Abstract:
A lithium-ion battery includes a plurality of generally planar positive and negative electrodes arranged in alternating fashion to form an electrode stack. Each of the electrodes includes a current collector having two opposed surfaces and an active material provided on at least one of the two opposed surfaces. The active material of the negative electrodes has a potential that is greater than 0.2 volts versus a reference electrode. The area of the current collectors of the positive electrodes covered by active material is not larger than the area of the current collectors of the negative electrodes covered by active material.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This application is a Continuation-In-Part of and incorporates herein by reference each of the following patent applications in their entirety: U.S. patent application Ser. No. 10/978,712, filed Oct. 29, 2004; U.S. patent application Ser. No. 10/979,043, filed Oct. 29, 2004; U.S. patent application Ser. No. 10/979,040, filed Oct. 29, 2004; U.S. patent application Ser. No. 10/976,508, filed Oct. 29, 2004; and U.S. patent application Ser. No. 11/260,853, filed Oct. 27, 2005, which claims priority from U.S. Patent Provisional Application No. 60/624,075, filed Oct. 29, 2004 and U.S. Provisional Patent Application No. 60/647,292, filed Jan. 26, 2005. 
     
    
     BACKGROUND  
       [0002]     The present invention relates generally to the field of lithium-ion batteries and methods of making such batteries.  
         [0003]     Lithium-ion batteries or cells include one or more positive electrodes, one or more negative electrodes, and an electrolyte provided within a case or housing. Separators made from a porous polymer or other suitable material may also be provided intermediate or between the positive and negative electrodes to prevent direct contact between adjacent electrodes. The positive electrode includes a current collector (e.g., aluminum such as an aluminum foil) having an active material provided thereon (e.g., LiCoO 2 ), and the negative electrode includes a current collector (e.g., copper such as a copper foil) having an active material (e.g., a carbonaceous material such as graphite) provided thereon. The active materials for the positive and negative electrodes may be provided on one or both sides of the current collectors.  
         [0004]     During charging and discharging of the battery, lithium ions move between the positive electrode and the negative electrode. For example, lithium ions flow from the negative electrode to the positive electrode during discharging of the battery, and in the opposite direction during charging.  
         [0005]     One issue associated with conventional secondary (i.e., rechargeable) lithium-ion batteries is lithium plating. Lithium plating, which is a well-known phenomenon characterized by a buildup of lithium metal on the negative electrode, may occur when the potential of the negative electrode drops to 0.0 volts (V) versus Li/Li + . Lithium plating can result in decreased battery capacity, since the buildup of lithium on the negative electrode decreases the amount of cyclable lithium available in the battery. Internal battery shorts may also result from lithium plating, for example, when the buildup of lithium becomes so significant that dendrites form on the negative electrode and extend to and make contact with an adjacent positive electrode.  
         [0006]     The issue of lithium plating is particularly problematic in batteries having negative active materials that exhibit relatively low potentials versus Li/Li + . Carbon-based active materials commonly used in lithium secondary batteries, for example, have an average potential of approximately 0.1 V versus Li/Li + .  
         [0007]     To reduce the likelihood of lithium plating, battery manufacturers typically provide excess negative electrode capacity to balance the positive electrode capacity, particularly at relatively high-current regions of the negative electrodes (e.g., edges of the electrodes). The additional negative active material provides additional intercalation sites for the cyclable lithium originating with the positive active material.  
         [0008]     One method of providing additional negative active material is to utilize negative electrodes that have different (e.g., larger) physical dimensions than the positive electrodes. Depending on the type of battery configuration involved (e.g., wound electrode, flat plate electrode, Z-fold electrode, etc.), the manner in which the dimensions of the negative electrode differ from that of the positive electrode may vary.  
         [0009]     For example, in a flat plate lithium battery that includes a plurality of positive and negative electrodes, lithium plating may occur at relatively high current density regions (e.g., near the four edges of the negative electrodes). To compensate for this tendency, conventional batteries are designed such that the negative electrodes (or the active material provided thereon) extend beyond the ends of the positive electrodes (or the active material provided thereon) in all directions by an amount sufficient to compensate for variations in the winding/assembly process (e.g., by an amount up to 1 millimeter or more).  
         [0010]      FIGS. 1-2  illustrate a portion of a battery  100  that includes a plurality of negative electrodes  110  (which include a current collector  112  and active material  114  provided thereon), a plurality of positive electrodes  120  (which include a current collector  122  and active material  124  provided thereon), and separators  130  and electrolyte between the positive and negative electrodes. Although shown as being coextensive with the active material  114  in  FIG. 1 , the separator(s) may be provided such that it extends beyond the edge of the adjacent electrode to prevent shorting between adjacent electrodes (i.e., the separator may extend above and below the edge of the active material  114  shown in  FIG. 1 ). The portion of the active material  114  that extends beyond the positive electrodes  120  is thus used to compensate for the tendency to plate lithium. The extent to which the negative electrodes extend beyond the edges of the positive electrodes (shown by arrows A-A and B-B in  FIG. 2 ) may be up to one millimeter (mm) or more.  
         [0011]     A similar design rule is used in the context of wound (e.g., jellyroll style) lithium batteries, as shown in  FIGS. 3-4 . As illustrated, a wound electrode battery  200  includes a negative electrode  210  (which includes a current collector  212  and active material  214  provided thereon) and a positive electrode  220  (which includes a current collector  222  and active material  224  provided thereon). Separators  230  are provided between the positive and negative electrodes. As illustrated in  FIGS. 3-4 , the top and bottom edges of the negative electrode  210  extend beyond the edges of the positive electrode  220  by a distance C that may be up to one millimeter or more. Additionally, because a leading edge  211  of the negative electrode is also a potential location where plating might occur, the leading edge  211  of the negative electrode  210  extends a distance D (e.g., up to one millimeter or more) beyond a leading edge  221  of the positive electrode  220 .  
