SACRIFICIAL ACTIVE MATERIAL OF A POSITIVE ELECTRODE FOR A LITHIUM-ION ELECTROCHEMICAL ELEMENT

Abstract

A sacrificial positive active material for a lithium-ion electrochemical element which is a compound of formula (Li.sub.2O).sub.x (MnO.sub.2).sub.y(MnO).sub.z(MO.sub.a).sub.t in which: x+y+z+t=1; 1xy0; 0.97x0.6; y0.45; x 0.17; y0; y+z>0; t0; 1a<3. M is selected from the group consisting of Fe, Co, Ni, B, Al, Ti, Si, V, Mo, Zr and a mixture thereof.

Claims

1. Compound of formula (Li.sub.2O).sub.x (MnO.sub.2).sub.y (MnO).sub.z (MOa).sub.t wherein x+y+z+t=1 1xy0 0.97x0.6 y0.45.Math.x0.17 y0 y+z>0 t0 1a<3 M is selected from the group consisting of Fe, Co, Ni, B, Al, Ti, Si, V, Mo, Zr and a mixture thereof.

2. Compound according to claim 1, crystallizing in the cubic system.

3. Compound according to claim 2, having an X-ray diffraction pattern, wherein the width at mid-height of the line at a 2-theta angle between 40 and 45 is greater than 1, the wavelength used being the Kalpha wavelength of copper.

4. Compound according to claim 1, wherein x0.7, preferably x0.8, preferably still x0.9.

5. Compound according to claim 1, wherein y=z+/0.05.

6. Compound according to claim 1, wherein y0.2, preferably y0.1.

7. Compound according to claim 1, wherein z0.4, preferably z0.3, preferably still z0.2, preferably still z0.1.

8. Compound according to claim 1, wherein t=0.

9. Composite material comprising the compound according to claim 1 and at least one crystalline phase Li.sub.2O.

10. Composite material comprising the compound according to claim 1 and containing from 1 to 10% by mass of carbon.

11. Electrode comprising a first electrochemically active material which is the compound according to claim 1 and at least one second electrochemically active material.

12. Electrode according to claim 11, wherein the second electrochemically active material is selected from the group consisting of: compound i) of formula Li.sub.xMn.sub.1yzM.sub.yM.sub.z PO.sub.4 (LMP), where M and M are different from each other and are selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo, with 0.8x1.2; 0y0.6; 0z0.2; compound ii) of formula Li.sub.xM.sub.2xyzwM.sub.yM.sub.zM.sub.wO.sub.2 (LMO2), where M, M, M and M are selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo, provided that M or M or M or M is selected from Mn, Co, Ni or Fe; M, M, M and M being different from each other; with 0.8x1.4; 0y0.5; 0z0.5; 0w0.2 and x+y+z+w<2; compound iii) of formula Li.sub.xMn.sub.2yzM.sub.yM.sub.zO.sub.4 (LMO), where M and M are selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo; M and M being different from each other, and 1x1.4; 0y0.6; 0z0.2; compound iv) of formula Li.sub.xFe.sub.1yM.sub.yPO.sub.4, where M is selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo; and 0.8x1.2; 0y0.6; compound v) of formula xLi.sub.2MnO.sub.3; (1x)LiMO.sub.2 where M is selected from Ni, Co and Mn and x1; and a mixture of one or more of the compounds i) to v).

13. Electrode according to claim 12 comprising the compound ii) and M is Ni; M is Mn; M is Co and M is selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo; with 0.8x1.4; 0<y0.5; 0<z0.5; 0w0.2 and x+y+z+w<2.

14. Electrode according to claim 12 comprising the compound ii) and M is Ni; M is Co; M is Al; and 1x1.15; y>0; z>0; w=0.

15. Electrode according to claim 14 comprising the compound ii) and x=1; 0.62xyz0.85; 0.10y0.25; 0.05z0.15.

16. Electrode according to claim 15 comprising the compound ii) of formula LiCoO.sub.2.

17. Electrode according to claim 12 comprising the compound iv) of formula Li.sub.xFe.sub.1yM.sub.yPO.sub.4, wherein M is selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo; and 0.8x1.2; 0y0.6;

18. An electrode according to claim 17 comprising the compound iv) of formula LiFePO.sub.4.

19. Electrode according to claim 12 comprising the compound iii) with x=1; 0y0.1; z=0 and M is Al.

20. Electrode according to claim 12 comprising the compound iii) of formula LiMn.sub.2O.sub.4.

21. Electrode according to claim 12, wherein the mass percentage of the compound is less than or equal to 5% based on the total mass of all electrochemically active materials, preferably less than or equal to 2%.

