WO1997001191A2

WO1997001191A2 - Nonaqueous battery with cathode active material mixture

Info

Publication number: WO1997001191A2
Application number: PCT/US1996/010760
Authority: WO
Inventors: Steven T. Mayer
Original assignee: Polystor Corporation
Priority date: 1995-06-22
Filing date: 1996-06-21
Publication date: 1997-01-09
Also published as: WO1997001191A3

Abstract

Composite cathodes (24) are disclosed which include a mixture of a first homogeneous cathode material and a second homegeneous cathode material, and binders as necessary. By choosing particular types and relative amounts of the first and second homogeneous cathode materials, the resulting composite cathodes can have matched formation and reversible cycling capacities (for a given anode material) and sloping discharge profiles. The first and second homogeneous cathode materials used in the composite cathodes disclosed herein may each be one of lithium molybdenum sulfides, lithium molybdenum oxides, lithium vanadium oxides, lithium chromium oxides, lithium titanium oxides, lithium tungsten oxides, lithium cobalt oxides, lithium nickel oxides, and lithium manganese oxides. Lithium ion cells having such composite electrodes are also disclosed herein. Such cells include a cell container (14), a non-aqueous electrolyte (34) and an anode (22) including a mixture of homogeneous graphitic carbon particles and homogeneous non-graphitic carbon particles.

Description

NONAQUEOUS ELECTROLYTE SECONDARY BATTERY

BACKGROUND OF THE INVENTION

The present invention relates to cathodes for use in electrochemical energy storage devices. More particularly, the invention relates to lithium insertion cathodes containing a mixture of two different homogeneous lithium transition metal containing materials.

Due to the increasing demand for portable electronic equipment, there is an increasing demand for rechargeable cells having high specific energies. In order to meet this demand, various type of rechargeable cells have been developed including improved nickel-cadmium aqueous batteries, various formulations of aqueous nickel metal hydride batteries, and, most recently, nonaqueous rechargeable lithium metal cells.

Although rechargeable lithium metal cells have high energy densities and specific energies, they have not gained wide-spread acceptance because they suffer from poor cycle life, discharge rate, and safety characteristics. As a result, some groups have developed rechargeable battery systems based on carbon anodes which intercalate lithium. Cells based upon such carbon intercalation systems are commonly referred to as "lithium ion," "lithium rocking chair," or "lithium intercalation" cells. Although these cells have a lower theoretical energy density than lithium metal cells, they are inherently safer and more rechargeable because they intercalate lithium ions rather than plate lithium metal.

Various cathodes have been studied and used in lithium ion batteries. These include lithium molybdenum sulfides, lithium molybdenum oxides, lithium vanadium oxides, lithium chromium oxides, lithium titanium oxides, lithium tungsten oxides, lithium cobalt oxides, lithium nickel oxides, and lithium manganese oxides (e.g., LiMnθ2 and LiMn2θ4). The preparation and use of lithium transition metal oxide cathodes are described in various publications including U.S. Patents Nos. 4,302,518 and 4,357,215 issued to Goodenough et al., which patents are incoφorated herein by reference for all purposes.

While these materials, particularly lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide (LiMn2θ4) have been found to be adequate cathode materials, they have some shortcomings. The most significant shortcomings derive from the fact that these materials must be used for both reversible cycling and formation in lithium intercalation cells. As a result, some extra cathode or anode material in excess of that required for reversible cycling must be provided. This excess increases the cost and reduces the specific energy of the cell. In a typical lithium ion cell having a lithium cobalt oxide cathode, the lithium cobalt oxide accounts for up to about 60% of the cell's material cost. Further, the excess of this material required for formation can reduce a cell's specific energy by about 10 to 15%.

Formation is a term that is used to describe electrode modification processes employed after a cell is assembled, but before it is reversibly cycled. Some cells do not require formation, while many others such as zinc nickel oxide and lithium ion cells do. In those cells that require formation, the process involves one or more irreversible cycles which drive an electrochemical reaction to produce some product that is necessary for optimal performance of the electrodes. After formation is complete, the cells may be cycled reversibly at expected capacities. In the case of lithium ion cells, a formation cycle irreversibly drives some lithium ions from the cathode material to a carbon anode where they are believed to form a surface film that has been found necessary to provide high energy cycling. The difficulty arises because the amount of cathode material required for formation is almost never the same as is required for subsequent reversible cycling. Thus, an excess or deficiency of cathode material must be provided in the cell.

In the case of lithium cobalt oxide used in a cell with a typical intercalation anode, extra oxide beyond the amount required to reversibly cycle against that anode is required for formation. Thus, some of the cathode material will be unused during each reversible cycle over the normal life of the cell. Lithium nickel oxide has the opposite problem — less of it is required for formation than for reversible cycling (against the same typical anode). Thus, if a cell is provided with an amount of lithium nickel oxide sufficient for formation, that cell will have insufficient cathode material to utilize the available anode material during subsequent reversible cycles. That is, the anode will be underutilized and thereby result in a reduction in the cell's specific energy. On the other hand, if more lithium nickel oxide is used in the cell (beyond that required for formation), some metallic lithium will plate onto the anode during formation.

In addition to formation/reversible cycling mismatch, other problems are associated with lithium metal oxide cathodes. First, the most widely used cathode material, lithium cobalt oxide, is quite expensive. Further, most lithium metal oxides have substantially flat discharge profiles (e.g., LiMn2θ4, LiCoθ2, LiNiθ2, and atomic mixtures of Mn, Co, and Ni oxides). That is, their voltage varies only slightly with state of charge until very nearly all of their capacity has been exhausted. An example of such behavior is illustrated in the LiCoθ2 discharge curve in Figure 4 which exhibits a characteristic sharp drop in voltage at the end of discharge. While such discharge characteristics provide high, relatively constant potentials during most of discharge, they fail to provide any indication as to when the battery is nearly completely discharged. Thus, there is no simple method of alerting users that such a battery must be recharged.

A flat discharge profile can also cause cells to perform poorly at high rates of discharge. This results from under-utilization of the electrode material which limits the rate, energy, and cycling performance of cells, as discussed in T. Fuller et al. J. Electrochem. Soc, 1, 114 (1994), incoφorated herein by reference for all puφoses.