         [0012]     In cases where an accordion-style fold (also referred to as a “Z fold” or a “zigzag” fold) is used for the electrodes, multiple design rules may be employed to mitigate lithium plating. For example, the top and bottom edges of the negative electrodes may extend beyond those of the positive electrodes in a manner similar to that described above with respect to wound electrodes. Accordian-style folding of the electrodes also provides an additional area of potential concern, however.  FIG. 5  illustrates a battery  300  having an electrode set  302  that is folded accordion-style. The electrode set  302  includes a negative electrode  310  (which includes a current collector  312  and active material  314  provided thereon), a separator  330 , and a positive electrode  320  (which includes a current collector  322  and active material  324  provided thereon). Because of the manner in which the electrode set  302  is folded, at certain localized areas within the battery  300 , there will be a greater amount of positive active material than negative active material. One such area is shown in  FIG. 5 , where the positive electrode  320  is outside the negative electrode  310  at a fold. Because there is more positive active material in this region than negative active material (owing to the larger radius, and hence greater surface area, of the positive electrode in this area), the negative active material would be unable to take in all of the cyclable lithium provided by the positive active material in this area unless steps are taken to mitigate against such a circumstance. As shown in  FIG. 5 , one possible solution is to mask the positive current collector  322  such that no positive active material  324  is provided in this region (denoted with reference numeral  326 ). One difficulty with such a solution is that it is relatively complicated to accurately mask regions on the electrodes to ensure that they will line up appropriately at locations of the folds.  
         [0013]     Another potential solution for accordion-style folded electrodes is shown in  FIG. 6 , which shows a portion of a battery  400  having a folded negative electrode  410  (which includes a current collector  412  and active material  414  provided thereon) with positive electrode plates  420  (which includes a current collector  422  and active material  424  provided thereon). By providing positive plates  420  interspersed within the folded electrode arrangement, there is additional negative active material near each of the rounded folded portions of the negative electrode  410  to take in any excess lithium from the positive active material  424 .  
         [0014]     Each of the various configurations described above suffers from various drawbacks. For example, each of the above-described configurations utilizes a larger negative electrode as compared to the positive electrode (with an associated amount of additional active material provided thereon), which increases the overall size of the battery and results in an increased materials cost. Additionally, each of these configurations requires careful alignment of the various components to ensure that the negative active material is properly positioned such that it will absorb excess lithium from the positive material, which complicates manufacturing processes and introduces increased labor and equipment costs. In certain cases, negative electrodes may be made even larger than are necessary to absorb the excess lithium in order to compensate for potential variability in manufacturing processes which may result in slight misalignments of the electrodes. Further still, the use of excess electrode area and active material results in batteries having lower energy density than might otherwise be obtained, since the excess electrode area does not fully contribute to the capacity of the battery.  
       SUMMARY  
       [0015]     An exemplary embodiment relates to a lithium-ion battery that includes a plurality of generally planar positive and negative electrodes arranged in alternating fashion to form an electrode stack. Each of the electrodes includes a current collector having two opposed surfaces and an active material provided on at least one of the two opposed surfaces. The active material of the negative electrodes has a potential that is greater than 0.2 volts versus a reference electrode. The area of the current collectors of the positive electrodes covered by active material is not larger than the area of the current collectors of the negative electrodes covered by active material.  
         [0016]     Another exemplary embodiment relates to a lithium-ion battery that includes an element comprising a plurality of alternating generally planar positive and negative electrodes each having a plurality of edges. The negative electrodes comprise an active material having a potential that is greater than 0.2 volts versus a reference electrode. The element comprises at least one edge region that comprises edges of the positive electrodes and edges of the negative electrodes. The active material provided on the negative electrodes does not extend beyond the edges of the positive electrodes in the edge region.  
         [0017]     Another exemplary embodiment relates to a lithium-ion battery that includes a plurality of alternating positive and negative electrodes, the positive and negative electrodes having a generally planar rectangular configuration and comprising a current collector with two opposed surfaces, at least a portion of at least one of the two opposed surfaces for each of the electrodes having an active material provided thereon. The active material provided on the negative electrodes has a potential that is greater than 0.2 volts versus a reference electrode. For at least a plurality of the negative electrodes, the area of the active material is not larger than the area of the active material on an adjacent positive electrode. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a cross-sectional view of a portion of a prior art lithium-ion battery having flat plate electrodes.  
         [0019]      FIG. 2  is a cross-sectional view of the battery shown in  FIG. 1  taken along line  2 - 2 .  
         [0020]      FIG. 3  is an isometric view of a portion of a prior art lithium-ion battery having wound electrodes.  
         [0021]      FIG. 4  is a cross-sectional view of the battery shown in  FIG. 3  taken along line  4 - 4 .  
         [0022]      FIG. 5  is a cross-sectional view of a prior art lithium-ion battery having accordion-style electrodes.  
         [0023]      FIG. 6  is a cross-sectional view of a prior art lithium-ion battery having an accordion-style negative electrode with positive electrode plates interspersed between the folds of the negative electrode.  
         [0024]      FIG. 7  is a cross-sectional view of a portion of a lithium-ion battery having flat plate electrodes according to an exemplary embodiment.  
         [0025]      FIG. 8  is a perspective view of a portion of the battery shown in  FIG. 7 .  
         [0026]      FIG. 9  is an isometric view of a lithium-ion battery having a wound electrode configuration according to an exemplary embodiment.  
         [0027]      FIG. 10  is a cross-sectional view of a portion of the battery shown in  FIG. 9 .  
         [0028]      FIG. 11  is a cross-sectional view of a lithium-ion battery having accordion-style electrodes.  
         [0029]      FIG. 12  is a cross-sectional view of a portion of a lithium-ion battery according to an exemplary embodiment.  
         [0030]      FIG. 13  is a cross-sectional view of a portion of a lithium-ion battery according to an exemplary embodiment.  
