204~7~
THIS INVENTION relates to a lithium manganese oxide compound
suitable for use as an electrode in an electrochemical cell; to
a method of making such compound; and to an electrochemical cell
employing such compound as its cathode.
According to one aspect of the invention there is provided
a lithium manganese oxide compound having the general formula
Li20 . yMnO2
in which:
y has a value which is >5; and
the Mn cations are substantially all tetravalent, the
compound, when coupled by a suitable electrolyte with lithium in
an electrochemical cell, providing an open circuit voltage of
<3,50V.
By 'substantially all tetravalent' is meant that the average
valency of the manganese cations will ~e at least +3,7 and
usually higher, eg 3,8-4,0.
The compound may have, as separate phases therein, a lithium
manganese oxide component having a spinel-type structure, and a
manganese oxide component. Instead, the compound may have an
essentially single-phase spinel-type structure.
Preferably the compound is one ~Jhich, l.ihen coupled with
lithium in a said cell, provides an open-circuit voltage of
<3,40V, more preferably <3,35V.
Stoichiometric spinel compounds have structures that can be
represented by the general formula A[B2]X4 in ~hich X atoms are
arranged in a cubic-close-packed fashion to form a negatively
2044732
charged anion array comprised of face-sharing and edge-sharing
tetrahedra and octahedra. In the formula A[B2]X4 the A atoms are
tetrahedral-site cations and the B atoms are octahedral-site
cations, ie the A cations and B cations occupy tetrahedral and
octahedral sites respectively. In the ideal spinel structure,
with the origin of the unit cell at the centre (3m), the close-
packed anions are located at the 32e positions of the space group
Fd3m. Each unit cell contains 64 tetrahedral interstices situated
at three crystallographically non-equivalent positions 8a, 8b and
48f, and 32 octahedral interstices situated at the
crystallographically non-equivalent positions 16c and 16d. In an
AtB2]X4 spinel the A cations reside in the 8a tetrahedral
interstices and the B cations in the 16d octahedral interstices.
There are thus 56 empty tetrahedral and 16 empty octahedral sites
per cubic unit cell.
Therefore, the B cations of the [B2]Xnn- host framework
structure may be regarded as being located at the 16d octahedral
positions and the X anions located at the 32e positions of the
spinel structure. The tetrahedra defined by the 8a, 8b and 48f
positions and octahedra defined by the l~c positions of the
spinel structure thus form the interstitial spaces of the
~B2~X4n- framework structure.
For example, a lithium manganese oxide compound which has
a spinel-type structure is Li[Mn2]O4 which is known to have been
used as a cathode in primary and rechargeable cells and batteries
with lithium as the active anode material. Li~Mn2]04 is typically
made by reacting a lithium salt with a manganese oxide at
temperatures above 700C. In its structure, half the Mn cations
are tetravalent and half are trivalent. Lithium can be removed
from the structure of Li[Mn2]O4 by means of a mineral acid such
as 1 Molar H2S04 or HCl, ~ith associated oxidation of the
trivalent Mn cations to form the manganese oxide phase known as
l-MnO2, which has a defect spinel-type structure ~lhich can be
represented in spinel notation by l O[Mn2]O4 (See Hunter -US
Patent 4 246 253). In l-MnO2 all the Mn cations are thus
tetravalent.
2044732
` Another example of a lithium manganese oxide compound which
has a spinel-type structure is Li4Mn5012, which has a more
complex cation distribution which can be represented in spinel
notation by Li[Li1/3Mn5/3]04. In this compound, as in A-Mno2, all
the Mn cations are tetravalent.
It will be appreciated that, using the formula Li20.yMnO2,
the stoichiometric ideal non-defect compound Li~MnsO12 can be
represented by Li20.yMnO2 in which y is 2,5; and the defect non-
stoichiometric spinel compound A-MnO2 can be represented by
Li20.yMnO2 in which y is infinite (ie the concentration of Li2o
is zero). The present invention concerns itself with compounds
of formula Li2O.yMnO2 with y>5 and with a concentration of Li2o
which is >0. The spinel-type compounds according to the invention
are defect non-stoichiometric spinel-type co~pounds, and the
expression 'spinel-type compounds' accordingly covers defect non-
stoichiometric spinel-type compounds.
