EP0968135A1

EP0968135A1 - Lithiated metal oxides

Info

Publication number: EP0968135A1
Application number: EP98914238A
Authority: EP
Inventors: Prashant N. Kumta; Akshay Waghray; Mandyam A. Sriram
Original assignee: Eveready Battery Co Inc
Current assignee: Edgewell Personal Care Brands LLC
Priority date: 1997-03-14
Filing date: 1998-03-09
Publication date: 2000-01-05
Also published as: AU6864098A; WO1998041476A1; JP2001526620A; CA2283942A1; CN1258264A

Abstract

A lithiated metal oxide of the general formula: LiayMbxM'b(1-x)Obz, where a is 1 or 2, b is 1 or 2, 0≤x≤1, 0∫y≤1, 1.8≤z≤2.2 and M and M' are multi-valent metals, a method of synthesizing such lithiated metal oxides, and a rechargeable lithium-based battery having at least one electrode in which such a lithiated metal oxide is used as an active material. The process comprises combining at least one lithium carboxylate or lithium alkoxide salt, at least one metal carboxylate salt and an alcohol solvent; reacting the combination in the presence of heat; drying the reaction mixture to remove volatile byproducts and solvent; and heat treating.

Description

Lithiated Metal Oxides

Field of the Invention

The invention relates to lithiated metal oxides, a method of synthesizing such lithiated metal oxides and the use of such as positive electrode materials for batteries.

Background of the Invention

Since the commercialization of lithium ion cells in 1990, rechargeable lithium ion batteries have become the focus of intense research activity as energy sources for portable equipment applications. As with rechargeable nonaqueous electrolyte batteries using lithium metal as the active material of the negative electrode, lithium ion batteries have the advantages of light weight, high energy density and rechargability. However, lithium ion batteries using an intercalation compound as a host structure for lithium ions in the negative electrode do not contain metallic lithium, thus avoiding potential hazards associated with the use of a highly reactive, flammable material such as lithium metal as an active ingredient. Intercalation compounds may also be used as a host structure for lithium ions in the positive electrode. There is variety of types of intercalation compounds suitable for use in lithium ion battery electrodes. Among these materials are certain metal oxides. These metal oxides may be in either the lithiated or delithiated form, depending on whether it is desired to use the material in the positive or negative electrode and whether it is desired to initially assemble the battey in a charged or discharged state. A suitable lithiated material is the spinel LiMn₂O , which is stable in air and water and exhibits potentials of about 4 volts against lithium. The delithiated spinel form is called λ-MnO₂ and was first identified by Hunter and described in the US Patent No. 4,312,930. LiMn₂O₄ has been synthesized in the past primarily by solid state methods that involve prolonged heat treatments of inorganic oxides, hydroxides, carbonates or nitrates of lithium and manganese in the temperature range of 700-800°C. These solid state methods are not only energy intensive but can also adversely affect the electrochemical properties of cells fabricated from these cathode materials. Chemical processes for manufacturing materials can offer significant advantages over the solid state techniques, mainly because of the chemical homogeneity that can be achieved at the molecular level. The reactions are conducted in solution, and good mixing at the molecular level, coupled with reduced diffusion distances, provides for excellent control of the stoichiometry in the synthesized material. In addition, the formation of metal-oxygen bonds at room temperature significantly reduces the time and temperature required in subsequent heat treatments. As a result, the final desired material can be synthesized at lower temperatures and/or in shorter reaction time periods than is typical with conventional solid state synthesis techniques. Furthermore, the ability to create the metal-oxygen bonds and extremely good mixing at the molecular level enable the formation of stoichiometric materials containing several components. By judicious selection of the starting materials, it is possible to control the reaction mechanisms and their kinetics so that amorphous and crystalline materials can be synthesized.

Among the various solution techniques, the sol-gel technique and chemical precipitation have received considerable attention for the synthesis of oxide ceramics and glasses. Conventional sol-gel processes are based on the use of metal alkoxides, which undergo hydrolysis and condensation reactions in solution to form an interconnected network of liquid and solid, called a gel. The gel is dried under ambient or supercritical conditions to form a xerogel or an aerogel, respectively. The use of metal alkoxides provides for excellent control of the chemical homogeneity, microstructure and temperature of formation of the crystalline ceramic. However, the high cost of metal alkoxide precursors preclude their use for synthesizing materials on a large scale, particularly for use in such applications as rechargeable batteries.

A chemical method for the synthesis of LiMn₂O is described by Barboux et al. in US Patent No. 5,135,732. This involves the use of manganese acetate, lithium hydroxide and an inorganic base, such as NH₄OH, to form a gel that is dried and heat treated. The addition of a base is essential to form the white gelatinous precipitate comprising Mn(OH)₂. Most of the LiOH that forms remains in solution, but some is also absorbed on the particles of metal hydroxide. Upon exposure to air the gel turns brown, indicating oxidation of manganese to higher oxidation states that contribute to inhomogeniety in the chemical composition. Bruce (British Patent No. 2,276,156) discloses a process in which a lithium containing solution (preferably LiOH) and a manganese containing solution (preferably manganese acetate), which also contains carbon and NH₄OH, are reacted to produce a lithiated manganese oxide. As in the process disclosed by Barboux, it is necessary to perform this reaction in an inert atmosphere.

A similar process for synthesizing LiMn₂O is described by Prabaharan et al. ("Low Temperature Synthesis of LiMn₂O for Secondary Lithium Batteries," Solid State Ionic Materials, World Scientific Publishing Co., pp. 409-414; and "Bulk Synthesis of Submicrometre Powders of LiMn O for Secondary Lithium Batteries," Journal of Materials Chemistry, Vol. 5, No. 7, 1995, pp. 1035-1037). The acetates of the two starting materials are dissolved in an organic solvent, such as methanol, to which an aqueous solution of carboxylic acid (acetic, citric or oxalic) is added. If citric or oxalic acid is added, insoluble manganese citrate or manganese oxalate precipitates, while lithium citrate or lithium oxalate remains in solution. The addition of acetic acid results in a clear solution without any precipitation. The resultant mixtures, when heated to evaporate the solvent, produce powders that are transformed on subsequent heat treatment to the spinel structure. The addition of carboxylic acids to the clear solution of metal carboxylates increases the concentration of the organics in the solution. Removal of these organic moeities from the precursor is very tedious and particularly detrimental to the synthesis of large quantities of the spinel phase LiMn₂O .

