GB2245264A

GB2245264A - Lithium manganese oxide

Info

Publication number: GB2245264A
Application number: GB9113020A
Authority: GB
Inventors: Michael Makepeace Thackeray
Original assignee: Technology Finance Corp Pty Ltd
Current assignee: Technology Finance Corp Pty Ltd
Priority date: 1990-06-18
Filing date: 1991-06-17
Publication date: 1992-01-02
Anticipated expiration: 2011-06-17
Also published as: FR2663468B1; GB2245264B; FR2663468A1; GB9113020D0; DE4119944A1; CA2044732A1; JPH04240117A

Abstract

A lithium manganese oxide compound Li2O.yMnO2 is provided in which y is >5 and the Mn cations are tetravalent. The compound, when coupled with lithium in an electrochemical cell provides an open circuit voltage of <3,5V. It is made by reacting together Li2O.yMnO2 reagent in which 2,5</=y</=4 and having a spinel-type structure with a manganese oxide reagent selected from lambda -MnO2 and electrolytically or chemically prepared manganese dioxide by mixing in finely divided form (</=250 mu m) and heating to 150-450 DEG C for >8 hours. The invention also provides an electrochemical cell having a lithium anode coupled by an electrolyte to a cathode which is said lithium manganese compound Li2O.yMnO2 in which y>5 and provides the open circuit voltage of <3,5V when coupled with lithium.

Description

1 MANGANESE OXIDE COMPOUND 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 L'20.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 be 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 which, when coupled with lithium in a said cell, provides an open-circuit voltage of <3,4OV, more preferably <3,35V.

Stoichiometric spinel compounds have structures that can be represented by the general formula A[B23X4 in which X atoms are arranged in a cubicclose-packed fashion to form a negatively 1 0 L5 2 charged anion array comprised of face-sharing and edge-sharing tetrahedra and octahedra. In the formula A[B21X4 the A atoms are tetrahedra 1-s ite cations and the B atoms are octahedra17site 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 (53m), 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 crystal lographically non-equivalent positions 8a, 8b and 48f, and 32 octahedral interstices situated at the crystal lographica 1 ly non-equivalent positions 16c and 16d. In an A[B21X4 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[B23Xn n- host f rainework structure may be regarded as being located at the 16d octahedral positions and the X anions located a' the 3.2e positions of 'the spinel structure. The tetrahedra defined by the 8a, 8b and 48f 2 0 positions and octahedra def ined by the 16c positions of the spinel structure thus form the interstitial spaces of the [B21X4 n- framework structure.

For example, a lithium manganese oxide compound which has a spinel-type structure is Li[Mn2104 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[Mn2104 is typically made by reacting a lithium salt with a manganese oxide at temperatures above 7000C. In its structure, half the Mn cations are tetravalent and half are trivalent. Lithium can be removed from the structure of LilMn2104 by means of a mineral acid such as 1 Molar H2S04 or HCl, with associated oxidation of the trivalent Mn cations to form the manganese oxide phase known as 1-Mn02, which has a defect spinel-type structure which can be represented in spinel notation by C11.O[Mn2104 (See Hunter -Us Patent 4 246 253). In 1-Mn02 all the Mn cations are thus tetravalent.

i 1 3 Another example of a lithium manganese oxide compound which has a spinel- type structure is L'4Mn5012, which has a more complex cation distribution which can be represented in spinel notation by Li[Lil/,Mn./3]04. In this compound, as in 1-Mn02, all the Mn cations are tetravalent.

It will be appreciated that, using the formula L'20.yMn02, the stoichionetric ideal non-defect compound Li4Mn5012 can be represented by L'20.yMn02 in which y is 2,5; and the defect nonstoichiometric spinel compound 1-Mn02 can be represented by L'20.yMn02 in which y is infinite (ie the concentration of L'20 is zero). The present invention concerns itself with compounds of formula L'20. yMn02 with y>5 and with a concentration of L'20 which is >O. The spinel- type compounds according to the invention are defect non-stoichiometric spinel-type compounds, and the expression 1 spinel-type compounds' accordingly covers defect nonstoichiometric spinel-type compounds.

