FIELD OF THE INVENTION

[0001] This invention relates to the field of batteries, and more specifically, to an electrode assembly for a lithium-ion battery cell for use in electric vehicles and other devices.

BACKGROUND

[0002] Electric vehicles ("EVs") typically use rechargeable or secondary batteries to provide electric power to electric motors for driving the wheels of the vehicle. Hybrid electric vehicles ("HEVs") are constructed similarly to EVs but also include internal combustion engines to drive on-board generators to supplement battery power. Lithium-ion batteries are often used in EV and HEV applications. Depending on the kind of electrolyte, lithium secondary batteries are divided into liquid electrolyte batteries and polymer electrolyte batteries. Batteries using liquid electrolyte are called lithium-ion batteries and batteries using a polymer electrolyte are called lithium polymer or lithium-ion polymer batteries. In addition, lithium secondary batteries are classified according to the shape of their cases into cylindrical, prismatic, and pouch types.

[0003] A typical lithium-ion battery includes a cathode and an anode which are insulated by a separator interposed therebetween. These are wound or stacked into a cylindrical ("jelly-roll") or prismatic ("flat" or "rectangular") electrode assembly which is inserted into a metal can or pouch. Injection of an electrolyte into the can or pouch completes the battery for liquid electrolyte type batteries. The cathode or cathode electrode has an active-material coating layer, which is a mixture of an active-material, a conducting agent, and a binder material, on one or both surfaces of a cathode current collector. Similarly, the anode or anode electrode has an active-material coating layer, which is a mixture of an active-material, a conducting agent, and a binder material, on one or both surfaces of an anode current collector. Again, the cathode and anode electrodes are wound or stacked with a plurality of separators interposed therebetween. Typically, the length and width of the anode electrode are greater than the length and width of a cathode electrode to prevent lithium metal plating on the anode electrode .

[0004] The cathode and anode current collectors are provided with tabs, leads, or grids (hereafter referred to as "grids") for connection to the battery load. Because the cathode is smaller in area than the anode, insulating tape is applied to the cathode grid where it faces the anode active-material to prevent short-circuits in the event of separator failure. In particular, each end of the cathode current collector includes an uncoated portion where the active-material layer is not formed. Usually, the electrode grids are provided at these uncoated portions. The insulation tape is attached to the boundary between the active-material layer and the uncoated portion of the cathode electrode. The insulation tape attached to the boundaries between the active-material layers and uncoated portions prevents short circuits that could be caused by contact between the grid of one electrode and the other electrode in the event of failure of the separator. [0005] There are several problems with prior lithium-ion batteries. For example, the use of insulating tape increases the thickness of the electrode stack at the grid location. This increase in thickness at the grid location may cause an increase in overall battery pack size. In addition, this increase in thickness at the grid location causes unevenness of the cell surface which in turn causes difficulties with respect to thermal control of the cell due to resulting gaps between cells and cooling/heading plates. See FIG. 1. Similarly, for "jelly-roll" type stacks, the increase in thickness of the electrode assembly where the insulating tape is applied may function as a pinching point of the jelly-roll stack resulting in distortion or twisting of the electrode assembly. In addition, increases in thickness of the cell in the cathode grid area in stack-type electrode assemblies may result in local distortion of the electrodes. These distortions may cause lithium plating which may have a detrimental effect on the life and safety of the battery.

[0006] A need therefore exists for an improved lithium-ion battery cell for use in electric vehicles and other devices. Accordingly, a solution that addresses, at least in part, the above and other shortcomings is desired.

SUMMARY OF THE INVENTION

[0007] According to one aspect of the invention, there is provided an electrode assembly for a battery, comprising: an anode having an anode height H _A ; a cathode having cathode active-material deposited on at least one side of a cathode current collector, the cathode having a cathode height H _c that is less than the anode height H _A , the cathode current collector having a cathode grid formed thereto; an insulating film formed on at least a portion of the cathode grid adjacent to the cathode active-material, the insulating film being formed from a polymer solution deposited and dried on the cathode grid, the insulating film having an insulating film height H _G being at least a difference between the anode height H _a and the cathode height H _c ; and, a separator interposed between the cathode and the anode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Features and advantages of the embodiments of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

