent to make a binder solution, and then to this binder solution is added an active electrode material and an electrically conductive material of a desired composition; the resulting solution is then sufficiently mixed (step S120). Here, the binder may be selected from among PVDF (poly-vinylidone fluoride), HFP (hexafluoropropylene) and so on, and the solvent may be chosen from among NMP, acetone and so on. In addition, the active electrode material may be selected from among LiCoO2, LiNixMnyCo(1-x-y)O2, LiMn2O4, LiNiO2 and so on, and the electrically conductive material may be carbon black.
If the active electrode material and the conductive material are added to the binder solution, and the resulting solution is sufficiently stirred, a small amount of the EC solution prepared in step S110 is then added to the binder solution, and then the binder solution is sufficiently stirred to make a slurry. That slurry may be used as an electrode binder to be coated onto the electrode (step S130). Here, an amount of the EC solution added to the binder solution is determined on the basis of an exact calculation of a ratio occupied by EC present in the electrolyte to be used in the battery.
If the slurry is made as an electrode binder to be coated on the electrode, the slurry is coated on a collector (commonly, aluminum foil is used as a cathode and copper foil is used as an anode in a lithium secondary battery) and the electrode binder is then dried sufficiently at a predetermined temperature (step S140). Before the slurry is coated onto the collector, a process of degrassing the slurry in a vacuum is preferably executed. In addition, although there are some differences, depending on features of the electrode, the temperature for drying the slurry already coated onto the collector is preferably kept within the range of approximately 120° C. to approximately 140° C. so that the organic solvent included in the slurry can not remain. Here, by means of the drying process, the organic solvent is evaporated from the slurry and removed, thereby making an electrode structure in which only active material, binder, electrically conductive material, and solid EC remain.
If the drying process is completed as mentioned above, the dried electrode structure is then compressed at a predetermined pressure to make a final electrode (step S150). Here, the pressure applied to the electrode structure is preferably in the range of approximately 500 kg/square centimeter to approximately 1500 kg/square centimeter, though it may be changed, depending on kind and usage of the electrode.
a through 2d sequentially illustrate the processes of manufacturing a lithium ion secondary battery using an electrode made by the method of the present invention.
Referring to
In addition, as shown in
After that, the cathode 202 of the cathode structure shown in
After that, as shown in
Here, an electrolyte of the lithium secondary battery is generally obtained by mixing EC (ethylene carbonate), PC (propylene carbonate), DEC (diethyl carbonate), DMC (dimethyl carbonate), EMC (ethyl methyl carbonate), and so on at appropriate ratios. The electrolyte used for making a lithium secondary battery using an electrode constructed according to the principles of the present invention already contains EC components, among other components, in the electrode, so that all of the components of the electrolyte, except EC, are mixed together and then aged for approximately ten hours so that EC may be sufficiently dissolved in the mixture.
In addition, most of the components of the electrolyte except EC, are in a liquid state at a room temperature, but at room temperature EC is in a solid state. When the electrode is manufactured, if an electrolyte that is free of the presence of EC is supplied, the electrolyte penetrates into the electrode. Accordingly, the EC is leached out from the electrode as an electrolyte, so that empty spaces generated by the leaching-out of the EC become micro-pores through the electrode, thereby improving a high current discharge capacity of the electrode. In addition, the micro-pores also provide a buffering function that relieves stress and strain which is caused to the active material when lithium ions are inserted or extracted, so that the life cycle of the battery is concomitantly improved.
Meanwhile,
a is a per-rate discharge graph obtained by measuring capacities of a lithium ion secondary battery assembled to incorporate an electrode containing EC as a function of changing current density after the battery has been charged to 4.3V.
The electrode was made with components LiCoO2: Super-P:PVDF in a ratio 94:3:3 percent by weight, and 7% of EC was added on the basis of the amount by weight of LiCoO2 when the electrode was manufactured. This amount of EC was selected to make the component of the electrolyte have a final ratio by weight of EC:PC:DEC:DMC=1:1:1:1, so the electrolyte actually supplied has a composition ratio percentage by weight of PC:DEC:DMC=1:1:1 without any EC being present among the final electrolyte components. The amount of electrolyte supplied was adjusted to 3.5 grams per one Ampere-hour of designed discharge capacity. If the thickness of the electrode is increased, the diffusion length of lithium ions is elongated, so the rate capability of the battery is decreased. Since the present invention is mainly focused on improvement of rate capability of batteries, a thick electrode with a thickness of about 300 millimeters was made and used in order to specify its improved degree. Considering that an electrode of a commercialized battery has a thickness of 145 mm or less, an electrode having about twice that thickness was used to measure battery characteristics.
b is a per-rate discharge graph of a lithium ion secondary battery assembled using an electrode that is free from EC such as a conventionally manufactured electrode, after a charge of 4.3V. The electrode was made with a composition of LiCoO2: Super-P:PVDF in a ratio percentage by weight of 94:3:3, and an electrolyte supplied that had a composition of EC: PC: DEC:DMC in a ratio percentage by weight of 1:1:1:1, so that the electrolyte actually supplied for the battery represented by
For reference, as used in
By comparing the graphs of
a and 4b show life cycles of batteries respectively, in which
As shown in
The foregoing paragraphs explain the details of a method for manufacturing a high power electrode for a lithium secondary battery. This method is capable of further improving the high current discharge capacity of a battery by forming micro-pores in the electrode for a lithium secondary battery in an inexpensive and easily implemented way, so that electrolyte may freely move into the pores while the battery is being manufactured.
As described above, the method for manufacturing a high power electrode for a lithium secondary battery enables the creation of a high power electrode by the expedient of forming micro-pores in the electrode with the use of EC, thereby substantially improving the life cycle and the discharge capacity of a battery incorporating the electrode. In addition, because the practice of the present invention enables the manufacture of a battery without the use of environmental hormones such as DBP, and does not require any separate extraction process that uses either methanol or ether, the present invention may reduce time and cost for the processes, improve workplace safety, and prevent environmental pollution.
Number | Date | Country | Kind |
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10-2004-0041258 | Jun 2004 | KR | national |