The processes disclosed by Barboux, Bruce and Prabaharan all have limitations in their application to large scale processes. The two step reaction mechanisms, involving the addition of a base or an organic acid, can lead to significant variation in the stoichiometry of the final product, adversely affecting battery performance, when the resultant material is used as an active material in secondary batteries. The need to perform the reactions in an inert atmosphere complicates the processes, increases the costs and imposes practical limitations on size.

Riley (US Patent No. 4,567,031) discloses a process of producing a mixed metal oxide by mixing oxygen-containing salts (e.g., nitrates, oxalates or acetates) of lithium and a metal in a solvent, concentrating the mixed solution, co-crystalizing the concentrated solution to produce a mixed salt of lithium and the metal, and calcining the mixed salt to produce the mixed metal oxide. In World Organization Publication No. 94/25,398 Bonneau teaches that the process disclosed by Riley produces a blend of the desired material and other lithium salts which results in poor performance when the material is used as an active material in a rechargeable battery. Bonneau further discloses that this problem may be avoided if at least one of the precursors is in suspension rather than in solution. Considering the problems inherent in the known methods of synthesis, it would be desirable to have a chemical process for synthesizing a lithiated multi-valent metal oxide with desired stoichiometry and structure that requires less energy for heat treatment than typical solid state processes, avoids the use of costly metal alkoxide starting materials, can be done in ambient atmospheres and facilitates removal of contaminating byproducts. It would also be desirable to have a lithiated multi-valent metal oxide having high specific capacity.

It would also be desirable to have a lithium ion secondary battery with excellent discharge capacity, long cycle life and little capacity fade during the majority of its useful life.

Summary of the Invention One aspect of this invention is a process for producing a lithiated multi-valent metal oxide of the general formula Li_ayMbχM'b(i-χ)Ob_z, where a is 1 or 2, b is 1 or 2, O≤x≤l, 0<y≤l, 1.8<z<2.2 and M and M' are multi-valent metals. The process includes dissolving one or more lithium carboxylate and/or lithium alkoxide salts and at least one multi-valent metal carboxylate salt in a solvent containing at least one alcohol, reacting in the presence of heat to produce a reaction mixture containing hydroxycarboxylates and byproducts, drying the reaction mixture to remove the byproducts and solvent and produce a reaction product, and heat treating the reaction product. As used in this specification, the term "reacting in the presence of heat" includes preheating the solvent before combining the salts, heating the solution during and/or after combining the salts, and heating the solution during drying.

Another aspect of this invention is a rechargeable lithium-based battery, containing one or more cells, each cell of which has a negative electrode, a positive electrode and an electrolyte enclosed in the cell casing; wherein the negative and/or positive electrode contains a lithiated multi-valent metal oxide active material of general formula Li_ayM_bxM'_b(i._x)O_bz, where a is 1 or 2, b is 1 or 2, O≤x≤l, 0<y<l, 1.8<z<2.2 and M and M' are multi-valent metals. The lithiated multi-valent oxide is produced by a process that includes combining one or more lithium carboxylate and/or lithium alkoxide salts, at least one multi-valent metal carboxylate salt and a solvent containing at least one alcohol, reacting the combination in the presence of heat to produce a reaction mixture containing hydroxycarboxylates and byproducts, drying the reaction mixture to remove the byproducts and solvent and produce a reaction product, and heat treating the reaction product.

Yet another aspect of this invention is a lithiated manganese oxide which is useful as an active electrode material for an electrochemical cell. Substantially all (at least about 90 percent) of the crystallites of the lithiated manganese oxide have a largest dimension of about 20 to about 140 nm.

Brief Description of the Drawings The invention will be more readily understood in reference to the following detailed description taken in conjunction with the accompanying figures wherein:

Figure 1 is a series of X-ray diffraction patterns for LiMn₂O produced using a range of heat treatment temperatures.

Figure 2 is a comparison of the charge/discharge performance of secondary lithium ion cells with LiMn₂O produced using a range of heat treatment temperatures as the active material of the positive electrode.

Figure 3 is an X-ray diffraction pattern for LiMn O .

Figure 4 is a set of curves showing the charge/discharge performance of secondary lithium cells with LiMn₂O₄ as the active material of the positive electrode. Figure 5 is an X-ray diffraction pattern for LiMn₂O₄.

Figure 6 is a set of curves showing the charge/discharge performance of secondary lithium cells with LiMn O as the active material of the positive electrode. Figure 7 is an X-ray diffraction pattern for LiMn₂O₄.

Figure 8 is a set of curves showing the charge/discharge performance of secondary lithium cells with LiMn₂O as the active material of the positive electrode. Figure 9 is an X-ray diffraction pattern for LiMn₂O₄. Figure 10 is an X-ray diffraction pattern for LiMn₂O .

Figure 11 is a graph showing specific capacity of secondary lithium cells with LiMn₂O₄ produced with two different spray decomposition furnace temperatures.

Figure 12 is a graph showing specific capacity of secondary lithium cells with LiMn₂O produced from solutions with a range of concentrations.

Figure 13 is a series of X-ray diffraction patterns for LiMn₂O produced from solutions with a range of concentrations.

Figure 14 is a scanning electron micrograph at one magnification of LiMn₂O produced using spray decomposition. Figure 15 is a scanning electron micrograph at a second magnification of LiMn₂O produced using spray decomposition.

Figure 16 is a scanning electron micrograph at a third magnification of LiMn₂O produced using spray decomposition.

Figure 17 shows the crystallite size distribution of LiMn₂O produced using spray decomposition.

Detailed Description of the Invention

One aspect of the present invention is a process that comprises the combination of (1) one or more lithium carboxylate or lithium alkoxide salts, (2) carboxylate salts of one or more multi-valent metals in the desired stoichiometric ratios of lithium and each of the metals, and (3) a solvent containing an alcohol and reacting the combinaton in the presence of heat to produce a reaction mixture containing hydroxycarboxylates of lithium and the multi-valent metals. The reaction mixture is dried to remove undesirable byproducts and solvent. The remaining reaction product is heat treated under controlled conditions to produce a lithiated metal oxide with the desired crystalline morphology. The lithiated metal oxide has the general formula Li_ayMb_XM'b₍ι-_X)Ob_Z, where a is 1 or 2, b is 1 or 2, O≤x≤l, 0<y<l, 1.8<z<2.2 and M and M' are multi-valent metals. Optionally, a portion of M or M' may be substituted with one or more other multi-valent metals, such as metals in Group IIIA and IIIB of the Periodic Table of the Elements. The amount of M or M' that may be substituted is not limited by the process of this invention, but by the thermodynamic solubility of the other multi-valent metal(s) in the crystal structure of the lithiated metal oxide.