Thus, the compounds of the present invention are not
stoichiometric spinel compounds, but are those in which defects
are created by varying the quantity of Li ions at the A sites,
such compounds being synthesized to have such defects by varying
the quantity of Mn cations in the framework structure. The
Li2O.yMnO2 compound in which y=5 can therefore be represented,
instead, in spinel notation as Li1_x[Mn2_z]Og in which x=0,273 and
y = 0,182. The compounds of the present invention, with y>5, can
in turn be represented in spinel notation as Li1_x~Mn2_z]04 in
which 0,273<x<1 and O~z<0,182.
In the present invention, the A sites are partially occupied
by Li cations, the B sites are partially occupied by Mn cations
and the X anions are 0 anions. The frame;ork structure is thus
a negatively charged [Mn2_zOz]04 structure wherein 0 represents
a vacancy and may be regarded as part of the interstitial spaces,
and said interstitial spaces are available for mobile Li cations,
for diffusion therethrough during electrochemical discharge and
charge reactions, as described hereunder.
20~47~
The aforegoing A[B2]X4 structure is known as a normal spinel
structure. It is possible, however, for the cations to be
rearranged into an arrangement wherein certain of the B cations
occupy tetrahedral sites normally occupied by A cations and
certain of the A cations occupy octahedral sites normally
occupied by B cations. If the fraction of the B cations occupying
tetrahedral sites is designated A, then in the normal spinel
structure the value of A is 0. If the value of ~ is 0,5, then the
spinel structure is known as an 'inverse spinel' structure, which
can be represented by the general formula B~AB]X4. Intermediate
values of l are common in compounds having spinel structures, and
l is not necessarily constant for a particular compound, but can
in some cases be altered by heat treatment under suitable
conditions.
For the purpose of the present specification the expression
'spinel-type structure' refers to defect spinels and includes,
in addition, normal spinel structures, also inverse spinel
structures and intermediate structures wherein o<A<o,5.
Single-phase compounds in accordance with Li20.yMnO2 with
2,5sy<5 are known, eg said Li4Mn50l2, and Li2Mn409 which can be
represented by Li20.yMnO2 in which y = 2,5 and y=4 respectively.
These compounds, and their manufacture and use in cells are
described in published British Patent Application GB 2 221 213A
and South African Patent 89/5273. The Applicant has, however,
been unable, using the techniques described therein, to obtain
single-phase spinel phases of Li20.yMnO2 in which y is >5, and
when y is >5 the Li20.y~nO2 produced by those techniques is
contaminated with impurity phases, suspected to be ~ nO2 andtor
Mn203 .
It should be noted that the compounds of the present
invention, as described above, can have up to 20% of their
manganese ions replaced by other metal ions, particularly
transition metal cations such as cobalt, by doping with o~ides
of such replacement metals, without affectin5 the properties of
the compounds of the present invention as regards their utility
204~73~
in electrochemical cells of the type described hereunder. This
doping can be effected by substituting said replacement metal
oxides for a proportion of the manganese oxide reagent employed
in the method, described hereunder, of making the compounds
according to the invention. Accordingly, in the compounds of the
present invention, at most 20% of the manganese cations may be
replaced by other metal cations.
The lithium manganese oxide compounds of the present
invention can be prepared by means of a solid state reaction
whereby a lithium manganese oxide reagent is reacted at an
elevated temperature with a suitable manganese oxide.
.
Thus, according to another aspect of the invention there is
provided a method of making a lithium manganese oxide product
which is a compound according to the present invention of the
formula Li20.yMnO2 as defined above in which y is >5, the method
comprising reacting together lithium manganese oxide reagent in
which the ionic ratio of lithium to manganese is 21:2,5 with a
manganese oxide reagent, the method comprising the steps of:
intimately mixing said reagents together in finely divided
form having a particle size of at most 250~m; and
heating the mixture so formed in an oxygen-containing
oxidizing atmosphere to a temperature in the range 150 - 450C,
for a period of at least 8 hours.
Naturally the proportions of the lithium manganese oxide
reagent and the manganese oxide reagent will be selected on a
stoichiometric basis so that the desired value of y is obtained
in the product.
Preferably, the reagents have a particle size of at most 50
~m, and a high specific surface of >20m2/g, the temperature being
240 - 400C, the heating being for a period of 8 -18 hours and
the oxidizing atmosphere being selected from oxygen, air and
mixtures thereof.