22. Electrochemical cell of lithium-ion type comprising a positive electrode according to claim 11.

23. Electrochemical cell according to claim 22, comprising: at least one negative electrode comprising a graphite-based active material, at least one positive electrode.

24. Electrochemical cell according to claim 22 comprising: at least one negative electrode comprising an active material selected from the group consisting of tin, silicon, carbon and silicon based-compounds, carbon and tin based-compounds, carbon, tin and silicon based-compounds; lithium titanium oxide compounds, such as lithium titanate. at least one positive electrode.

25. Process for manufacturing the compound according to claim 1, comprising the steps of: a) provision of a mixture of Li.sub.2O, MnO, MnO.sub.2 and optionally MO; b) grinding the mixture in an inert atmosphere; c) heating the mixture in an inert atmosphere at a temperature between 800 and 1000 C.; d) cooling of the mixture to room temperature; e) grinding the mixture in an inert atmosphere; f) possibly adding carbon to the mixture; g) possibly grinding the mixture in an inert atmosphere.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0051] FIG. 1a schematically shows the capacities of the positive electrode and the negative electrode of a lithium-ion electrochemical cell according to the prior art in which the irreversible capacity of the negative electrode is greater than that of the positive electrode.

[0052] FIG. 1b schematically represents the capacities of the positive electrode and the negative electrode of a lithium-ion electrochemical cell according to the invention in which the difference between the irreversible capacity of the positive electrode and the irreversible capacity of the negative electrode has been reduced compared with the situation illustrated in FIG. 1a.

[0053] FIG. 2 is a ternary diagram (Li.sub.2O, MnO, MnO.sub.2) showing the compositional domain to which the compound according to the invention belongs and the position in that domain of the compounds in the examples in Table 1 below.

[0054] FIG. 3 is an X-ray diffraction pattern of a powder of the compound from example 1 Li.sub.9MnO.sub.6 according to the invention.

[0055] FIG. 4 is an X-ray diffraction pattern of a powder of the compound from example 7 Li.sub.10Mn.sub.2O.sub.7 according to the invention.

[0056] FIG. 5 shows the galvanostatic cycling curve of the compound of example 1 in a half cell of the Swagelok type versus lithium.

[0057] FIG. 6 shows the galvanostatic cycling curve of the compound in example 7 in a half cell of the Swagelok type versus lithium.

[0058] FIG. 7 is a ternary diagram (Li.sub.2O, MnO, MnO.sub.2) showing the position of the compounds in Table 3 below as counter-examples.

[0059] FIG. 8a shows the cycling curve of a reference electrode A and an electrode B including the compound from example 1. The abscissa axis is graduated in mAh/cm.sup.2.

[0060] FIG. 8b shows the cycling curve of a reference electrode A and an electrode B including the compound from example 1 under the same conditions as those in FIG. 8a. The abscissa axis is graduated in mAh/g.

DESCRIPTION OF DETAILED EMBODIMENTS

[0061] The compound according to the invention has the formula

(Li.sub.2O).sub.x(MnO.sub.2).sub.y(MnO).sub.z(MOa).sub.t

wherein
x+y+z+t=1
1xy0
0.97x0.6
y0.45.Math.x0.17
y0
y+z>0
t0
1a<3
M is selected from the group consisting of Fe, Co, Ni, B, Al, Ti, Si, V, Mo, Zr and a mixture thereof.

[0062] This formula defines a composition range. This composition range can be represented in a ternary diagram. Assuming that cell M is absent from the compound, the formula of the compound according to the invention can be rewritten:

(Li.sub.2O).sub.x (MnO.sub.2).sub.y (MnO).sub.z. The three pure components Li.sub.2O, MnO.sub.2 and MnO are the three vertices of this ternary diagram.

[0063] Five criteria are imposed on parameters x, y and z. They define the composition range of the compound according to the invention in the ternary diagram. These criteria are as follows:

Criterion 1: x0.6
Criterion 2: y0.45.Math.x0.17
Criterion 3: x0.97
Criterion 4: y+z>0
Criterion 5: 1xy0

[0064] FIG. 2 shows the composition domain in the ternary diagram. This domain is delimited by a thick line. The points included in this domain meet the five criteria above.