         [0031]      FIG. 14  is a cross-sectional view of a portion of a lithium-ion battery according to an exemplary embodiment.  
         [0032]      FIG. 15  is a cross-sectional view of a portion of a lithium-ion battery according to an exemplary embodiment.  
         [0033]      FIG. 16  is a photograph showing a portion of a lithium-ion battery using a carbonaceous negative active material.  
         [0034]      FIG. 17  is a photograph showing a portion of a lithium-ion battery using a lithium titanate negative active material.  
         [0035]      FIG. 18  is a schematic view of an implantable medical device (IMD) provided within the body of a patient according to an exemplary embodiment.  
         [0036]      FIG. 19  is a schematic view of another implantable medical device (IMD) provided within the body of a patient according to an exemplary embodiment.  
         [0037]      FIG. 20  is a photograph showing a portion of a lithium-ion battery using a carbonaceous negative active material and employing a design rule in which the negative electrode extends beyond the positive electrode. 
     
    
     DETAILED DESCRIPTION  
       [0038]     According to an exemplary embodiment, a lithium-ion battery utilizes a negative active material that is configured to provide enhanced flexibility in the physical design of the battery. For example, the use of certain materials may allow for the elimination of relatively complicated and expensive design rules which are intended to prevent lithium plating at the negative electrode. Various advantages may be obtained from eliminating such design rules, including, for example, better overall energy density for the batteries, simpler and more inexpensive manufacturing processes, material reduction and cost savings, and the like.  
         [0039]     According to various exemplary embodiments as will be described in greater detail below, a lithium-ion battery includes a housing (e.g., a can, case, casing, etc.) in which one or more positive electrodes, one or more negative electrodes, and an electrolyte is provided. One or more separators made from a porous polymer or other suitable material may also be provided intermediate or between the positive and negative electrodes to prevent direct contact between adjacent electrodes. The positive and negative electrodes each include a current collector and an active material provided on one or more sides thereof.  
         [0040]     According to an exemplary embodiment, the battery housing is made of stainless steel or another metal (e.g., titanium or titanium alloys, aluminum or aluminum alloys, etc.). According to other exemplary embodiments, the battery housing may be made of a plastic material or a plastic-foil laminate material (e.g., an aluminum foil provided intermediate a polyolefin layer and a polyester layer).  
         [0041]     Any suitable electrolyte may be used depending on desired performance characteristics of the battery and other factors. According to various exemplary embodiments, the electrolyte may be a lithium salt dissolved in one or more non-aqueous solvents, a lithium salt dissolved in a polymeric material such as poly(ethylene oxide) or silicone, a solid state electrolyte such as a lithium-ion conducting glass, or an ionic liquid such as an N-methyl-N-alkylpyrrolidinium bis(trifluoromethanesulfonyl)imide salt.  
         [0042]     According to one exemplary embodiment, the electrolyte is a 3:7 mixture of ethylene carbonate to ethylmethyl carbonate (EC:EMC) in a 1.0 M salt of LiPF 6 . According to another particular exemplary embodiment, the electrolyte includes a polypropylene carbonate solvent and a lithium bis-oxalatoborate salt (sometimes referred to as LiBOB). According to other exemplary embodiments, the electrolyte may comprise one or more of a PVDF copolymer, a PVDF-polyimide material, and organosilicon polymer, a thermal polymerization gel, a radiation cured acrylate, a particulate with polymer gel, an inorganic gel polymer electrolyte, an inorganic gel-polymer electrolyte, a PVDF gel, polyethylene oxide (PEO), a glass ceramic electrolyte, phosphate glasses, lithium conducting glasses, lithium conducting ceramics, and an inorganic ionic liquid or gel, among others.  
         [0043]     According to an exemplary embodiment, the separator is a polymeric material such as a polypropylene/polyethelene copolymer or another polyolefin multilayer laminate that includes micropores formed therein to allow electrolyte and lithium ions to flow from one side of the separator to the other. The thickness of the separator is between approximately 10 micrometers (μm) and 50 μm according to an exemplary embodiment. According to a particular exemplary embodiment, the thickness of the separator is approximately 25 μm and the average pore size of the separator is between approximately 0.02 μm and 0.1 μm.  
         [0044]     The positive electrode(s) include a current collector made of a conductive material such as a metal. According to an exemplary embodiment, the positive current collector is formed from aluminum or an aluminum alloy, and may be provided as a thin foil, a grid (e.g., a mesh grid, an expanded metal grid, a photochemically etched grid, etc.), or any other suitable configuration. The current collector may also have an electrically conductive member or element (e.g., a tab) coupled thereto and extending therefrom. The tab may be formed from aluminum or an aluminum alloy (or any other suitable material), and may serve as a terminal for the battery according to an exemplary embodiment.  
         [0045]     The positive current collector has a layer of active material provided on one or both sides thereof. The active material layer provided on the positive current collector (hereinafter referred to as the “positive active material”) may be made up of a single active material or may include multiple active materials (either provided as separate layers or intermixed). The positive active material may also include a binder material that includes a material such as polyvinylidine fluoride (PVDF) and/or an elastomeric polymer such as styrene butadiene rubber (SBR) (carboxymethylcellulose may be provided by itself with a water solvent or may be provided in conjunction with the SBR). The positive active material may also include a conductive additive such as carbon black.  
