Lithium and Manganese-(III/IV) containing spinels and method of manufacture

0001The present invention relates to a process for the preparation of spinels containing lithium and manganese (III / IV) by reacting stoichiometric amounts of a lithium compound, a manganese compound and optionally a further metal compound at 200 to 800 ° C. under conditions under which the manganese has an average oxidation state from 3.5 to 4.0.

0002The invention further relates to spinels of this type with a new morphology, their use as cathode material for electrochemical cells and electrochemical cells which contain these spinels as cathode material.

0003Spinels containing lithium and manganese (III / IV) and the use of such compounds as cathode material in electrochemical cells are generally known, for example from DE-A 4328755.

0004In the simplest case, LiMn₂O Li, the manganese is present in these spinels in an average oxidation state of 3.5.

0005With compounds that can incorporate lithium cations into their lattice, such as graphite, these spinels react reversibly with the expansion of the small lithium atoms from the crystal lattice, in which manganese III ions are oxidized to manganese IV ions. This reaction can be used in an electrochemical cell for storing electricity by separating the lithium ion-absorbing compound (anode material) and the manganese spinel by an electrolyte through which the lithium cations migrate from the spinel into the anode material.

0006To charge the cell, electrons flow from an external voltage source and lithium cations through the electrolyte from the spinel to the anode material. When the cell is used, the lithium cations flow through the electrolyte, while the electrons flow from the anode material to the spinel through a useful resistance.

0007For this reaction, not only the spinel LiMn₂O₄ comes into consideration, but also, as is generally known, spinels with other metal cations and deviating valences.

0008Other metals A are, for example, cobalt or nickel, which partly replace the manganese and the lithium in the lattice or which can also be incorporated in the lattice. With these cations A, the electrical properties of a Li-Mn cell can be modified, for example with regard to the voltage and the voltage drop.

0009These spinels are produced discontinuously, as described, for example, in DE-A 4328755, by reacting a lithium compound, a manganese compound and, if appropriate, a further metal compound at elevated temperatures in a known very slow solid diffusion reaction.

0010To shorten the reaction times to 48 to 96 hours at temperatures from 300 to 750 ° C., grinding the powder in an inert solvent is described.

0011Despite the technical effort, poor space-time yields can be achieved due to the long reaction times and the grinding step.

0012The object of the invention was therefore to provide an economical and technically simple process for the production of spinels containing lithium and manganese (III / IV).

0013We have found that this object is achieved by a process for the preparation of the abovementioned spinels by reacting stoichiometric amounts of a lithium compound, a manganese compound and optionally another metal compound at 200 to 800 ° C. under conditions under which the manganese takes on an oxidation state of 3.5 to 4.0. the reaction being carried out by mixing the reactants.

0014Furthermore, new spinels of this type, their use as cathode material for electrochemical cells and electrochemical cells which contain them as cathode material have been found.

0015Known apparatuses can be used to carry out the process, in which solid reactions are carried out by mixing the reaction mixture. For example, rotating balls, screw conveyors and above all rotary tube furnaces are suitable.

0016The rotary tube furnaces preferably contain rotating internals with which the reactants are mixed, scraped off the inner tube wall and at the same time conveyed through the tube in the direction of the axis of rotation of the tube. Furnaces with a rotating drum and the corresponding fixed fittings are also possible. More about these reaction apparatuses, which must be provided with the necessary heating devices, is, for example, Ullmann's Encyclopedia of Industrial Chemistry, 5. Ed., VCH Verlagsgesellschaft mbH, Weinheim, 1992, Vol. B4, pp. 107-111, so that further explanations are not necessary.

0017The reaction is carried out at temperatures from 200 to 800 ° C., in particular 400 to 750 ° C., with reaction times of 0.5 to 10 hours, preferably 0.5 to 6 hours, in particular 1 to 4 hours.

0018As lithium compounds, lithium oxide or substances which decompose under the reaction conditions into oxides containing lithium, 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 laurate, lithium tartrate, preferably lithium oxalate or lithium acetate, or containing lithium Complex compounds such as lithium acetylacetonate. Mixtures of such compounds are also suitable.

0019Suitable manganese compounds are substances which, under the reaction conditions, convert to oxides containing manganese (III / IV), 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 carborylates, for example manganese II acetate, manganese III acetate, manganese II tartrate, manganese II citrate or preferably manganese II oxalate, Manganese-containing complex compounds, for example manganese-II-acetylacetonate or mixtures of such compounds.

0020Mixed compounds containing lithium and manganese can also be used, as can be obtained in a manner known per se, for example from a solution containing a lithium and a manganese compound, by co-precipitation or removal of the solvent.

