Abstract:
An implantable medical device comprises a housing, circuitry enclosed in the housing and configured at least to transmit therapeutic stimulatory pulses to a patient, and an electrochemical cell enclosed therein. The electrochemical cell comprises an electrode assembly having a negative electrode, and a positive electrode.

Description:
FIELD OF THE INVENTION  
       [0001]     Most of the disclosures generally relate to batteries, and more particularly relate to lithium ion rechargeable batteries having electrodes formed from compressed composite films.  
       BACKGROUND OF THE INVENTION  
       [0002]     Implantable medical devices (IMDs) are commonly used today to provide therapies to patients suffering from various ailments. One type of IMD, a neurostimulator, delivers mild electrical impulses to neural tissue using an electrical lead. For example, electrical impulses may be directed to specific neural sites to provide pain relief and to reduce the need for pain medications and/or repeat surgeries. Neurostimulators may also be utilized to treat other conditions such as incontinence, sleep disorders, movement disorders such as Parkinson&#39;s disease and epilepsy, and other psychological, emotional, and other physiological conditions.  
         [0003]     A neurostimulator is commonly implanted in the abdomen, upper buttock, or pectoral region of a patient, depending in part on the therapy that the neurostimulator is to provide. A lead assembly extends from the neurostimulator to electrodes that are positioned on or near an area of a targeted tissue such as the spinal cord or brain. A lead extension may be coupled to the neurostimulator at a proximal end thereof, and coupled to the lead assembly at a distal end thereof. The implanted neurostimulation system is configured to deliver mild electrical pulses to the spinal cord. The electrical pulses are delivered through the lead assembly to the electrodes.  
         [0004]     An ideal power source for a neurostimulator or other IMD is also very small so it can supply energy from a small package volume. Lithium ion batteries are becoming recognized as a workable power source option for implantable neurostimulators, and other IMDs that may require recharging such as monitors, sensors, and drug pumps. High energy density, safety, and reliability associated with lithium ion batteries have also led to their selection for use in artificial hearts and in implantable cardiac devices such as left ventricular assist devices. Such implantable devices are being built increasingly smaller, and lithium batteries are correspondingly being built smaller and thinner, and with increasingly higher energy densities.  
         [0005]     A lithium ion battery includes a positive electrode that has an electrochemically higher potential, and a negative electrode that has an electrochemically lower potential. Conventional active materials for a positive electrode in lithium ion batteries include LiCoO 2 , LiNi X CO (1-X) O 2 , LiMn 2 O 4 , and LiNi 0.33 Mn 0.33 Co 0.33 O 2 . LiCoO 2  is a particularly suitable positive electrode active material for lithium ion batteries because LiCoO 2  itself has a relatively high energy density. Further, since charging is carried out through the de-intercalation of lithium ions from the crystalline structure of the LiCoO 2  active material, and discharging is carried out by the intercalation of lithium ions into the crystalline structure of the active material, the lithium ion battery has an optimal voltage plateau in the battery and electrode discharge curves.  
         [0006]     One method of manufacturing a dense LiCoO 2  positive electrode for a lithium ion battery includes depositing a coating of the LiCoO 2  active material, a binder, and a solvent onto a foil substrate. The coating is dried and cured, and then compressed in a calender. Typically, the compressed density of the positive electrode coating for a lithium battery is about 3.09 g/cm 3 . For example, in a conventional arrangement the coating is deposited to an initial loading density of 22 mg/cm 2  on each side of a substrate at a thickness of 105 μm, and compressed to a thickness of 71.5 μm to yield a final density of about 3.09 g/cm 3 .  
         [0007]     Battery manufacturers are continuously seeking for technology advancements that will improve a battery&#39;s energy density and power density, and will also decrease a battery&#39;s capacity fade. It is conventionally thought that a positive correlation exists between the extent that the positive electrode active material is compressed and the energy density. However, it is also conventionally understood that compressing higher than 3.09 g/cm 3  will reduce the battery power capability and the battery cycle life since high compression reduces the accessibility of electrolyte through the electrode pores.  
         [0008]     Accordingly, it is desirable to overcome the limitations associated with conventional batteries.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     An implantable medical device comprises a housing, circuitry enclosed in the housing and configured at least to transmit therapeutic stimulatory pulses to a patient, and an electrochemical cell enclosed therein. The electrochemical cell comprises an electrode assembly having a negative electrode, and a positive electrode. The positive electrode includes an alkali metal active material compressed to a density ranging between 3.3 and 3.7 g/cm 3 . In one preferred embodiment, the alkali metal active material is compressed to 3.5 g/cm 3 .  