Thus, the compounds of the present invention arenot stoichionetric 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 L'20.yMn02 compound in which y=5 can therefore be represented, instead, in spinel notation as Lil-x[Mn2-z104 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 Lil_x[Mn2-z104 in which 0,273<x<1 and 0<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 framework structure is thus a negatively charged [Mn2-zoz]04 structure wherein D 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.

1 4 The aforegoing A[B21X4 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 1, then in the normal spinel structure the value of A is 0. If the value of 1 is 0, 5, then the spinel structure is known as an 'inverse spinel 1 structure, which can be represented by the general formula B[AB]X4. Intermediate values of I are common in compounds having spinel structures, and A 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 Ispinel-type structure' refers to defect spinels and includes, in addition, normal spinel structures, also inverse spinel structures and intermediate structures wherein 0<1<0,5.

Single-phase compounds in accordance with L'20.yMnO2 with 2,5:y:55 are known, eg said L'4Mn_,012 and L'2Mn4O9 which can be represented by L'20. 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 L'20.yMnO2 'in which y is >5, and when y is >5 the L'20.yMnO2 produced by those techniques is contaminated with impurity phases, suspected to be y-Mn02 and/or 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 oxides of such replacement metals, without affecting the properties of the compounds of the present invention as regards their utility 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 L'20.yMn02 as defined above in which y is >S, the method comprising reacting together lithium manganese oxide reagent in which the ionic ratio of lithium to manganese is:1:2,5 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 250gm; and heating the mixture so formed in an oxygen-containing oxidizing atmosphere to a temperature in the range 150 4500C, 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 gin, and a high specific surface of >20nl/g, the temperature being 240 - 4000C, the heating being for a period of 8 -18 hours and the oxidizing atmosphere being selected from oxygen, air and mixtures thereof.

6 The lithium manganese oxide reagent preferably is of the formula Li20. yMn02 in which 2,55y:54, having a spinel-type structure, the manganese oxide reagent being a manganese dioxide reagent selected from the group consisting of 1-Mn02, electrolytically prepared manganese dioxide (EMD), chemically prepared manganese dioxide (CMD) and mixtures thereof.

The manganese dioxide reagent may be I-MnO2, the heating being carried out at a temperature of 240-4000C to obtain a substantially single-phase product.

is 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 350-4000C 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 L'20.yMn02f the value of y is 2,5, so that the reagent can be represented by the formula L'4Mn.012.

The manganese dioxide reagent and said L'20.yMn02 reagent in which 2,5:5y:54 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 L'20.yMn02 will be oxygen-deficient.

The L20.yMn02 reagent in which 2,5Sy:54 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.

e 7 The reaction between the L'20.yMnO2 reagent and the manganese dioxide reagent is preferably, as indicated above, carried out at a temperature of 240 - 4000C, for a period of eg 8 hrs - 1 week, 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 I-MnO2 transforms in the absence of lithium to P-Mn02 at 2700C, a heating temperature of 240 - 27011C is convenient therefor (although high temperature can be employed), whereas for electrolytically and/or chemically prepared manganese dioxides, temperatures of 375 - 4000C are typically used.

By varying the mole ratio between the L'20.yMnO2 reagent and the manganese dioxide reagent, the value of y in the L'20.yMnO2 product can be varied, from a value of 5 up to considerably higher values where the proportion of L'20 is extremely small, being a fraction of a percent or less. The method can thus be used to prepare high quality L'20.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 L'20.yMnO2 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 250g, preferably at most SOA. 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 product in the form of a solid unitary artifact, as opposed to powder.

8 As indicated above, the lithium manganese 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 lithiumcontaining 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 with the present invention and of formula L'20'yMnO2 in which y has a value of >5 as described above.

Such cells can accordingly be represented schematically by:

Li (anode) / electrolyte/L'20. yMn02 (cathode) is 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 L'C'04, LiAsF6 or LAF4, dissolved in an organic solvent such as propylene carbonate, dimethoxyethane, mixtures thereof or the like.