0021If, as is preferred, compounds of divalent manganese are used, the use of oxidizing agents containing oxygen is required, for example manganese dioxide (manganese dioxide) or, most simply, air. If, on the other hand, a higher-value manganese compound is used, it is expedient to use an appropriate amount of a Mn-II compound or a reducing agent such as carbon monoxide, or the easiest way to adjust the manganese value is by means of the oxygen partial pressure of a gas containing oxygen.

0022If the spinel is to contain heteroatoms, the corresponding amounts of oxides or salts of these metals are used in the process according to the invention.

0023In general, the spinels preferably correspond to formula I.         Li_x (Mnⁿ) ₂ A_y O_e.g. I. in which the variables have the following meaning:<dl id="dl0001" compact="compact"><dt>x</dt><dd>0.5 to 1.6</dd><dt>n</dt><dd>the average oxidation level of the manganese in the range 3.5 to 4.0</dd><dt>A</dt><dd>the equivalent of a metal cation</dd><dt>y</dt><dd>0 to 0.4</dd><dt>e.g.</dt><dd>the oxygen equivalents resulting from the amount of the other components</dd></dl> The most 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.

0024A lithium compound, a manganese compound and optionally a further metal compound can be added separately to the reaction apparatus or advantageously mixed before the reaction.

0025If the oxalate is used as the lithium compound and especially as the manganese compound, spinels are obtained in the form of preferably not yet known, preferably elongated particles with axial ratios (length / thickness) of 2: 1 to 20: 1, in particular 3: 1 to 10: 1, which are particularly suitable as cathode material in electrochemical parts because they are easier to disperse in the known production of the cathode material.

0026For the use of the spinels as cathode material in electrochemical cells, single-phase products are desired to achieve good cycling behavior, the reversible electrochemical incorporation or removal of lithium ions in the spinel lattice or from the lattice being compensated for under the cycling behavior understands the charge by changing the oxidation state of the manganese or any other metal ions that may be present.

0027By suitable selection of the reaction conditions and starting materials, spinels with a wide variety of specific surfaces can be produced by the process according to the invention. Both spinels with high specific surfaces (measured according to DIN 66 132), such as 5 to 12 m² / g, and those with low specific surfaces, such as 0.1 to 4 m² / g, can be obtained.

0028The choice of the specific surface can influence essential technical parameters such as dispersibility, bulk density, packing density in the electrode or the stability of the spinel, in particular against chemical decomposition. Products of this type can advantageously be admixed with an atomic ratio of lithium to manganese of 0.5 to 2 to 1.6 to 2, preferably 0.8 to 2 to 1.3 to 2, in particular 0.9 to 2 to 1.12 2, usually have and have a capacity of 100 to 140 mAh / g.

0029To produce the cathode material, the spinels are processed in a manner known per se.

0030In electrochemical cells, this cathode material can be used in a manner known per se against an anode which absorbs lithium cations.

0031Suitable electrolytes in a manner known per se are organic compounds, preferably esters such as ethylene carbonate and propylene carbonate, or mixtures of such compounds.

0032Such electrochemical cells generally deliver a voltage of 3.5 to 4.5 V.

0033The lithium content of the spinels was determined by atomic absorption spectrometry, the total manganese content after oxidation to manganese IV as Mn₂P₂O₇ and the manganese III / IV content by reaction with hydrochloric acid and measurement of the chlorine gas formed. The axial ratio (length / thickness) was determined by scanning electron micrographs. The electrical capacity of the spinels was measured in a cell with button cell geometry against a lithium anode with LiClO₄ in propylene carbonate as the electrolyte in a manner known per se.

Examples 1-3

0034In a rotary tube with an inner diameter of 55 mm and a length of 700 mm heated to 675 ° C, which was provided with webs to mix the reaction mixture with a static screw conveyor, mixtures of a lithium and a manganese compound with an air supply of 200 l / Hour implemented. The speed of the tube was controlled so that the residence time of the reaction mixture in the heated zone was 2 hours.

0035The details of these tests and their results are shown in the table below.

0036The spinels had a capacity of 100-110 mAh / g.

0037The elongated spinels had an axial ratio (length / thickness) of 3: 1 to 10: 1.

Examples 4-15

003870 g of mixtures of a lithium and a manganese compound were reacted in a 250 ml rotary flask with an air supply of 50 l / hour. The residence time of the reaction mixture was 2 hours.

0039The details of these tests and their results are shown in the table below.

0040The spinels had a capacity of 100-110 mAh / g.

Comparative example

0041A mixture of lithium carbonate and manganese carbonate was heated in a crucible to 675 ° C. in air for 2 hours.

0042No spinel suitable for the production of cathode material was found.

0043The details of this experiment and its result are shown in the table below.<tables id="tabl0001" num="0001"><img file="EP0717455A1_D0001.tif" /></tables>