Another aspect of this invention is a rechargeable lithium-based battery having a lithiated multi-valent metal oxide as an active material of at least one electrode, wherein the lithiated multi-valent metal oxide is produced using the process of this invention.

Another aspect of this invention is a lithiated manganese oxide, useful as an active material of an electrode of a rechargeable lithium-based battery.

In the process of this invention one or more lithium carboxylate salts, one or more lithium alkoxide salts or a combination thereof is used as one starting material. Suitable lithium salts are those which are soluble in the solvent to be used and whose undesired byproducts (e.g., alkyl carboxylates) are readily removed by drying. Undesirable byproducts preferably have boiling points or decomposition temperatures no greater than about 700°C, more preferably no greater than about 500°C. For this reason preferred lithium salts typically have relatively small alkyl or aryl groups, for example, an alkyl having no more than four carbon atoms. Preferred lithium salts include lithium acetate, lithium formate and lithium methoxide.

At least one multi-valent metal carboxylate for each multi-valent metal (M and M') in the desired lithiated multi-valent metal oxide is used as another starting material in the process of this invention. The multi-valent metals may be any multi-valent metals but are preferably transition metals, more preferably manganese, cobalt or nickel. The carboxylate group may contain a hydrogen atom, an alkyl group or an aryl group. The carboxylate group(s) selected should be a group(s) whose undesirable byproducts (e.g., alkyl carboxylates) of the process of this invention can be easily removed by drying. Undesirable byproducts preferably have boiling points or decomposition temperatures no greater than about 700°C, more preferably no greater than about 500°C. For this reason preferred carboxylate groups typically have relatively small alkyl or aryl groups, for example, an alkyl group having no more than four carbon atoms. Preferred carboxylate groups include formates, acetates and acetonates. More than one metal carboxylate may be used to produce a lithiated multi-valent metal oxide containing more than one multi- valent metal. The multi-valent metal carboxylate(s) selected for use must be soluble in the solvent to be used. Suitable solvents for the process of this invention are those in which the starting materials can be dissolved The solvent(s) are volatile at a low temperature, preferably with a boiling point of no more than about 100°C, to facilitate removal of excess solvent by evaporation during the drying step of the process Preferred solvents are those whose undesirable byproducts (e g , alkyl carboxylates) are easily removed by drying Undesirable byproducts preferably have boiling points no greater than about 700°C, more preferably no greater than about 500°C Preferable solvents comprise one or more alcohols The alcohol may be mixed with water or alcohol alone may be used Ethanol, methanol, isopropyl alcohol and methoxyethanol are preferred alcohols The presence of alcohol and heat results in esterification of the starting materials and the formation of the desired hydroxycarboxylate precursors, as well as alkoxide byproducts that are easily removed during drying

The presence of heat during the reaction of the combined salts and solvent is necessary to avoid byproducts that are not easily removed during drying Preferably the temperature during the reaction is at least about 50°C If the reaction and drying are done separately, the preferred maximum temperature during the reaction step is about the boiling point of the solvent Carboxylic acids are produced during the reaction Carboxylic acids would be difficult to remove from the reaction mixture, but they react with the alcohol to form the easily removable byproducts alkyl carboxylate and water This is important since formation of phase pure material is highly dependent upon efficient removal of the carboxylate and/or alkoxy groups during drying

The reaction mixture is dried Any suitable drying method known in the art may be used Evaporation by spray drying or rotary drying may be used Spray drying is a preferred method It is also preferred to dry the material in air rather than in an inert atmosphere to minimize cost If methanol is used as a solvent, its fire and explosion hazards must be taken into consideration in selecting the drying method and in the design of the equipment used

The dried reaction product is heat treated at from about 500°C to about 800°C An important consideration in selecting a heat treatment temperature is the lithiated metal oxide structure that is desired A variety of heat treatments may be used as part of this invention The composition, structure and morphology of the lithiated metal oxide are dependent upon the types and quantities of starting materials, the solvent used, the conditions during reaction and drying and the heat treatment temperature.

It is possible to perform the reacting and drying steps as a continuous operation to simplify the process if the heating temperature and duration are sufficient for both the desired reactions and drying to occur. When reacting and drying are performed as a continuous operation, a preferred method is evaporative decomposition. The reaction mixture can be either sprayed or nebulized ultrasonically to form a mist of controlled droplet sizes to produce a powder of the desired morphology and particle size. The powder may then be heat treated as described above. Drying and heat treating may also be performed as a continuous operation if the temperature is high enough and the time is long enough to produce the desired lithiated metal oxide structure, or the reacting drying and heat treating steps can all be performed as a continuous operation. As used herein, performing specified steps of the process as a continuous operation means that those steps occur sequentially, with substantially no interruption between the steps, other than the time required for the materials to flow through the equipment. The steps of the continuous operation may be performed in a single vessel. For example the reacting and drying steps may be performed in a spray drying unit, where the spray droplets are heated sufficiently to produce the desired reaction mixture and dry the mixture. Several equipment components may be connected together in such a way that material flows through several temperature zones. For example, a spray drying unit and a long furnace may be connected in series, with a flow of air or gas therethrough, such that reacting and drying take place while material is in the spray drying unit and heat treating takes place as material travels through to furnace.

The lithiated metal oxide made using the process of this invention may be ground to the desired particle size distribution and used as the active material of a rechargeable lithium-based battery electrode. Methods of making such electrodes are well known in the art. Generally, the active material is mixed with a suitable binder material, such as an ionically conductive polymer or polymerizable material, a conductive material, such as acetylene black or graphite, and a solvent, either aqueous or nonaqueous. Various additives may also be used. The electrode mixture may, for example, be formed into pellets, shaped into other geometries or applied or coated onto current collectors, depending on the size, shape and design of the cell. Typically the electrodes thus formed are dried before being used in cells, unless the solvent used in the electrode mixture is the same solvent as used in the cell electrolyte.

