Note: Descriptions are shown in the official language in which they were submitted.
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, PROCESS FOR PRODUCING A POSITIVE ELECTRODE
FOR LITHIUM SECONDARY BATI'ERIES
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Backqround of the Invention
The present invention generally relates to a process
for producing a positive electrode for lithium secondary
~~ batterles, particularly those with an active material composed of
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a lithium manganese oxide having a spinel structure.
Lithium-manganese spinels are presently advantageously
used as positive electrode material in electrochemical cells of
the Li/LixMn2O4 type. The charge/discharge mechanism of 5uch
cells is based on the capacity of the manganese oxide to
reversibly intercalate and deintercalate active Li~ ions in its
spinel lattice, which forms an open skeletal structure.
Consequently, x will vary in parallel with progressive charging
~:i! or discharging.
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To prepare a LiMn204 material from MnO2, one usually
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begins with a commercially available manganese dioxide which is
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~ first thoroughly ground in a mortar with a solid lithium
`~ compound, such as lithium hydroxide, lithium carbonate or lithium
~``j iodide, and which is then heated for the purpose of sintering.
The sinter cake may optionally be comminuted and provided with a
`~ 20 conductive agent (graphite) and a binder before it is press-
¦ molded to form the finished cathode pellet.
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The commercially available manganese oxides (manganese
blacks) form a suitable starting material for the preparation of
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"! lithium-manganese spinels insofar as they have for a long time
been used in the battery industry, their electrochemical behavior
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has been thoroughly researched, they are an inexpensive raw
matarial and they are environmentally compatible. However, such
cathode ~ubstances, produced in the manner described, have to
date continued to suffer from a lack of adequate cycling
stability. q~he dischargeable capacities of such substances
decrease noticeably in the course of their cyclic service life,
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~ in particular after a prior high-temperature aging.
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~ Various measures have been proposed for stabilizing the
'7~l host structure of a manganese spinel. To this end, it has been
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~i attempted to substitute some of the manganese in the lattice with
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foreign metals including Co, Ni and Fe. Japanese Published
Specification 4-169065 suggests the production of a positive
electrode material for lithium batteries from MnO2 and lithium
`~3l salts of ~ormic acid and acetic acid, which are mixed with the
manganese oxide ~instead of LioH-H2o or Li2Co3). ~he mixture is
~j 20 then heat-treated at relatively low temperatures of between 200C
;.1
and 500c. However, in each case, an unqualified success has not
resulted.
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Summary of the Invention
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: It is therefore the primary object of the present
invention to provide a process for producing a lithium-manganese
~ spinel of the Li~Mn2O4 type which is cyclically stable as a
-. 5 cathode material in lithium cells containing organic
,
; electrolytes, and which is suitable as a counterelectrode to a
~l negative lithium intercalating electrode (Li in a carbon matrix),
... ! even in systems where lithium is intercalated in both electrodes
(1.e., so-called "SWING" systems).
These and other ob~ects which will be apparent are
achieved in accordance with the present invention by a process
~`' for producing an active material for a positive electrode for
lithium secondary batteries, where the active material is
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composed of a lithium manganese oxide having a spinel structure,
which process includes mixinq manganese dioxide with lithium
~ormate and/or lithium acetate, heating the resulting mixture to
a temperature of from 550C to 800C for between 10 and 100
hours, and if necessary, grinding the heated mixture.
For further discussion of the process of the present
invention, reference is made to the following detailed
description.
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; Detailed Description of Preferred Embodiments
Initially, in a first step, pure manganese oxide is
lntlmately mixed with lithium formate (HcooLi) or lithium acetate
(CH3CooLi), or optionally, with both of these salts.
The manganese dioxide which is preferably used for such
purposes is commercially prepared by a chemical reaction, usually
` by precipitation from a manganese salt solution. Such products
are commercially available as "chemical" manganese dioxide (CMD).
,~i However, pure ~-MnO2, which occurs naturally as pyrolusite, is
~3 l0 also suitable as a starting material.
`i The foregoing constituents are mixed in quantitative
proportions that achieve a molar ratio of manganeise to lithium of
¦ 2:x, where 0.5 < x < 1.5, and preferably 0.9 < x < 1.2. A
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particularly intimate mixing is obtained if a suspension of
~ 15 manganese black 18 produced in an aqueous solution of the
.1 appropriate lithlum salts and the resulting solution is then
;~ sub;ected to spray drying.
The mixture obtained (in the latter case, the dry
residue obtained) is heated in air at a temperature of from 550~C
to 800C, and preferably from 600C to 750C, for between l0 and
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l00 hours. Resulting from the vigorous gas evolution
~ accompanying the reaction of the mixture's components, this
`: thermal treatment produces an extremely finely crystalline
~: lithium-manganese spinel. The small particle size of this
.~ LA~b~ preparation corresponds to a relatively large BET surface area
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(i.e., in excess of 5 m2/g). This, in turn, imparts good high-
current properties to the manganese spinel produced in accordance
with the present invention.
The product is then ground, if necessary, and processed
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~ 5 to produce the desired electrode. This grinding step can be
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performed either concurrently with the heating and mixing steps,
or if desired, at a later stage in the manufacturing process.
j The active electrode material which results meets the
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requirement of good cycle stability in batteries having positive
LiMn20~ electrodes and negative Li or Li-intercalating
electrodes. The resulting active electrode material is also
¦ notable for high-temperature stability, and is therefore even
suitable for use in galvanic solid-electrolyte cells which are
I operated in temperature ranges above room temperature.
~i 15 The capacity to dope the resulting lithium-manganese
spinels with lithium, to a Ievel far beyond the stoichiometric
composition (not more than x = 1.5), is also extremely
advantageous. This is especially so relative to the production
of cells having posltive and negative lithium-intercalating
electrodes (i.e., SWING systems) .
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When finally assembled and in a sealed state, the
electrodes of the resulting cells are usually uncharged (i.e.,
all available lithium is incorporated in the positive spinel or
metal oxide electrode, while the host structure of the negative
electrode is drained of lithium). During the first charging
UWfff~S cycle, lithium is discharged from the positive host lattice and
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incorporated into the negative host lattice, which is preferably
a carbon matrix. Some of the lithium is now retained by the
carbon matrix to form a covering layer (i.e., by an irreversible
chemical reaction). In this way, a certain quantity of lithium
is withdrawn from the intercalation mechanism (by the carbon
material, which is not inert from the outset). However, a
', lithium-manganese spinel made with a hyper-stoichiometric amount
of lithium makes it possible to saturate the reactivity of the
~l carbon electrode, so that the intercalation and deintercalation
; lO of lithium in the carbon matrix can proceed virtually without
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`' loss, i.e., reversibly.
It will therefore be understood that various changes in
'¦ the details, materials and arrangement of parts which have been
t~l herein described and illustrated in order to explain the nature
of this invention may be made by those skilled in the art within
the principle and scope of ~hé invention as expressed in the
' following claims.
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