[0065] The applicant found that the theoretical capacity of the compound according to the invention increased with the Li.sub.2O content. Therefore, x is preferably greater than or equal to 0.7, preferably still x is greater than or equal to 0.8, preferably still x is greater than or equal to 0.9.

[0066] The applicant also found that when the quantity of MnO is close to that of MnO.sub.2, a high capacity was obtained. Close to means y=z0.05. In the ternary diagram in FIG. 2, this condition is met for the points on the vertical bisector of the triangle passing through the Li.sub.2O vertex.

[0067] According to a preferred embodiment, the compound according to the invention is devoid of the cell M, i.e. t=0.

[0068] The compound according to the invention is generally nanostructured. The size of the crystallites is typically less than 50 nm, preferably less than or equal to 10 nm, preferably even less than or equal to 5 nm. This nanostructure can be demonstrated by using the X-ray diffraction technique. The X-ray diffraction pattern on a powder of the compound has a peak at a 2-theta angle between 40 and 45, the width at mid-height of which is greater than 1. The wavelength used for the measurement is the Kalpha wavelength of copper. The width at mid-height can be greater than or equal to 2, greater than or equal to 3. It has been observed that the width at mid-height of the peak increases as the size of the crystallites decreases. Typically, a width at mid-height peak of about 2 corresponds to a crystallite size of about 5 nm.

[0069] The compound according to the invention generally crystallizes in the cubic system. The cubic system can be demonstrated by using the Rietveld refinement method which uses the X-ray diffraction technique.

[0070] A second Li.sub.2O phase is usually present with the compound of the invention. The X-ray diffraction pattern characteristic of the presence of the Li.sub.2O phase has peaks at the following 2-theta angles: 34, 39, 56, 71 and 841, the angle being obtained using the Kalpha wavelength of copper.

[0071] The compound according to the invention is generally used in mixture with at least one second electrochemically active material. This second electrochemically active material can be selected from the group consisting of: [0072] compound i) of formula Li.sub.xMn.sub.1yzM.sub.yM.sub.zPO.sub.4 (LMP), where M and M are different from each other and are selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo, with 0.8x1.2; 0y0.6; 0z0.2; [0073] compound ii) of formula Li.sub.xM.sub.2-x-y-z-wM.sub.yM.sub.zM.sub.wO.sub.2 (LMO2), where M, M, M and M are selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo, provided that M or M or M or M is selected from Mn, Co, Ni or Fe; [0074] M, M, M and M being different from each other; with 0.8x1.4; 0y0.5; 0z0.5; 0w0.2 and x+y+z+w<2; [0075] compound iii) of formula Li.sub.xMn.sub.2yzM.sub.yM.sub.zO.sub.4 (LMO), where M and M are selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo; M and M being different from each other, and 1x1.4; 0y0.6; 0z0.2; [0076] compound iv) of formula Li.sub.xFe.sub.1yM.sub.yPO.sub.4, where M is selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo; and 0.8x1.2; 0y0.6; [0077] compound v) of formula xLi.sub.2MnO.sub.3; (1x)LiMO.sub.2 where M is selected from Ni, Co and Mn and x1
or a mixture of compounds i) to v).

[0078] The compound according to the invention may be used in combination with one or more electrochemically active materials which may or may not be part of compounds i) to v).

In an embodiment, the lithium-ion electrochemical cell comprises: [0079] at least one negative graphite-based electrode, [0080] at least one positive electrode comprising the compound according to the invention in admixture with one or more of the compounds i) to v).
In a preferred embodiment, the electrochemical cell comprises: [0081] at least one negative graphite-based electrode, [0082] at least one positive electrode comprising the compound according to the invention in admixture with a compound iv) as described above. The compound iv) is advantageously LiFePO.sub.4. The use of a compound according to the invention in an electrochemical cell comprising a positive electrode comprising a type iv) compound allows to increase the energy density of the cell by about 8 to 9%.
In an embodiment, the electrochemical cell comprises: [0083] at least one negative silicon-based electrode, [0084] at least one positive electrode comprising the compound according to the invention in admixture with another electrochemically positive active material.

[0085] Generally, carbon is mixed with the compound according to the invention and any other electrochemically active material present in the positive electrode. The mass proportion of carbon used generally represents 3 to 10% of the sum of the masses of the positive active materials.

