WO1999000860A1

WO1999000860A1 - Transition manganese oxide cathodes

Info

Publication number: WO1999000860A1
Application number: PCT/US1998/013226
Authority: WO
Inventors: David L. Chua; Hsiu-Ping Lin; Hung-Chieh Shiao
Original assignee: Maxpower, Inc.
Priority date: 1997-06-27
Filing date: 1998-06-25
Publication date: 1999-01-07

Abstract

An improved cathode material for use in primary and secondary non-aqueous active metal cells is described and includes an amount of conductive carbon combined with an amount of LixMn4Oz (0≤x≤5 and 8≤z≤9).

Description

TRANSITION MANGANESE OXIDE CATHODES

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates generally to primary or secondary non-aqueous electrochemical cells or batteries utilizing a lithium ion containing anode in combination with a liquid-based or polymer-based electrolyte and chargeable metal oxide cathode materials. More particularly, the invention relates to novel transition metal oxide, namely, transition manganese oxide cathode materials for use in both secondary (rechargeable) and primary batteries.

II. Related Art Traditional lithium based rechargeable batteries using lithium metal as the anode have been found to have limited cycle life due to poor integrity of the plated lithium morphologies in safety issues due to high surface area lithium dendrite. When the anodes are subjected to charge and discharge cycles, a certain amount of the metallic lithium is electro-deposited onto the current collecting substrate as a powder material, which then converts into a dendrite-shaped crystalline form during the course of the charge cycle. These dendrite crystals are highly conductive, relatively sharp, hard deposits and the material is of sufficient strength to penetrate even the strongest conventional separator material or to form through the fine holes or interstices in fabric separator structures building to eventually contact a positive electrode causing undesirable cell shorts. One approach to reducing or eliminating dendrite formation in rechargeable lithium-based batteries involves the use of a carbonaceous material susceptible of being intercalated and de-intercalated, i.e., that reversibly intercalates, in combination with lithium containing metal complex oxide cathodes. This lithiated cathode material provides a source of lithium during the first charging of this type of battery and the intercalation and de- intercalation of lithium in the carbonaceous material during the charge and discharge half cycles greatly reduces dendrite formation. The problem of dendrite formation has been addressed, for example, in U.S. Patent 5 292 601 to Sugeno et al, in which dendrite formation is inhibited by the use of lithium in the form of a carbon-lithium intercalation compound for the negative electrode. A system using a carbonaceous anode and a rechargeable lithium system is also described in European Patent Application 0 634 805 Al, published June 23, 1994.

With respect to the cathode materials, however, there remains a need for a material with higher lithium capacity which, in turn, would produce an energy density performance that is significantly higher than the traditional nickel- cadminium and nickel-zinc battery systems or than even the more current nickel metal hydride and the Li ion rechargeable systems. This need is particularly significant where battery size is important and reduction in battery size for a given power output, i.e., a greater energy density, would represent a significant advance in the art. Specialized batteries for medical uses, specifically, implantable Pacer batteries and externally charged Ventricular Assist Devices (VAD) would particularly benefit from such innovations. The present invention addresses this shortcoming by utilizing cathode intercalation materials of superior lithium capacity. such materials include certain transition metal oxides, notably manganese oxides. Particularly, the present invention enables the cycling of Li_xMn₄O_z where 8< z < 9 in a manner which yields an energy density (discharge capacity) > 250 mAh/g.

Accordingly, it is a primary object of the present invention to provide a higher energy density by employing more efficient cathode material for lithium-based primary and secondary batteries.

Another object of the invention is to provide a new cathode material of higher energy density useful for implantable lithium-based primary Pacer batteries and secondary VAD batteries.