It should be noted that mixed atomic oxides have been proposed as cathode materials in an attempt to address some problems with lithium transition metal oxides. The mixed oxides are compounds having the general formula LiMa_xMbi-._xO2. where Ma and Mb are two different transition metals. Such materials are described in such publications as Ohzuku et al., "Comparative Study of LiCoθ2, LiNiι/2Coj_/2θ2 and LiNiθ2 for 4 Volt Secondary Lithium

Cells", Electrochim Acta, 38, 1159-1167 (1993); Gummow et al. "Lithium-cobalt-nickel-oxide cathode materials prepared at 400°C for rechargeable lithium batteries", Solid State Ionics, 53-56, 681-687 (1992); and Reimers et al. "Effects of Impurities on the Electrochemical Properties of L1C0O2", J. Electrochem. Soc. 140, 2752-2754 (1993), each of which are incoφorated herein by reference for all puφoses. Unfortunately, such materials have suffered from poor cycle life due, possibly, to unstable lattice structures which collapse or rearrange after a few cycles.

In view of the above, there is a need for lithium insertion cathode materials which overcome the problems associated with prior insertion cathode materials.

SUMMARY OF THE INVENTION

The present invention provides improved cathode materials containing mixtures of two or more distinct and homogeneous cathode materials. These materials are preferably chosen such that the resulting cathode will have matched formation and reversible cycling capacities for a given anode material. Thus, the materials will typically be chosen such that one of them deintercalates on formation more lithium than necessary for reversible cycling and another of them deintercalates on formation less lithium than necessary for reversible cycling (both against the same anode material). By combining these materials in certain specified amounts, the resulting cathode will have a matched formation and reversible cycling capacity. Thus, the problems of unused anode or cathode capacity and potential lithium plating are eliminated. Further, the cathode material cost can be reduced by employing a less expensive cathode material (e.g., lithium nickel oxide) in combination with a more expensive cathode material (e.g., lithium cobalt oxide), while improving cathode performance over either of the constituent materials. Still further, the resulting cathode materials have been found to have somewhat sloped discharge profiles. Thus, the state of charge of a lithium ion cell can be more easily ascertained by monitoring the cell voltage.

The term "mixture" is used herein in the sense commonly employed in the chemical arts. Thus, a mixture of oxides in accordance with this invention is composed of distinct chemical species, and, in theory, can be separated by physical means. Typically, the mixtures of this invention will include "particles" of a first chemically distinct cathode material interspersed with "particles" of a second chemically distinct cathode material. However, other forms are possible, so long as there are shaφ phase boundaries between the distinct component materials. Various forms of both constituent cathode materials may be employed in the electrodes of this invention. Such particles may each assume various shapes such has fibers, plates, spheres, crystallites, etc. In each case, the moφhology of the particle may be somewhat smooth, rough, jagged, porous, fractious, etc. Still further, the size of the particles may vary widely from dust at one extreme to large continuous structures at the other extreme. In the latter case, the material making up the large structure will be interspersed with smaller particles of the material making up other component.

One aspect of the present invention provides a lithium insertion cathode material that may be characterized as including a mixture of two or more homogeneous lithium ion insertion materials, wherein the mixture is characterized by two ratios. The first ratio is the ratio between the per mass capacities ofthe mixture and a specified anode material required to reversibly charge and discharge a lithium ion cell, and the second ratio is the ratio between the per mass capacities of the mixture and the specified anode material required to put the lithium ion cell through formation. In this aspect of the invention, the relative amounts of the two or more homogeneous lithium ion insertion materials in the mixture are chosen such that the first and second ratios are substantially the same. Thus, the amount of cathode material required for formation of a cell is substantially the same as the amount required for reversibly cycling that cell. Preferably, the first and second ratios are within about 5 % (and more preferably within about 1 %) of one another.

Another aspect of the invention provides a lithium insertion cathode material which may be characterized as including a mixture of a first homogeneous lithium and transition metal containing material and a second homogeneous lithium and transition metal containing material, provided that such mixture is in a form that can (i) reversibly donate lithium ions during charge in a lithium ion cell, and (ii) reversibly incoφorate lithium ions during discharge in a lithium ion cell.

Regardless of how the resulting cathode material is characterized, the homogeneous cathode materials that make up the mixture preferably are each one of a lithium molybdenum oxide, a lithium vanadium oxide, a lithium chromium oxide, a lithium titanium oxide, a lithium tungsten oxide, a lithium cobalt oxide, a lithium nickel oxide, or a lithium manganese oxide. More preferably, the materials are each one of LiCoθ2, LiNiθ2, LiMnθ2 or LiMn2θ4. In especially preferred embodiments, the mixture includes either (i) LiCoθ2 and LiNiθ2 or (ii) LiCoθ2 and LiMn2θ4 such that the LiCoθ2 constitutes about 10 to 90 percent by mass of the mixture. Regardless of the chemical composition of the mixture, if that mixture is made from particles of the homogeneous components, the particles preferably have an average diameter of less than about 50 μm.

The cathodes of this invention typically include the following components in addition to the electrochemically active mixtures described above: a binder, an electronic conductor, and in some cases a material which protects against corrosion, and a material that controls the acidity/basicity of the electrolyte solution. Preferably, the resulting cathode material is provided as a film affixed to a current collector.

Yet another aspect of the invention provides a lithium ion cell which may be characterized as including (a) a cell container; (b) an anode provided within the cell container and capable of intercalating lithium during charge and deintercalating lithium during discharge; (c) an electrolyte conductive to lithium ions and provided within the cell container; and (d) a cathode provided within the cell container and capable of taking up lithium on discharge and releasing lithium on charge. The cathode includes a mixture of two or more homogeneous lithium ion insertion materials characterized by two ratios as described above. Preferably the anodes in such cells include a mixture of homogeneous graphitic carbon particles and homogeneous non-graphitic carbon particles. Further, the electrolytes in such cells preferably include one or more of the following solvents: propylene carbonate, ethylene carbonate, 1 ,2-dimethoxyethane, 1,2- diethoxy ethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolan, 4- methyl 1,3-dioxolan, diethyl ether, sulfolane, acetonitrile, propionitrile, glutaronitrile, dimethyl carbonate, diethyl carbonate, anisole, and mixtures thereof. Such solvents may be provided with one or more of the following salts: LiN(CF3S02)2, LiAsFό, LiPFό, L-BF4, LiB(C6H5)4, LiCl, LiBr, CH3SO3Li, and CF3SO3L The electrolyte may also include a polymer or gelling agent.

In one especially preferred embodiment, the electrolyte includes a mixture of ethylene carbonate and dimethyl carbonate and dissolved LiN(CF3SO2)2 at about 0.5 to 1.2 M and dissolved LiAsFό or LiPF6 at about 0.1 to 0.4 M, provided that the total concentration of Li(CF3Sθ2)2N and LiAsF6 or LiPF6 does not exceed the solubility limit of lithium in the solvent.