[0086] The process used to obtain the compound according to the invention is a mechanical alloying process. Mechanical alloying refers to all material techniques in which the energy for activating the chemical reaction between precursors is provided by mechanical means. The manufacturing process includes the steps of:

a) provision of a mixture of Li.sub.2O, MnO, MnO.sub.2 and optionally MO;
b) grinding the mixture in an inert atmosphere;
c) heating the mixture in an inert atmosphere at a temperature between 800 and 1000 C.;
d) cooling of the mixture to room temperature;
e) grinding the mixture in an inert atmosphere;
f) possibly adding carbon to the mixture;
g) optionally grinding the mixture in an inert atmosphere if carbon has been added in step f).

[0087] According to an embodiment, step c) of heating is carried out for a period of 3 to 5 hours.

[0088] Preferably, step c) of heating the mixture is carried out at a temperature ranging from 850 to 950 C., preferably 900 C., for a period ranging from 3.5 h to 4.5 h, preferably 4 h. The temperature rise to the desired temperature is carried out gradually over a period of 4 to 6 hours, preferably 5 hours.

[0089] Preferably, step d) of cooling to room temperature is carried out gradually over a period of 4 to 6 hours, preferably 5 hours.

[0090] According to an embodiment, step e) of grinding the mixture is carried out for a period of 13 to 17 hours.

[0091] Preferably, step e) of grinding the mixture is carried out over a period of about 15 hours.

[0092] According to an embodiment, step g) of grinding the mixture is carried out for a period of 4 to 6 hours.

[0093] The process according to the invention makes it possible to obtain a stabilization of the cubic phase as well as a nanostructuring of the compound.

EXAMPLES

[0094] Thirteen examples of compounds according to the invention have been synthesized. Their composition is shown in Table 1 below. The procedure used to synthesize them is as follows:

[0095] First, the precursors Li.sub.2O, MnO, MnO.sub.2 and MO, if applicable, were ground in a mortar in stoichiometric proportions in a glove box under argon atmosphere (total mass of precursors: 5 g). Then, a heat treatment in a sealed tube was carried out for 4 hours at 900 C. The temperature rise to 900 C. occurred in 5 hours, as did the temperature drop to 25 C.

[0096] The resulting material was then ground for 15 hours under argon atmosphere, in 10 mL WC tungsten carbide bowls with 4 WC balls 10 mm in diameter, using a FRITSCH planetary mill. The grinding speed is 700 rpm. After 5 hours, 10 hours and 15 hours, the grinding was stopped and the walls of the bowl were scraped in the glove box to homogenize the powder which is compacted on the walls. In order to avoid heating during the 5 consecutive hours of grinding, ten 30-minute cycles with a 5-minute break were carried out, alternating the direction of rotation of the grinding bowls.

[0097] Carbon was added to the material already ground for 15 hours at a rate of 5% by mass with respect to the mass of the material already ground for 15 hours. Then, the mixture containing the carbon was ground again for a further 5 hours, under the same conditions as before (10 cycles of 30 minutes at a speed of 700 rpm, alternating the direction of rotation of the grinding).

TABLE-US-00001 TABLE 1 Compounds according to the invention Degree of Theoretical Example oxidation capacity* number Formula xLi.sub.2O yMnO.sub.2 zMnO tMO.sub.a M a of Mn (mAh/g) 1 Li.sub.9MnO.sub.6 0.818 0.091 0.091 0.000 3 1130 2 Li.sub.19MnO.sub.11 0.905 0.048 0.048 0.000 3 1403 3 Li.sub.18Mn.sub.2O.sub.13 0.818 0.182 0.000 0.000 2 1089 4 Li.sub.38Mn.sub.2O.sub.23 0.905 0.095 0.000 0.000 2 1373 5 Li.sub.60MnO.sub.32 0.968 0.032 0.000 0.000 2 1635 6 Li.sub.6Mn.sub.2O.sub.5 0.600 0.000 0.400 0.000 4 694 7 Li.sub.10Mn.sub.2O.sub.7 0.714 0.000 0.286 0.000 4 920 8 Li.sub.18Mn.sub.2O.sub.11 0.818 0.000 0.182 0.000 4 1174 9 Li.sub.38Mn.sub.2O.sub.21 0.905 0.000 0.095 0.000 4 1435 10 Li.sub.12Mn.sub.4O.sub.11 0.600 0.100 0.300 0.000 3.5 671 11 Li.sub.9Mn.sub.0.5Al.sub.0.5O.sub.6 0.818 0.045 0.045 0.091 Al 1.5 3 1209 12 Li.sub.9Mn.sub.0.5V.sub.0.5O.sub.6 0.818 0.000 0.091 0.091 V 2.0 4 1141 13 Li.sub.9Mn.sub.0.5Ti.sub.0.5O.sub.6 0.818 0.000 0.091 0.091 Ti 2.0 4 1149 *theoretical capacity (mAh/g) equal to the number of lithium atoms in the formula of the compound under consideration 26800/molecular weight of the compound

[0098] The position of compounds 1-10 in a ternary diagram is shown in FIG. 2. A fourteenth example of a compound that is part of the invention but not synthesized is Li.sub.190Mn.sub.5O.sub.102.5 which has a theoretical mass capacity of 1500 mAh/g.