A further object of the invention is to increase the energy density by using Li_xMn₄0₂ where 1.8 < x ≤ 2.2 and 8< z < 9 and more specifically, 1.9 < x < 2.1 and 8.3 ≤ z ≤ 8.7 cathodes in combination with either lithium-based or lithium-ion-based anodes by increasing the available electron exchange between the charged and discharged material. Yet still another object of the present invention is to provide Li_xMn₄O_z 0.5 < x < 4.5 , 8 < z < 9 cathodes for use in combination with either lithium-based or lithium-ion- based anodes in a higher energy density, non-aqueous electrochemical battery. Other objects and advantages of the invention will occur to those skilled in the art upon familiarization with the descriptive materials contained in the specification, drawings and claims herein.

SUMMARY OF THE INVENTION In accordance with the present invention, an improved lithium, manganese oxide cathode material is provided for use in combination with either lithium-based or lithium- ion-based anode materials in both primary and secondary non-aqueous cells and batteries. The combinations of the invention enable the realization of very high energy densities which may exceed 250 mAh/g. The present invention involves the use of Li_xMn₄O_z as the cathode material. In the fully charged state for the material of the invention x is generally in the range of 0.5 to 1; and in the fully discharged state, x is in the range of 4 to 4.5.

In the current invention, the material, Li_xMn₄O_z, 1.8 < x < 2.2 and 8 < z < 9 and more specifically, 1.9 < x < 2.1 and 8.3 < z < 8.7 is prepared by a solid-state reaction process. In particular, it has been found that lithium- manganese oxide materials can be synthesized from any decomposable, i.e., calcinable, Li and Mn salts, including carbonates, hydroxides, oxides, nitrates and acetates. The solid state reaction is a diffusion controlled process. In order to obtain a completely reacted and uniform product, both intimate contact between reacting species and uniform distribution of each species are needed.

For example, using LiOH»H₂0 and MnC0₃ in powdered form, as the decomposable Li and Mn salts, the intimately mixing of the two different powders, has been achieved by first dissolving the LiOH^»H₂0 in de-ionzed (D.I.) water, then, mixing the MnC0₃ powder into the slurry and homogenizing the slurry at 4,000 - 6,000 rpm for 5 - 30 minutes. After evaporation of the D.I. water at 100°C, the soft powder cake is blended as by using a Waring high speed blender for 1 to

5 minutes. Using this mixture, Li_xMn₄0_z, can be synthesized at 350 to 450°C by reaction of stoichiometric amounts (for example, in a mole ratio of 1 to 2) of thoroughly mixed

LiOH»H₂0 and MnC0₃ powders in an oxygen-rich environment.

2 LiOH»H₂0 + 4 MnC03₃ + (z-5) 0₂ > Li₂Mn₄O_z + 4 C0₂ + 3 H₂0 The reaction conditions including calcination temperature (from 350 to 500°C) , dwell time (20 to 60 hours) , and reaction environments (0₂ and/or air) have been evaluated for the solid state reaction process. Some of the reaction condition ranges are listed as follows: Atmosphere air and/or 0₂

Reaction Temp 350 - 500°C Heating Rate 1 - 5 °C/min Reaction Time 20 to 60 hours Cooling Rate 1 - 10 °C/min A cathode having the composition, for example, of 87% Li_xMn₄O_z, 10% conductive carbon and 3% polyvinylidene fluoride (PVDF) binder, is used in a three-electrode (Li- cathode-Li) electrochemical cell stack. The cell is then sealed in an aluminized trilaminate plastic bag. Electrolyte solution (e.g., 1.0 M LiPF₆/ethylene carbonate (EC) : dimethyl carbonate (DMC) :diethyl carbonate (DEC) at volume ratios, for example, at 50/40/10) can then be injected into the bag using a syringe. This electrochemical cell can then be charged at constant current to the predetermined levels of state-of-charge, or discharged at constant current to the pre-determined levels of state-of- discharge to obtain Li_xMn₄0₉ (0.5 < x < 1) and Li_xMn₄0₉ (4 < x < 4.5), respectively. To insure reaction uniformity, low discharge current density should be used, typically, 0.5 ma/cm².