Designs, materials and methods of manufacture of rechargeable lithium-based cells and batteries are well known in the art. Rechargeable batteries of this invention may use a lithiated metal oxide as the active material of one or both cell electrodes. Any suitable material known in the art may be used as the active material of the second electrode.

Suitable active materials of the negative electrode include lithium metal, alloys of lithium and other metals, such as aluminum, and lithium insertion compounds, such as carbonaceous materials, amorphous silicon oxides and metal oxides, chalcogenides and oxysulfides. Preferable negative electrode active materials are lithium insertion compounds, more preferably carbonaceous materials, such as graphite, amorphous carbon, mesophase carbon and mixtures thereof, as well as amorphous silicon oxides. Rechargeable lithium ion batteries are normally manufactured in a discharged state, that is, with the lithium ions contained in the positive electrode active material. It is possible, however, to produce such batteries in a charged or partially charged state, with all or part of the lithium contained in the negative electrode active material. When this is done, a lithiated or partially lithiated material, such as the lithiated multi-valent metal oxide of this invention, may be used as the active material of the negative electrode. Suitable active materials of the positive electrode include carbonaceous insertion compounds, metal oxides and selenides and lithiated metal oxides. Lithiated metal oxides are preferred active materials of the positive electrode.

As an electrolyte, any suitable nonaqueous electrolyte known in the art, either liquid or solid, may be used. Suitable electrolytes include but are not limited to liquid electrolytes comprising one or more lithium-containing salts in solution with one or more organic solvents and polymeric electrolytes comprising one or more lithium-containing salts in an ionically conductive polymer or polymer blend.

Batteries of this invention may contain one or more cells. The cells may be of any suitable geometry, including flat cells, coin cells, cylindrical cells or prismatic cells. Electrode assemblies, comprising a negative electrode and a positive electrode, may be of a flat, bobbin or spiral wound construction with a suitable separator. Electrode assemblies having a spiral wound construction may have either a round or essentially oval cross section. Cells of this invention have casings which are hermetically sealed (including those with metal containers and plastic gaskets) and may include pressure relief vent mechanisms to prevent cell rupture under conditions which produce high internal cell pressure. In a first embodiment of this invention lithium acetate and the acetate(s) of one or more multi-valent metals in the desired stoichiometric ratios are dissolved and mixed in a solution of about 10% by volume ethanol in water. Upon reacting the solution in the presence of heat, acetic acid is released and hydroxyacetates of the multi-valent metals and lithium hydroxide are formed. The acetic acid reacts with the alcohol to form low boiling alkyl acetates and water, providing for easy removal of the acetate groups during the drying step. Using ethanol in water as the solvent avoids potential fire and explosion hazards. The xerogel reaction product is then heat treated at temperatures from about 500°C to about 800°C to produce lithiated multi-valent metal oxide. Example 1 illustrates the use of this embodiment to produce Li_nMn_mO (where n is from about 0.8 to 1.2 and m is from about 1.8 to 2.2), nominally referred to as LiMn₂O₄ hereinafter, and describes the performance of secondary lithium batteries made with this material as the active material of the positive electrode. In this example the reacting and drying steps are combined. Lithiated multi-valent metal oxides containing more than one multi-valent metal can also be produced using this embodiment of the invention. A second embodiment of this invention is similar to the first embodiment, except that the lithium acetate and the acetate(s) of the multi-valent metal(s) are dissolved and mixed in pure methanol. The hydroxyacetate mixture is dried to form a xerogel. Methanol is preferable to ethanol as a solvent, since the removal of acetate groups from the mixture is facilitated by the formation of the lower boiling methyl acetate. Example 2 illustrates the use of this embodiment to produce LiMn₂O and describes the performance of secondary lithium batteries made with this material as the active material of the positive electrode.

In a third embodiment of this invention lithium acetate and one or more multi- valent metal formates in the desired stoichiometric ratios of lithium and the multi-valent metals are dissolved and mixed in a solution with water and ethanol. The mixture is dried to form a xerogel and heat treated to produce a lithiated multi-valent metal oxide. Example 3 illustrates the use of this embodiment to produce LiMn₂O₄ and describes the performance of secondary lithium batteries made with this material as the active material of the positive electrode. This embodiment may be used to produce lithiated oxides of other multi-valent metals, such as those of cobalt and nickel, and mixed oxides comprising more than one multi-valent metal. In Example 3 an alternative method of heat treating is also illustrated.

In a fourth embodiment of this invention a lithium alkoxide, such as lithium methoxide, and one or more multi-valent metal acetylacetonates, such as manganese(II)acetylacetonate (Mn-acac), in quantities with the desired stoichiometric ratios of lithium and the multi-valent metals, are used as the starting materials. A suitable nonaqueous solvent, such as methanol or methoxyethanol, is used to dissolve the starting materials. The lithium alkoxide solution is dissolved in solvent in an inert atmosphere to prevent hydrolysis of the alkoxide. The multi-valent metal acetylacetonate solution is prehydrolyzed and added to the lithium alkoxide solution. The solution is prehydrolyzed to replace only one of the acetylacetonate groups attached to the multi-valent metal in order to accelerate the rate of hydrolysis. This is done by the addition of the desired stoichiometric amount of water and refluxing the solution for 3 hours. This is possible due to the greater hydrolytic stability of acetylacetonates compared to that of alkoxides. The combined solution is mixed and refluxed for 2 hours to ensure molecular mixing as well as to initiate the polymerization and condensation of lithium alkoxide and the prehydrolyzed multi-valent metal acetylacetonate. The refluxed reaction mixture is dried to form a xerogel, which is then heat treated. This embodiment is advantageous because lithium multi-valent metal oxide with the desired structure requires a lower temperature of heat treatment. Example 4 illustrates the use of this embodiment to produce LiMn₂O and describes the performance of secondary lithium batteries made with this material as the active material of the positive electrode.