[0099] The compound of example 1 of formula Li.sub.9MnO.sub.6 was studied for its crystallographic structure. An X-ray diffraction pattern was performed on a powder of this compound. It is shown in FIG. 3, which shows a peak at a 2-theta angle of 43.5. This peak has a mid-height width of about 3, indicating that the compound is nanostructured. In addition, the Rietveld refinement of the pattern indicates that this compound crystallizes in the cubic system. It also indicates the presence of a secondary phase Li.sub.2O.

[0100] The compound of example 7 of formula Li.sub.10Mn.sub.2O.sub.7 was also studied for its crystallographic structure. An X-ray diffraction pattern was performed on a powder of this compound. It is shown in FIG. 4, which shows a peak at a 2-theta angle of 41.5. This peak has a width at mid-height of about 3. This indicates that the compound is nanostructured.

[0101] For comparison, the compound Li.sub.6MnO.sub.4 described in document WO 2015/011883, cited in the discussion of prior art, is not nanostructured. Indeed, referring to FIG. 14 of document WO2015/011883, it can be noted that the peak at a 2-theta angle between 40 and 45 has a half-height width of less than 0.3, which means that the size of the crystallites is greater than 50 nm.

Electrical Tests:

[0102] The electrical tests were performed using Swagelok-type laboratory electrochemical cells. The positive electrode has a surface area of 1.14 cm.sup.2 and consists of from 20 to 30 mg of a mixture comprising 72% by mass of electrochemically active material and 28% by mass of carbon. The mixture was previously mixed with the FRITSCH planetary mill for 30 minutes at 450 rpm. The negative electrode consists of a lithium disc in large excess of capacity compared with the positive electrode. The thickness of the lithium disc is 500 m. The two electrodes are electrically isolated by two layers of borosilicate glass fibre (Whatman) separators. The electrolyte used is composed of 1 M LiPF.sub.6 dissolved in a mixture of ethylene carbonate and methyl ethyl carbonate EC:MEC in a mass ratio of 3:7.
The charge currents were calculated to extract one Li atom per compound formula in 20 hours.
The cut-off voltage of the first charge is 4.4V and 1.2V for the discharge.
The cut-off voltage of the second charge is 4.6V and 1.2V for the discharge.
The cut-off voltage for the third charge is 4.8V and 1.2V for the discharge.
The results of the electrical tests are summarized in Table 2 below