         [0010]     A method is also provided for manufacturing a positive electrode for an electrochemical cell. The method comprises the steps of preparing an active material mixture comprising an alkali metal, and compressing the active material mixture onto a metal foil to a density ranging between 3.3 and 3.7 g/cm 3 . In one preferred embodiment, the active material mixture is compressed to 3.5 g/cm 3 . 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0012]      FIG. 1  is a perspective view of a neurostimulator assembly associated with a patient, the neurostimulator assembly having a plurality of leads coupled to electrodes that are disposed near a patient&#39;s spine;  
         [0013]      FIG. 2  is a cross-sectional view of a battery disposed in a neurostimulator, particularly depicting the windings of an electrode assembly around a mandrel;  
         [0014]      FIG. 3  is a partially cut-away side view of a positive electrode assembly;  
         [0015]      FIG. 4  is a cross-sectional view of the positive electrode assembly illustrated in  FIG. 3 , taken along line  4 - 4  in  FIG. 3 ;  
         [0016]      FIG. 5  is a flow chart outlining a method of manufacturing a positive electrode assembly;  
         [0017]      FIG. 6  is a cross-sectional view of a positive electrode and a negative electrode positioned side-by-side with a separator disposed between the two electrodes;  
         [0018]      FIG. 7  is a graph that includes plots of experimental and expected data for the capacity density of a composite electrode coating as a function of coating mass density;  
         [0019]      FIG. 8  is a graph that plots discharge capacity data sets for a series of accelerated battery charging and discharging cycles for a battery having active material in the positive electrode compressed to a conventional density, and also for a battery having active material in the positive electrode compressed to a higher than conventional density; and  
         [0020]      FIG. 9  is a graph that plots discharge capacity data sets for a series of weekly battery charging and discharging cycles for a battery having active material in the positive electrode compressed to a conventional density, and also for a battery having active material in the positive electrode compressed to a higher than conventional density. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. While the following description is set forth with reference to a neurostimulator, the principles of the claimed invention may be implemented in other IMDs as well.  
         [0022]     The neurostimulator typically includes a hermetically sealed housing that is impervious to body fluids. The neurostimulator also includes a connector header for making electrical and mechanical connection with one or more leads bearing electrodes adapted to be located at or near targeted tissue. The housing is typically formed of a suitable body-compatible material approved for medical, use, such as titanium. Typically, the housing is formed having major opposed surfaces joined together by sides enclosing an interior housing chamber or cavity and having electrical feedthroughs extending therethrough and into the connector header. The cavity houses receives an electrochemical cell and both high voltage (HV) and low voltage (LV) electronic circuitry, which can comprise ICs, hybrid circuits and discrete components, e.g. a step-up transformer and at least one high voltage output capacitor.  
         [0023]      FIG. 1  depicts a spinal cord stimulation (SCS) assembly implanted in a patient. The assembly includes a neurostimulator  300 , a lead extension  322  having a proximal end coupled to the neurostimulator  320 , and a lead  324  having a proximal end coupled to the distal end of the extension  322  and a distal end coupled to one or more electrodes  326 . A lead connector at the proximal end of lead  324  may be coupled directly to the neurostimulator  320  instead of the indirect connection by way of the lead extension  322  depicted in  FIG. 1 . The neurostimulator  320  may be implanted in the abdomen of a patient  328 , and the lead  324  placed along the spinal cord  330 . As stated previously, the neurostimulator  320  may have one or more leads, each lead having a plurality of electrodes. The neurostimulator  320  includes an electrochemical cell as a power source, and electronics for sending precise, electrical pulses to the spinal cord to provide a desired therapy.  
         [0024]     Although the lead  324  is positioned to stimulate a specific site in the spinal cord  330  in  FIG. 1 , it may also be positioned on or near other tissue such as the peripheral nerve or adjacent neural tissue ganglia, the brain, and muscle tissue.  
         [0025]     Furthermore, electrodes  326  may be epidural, intrathecal or placed into spinal cord  330  itself.  
         [0026]      FIG. 2  is a cross-sectional view of a battery  200  disposed in the housing  336  of an IMD. The IMD may be a neurostimulator such as that depicted in  FIG. 1 , an implantable cardiac defibrillator, a spinal or deep brain stimulator, a sensor, a drug pump, an artificial heart, and so forth. The battery  200  may be encased in stainless steel or other metal such as titanium, aluminum, or alloys thereof.  