         [0046]     According to an exemplary embodiment, the positive active material is a material or compound that includes lithium that may be doped and undoped during discharging and charging of the battery. The positive active material may include any one or more of the following materials: lithium cobalt oxide (LiCoO 2 ); lithium manganese oxide (LiMn 2 O 4 ); LiCo x Ni (1-x) O 2  (0.05≦x≦0.8); LiAl x Co y Ni (1-x-y) O 2  (0.05≦x≦0.3, 0.1≦y≦0.3); LiM x ′M 1-x ″O 2  (where M′ and M″ are metals such as Co, Ni, Mn, Fe, Cr, V, Zn, Al, Cd, and mixtures thereof); Li 1-x MO 2  (where M is a metal such as cobalt, nickel, or manganese and 0≦x≦0.5); LiM′ x M″ 1-x PO 4  (where 0≦x≦1 and M′ and M″ =Fe, Co, Ni, Mn, Cu, V. Ti, Cr, Zn, Cd, and mixtures thereof); and any other suitable active material such as those described in U.S. patent application Ser. No. 10/978,712 filed Oct. 29, 2004, the entire disclosure of which is incorporated herein by reference. According to a particular exemplary embodiment, the positive active material includes lithium cobalt oxide.  
         [0047]     The thickness of the positive active material may differ according to various exemplary embodiments. According to one exemplary embodiment, the thickness of the positive active material is between approximately 0.1 μm and 3 mm. According to another exemplary embodiment, the thickness of the positive active material is between approximately 25 μm and 300 μm. According to a particular exemplary embodiment, the thickness of the positive active material is approximately 75 μm.  
         [0048]     The negative electrode(s) includes a current collector made of a conductive material such as a metal. According to an exemplary embodiment, the negative current collector is formed from copper, titanium, nickel, aluminum, or alloys thereof, and may be provided as a thin foil, a grid (e.g., a mesh grid, an expanded metal grid, a photochemically etched grid, etc.), or any other suitable configuration. The negative current collector may also have an electrically conductive member or element (e.g., a tab) coupled thereto and extending therefrom. The tab may be formed from nickel or a nickel alloy, vanadium or a vanadium alloy, or any other suitable material, and may serve as a terminal for the battery according to an exemplary embodiment. According to a particular exemplary embodiment, the tab may couple the negative current collector to the battery housing.  
         [0049]     The negative current collector has a layer of active material provided on one or both sides thereof. The active material layer provided on the negative current collector (hereinafter referred to as the “negative active material”) may be made up of a single active material or may include multiple active materials (either provided as separate layers or intermixed). The positive active material may also include a binder material (e.g., polyvinylidine fluoride (PVDF) or an elastomeric polymer) and/or a conductive additive such as carbon black, graphite, or a mixture thereof.  
         [0050]     According to an exemplary embodiment, the negative active material is selected such that it has an average potential that is greater or equal to approximately 0.2 V versus Li/Li +  (e.g., according to one particular exemplary embodiment, the negative active material has an average potential that is greater or equal to approximately 0.3 V versus Li/Li + ; according to a particularly preferred embodiment, the negative active material is a titanate material having an average potential that is greater or equal to approximately 1.5 V versus Li/Li + ). The inventors have unexpectedly discovered that the use of negative electrode materials that possess a relatively high average potential versus Li/Li +  reduces the likelihood of lithium plating. According to one exemplary embodiment, such a negative active material is used in conjunction with a positive active material that has an average potential of greater than approximately 3 V versus Li/Li +  (e.g., LiCoO 2 ).  
         [0051]     According to an exemplary embodiment, the negative active material is a lithium titanate material such as Li 4 Ti 5 O 12  (referred to alternatively as Li 4/3 Ti 5/3 O 4 ) having a spinel structure. Other lithium titanate materials which may be suitable for use as the negative active material may include one or more of the following lithium titanate materials: LiTi 2 O 4 , Li 2 TiO 3 , Li x Ti y O 4  (where 1.0≦x≦1.6 and 1.6≦y≦2.0), Li 4 M x Ti 5-x O 12  (where 0≦x≦1.0 and M is a metal such as vanadium, iron, aluminum, nickel, cobalt, copper, chromium, molybdenum, niobium or combinations thereof) and LiM′M″XO 4  (where M′ is a metal such as nickel, cobalt, iron, manganese, vanadium, copper, chromium, molybdenum, niobium, or combinations thereof, M″ is an optional three valent non-transition metal, and X is zirconium, titanium, or a combination of these two). Note that such lithium titanate materials may be used in any state of lithiation (e.g., Li 4+x Ti 5 O 12 , where 0≦x≦3).  
         [0052]     One advantage of using a lithium titanate material instead of a carbonaceous material is that it is believed that the use of a lithium titanate material allows for charging and discharging of the battery at higher rates than is capable using carbonaceous materials. Lithium titanate materials are also believed to offer superior cycle life because they are so called “zero-strain” materials. Zero strain materials have crystal lattices which do not experience shrinkage or contraction with lithium doping/de-doping, making them free from strain-related degradation mechanisms.  
         [0053]     Another advantageous feature of using a lithium titanate material is that it is believed that when used in a negative electrode of a lithium-ion battery, such materials will cycle lithium at a potential plateau of about 1.5 V versus a lithium reference electrode. This is substantially higher than graphitic carbon, which is traditionally used in lithium ion batteries, and cycles lithium down to about 0.1 V in the fully charged state. As a result, the battery using lithium titanate is believed to be less likely to result in lithium plating during charging.  
         [0054]     Being free from the risk lithium plating, cells with lithium titanate negative electrodes may also be charged at rates that exceed those with carbon negative electrodes. For example, a common upper limit for the rate of charge in lithium ion batteries is about 1 C (meaning that the battery can be fully charged from the discharged state in one hour). Conversely, it has been reported in literature that lithium titanate may be charged at rates up to 10 C (i.e., attaining full charge in 1/10 hour, or six minutes). Being able to recharge a battery more quickly substantially increases the functionality of devices that employ such a battery. A further advantage of the higher potential of the lithium titanate material is that it avoids decomposition of organic solvents (such as propylene carbonate) commonly used in lithium ion batteries. In so doing, it may reduce negative consequences such as formation of gas, cell swelling, reduction of reversible battery capacity, and buildup of resistive films which reduce battery power.  
