Patent Application
20070072082
Most of the disclosures generally relate to batteries, and more particularly relate to lithium ion rechargeable batteries having electrodes formed from compressed composite films.
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's disease and epilepsy, and other psychological, emotional, and other physiological conditions.
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.
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.
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 LiCoO2, LiNiXCO(1-X)O2, LiMn2O4, and LiNi0.33Mn0.33Co0.33O2. LiCoO2 is a particularly suitable positive electrode active material for lithium ion batteries because LiCoO2 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 LiCoO2 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.
One method of manufacturing a dense LiCoO2 positive electrode for a lithium ion battery includes depositing a coating of the LiCoO2 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/cm3. For example, in a conventional arrangement the coating is deposited to an initial loading density of 22 mg/cm2 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/cm3.
Battery manufacturers are continuously seeking for technology advancements that will improve a battery's energy density and power density, and will also decrease a battery'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/cm3 will reduce the battery power capability and the battery cycle life since high compression reduces the accessibility of electrolyte through the electrode pores.
Accordingly, it is desirable to overcome the limitations associated with conventional batteries.
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/cm3. In one preferred embodiment, the alkali metal active material is compressed to 3.5 g/cm3.
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/cm3. In one preferred embodiment, the active material mixture is compressed to 3.5 g/cm3.
The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and
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.
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.
Although the lead 324 is positioned to stimulate a specific site in the spinal cord 330 in
Furthermore, electrodes 326 may be epidural, intrathecal or placed into spinal cord 330 itself.
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.
In the particular embodiment depicted in
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.
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
A connector tab 70 associated with the positive electrode 10 protrudes from the electrode assembly 100 as depicted in
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.
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.
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 LiCoO2, LiNixCo(1-x)O2, LiMn2O4, and LiNixMnyCozO2 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.
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.
The binder concentration ranges between about 2% and about 10% by weight.
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.
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.
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, SnO2, 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.
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).
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 LiPF6, 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 LiPF6 as the lithium salt.
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.
A procedure for fabricating the positive electrode assembly 10 will now be discussed in conjunction with
After forming the electrode assembly 10, a porous separator is provided between the positive and negative electrodes.
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/cm3 and about 3.7 g/cm3, and preferably about 3.5 g/cm3. 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/cm3 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/cm3 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/cm3 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/cm3 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/cm3 as established in the following examples and data summarized in FIGS. 7 to 9.
The data points in
Particularly high capacity retention (i.e., low capacity fade) values were obtained when the composite was compressed to about 3.5 g/cm3 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.
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.