These and other features of the present invention will be presented in more detail in the following specification of the invention and in the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph showing how a cathode composition providing maximal theoretical specific energy (against a given anode) can be indentified by plotting a ratio of reversible cycling ratios (first ratio) and formation ratios (second ratio) as a function of cathode composition.

Figure 2a is a graph showing how a cathode composition providing maximal theoretical specific energy (against a given anode) can be indentified by plotting specific energy as a function of LiCoθ2-LiMn2θ4 cathode composition for anode capacity limited and cathode capacity limited cells. Figure 2b is a graph showing how a cathode composition providing maximal theoretical specific energy (against a given anode) can be indentified by plotting specific energy as a function of LiCoθ2-LiNiθ2 cathode composition for anode capacity limited and cathode capacity limited cells.

Figure 3 is an illustration of an experimental apparatus employed to test the carbon-based intercalation electrodes of the present invention.

Figure 4 is a graph displaying voltage (versus a lithium reference electrode) as a function of fractional discharge capacity (lithium insertion) for low-rate discharge of three electrodes: (1) a lithium cobalt oxide cathode, (2) a lithium nickel oxide cathode, and (3) a composite cathode containing lithium cobalt oxide and lithium nickel oxide.

Figure 5 is a graph displaying voltage (versus a lithium reference electrode) as a function of fractional discharge capacity (lithium insertion) for high-rate discharge of three electrodes: (1) a lithium cobalt oxide cathode, (2) a lithium nickel oxide cathode, and (3) a composite cathode containing lithium cobalt oxide and lithium nickel oxide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. Composite Cathode Compositions

Composite cathodes of this invention include a mixture of two or more chemically- homogeneous electrochemically-active cathode materials. The cathode materials making up the composite should each be capable of ( 1 ) intercalating lithium ions on discharge in a cell having a lithium ion intercalating anode (e.g., a carbon anode), and (2) deintercalating lithium ions on charge in the cell. Preferably, the relative amounts of the two or more cathode materials are chosen such that the cell is optimized for both formation and reversible cycling. That is, the relative amounts are selected such that the total mass of cathode material required for formation of a lithium intercalation anode is about equal to the mass of cathode material required to reversibly discharge the lithium intercalation anode. This optimizes the cell's energy density by utilizing substantially all ofthe anode and cathode materials in each cell cycle.

The following table and examples quantitatively illustrate the advantages of combining cathode materials in accordance with the present invention. In this example, the formation and reversible cycling behaviors of three typical cathode materials, LiCoθ2, LiNiθ2, and LiMn2θ4 are compared and used to develop a composite cathode which is optimized for a lithium ion cell having a graphite anode. The result is a cell that has no excess cathode material (composite of this invention) or anode material (e.g., graphite). Table

Material Reversible First Cycle Ratio of Rev. Ratio of 1st

Charge Charge Cap. to Cycle Cap.

Capacity Capacity Carbon Rev. to Carbon

Cap. Form. Cap

Graphite 372 432 —

LiCoθ2 137 147 0.368 0.340

LiNiθ2 132 176 0.355 0.407

LiMn2θ4 119 150 0.320 0.347

The second and third columns of Table 1 provide typical per mass capacities for graphite and the three different pure lithium transition metal oxides. Each listed capacity has the units mA-hr/gm. The second column "Reversible Capacity" provides the per mass capacity of each material for reversible cycling after formation. Thus, during reversible cycling, one gram of graphite will provide (or accept) up to 372 mA-hr of charge, while one gram of lithium cobalt oxide will accept (or provide) up to 137 mA-hr of charge. In a cell having a graphite anode and a LiCoθ2 cathode, the anode and cathode material will be matched for reversible cycling if they are provided in a per mass ratio of 137:372 graphite to LiCoθ2- That is, for every gram of LiCoθ2 in the cell, 137/372 or 0.368 gram of graphite should be also be used. The fourth column of Table 1 shows the per mass ratios of anode to cathode materials required to match these materials for reversible cycling.

The third column of Table 1 "First Cycle Charge Capacity" provides the per mass capacity of graphite and the cathode materials for formation plus reversible charging. This is the capacity for a cell's first charge in which sufficient charge must be passed for reversible charging plus formation of the carbon anode. Throughout most of the discussion herein, this charging cycle will simply be referred to a "formation cycle." As can be seen, the first cycle capacities are all higher than the corresponding reversible cycling capacities presented in the second column. For example, during a formation cycle, one gram of graphite will accept 432 mA-hr of charge, while one gram of lithium nickel oxide will provide 176 mA-hr of charge. In contrast, that same gram of graphite will accept at most only 372 mA-hr of charge and the same gram of LiNiθ2 will provide at most only 132 mA-hr of charge on reversible cycling. In a cell having a graphite anode and a LiNiθ2 cathode, the anode and cathode material will be matched for formation if they are provided in a per mass ratio of 176:432 graphite to LiCoθ2- That is, for every gram of LiNiθ2 in the cell, 176/432 or 0.407 gram of graphite should be also be used. The fifth column of Table 1 shows the per mass ratios of anode to cathode materials required to match these materials for formation.

As can be seen by comparing the values in columns four and five of Table 1, no cathode material has matching reversible cycling and formation requirements vis-a-vis graphite. Thus, as stated above, providing a cell with amounts of anode and cathode materials necessary to match for formation, will provide either too little or too much cathode material for reversible cycling. In the case of LiCoθ2, matching for formation will provide excess cathode material for reversible cycling. One gram of graphite will require 2.94 (1/0.340) grams of LiCoθ2 for formation, but that same gram of graphite requires only 2.71 (1/0.368) grams of LiCoθ2 for reversible cycling. In the case of LiNiθ2 and LiMn2θ4, matching for formation will provide too little cathode material for reversible cycling. This analysis shows that a graphite-LiCoθ2 cell matched for formation will have unused LiCoθ2 during reversible cycling and a graphite-LiNiθ2 cell matched for formation will have unused graphite during reversible cycling. In either case, the specific energy of the cell will be below that theoretically possible because of the excess anode or cathode material that does not participate in the cell's energy producing electrochemical reactions.

Ideally, a cell should be provided with anode and cathode materials that are matched for both reversible cycling and formation. That is, the electrode materials should have identical values of the first ratio (column 4) and the second ratio (column 5). This invention, provides matched cells by providing composite cathode materials designed to have identical first and second ratios for a given anode material. The invention provides such composite cathode materials as a simple mixture of two or more component materials. The relative amounts of the two or more materials may be determined empirically or through analysis.