Accordingly, the anode may be selected from the group consisting of lithium metal, lithium alloys with other metals, lithium alloys with nonmetals, lithium/carbon intercalation compounds and mixtures thereof, the electrolytes being a room temperature electrolyte and comprising a member of the group consisting of LiC104. LiAsF6, LiBF6 and mixtures thereof, 9 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 of example, with reference to the following Example which describes the making and characterization of L'20.yMn02 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 20 for the 20 range 10- 700 using CuKa radiation, for a A-Mn02 reagent used in the method of the present invention; Figure 2 shows a similar trace, for a L'2Mn409 (L'20.yMnO2 in which y =4) reagent used in the method of the present invention to prepare a control compound; Figure 3 shows a similar trace, for a L'4Mn5012 (L'20.yMn02 in which y = 2, 5) reagent used in the method of the present invention to prepare product compounds according to the invention; Figure 4 shows a similar trace, for a control L'20.yMn02 compound, made in accordance with the method of the present invention, in which y=s; Figures 5 - 9 show similar traces, for various L'20.yMn02 product compounds, in accordance with the invention and made in accordance with the method of the invention, in which y varies from 7,5 - 10; Figures 10 12 show discharge curves which are plots of voltage against capacity for control electrochemical cells operated at room temperature (20 - 2SOC) having lithium as anode material and respectively having V-Mn02. the 1-Mn02 reagent whose trace is shown in Figure 1 and the control compound Li20.yMnO2 in which y = 5, whose trace is shown in Figure 4, as their cathodes, the electrolyte being 1 Molar LiC104 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 respectively 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 following table, Table 1. In each case, for each Figure it is indicated whether the compound is a reagent, a control compound or a lithium manganese oxide product compound in accordance with the invention; the value of y if that compound is expressed by the formula L'20.yMn02; 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 abbreviated to EMD, and chemically prepared manganese dioxide is abbreviated to CMD. The control employed L'2Mn409 as its lithium manganese oxide reagent, and the lithium manganese oxide product compounds according to the invention all employed L'4Mn5012 as their lithium manganese dioxide reagent.

TABLE 1

Mole Ratio (Lithium Manganese Manganese Figure No Reagent/Control Value of y in Dioxide reagent Oxide Reagent:

/Invention 1-120.yMn02 used Manganese Dioxide Reagent) 1 Reagent 2 Reagent 4 3 Reagent 2,5 4 Control 5;L-Mno2 1:1 Invention 8,25;L-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 following table, Table 2, further details are set forth for the compounds whose traces are shown in Figures 1 - 9.

The control compound and product compounds according to the invention were prepared in accordance with the methods given 1 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 V-Mn02) reagent compounds for comparative purposes, the EMD and CMD being heated in air at 4000C, for about 8 hours beforehand; and it is to be noted that the reagent compound of Figure 1 was dried in air at 1200C 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 Theoretical Capacity reaction ('C) Voltage (V) (mAh/g) EMID - 3,56 308 (reagent) - 3,56 308 CMD - 3,62 308 (reagent) - - - 1 - 4,12 308 - 4,13 308 2 - 3,35 213 - 3,34 213 3 - 3,45 163 - 3,46 163 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 3,48 268 Table 2 illustrates that L'20.yMn02 compounds can be made with y>5 in accordance with the present invention, 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 gA/cm 2 down to a cut-off voltage of 2V, j 12 and in the following table, Table 3, cell capacity down to said cut-of f voltage of 2V is shown f or the cells whose discharge curves are shown in Figures 10-17.

TABLE3

Figure No Capacky (to a 2V cut off at 50OpA discharge current (mAh/g) 203 11 186 12 115 13 112 14 209 204 16 156 17 186 is EXAMPLE 1 (CONTROL) In this example a L'20. yMn02 reagent compound was initially prepared in accordance with the f ormula L'2Mn409 (ie L'20.yMn02 in which y=4) by milling together L'2C03 and MnC03 in a suitable mole ratio of L'2C03:MnCO3 to obtain a mixture with an atomic ratio of Li:Mn of 1:2, and a particle size of <5Og. The mixture was heated at 4200C for 5 hours in air to produce the L'2Mn409 which had an essentially singlephase spinel-type structure (see Figure 2).

