Lithium- and manganese(iii/iv)-containing spinels

The present invention relates to a process for the preparation of lithium- and manganese(III/IV)-containing spinels by reacting stoichiometric amounts of a lithium compound, of a manganese compound and, if required, of a further metal compound at from 200 to 800’C under conditions under which the manganese assumes an average oxidation state of from 3.5 to 4.0.

The present invention furthermore relates to spinels of this type having novel morphology, their use as cathode material for electrochemical cells and electrochemical cells which contain these spinels as cathode material.

Lithium- and manganese(III/IV)-containing spinels and the use of such compounds as cathode material in electrochemical cells are generally known, for example from DE-A 4328755.

In the stoichiometrically simplest case of LiMn2O₄, the manganese is present in an average oxidation state of 3.5 in these spinels.

These spinels undergo reversible reaction with compounds which are capable of incorporating lithium cations in their lattice, such as graphite, with elimination of the small lithium atoms from the crystal lattice, manganese(III) ions being oxidized to manganese(IV) ions in said lattice. This reaction can be used in an electrochemical cell for storing electric power by separating the compound (anode material) which takes up lithium ions and the manganese spinel by an electrolyte through which the lithium cations migrate from the spinel into the anode material.

To charge the cell, electrons flow through an external voltage source and lithium cations through the electrolyte from the spinel to the anode material. During the use of the cell, the lithium cations flow through the electrolyte, whereas the electrons flow through an effective resistance from the anode material to the spinel.

However, it is not only the spinel LiMn2O₄ which is suitable for this reaction but also, as is generally known, spinels having further metal cations and other valencies.

Further metals A are, for example, cobalt and nickel, which partly replace the manganese and the lithium in the lattice or which may be incorporated additionally in the lattice. By means of these cations A, the electrical properties of an Li-Μη cell can be modified, for example with regard to the voltage and the voltage drop.

As described, for example, in DE-A 4328755, these spinels are prepared batchwise by reacting a lithium compound, a manganese compound and, if required, a further metal compound at elevated temperatures in a solid-state diffusion reaction which is known to be very slow.

Milling of the powder in an inert solvent has been described for shortening the reaction times to 48-96 hours at from 300 to 750’c.

In spite of the technical complexity, however, only poor spacetime yields can be achieved owing to the long reaction times and the milling step.

It is an object of the present invention to provide an economical and technically simple process for the preparation of lithiumand manganese(III/IV)-containing spinels.

We have found that this object is achieved by a process for the preparation of the above-mentioned spinels by reacting stoichiometric amounts of a lithium compound, of a manganese compound and, if required, of a further metal compound at from 200 to 800’c under conditions under which the manganese assumes an oxidation state of from 3.5 to 4.0, which comprises carrying out the reaction with mixing of the reactants.

We have also found novel spinels of this type, their use as cath30 ode material for electrochemical cells and electrochemical cells which contain them as cathode material.

The process can be carried out using known apparatuses in which solid-state reactions are carried out with mixing of the reactants. For example, rotating bulbs, screw conveyors and in particular rotary tubular furnaces are suitable.

The rotary tubular furnaces preferably contain rotating baffles, by means of which the reactants are thoroughly mixed, scraped off the inner wall of the tube and at the same transported through the tube in the direction of the axis of rotation of the tube. Furnaces having rotating drums and the correspondingly stationary baffles are also possible. Further details of these reaction apparatuses, which must be provided with the required heating means, are described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., VCH Verlagsgesellschaft mbH,

Weinheim, 1992, Vol. B4, pages 107-111, so that further statements in this context are unnecessary.

The reaction is carried out at from 200 to 800°C, in particular from 400 to 750’C, resulting in reaction times of from 0.5 to 10, preferably from 0.5 to 6, in particular from 1 to 4, hours.

The lithium compounds used may be lithium oxide or substances which decompose under the reaction conditions into lithium-containing oxides, such as inorganic lithium salts, for example lithium nitrate, lithium hydroxide or, preferably, lithium carbonate, organic lithium compounds, such as lithium carboxylates, for example lithium acetate, lithium laurate, lithium tartrate or, preferably, lithium oxalate, or lithium-containing complexes, such as lithium acetylacetonate. Mixtures of such compounds are also suitable.