In the simplest case, the first and second ratios are considered to vary linearly with mass fraction of component cathode materials. Figure 1 is a graph illustrating how an ideal cathode material composition can be derived using this assumption. In Figure 1, the ordinate represents the ratio "R" of the first and second ratios (i.e., first ratio/second ratio), and the abscissa represents the mass fraction of LiCoθ2 in a mixed cathode material. In this example, the first and second ratios are obtained assuming that the anode material is graphite. The value of R is plotted for both LiNiθ2-LiCoθ2 and LiMn2θ4-LiCoθ2 mixed cathode materials. The point at which a line equals 1, corresponds to the composition at which first and second ratios are equal, and at which a cell's specific energy is maximal. In the example presented, the ideal cathode composition should include about 48 weight % LiCoθ2 for LiMn2θ4-LiCoθ2 mixed cathodes, and about 67% weight % LiCoθ2 for LiNiO2-LiCoO2 mixed cathodes.

Another way to illustrate this approach is provided in Figures 2a and 2b. Referring first to

Figure 2a, a plot of theoretical specific energy versus mass fraction of LiCoθ2 for a LiCoθ2- LiMn2θ4 composite electrode is shown. The specific energy is that of a cell constructed of a graphite anode and a composite cathode. In this plot, the solid line represents the cell's theoretical specific energy as calculated based upon a cathode limited cell design and the dashed line represents the cell's specific energy based upon an anode limited cell design. The point of intersection represents the ideal composition of a mixed cathode material. As can be seen, the specific energy plots intersect at a composition of about 48 mass per cent LiCoθ2- Again, this composition represents the composite cathode material in which the formation and reversible cycling requirements match. The composition also represents the maximum theoretical specific energy that can be achieved with a mixture of the component cathode materials. In this case, that maximum specific energy is about 361 Watt-hr/kg.

Figure 2b displays the theoretical specific energy versus composite cathode composition for graphite and LiCoO2-LiNiO2 cells. Again, the solid line represents the cell's theoretical specific energy as calculated based upon a cathode limited cell design and the dashed line represents the cell's specific energy based upon an anode limited cell design. As can be seen, the specific energy plots intersect at a composition of about 67 mass per cent LiCoθ2 corresponding to a maximum specific energy is about 367 Watt-hr/kg.

The theoretical specific energy of a cell is the cell's total reversible electrochemical energy divided by the anode and cathode masses. The actual specific energy which is the cell's measured energy output divided by the cell's total mass is somewhat smaller because, among other reasons, the theoretical specific energy does not account for the mass ofthe cell electrolyte, casing, etc. In Figures 2a and 2b, the theoretical specific energy is determined as follows.

In the optimal case, as provided by this invention, the total capacities of the anode and cathode materials match for reversible cycling. In such case, there is no extra unused anode or cathode material. In Figures 2a and 2b, the anode limited case (dotted lines), assumes that the cell has excess cathode material that is not used. This is the typical case in a cell employing a pure LiCoθ2 cathode, where additional LiCoθ2 is included in the cell to ensure that formation is completed. In the cathode limited case (solid lines), the cell has excess anode material that is not used during reversible cycling. This is typically the case in a cell employing a pure LiNiθ2 cathode, where the amount of LiNiθ2 in the cell is limited to ensure that lithium plating does not occur during formation.

The lines presented in Figures 2a and 2b were calculated as follows. First, a specified amount of anode material (e.g., one gram) is assumed to be used in each cell. In the anode limited case, a cell's specific energy is the quotient of (1) the product of the cell's capacity (based upon the amount of anode material in the cell) and cell potential, and (2) the sum of the anode material and the cathode masses in the cell. In calculating the masses, the extra mass of cathode material required for formation must be included. In the cathode limited case, the specific energy is again the quotient of the cell's capacity times potential and the sum of the electrode masses. In this case, however, the cell's capacity is limited by the amount of cathode material in the cell. Further, the extra mass is provided by the anode not the cathode. In the curves presented in Figures 2a and 2b, the cell discharge voltages were taken as averages over the entire discharge and were as follows: 3.75 volts for graphite-LiCoθ2; 3.60 volts for graphite-LiNiθ2; and 3.83 volts for graphite-LiMn2θ4.

Stated symbolically, the specific energy for the cathode limited case was calculated by the following expression:

E = MC_DCV_avg /[l + M]

where E is the theoretical specific energy ofthe cell (in Whr/kg), M is the ratio ofthe masses of the cathode material in the cell to anode material in the cell (assuming a one gram basis of anode material in the cell, this is the total mass of cathode material in the cell in grams), C_DC is the reversible capacity ofthe cathode material (in mAhr/gm), and V_avg is the average cell voltage (in volts). The corresponding expression for the anode limited case is as follows:

E = C_DAV_avg/[l + M]

where C_DA is the capacity of one gram of anode material (in mA hr) and all other variable are as defined above.

It should be understood that the values presented in Table 1 and in Figs. 1, 2a and 2b are appropriate only for certain specific to materials: graphite and the listed cathode materials. Most other anode and cathode starting materials could be used to similar advantage but in different proportions. That is, most other cathode/anode combinations will have different values for the above defined first and second ratios. For example, some non-graphitic carbons have lower capacity requirements for formation and therefore have increased values for the second ratio when combined with the above cathode materials. Such materials may also have a different value for the first ratio. Of course, if other cathode materials are employed, the first and second ratios also will typically be different from those calculated above. In fact, two cathode materials of the same stoichiometry (e.g., LiCoθ2) but different preparation or processing methods may yield different first and second ratios when used in combination with a given anode material.

This highlights a general advantage of this invention. That is, the invention affords substantial flexibility in preparing cathode materials optimized for high specific energies. In general, any cathode material having identified first and second ratios for a given anode material can be optimized by mixing with another cathode material chosen to adjust the first and second ratios toward a common value. In this manner, promising cathode materials that have divergent first and second ratios can be improved by adding one or more appropriate supplemental cathode materials. It should be borne in mind that in designing a composite cathode, two components should be chosen having complementary first and second ratios. That is, if one component material has a first ratio that is greater than its second ratio, another component material should have second ratio that is greater than its first ratio. This ensures that mixtures of the materials will be better matched for formation and reversible cycling than the pure component materials.

Obviously, an important aspect of the invention involves selecting relative amounts of the two or more homogeneous lithium ion insertion materials such that the first and second ratios are substantially the same. Generally, any mixture which brings the first and second ratios closer together in comparison to the unmixed materials is within the scope of this invention. However, for many composite cathode materials such as mixed lithium transition metal oxides, the mixture will be chosen such that the first and second ratios are preferably within about 5% of one another, and more preferably within about 1% of one another.

