Abstract:
A battery having an electrode mix comprising water, a conductive additive and a poly(acrylonitrile-co-acrylamide) co-polymer as a binder. The use of poly(acrylonitrile-co-acrylamide) co-polymer eliminates the need for a separate thickener.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/701,001, filed Feb. 5, 2010, now issued as U.S. Pat. No. 8,076,026. 
    
    
     FIELD OF INVENTION 
     The present invention relates to a water soluble binder for use in a rechargeable lithium ion battery and the battery in which the binder is used. 
     BACKGROUND 
     Rechargeable batteries use polymer binders to hold together the active particulate material and adhere this particulate material to the current collector in the fabrication of electrodes. The binder is generally comprised of one or more polymers. The binders commonly used in commercial li-ion batteries are polyvinyledene fluoride (PVDF), ethylene-propylene and a diene (EPDM). These polymers are generally insoluble in water and, thus are dissolved in an organic solvent such as N-methyl pyrrolidone (NMP). The organic solvent additionally serves as a dispersion medium for the active materials. Some disadvantages of using organic solvents are that they have relatively high cost, can possess negative environmental impacts, and pose disposal issues. Further, PVDF is highly unstable and tends to break down at high temperatures. 
     Known water soluble binders, such as carboxy methyl cellulose (CMC), require a thickening agent to control the viscosity of the binder. Further, they exhibit only marginal adhesion capability. Polytetrafluoroethylene (PTFE) and styrene butadiene rubber (SBR) based water soluble binders also exhibit poor adhesion and do not exhibit good cycle life. Further, SBR binders exhibit high expandability and undesirable agglomeration characteristics resulting in poor dispersion, poor performance, and high electrode resistance. 
     Accordingly, there is a need for a water soluble binder in rechargeable lithium batteries. This water soluble binder should exhibit good adhesion in the absence of thickening or wetting agents, low resistance and good chemical and electrochemical stability. 
     SUMMARY 
     Briefly, the present invention provides a water-soluble binder comprising a poly(acrylonitrile-co-acrylamide) polymer and water. 
     The invention further includes a battery comprising a positive electrode mix which contains a positive electrode active material, a water soluble binder comprising a poly (acrylonitrile-co-acrylamide) polymer and water, and a conductive additive. The battery also includes a negative electrode mix comprising a negative electrode active material, a water soluble binder comprising a poly(acrylonitrile-co-acrylamide) polymer and water, and a conductive additive. The battery further includes an electrolyte. 
     The present invention also includes a battery comprising a positive electrode mix which contains a positive active material selected from the group consisting of LiCoO 2 , LiNiO 2 , LiNi 1/3 Co 1/3 Mn 1/3 O 2 , LiNi 0.8 Co 0.15 Al 0.05 O 2 , Li 1+x Ni 1/3 Co 1/3 Mn 1/3 O 2 , LiMn 2 O 4  LiFePO 4  coated with graphite and/or carbon and Li 2 Mn 2 O 4 . The positive electrode mix also includes a water soluble binder comprising a poly(acrylonitrile-co-acrylamide) polymer and water, as well as a conductive additive or additives selected from the group consisting of carbon black, acetylene black, carbon fibers, coke, high surface area carbon and graphite. The negative electrode mix comprises a negative active material selected from the group consisting of graphite, hard carbon, silicon, tin and lithium titanate. The negative electrode mix also includes, a binder comprising a poly(acrylonitrile-co-acrylamide) polymer and water, as well as the conductive additive or additives. The battery further comprises an electrolyte comprised of a lithium salt in cyclic and linear carbonates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is 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 shown. In the drawings: 