         [0055]     Other materials having cycling potentials that exceed that of lithium by several hundred millivolts and which may be suitable for use as the negative active material include the materials listed in Table 1. Such materials may be used alone or in combination with the lithium titanates described above and/or any of the other materials listed in Table 1.  
                                                                   TABLE 1                                       Cycling Potentials (vs Li)            Class   Compound   Vmin   Vmax   Vavg                    Oxides   TiO 2  (Anatase)   1.4   2   1.80       Oxides   WO 2     0.6   1.3   0.80       Oxides   WO 3     0.5   2.6   1.0       Oxides   MoO 2     1.3   2   1.60       Oxides   Nb 2 O 5     1.0   2   1.50       Oxides   LiWO 2             0.75       Oxides   Li x MoO 2     0.8   2   1.60       Oxides   V 6 O 13             2.30       Oxides   Li 6 Fe 2 O 3             0.75       Oxides   LiFeO 2     1.0   3.0   2.0       Oxides   Fe 2 O 3     0.2   2.0   0.75       Oxides   MO where M = Co, Ni, Cu or Fe           0.8-1.5       Sulfides   FeS 2     1.3   1.9   1.65       Sulfides   MoS 2             1.75       Sulfides   TiS 2             2.00       Alloys   Sn—Bi           0.75       Alloys   Alloys comprising of Al, Si or Sn           0.30           and other elements       Alloys   Sn—Co—C           0.30       Alloys   Sb           0.90       Alloys   NbSe 3             1.95       Alloys   Bi           0.80       Alloys   In           0.60       Alloys   Li x Al           0.36       Alloys   Li x Sn   0   1.3   0.50       Alloys   Sn—Sb           0.0-1.0       Polymers   Poly(phenylquinoline)           1.50       Polymers   Polyparaphenylene           0.70       Polymers   Polyacetylene           1.00       Vanadates   Li x MVO 4  where M = Ni, Co,           2.0-0.5           Cd, Zn                  
 
         [0056]     According to a particular exemplary embodiment, a negative active material such as Li 4 Ti 5 O 12  is coated on an aluminum foil using a conductive carbon filler and a PVDF, NMP (Polyvinylidene fluoride, N-methylpyrrolidone) binder solution. The binder may also include (in addition to or in place of the foregoing materials) an elastomeric polymer such as styrene butadiene rubber (SBR) (carboxymethylcellulose may be provided by itself with a water solvent or may be provided in conjunction with the SBR). The negative electrode coatings are made using the negative electrode material, the conductive carbon and the binder solution.  
         [0057]     According to various exemplary embodiments, the thickness of the negative active material is between approximately 0.1 μm and 3 mm. According to other exemplary embodiments, the thickness of the active material may be between approximately 25 μm and 300 μm. According to another exemplary embodiment, the thickness of the active material may be between approximately 20 μm and 90 μm, and according to a particular exemplary embodiment, approximately 75 μm.  
         [0058]     According to an exemplary embodiment, negative active materials such as those described above may allow for more flexible design rules to be employed in the design and manufacture of lithium-ion batteries. For example, because the use of a lithium titanate material reduces the risk of lithium plating at the negative electrode, lithium-ion battery designs may be modified such that the edges of the negative electrodes need not extend beyond the edges of the positive electrodes at all locations within the battery. This in turn allows for a reduction in negative electrode area (relative to the positive electrode area) and potentially improved energy density, while still providing protection against lithium plating.  
         [0059]     It should be understood by those reviewing the present application that with respect to the various exemplary embodiments described herein, it is contemplated by the inventors that the need to provide excess negative active material in high current regions (e.g., edges of electrodes) may be eliminated using higher potential negative active materials. Where references are made to providing negative electrodes that have electrode edges that are coextensive (or which do not extend as far as) the adjacent edges of the positive electrodes, it should also be understood that similar advantages may be obtained by providing negative electrodes having negative active materials that are coextensive (or which does not extend as far as) the positive active material on the adjacent positive electrode (e.g., where the negative active material and/or positive active material are not coated to the edge of their respective electrodes). Examples of such configurations are provided in  FIGS. 12-15  and described below. Further, because there is necessarily some variability in the battery assembly process that may cause electrodes to shift during the assembly process relative to each other, references to negative electrodes and/or negative active materials not extending as far as the positive electrodes and/or positive active materials should be interpreted to mean that the negative electrodes and/or negative active materials do not extend as far as the positive electrodes and/or positive active materials for all or a portion of the relevant area (e.g., a statement that the negative active material does not extend as far as the positive active material toward the top edge of a wound cell element should be interpreted to mean that the negative active material does not extend as far as the positive active material along at least a portion of the top edge of the wound cell element).  
         [0060]     For example,  FIG. 7  is a cross-sectional view of a portion of a lithium-ion battery  500  having flat plate electrodes having a generally rectangular (e.g., a square) geometry according to an exemplary embodiment, and  FIG. 8  is a perspective view of a portion of the battery  500 . The battery  500  includes a housing or casing  502  (e.g., a can) that may be made of any suitable material as described above. The battery  500  includes a plurality of generally planar negative electrodes  510  and positive electrodes  520 , with separators  530  provided between the positive and negative electrodes. The negative electrodes  510  include current collectors  512  and an active material  514  provided on one or both sides thereof, and the positive electrodes  520  include current collectors  522  and an active material  524  provided on one or both sides thereof. Although illustrated as having a generally rectangular planar configuration, according to other exemplary embodiments, the electrodes may be circular or may have other suitable shapes.  
         [0061]     As illustrated in  FIGS. 7-8 , the negative electrodes  510  do not extend beyond the edges of the positive electrodes  520  along their outer perimeter (as they do, for instance, in the batteries illustrated in  FIGS. 1-2 ). According to an exemplary embodiment, the negative electrodes  510  have the same area as the positive electrodes  520  (i.e., the top, bottom, and side edges are coextensive or coterminous) and the active materials  514 ,  524  on both the negative electrodes  510  and positive electrodes  520  extend to the ends of their respective current collectors  512 ,  522 . According to one exemplary embodiment, the active material provided on the negative electrode does not extend beyond the active material provided on the positive electrode near at least one of the edges of the electrodes. According to another exemplary embodiment, the area of the active material provided on the negative electrode is less than or equal to the area of the active material provided on the positive electrode.  