TABLE-US-00002 TABLE 2 Electrical test results for compounds according to the invention Total charged Theoretical capacity Example capacity at 4.4 V number Formula xLi.sub.2O yMnO.sub.2 zMnO tMO.sub.a M a (mAh/g) (mAh/g) 1 Li.sub.9MnO.sub.6 0.818 0.091 0.091 0.000 1130 1125 2 Li.sub.19MnO.sub.11 0.905 0.048 0.048 0.000 1403 813 3 Li.sub.18Mn.sub.2O.sub.13 0.818 0.182 0.000 0.000 1089 641 4 Li.sub.38Mn.sub.2O.sub.23 0.905 0.095 0.000 0.000 1373 676 5 Li.sub.60MnO.sub.32 0.968 0.032 0.000 0.000 1635 640 6 Li.sub.6Mn.sub.2O.sub.5 0.600 0.000 0.400 0.000 694 630 7 Li.sub.10Mn.sub.2O.sub.7 0.714 0.000 0.286 0.000 920 797 8 Li.sub.18Mn.sub.2O.sub.11 0.818 0.000 0.182 0.000 1174 633 9 Li.sub.38Mn.sub.2O.sub.21 0.905 0.000 0.095 0.000 1435 796 10 Li.sub.12Mn.sub.4O.sub.11 0.600 0.100 0.300 0.000 671 781 11 Li.sub.9Mn.sub.0.5Al.sub.0.5O.sub.6 0.818 0.045 0.045 0.091 Al 1.5 1209 691 12 Li.sub.9Mn.sub.0.5V.sub.0.5O.sub.6 0.818 0.000 0.091 0.091 V 2.0 1141 931 13 Li.sub.9Mn.sub.0.5Ti.sub.0.5O.sub.6 0.818 0.000 0.091 0.091 Ti 2.0 1149 954 Criterion Total 2: 0.45x- charged Crierion 0.17- Criterion Criterion capacity 1: y 0 3: 0.97- Criterion 5: Example at 4.8 V x-0.6 0 0.45x- x 0 4: y + z > 0 1-x-y 0 All number (mAh/g) x-0.6 0.17-y 0.97-x y + z 1-x-y criteria 1 1124 0.218 0.107 0.152 0.182 0.091 YES 2 893 0.305 0.190 0.065 0.095 0.048 YES 3 641 0.218 0.016 0.152 0.182 0.000 YES 4 676 0.305 0.142 0.065 0.095 0.000 YES 5 769 0.368 0.233 0.002 0.032 0.000 YES 6 643 0.000 0.100 0.370 0.400 0.400 YES 7 829 0.114 0.151 0.256 0.286 0.286 YES 8 633 0.218 0.198 0.152 0.182 0.182 YES 9 876 0.305 0.237 0.065 0.095 0.095 YES 10 819 0.000 0.000 0.370 0.400 0.300 YES 11 925 0.218 0.153 0.152 0.091 0.136 YES 12 910 0.218 0.198 0.152 0.091 0.182 YES 13 957 0.218 0.198 0.152 0.091 0.182 YES
Table 2 shows that the compounds according to the invention have, for a charging voltage of 4.4V, a total charged capacity of at least 630 mAh/g, up to 1200 mAh/g.

[0103] The galvanostatic cycling of the compound in example 1 of formula Li.sub.9MnO.sub.6, in half-cell versus lithium (Swagelok type) was performed. The resulting cycling curve is shown in FIG. 5, which shows that at a charge rate corresponding to the extraction of a lithium atom in 20 hours per compound formula, nearly 9 lithium atoms per compound formula are extracted from the structure (1125 mAh/g) at the first charge and 1.64 lithium atoms are reinserted during the first discharge (206 mAh/g). This result shows the strong irreversible capacity of this material since out of the 9 lithium atoms per compound formula extracted during charging, only 1.64 atoms per compound formula are able to reinsert into the compound structure during the discharge following this first charge. It can be seen that the experimental capacity under charge is very close to the theoretical capacity despite the presence of a significant amount of Li.sub.2O. This result means that the nanostructured composite composed of cubic phase and Li.sub.2O makes it possible to electrochemically activate the oxidation of Li.sub.2O. Finally, one can note that the charged capacity of 1125 mAh/g at 4.4V of the compound in example 1 is much higher than the charged capacity of compound Li.sub.6MnO.sub.4 described in document WO 2015/011883, cited in the discussion of the prior art, which is indeed less than 450 mAh/g.

[0104] The galvanostatic cycling curve of the compound in example 7 of formula Li.sub.10Mn.sub.2O.sub.6, in half-cell versus lithium (Swagelok type) is shown in FIG. 6. This figure shows that at a charging rate corresponding to the extraction of a lithium atom in 20 hours per compound formula, nearly 8.6 lithium atoms per compound formula are extracted from the structure (797 mAh/g) at the first charge and 2.8 lithium atoms per compound formula are re-inserted at the 1st discharge (260 mAh/g).

[0105] Compounds not part of the invention were synthesized and their capacity were measured. The position of these compounds in a ternary diagram is shown in FIG. 7. These compounds are outside the compositional domain materialized by a thick line. Table 3 shows the formulas of these compounds as well as the measured charged capacities.