In a fifth embodiment of this invention, acetates of lithium and one or more multi- valent metals are used as starting materials and alcohol and water is used as the solvent. Evaporative decomposition is used as the drying method. The reaction mixture is either sprayed or ultrasonically nebulized to form a mist with controlled droplet sizes. Decomposing the mist in a furnace produces a powder with a fine texture, high surface area and unique morphology. The particle size distribution of the final material can be controlled by changing the droplet sizes. The nozzle orifice must be of sufficient size to prevent clogging. The decomposition temperature may be from about 300°C to about 750°C. The temperature used may affect the flow characteristics of the final material. In addition, if the residence time of the mist in the furnace is sufficient and the temperature is high enough, a separate heat treatment step is not necessary. The specific capacity of the lithiated multi-valent metal oxide is affected by the concentration of the starting materials in the solution as well as other factors in the synthesis of the material and the manufacture of electrodes and batteries using those electrodes. This embodiment may also be used with the starting materials and solvents discussed in embodiments 2 through 4, though the use of methanol as the solvent would require a more robust equipment design due to the flammable nature of the solvent. Example 5 illustrates the use of this embodiment with ultrasonic nebulization as the means of forming the mist, and Example 6 illustrates the use of this embodiment with spraying to produce the mist. Although lithium acetate dihydrate and manganese acetate tetrahydrate are mixed in ethanol and water in Examples 5 and 6, evaporative decomposition may be used with any of the starting materials and solvents of this invention. The evaporative decomposition method provides flexibility in controlling particle size, morphology and phase purity, all of which may affect the electrochemical behavior of the resultant lithiated metal oxide. Combination of the drying, removal of unwanted byproducts and heat treatment into a single step which can be performed as a continuous operation is also possible by using evaporative decomposition.

It is preferred that the lithiated multi-valent metal oxide have a small deficiency in the amount of the multi-valent metal in the crystalline structure (i.e., a small amount of a second phase of the lithiated multi-valent metal oxide is present). High heat treatment temperatures (e.g., 700°C and higher) tend to produce highly phase pure materials, while lower heat treatment temperatures tend to produce materials with a small amount of a second phase, or a deficiency of multi-valent metal in the crystalline structure. A preferred lithiated multi-valent metal oxide is LiMn₂O having deficiency in manganese in the spinel structure, more preferably a deficiency of about 5-10 percent, and most preferably a deficiency of about 7 percent when compared with the phase pure spinel material. Preferably the deficiency in manganese is due to the presence of a small amount of Mn₂O₃. Example 1

30 g of lithium acetatedihydrate is dissolved in 250 ml of doubly distilled deionized water In another beaker 144 g of manganese(II)acetate tetrahydrate (1 2 molar ratio of lithium manganese) is dissolved in 1, 100 ml of doubly distilled deionized water Both the solutions are then mixed together along with 150 ml of 200 proof ethanol (10 vol% of the total liquid volume) and stirred and the solution is heated as it is sprayed into a spray drying unit at about 200°C The presence of a fruity smell suggests that an esterification reaction has occurred, resulting in the formation of ethyl acetate The formation of ethyl acetate is beneficial since it has a far lower boiling point than acetic acid and can be removed early in the drying step The resultant light pink colored powder is very fine and is stored for further heat treatments Controlled heat treatments of four separate samples are performed in a box furnace in air for 2 hours at 500°C, followed by 2 hours at 500°C, 600°C, 700°C and 800°C, respectively, to produce LiMn₂O₄ Each heat treated sample is ground in an agate mortar and pestle and the final powders are analyzed for their structure and phase evolution using X-ray diffraction (Rigaku θ/θ diffractometer employing Cu-Kα radiation) The morphology and micro structure of the powders are examined using a scanning electron microscope (CamScan Series IV) The morphology of the sample calcined at 500°C consists of large soft agglomerates several microns in size which are in turn composed of extremely fine (< 1 μm) particles on the surface These soft agglomerates are formed from small hard agglomerates that undergo necking during the heat treatment process As a result, the soft agglomerates display a fine substructure Such morphologies are unique and have a useful influence on the electrochemical characteristics of cells made using this material as an active electrode material in a secondary lithium ion battery The X-ray diffraction (XRD) patterns in Figure 1 show the evolution of the pure spinel phase as the temperature during the second 2 hours of heat treatment is varied from 500°C to 800°C The absence of a distinguishable peak in the XRD pattern at 2Θ of about 33 is indicative of phase pure spinel LiMn₂O , produced with heat treatment temperatures of 700°C or higher The presence of a small peak at this point in the XRD patterns of the samples heat treated at less than 700°C shows the presence of Mn₂O₃, indicating a deficiency of manganese in the spinel structure. The exact structure is obtained by conducting Rietveld refinement of the patterns The Rietveld refinements, summarized in Table 1 , show that material heat treated at less than 700°C has a deficiency of about 7 percent in manganese (0.93 Mn occupancy) when compared to the single phase spinel material. This deficiency of about 7 percent in manganese corresponds to about a 14 percent excess of lithium in the spinel structure.

Table 1

Heat

Treatment Lattice Oxygen Mn

Temperature Parameter Coordinate Occupancy

500°C 8.217 0.262 0.93

600°C 8.227 0.262 0.93

700°C 8.243 0.263 *

800°C 8.245 0.262 *

* Mn occupancy refined for two-phase samples only Lattice parameter is 8.24762(16) - JCPDS card PDF-2, sets 1-42, database #35-782

The LiMn O samples are characterized in lithium ion cells using a conventional three electrode test cell with a coke negative electrode and a lithium metal reference electrode. A positive electrode mix is prepared from binder (5.34 dry wt.%), carbon black (7.59 dry wt.%) and LiMn₂O₄ (87.06 dry wt.%) by dissolving ethylene/propylene copolymer binder (60% ethylene, from Scientific Polymer Products, Ontario, NY 14519) in trichloroethylene, to which is added a mixture of LiMn₂O and carbon black powder (Super S, manufactured by MMM Carbon, Willebroek, Belgium). The positive electrode mixture, having pancake mix consistency, is tape cast (coating thickness about 0.006 in. (0.152 mm)) onto a 1 mil (0.0254 mm) thick aluminum foil that is dried in air before punching 1 cm² positive electrode disks. These disks are dried in a vacuum oven at 160°C for 16 hours before use.