         [0062]      FIGS. 9-10  illustrate a lithium-ion battery  600  having a wound electrode configuration according to an exemplary embodiment. The battery  600  includes a housing or casing  602  (e.g., a can) in which the wound cell element  604  is provided during assembly. As shown in  FIG. 9 , the element  604  is partially unwound to illustrate the components of the element.  
         [0063]     The battery  600  includes a negative electrode  610  and positive electrode  620 , with separators  630  provided between the positive and negative electrodes (according to an exemplary embodiment, two separators are utilized so that when the electrodes are wound, the positive and negative electrodes do not come into direct contact at any point, which may result in battery shorting). The negative electrode  610  includes a current collector  612  and an active material  614  provided thereon, and the positive electrode  620  includes a current collector  622  and an active material  624  provided thereon.  
         [0064]     It should be noted that while the exemplary embodiment shown in  FIG. 9  illustrates a battery in which the electrodes are wound to form a generally cylindrical cell element, other configurations may also be possible for wound or wrapped electrode batteries. For example, the shape of the cell element may be that of an oval or elongated flattened winding (e.g., having a generally rectangular winding with rounded corners) according to other exemplary embodiments. A mandrel or other structure may be used to define the shape which the cell element may take (e.g., by wrapping the electrodes around the mandrel).  
         [0065]     As shown in  FIGS. 9-10 , the height of the negative and positive electrodes are substantially equal. That is, the top and bottom edges of the negative electrode  610  do not extend beyond the top and bottom edges of the positive electrode  620 . According to an exemplary embodiment, the top and bottom edges of the positive and negative electrodes are coextensive or coterminous. As also shown in  FIGS. 9-10 , the leading edge of the positive and negative electrodes are substantially coextensive or coterminous (i.e., the leading edge of the negative electrode does not extend beyond that of the positive electrode).  
         [0066]      FIG. 11  is a cross-sectional view of a lithium-ion battery  700  having an accordion-style electrode configuration (also referred to as a “Z fold” or a “zigzag” fold configuration) according to an exemplary embodiment. The battery  700  includes a housing or casing  702  (e.g., a can) in which a folded electrode set  704  is provided during assembly.  
         [0067]     The battery  700  includes a negative electrode  710  and positive electrode  720 , with a separator  730  provided between the positive and negative electrodes. The negative electrode  710  includes a current collector  712  and an active material  714  provided on one or both sides thereof, and the positive electrode  720  includes a current collector  722  and an active material  724  provided on one or both sides thereof.  
         [0068]     As illustrated in  FIG. 11 , the positive active material  724  may be applied along substantially the entire length of the positive electrode  720 , without the need to mask a portion of the current collector  720  in the region  726  (see, e.g.,  FIG. 5 , in which a portion of the current collector  322  is not coated with active material in a region  326 ). Additionally, the top and bottom edges of the positive electrode  720  and negative electrode  710  may be coextensive or coterminous (i.e., instead of the edge of the negative electrode  710  extending beyond that of the positive electrode).  
         [0069]     While  FIGS. 7-11  illustrate exemplary embodiments in which the edges of the positive and negative electrodes are substantially coextensive or coterminous, those reviewing this disclosure will appreciate that other possibilities may be possible.  FIGS. 12-15  illustrate portions of lithium-ion batteries according to various other exemplary embodiments. It should be understood that the embodiments illustrated in  FIGS. 12-15  may be applied to batteries of any of a variety of configurations (e.g., flat plate electrode, wound electrode, Z-fold electrode, etc.). To this end, the edges of the various electrodes shown in  FIGS. 12-15  may be applied to top and bottom edges of electrodes, leading and trailing edges of wound batteries, and the like.  
         [0070]      FIG. 12  is a cross-sectional view of a portion of a lithium-ion battery  800  that includes a negative electrodes  810 , a positive electrode  820 , and a separator  830  provided between the positive and negative electrodes. The negative electrode  810  includes a current collector  812  and an active material  814  provided thereon, and the positive electrode  820  includes a current collector  822  and an active material  824  provided thereon. As shown in  FIG. 12 , the edge of the positive electrode  820  extends beyond the edge of the negative electrode  810 . According to an exemplary embodiment, the positive electrode  820  may extend beyond the edge of the negative electrode  810  by between approximately 0 and 5 millimeters.  
         [0071]      FIG. 13  is a cross-sectional view of a portion of a lithium-ion battery  900  that includes a negative electrode  910 , a positive electrode  920 , and a separator  930  provided between the positive and negative electrodes. The negative electrode  910  includes a current collector  912  and an active material  914  provided thereon, and the positive electrode  920  includes a current collector  922  and an active material  924  provided thereon. As shown in  FIG. 13 , the edge of the negative current collector  914  is coextensive or coterminous with the edge of the positive electrode  920 , while the negative active material  914  terminates before the edge of the negative current collector  912 . According to an exemplary embodiment, the negative active material  914  terminates between approximately 0 and 5 millimeters before the edge of the negative current collector  912 . Because it is the negative active material  914  that is responsible for intercalating lithium, it is thus possible to use negative current collectors that have the same dimensions as the positive current collectors while using less active material by not coating the negative material all the way to its edge. The amount of negative active material (e.g., the amount by which the active material terminates before the edge of the current collector) may be selected so that there is an adequate or optimal amount of negative active material to intercalate the active lithium in the battery without resulting in lithium plating at the edges of the electrodes.  