In addition to providing higher energy densities, the composite cathodes of this invention provide an improved ability to monitor a cell's state of charge. This is because such cell's exhibit a discharge profile that is more sloped than their single component counteφarts. As illustrated in Figures 4 and 5, the discharge profiles of the single component cathodes drop off very shaφly near the end of discharge. In contrast, the discharge profiles of the composite cathodes drop off much more gradually near the end of discharge. Thus, it becomes fairly easy to engineer a mechanism to alert users that a battery is nearly discharged. When such cell discharge detection mechanism determines that the cell voltage drops to a prespecified level, it signals that the cell should be recharged.

While the composite cathode materials of this invention might be fabricated from any homogeneous lithium insertion cathode materials, preferred materials are lithium transition metal oxides or sulfides. Examples of suitable oxides and sulfides include lithium molybdenum sulfides, lithium molybdenum oxides, lithium vanadium oxides, lithium chromium oxides, lithium titanium oxides, lithium tungsten oxides, lithium cobalt oxides, lithium nickel oxides, and lithium manganese oxides (e.g., L-Mnθ2 and LiMn2θ4). From within this group, especially preferred component materials are LiCoθ2, LiNiθ2, LiMnθ2, and LiMn2θ4.

Preferred composite cathodes include (1) LiCoθ2 and LiNiθ2 mixtures and (2) LiCoθ2 and LiMn2θ4 mixtures. By way of example, the range of concentrations for LiCoθ2 in a mixed cathode containing LiCoθ2 and LiNiθ2 or containing LiCoθ2 and LiMn2θ4 is preferably between about 10 and 90 wt% LiCoθ2, and more preferably preferably between about 55 and 75 wt% LiCoθ2. In one specific example, for anode material containing a mixture of graphitic and non-graphitic carbons, a LiCoθ2 and LiNiθ2 mixed cathode contains about 30 wt% LiCoθ2- For the same anode material and a LiCoθ2 and LiMn2θ4 mixed cathode contains about 6.4 wt% LiCoθ2. The anode material used in this case contained 50% graphite and 50% nongraphitic carbon (by weight) obtained by pyrolyzing polyacrylonitrile at 1050°C. This anode material has a first cycle charge capacity of about 325 mA-hr/gm and has a reversible cycling capacity of about 260 mA-hr/gm. 2. Methods of Making Composite Cathodes

The component cathode materials employed to make composites of this invention preferably have average particle diameters of not greater than about 50 μm, and more preferably between about 1 and 20 μm. If necessary, the component materials may be ground and sieved, as with a hammer, ball, or attritive mill, to achieve the desired particle sizes. Often it will be enough to simply remove the particles that are larger than the upper the end of the desired range. Cathodes may be prepared from the cathode materials by any suitable method. In prefeπed embodiments, the cathode material components are first mixed and then combined with one or more of the following additives: a binder, an electronic conductor, an additive to prevent coπosion of cell metal components and a material to control the acidity or basicity of the electrolyte. Suitable binders are substantially unreactive and insoluble in a cell's electrolyte at the voltages which the cathode experiences within the cell. Such binders include polyvinylidene difluoride ("PVDF"), polytetrafluoroethane ("PTFE"), and ethylene propylene diene monomer ("EDPM"). Suitable electronic conductors include carbon and electronically conducting polymers. In a prefeπed embodiment the electronic conductor is a mixture of graphitic and non-graphitic carbons having in the range of about 95 to 50 wt% graphite, and more preferably in the range of about 65 to 80 wt% graphite. Suitable coπosion preventing additives include aluminum oxide And suitable buffers include lithium carbonate.

The above described cathode components can be formed into a cathode by various techniques. In especially prefeπed embodiments, the binder is provided in a solvent. By way of example, suitable solvents include cyclohexane with ethylene propylene diene monomer (EPDM) and dimethyl formamide (DMF) or dimethyl acetamide (DMA) with polyvinylidene difluoride (PVDF). Initially, all solid components of the cell — including lithium transition metal oxides, electronic conductors, etc. — are mixed. Next, they are combined in a slurry using a solution of the binder. In one relatively simple process, the cathode slurry is applied to a metal support which acts as a cuπent collector for the completed electrode. Preferably, the slurry is first applied as a thin film onto a aluminum foil substrate, the solvent of the slurry is then evaporated, the temperature of the composite is then heated to the melting point of the polymer binder, allowed to cool, and finally the composite is compressed onto the foil (e.g., by using a compression roller). The resulting structure is then simply sized for use in an electrochemical cell, and optionally preprocessed in another manner to provide the desired physical-chemical properties of an electrode. Such procedures are well known to those in the skill of the art. In a further prefeπed embodiment, the composite electrode is reheated after the compression step to allow the polymer binder to melt a second time. The composite may then compressed onto the foil a second time (e.g., by using a compression roller). This gives the finished electrode which may cut into sizes necessary for testing or cell assembly. Various cuπent collectors may be employed with the electrodes of the present invention. Preferably, the cuπent collector is a metal foil, metal screen, or an expanded metal screen (e.g., "Exmet" ™). If the cuπent collector is a foil, adhesion of the composite cathode mixture to the cuπent collector may be enhanced by roughening the cuπent collector's surface. Suitable methods of roughening the surface include mechanical roughening (e.g., with steel wool), chemical etching, and electrochemically etching, as are all known in the art.

3. Cells Containing Composite Cathodes

After the insertion cathode has been prepared, it is assembled in a lithium intercalation cell. Typically, the cell will include (1) a cell container, (2) a composite cathode prepared as described above, (3) an intercalation anode capable of reversibly taking up lithium on charge and releasing lithium on discharge, (4) an electrolyte conductive to lithium ions, and (5) a separator between the anode and cathode.

Conventional cell containers having venting capability may be used to fabricate cells from the composite cathodes of this invention. Those of skill in the art will recognize the required properties of a cell container. It should be sized to compactly hold the various cell components and should be made of materials that are impervious to and chemically resistant to the other cell components at operating cell potentials.