For the purpose of evaluating the performance of electrodes made with lithiated metal oxides made according to this invention, latex bonded coke negative electrodes are made using the following procedure, using the materials shown in Table 2. Polyacrylamide is dissolved in 9 ml deionized water by stirring for about 2 hours. Coke and acetylene black are micromilled together for 2 minutes. Latex binder is added to the polyacrylamide solution and stirred until homogeneous The coke/acetylene black mixture is gradually added to the solution while stirring Two ml of additional deionized water is added, and stirring continued for about 1 5 hours The positive electrode mixture is coated onto one side of 0 4 mil (10 2 mm) thick copper foil to produce a coating thickness of about 0 007 in (0 178 mm) The coated foil is dried in air and then cut into 1 cm² negative electrode disks The negative electrode disks are dried in a vacuum oven at 160°C for 16 hours before use

Table 2 Ingredient Quantity Source

Calcined petroleum coke 9 200 g (XP- 13, Conoco, Inc , Houston, TX 77252)

Acetylene black 0 200 g (Chevron Chemical Co , Houston, TX 77253)

Latex binder 0 525 g (Rovene 4076, Rohm and Haas, Phila , PA 19105)

Polyacrylamide 0 075 g (Cyanamer N-300 LMW, Cytec Industries, Inc , West Patterson NJ 07424)

Negative electrodes are titrated with the lithium reference so that excess lithium is available in the negative electrode before charging the positive electrode Electrolyte (1M LiPF₆ in 3 1 by weight ethylene carbonate to dimethyl carbonate) is soaked onto a separator (Grade DR2, Whatman, Inc , Haverhill, MA 01835) that has been predried under vacuum at 250°C for 16 hours Cell components are assembled into test fixtures in an argon filled glove box

The cells are tested using a constant current of 0 25 mA in the voltage range of 4 6 and 3 IV over ten charge/discharge cycles Each cell is first charged to remove the lithium ions within the LiMn₂O so that the positive electrode becomes the open structure of spinel λ-MnO₂ The results for each of the LiMn₂O heat treatment conditions (with the final 2 hours of heat treatment at 500°C, 600°C, 700°C and 800°C) are shown in Figure 2, in which cell potential (volts) is plotted as a function of specific capacity (mAh/g) for the first and tenth cycles The single phase spinel materials, heat treated at 700°C and above, provide higher discharge capacities but much more fade (loss in capacity from one cycle to another) than those samples heat treated at lower temperatures and having a deficiency of manganese in the spinel structure The cells made with LiMn₂O calcined to 600°C have an initial discharge capacity of about 120 mAh/g of LiMn₂O , which is about 80% efficient in lithium utilization, and give a greater discharge capacity after ten cycles (less than 2 percent cumulative fade) than cells made with LiMn₂O calcined at the other temperatures Example 2

5 g of lithium acetatedihydrate is dissolved in 200 ml of methanol, followed by the addition of 24 g of manganese(II)acetate tetrahydrate (1 2 molar ratio of lithium manganese) Upon stirring, a clear solution is obtained The solution is heated with stirring in a Buchi rotary evaporator A fruity smell is observed, suggesting the occurrence of an esterification reaction and the formation of methyl acetate The formation of methyl acetate is beneficial since it has a far lower boiling point than acetic acid and can be removed early in the drying step The operating pressure initially is set at 950 mbar using an inert nitrogen atmosphere It was initially thought that an inert atmosphere is essential to prevent the oxidation of manganese, however, subsequent experiments have shown that this is not required, and the use of normal air leads to essentially the same results The temperature of the evaporator is set between 100°C and 140°C, and the solution is rotated at 100 rpm until the remaining powder appears to be dry To complete the drying stage, the pressure is reduced to 1 mbar, the temperature is set at 180°C and drying is continued for 30 minutes The total drying time varies, depending upon the type of solvent, its volatility and sample batch size For example, the use of methanol rather than ethanol reduces the drying time significantly due the lower boiling point of methyl acetate The resultant dried xerogel is ground into a fine powder using an alumina mortar and pestle The powder is then heat treated in a box furnace heating at 2°C/minute to a temperature of 500°C, followed by soaking at 500°C for 2 hours and cooling at the rate of 2°C/minute The heat treated product is ground in an agate mortar and pestle, and the ground powders are analyzed for their structure and phase evolution using XRD The morphology and microstructure of the powders are examined using a scanning electron microscope The morphology is similar to that described in Example 1 The XRD pattern for the powder made according to this embodiment is shown in Figure 3 Cells are made and tested as described in Example 1 The results for the first 5 cycles are shown in Figure 4 The initial discharge capacity is about 2% higher than that of the cell containing the 500°C calcined sample of Example 1, probably due to the more efficient removal of acetate ions during drying as a result of using pure methanol as the solvent

Example 3 Lithium acetate and manganese(II)formate are dissolved in water and used as starting materials The process of reacting, drying and heat treating described in Example 1 is used, holding the reaction product at 500°C for 2 hours under a 0 5 liter/minute flow of air during heat treatment A scanning electron micrograph shows hard agglomerates about 1 μm in size combined into soft agglomerats of about 3 to 5 μm in size The primary particles show a rough topography comprising several convoluted submicron size channels The rough topography which describes the morphology of the spinel powders is indicative of the different sample microstructures that can be obtained using different embodiments of the sol-gel process of this invention The XRD pattern shown in Figure 5 has no distinct peak at 2Θ of 33, indicating that the material produced is phase pure LiMn₂O spinel After grinding the LiMn₂O , cells are made and tested as described in Example 1 The results are shown in Figure 6 The initial discharge capacity (about 75 mAh/g) is lower and capacity fade is greater than for the cells in Examples 1 and 2

Example 4 Manganese(II)acetylacetonate (Mn-acac) is dissolved in methoxyethanol, and the solution is prehydrolyzed In an argon filled glove box lithium methoxide is dissolved in methoxyethanol and mixed with the prehydrolyzed Mn-acac solution, and the mixture is refluxed for 1 hours at 60°C The mixture is then dried in a rotary evaporator, in a manner similar to that described in Example 2, to form a xerogel The xerogel is heat treated under various conditions to produce phase pure or nearly phase pure spinel structures Heat treatment for 2 hours in air in a box furnace set at 500°C produces a LiMn₂O with a unique crystalline morphology The crystallites of the spinel obtained consist of fine connected regular octahedra about 1 to 1 5 μm in size and are different from the crystallites of spinels obtained by the embodiments described in Examples 1 through 3 These crystallites are indicative of the excellent control of particle size and shape that is obtained using different embodiments of this sol-gel based chemical process and also demonstrate good control of phase purity and stoichiometry This material has a deficiency of manganese in the spinel structure, as shown by XRD (Figure 7). Unlike the materials heat treated at less than 700°C in the previous examples, in which the manganese deficiency is a result of the presence of a small amount of Mn₂O₃, the manganese deficiency in this material is due to the presence of Mn₃O₄. After grinding the LiMn₂O , cells are made and tested as described in Example 1. The initial discharge capacity is about 105 mAh/g, with minimal fade after cycling, as shown in Figure 8.