         [0072]      FIG. 14  is a cross-sectional view of a portion of a lithium-ion battery  1000  that includes a negative electrodes  1010 , a positive electrode  1020 , and a separator  1030  provided between the positive and negative electrodes. The negative electrode  1010  includes a current collector  1012  and an active material  1014  provided thereon, and the positive electrode  1020  includes a current collector  1022  and an active material  1024  provided thereon. As shown in  FIG. 14 , the negative active material  1014  does not extend to the edge of the negative current collector  1012  but is substantially coextensive or coterminous with the positive electrode  1020  and the positive active material  1024  provided thereon. According to an exemplary embodiment, the negative active material  1014  terminates between approximately 0 and 5 millimeters before the edge of the negative current collector  1012 . The amount of negative active material (e.g., the amount by which the active material terminates before the edge of the current collector) may be selected so that there is an adequate or optimal amount of negative active material to intercalate the active lithium in the battery without resulting in lithium plating at the edges of the electrodes.  
         [0073]      FIG. 15  is a cross-sectional view of a portion of a lithium-ion battery  1100  that includes a negative electrodes  1110 , a positive electrode  1120 , and a separator  1130  provided between the positive and negative electrodes. The negative electrode  1110  includes a current collector  1112  and an active material  1114  provided thereon, and the positive electrode  1120  includes a current collector  1122  and an active material  1124  provided thereon. As shown in  FIG. 15 , the positive current collector  1112  extends beyond the edge of the positive electrode  1120 . The negative active material  1114  does not extend to the edge of the negative current collector  1112 , and also does not extend such that it is coextensive or coterminous with the positive electrode  1120  and the active material  1124  provided thereon. According to an exemplary embodiment, the negative active material  1114  terminates between approximately 0 and 5 millimeters before the edge of the negative current collector  1112 . The amount of negative active material (e.g., the amount by which the active material terminates before the edge of the current collector) may be selected so that there is an adequate or optimal amount of negative active material to intercalate the active lithium in the battery without resulting in lithium plating at the edges of the electrodes.  
         [0074]      FIGS. 16-17  illustrate one advantage of using negative active materials such as those described above (e.g., lithium titanate active materials).  FIG. 16  is a photograph showing the edge of a lithium-ion battery in which a carbonaceous negative active material that included mesocarbon microbeads was used on the negative current collector. In the example shown in  FIG. 16 , the edge of the negative electrode did not extend beyond the edge of the positive electrode (i.e., their edges were coextensive or coterminous). Extended battery cycling was performed, after which the battery was cut apart and viewed. As shown by the bright spots on the edges of the negative electrodes in  FIG. 16 , relatively severe lithium plating occurred during cycling of the battery. Notably, if the lithium-ion battery with a carbonaceous negative active material is built following the design rule that the negative electrode must extend beyond the positive electrode (e.g., by up to approximately 1 millimeter or more), lithium plating will generally not occur, as illustrated in  FIG. 20 .  
         [0075]     The battery shown in  FIG. 17 , in contrast to that shown in  FIG. 16 , utilized a lithium titanate (Li 4 Ti 5 O 12 ) active material on the negative electrode. Similar to the battery shown in  FIG. 16 , the edges of the negative electrode were coextensive or coterminous with those of the positive electrode. The battery was then subjected to extended cycling (100 cycles at an accelerated charge discharge rate). As shown in  FIG. 17 , no lithium plating is readily apparent at the edges of the negative electrodes.  
         [0076]     As described above, one advantageous feature associated with the use of negative active materials that have potentials that greatly exceed that of carbon (e.g., lithium titanate materials) is that design rules requiring that the negative active material extend beyond the positive active material by up to one millimeter or more may be eliminated. The inventors have discovered that the use of such high potential materials may also allow the production of batteries in which the positive active material and/or positive electrode may extend beyond the negative active material and/or negative electrode by several millimeters without resulting in lithium plating (e.g., up to 10 millimeters in some cases). This provides enhanced flexibility in the design of lithium-based batteries in addition to allowing for variability in the manufacturing/assembly process (e.g., in the past, negative electrodes had to extend beyond the positive electrodes to compensate for potential misalignments between the electrodes that may result in lithium plating; the use of higher potential materials may eliminate the need to compensate for such variability).  
         [0077]     The batteries described herein may find utility in a variety of applications, including in implantable medical devices (IMDs).  FIG. 18  illustrates a schematic view of a system  1200  (e.g., an implantable medical device) implanted within a body or torso  1232  of a patient  1230 . The system  1200  includes a device  1210  in the form of an implantable medical device that for purposes of illustration is shown as a defibrillator configured to provide a therapeutic high voltage (e.g., 700 volt) treatment for the patient  1230 .  
         [0078]     The device  1210  includes a container or housing  1214  that is hermetically sealed and biologically inert according to an exemplary embodiment. The container may be made of a conductive material. One or more leads  1216  electrically connect the device  1210  and to the patient&#39;s heart  1220  via a vein  1222 . Electrodes  1217  are provided to sense cardiac activity and/or provide an electrical potential to the heart  1220 . At least a portion of the leads  1216  (e.g., an end portion of the leads shown as exposed electrodes  1217 ) may be provided adjacent or in contact with one or more of a ventricle and an atrium of the heart  1220 .  
         [0079]     The device  1210  includes a battery  1240  provided therein to provide power for the device  1210 . The size and capacity of the battery  1240  may be chosen based on a number of factors, including the amount of charge required for a given patient&#39;s physical or medical characteristics, the size or configuration of the device, and any of a variety of other factors. According to an exemplary embodiment, the battery is a 5 mAh battery. According to another exemplary embodiment, the battery is a 300 mAh battery. According to various other exemplary embodiments, the battery may have a capacity of between approximately 1 and 1000 mAh.  