TABLE-US-00003 TABLE 3 Electrical test results for counter-examples Total Degree charged Counter of Theoretical capacity example oxidation capacity at 4.4 V number Formula xLi.sub.2O yMnO.sub.2 zMnO tMO.sub.a M a of Mn (mAh/g) (mAh/g) 1 Li.sub.3MnO.sub.3 0.600 0.200 0.200 0.000 3 676 370 2 Li.sub.2MnO.sub.3 0.500 0.500 0.000 0.000 2 589 335 3 Li.sub.6Mn.sub.2O.sub.7 0.600 0.400 0.000 0.000 2 758 620 4 Li.sub.10Mn.sub.2O.sub.9 0.714 0.286 0.000 0.000 2 985 489 5 Li.sub.10Mn.sub.5O.sub.14 0.500 0.400 0.100 0.000 2.4 557 277 6 Li.sub.6Mn.sub.4O.sub.7 0.429 0.000 0.571 0.000 4 367 272 7 Li.sub.2MnO.sub.2 0.500 0.000 0.500 0.000 4 458 326 8 Li.sub.10Mn.sub.7O.sub.13 0.417 0.083 0.500 0.000 3.71 366 360 9 Li.sub.10Mn.sub.5O.sub.11 0.500 0.100 0.400 0.000 3.6 479 362 10 Li.sub.9TiO.sub.6 0.818 0.000 0.000 0.182 Ti 2 1122 304 11 Li.sub.2O 1.000 0.000 0.000 0.000 1787 200 Criterion 2: Total 0.45x- Criterion charged Criterion 0.17- 3: Criterion Counter capacity 1: y 0 0.97- Criterion 5: example at 4.8 V x-0.6 0 0.45x- x 0 4: y + z > 0 1-x-y 0 all number (mAh/g) x-0.6 0.17-y 0.97-x y + z 1-x-y criteria 1 416 0.000 0.100 0.370 0.400 0.200 NO 2 331 0.100 0.445 0.470 0.500 0.000 NO 3 620 0.000 0.300 0.370 0.400 0.000 NO 4 621 0.114 0.134 0.256 0.286 0.000 NO 5 360 0.100 0.345 0.470 0.500 0.100 NO 6 358 0.171 0.023 0.541 0.571 0.571 NO 7 350 0.100 0.055 0.470 0.500 0.500 NO 8 420 0.183 0.066 0.553 0.583 0.500 NO 9 433 0.100 0.045 0.470 0.500 0.400 NO 10 443 0.218 0.198 0.152 0.000 0.182 NO 11 250 0.400 0.280 0.030 0.000 0.000 NO
Table 3 shows that the counter-examples have, for a charging voltage of 4.4V, a total charging capacity not exceeding 620 mAh/g. None of these compounds meet all five criteria.
Validation in Mixture with a Cathode Active Material with Low Irreversible Capacity LiFePO.sub.4:

[0106] The validation of the invention was carried out by comparing two positive electrodes, one based on LiFePO.sub.4 (electrode A) and the other by replacing 2% of this compound with the Li.sub.9MnO.sub.6 compound of 1125 mAh/g capacity (electrode B). This substitution creates an irreversible capacity of about 20 mAh/g of positive active material.

[0107] The reference electrode A was manufactured with the following mass composition: 77% LiFePO.sub.4/23% carbon.

[0108] The electrode B according to the invention was manufactured with the following mass composition: 75.5% LiFePO.sub.4/1.5% Li.sub.9MnO.sub.6/23% carbon. The mass of Li.sub.9MnO.sub.6 represents about 2% of the sum of the masses of positive active materials.

[0109] The mixture of active material(s) and carbon is deposited on a current collector of the positive electrode at a rate of 12.65 mg/cm.sup.2.

[0110] The electrical tests performed on both electrodes are as follows:

1: Charging at a rate allowing to extract a Li atom by compound formula in 20 hours, up to 4V.
2: Constant voltage charging supplement (4V) with an end of charge current corresponding to C/1300
3: Discharge up to 2V at a C/10 rate

[0111] The charge-discharge curves for each of these electrodes have been plotted. They are shown in FIGS. 8a and 8b. FIG. 8a shows that: [0112] during the first charge, the charged capacity of electrode A is 1.85 mAh/cm.sup.2 and the charged capacity of electrode B is 2.15 mAh/cm.sup.2, i.e. an additional capacity of about 0.3 mAh/cm.sup.2 for electrode B containing the compound according to the invention. [0113] the capacity discharged after the first charge is the same for electrode A and electrode B. It is about 1.85 mAh/cm.sup.2.

[0114] FIG. 8b shows the cycling curve of electrodes A and B under the same conditions as those in FIG. 8a, with the difference that the abscissa axis is graduated in mAh/g and the starting points of the discharge curves of electrodes A and B are not overlapped.

The total charged capacity during the first charge for electrode A is 165 mAh/g and that of electrode B is 185 mAh/g. The capacity measured during the first discharge is the same for both electrodes, which means that the irreversible capacity has been increased by 20 mAh/g as expected.