[0009] FIG. 1 is a cross sectional view illustrating an electrode stack for a lithium-ion battery in accordance with the prior art;

[0010] FIG. 2 is a cross sectional view illustrating a lithium- ion battery in accordance with an embodiment of the invention;

[0011] FIG. 3 is a top view illustrating an exemplary rectangular anode and cathode for the battery of FIG. 2 in accordance with an embodiment of the invention;

[0012] FIG. 4 is a side view illustrating the application of a polyimide precursor to a cathode electrode for the formation of a polyimide film in accordance with an embodiment of the invention; [0013] FIG. 5 is a graph illustrating a general thermogravimetric analysis of homopolymer PVDF;

[0014] FIG. 6 is a top view illustrating the placement of the insulting film on the cathode grid in accordance with an embodiment of the invention;

[0015] FIG. 7 is a graph illustrating the thermal stability of a battery based on the SAE J2464 Standard without a polyimide film formed on its cathode; and,

[0016] FIG. 8 is a graph illustrating the thermal stability of a battery based on the SAE J2464 Standard with a polyimide film formed on its cathode in accordance with an embodiment of the invention .

[0017] It will be noted that throughout the appended drawings, like features are identified by like reference numerals. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0018] In the following description, details are set forth to provide an understanding of the invention. In some instances, certain circuits, structures and techniques have not been described or shown in detail in order not to obscure the invention.

[0019] For reference, a lithium-ion (or "Li-ion") battery is a rechargeable battery in which lithium ions Lit move between the anode and cathode, creating electricity flow useful for electrical applications such as powering EVs and HEVs . In the discharge cycle, lithium ions ("Li+") in the anode (e.g., a graphite material) are deintercalated. Deintercalated lithium ions Li+ move through a porous separator membrane and are intercalated in the cathode (e.g., a lithium metal oxide). At the same time, electrons are released from the anode. This becomes electric current traveling to an outside electric circuit. When charging, lithium ions Li+ move from the cathode to the anode through the separator. Since this is a reversible chemical reaction, the battery can be recharged.

[0020] FIG. 1 is a cross sectional view illustrating an electrode stack 100 for a lithium-ion battery in accordance with the prior art. A typical lithium-ion battery includes a cathode 230 and an anode 210 which are insulated by a separator 220 interposed therebetween. The cathode and anode current collectors are provided with grids 250 for connection to the battery load. Because the cathode 230 is smaller in area than the anode 210, insulating tape 110 is applied to the cathode grid 250 where it faces the anode active-material to prevent short-circuits in the event of separator failure. In particular, each end of the cathode current collector includes an uncoated portion where the active-material layer is not formed. Usually, the electrode grids 250 are provided at these uncoated portions. The insulating tape 110 is attached to the boundary between the active-material layer and the uncoated portion of the cathode electrode. The insulation tape 110 attached to the boundaries between the active-material layers and uncoated portions prevents short circuits that could be caused by contact between the grid of one electrode and the other electrode in the event of failure of the separator 220.

[0021] Also as mentioned above, several problems with prior lithium-ion batteries have been identified. For example, the use of insulating tape 110 increases the thickness of the electrode stack 100 at the grid location 130. This increase in thickness at the grid location may cause an increase in overall battery pack size. In addition, this increase in thickness at the grid location 130 causes unevenness of the cell surface 120 which in turn causes difficulties with respect to thermal control of the cell due to resulting gaps between cells and cooling/heading plates. See FIG. 1. Similarly, for "jelly-roll" type stacks, the increase in thickness of the electrode assembly 100 where the insulating tape 110 is applied may function as a pinching point of the jelly-roll stack resulting in distortion or twisting of the electrode assembly. In addition, increases in thickness of the cell in the cathode grid area in stack-type electrode assemblies may result in local distortion of the electrodes. These distortions may cause lithium plating which may have a detrimental effect on the life and safety of the battery.

