Patent Application
20120064229
The present invention relates to aqueous electrode slurries for lithium ion cells.
The aqueous based process of manufacturing positive active material electrodes for Li-ion cells using LiCoO2, LiNiCoAlO2, LiNiCoMnO2, lithium transition metal oxides containing nickel and cobalt compounds and combinations thereof are practically impossible because the pH of the slurry is too high. A positive electrode slurry made using a conventional aqueous based process typically yields a high pH of over 11.8, which adversely affects the surface property of the oxide based cathode material. In addition to the surface property modification, the high pH also adversely affects the dispersion of the active material, adhesion to the current collector and the micro structure of the aluminum current collector.
It would be beneficial to develop an electrode slurry having a sufficiently low pH that does not adversely affect the positive active material during the manufacturing of a positive electrode.
In one respect, the invention comprises a slurry for a positive electrode for an electrochemical cell, the slurry comprising: a positive active material consisting essentially of a lithium transition metal oxide, a water-soluble polymeric binder, and water, wherein the slurry has a pH in the range of 7 to 11 and would have a pH of at least 11.8 in the absence of the water-soluble polymeric binder.
In another respect, the invention comprises a method of forming an electrochemical cell electrode comprising the steps of:
a) forming a slurry comprising a positive active material, a water-soluble polymeric binder, and water, the slurry having a pH equal to or greater than 11.8 in the absence of the water-soluble polymeric binder;
b) providing a sufficient amount of the water-soluble binder in the slurry to reduce the pH of the slurry to between 7 and 11;
c) coating at least a portion of a current collector with the slurry; and
d) drying the slurry onto the current collector.
The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawing. For the purpose of illustrating the invention, there are shown in the drawing certain embodiments of the present invention. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:
In describing the embodiments of the invention illustrated in the drawings, specific terminology will be used for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, it being understood that each specific term includes all technical equivalents operating in similar manner to accomplish similar purpose. It is understood that the drawings are not drawn exactly to scale. In the drawings, similar reference numbers are used for designating similar elements throughout the several figures.
The following describes particular embodiments of the present invention. It should be understood, however, that the invention is not limited to the embodiments detailed herein. The invention pertains to the manufacture of electrodes used in Li-ion cells. Electrodes manufactured according to the present invention include a current collector that is coated with an electrode mix slurry that is coated onto the current collector and then dried.
Referring to
In an exemplary embodiment, positive electrode mix 110 consists essentially of a positive active material, a water-soluble polymeric binder, and, optionally, a conductive additive. All of the materials are mixed together in a water medium to make a slurry.
In an exemplary embodiment, the positive active material is selected from the group consisting of LiFePO4, LiNiCoAlO2, LiMn2O4, LiCoO2, LiNiCoMnO2, lithium transition metal oxides containing nickel and cobalt compounds, LiNiyCoxMzO, where M=Mn, Al, Sn, In, Ga or Ti and 0.15<x<0.5, 0.5<y<0.8 and 0<z<0.15, Li[Li(1-2y)/3/NiyMn(2-y)/3]O2, Li[Li(1−y)/3COyMn(2-2y)/3]O2 and Li[NiyCo1-2yMny]O2 where x=(2−y)/3 and 0<y<0.5, LiNiCoO2.MnO2, lithium rich compounds Li1+y(Ni1/3Co1/3Mn1/3)1−yO2, where x=0-0.33, y=(x/(2+x)) and xLi2MnO3(1−x)Li(NiCoMn)O2 and Li(1+y)(Ni0.5CO0.2Mn0.3)1−yO2, where x=0−0.33, y=(x/(2+x)), and LiMPO4, where M is one or more of the first row transition-metal cations selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, and combinations thereof.