204~732
The lithium manganese oxide reagent preferably is of the
formula Li20.yMnO2 in which 2,5sys4, having a spinel-type
structure, the manganese oxide reagent being a manganese dioxide
reagent selected from the group consisting of ~-MnO2,
electrolytically prepared manganese dioxide (EMD), chemically
prepared manganese dioxide (CMD) and mixtures thereof.
The manganese dioxide reagent may be A-Mno2~ the heating
being carried out at a temperature of 240-400C to obtain a
substantially single-phase product.
Instead, the manganese dioxide reagent may be selected from
electrolytically prepared manganese dioxide, chemically prepared
manganese dioxide and mixtures thereof, the heating being at a
temperature of 350-400C to obtain a product which comprises, in
addition to a component having a spinel-type phase, a component
having a manganese dioxide phase.
Preferably, when the lithium manganese dioxide reagent is
of the formula Li2O.yMnO2, the value of y is 2,5, so that the
reagent can be represented by the formula Li4Mn5012.
The manganese dioxide reagent and said Li20.yMnO2 reagent in
which 2,5sys4 need not be stoichiometric and the valency of the
Mn therein need not be exactly +4 and may be <+4 depending on the
temperature of preparation of the reagents. This valency may thus
be 3-4, but is preferably 3.5-4, more preferably 4. In cases
where the valency of the Mn is <4 the manganese dioxide reagent
and Li20.yMnO2 will be oxygen-deficient.
The Li20.yMnO2 reagent in which 2,5<y<4 can be obtained by
reacting together a lithium salt and a manganese salt according
to the method described in published British Patent Application
GB 2 221 213A and South African Patent 89/5273, and the lithium
salt and manganese salt used to make this reagent are preferably
anhydrous.
204~73~
The reaction between the Li2O.yMnO2 reagent and the
manganese dioxide reagent is preferably, as indicated above,
carried out at a temperature of 240 - 400C, for a period of eg
8 hrs - 1 weeX, preferably 8 - 18 hours, the heating period
being, broadly, inversely related to the temperature. The heating
temperature to an extent depends on the manganese dioxide reagent
used. As A-Mno2 transforms in the absence of lithium to ~-MnO2 at
270C, a heating temperature of 240 - 270C is convenient
therefor (although high temperature can be employed), whereas for
electrolytically and/or chemically prepared manganese dioxides,
temperatures of 375 - 400C are typically used.
By varying the mole ratio between the Li20.yMnO2 reagent and
the manganese dioxide reagent, the value of y in the Li2O.yMnO2
product can be varied, frcm a value of 5 up to considerably
higher values where the proportion of Li2o is extremely small,
being a fraction of a percent or less. The method can thus be
used to prepare high quality Li2O.yMnO2 in which y is 5 (which
product is described in South African Patent 89/5273, but of
substantially enhanced purity as regards its single-phase
character), and the method is indeed the only method which the
Applicant has found whereby said Li20.y~lnO2 can be made with y>5
and which provides said open circuit voltage of <3,5 when coupled
electrochemically with lithium.
Conveniently the mixing is by dry milling, to obtain said
particle size of at most 250~, preferably at most 50~. Instead,
however the mixing may be by making up a slurry of said reagents
in a suitable liquid by wet milling, which slurry can be dried
to provide the mixture of said reagents which is heated.
If desired, the method may include the step, prior to the
heating, of consolidating the mixture (after drying if necessary)
by pressing it at a suitable pressure, eg 2-10 bars (ie 200 -
1000 kPa), to form a green artifact which, after heating, forms
a product in the form of a solid unitary artifact, as opposed to
a powder.
20~732
- As indicated above, the lithium ~anganese oxide compounds
of the present invention have utility as insertion electrodes in
both primary and secondary electrochemical cells having lithium
as their electrochemically active anode material.
Thus, according to a further aspect of the invention there
is provided an electrochemical cell which comprises an anode
selected from lithium-containing materials, a cathode and a
suitable electrolyte whereby the anode is electrochemically
coupled to the cathode, the cathode comprising a lithium
manganese oxide compound in accordance~Jith the present invention
and of formula Li20.yMnO2 in which y has a value of >5 as
described above.
Such cells can accordingly be represented schematically by:
.