         [0027]     Other exemplary battery cases are made of plastic materials, or a plastic-foil laminate material such as an aluminum foil provided between a polyolefin layer and a polyester layer.  
         [0028]     In the particular embodiment depicted in  FIG. 2 , the electrode assembly  100  is wound to fit into a prismatic cell. However, other winding arrangements may be utilized to accommodate various battery shapes, sizes, and configurations. One exemplary prismatic battery case has dimensions of about 30 to 40 mm by about 20 to 30 mm by about 5 to 7 mm.  
         [0029]     The battery  200  includes an electrode assembly  100 , including a positive electrode assembly  10  wound with a negative electrode assembly  50  around a mandrel  40 . The mandrel may be removed before the battery  200  is used.  
         [0030]     Separators are provided intermediate or between the positive and negative electrodes, although for the sake of clarity the electrodes  10  and  50  are depicted without separators in  FIG. 2 . Other electrode arrangements include flat, planar, and folded configurations.  
         [0031]     A connector tab  70  associated with the positive electrode  10  protrudes from the electrode assembly  100  as depicted in  FIGS. 3 and 4 . Another connector tab  62  associated with the negative electrode  50  protrudes from the electrode assembly  100  and is spaced apart from the connector tab  70  that is associated with the positive electrode  10  to avoid inadvertent short circuits in the completed battery  200 . The connector tabs  62  and  70  are preferably positioned to be close to their intended connection point when the electrode assembly  100  is connected to a feedthrough in a neurostimulator or other IMD. In an exemplary embodiment, the negative electrode  50  is coupled to an intended connection point using nickel or nickel alloy tabs. In another exemplary embodiment, the positive electrode  10  is coupled to an intended connection point using aluminum or aluminum alloy tabs.  
         [0032]     FIGS.  3  to  4  depict an elongated positive electrode assembly  10 , which includes a metal foil that functions as a current collector  15  onto which layers  20  and  25  of a composite that includes an active material are pressed. The positive electrode assembly  10  is thin and flat, and has an essentially uniform width. The connector tab  70  projects from the current collector edge, although more tabs may be included depending on the conductivity and current distribution for the active material.  
         [0033]     The current collector  15  is a conductive metal foil that is associated with active material in layers  20  and  25 . An exemplary current collector is an aluminum or aluminum alloy foil. An exemplary foil has a thickness between about 5 μm and about 75 μm. Other exemplary current collectors are formed in various grid configurations including a mesh grid, an expanded metal grid, and a photochemically etched grid.  
         [0034]     The positive electrode active material in layers  20  and  25  is a material or compound that includes lithium. Exemplary active materials in the layers  20  and  25  are compounds that include in the molecule lithium, one or more transition metals, and oxygen. Examples of such compounds include LiCoO 2 , LiNi x Co (1-x) O 2 , LiMn 2   O   4 , and LiNi x Mn y Co z O 2  wherein x+y+z=1. In many cases it is desirable to include a small amount of a conductivity enhancer to the composite layers  20  and  25 . Exemplary conductivity enhancers include various forms of carbon, including graphite and carbon black. Other suitable conductivity enhancers include electronically conductive polymers. The conductivity enhancer concentration ranges between about 0.5% and about 10% by weight.  
         [0035]     Further, exemplary composite layers  20  and  25  include a binder. Suitable binders include polymers such as polyvinylidine fluoride, polyvinylidene difluoride (PVDF)-hexafluoro propylene (HFP) copolymer, and styrene-butadiene rubber.  
         [0036]     The binder concentration ranges between about 2% and about 10% by weight.  
         [0037]     The active material layers  20  and  25  are between about 30 μm and about 5 mm thick. Exemplary layers  20  and  25  are between about 30 μm and 300 μm thick, and the layers are most preferably about 70 μm thick.  
         [0038]     The negative electrode  50  also includes a current collector, which is also made of a conductive material such as a metal associated with an active material. Exemplary negative electrode current collectors include a film of a metal such as copper, a copper alloy, titanium, a titanium alloy, nickel, and a nickel alloy. The negative electrode current collector may be formed in any of the above-described current collector configurations, such as a foil or a grid. An exemplary negative electrode current collector foil is between about 100 nm and 100 μm in thickness, preferably between about 5 μm and about 25 μm in thickness, and most preferably about 10 μm in thickness.  