[0022] FIG. 2 is a cross sectional view illustrating a lithium- ion battery 260 in accordance with an embodiment of the invention. FIG. 3 is a top view illustrating an exemplary rectangular anode 210 and cathode 230 for the battery 260 of FIG. 2 in accordance with an embodiment of the invention. And, FIG. 4 is a side view illustrating the application of a polyimide precursor 241 to a cathode grid 250 for the formation of a polyimide film 240 in accordance with an embodiment of the invention. The battery 260 may be used in EVs, HEVs, or in a variety of other battery applications. The battery 260 includes a stack 200 of layers which form Li-ion cells. Each electrode assembly or cell (e.g., 201) includes an anode 210, a separator 220, and a cathode 230. The current collectors of the cathodes 230 and the current collectors of the anodes 210 are coupled together in parallel to form respective positive (+) and negative (-) terminals 255, 355 for the battery 260. To complete the battery 260, the stack 200 is housed in a container, package, or can 270 with electrolyte 280 for mounting in an EV, HEV, or other electronic device.

[0023] According to one embodiment, the anodes and cathodes 210, 230 of the stack 200 may be rectangular in overall shape or may have corners that are rectangular in shape or radius (see FIG. 3) . In addition, the stack 200 may be flat as shown in FIGS. 2 and 3 or it may be of the "jelly-roll" type.

[0024] Each cathode 230 may be formed by depositing and drying a slurry, being a mixture of active-material, conductive carbons, binder, and solvent, to produce a coating of active- material 231, 232 on at least one side 291, 292 of a cathode current collector 290. The current collector 290 may be shaped or formed to include a cathode grid 250 by stamping, punching, or tab notching and cutting. Each anode 210 may be formed in a similar manner having an anode grid 350.

[0025] According to one embodiment, the battery 260 may be based on lithium metal oxide chemistry. In particular, according to one embodiment, the cathode current collector 290 may be formed from aluminum, the cathode active material 231, 232 may be formed from a lithium nickel manganese cobalt oxide ("NMC") powder, the separator 220 may be formed of a micro- porous PE membrane, the anode active-material (not shown) may be formed from an artificial graphite powder, and the anode current collector 350 may be formed from copper. The stack 200 may be immersed in an electrolyte 280 within the container 270 composed of a lithium salt and organic solvents which is absorbed for the most part into the active-materials of the cathode 230, anode 210, and separator 220 which are typically porous in nature.

[0026] For reference, in addition to use in EVs and HEVs, Li-ion batteries are common in consumer electronics. Handheld electronics mostly use Li-ion batteries based on lithium cobalt oxide ("LCO") , which offers high energy density, but have well-known safety concerns, especially when damaged. Lithium iron phosphate ("LFP") , lithium manganese oxide ("LMO") , and lithium nickel manganese cobalt oxide ( "NMC" ) offer lower energy density, but longer lives and inherent safety. These chemistries are widely used for electric tools, medical equipment, and other applications. NMC in particular is a generally used for automotive applications such as in EVs and HEVs. Lithium nickel cobalt aluminum oxide ("NCA") is a specialty design aimed at particular niche applications.

[0027] Referring to FIG. 4, rather than tape 110, the grid 250 of the cathode 230 has an insulating film (e.g., a polyimide film) 240 formed thereon to insulate the grid 250 of the cathode 230 from the anode 210. In FIG. 4, the insulating film 240 is shown as being formed on one side 251, 231 of the cathode 230. However, of course, it may also be formed on the other side 252, 232 of the cathode 230. According to one embodiment, the insulating film 240 may be a polyimide film. Polyimide films, which are formed by the polymerization of a polyimide polymer precursor (poly(amic acid)), have good thermal and chemical stability when compared with prior art insulating tapes 110 which are typically made from polypropylene. Polyimide films also have good electrical insulating characteristics. As shown in FIG. 4, a polymer solution (e.g., a polyimide polymer precursor solution) 241 is applied 410 to at least one side 251 of the cathode grid 250 at the point 430 where the cathode active-material 231 coating begins. The polymer solution 241 may be applied by nozzle, spray, or brush to the cathode current collector 290 when it is in strip roll form prior to stamping, punching, or tab notching and cutting. Alternatively, it may be applied after stamping. As a further alternative, the polymer solution 241 and cathode active-material slurry 231 may be applied to the cathode current collector 290 and dried at the same time. According to one embodiment, after application 410, the polymer solution 241 levels off 420 automatically due to gravity. This leveling reduces the thickness increase of the cathode 230 at the grid 250 over prior art tape designs. In addition, electrode distortion at the grid 250 is also reduced over prior art tape designs. The polymer solution 241 is dried or cured (e.g., by heating) to produce the final insulting film 240 on the cathode grid 250.