         FIG. 1  is a schematic view of a battery formed in a jellyroll configuration according to an exemplary embodiment of the present invention; 
         FIG. 1A  is a schematic view of the battery of  FIG. 1  with the electrolyte; 
         FIG. 2  is a cross-sectional representation of a prismatic electrochemical cell according to an exemplary embodiment of the present invention; 
         FIG. 3  is a schematic representation of a positive electrode, a separator and a negative electrode-bi-cell configuration of the exemplary embodiment illustrated in  FIG. 1 ; 
         FIG. 4A  is a charge/discharge curve for a LiFePO 4 /Graphite Li-ion cell according to an exemplary embodiment of the present invention; and 
         FIG. 4B  is a cycle life curve for the LiFePO 4 /Graphite Li-ion cell whose charge/discharge curve is illustrated in  FIG. 4A ; 
         FIG. 5A  is a charge/discharge curve for a LiNi 1/3 O 1/3 Mn 1/3 O 2 /Graphite Li-ion cell according to another exemplary embodiment of the present invention; and 
         FIG. 5B  is a cycle life curve for LiNi 1/3 Co 1/3 Mn 1/3 O 2 /Graphite Li-ion cell whose charge/discharge curve is illustrated in  FIG. 5A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
     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. Generally, the following disclosure refers to lithium ion batteries and a water soluble binder for use in lithium ion batteries. 
     Referring to  FIGS. 1 and 1A , a rechargeable battery  100  according to an exemplary embodiment of the present invention includes a positive electrode  112  formed from a positive electrode mix  110 , a negative electrode  122  formed from a negative electrode mix  120 , and an electrolyte  130 . While  FIG. 1  illustrates battery  100  formed in a “jellyroll” configuration, those skilled in the art will recognize that other formations, such as, for example, a prismatic configuration, which is illustrated in  FIG. 2 , may also be used within the teaching of the present invention. 
     Positive electrode mix  110  exhibits a reduction potential and negative electrode mix  120  has an oxidation potential. Electrolyte  130  transfers ions between positive electrode mix  110  and negative electrode mix  120  within battery  100 . Separator  140  separates positive electrode mix  110  from negative electrode mix  120 . 
     Positive electrode mix  110  includes a positive electrode active material selected from the group consisting of LiCoO 2 , LiNiO 2 , LiNi 1/3 Co 1/3 Mn 1/3 O 2 , LiNi 0.8 Co 0.15 Al 0.05 O 2 , Li 1+x Ni 1/3 Co 1/3 Mn 1/3 O 2 , LiMn 2 O 4 , LiFePO 4  coated with at least one of graphite and carbon and Li 2 Mn 2 O 4 , and LiMPO 4  where M is one or more of the first row transition-metal cations selected from the group consisting of Fe, Mn, Ni, Ti, Co and combinations thereof. In an exemplary embodiment, the positive electrode active material is about 80-95 percent (by weight) of the positive electrode mix  110 . 
     Positive electrode mix  110  further includes a conductive additive or additives selected from the group consisting of carbon black, acetylene black, carbon fibers, coke, high surface area carbon, graphite and combinations thereof. In an exemplary embodiment, the conductive additive material is about 1-10 percent (by weight) of the positive electrode mix  110 . 
     Positive electrode mix  110  also includes a water soluble binder polymer comprising a poly(acrylonitrile-co-acrylamide) polymer and water. The binder is used to bind the positive electrode active material and the conductive additive together to form slurry. The slurry is coated on an aluminum current collector or a carbon coated aluminum current collector to form positive electrode  112 . The slurry pH ranges between about 7 and about 9 for a LiFePO 4  based system and for other positive electrode mixes, the pH ranges between about 10 and about 12. 
     An exemplary poly(acrylonitrile-co-acrylamide) polymer and water binder has the chemical formula: 
     