Separately, a 1-Mn02 reagent was made by reacting stoichiometric LiMn204 with 1 Molar HCl at 250C for 24 hours (see Figure 1).

The L'2Mn409 and 1-Mn02 were intimately mixed by milling to a particle size of <Sog followed by heating at 2400C in air for 16 hours. The L'2Mn409:1-Mn02 mole ratio was selected to give a Li:Mn atomic ratio of 2:5 to obtain L'20.yMn02 in which y=5 (see Figure 4).

EXAMPLE 2 (INVENTION) Example 1 was repeated except that the L'20.yMn02 reagent compound initially prepared was L'20.yMn02 'n 13 which y = 2, 5 (ie L'4Mn.012). The L'4Mn.5012 and 1-Mn02 were heated together at 4000C for 10 hours, in a mole ratio of 1:11,5 to obtain a L'20.yMn02 product compound in which y is 8,25 (see Figure 5).

The L'4Mn.012 was prepared in the same f ashion as said L'2Mn409 reagent of Example 1 but using a starting mixture of L'2C03 and MnC03 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 4200C for a period of 5 hours.

EXAMPLE 3 (INVENTION) Example 2 was repeated using EMD instead of 1-Mn02, the reaction of the L'4Mn.012 and EMD being at 3750C for 48 hours to obtain a L'20.yMn02 product compound in which y was 7,5, the mole ratio of L'4Mn.012:WD being 1:10 (see Figure 6).

EXAMPLE 4 (INVENTION) Example 3 was repeated, the reaction of the L'4Mn.012 and EMD being at 3500C for 16 hours in a mole ratio of L'4MnSO12:EMD of 1:12,5, to obtain a L'20.yMn02 product compound in which y was 8,75 (see Figure 7).

EXAMPLE 5 (INVENTION) Example 3 was repeated, using CMD instead of EMD, except that the reaction of the L'4Mn.012 with CMD was at 4000C for 24 hours (see Figure 8).

EXAMPLE 6 (INVENTION) Example 5 was repeated, except that the reaction of the L'4Mn5012 with the CMD took place at 3500C for a period of 3 days with a mole ratio of L'4Mn.012: CMD of 1:15 (see Figure 9).

1 14 X-ray diffraction traces were prepared from the aforegoing and are shown in the accompanying drawings as set forth in the tables above.

Figures I to 3 respectively demonstrates the essentially single-phase character of the I-MnO2, L'2Mn4O9 and L'4Mn.0.12 in question.

Figure 4 shows that the control L'20.5MnO2 prepared in accordance with the method of the present invention also has a single-phase character, in which only negligible traces of MR203 impurity are discernible.

Figure 5 shows that the L'20.8,25Mn02 of the invention, prepared according to the method of the invention, has a predominantly spinel-type character, in which no more than acceptably small traces of V-Mn02, P-Mn02 and Mn203 are discernable as impurities.

Figures 6, 7, 8 and 9 show that the products of the invention, L'20-yMn02 with y >5 contain a significant component having a spinel-type structure, and, in addition, a V-Mn02 related phase that it believed to contain a minor proportion of lithium ions.

An advantage of the invention is that the L'20.yMn02 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 5 (y = 8,25) and Figure 9 (y = 10) can have significantly higher capacities than the L'20.yMn02 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 1-Mn02 reagents (see Figures 12 and 13) are attributed to the low surf ace area of this reagent (less than 10 m2 /g). Higher capacities can be expected if 1-Mn02 reagents with higher specific surfaces are used.

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 V, but for the cycling experiments shown in Figure 14 the upper and lower voltage limits were set at 3,8 V and 2,0 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 f urther 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 X-Mn02 as cathodes, all of which are >3,5V (see Table 2). For example, Table 2 shows that I-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 3,62 V respectively. However, after reaction with a Li20.yMnO2 reagent such as Li20.2,5MnO2 (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.