Li(anode)/electrolyte/Li20.yMnO2(cathode)
Apart from lithium itself, suitable lithium-containing
anodes which can be employed include suitable lithium-containing
alloys with other metals or non-metallic elements, examples being
lithium/aluminium alloys and lithium/silicon alloys wherein the
lithium:aluminium and lithium:silicon ratios are those typically
employed in the art, and lithium/carbon anodes in which lithium
is intercalated into a carbonaceous structure, eg a graphite
structure.
The electrolyte is conveniently a room-temperature
electrolyte such as LiC104, LiAsF6 or LiBF4, dissolved in an
organic solvent such as propylene carbonate, dimethoxyethane,
mixtures thereof or the like.
Accordingly, the anode may be selecte~ from the group
consisting of lithium metal, lithium alloys ~ith other metals,
lithium alloys ~lith non-metals, lithium/caroon intercalation
compounds and mixtures thereof, the electrolytes being a room
temperature electrolyte and comprising a me~ber of the group
consisting of LiC10~, LiAsF6, LiBF~ and miY.tures thereof,
2~732
dissolved in an organic solvent selected from the group
consisting of propylene carbonate, dimethoxymethane and mixtures
thereof.
The invention will now be described, by way o~ example, with
reference to the following Example which describes the making and
characterization of Li2O.y~nO2 according to the present
invention, and with reference to the accompanying drawings, in
which:
Figure 1 shows an X-ray diffraction pattern trace in counts
per second plotted against 2~ for the 26 range 10-70 using CuKa
radiation, for a A-MnO2 reagent used in the method of the present
invention;
Figure 2 shows a similar trace, for a Li2Mn4Og (Li2O.yMnO2
in which y =~) reagent used in the method of the present
invention to prepare a control compound;
Figure 3 shows a similar trace, for a Li4MnsOl2 (Li2O.yMn32
in which y = 2,5) reagent used in the method of the present
invention to prepare product compounds according to the
invention;
Figure ~ shows a similar trace, for a control Li2O.yMnO2
compound, made in accordance with the method of the present
invention, in which y=5;
Figures 5 - 9 show similar traces, for various Li2O.yMnO2
product compounds, in accordance with the invention and made in
accordance with the method of the invention, in which y varies
from 7,5 - lO;
Figures 10 - 12 show discharge curves which are plots of
voltage against capacity for control electrochemical cells
operated at room temperature (20 - 25C) havi.ng lithium as anode
material and respectively having y-MnO2, the l-Mno2 reagent whose
trace is shown in Figure 1 and the control compound Li2O.yMnO2 in
which y = 5, whose trace is shown in Figuxe ~, as their cathodes,
the electrolyte being 1 Molar LiClO~ in propylene carbonate
/dimethoxyethane mixed in a 1:1 volumetric ratio ; and
Figures 13 - 17 show similar discharge curves for similar
cells in accordance with the invention in which the cathodes
20~732
11
r~spectively are the lithium manganese oxide product compounds
whose traces are shown in Figures 5 - 9.
Details of the various compounds whose traces are shown in
these Figures are set forth in the follo~ing table, Table l. In
each case, for each Figure it is indicated whether the compound
is a reagent, a control compound or a lithium ~anganese oxide
product compound in accordance with the invention; the value of
y if that compound is eYpressed by the formula Li20.yMnO2; the
manganese dioxide reagent from which it was made, if it was made
from reagents in accordance with the method of the invention; and
the mole ratio of the reagents used. Electrolytically prepared
manganese dioxide is a~breviated to EMD, and chemically prepared
manganese dioxide i~ abbreviated to CMD. The control employed
Li2Mn409 as its lithium manganese oxide reagent, and the lithium
manganese oxide product compounds according to the invention all
employed Li4Mn50l2 as their lithium manganese dioxide reagent.
TABLE 1
Mole Ratio
(Lithium
Manganese Manganese
Figure No Reagent/Control Value of y in Dioxide re2gent Oxide Reagent:
/I nvention Li2O .yMnO2 used Manganese
Dioxide
Reagent)
1 Reagent ~
2 O 2 Reagent 4 .
3 Reagent 2,5
4 Control 5 1 MnO2 1:1
Invention 8,25 ~.-MnO2 1:11,5
6 Invention 7,5 EMD 1:10
7 Invention 8,75 EMD 1:12,5
8 Invention 7,5 CMD 1:10
9 Invention 10 CMD 1:15
~ _ _ _
In the follo~ing table, Table 2, further details are set
forth for the co~pounds T~hose traces are shoT~in in Figures 1 - 9.
