Note: Descriptions are shown in the official language in which they were submitted.
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Lithium- and manganese(III/IV)-containing spinels
The present invention relates to a process for the preparation of
lithium- and manganese(III/IV)-containing spine~s by reacting
stoichiometric amounts of a lithium compound, of a manganese com-
pound 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 4Ø
The present invention furthermore relates to spinels of this type
having novel morphology, their use as cathode material for elec-
trochemical 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.
20 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 lithlum ions and the
30 manganese spinel by an electrolyte through which the lithium cat-
ions 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 elec-
trons flow through an effective resistance from the anode
material to the spinel.
40 However, it is not only the spinel LiMn2Og 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-Mn cell
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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.
10 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 space-
time 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 lithium-
and 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 stoichio-
metric 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 reac-
tion with mixing of the reactants.
We have also found novel spinels of this type, their use as cath-
30 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 reac-
tants. For example, rotating bulbs, screw conveyors and in par-
ticular rotary tubular furnaces are suitable.
The rotary tubular furnaces preferably contain rotating baffles,
by means of which the reactants are thoroughly mixed, scraped off
40 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 ap-
paratuses, 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,
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Weinheim, 1992, Vol. B4, pages 107-111, so that further state-
ments 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-con-
10 taining oxides, such as inorganic lithium salts, for examplelithium nitrate, lithium hydroxide or, preferably, lithium car-
bonate, 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
20 oxides, such as inorganic manganese salts, for example manga-
nese(II) hydroxide, manganese(III) hydroxide, manganese(IV)
hydroxide, manganese(II) nitrate, manganese oxides or, prefer-
ably, 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-contain-
ing complexes, for example manganese(II) acetylacetonate, or mix-
tures of such compounds.
30 It is also possible to use mixed lithium- and manganese-contain-
ing compounds, as can be obtained in a manner known per se, for
example from a solution containing a lithium compound and a man-
ganese compound, by coprecipitation or removal of the solvent.
If compounds of divalent manganese are used as starting materi-
als, 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 hav-
ing a higher valency is used, a corresponding amount of an Mn(II)
40 compound or a reducing agent, such as carbon monoxide, is expedi-
ently concomitantly used, or the manganese valency is most simply
established by means of the oxygen partial pressure of an oxygen-
containing gas.
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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
LiX (Mnn)2 Ay Oz
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
20 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 fur-
ther metal compound can be introduced separately into the reac-
tion 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
30 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
40 cycling behavior being understood as meaning the reversible elec-
trochemical 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.
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In accordance with the process according to the invention, spi-
nels 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) such 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 influ-
10 ence 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 atom-
ic ratio of lithium to manganese of from 0.5:2 to 1.6:2, prefer-
ably 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 pro-
20 cessed 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 cat-
ions.
Suitable electrolytes are known to be organic compounds, prefer-
ably esters, such as ethylene carbonate and propylene carbonate,
or mixtures of such compounds.
30 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 ab-
sorption spectrometry, the total manganese content was determined
as MnzP2O7 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
40 measured in a manner known per se in a cell having button cell
geometry, against a lithium anode with LiC104 in propylene
carbonate as electrolyte.
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Examples 1-3
Mixtures of a lithium compound and a manganese compound were
reacted with a supply of 200 l/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
70 g of mixtures of a lithium compound and a manganese compound
were reacted in a 250 ml rotating flask with a supply of
50 l/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
~0 table below.
Table
Starting materials Reaction Spinels
Li Mn Li/Mn T Li Mn-III Mn-IV Morphology
compound compound atom/atom C % by wt. % by wt. % by wt.
Ex. 1 carbonate carbonate 1.10:2 675 4.0 27.0 32.0 isometric
Ex. 2 carbonate oxalate1.08:2 675 4.0 27.1 32.2 elongated
Ex. 3 oxalate oxalate 1.10:2 675 4.1 25.0 33.6 elongated
Ex. 4 carbonate carbonate 1.00:2 650 3.6 26.6 33.8 isometric
Ex. 5 carbonate carbonate 1.00:2 675 3.5 34.8 25.2 isometric
Ex. 6 carbonate carbonate 1.00:2 700 3.5 36.7 23.8 isometric
Ex. 7 carbonate carbonate 1.04:2 650 3.7 30.5 29.1 isometric
Ex. 8 carbonate carbonate 1.04:2 675 3.7 31.0 28.7 isometric
Ex. 9 carbonate carbonate 1.04:2 700 3.7 32.1 27.7 isometric ~7
Ex. 10 carbonate carbonate 1.08:2 650 3.8 28.3 30.8 isometric
Ex. 11 carbonate carbonate 1.08:2 675 3.7 28.8 30.6 isometric r~
Ex. 12 carbonate carbonate 1.08:2 700 3.8 31.0 28.9 isometric CJQ
Ex. 13 carbonate carbonate 1.10:2 650 3.8 28.2 31.3 isometric
Ex. 14 carbonate carbonate 1.10:2 675 4.0 28.4 30.7 isometric
Ex. 15 carbonate carbonate 1.10:2 700 3.9 30.1 29.3 isometric
Comp. carbonate carbonate 1.08:2 675 No spinel suitable as cathode material was
Example formed