[0028] According to one embodiment, the polyimide film 240 may be formed on the grid 250 using a polyimide resin soluble in solvents (e.g., N-Methyl-2-pyrrolidone (NMP) or

Dimethylacetamide (DMAc) ) having a polymer concentration of 20% (e.g., RIKACOAT™ PN-20, SN-20, or EN-20, manufactured by New Japan Chemical Co., Ltd.). In this embodiment, the imidization (polymerization) of the precursor is no longer required. Drying the solvent is enough to form a robust polyimide film with good heat resistance, chemical resistance, and electric insulation properties. [0029] A sample polyimide film 240 using RIKACOAT™ was prepared as follows: 100 g of PN-20 polymer solution was diluted by adding 100 g of NMP solvent for adjustment of solution viscosity and this diluted polymer solution was coated with a brush on the cathode grid 250 and dried at 70 C for 10 minutes followed by subsequent drying at 100 C for 10 minutes to dry the solvent out completely.

[0030] According to another embodiment, the polymer solution 241 may be or include U-Varnish™ which is available from UBE America Inc. U-Varnish™ is a solution of polyimide precursor poly (amic acid) . It is coated on a substrate and heated to a high temperature to vaporize the solvent, accelerate the imidization process, and make a coated film with good heat resistance, chemical resistance, and electric insulation properties. The solvent used in U-Varnish™ is NMP (N- ethyl- 2 -Pyrrolidone ) . In some cases, dilution of the polymer solution is required to optimize the coating process. To dilute the polymer solution 241 , N-Methyl-2-pyrrolidone (NMP) or Dimethylacetamide (DMAc) may be used.

[0031] According to another embodiment, the polymer solution 241 may include a dye (such as Dianthrone) to enable a deposition quality check by an ultra-violet ("UV") or visible light detector. Such quality checks are important for improving battery reliability. [0032] With respect using U-Varnish™ as the polymer solution 241, it may be cured by heating in a hot environmental oven as follows (best mode) : (1) 120 C for 20 minutes; (2) 150 C for 5 minutes; (3) 200 C for 5 minutes; and, (4) 250 C for 5 minutes. Infrared ("IR") heating may be used to localize thermal exposure of the electrode 230 from step (3) as will be discussed further below in the context of FIG. 5. Accordingly to one embodiment, the polymer solution 241 is dried above 150 C. [0033] Again with respect using U-Varnish™ as the polymer solution 241, several samples were prepared to determine the best curing conditions. Sample 1 was prepared by casting polymer solution and drying at 120 C for 20 minutes. Sample 2 was prepared by casting polymer solution and drying at 120 C for 20 minutes and at 150 C for 5 minutes. Sample 3 was prepared by casting polymer solution and drying at 120 C for 20 minutes, at 150 C for 5 minutes, and at 200 C for 5 minutes. Sample 4 was prepared by casting polymer solution and drying at 120 C for 20 minutes, at 150 C for 5 minutes, at 200 C for 5 minutes, and at 250 C for 5 minutes. The heating for each sample was performed in a hot environmental oven. Test results have shown that the curing or drying temperature is important to obtaining desirable film properties. With respect to film robustness (i.e., adhesion to foil or collector 290), the following properties were observed: under 120 C (Sample 1 - poor); 120 C to 150 C (Sample 2 - moderate); 150 C to 200 C (Sample 3 - good) ; and, above 200 C (Sample 4 - good) . With respect to electrical isolation (i.e., electrical resistance on the film surface with greater than 50 ΜΩ representing a good characteristic) , the following properties were observed: under 120 C (Sample 1 - poor) ; 120 C to 150 C (Sample 2 - moderate); 150 C to 200 C (Sample 3 - good); and, above 200 C (Sample 4 - good) . These results were observed after exposure at high temperature (85 C) with an electrolyte present for one week .