         [0039]     Negative electrode active materials may include a carbonaceous material such as petroleum coke, carbon fiber, mesocarbon microbeads (MCMB), elements and oxides that are reactive with Li such as Sn, SnO 2 , and Si, and combinations of such active materials. A particular exemplary active material is MCMB. An additive such as carbon black may be included in the active material, along with a binder such as polyvinylidine fluoride (PVDF) or an elastomeric polymer. An exemplary active material ranges between about 0.1 μm and about 3 mm in thickness, preferably between about 25 μm and about 300 μm in thickness, and most preferably between about 20 μm and about 90 μm in thickness.  
         [0040]     An electrolyte is provided between the positive and negative electrodes  10  and  50  to provide a medium through which lithium ions may travel. An exemplary electrolyte is a liquid that includes a lithium salt dissolved in one or more non-aqueous solvents. Other exemplary electrolytes include a lithium salt dissolved in a polymeric material such as poly(ethylene oxide) or silicone, an ionic liquid such as N-methyl=N-alkylpyrrolidinium bis(trifluoromethanesulfonyl)imide salts, and a solid state electrolyte such as a lithium-ion conducting glass such as lithium phosphorous oxynitride (LiPON).  
         [0041]     Various other electrolytes may be used, such as a mixture of propylene carbonate, ethylene carbonate, and diethyl carbonate (PC:EC:DEC) in a 1.0 M salt of LiPF 6 , and a polypropylene carbonate solvent and a lithium bis-oxalatoborate salt. Other electrolytes may include one or more of a PVDF copolymer, a PVDF-polyimide material, an 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. Some exemplary electrolytes include a mixture of organic carbonates with 1 molar LiPF 6  as the lithium salt.  
         [0042]     The electrolyte includes one or more additives that are intended to reduce the occurrence of capacity fade in the battery  200 . Exemplary additives that may be used include organoboron compounds such as trimethoxyboroxine (TMOBX) and its derivatives.  
         [0043]     A procedure for fabricating the positive electrode assembly  10  will now be discussed in conjunction with  FIG. 5 . An active material is prepared as step  52  by sifting or otherwise dividing the active material into a fine powder. Although several active materials are suitable, in this example the active material is lithium cobalt oxide LiCoO 2 . The active material is mixed with a binder as step  54 . In this example the polymeric binder is polyvinylidine fluoride. As the active material and binder are mixed, a conductivity enhancer such as graphite powder may be combined as well. The mixture is mixed with an organic solvent as step  56 . The solution including at least the solvent, active material, and the binder is then cast onto an aluminum foil substrate as an active material coating as step  58 . The solvent-cast coating is heated and dried until the polymeric binder is cured as step  60 . The composite-coated aluminum foil is then compressed in a calender as step  62 . The assembly is compressed until the composite reaches a density ranging between about 3.3 g/cm 3  and about 3.7 g/cm 3 , and preferably until the composite reaches a density of about 3.5 g/cm 3 .  
         [0044]     After forming the electrode assembly  10 , a porous separator is provided between the positive and negative electrodes.  FIG. 6  depicts a separator  35  disposed between the positive electrode  10  and the negative electrode  50 . The separator  35  is formed from a porous polymer such as a polyolefin, with exemplary materials including polyethylene or polypropylene.  
         [0045]     As previously discussed, an exemplary positive electrode assembly includes a composite of active material and a polymeric binder that is compressed to a final density ranging between about 3.3 g/cm 3  and about 3.7 g/cm 3 , and preferably about 3.5 g/cm 3 . It is conventionally understood that a positive correlation exists between the extent that the positive electrode active material is compressed and the energy density. Indeed, comparative tests on both positive electrodes compressed to 3.09 g/cm 3  and the exemplary highly compressed positive electrodes provided data revealing that highly compressed electrodes have about 16% higher energy density. However, compressing the electrode to more than 3.09 g/cm 3  likely reduces the battery power capability and life cycle since high compression may have a detrimental effect on the battery electrical properties. This is because compression higher than 3.09 g/cm 3  could cause damage to the particles of active material and could reduce the amount of void space that allows electrolyte to permeate the active material. A relatively unimpeded ionic transport of electrolyte through an open pore structure is important for optimal battery function at high power. Consequently, positive electrode compression to densities higher than 3.09 g/cm 3  is conventionally thought to decrease battery power and current density, and increase impedance values. Yet, unexpectedly high capacity density values result from compressing the composite above 3.09 g/cm 3  as established in the following examples and data summarized in FIGS.  7  to  9 .  