In accordance with the present invention, the water-soluble polymeric binder functions as both a binder and as a pH reducing agent. Accordingly, it is important that the water-soluble polymeric binder provide a sufficient reduction in the pH of the slurry when provided in a proportion that will exhibit the desired binding characteristics, so as to provide a robust coating on positive electrode 112. It has been discovered that certain carboxylic acid-containing polymers are well-suited for this application. Examples of suitable carboxylic acid-containing polymers include polylactic acid (PLA), polyacrylic acid (PAA), polysuccinic acid, poly maleic acid and anhydride, poly furoic (pyromucic acid), poly fumaric acid, poly sorbic acid, poly linoleic acid, poly linolenic acid, poly glutamic acid, poly methacrylic acid, poly licanic acid, poly glycolic acid, poly aspartic acid, poly amic acid, poly formic acid, poly acetic acid, poly propoionic acid, poly butyric acid, poly sebacic acid, and copolymers thereof.
In an exemplary embodiment, the optional conductive additive may be carbon black, actylene black, or graphite.
The positive electrode mix 110 prepared by mixing between about 10 and about 90 percent (by weight) of positive active material, between about 0 and about 20 percent (by weight) of conductive additive, and between about 1 and about 10 percent (by weight) of the water-soluble polymeric binder. Water is added to form the slurry, which preferrably has a pH between about 7 and about 10. When LiMn2O4, LiFePO4 or any other lithium transition metal oxide is used as the positive active material, with the natural pH is ranging from 7 to 11, a LiOH solution is preferably added to the water-soluble polymeric binder in an amount sufficient to neutralize the pH prior to adding the positive active material.
When the slurry reaches the desired pH, the slurry is coated onto a current collector, such as, for example, aluminum foil, carbon coated aluminum foil, or nickel foil, and dried to form a positive electrode.
An electroactive negative electrode mix 120 includes a mix of negative active material, a water soluble binder, and, optionally, a conductive additive and/or a thickener. All of the materials are mixed together in a water medium to make a slurry.
In an exemplary embodiment, the negative active material is selected from the group consisting of graphite, hard carbon, silicon, silicon alloy, tin, tin alloy, and lithium titanate and any combination thereof. In an exemplary embodiment, the optional conductive additive material is selected from the group consisting of carbon black, actylene black and graphite. Exemplary binder material and the optional thickener are both disclosed above with respect to the positive electrode.
Negative active material mix 120 is prepared by mixing between about 10 and about 95 weight percent of active material, between about 0 and about 20 weight percent of conductive additive, and between about 1 and about 10 weight percent of binder polymer. Water is added to the mix to form a slurry. Between about 0 and about 10 weight percent of the thickener may be added to the slurry. The water soluble binder may optionally be used to make the negative electrode slurry without adding any thickening agent to control the viscosity.
The negative electrode slurry pH is between about 7 and about 10 and does not need polymeric acids to control the pH of the slurry. The slurry is coated on a copper current collector and dried to form a negative electrode.
After the positive and the negative electrodes are formed, the electrodes are used to form a cell 100, schematically illustrated in
The following example is provided for the purpose of illustrating a specific implementation of the invention and is not intended to limit the scope of the invention in any way. A positive electrode mix was prepared by mixing a powdered positive active material (90% by weight) consisting of LiNiCoAlO2, manufactured by Toda Corporation, a water-soluble binder (6% by weight) consisting of poly acrylic acid, purchased from Aldrich Chemicals (35% solid and a molecular weight of 250,000), and a conductive additive (4% by weight) consisting of Super P®, manufactured by Timcal Graphite & Carbon. The positive electrode mix was mixed with a water solution for about 2 hours to form the slurry. After mixing thoroughly, the pH of the slurry was between 10 and 11. The slurry was then coated on a Nickel current collector to form the positive electrode. The positive electrode was then cut into an appropriate size and dried in a vacuum oven until the moisture was below about 1000 ppm and most preferably below about 200 ppm.
Lithium half cells using positive electrodes made in accordance with the above-described example were built for capacity evaluation. The cells were then filled with electrolyte.
While the principles of the invention have been described above in connection with preferred embodiments, it is to be clearly understood that this description is made only by way of example and not as a limitation of the scope of the invention.