The control compound and product compounds according to the
invention were prepared in accordance with the methods given
20~4732
respectively in Examples 1 and 2 - 6 hereunder. In Table 2 are
given the reaction temperatures for making the control compound
and product compounds according to the invention of Figure 4 and
Figures 5 - 9; the initial open circuit voltage of the compounds
in question when loaded, as cathodes into cells of the type whose
discharge curves are shown in Figures 10 -17; and the theoretical
capacity of the compounds as cathodes in such cells. Table 2 also
shows values for EMD and CMD (both of which are ~-MnO2) reagent
compounds for comparative purposes, the EMD and CMD being heated
in air at 400C, for about 8 hours beforehand; and it is to be
noted that the reagent compound of Figure 1 was dried in air at
120C for about 8 hours. It is to be noted that a number of tests
were repeated, the repeat values also being shown in Table 2.
TABLE 2
Figure No Temperature of Initial Open Circuit Theore~ical Capacity
. reaction ( C) Voltage (V)(mAh/g)
I I
EMD 3,56 308
(reagent) 3,56 308
CMD . 3,62 308
(reagent)
~ ; ~ a
4 400 3,37 231
~ 400 3,41 260
6 375 3,40 256
. 7 380 3,34 263
8 400 3,42 256
9 350 3,43 268
Table 2 illustrates that Li20.y~nO2 compounds can be made
with y>5 in accordance with the present in~ention, which
compounds give initial voltages of <3,5V when coupled with Li/Li+
in an electrochemical cell of the type in question.
With regard to the discharge curves shown in Figures 10- 17,
it is to be noted that the cells in question were operated at a
discharge current of 500 ~A/cm2 down to a cut-off voltage of 2V,
2044732
13
and in the following table, Table 3, cell capacity down to said
cut-off voltage of 2V is shown for the cells whose discharge
curves are shown in Figures 10-17.
TABLE3
Figure No Capacity (to a 2V cut off at 500~A discharge
current (mAh/s)
Ir1~3 ~ 211~8~362
1 1s 26
EXAMPLE 1 (CONTROL)
In this example a Li2O.yMnO2 reagent compound was
init.ially prepared in accordance with the formula
Li2Mn4Og (ie Li2O.yMnO2 in which y=4) by milling
together Li2co3 and MnCO3 in a suitable mole ratio of
Li2CO3:MnCO3 to obtain a mixture with an atomic ratio
of Li:Mn of 1:2, and a particle size O.c <50~. The
: mixture was heated at 420C for 5 hours in air to
produce the Li2Mn4Og which had an essentially single~
phase spinel-type structure (see Figure 2).
Separately, a A~MnO2 reagent was made by reacting
stoichiometric LiMn2O4 with 1 Molar HCl at 25C for 24
hours (see Figure 1).
.
The Li2Mn4Og and i.-MnO2 were intimately mixed by
milling to a particle size of <50~ followed by heating
at 240C in air for 16 hours. The Li2Mn~os:A-Mno2 mole
ratio was selected to give a Li:Mn atomic ratio of 2:5
to obtain Li2O.yMnO2 in which y=5 (see Figure 4).
EXAMPLE 2 (INVENTION)
Example 1 was repeated except that the I.i2O.yMnO2
reagent compound initially prepared was Li2O.yMnO2 in
2~732
14
which y = 2,5 (ie Li4Mn5O12). The Li4Mn5O12 and 1-MnO2
were heated together at 400C for 10 hours, in a mole
ratio of 1:11,5 to obtain a Li2O.yMnO2 product compound
in which y is 8,25 ~see Figure 5).
The Li4MnsOl2 was prepared in the same fashion as said
Li2Mn4O9 reagent of Example 1 but using a starting
mixture of Li2Co3 and MnCO3 in which the molar ratio
was such as to obtain, in the mixture, a Li:Mn atomic
ratio of 4:5, the heating being at 420C for a period
of 5 hours.