Example 5 Lithium acetate dihydrate and manganese (II) acetate tetrahydrate are each dissolved in doubly distilled deionized water, and the two solutions mixed together with ethanol, as described in Example 1. The solution is ultrasonically nebulized to form a mist with droplet sizes that are not larger than 1 μm. This is accomplished by the use of a nebulizer equipped with an ultrasonic transducer (Holmes Products Corp., Milford, MA). The nebulized droplets are then reacted and dried as they pass through a furnace, in air at ambient atmospheric pressure and a temperature of about 300°C to about 500°C. The LiMn₂O produced has a very fine texture, a high surface area and spherical hard agglomerates with diameters of 1 to 5 μm. If the material is not heat treated, phase purity of the spinel structure suffers, as illustrated by the XRD pattern in Figure 9.

Example 6 As in Example 5, stoichiometric amounts of lithium acetate dihydrate and manganese (II) acetate tetrahydrate are dissolved in doubly distilled deionized water and mixed together with ethanol to obtain a clear solution. In this case, however, spraying rather than ultrasonic nebulization is used to form a mist, having droplet sizes larger than those produced by ultrasonic nebulization in Example 5. Spraying is preferred because it is a faster process. The solution is pumped into a spray nozzle using a peristaltic pump and atomized using compressed air at a pressure of 1.5 Kgf/cm². The mist is reacted and dried in the spray drying chamber utilizing additional air, heated to 230°C. The flow rate of the air is controlled by the speed of the aspirator fan. At the lower end of the spray drying unit, the powder produced is passed through a decomposition tube 4 ft. long and 1 in. inside diameter, with the temperature set at 600°C to 800°C, preferably at 750°C. The tube is located within a 3 -zone tube furnace, with the central zone held at the set temperature and the end zones at 40°C higher than the central zone. The powder is collected using a cyclone separator, and hot air exiting the separator is cooled using a water cooled heat exchanger and exhausted into a fume hood. The powder may optionally be heated in the collection chamber to 300±50°C to induce further decomposition if necessary. The powder is further heat treated in order to ensure complete decomposition and reproducibility. Heat treatment is done by spreading 100 g of the powder on a stainless steel plate, heating at a rate of 2°C/min., holding at 600°C for 2 hours and cooling at a rate of 2°C/min. The material produced has a morphology similar to that of the material produced in Examples 1 and 2. The XRD patterns of material produced with decomposition tube furnace settings of 750°C and 800°C are shown in Figure 10. Both patterns show the presence of a small amount of Mn₂O phase impurity. Cells made with material produced at these two furnace settings and tested as in the previous examples give the specific capacities shown in Figure 11. There is little fade with either sample, but the specific capacity is higher with material produced at a furnace setting of 750°C.

When material produced using the spray decomposition process described in Example 6 is heat treated at temperatures above 600°C, phase pure spinel LiMn₂O is produced. This material has a deficiency of lithium throughout the spinel structure. When the LiMn₂O is used in cells, the initial capacity is high (about 120 mAh/g), but there is greater fade than with material heat treated at 600°C.

The concentration of the starting materials in the solvent can be varied when using the spray decomposition process described in Example 6. The optimum concentration is dependent upon the spray decomposition equipment and other process variables. Solution concentrations of 0.196, 0.261 and 0.392 mols/1, decomposed in a furnace set at 750°C and heat treated at 600°C produce spinel phase LiMn₂O with a manganese deficiency of about 7%. When used as the active positive electrode material in cells, a trend in specific capacity is observed (Figure 12), with the lower solution concentrations resulting in materials with better specific capacities, though the initial specific capacity is above 105 mAh g and fade is minimal in cells with LiMn₂O produced using all three solution concentrations. XRD patterns show the presence of a small amount of Mn₂O₃ in the material produced with all three of the solution concentrations used (Figure 13). When solid state preparations of lithiated metal oxides are used, the morphology of the resultant material may be controlled by changing the heat treatment method, but with spray decomposition method described in Example 6, the crystal structure can also be controlled by varying the concentration of the solution, temperature of the decomposition furnace and heat treatment conditions used to heat treat the spray decomposed powders (such as temperature, time and atmosphere As used herein, morphology refers to the shape and form of agglomerates of material Crystal structure refers to the size and form of crystallites, which are single crystals of the material Hard agglomerates are made of a plurality of crystallites, and soft agglomerates are made of a plurality of hard agglomerates Soft agglomerates can be ground (e g , by hand grinding, ball milling, normal attrition milling or by exposure to ultrasonic energy) into smaller soft agglomerates and/or hard aggolmerates, but hard agglomerates can only be broken apart by special methods such as high energy attrition milling for extended periods of time (e g , 1200 impacts/sec in a Spex mill) Since crystallites cannot be broken apart by normal processing methods, their size and shape are characteristic of the material LiMn₂O made using spray decomposition as described in Example 6 has a specific surface area of about 6 m /g (BET method, N₂ adsorption) and soft agglomerates that vary in size from a few microns to a few hundred microns, the size distribution depends in part on the amount of grinding which is done The hard agglomerates vary from about 1 micron to about 5 microns The largest dimension of individual crystallites is typically about 20 to about 140 nm Preferably at least about 90 percent, more preferably at least about 95 percent, of all crystallites have a largest dimension of about 20 to about 140 nm Figures 14, 15 and 16 are scanning electron micrographs of this LiMn₂O₄ which were taken at increasing magnification and highlight the soft agglomerates, hard agglomerates and crystallites, respectively The data on crystallite sizes in Figure 17 was obtained using a scanning electron micrograph, measuring the largest dimension of approximately 160 crystallites from at least seven different areas of the sample The material has a small average crystallite size and tight size distribution The crystal structure of the LiMn₂O₄ of this invention has an average crystallite size of preferably about 50 to 90 nm, more preferably about 60 to 80 nm and most preferably about 70 to 75 nm The crystallite size standard deviation is preferably less than about 30 nm, more preferably less than about 25 nm and most preferably less than about 20 nm The BET specific surface area is preferably between about 4 and about 10 m²/g, more preferably between about 5 and about 8 m²/g and most preferably about 6 m²/g. The material of this invention is advantageous because of the control of the Li to Mn ratio achieved using the process of this invention, and the material produced as described in Example 6 has good discharge capacity (120 mAh/g) with very little fade. In Example 6 the reacting and drying steps are performed as a continuous operation and the heat treatment step is done separately, but the same results are achieved by performing the reacting, drying and heat treating as a continuous operation, using a spray drying unit and decomposition tube furnace connected together in series. The combined salts and solution are sprayed into the spray drying chamber through a nozzle. Gas, heated to about 500°C to 800°C, is flowed into the spray dryer to accelerate drying, carry the dried material into the tube furnace and minimize the required length of the furnace. The gas flow rate is controlled to give material a residence time in the furnace of from about 1 second to about 10 seconds. The heat treated lithiated metal oxide powder is collected and the gases exhausted.