1 16

Claims

1. A lithium manganese oxide compound having the general formula:

L'20.yMn02 in which:

y has a value of >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 <3,50 V.

2. A compound as claimed in claim 1, which has, as separate phases therein, a lithium manganese oxide component having a spinel-type structure, and a manganese oxide component.

3. A compound as claimed in claim 2, which has an essentially singlephase spinel-type structure.

4. A compound as claimed in any one of claims 1 - 3 inclusive, which, when coupled with lithium in a said cell, provides an open circuit voltage of <3,40 V.

5. A compound as claimed in claim 4, in which said open circuit voltage is <3,35 V.

6. A compound as claimed in any one of the preceding claims, in which at most 20% of the manganese cations are replaced by other metal cations.

7. A method of Tnaking a lithium manganese oxide product which is a compound having the general formula L'20.yMn02 as claimed in claim 1, the method comprising reacting together a lithium manganese oxide reagent in which the ionic ratio of lithium to A 1 17 manganese is k1:2,5 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 25Ogm; and heating the mixture so formed in an oxygen-containing oxidizing atmosphere to a temperature in the range 150-4500C for a period of at least 8 hours.

8. A method as claimed in claim 7, in which the reagents have a particle size of at most 5Ogm and a specific surface >20 M2 /g, the temperature being 240-4000C, the heating being for a period of 8 -18 hours, and the oxidizing atmosphere being selected from air, oxygen and mixtures thereof.

9. A method as claimed in claim 7 or claim 8, in which the lithium manganese oxide reagent is of the formula L'20.yMn02 in which 2,5:sy:54, having a spinel-type structure, the manganese oxide reagent being a manganese dioxide reagent selected from the group consisting of 1-Mn02, electrolytically prepared manganese dioxide, chemically prepared manganese dioxide and mixtures thereof.

10. A method as claimed in claim 9, in which the manganese dioxide reagent is I-MnO2, the heating being carried out at a temperature of 240- 4000C to obtain a substantially single-phase product.

11. A method as claimed in claim 9, in which the manganese dioxide reagent is selected from electrolyticallyprepared manganese dioxide, chemically prepared manganese dioxide and mixtures thereof, the heating being at a temperature of 350-4000C to obtain a product which comprises, in addition to a component having a spinel-type phase, a component having a manganese dioxide phase.

12. A method as claimed in any one of claims 9 - 11 inclusive, in which the value of y in the L'20.yMn02 reagent is 2,5.

1 1 1 t 18

13. A method as claimed in any one of claims 7 - 12 inclusive, which includes the step, prior to the heating, of consolidating the mixture by pressing it at a pressure of 2 - 10 bars (200 1000 kPa) to form a green artifact which, after heating, forms a product in the form of a solid unitary artifact.

14. An electrochemical cell which comprises an anode selected from lithium-containing materials, a cathode and an electrolyte whereby the anode is electrochemically coupled to the cathode, the cathode comprising a lithium manganese oxide compound as claimed in claim 1.

is

15. A cell as claimed in claim 14, in which the anode is selected from the group consisting of lithium metal, lithium alloys with other metals, lithium alloys with non-metals, lithium/carbon intercalation compounds and mixtures thereof, the electrolyte being a room temperature electrolyte and comprising a member of the group consisting of L'Cl04, LiAsF6, LAF6 and mixtures thereof, dissolved in an organic solvent selected from the group consisting of propylene carbonate, dinethoxyethane and mixtures thereof.

16. A lithium manganese oxide compound as claimed in claim 1, substantially as described herein.

17. A method as claimed in claim 7, substantially as described herein.

18. An electrochemical cell as substantially as described herein.

claimed in claim 14, Published 1991 at The Patent Office. Concept House. Cardiff Road. Newport, Gwent Npq I R U1 Further co M be obtained from e ay Sales Branch, Unit 6. Nine Mile Point, Cwrafelinilach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniquTes ltd. St Mary Cray, Kent.