EXAMPLE 3 (INVENTION)
Example 2 was repeated using EMD instead of A-Mno2,
the reaction of the Li4Mn5O12 and EMD being at 375C
for 48 hours to obtain a Li2O.yMnO2 prsduct compound in
which y was 7,5, the mole ratio of LiAMnsOi2:EMD being
1:10 (see Figure 6).
EX.~MPLE 4 (INVENTION)
Example 3 was repeated, the reaction of the Li4Mn5O12
and EMD being at 350~C for 16 hours in a mole ratio of
Li4Mn5O12:EMD of 1:12,5, to obtain a Li20.y~.-InO2 product
compound in which y was 8,75 (see Figure 7).
EXAMPLE 5 (INVENTION)
Example 3 was repeated, usiny C;~3 in5tead of EMD,
except that the reaction of the Li~Mn;012 with CMD was
at 400C for 24 hours (see Figure ~).
EXAMPLE 6 iINVENTION)
Example 5 was repeated, except tna the reaction of
the Li4Mn5O12 with the CMD took place at 350C for a
period of 3 days with a mole ratio or Li~Mr5Ol2:CMD of
1:15 (see Figure.9).
20~732
X-ray diffraction traces were prepared from the aforegoing
and are shown in the accompanying drawings as set forth in the
tables above.
Figures 1 to 3 respectively demonstrates the essentially
5single-phase character of the A-Mno2~ Li2Mn4Og and Li4Mn5O12 in
question.
Figure 4 shows that the control Li2O.5MnO2 prepared in
accordance with the method of the present invention also has a
single-phase character, in which only negligible traces of Mn2O3
10impurlty are discernible.
Figure 5 shows that the Liz0~8~25MnO2 of the invention,
prepared according to the method of the invention, has a
.
predominantly spir.el-type character, in which no more than
acceptably small traces of ~-MnO2, ~-MnO2 and Mn2O3 are
discernable as impurities.
Figures 6, 7, 8 and 9 show that the products of the
invention, Li2O.yMnO2 with y >5 contain a significant component
having a spinel-type structure, and, in addition, a y-MnO2-
related phase that it believed to contain a minor proportion of
lithium ions.
An advantage of the invention is that the Li2O.yMnO2 product
compounds according to the present invention, as electrodes in
cells wherein they are coupled with Li/Li~ anodes, can have
significantly high electrode capacities - see Table 2 which shows
that the compounds of particularly Figure S (y = 8,2S) and Figure
9 (y = 10) can have significantly higher capacities than the
Li2O.yMnO2 reagents with y=4 and y = 2,5 respectively, (Figures
2 and 3) and the control (Figure 4 in which y = 5). It should be
noted, however, that the relatively low capacities obtained using
A-Mno2 reagents (see Figures 12 and 13) are attributed to the low
surface area of this reagent (less than 10 m2/g). Higher
capacities can be expected if A-MnO2 reagents with higher
specific surfaces are used.
.
.
'
,,
2~73~
16
While cells according to the invention have shown
particularly high capacities during the first or initial
discharge cycles thereof, cell capacities in excess of 140 mA-h/g
have also been achieved on repeated charge/discharge cycling of
these cells, as illustrated by Figure 14 which shows the first
6 discharge cycles of the cell in question, it should be noted
that the initial open circuit voltage of the cell was 3,40 Vl but
for the cycling experiments shown in Figure 14 the upper and
lower voltage limits were set at 3,8 V and 2lO V respectively.
This confirms that the product compounds of the present invention
can have utility in both primary and rechargeable (secondary)
electrochemical cells.
It is a further advantage that the initial open circuit
voltages of <3,5 V of the cells of the present invention are
substantially less then those of known cells employing EMD, CMD
or A-MnO2 as cathodes, all of which are >3,5V (see Table 2). For
example, Table 2 shows that A-Mno2 cathodes deliver an initial
open circuit voltage against lithium of 4,12 V and EMD and CMD
deliver initial open circuit voltages of 3,56 V and 3l62 V
respectively. However, after reaction with a Li20.yNnO2 reagent
such as Li2O.2,5MnO2 (y = 2,5) as described hereinbefore, the
open circuit voltages drop to below 3,5 V. Cells according to the
present invention, as contrasted with said known cells, can thus
be made which do not need to be partially predischarged by the
manufacturer before they are stored, to resist self-discharge and
promote an acceptable shelf-life.