Suitable manganese compounds are substances which are converted under the reaction conditions into manganese(III/IV)-containing oxides, such as inorganic manganese salts, for example manganese (II) hydroxide, manganese(III) hydroxide, manganese(IV) hydroxide, manganese(II) nitrate, manganese oxides or, preferably, manganese (II) carbonate, organic manganese compounds, such as manganese carboxylates, for example manganese(II) acetate, manganese (III) acetate, manganese(II) tartrate, manganese(II) citrate or, preferably, manganese(II) oxalate, manganese-containing complexes, for example manganese(II) acetylacetonate, or mixtures of such compounds .

It is also possible to use mixed lithium- and manganese-containing compounds, as can be obtained in a manner known per se, for example from a solution containing a lithium compound and a manganese compound, by coprecipitation or removal of the solvent.

If compounds of divalent manganese are used as starting materials, as is preferred, the presence of oxygen-containing oxidizing agents is required, for example of manganese dioxide or, most simply, of air. If, on the other hand, a manganese compound having a higher valency is used, a corresponding amount of an Mn(II) compound or a reducing agent, such as carbon monoxide, is expediently concomitantly used, or the manganese valency is most simply established by means of the oxygen partial pressure of an oxygencontaining gas.

If the spinel is to contain heteroatoms, the corresponding amounts of oxides or salts of these metals are concomitantly used in the novel process.

In general, the spinels are preferably of the formula I

Li_x (Mnⁿ)j A_y O_z I where x is from 0.5 to 1.6, n is the average oxidation state of the manganese in the range from 3.5 to 4.0,

A is one equivalent of a metal cation, y is from 0 to 0.4 and z is the number of oxygen equivalents determined by the amount of the other components.

Particularly suitable metal equivalents A are those of cobalt and nickel. They are preferably used in oxidic form or in the form of salts with the same anions as the lithium and manganese salts. Mixtures of such compounds are also suitable.

A lithium compound, a manganese compound and, if required, a further metal compound can be introduced separately into the reaction apparatus or advantageously mixed before the reaction.

If the oxalate is used as the lithium compound and in particular as the manganese compound, spinels are obtained in the form of previously unknown, preferably elongated particles which have axial ratios (length/width) of from 2:1 to 20:1, in particular from 3:1 to 10:1, and are particularly suitable as cathode material in electrochemical parts because they can be more readily dispersed in the conventional preparation of the cathode material.

With regard to the use of the spinels as cathode material in electrochemical cells, products shown to be single-phase by X-ray analysis are desirable for achieving good cycling behavior, the cycling behavior being understood as meaning the reversible electrochemical incorporation of lithium ions into, or elimination of lithium ions from, the spinel lattice with compensation of the charge by a change in the oxidation state of the manganese or of the further metal ions which may be present.

In accordance with the process according to the invention, spinels having widely varying specific surface areas can be produced by appropriate selection of the reaction conditions and starting materials. For example, spinels having high specific surface areas (measured in accordance with DIN 66 132) süch as 5 to 12 m²/g can be obtained, as well as spinels with low specific surface areas such as 0.1 to 4 m²/g.

By selecting the specific surface area it is possible to influence essential application parameters such as dispersibility, bulk density, packing density in the electrode and the stability of the spinel, in particular against chemical decomposition.

Such products can advantageously be obtained by means of an atomic ratio of lithium to manganese of from 0.5:2 to 1.6:2, preferably from 0.8:2 to 1.3:2, in particular from 0.9:2 to 1.12:2, and usually have a capacitance of from 100 to 140 mAh/g.

For the preparation of the cathode material, the spinels are processed in a manner known per se.

In electrochemical cells, this cathode material can be used in a manner known per se opposite an anode which takes up lithium cations .

Suitable electrolytes are known to be organic compounds, preferably esters, such as ethylene carbonate and propylene carbonate, or mixtures of such compounds.

Such electrochemical cells deliver, as a rule, a voltage of from 3.5 to 4.5 V.

The lithium content of the spinels was determined by atomic absorption spectrometry, the total manganese content was determined as Μη2Ρ2θ7 after oxidation to manganese(IV) and the content of manganese(III/IV) was determined by reaction with hydrochloric acid and measurement of the chlorine gas formed. The axial ratio (length/width) was determined by means of scanning electron micrographs. The electrical capacitance of the spinels was measured in a manner known per se in a cell having button cell geometry, against a lithium anode with LÏC1O4 in propylene carbonate as electrolyte.