         [0080]     According to other exemplary embodiments, more than one battery may be provided to power the device  1210 . In such exemplary embodiments, the batteries may have the same capacity or one or more of the batteries may have a higher or lower capacity than the other battery or batteries. For example, according to an exemplary embodiment, one of the batteries may have a capacity of approximately 500 mAh while another of the batteries may have a capacity of approximately 75 mAh.  
         [0081]     According to an exemplary embodiment, the battery may be configured such that it may be charged and recharged using an inductive charging system in which a primary or external coil is provided at an exterior surface of a portion of the body (either proximate or some distance away from the battery) and a secondary or internal coil is provided below the skin adjacent the primary coil.  
         [0082]     According to another exemplary embodiment shown in  FIG. 19 , an implantable neurological stimulation device  1300  (an implantable neuro stimulator or INS) may include a battery  1302  such as those described above with respect to the various exemplary embodiments. Examples of some neuro stimulation products and related components are shown and described in a brochure titled “Implantable Neurostimulation Systems” available from Medtronic, Inc.  
         [0083]     An INS generates one or more electrical stimulation signals that are used to influence the human nervous system or organs. Electrical contacts carried on the distal end of a lead are placed at the desired stimulation site such as the spine or brain and the proximal end of the lead is connected to the INS. The INS is then surgically implanted into an individual such as into a subcutaneous pocket in the abdomen, pectoral region, or upper buttocks area. A clinician programs the INS with a therapy using a programmer. The therapy configures parameters of the stimulation signal for the specific patient&#39;s therapy. An INS can be used to treat conditions such as pain, incontinence, movement disorders such as epilepsy and Parkinson&#39;s disease, and sleep apnea. Additional therapies appear promising to treat a variety of physiological, psychological, and emotional conditions. Before an INS is implanted to deliver a therapy, an external screener that replicates some or all of the INS functions is typically connected to the patient to evaluate the efficacy of the proposed therapy.  
         [0084]     The INS  1300  includes a lead extension  1322  and a stimulation lead  1324 . The stimulation lead  1324  is one or more insulated electrical conductors with a connector  1332  on the proximal end and electrical contacts (not shown) on the distal end. Some stimulation leads are designed to be inserted into a patient percutaneously, such as the Model 3487A Pisces-Quad® lead available from Medtronic, Inc. of Minneapolis Minn., and stimulation some leads are designed to be surgically implanted, such as the Model 3998 Specify® lead also available from Medtronic.  
         [0085]     Although the lead connector  1332  can be connected directly to the INS  1300  (e.g., at a point  1336 ), typically the lead connector  1332  is connected to a lead extension  1322 . The lead extension  1322 , such as a Model 7495 available from Medtronic, is then connected to the INS  1300 .  
         [0086]     Implantation of an INS  1300  typically begins with implantation of at least one stimulation lead  1324 , usually while the patient is under a local anesthetic. The stimulation lead  1324  can either be percutaneously or surgically implanted. Once the stimulation lead  1324  has been implanted and positioned, the stimulation lead&#39;s  1324  distal end is typically anchored into position to minimize movement of the stimulation lead  1324  after implantation. The stimulation lead&#39;s  1324  proximal end can be configured to connect to a lead extension  1322 .  
         [0087]     The INS  1300  is programmed with a therapy and the therapy is often modified to optimize the therapy for the patient (i.e., the INS may be programmed with a plurality of programs or therapies such that an appropriate therapy may be administered in a given situation).  
         [0088]     A physician programmer and a patient programmer (not shown) may also be provided to allow a physician or a patient to control the administration of various therapies. A physician programmer, also known as a console programmer, uses telemetry to communicate with the implanted INS  1300 , so a clinician can program and manage a patient&#39;s therapy stored in the INS  1300 , troubleshoot the patient&#39;s INS system, and/or collect data. An example of a physician programmer is a Model 7432 Console Programmer available from Medtronic. A patient programmer also uses telemetry to communicate with the INS  1300 , so the patient can manage some aspects of her therapy as defined by the clinician. An example of a patient programmer is a Model 7434 Itrel® 3 EZ Patient Programmer available from Medtronic.  
         [0089]     According to an exemplary embodiment, a battery provided as part of the INS  1300  may be configured such that it may be charged and recharged using an inductive charging system in which a primary or external coil is provided at an exterior surface of a portion of the body (either proximate or some distance away from the battery) and a secondary or internal coil is provided below the skin adjacent the primary coil.  
         [0090]     While the medical devices described herein (e.g., systems  1200  and  1300 ) are shown and described as a defibrillator and a neurological stimulation device, it should be appreciated that other types of implantable medical devices may be utilized according to other exemplary embodiments, such as pacemakers, cardioverters, cardiac contractility modules, drug administering devices, diagnostic recorders, cochlear implants, and the like for alleviating the adverse effects of various health ailments.  
         [0091]     It is also contemplated that the medical devices described herein may be charged or recharged when the medical device is implanted within a patient. That is, according to an exemplary embodiment, there is no need to disconnect or remove the medical device from the patient in order to charge or recharge the medical device.  
         [0092]     It should be noted that references to “front,” “back,” “upper,” and “lower” in this description are merely used to identify various elements as are oriented in the FIGURES, with “front” and “back” being relative the vehicle in which the battery assembly is placed.  
         [0093]     For the purpose of this disclosure, the term “coupled” means the joining of two members directly or indirectly to one another. Such joining may be stationary in nature or moveable in nature. Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another. Such joining may be permanent in nature or may be removable or releasable in nature.  
         [0094]     It is also important to note that the construction and arrangement of the batteries and cells as shown in the various exemplary embodiments is illustrative only. Although only a few embodiments of the present inventions have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. For example, elements shown as integrally formed may be constructed of multiple parts or elements, the position of elements may be reversed or otherwise varied, and the nature or number of discrete elements or positions may be altered or varied. Accordingly, all such modifications are intended to be included within the scope of the present invention as defined in the appended claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the various exemplary embodiments without departing from the scope of the present inventions as expressed in the appended claims.