The material used as the intercalation cell anode should exhibit high capacity, good reversibility of lithium insertion, and a high average discharge voltage so as to achieve the largest possible energy of the cell. In addition, the material should exhibit a relatively low formation capacity. Such materials include, by way of example, graphitic carbons, non-graphitic carbons, and mixtures of graphitic and non-graphitic carbons. The latter are particularly prefeπed anodes for use with this invention. They are described in some detail in US Patent Application No. 08/386,062, entitled "NONAQUEOUS ELECTROLYTE SECONDARY BATTERY" and filed on February 7, 1995, naming S. Mayer as inventor. That application is incoφorated herein by reference for all puφoses. Briefly, such composite anodes include mixtures of homogeneous graphitic carbon particles, homogeneous non-graphitic carbon particles, and binders as necessary. Such electrodes can be formulated to have high capacities, low electrode potentials, and other desirable properties of graphite, and, at the same time, have discharge profiles in which the electrode potential varies significantly with the degree of intercalation. Thus, lithium ion cells employing such anodes will perform well at high rates of discharge.

Prefeπed intercalation anodes include at least about 25 mass percent homogeneous graphitic carbon particles, more preferably at least about 50 mass percent homogeneous graphitic carbon particles, and most preferably about 75 mass percent homogeneous graphitic carbon particles. Of course, the optimal ratios may vary quite a bit depending upon the carbon constituents of the mixture and the desired properties of the electrode. It is generally desirable that the mixture result in electrodes having a potential which varies significantly with state of charge (state of deintercalation). Preferably, the mixture should be chosen such that the resulting electrode has an open circuit potential of that varies by at least about 0.25 volts from a fully charged state in which the electrode is fully intercalated to a state of charge at about 90% deintercalation. For comparison, a pure graphite intercalation electrode generally varies by only about 180 mV during discharge.

The graphite used in this invention is a high purity natural graphite or a synthetic graphite having a high degree or anisotropic moφhological structure similar to natural graphite and very good compressibility and electrical conductivity. Suitable graphite includes, for example, SFG synthetic Graphites from Lonza Inc. of Fairlawn, NJ, Graphite KS (a round shaped particle) from Lonza, Graphite T (having a flake-shaped particle with higher surface area) also from Lonza, and grade B6-35 or 9035 from Superior Graphite Co. of Chicago, 111. Non-graphitic carbons of widely ranging properties may be employed in this invention. In general, the non-graphitic carbons should provide intercalation electrodes having sloping deintercalation profiles. The intercalation electrodes should also have a reasonably high capacity and a reasonably low voltage.

An organic electrolyte for use in the cell may include any of various acceptable compounds and salts. Suitable organic electrolytes for use in intercalation cells include one or more of the following: propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxy ethane, γ- butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolan, 4-methyl 1,3-dioxolan, diethyl ether, sulfolane, acetonitrile, propionitrile, dimethyl carbonate, diethyl carbonate, anisole, and mixtures or combinations thereof. Suitable electrolyte salts include one or more of the following: lithium bis-trifluoromethane sulfonimide (Li(CF3SO2)2N available from 3M Coφ. of Minnesota), LiAsF6, LiPF6, L1BF4, LiB(C6H5)4, LiCl, LiBr, CH3S03Li, and CF3SO3Li. In a prefeπed embodiment, the electrolyte includes a mixture of ethylene carbonate and dimethyl carbonate as the solvent together with Li(CF3SO2)2 , and LiAsFβ or LiPF6- In a particularly prefeπed embodiment, the electrolyte includes a mixture of (1) ethylene carbonate and dimethyl carbonate as solvent, (2) dissolved Li(CF3SO2)2 (about 0.5 to 1.2 M), and (3) dissolved LiAsF or LiPFό (either of which is present in a concentration of about 0.1 to 0.4 M). The total concentration of Li(CF3SO2)2N and LiAsF6 or LiPF6 should not exceed the solubility limit of lithium in the solvent. Thus, the total concentrations of these salts will generally be maintained below about 1.5 M.

Various separators known and widely-used in the art may be employed in the cells of this invention. Two particularly prefeπed separator materials are Celgard 2400 and Celgard 2500 available from Hoechst Celanese of Dallas, Texas. These materials are thin and inert and resist puncture.

u 4. Examples

The following examples compare discharge curve profiles for LiCoθ2, LiNiθ2, and composite electrodes made from LiCoθ2 and LiNiθ2- The graphs presented as Figures 4 and 5 show electrode voltage versus fractional lithium insertion (discharge capacity) for each electrode. In each graph, the data for the composite electrode was derived from an electrode made from a 50:50 mixture of the two component cathode materials. The examples illustrate that generally all three cathodes have voltage versus discharge capacity curves that remain relatively flat throughout most of discharge. But, unlike the cathodes made from pure component materials, the composite cathode begins a sloping discharge profile well before it is completely discharged. This behavior can be exploited to make a system that detects when a cell should be recharged.

While several combinations of cathode materials, polymer binder, and solvent can be used to fabricate electrodes in this invention, all examples described below use a cathode prepared generally as follows. 60 gm of cathode oxide (either pure or a mixture for the composite cathodes of this invention) from FMC Coφoration of Gastonia, NC was combined with 6.48 gm SFG- 15 graphite from Lonza Coφoration from Fairlawn, NJ and 2.52 gm of Vulcan Carbon Black from Cabot Coφoration of Billerica, MA in a large, 1 liter beaker. Next 39.4 ml (39.75 gm) of binder solution (1 gm PVDF/10 cc Dimethylformanide (DMF)) and 20.7 ml (19.87 gm) of additional DMF were added, covered, and mixed using an electronic stiner for 1 hour. This resulted in a uniform slurry suspension. The PVDF was obtained from Alf Atochem of Philadelphia, PA and the DMF was obtained from DuPont Coφoration of Wilmington, DE.

Aluminum foil was placed on a glass plate and held in place using tape. A small amount of slurry was placed on one end of the foil using a syringe. A metallic rod wound with wire (known as a "Mayer Rod") was used to evenly spread the thin film of slurry onto and across the foil. The slurry film was then partially dried by blowing forced hot air across its surface (using a hair dryer) until the surface appeared dry. Next, the foil/film was placed onto a 200°C hot plate where the remaining solvent was removed, and the PVDF polymer was melted. After removing the foil/film from the hot plate and allowing it to cool, it was run through a set of compression roller (force on electrode about 2000 lb/inch) several times. The resulting compressed film and attached foil constituted an electrode. The electrode was cut into small samples, weighed, and surrounded in aluminum expanded metal for electrochemical testing.

Figure 3 illustrates a cell 10 employed in the examples described herein. The cell includes a test tube 14 which together with a screw-in top 18 serves as the cell container. Screw-in top 18 also provides the necessary electrical connections for a composite cathode working electrode 20, a lithium counter electrode 22, and a lithium reference electrode 24. The working electrode assembly 20 includes a porous nylon separator placed around both a composite cathode 26 and a piece of expanded aluminum metal 30 (of a size substantially larger than that of the electrode). The cell contained 50 cm- of an electrolyte 34 containing 0.5 M Li(CF3Sθ2)2N and 0. IM LiAsFό in a 50:50% (by volume) solution of dimethylcarbonate ("DMC") and ethylene carbonate ("EC"). The tests were run at room temperature (about 18° C).