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, wherein like numerals are utilized to designate like parts throughout the same:

FIGURE 1 is an exploded schematic view of a cell including a cathode in accordance with the invention;

FIGURE 2 is a schematic view showing the cell of FIGURE 1 in the assembled configuration;

FIGURE 3 is a graphical representation of cell charge capacity over several cycles in the cut off potential range vs. Li of 3.75 - 1.5V; and

FIGURE 4 is a graphical representation showing cell discharge capacity for cathode samples synthesized at various temperatures over the temperature range of 375 to 450°C.

FIGURE 5 is a flow chart for an example of the Li_xMn₄0_z synthesis process. DETAILED DESCRIPTION

The invention involves using innovative transition manganese oxide cathode technologies for both rechargeable and primary implantable batteries. Specific examples of uses include implantable pacer batteries and in an external charger for a Ventricular Assist Device (VAD) . The invention extends the operating range of lithium concentrations of Li_xMn₄0_z from 1.0 < x -< 4.0 to 0.5 -< x -< 4.5 and possibly to 0 ≤ x ^ 5. The Li₂Mn₄O_z material is known to have a defect spinel structure with cubic symmetry. This spinel framework provides a three- dimensional interstitial space for Li ion diffusion in and out and is retained during lithiation and delithiation. It has been observed that cells incorporating Li₂Mn₄0₉ as a cathode and Li as the anode can be cycled with reversible electrochemical performance realized between the lithium compositional range (x value) of 2 to 5 at a 3.0 volt level.

By means of the present invention reversibility in the lithium compositional range below 2, i.e., of 2 to 0 has also been demonstrated as feasible for cathode material synthesized as Li_xMn₄O_z. The operating range of Li_xMn₄O_z cathode can now be maximized between x = 0 to 5. Delineated below are the alternate net reaction routes for Li₂Mn₄0₂, when, for example, x=2 is used as the starting cathode material:

Discharge Li₂Mn₄O_z + x Li ** Li_2+xMn₄O_z x < 3 E = 3V

Charge

Charge Li₂Mn₄O_z ** y Li + Li₂__yMn₄0, y ≤ 2 E = 4V Discharge

For the Li-based pacer battery application, the optimum cathode candidate is Li₀Mn₄O_z where O < x < 0.5, and the net electrochemical reaction during discharge is :

Discharge

Li₀Mn₄O_z + x Li ^→ Li_xMn₄O_z 0 < x < 5

The invention contemplates in one embodiment Li_xMn₄0₂, where 0.5 < x < 2.0 and where 8.3 < z < 8.7, can be employed as a cathode for a fully charged implantable lithium-based primary pacer battery. The maximum theoretical capacity of Li_xMn₄O_z cathode operating between lithium concentration limits of 0 to 5 is 337 mAh/g, of which 142 mAh/g is delivered at approximately 4.0 volts, and 213 mAh/g is delivered at about 3.0 volts. Integration of the Li_xMn₄O_z where 0.5 < x < 1.0 with a plasticized polymer-based electrolyte (PPE) , or solid polymer-based electrolyte and lithium metal as the anode is contemplated for an advanced pacer battery. In another embodiment Li_xMn₄0₉, where 4.0 < x < 5.0, can be used as a cathode for a fully discharged advanced rechargeable battery that can be used as the external power supply for a VAD device. This cathode in combination with the Li ion anode or lithium metal anode is projected to deliver similar capacity as Li_xMn₄O_z mentioned above with extended cycle life capability.