[0034] According to alternate embodiments, a polymer solution 241 may be cast and dried to form the insulating film 240. For example, a PVDF dispersed solution may be cast and dried to form a PVDF polymer film. In addition, UV or heat curable polymer solutions may be cast and dried to form polymer films. For example, a silicone polymer solution, an epoxy polymer solution, an acrylic polymer solution, a cyanoacrylate polymer solution, or a rubber-based polymer solution may be used to form a silicone polymer film, an epoxy polymer film, an acrylic polymer film, a cyanoacrylate polymer film, or a rubber-based polymer film, respectively.

[0035] FIG. 5 is a graph illustrating a general thermogravimetric analysis of homopolymer PVDF (polyvinylidene fluoride) . PVDF is the standard binder material used in the production of electrodes for lithium-ion batteries. The general binder PVDF for the cathode 230 may be stable up to approximately 400 C. In general, if the whole cathode 230 is exposed to a higher temperature for a longer time to improve the quality of polymide film formation, local (partial) crystallization of the binder may begin. Crystallization of the binder makes the electrode 230 brittle. This must be taken into consideration with respect to the drying or curing of the polymer solution 241. As such, localized heat exposure (e.g., IR heating) may be beneficial.

[0036] FIG. 6 is a top view illustrating the placement of the insulting film 240 on the cathode grid 250 in accordance with an embodiment of the invention. FIG. 6 shows the stacking of the anode 210 and cathode 230 shown unstacked in FIG. 3. In FIG. 6, the cathode 230 has a height H _c , the anode 210 has a height H _A (with H _A being greater than H _c ) , the cathode grid 250 has a height H _Gr i _d , and the insulting film 240 has a height H _G (with H _Grid being greater than or equal to H _G ) . For ease of manufacture, the insulting film 240 may be disposed on the entire grid 250 (i.e., H _G = H _Gr i _d ) . However, tests have shown that this is undesirable as it results in increased battery or cell impedance as well as increased vibration test failure rates as the film 240 may interfere with the welding of the grids to form the battery terminal 255. That is, the setting of H _G = H _Gr i _d results in poor welding quality and increased battery or cell impedance. As such, it is desirable to maintain a portion 620 of the grid 250 (i.e., H _G < H _Grid ) uncoated by insulting film 240 for welding purposes. In particular, in general, as the size of the cathode 230 is generally smaller than that of the anode 210 (i.e., H _c < H _A ) , a portion 610 of the cathode grid 250 is exposed to the anode active material which is problematic as described above. Therefore, in general, to meet minimum requirements, the insulating film 240 should have a height H _G that is greater than or equal to the difference between the height of the anode H _¾ and the height of the cathode H _c (i.e., H _G ≥ H _A -H _C ) . Theoretically, the insulting film 240 may cover a portion of the grid 250 as long as it does not cover the entire grid (i.e., H _G < H _Grid ) . For improved cell safety, it has been found that the height of the insulting film H _G should be set to at least the difference between the height of the anode H _A and the height of the cathode H _c (i.e., H _G = H _A -H _C ) . According to one embodiment, the insulating film height H _G may be set to double the difference between the anode height H _A and the cathode height H _c (i.e., H _G = 2* (H _A -H _C ) ) . Manufacturing tolerances should also be taken into account in setting these heights.