Claims

WE CLAIM:

1. A process for producing a lithiated multi-valent metal oxide of the general formula Li_ayM_bxM'_b(1-_X)O_bz, where a is 1 or 2, b is 1 or 2, OΓëñxΓëñI, 0<yΓëñl, 1.8ΓëñzΓëñ2.2 and M and M' are multi-valent metals, comprising the steps: (a) combining at least one lithium-containing salt, selected from the group consisting of lithium carboxylates and lithium alkoxides, and at least one multi-valent metal carboxylate salt in an alcohol solvent;

(b) reacting the combination of step (a) in the presence of heat to produce a reaction mixture comprising hydroxycarboxylates, and byproducts; (c) drying said reaction mixture to remove said byproducts and said solvent, thereby producing a reaction product; and (d) heat treating said reaction product.

2. The process according to claim 1, wherein at least one of M and M' is selected from the group consisting of manganese, nickel and cobalt.

3. The process according to claim 1, wherein a portion of at least one of M and M' is substituted with another multi-valent metal.

4. The process according to claim 1, wherein said lithium salt contains no more than four carbon atoms per molecule.

5. The process according to claim 4, wherein said lithium salt is selected from the group consisting of lithium acetate, lithium formate, lithium methoxide and combinations thereof.

6. The process according to claim 1, wherein said multi-valent metal carboxylate salt contains no more than four carbon atoms per molecule.

7. The process according to claim 6, wherein said multi-valent carboxylate salt is selected from the group consisting of formates, acetates, acetonates and combinations thereof.

8. The process according to claim 1, wherein said solvent has a boiling point no greater than about 100┬░C.

9. The process according to claim 8, wherein said solvent comprises an alcohol selected from the group consisting of methanol, ethanol, isopropyl alcohol, methoxyethanol and combinations thereof.

10. The process according to claim 1, wherein said byproducts have boiling points or decomposition temperatures no greater than about 700┬░C.

11. The process according to claim 1, wherein said reacting is performed at a temperature of at least about 50┬░C.

12. The process according to claim 1, wherein the process further comprises a continuous operation.

13. The process according to claim 12, wherein said continuous operation comprises evaporative decomposition.

14. The process according to claim 1, wherein the heat treatment temperature is from about 500┬░C to about 800┬░C.

15. The process according to claim 14, wherein the heat treatment temperature is from about 600┬░C to about 800┬░C.

16. A rechargeable lithium-based battery, comprising one or more cells, each of said cells comprising a negative electrode, a positive electrode and an electrolyte in a cell casing, said negative electrode comprising a first active material, said positive electrode comprising a second active material, wherein at least one of said first active material and second active material comprises a lithiated multi-valent metal oxide of the general formula

Li_ayM_b╧çM'b(i._X)O_b/, where a is 1 or 2, b is 1 or 2, OΓëñxΓëñI, 0<y<l, 1.8<zΓëñ2.2 and M and M' are multi-valent metals, wherein said lithiated multi-valent metal oxide is produced by a process comprising the steps:

(a) combining at least one lithium-containing salt, selected from the group consisting of lithium carboxylates and lithium alkoxides, and at least one multi-valent metal carboxylate salt in an alcohol solvent;

(b) reacting the combination of step (a) in the presence of heat to produce a reaction mixture comprising hydroxycarboxylates, and byproducts;

(c) drying said reaction mixture to remove said byproducts and said solvent, thereby producing a reaction product; and

(d) heat treating said reaction product.

17. The rechargeable lithium-based battery according to claim 16, wherein at least one of M and M' is selected from the group consisting of manganese, nickel and cobalt.

18. The rechargeable lithium-based battery according to claim 16, wherein a portion of at least one of M and M' is substituted with another multi-valent metal.

19. The rechargeable lithium-based battery according to claim 16, wherein said negative electrode comprises a carbonaceous lithium intercalation compound.

20. The rechargeable lithium-based battery according to claim 16, wherein said electrolyte is a liquid electrolyte.

21 The rechargeable lithium-based battery according to claim 16, wherein said electrolyte is a polymer electrolyte

22 The rechargeable lithium-based battery according to claim 16, wherein said process further comprises a continuous operation

23 A lithiated manganese oxide useful as an active electrode material for an electrochemical cell and having crystallites, wherein a largest dimension of at least about 90 percent of said crystallites is from about 20 to about 140 nm

24 The lithiated manganese oxide according to claim 23, having hard agglomerates of crystallites of from about 1 micron to about 5 microns

25 The lithiated manganese oxide according to claim 24, having a BET specific surface area of about 5 m²/g to about 10 m²/g

26 The lithiated manganese oxide according to claim 25, produced by a process comprising the steps (a) combining at least one lithium-containing salt, selected from the group consisting of lithium carboxylates and lithium alkoxides, and at least one multi-valent metal carboxylate salt in an alcohol solvent,

(b) reacting the combination of step (a) in the presence of heat to produce a reaction mixture comprising hydroxycarboxylates, and byproducts, (c) drying said reaction mixture to remove said byproducts and said solvent, thereby producing a reaction product, and

(d) heat treating said reaction product

27. The lithiated manganese oxide according to claim 26, wherein said process further comprises a continuous operation.