In the following two examples, the working electrodes were prepared as followed. In "low rate" discharge experiments, the working electrode was depleted of lithium at a rate of 20.3 mA/gm cathode material until the potential reached 4.2 V (versus the lithium reference electrode), after which the potential was maintained at that value for 8 hours while the cuπent decreased and the electrode reached equilibrium. After these steps (both of which are charge steps with respect to the lithium metal electrode), the working electrode was replenished with lithium at a constant cuπent of 20.3 mA/gm cathode until the potential reached 2.5 V (a discharge step). This discharge required 7 hours. The voltage versus discharge capacity (mA-hr/gm) for this cell is shown in the curves of Figure 4.

In "high rate" discharge experiments, the working electrode was depleted of lithium at a rate of 79.2 mA gm cathode material until its potential reached 4.2 V (versus the lithium reference electrode), after which the potential was maintained at that value for 2 hours while the cuπent decreased and the electrode approached equilibrium. After these steps (both of which are charge steps with respect to the lithium metal electrode), the working electrode was replenished with lithium at a constant current of 79.2 mA/gm cathode until the potential reached 2.5 V (a discharge step). This discharge required 1.5 hours. The voltage versus discharge capacity (mA-hr/gm) for this cell is shown in the curves of Figure 5.

In Figures 4 and 5, the voltage versus lithium insertion (displayed as "discharge capacity") for the LiCoθ2 electrode is shown as the solid curve, for the LiNiθ2 electrode is shown as the dashed curve, and for the composite LiCoO2-LiNiO2 cathode is shown as the dotted curve. As can be seen from the curves in Figure 4, the "low-rate" voltage versus discharge capacity curves for all three cathodes remain relatively flat throughout most of discharge. When the composite cathode is nearly two-thirds discharged, however, its curve begins sloping downward. In contrast, the cathodes made from the component materials LiCoθ2 and LiNiθ2 have rather flat discharge profiles until they are nearly 90% discharged. Similar results for the "high-rate" voltage versus discharge capacity curves are shown in Figure 5. Again most notably, the composite cathode begins a gradual decrease in voltage with further discharge when it is nearly two-thirds discharged. As mentioned above, this property can be utilized to develop simple and accurate detection systems for alerting users that recharge will be necessary.

Although the foregoing invention has been described in some detail for puφoses of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. For instance, although the specification has primarily described a process for preparing electrodes for use in lithium ion cells, the composite cathode materials disclosed herein may have other applications as well. For example, they may; be used in lithium metal cells as well. Further, other sodium, potassium, etc. based composite cathode materials prepared in accordance with this invention may be employed for non-lithium cells. Further, the general approach of mixing cathode materials to obtain composite cathodes having matched formation and reversible cycling requirements can be applied to other systems besides lithium insertion cathodes, and is even applicable to composite anodes. Therefore, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.

Claims

1. A lithium insertion cathode material, said electrode comprising a mixture of a first homogeneous lithium and transition metal containing material and a second homogeneous lithium and transition metal containing material, wherein the mixture is provided in a form that can (i) reversibly donate lithium ions during charge in a lithium ion cell, and (ii) reversibly incoφorate lithium ions during discharge in a lithium ion cell wherein the first and second homogeneous lithium and transition metal containing materials are provided in relative amounts selected such that the total amount of cathode material required for formation of a lithium intercalation anode is about equal to the amount of cathode material required to reversibly discharge said lithium intercalation anode.

2. The lithium insertion cathode material of claim 1 wherein the first and second homogeneous lithium and transition metal containing materials are oxides.

3. The lithium insertion cathode material of claim 2 wherein each of the first and second homogeneous lithium and transition metal containing materials are selected from the group consisting of lithium molybdenum oxides, lithium vanadium oxides, lithium chromium oxides, lithium titanium oxides, lithium tungsten oxides, lithium cobalt oxides, lithium nickel oxides, and lithium manganese oxides.

4. The lithium insertion cathode material of claim 3 wherein each of the first and second homogeneous lithium and transition metal containing materials are selected from the group consisting of LiCoO2, LiNiO2, LiMnO2 and LiMn2O4.

5. The lithium insertion cathode material of claim 4 wherein the first homogeneous lithium and transition metal containing material is LiCoO2 and the second homogeneous lithium and transition metal containing material is LiNiO2.

6. The lithium insertion cathode material of claim 6 wherein the first homogeneous lithium and transition metal containing material is LiCoO2 and the second homogeneous lithium and transition metal containing material is LiNiO2, and wherein the LiCo02 constitutes about 10 to 90 percent by mass of the mixture.

7. The lithium insertion cathode material of claim 6 wherein the first homogeneous lithium and transition metal containing material is LiCoO2 and the second homogeneous lithium and transition metal containing material is LiMn2O4, and wherein the LiCo02 constitutes about 10 to 90 percent by mass of the mixture.

8. The lithium insertion cathode material of claim 1 further comprising a binder of a type and in an amount that effectively binds the first and second homogeneous lithium and transition metal containing materials.

9. The lithium insertion cathode material of claim 9 wherein the binder is selected from the group consisting of polyvinylidene difluoride, polytetrafluoroethane, and ethylene propylene diene monomer.

10. The lithium insertion cathode material of claim 9 further comprising one or more materials selected from the group consisting of an electronic conductor, a material which protects against coπosion, and a material that controls acidity.

11. The lithium insertion cathode material of claim 1 1 wherein the cathode material includes a mixture of graphitic and non-graphitic carbons as an electronic conductor.

12. The lithium insertion cathode material of claim 1 wherein the first and second homogeneous lithium and transition metal containing materials are provided as particles having average particle sizes of not greater than about 50 am.

13. The lithium insertion cathode material of claim 11 wherein the first and second homogeneous lithium and transition metal containing materials are provided as particles having average particle sizes of between about 1 and 20 ,um.

14. The lithium insertion cathode material of claim 1 wherein the mixture of the first and second homogeneous lithium and transition metal containing materials serves as a film affixed to a cuπent collector.