[0037] FIG. 7 is a graph illustrating the thermal stability of a battery based on the SAE J2464 Standard without a polyimide film formed on its cathodes. And, FIG. 8 is a graph illustrating the thermal stability of a battery based on the SAE J2464 Standard with a polyimide film 240 formed on its cathodes 230 in accordance with an embodiment of the invention. The SAE J2464 Standard (i.e., Doughty, D. , "EV and HEV Rechargeable Energy Storage System (RESS) Safety and Abuse Testing Procedure", SAE Technical Paper 2010-01-1077, 2010) is incorporated herein by reference. FIGS. 7 and 8 show that preventing cathode collector 290 exposure by adding a polyimide film 240 increases battery safety. A comparison of the timing of thermal runaway events shows an improvement in battery cell safety. With respect to temperature to thermal run-away, the following values were observed for a battery with a polyimide film formed on its cathodes and for a battery without a polyimide film formed on its cathodes, respectively: 135 C and 125 C. With respect to time to thermal run-away, the following values were observed for a battery with a polyimide film disposed on its cathodes and for a battery without a polyimide film disposed on its cathodes, respectively: 516 minutes and 434 minutes. And, with respect to voltage before thermal run-away, the following values were observed for a battery with a polyimide film disposed on its cathodes and for a battery without a polyimide film disposed on its cathodes, respectively: 4.08 V and 4.09 V. These results were observed for a battery having cells prepared with PE separators 220 available from W-Scope Korea Corporation. [0038] Thus, according to one embodiment, there is provided an electrode assembly 201 for a battery 260, comprising: an anode 210 having an anode height H _A ; a cathode 230 having cathode active-material 231, 232 deposited on at least one side 291, 292 of a cathode current collector 290, the cathode 230 having a cathode height H _c that is less than the anode height H _A , the cathode collector 290 having a cathode grid 250 formed thereto; an insulating film 240 formed on at least a portion 610 of the cathode grid 250 adjacent to the cathode active- material 231, the insulating film 240 being formed from a polymer solution 241 deposited and dried on the cathode grid 250 (e.g., on the portion 610), the insulating film 240 having an insulating film height H _G being at least a difference between the anode height H _A and the cathode height H _c ; and, a separator 220 interposed between the cathode 230 and the anode 210.

[0039] In the above electrode assembly 201, the polymer solution 241 may be a polyimide polymer precursor solution or a soluble polyimide polymer solution and the insulating film 240 may be a polyimide film. The polymer solution 241 may be dried in a hot environmental oven or cured by exposure to heat to form the insulating film 240. The polymer solution 241 may be a PVDF polymer dissolved in a solvent and the insulating film 240 may be a PVDF film. The polymer solution 241 may be a silicone polymer solution and the insulating film 240 may be a silicone polymer film. The polymer solution 241 may be an epoxy polymer solution and the insulating film 240 may be an epoxy polymer film. The polymer solution 241 may be an acrylic polymer solution and the insulating film 240 may be an acrylic polymer film. The polymer solution 241 may be a cyanoacrylate polymer solution and the insulating film 240 may be a cyanoacrylate polymer film. The polymer solution 241 may be a rubber-based polymer solution and the insulating film 240 may be a rubber-based polymer film. The polymer solution 241 may include a dye detectable by UV or visible light. The cathode active-material 231, 232 as a slurry and the polymer solution 241 may be deposited on the cathode current collector 290 simultaneously. The polymer solution 241 on the cathode grid 250 may be deposited after forming the cathode grid 250. The polymer solution 241 may be leveled on the cathode grid 250 (e.g., on the portion 610) by gravity. The cathode grid 250 may be formed by at least one of stamping, punching, and tab notching and cutting. The insulting film height H _G may be less than an overall height of the cathode grid H _Grid . The insulating film height H _G may be double the difference between the anode height H _¾ and the cathode height H _c . The corners of the anode 210 and the cathode 230 may be rectangular in shape or radius. The anode 210, the cathode 230, and the separator 220 may be at least one of stacked and rolled. The battery 260 may be a lithium-ion battery. The battery 260 may have at least one electrode assembly 201 and a container 270 for housing the at least one electrode assembly 201 and an electrolyte 280. The battery 260 may be used for powering an electric motor of an electric vehicle ("EV") . The battery 260 may be used for powering an electric motor of a hybrid electric vehicle ("HEV") . And, the battery 260 may be used for powering an electronic device.

[0040] The above embodiments may contribute to an improved Li- ion battery 260 and may provide one or more advantages. For example, thickness and distortion of the electrode assembly 201 at the grid 250 may be reduced by using an insulating film 240 formed from a polymer solution 241 rather than insulating tape 110 as in prior designs. [0041] The embodiments of the invention described above are intended to be exemplary only. Those skilled in this art will understand that various modifications of detail may be made to these embodiments, all of which come within the scope of the invention .