[0115] The increase in the mass capacity of an electrochemical cell of the lithium-ion type by the use of a compound according to the invention in the positive electrode can be verified by calculation. The assumptions used are as follows:

A reference electrochemical cell R comprises: [0116] a positive reference electrode A containing an active material consisting of 100% LiFePO.sub.4. [0117] a negative electrode containing an active material consisting 100% of a C/Si composite.
An electrochemical cell I according to the invention comprises: [0118] a positive electrode B containing an active material consisting of 98% LiFePO.sub.4 and 2% Li.sub.9MnO.sub.6, which is a compound according to the invention. [0119] a negative electrode containing an active material consisting 100% of a C/Si composite.

[0120] LiFePO.sub.4 used in the two cells R and I described above has a total mass capacity of 165 mAh/g, of which 9% is the irreversible mass capacity, or 15 mAh of irreversible capacity.

[0121] The C/Si composite used in the two cells R and I described above has a total mass capacity of 1266 mAh/g, of which 21% is the irreversible mass capacity, or 266 mAh/g of irreversible capacity.

[0122] The Li.sub.9MnO.sub.6 compound has a total mass capacity of 1125 mAh/g.

[0123] On the basis of the numerical values of the capacities described above and the assumption that the total capacity of the negative electrode is equal to the total capacity of the positive electrode, the mass capacity of the cells R and I expressed per gram of positive and negative material can be calculated. The results of this calculation are shown in Tables 4a and 4b below:

TABLE-US-00004 TABLE 4a Active material /+/ Active material // irreversible Irreversible Total mass total mass mass mass capacity capacity capacity capacity description (mAh/g) (mAh/g) description (mAh/g) (mAh/g) Cell R A 100% LiFePO.sub.4 15 165 C/Si composite 266 1266 Cell I B 98% LiFePO.sub.4 + 2% Li.sub.9MnO.sub.6 35 185 C/Si composite 266 1266

TABLE-US-00005 TABLE 4b Cell containing 1 gram of positive active material and equal capacities of the positive and negative electrodes Cell Cell total Cell mass total capacity irreversible capacity /+/ mass of // electrode /+/ electrode (mAh) capacity Cell (mAh per gain material // active irreversible irreversible (=total // (=max reversible g active cell R mass material materials capacity capacity electrode (irrev//; capacity material vs. cell description (g) mass (g) /+/ and // (mAh) (mAh) capacity) irrev/+/)) (mAh) /+/ and //) I (%) Cell A 100% 1 0.13 1.13 35 15 165 35 130 115 R LiFePO.sub.4 Cell I B 98% 1 0.15 1.15 39 35 185 39 146 128 10% LiFePO.sub.4 + 2% Li.sub.9MnO.sub.6

[0124] Table 4b shows that the capacity of cell I is 128 mAh per gram of positive and negative active materials while that of cell R is only 115 mAh/g, representing a mass capacity gain of about 10%.

[0125] The capacities of the positive and negative electrodes of the reference cell R are shown schematically in FIG. 1a. The positive electrode containing 1 gram of positive material has a total capacity of 165 mAh and an irreversible capacity of 15 mAh (left rectangle). The negative electrode has an identical total capacity of 165 mAh and an irreversible capacity of 35 mAh (right rectangle).

[0126] The capacities of the positive and negative electrodes of cell I according to the invention are shown schematically in FIG. 1b. In the positive electrode of cell I, 2% LiFePO.sub.4 was substituted by Li.sub.9MnO.sub.6. The irreversible capacity provided by LiFePO.sub.4 is 14.7 mAh, that provided by Li.sub.9MnO.sub.6 is 20.3 mAh, i.e. a total irreversible capacity of 35 mAh. The total capacity of the positive electrode is increased to 185 mAh due to the increase in capacity provided by Li.sub.9MnO.sub.6 (left rectangle). The irreversible capacity of the negative electrode has increased from 35 mAh to 39 mAh, due to the increase in the total capacity of the positive electrode and the assumption that the total capacity of the negative electrode is maintained at the same value as that of the positive electrode. The capacity of the negative electrode is shown in the right-hand rectangle in FIG. 1b.

[0127] The comparison of FIG. 1b with FIG. 1a shows that the addition of only 2% of Li.sub.9MnO.sub.6 compound has significantly reduced the difference between the irreversible capacity of the positive electrode and the irreversible capacity of the negative electrode. Table 4b above shows that this reduction in difference is accompanied by an increase in the reversible mass capacity of the cell (expressed per gram of positive and negative active materials) since the reversible mass capacity of cell I has increased by 10% compared with that of cell R. Therefore, the principle of the invention is well verified.