Examples 1-3

Mixtures of a lithium compound and a manganese compound were reacted with a supply of 200 1/hour of air in a rotating tube which had an internal diameter of 55 mm and a length of 700 mm, heated to 675°C, and which was provided with a static transport screw with webs for thorough mixing of the reaction mixture. The rotary speed of the tube was controlled so that the residence time of the reaction mixture in the heated zone was 2 hours.

The details of these experiments and their results are shown in the table below.

The spinels had a capacitance of 100-110 mAh/g.

The elongated spinels had an axial ratio (length/width) of from 3:1 to 10:1.

Examples 4-15 g of mixtures of a lithium compound and a manganese compound were reacted in a 250 ml rotating flask with a supply of 50 1/hour of air. The residence time of the reaction mixture was 2 hours.

The details of these experiments and their results are shown in the table below.

The spinels had a capacitance of 100-110 mAh/g.

Comparative example

A mixture of lithium carbonate and manganese carbonate was heated in a crucible for 2 hours at 675’C under air.

A spinel suitable for the preparation of cathode material was not obtained.

The details of this experiment and its result are shown in the 40 table below.

Table

<td> Morphology</td><td> isometric</td><td> elongated</td><td> elongated</td><td> isometric</td><td> isometric.</td><td> isometric</td><td> isometric</td><td> isometric</td><td> isometric</td><td> isometric</td><td> isometric</td><td> isometric</td><td> isometric</td><td> isometric</td><td> isometric</td><td rowspan="4"> No spinel suitable as cathode material was formed</td>

<td rowspan="3"> Spinels Li Mn-III Mn-IV % by wt. % by wt. % by wt.</td><td> 32.0</td><td> 32.2</td><td> 33.6</td><td> 33.8</td><td> 25.2</td><td> 23.8</td><td> 29.1</td><td> 28.7</td><td> 27.7</td><td> 30.8</td><td> 30.6</td><td> 28.9</td><td> co rM CO</td><td> 30.7</td><td> 29.3</td>

<td> 27.0</td><td> 27.1</td><td> 25.0</td><td> 26.6</td><td> 34.8</td><td> 36.7</td><td> 30.5</td><td> 31.0</td><td> 32.1</td><td> 28.3</td><td> 28.8</td><td> 31.0</td><td> CM 00 CM</td><td> 28.4</td><td> 30.1</td>

<td> 4.0</td><td> 4.0</td><td> 4.1</td><td> 3.6</td><td> 3.5</td><td> 3.5</td><td> 3.7</td><td> 3.7</td><td> 3.7</td><td> 3.8</td><td> 3.7</td><td> 3.8</td><td> 00 co</td><td> 4.0</td><td> 3.9</td>

<td> Reaction T °C</td><td> in r~ Ό</td><td> 675</td><td> 675</td><td> 650</td><td> 675</td><td> 700</td><td> 650</td><td> 675</td><td> 700</td><td> 650</td><td> 675</td><td> 700</td><td> o in co</td><td> 675</td><td> 700</td><td> If) Γ* kD</td>

<td> Li/Mn atom/atom</td><td> 1.10:2</td><td> 1.08:2</td><td> 1.10:2</td><td> 1.00:2</td><td> 1.00:2</td><td> 1.00:2</td><td> 1.04:2</td><td> 1.04:2</td><td> 1.04:2</td><td> 1.08:2</td><td> 1.08:2</td><td> 1.08:2</td><td> CM o r—&lt; rH</td><td> 1.10:2</td><td> 1.10:2</td><td> 1.08:2</td>

<td rowspan="2"> Starting materials Li Mn compound compound</td><td> carbonate</td><td> oxalate</td><td> oxalate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td>

<td> carbonate</td><td> carbonate</td><td> oxalate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td><td> carbonate</td>

<td></td><td> Ex. 1</td><td> Ex. 2</td><td> Ex. 3</td><td> Ex. 4</td><td> Ex. 5</td><td> Ex. 6</td><td> Ex. 7</td><td> Ex. 8</td><td> Ex. 9</td><td> Ex. 10</td><td> Ex. 11</td><td> Ex. 12</td><td> CO rH &gt;&lt;: w</td><td> Ex. 14</td><td> Ex. 15</td><td> Comp . Example</td>