15. A lithium insertion cathode material for use in a lithium ion cell of the type having a specified anode material which must undergo one or more formation cycles, the cathode material comprising: a mixture of two or more homogeneous lithium ion insertion materials, wherein the mixture is characterized by two ratios, a first ratio which is the ratio between the per mass capacities of said mixture and said specified anode material required to reversibly charge and discharge said lithium ion cell, and a second ratio which is the ratio between the per mass capacities of said mixture and said specified anode material required to put the lithiurrrion cell through formation, and wherein relative amounts of the two or more homogeneous lithium ion insertion materials in the mixture are chosen such that the first and second ratios are substantially the same.

16. The lithium insertion cathode material of claim 16 wherein the first and second ratios are within about 5 % of one another.

17. The lithium insertion cathode material of claim 17 wherein the first and second ratios are within about 1 % of one another.

18. The lithium insertion cathode material of claim 16 wherein each of the two or more homogeneous lithium ion insertion materials are selected from the group consisting of lithium molybdenum sulfides, lithium molybdenum oxides, lithium vanadium oxides, lithium chromium oxides, lithium titanium oxides, lithium tungsten oxides, lithium cobalt oxides, lithium nickel oxides, and lithium manganese oxides.

19. The lithium insertion cathode material of claim 19 wherein the two or more homogeneous lithium ion insertion materials are LiCoO2 and LiNiO2.

20. The lithium insertion cathode material of claim 20 wherein the LiCoO2 constitutes about 10 to 90 percent by mass of the mixture.

21. The lithium insertion cathode material of claim 19 wherein the two or more homogeneous lithium ion insertion materials are LiCoO2 and LiMn204.

22. The lithium insertion cathode material of claim 22 wherein the LiCoO2 constitutes about 10 to 90 percent by mass of the mixture.

23. The lithium insertion cathode material of claim 16 further comprising a binder of a type and in an amount that effectively binds the first and second homogeneous lithium transition metal containing materials, wherein the binder is selected from the group consisting of polyvinylidene difluoride, polytetrafluoroethane, and ethylene propylene diene monomer.

24. The lithium insertion cathode material of claim 24 further comprising one or more materials selected from the group consisting of an electronic conductor including a mixture of graphitic and non-graphitic carbons.

25. The lithium insertion cathode material of claim 16 wherein the first and second homogeneous lithium and transition metal containing materials are provided as particles having average particle sizes of not greater than about 50 ~m.

26. The lithium insertion cathode material of claim 16 wherein the mixture of the first and second homogeneous lithium and transition metal containing materials serves as a film affixed to a cuπent collector.

27. A lithium ion cell comprising:

(a) a cell container;

(b) an anode provided within the cell container and capable of intercalating lithium during charge and deintercalating lithium during discharge, the anode including a specified anode material;

(c) an electrolyte conductive to lithium ions and provided within said cell container; and

(d) a cathode provided within the cell container and capable of taking up lithium on discharge and releasing lithium on charge, the cathode including a mixture of two or more homogeneous lithium ion insertion materials, wherein the mixture is characterized by two ratios,

a first ratio which is the ratio between the per mass capacities of said mixture and said specified anode material required to reversibly charge and discharge said lithium ion cell, and

a second ratio which is the ratio between the per mass capacities of said mixture and said specified anode material required to put the lithium ion cell through formation, and

wherein relative amounts of the two or more homogeneous lithium ion insertion materials in the mixture are chosen such that the first and second ratios are substantially the same.

28. The lithium ion cell of claim 28 wherein the first and second ratios are within about 5 % of one another.

29. The lithium ion cell of claim 29 wherein the first and second ratios are within about 1

% of one another.

30. The lithium ion cell of claim 28 wherein each of the two or more homogeneous lithium ion insertion materials are selected from the group consisting of lithium molybdenum sulfides, lithium molybdenum oxides, lithium vanadium oxides, lithium chromium oxides, lithium titanium oxides, lithium tungsten oxides, lithium cobalt oxides, lithium nickel oxides, and lithium manganese oxides.

31. The lithium ion cell of claim 31 wherein the two or more homogeneous lithium ion insertion materials are LiCoO2 and LiNiO2 or are LiCoO2 and LiMn2O4.

32. The lithium ion cell of claim 32 wherein the LiCo02 constitutes about 10 to 90 percent by mass of the mixture.

33. The lithium ion cell of claim 28 wherein the anode includes a mixture of homogeneous graphitic carbon particles and homogeneous non-graphitic carbon particles.

34. The lithium ion cell of claim 28 wherein the electrolyte includes one or more of the following: polyacrylonitrile, polyvinylidene difluoride, and polyethyleneoxide.

35. The lithium ion cell of claim 28 wherein the electrolyte includes one or more of the following: propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, l,2diethoxyethane, g-butyrolactone, tetrahydrofuran, 2-methy ltetrahydrof uran, 1,3-dioxolan, 4methyl 1,3-dioxolan, diethyl ether, sulfolane, acetonitrile, propionitrile, glutaronitrile, dimethyl carbonate, diethyl carbonate, anisole, and mixtures thereof.

36. The lithium ion cell of claim 36 wherein the electrolyte further includes one or more of the following salts: LiN(CF3S02)2, LiAsFό, LiPF6, LiBF4, LiB(C6Hs)4, LiCl, LiBr, CH3SO3Li, and CF3SO3Li.

37. The lithium ion cell of claim 37 wherein the electrolyte includes a mixture of ethylene carbonate and dimethyl carbonate and dissolved LiN(CF3SO2)2 at about 0.5 to 1.2 M and dissolved LiAsFό or LiPFό at about 0.1 to 0.4 M, wherein the total concentration of Li(CF3SO2)2N and LiAsFό or LiPFό does not exceed the solubility limit of lithium in the solvent.

38. A method of preparing a lithium insertion cathode, said method comprising the following steps:

(1) applying a slurry film to a cuπent collector, the slurry containing a binder, a solvent for the binder, and a mixture of a first homogeneous lithium and transition metal containing material and a second homogeneous lithium and transition metal containing material, wherein the mixture is provided in a form that can (i) reversibly donate lithium ions during charge in a lithium ion cell, and (ii) reversibly incoφorate lithium ions during discharge in a lithium ion cell;

(2) evaporating the solvent from the slurry to leave a composite on the cuπent

(3) compressing the composite on the cuπent collector to form said lithium

39. The method of claim 39 further comprising a step of heating the composite to a temperature aufficient to cause said binder to flow after the step of compressing.

40. The method of claim 40 further comprising a step of recompressing the composite after the step of heating the composite.

41. The method of claim 39 wherein the binder is selected from the group consisting of polytetrafluoroethylene, polyvinylidene difluoride, and ethylene propylene diene monomer.

42. The method of claim 42 wherein the binder is polyvinylidene difluoride and the solvent for the binder is dimethyl formamide or dimethyl acetamide.