A Li ion-based rechargeable system can be used as the external charger for an implanted VAD device. For this application Li₅Mn₄0₉ is the optimum cathode material. Li- based rechargeable batteries use carbon or graphite material (Li ion) as an anode to couple with lithiated cathode material. This lithiated cathode material provides the source of lithium during the first charging of the battery. This technology has demonstrated a life greater than 1000 cycles and is a much safer system compared to the Li anode batteries. The equation representing the net electrochemical reaction is shown as follows:

Charge Li₅Mn₄O_z ** x LiC₆_₁₂ + Li₅._xMn₄0₉ 0 < x < 5 Discharge

Li₂Mn₄0₉ is an intermediate compound for the lithium operating range of interest. When this material is used as the starting cathode active material, an initial partial charge or discharge step is required for a full capacity delivery. In a primary system, a partial charging process is needed to form fully charged cathode Li₀Mn₄O₉. This charging process complicates the cell design does require some volume compensation to accommodate for negative electrode expansion due to plated lithium. The rechargeable system embodiment, requires an amount of excess lithium metal in the cell if lithium metal is used as the anode and if Li₂Mn₄0₉ is coupled with a carbon (Li ion) anode as the host structure does not have enough lithium to supply the full capacity corresponding to the 4.0 volt and 3.0 volt level.

A typical example of the synthesis of the solid state cathode of the invention will next be described. Reference is made to Figure 5. EXAMPLE 1

Synthesis of Li₂Mn₄O_z

Li₂Mn₄O_z was prepared by first dissolving 44.6 grams of LiOH»H₂0 in 300 grams de-ionized water as at 10. 244.6 grams of MnC0₃ powder were then mixed into the slurry at 12 and homogenized at 5,000 rpm for 20 minutes at 14 to form a slurry at 16. After drying by evaporation of the D.I. water which may be accomplished by spray drying, freeze drying or oven drying at 100°C to 110°C as at 18 for several hours to dry, the powder was then blended in a Waring high speed blender to micronize the sample cake to form powders at 20. Li_xMn₄0_z can be synthesized at 21 from the prepared fine homogenous powder of the step at 20 by reaction of stoichiometric amounts 350 to 450°C (for example, in a mole ratio of 1:1.9, 1:2 or 1:2.1) of the thoroughly mixed LiOH»H₂0 and MnCQ powders in an oxygen-rich environment. Analysis may be made by thermo gravimetric analysis (TGA) as at 22.

2 LiOH»H₂0 + 4 MnC0₃ + 1/2 (z-5) 0₂ > Li₂Mn₄O_z + 4 C0₂ + 3 H₂0

The calcination temperature (preferably from 400 to 450°C) , dwell time (20 to 60 hours and preferably 20 to 40 hours) , and reaction environments (0₂) have been evaluated for the solid state reaction process. The composition can be verified as by x-ray diffraction at 24 and used for actual electrochemical evaluation as at 26 and as will be described below. The construction of one experimental cell is shown generally in FIGURES 1 and 2 sandwiched between a pair of glass covers 30 and 32 and including an aluminum cathode grid 34 with electrical lead 36. A thin sheet of cathode composite cathode active material is illustrated at 38 and the layers of a composite separator material such as Cellgard 2300 separator material and noted by a composite representation 40. The anode consists of a thin layer of lithium foil imbedded on a nickel grid current collector as at 42 which, in turn, is connected to an electrical lead 44. As shown in FIGURE 2, the cell components may be fastened together in sandwich fashion as by wire retainers at 46.

The electrochemical cell shown in Figures 1 and 2 or similar cells can also be used to assess the electrical characteristics and cycle performance of the Li₂Mn₄O_z cathode. To assess cathode performance, the electrode design is made cathode limited. KS-44 graphite from Lonza can be used for the Li ion anode, 1.5 M LiPF₆/EC+DMC electrolyte solution, and teflonated Li₂Mn₄O_z/C cathode. Li_xMn₄0_z cathodes were prepared by mixing 83 weight percent Li_xMn₄O_z, prepared as in Example 1, 10 weight percent conductive carbon and 7 weight percent Teflon. The mix was then kneaded and rolled into a thin sheet, 4 to 6 mil thick. A cell was assembled by layering an aluminum grid, a cathode, two layers of Cellgard 2300 separators, a lithium foil and a nickel grid.

