HK1000079A1 - Process for preparing a positive electrode for secondary lithium batteries - Google Patents

Abstract

A lithium-manganese oxide LixMn2O4 having a spinel structure as cathode material for secondary lithium batteries is obtained by mixing manganese dioxide with lithium formate and/or lithium acetate (HCOOLi, CH3COOLi) and subsequently heating the mixture for 10 to 100 hours at preferably 600 DEG C to 750 DEG C. The mixture components contain manganese and lithium in a molar ratio Mn : Li = 2: x, where 0.5 < x < 1.5. The end product is extremely finely crystalline and is distinguished by good cycle stability.

Description

The invention relates to a method for the manufacture of a positive electrode for lithium secondary batteries, the active material of which consists of a spindle-shaped lithium manganese oxide formed by mixing manganese dioxide with lithium formate and/or lithium acetate and then heating the mixture.

Lithium manganese spinels are now used with great advantage as positive electrode material in electrochemical cells of the Li/LixMn2O4 type. The charging/discharging mechanism of such elements is based on the ability of manganese oxide to reversibly intercalize and deintercalize active Li+ ions in its spinel lattice, which forms an open skeletal structure.

The preparation of a LiMn2O4 material from MnO2 is usually based on a commercial manganese dioxide which is thoroughly ground in a mortar with a solid lithium compound, e.g. lithium hydroxide, lithium carbonate or lithium iodide, and then heated for sintering.

The sinter cake may be crushed and, if necessary, provided with a conductive agent (graphite) and a binder before being sold into a finished cathode pellet.

The commercial manganese oxides (brownstones) are a good starting material for the preparation of lithium manganese spinels, since they have been used for a long time in the battery industry, their electrochemical behaviour has been thoroughly studied, they are a cheap raw material and do not pose any problems in terms of environmental impact.

However, the cathode materials produced in the manner described have so far been characterised by a lack of cyclic stability and a gradual decrease in discharge capacities over the life of the cycles, particularly after prior high temperature storage.

There has therefore been no shortage of proposals to stabilise the host structure of manganese spinel, for example by substituting some of the manganese in the lattice with foreign metals, including Co, Ni, Fe.

JP-OS 4-169065 is also known to produce a positive electrode material for lithium batteries from MnO2 and lithium salts of formic acid and acetic acid, with which manganese oxide is mixed instead of LiOH·H2O or Li2CO3.

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The problem is solved according to the invention by a process as defined in claim 1.

The first step is to prepare a deep mixture of pure manganese oxide and lithium formic acid, HCOOLi, or lithium acetic acid, CH3COOLi, and, where appropriate, both salts, and to obtain a particularly deep mixture by making a suspension of the brownstone in an aqueous solution of the corresponding lithium salts and then subjecting it to spray drying.

Err1:Expecting ',' delimiter: line 1 column 180 (char 179)

The above ingredients are mixed in such proportions as to give a molar ratio Mn: Li = 2: x with 0.5 < x < 1.5, preferably 0.9 < x < 1.2.

The resulting mixture, in the latter case the dry residue obtained, is heated in a second step between 10 and 100 hours in air to 600°C to 750°C. This heat treatment results, due to the strong gas development associated with the implementation of the mixture components, in an extremely fine crystalline lithium-manganese spinel.

The product is then ground and processed into an electrode.

The active electrode material thus produced meets the requirement for good cycle resistance in batteries with positive LixMn2O4 electrodes and negative Li or Li intercalating electrodes.

Finally, especially with regard to the production of cells with positive and negative lithium intercalating electrodes (swing system), the capability of the present lithium-manganese spindle to be doped with lithium far beyond the stoichiometric composition (maximum x = 1.5) is extremely advantageous.

The electrodes of such cells in the pre-mounted and closed state are usually uncharged, i.e. all available lithium is stored in the positive spinel or metal oxide electrode, while the host structure of the negative electrode is empty of lithium.

The first charge involves the removal of lithium from the positive host grid and its incorporation into the negative host grid, preferably a carbon matrix, from which a portion of the lithium is now held in place by the formation of a coating layer, i.e. by an irreversible chemical reaction, thus removing a certain amount of lithium from the intercalation mechanism by the chemically non-indifferent carbon material from the beginning.

However, by means of a lithium-manganese spinel according to the invention with a superstoichiometric amount of lithium, it is possible to saturate the reactivity of the coal electrode so that the inercalation and deintercalation of lithium in the coal matrix can be practically lossless, i.e. reversible.

Claims (3)

1. Process for preparing a positive electrode for secondary lithium batteries, whereof the active material comprises a lithium manganese oxide having spinel structure, which is formed by spray-drying a suspension of manganese dioxide in an aqueous solution of lithium formate and/or lithium acetate, followed by heating the mixture, with the mixture being heated at a temperature of from 600 to 750°C for between 10 and 100 hours and then ground.
2. Process according to Claim 1, in which the manganese oxide is a chemically generated product (CMD) or a β-MnO₂ (pyrolusite).
3. Process according to one of Claims 1 or 2, in which the constituents of the mixture contain manganese and lithium in a molar ratio of Mn : Li of 2 : x, where 0.5 < x < 1.5, preferably 0.9 < x < 1.2.