       
                 
         
             
             
         
      
     
     In another exemplary embodiment, the mole ratio of acrylonitrile units to acrylamide units (m:n) is between about 3:1 and 1:1. In another exemplary embodiment, the mole ratio of acrylonitrile units to acrylamide units (m:n) is about 2:1. In an exemplary embodiment, an average molecular weight of the polymer is between about 10,000 and 300,000. In another exemplary embodiment, the average molecular weight of the polymer is between about 100,000 and 200,000. In an exemplary embodiment, the water soluble binder is about 1-10 percent (by weight) of positive electrode mix  110 . 
     The water soluble binder is used to make the slurry in the absence of a thickening agent or any external reagent to control its viscosity. An exemplary binder has a viscosity ranging from 6000-10000 cp. The water soluble binder is also provided in the absence of a wetting agent or any other additives to improve the active material and conductive additive dispersion. 
     Negative electrode mix  120  comprises a negative electrode active additive or additives material selected from the group consisting of graphite, hard carbon, silicon, tin and lithium titanate. In an exemplary embodiment, the negative electrode active material is about 80-95 percent (by weight) of the negative electrode mix  120 . 
     Negative electrode mix  120  further includes a conductive additive selected from the group consisting of carbon black, acetylene black, carbon fibers, coke, high surface area carbon, graphite and combinations thereof. In an exemplary embodiment, the conductive additive material is about 0-10 percent (by weight) of the negative electrode mix  120 . 
     Negative electrode mix  120  further comprises the water soluble binder as described above with respect to the positive electrode mix  110 . In an exemplary embodiment, the water soluble binder is about 1-10 percent (by weight) of the negative electrode mix  120 . 
     An exemplary electrolyte  130  may be comprised of lithium salts such as LiBF 4 , LiPF 6 , LiBOB, LiTFSI or LiFSI or mixtures thereof in cyclic and linear carbonates. 
     To form battery  100 , positive electrode mix  110  is coated on an aluminum current collector or a carbon coated aluminum current collector  111 , forming a positive electrode  112 . The negative electrode mix  120  is coated on a copper current collector  121 , forming negative electrode  122 . Positive electrode  112  and negative electrode  122  are each then compressed or calendared for specific thickness. Electrodes  112 ,  122  are stacked as shown in  FIG. 3 . The stack is dried in a vacuum oven until the moisture is below 2000 ppm, and most preferably below 200 ppm. The electrode stack may be inserted into a polyethylene or polypropylene cell housing  150 , shown in  FIG. 2 , and filled with electrolyte  130 , forming battery  100 . Battery  100  is then charged and discharged to complete the forming process. 
     EXAMPLES 
     The following examples are given purely as an illustration and should not be interpreted as constituting any kind of limitation to the invention. 
     Positive electrode mix  110  was prepared first by dissolving poly (acrylonitrile-co-acrylamide) polymer binder in water. The ratio of binder to water ranges from 15 to 20%. A positive active powder (carbon and/or graphite coated LiFePO 4  manufactured by Phostech Lithium, Canada and LiNi 1/3 Co 1/3 Mn 1/3 O 2  manufactured by 3M corporation, USA) with an appropriate amount of conductive additive such as for example Super P®, manufactured by Timcal Graphite &amp; Carbon located in Switzerland, was mixed with the binder in water solution for about 2 hrs. The pH of the slurry for LiFePO 4  positive mix was between about 7 and about 9 and for LiNi 1/3 Co 1/3 Mn 1/3 O 2 , the pH range was between about 10 and 12. The homogeneously mixed slurry was then coated on the aluminum current collector or a carbon coated aluminum current collector  111  to form positive electrode  112 . Positive electrode  112  was 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. 
     Negative electrode mix  120  was prepared first by dissolving poly (acrylonitrile-co-acrylamide) polymer binder in water. The ratio of binder to water ranged from 15% to 20%. A negative active powder (graphite) with an appropriate amount of conductive additive (Super P®) was mixed with the binder in water solution and mixed for about 2 hrs. The pH of the slurry was between about 7 and 9. The homogeneously mixed slurry was then coated on to copper current collector  121  to form negative electrode  122 . Negative electrode  122  was 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. 
     The cells were built as described in  FIGS. 1-3 . The cells were then filled with electrolyte  130 . The Li-ion cells were in discharged state and had a potential of a few millivolts.  FIGS. 4A and 5A  each illustrate a charge/discharge curve for a LiFePO 4  and LiNi 1/3 Co 1/3 Mn 1/3 O 2  cathode and a graphite anode, respectively.  FIGS. 4B and 5B  each illustrate a cycle life curve for the LiFePO 4  and LiNi 1/3 Co 1/3 Mn 1/3 O 2  cathode and a graphite anode, respectively. 
     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.