As shown in Figure 3, one cell was cycled for 29 cycles at a current density of 0.025 mA/cm² between 1.5 and 3.75 V. The charge capacity initially approached 180 mAh/g and remained above 160 mAh/g throughout the test.

Figure 4 depicts performance of solid state cathode samples synthesized at various calcination or synthesis temperatures. As borne out by that figure, the highest discharge capacity (initially above 250 mAh/g) was achieved for synthesis temperatures of about 450°C. In all cases, however, the discharge capacity remains fairly constant throughout the 11 or 12 test cycles depicted in the figure. In accordance with the present invention, it is believed that the feasibility of increasing the net electron exchange of Li_xMn₄O_z can be increased to a range of from about 3.5 net electrons to a value - 5. This should result in a rate increase in the available energy density of both primary and secondary cells utilizing the technology.

Cathodes constructed using the cathode active material of the invention are generally constructed using a composition of from about 75% to 85% Li_xMn₄O_z, between about 15% and 10% conductive carbon and between about 10% and 5% of a binder material. Specific examples are meant to be examples only and are by no means intended to limit the scope of the invention.

This invention has been described herein in considerable detail in order to comply with the Patent Statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use embodiments of the example as required. However, it is to be understood that the invention can be carried out by specifically different devices and that various modifications can be accomplished without departing from the scope of the invention itself.

Claims

CLAIMS I CLAIM:

1. An improved cathode material for use in electrochemical cells selected from the group consisting of primary and secondary non-aqueous active metal cells including an amount of conductive carbon combined with an amount of Li_xMn₄O_z wherein 0 < x < 5 and 8 ≤ z ≤ 9.

2. The cathode material of claim 1 wherein the cell is a primary cell and 0.5 < x < 1.0 in the fully charged state.

3. The cathode material of claim 1 wherein the cell is a secondary cell and 4.0 < x < 4.5 in the fully discharged state.

4. The cathode material of claim 1 wherein the net electron exchange between the charged and discharged state is < 3.5.

5. An electrochemical cell selected from the group consisting of primary and secondary non-aqueous electrochemical cells including an anode containing material selected from the group consisting of lithium and lithium ion and a cathode including an amount of conductive carbon combined with an amount of Li_xMn₄0_z wherein O < x < 5 and 8 < z < 9.

6. The electrochemical cell of claim 5 wherein the cell is a primary cell and 0.5 < x < 1.0 in the fully charged state.

7. The electrochemical cell of claim 5 wherein the cell is a secondary cell and 4.0 < x < 4.5 in the fully discharged state.

8. The electrochemical cell of claim 5 wherein the net electron exchange between the charged and discharged state is < 3.5.

9. The electrochemical cell of claim 5 wherein the cathode has a composition of from between 75% and 85% Li_xMn₄O_z, between 15% and 10% conductive carbon and between 10% and 5% binder.

10. The cathode of claim 9 wherein the binder is polyvinylidene fluoride.

11. The method of making an improved cathode material for use in electrochemical cells selected from the group consisting of primary and secondary non-aqueous electrochemical cells, the method comprising the following steps:

(a) obtaining an intimate homgeneous mixture of amounts of finely divided decomposable lithium and manganese salts in desired stoichiometric ratio; and

(b) Calcining mixture of (a) at a temperature from 350 to 500°C in an oxygen-rich atmosphere.

12. The method of claim 11 wherein the decomposable lithium and manganese salts are LiOH»H₂0 and MnC0₃.

13. The method of claim 12 wherein the reaction time for calcination is from about 20 to about 60 hours.

14. The method of claim 12 wherein the calcination temperature is about 450°C.

15. The method of claim 12 wherein step (a) further comprises forming a slurry of LiOH»H₂0 in de-ionized water with MnC0₃ powder, removing the de-ionized water and further mixing the powder cake obtained at high speed to micronize the cake to fine powders wherein the powders are mixed in the desired stoichiometric amounts.

16. The method of claim 15 wherein the calcination temperature is about 450°C.