         [0046]      FIG. 7  graphically depicts plots of experimental and expected data for the capacity density of a composite electrode coating (measured in mAh/cm 3 ), as a function of coating mass density (measured in g/cm 3 ). The experimental data represents a test for positive electrodes that were made by coating a slurry containing LiCoO 2  powder, graphite powder, polyvinylidine fluoride (PVDF) and n-methyl pyrrolidone (NMP) solvent onto a 20 micron thick layer of Al foil. The cured composition of the coating was 90% LiCoO 2 , 6% powdered graphite and 4% PVDF. The cured coating was calendered to thicknesses ranging from 58 to 70 microns, such that the coating density ranged from 3.09 to 3.76 g/cm 3 . Disks of 2 cm 2  were punched from these coatings and assembled into 2032-type coin cells. The cells were cycled versus a Li negative electrode at 0.2 mA between 4.2 and 3.0 V, in an electrolyte consisting of 1 M LiPF 6  in a mixture of propylene carbonate (PC), ethylene carbonate (EC) and diethyl carbonate (DEC).  
         [0047]     The data points in  FIG. 7  indicate experimentally measured values, and the solid line represents the theoretical result. The theoretical result was obtained by multiplying 427 mAh by the quotient ρ/3.09, wherein ρ is the mass density and 427 mAh is the capacity density measured when ρ is equal to 3.09 g/cm 3 . As depicted in  FIG. 7 , the experimental values fall above the theoretical line, indicating that densifying the electrodes above 3.09 g/cm 3 , results in a capacity density that is greater than what one would expect simply based on a linear scaling with the mass density.  
         [0048]     Particularly high capacity retention (i.e., low capacity fade) values were obtained when the composite was compressed to about 3.5 g/cm 3  as exemplified by the data presented in the graphs of FIGS.  8  to  9 . The improvement is further surprising in view of the traditional understanding that undesirable surface reactions on the negative electrode active material predominantly affect the capacity fade rate.  
         [0049]      FIG. 8  is a graph that represents discharge capacity data sets for a series of accelerated battery charging and discharging cycles. Specifically, the graph compares the discharge capacity for a representative battery having active material in the positive electrode compressed to 3.09 g/cm 3  with a battery having active material in the positive electrode compressed to 3.5 g/cm 3 . Both batteries had identical chemical compositions, and the active material composites in each of the positive electrodes consisted of LiCoO 2  at a concentration of 90% by weight, the polymeric binder polyvinylidine fluoride at a concentration of 4% by weight, and graphite powder as a conductivity enhancer at a concentration of 6% by weight. Electrodes were coated onto both sides of a 20 micron layer of Al foil and assembled into wound prismatic Li ion batteries using a conventional negative electrode, porous separator and electrolyte. The batteries were cycled at an accelerated rate by charging at 150 mA to 4.15 V and discharging at 75 mA to 2.75 until each battery had experienced more than 1000 charging and discharging cycles. Every 100 th  cycle, the cells were subject to slower cycles that consisted of a 50 mA charge and 12.5 mA discharge. These slower cycles show up as spikes in the capacity data, because they are less subject to impedance-related capacity loss. As depicted in  FIG. 8 , although both batteries initially had similar discharge capacities, the battery with the positive electrode active material compressed to 3.5 g/cm 3  consistently exhibited higher discharge capacity values than the battery having active material compressed to 3.09 g/cm 3 .  
         [0050]      FIG. 9  is a graph that represents discharge capacity data sets for batteries that experienced a series of weekly charge and discharge cycles. The batteries had identical compositions that were also identical to the batteries that underwent accelerated discharging cycles at 75 mA discharge rates, and weekly discharging cycles at 2 mA discharge rates for nearly two years. Each of the batteries experienced a discharge over a period of a week, for over 100 charge and discharge cycles. As depicted in  FIG. 9 , although both batteries initially had similar discharge capacities, the battery with the positive electrode active material compressed to 3.5 g/cm 3  consistently exhibited higher discharge capacity values than the battery having active material compressed to 3.09 g/cm 3 . The degree of reduction in capacity fade rate ranges from 35 to 41%, with the degree of capacity fade rate reduction being more profound at higher charge and discharge rates. Table 1 summarizes the cycle data for both the accelerated and weekly cycles.  
                                                             TABLE 1                               First   Cycled   Relative                   Cycle   Ca-   Capacity   Relative               Capacity   pacity   Lost since   Benefit of       Group   Cycle Type   (Ah)   (Ah)   Formation   Densification                                3.5 g/cm 3     Accelerated-   0.358   0.300   16%   41.0%       3.1 g/cm 3     801 cycles   0.359   0.260   27%       3.5 g/cm 3     Weekly-41   0.358   0.341   5%   35.3%       3.1 g/cm 3     cycles   0.359   0.333   7%                    
         [0051]     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.