ELECTRODE MATERIAL OF FORMULA LiMnxCo1-xBO3 AND PRODUCTION METHOD THEREOF

Abstract

The present invention relates to an electrode material of formula LiMn.sub.xCo.sub.1-xBO.sub.3, where 0<x<1, and to a method of preparing the same comprising independently preparing a manganese borate and a cobalt borate and then simultaneously thermally treating them under an inert atmosphere, in the presence of a precursor of lithium and of boric acid.

Claims

1. An electrode material of formula LiMn.sub.xCo.sub.1-xBO.sub.3, wherein 0<x<1.

2. The electrode material of claim 1, wherein 0<x≦0.7.

3. A method of solid-state preparation of a material of formula LiMn.sub.xCo.sub.1-xBO.sub.3, with 0<x<1, comprising the steps of: a) preparing a manganese borate from a manganese compound and a boron compound by: a1) milling of a mixture of a manganese compound and of a boron compound; a2) thermal treatment of the mixture thus obtained, under an inert atmosphere, at a temperature in the range from 300 to 900° C.; b) preparing a cobalt borate from a cobalt compound and a boron compound by: b1) milling of a mixture of a cobalt compound and of a boron compound; b2) thermal treatment of the mixture thus obtained, under an oxidizing atmosphere, at a temperature in the range from 300 to 1,000° C.; c) preparing a mixture containing the manganese borate, the cobalt borate, a precursor of lithium, and boric acid; d) thermally treating the mixture under an inert atmosphere; e) obtaining a material of formula LiMn.sub.xCo.sub.1-xBO.sub.3 with 0<x<1.

4. The method of claim 3, wherein the manganese compound is selected from the group consisting of: manganese oxalate; manganese carbonate; and manganese oxide (II).

5. The method of claim 3, wherein the cobalt compound is selected from the group consisting of: cobalt oxalate; cobalt carbonate; and cobalt oxide (II).

6. The method of claim 3, wherein the boron compound is boron oxide or boric acid.

7. The method of claim 3, wherein the lithium precursor is lithium carbonate or lithium hydroxide.

8. The method of claim 3, wherein the thermal treatment of step d) is carried out at a temperature in the range from 300 to 900° C., for a duration in the range from 30 to 1,200 minutes.

9. The method of claim 3, wherein it comprises the steps of: a) preparing a manganese borate from a manganese compound and a boron compound, by thermal quenching under an inert atmosphere at a temperature in the range from 600 to 750° C. for a duration in the range from 5 to 20 minutes; b) preparing a cobalt borate from a cobalt compound and a boron compound, by thermal quenching under an oxidizing atmosphere at a temperature in the range from 700 to 850° C. for a duration in the range from 5 to 20 minutes; c) preparing and milling a mixture containing the manganese borate, the cobalt borate, a precursor of lithium, and boric acid; d) thermally quenching the mixture under an inert atmosphere, at a temperature in the range from 400 to 550° C. for a duration in the range from 15 to 120 minutes; e) obtaining a material of formula LiMn.sub.xCo.sub.1-xBO.sub.3, with 0<x<1.

10. A lithium-ion battery comprising a cathode, having an electronically-active material that is the material of claim 1.

11. A lithium-ion battery comprising a cathode having an electronically-active material that is the material of claim 2.

12. The method of claim 3, wherein the thermal treatment of step d) is carried out at a temperature in the range from 400 to 700° C., for a duration in the range from 30 to 1,200 minutes.

13. The method of claim 4, wherein the cobalt compound is selected from the group consisting of: cobalt oxalate; cobalt carbonate; and cobalt oxide (II).

14. The method of claim 4, wherein the boron compound is boron oxide or boric acid.

15. The method of claim 13, wherein the boron compound is boron oxide or boric acid.

16. The method of claim 4, wherein the lithium precursor is lithium carbonate or lithium hydroxide.

17. The method of claim 13, wherein the lithium precursor is lithium carbonate or lithium hydroxide.

18. The method of claim 14, wherein the lithium precursor is lithium carbonate or lithium hydroxide.

19. The method of claim 15, wherein the lithium precursor is lithium carbonate or lithium hydroxide.

20. The method of claim 3, wherein the boron compound is boron oxide or boric acid and the lithium precursor is lithium carbonate or lithium hydroxide.

Description

DESCRIPTION OF THE DRAWINGS

[0104] FIG. 1 corresponds to the diffractograms of the LiMn.sub.xCo.sub.1-xBO.sub.3 compounds when x=0; 0.3; 0.5; 0.7; and 1.

[0105] FIG. 2 corresponds to an enlarged view of the diffractograms of the LiMn.sub.xCo.sub.1-xBO.sub.3 compounds when x=0; 0.3; 0.5; 0.7; and 1.

[0106] FIG. 3 is an image obtained by scanning electron microscopy (SEM) of the LiMn.sub.0,5Co.sub.0,5BO.sub.3 compound according to a specific embodiment of the invention.

[0107] FIG. 4 corresponds to the graph of the reversible specific capacity of the LiMn.sub.xCo.sub.1-xBO.sub.3 compound (x=0; 0.3; 0.5; 0.7; and 1) according to the number of cycles.

[0108] FIG. 5 is an image obtained by scanning electron microscopy (SEM) of the LiMn.sub.0,7Co.sub.0,3BO.sub.3 compound according to a specific embodiment of the invention

[0109] FIG. 6 is an image obtained by scanning electron microscopy (SEM) of the LiMn.sub.0,7Co.sub.0,3BO.sub.3 compound according to a specific embodiment of the invention.

[0110] FIG. 7 corresponds to the diffractogram of the LiMn.sub.0,7Co.sub.0,3BO.sub.3 compound according to two specific embodiments of the invention.

[0111] FIG. 8 corresponds to the C/20 galvanostatic cycling for the LiMn.sub.0,7Co.sub.0,3BO.sub.3 compound obtained according to two embodiments of the invention.

[0112] FIG. 9 corresponds to the reversible specific capacity of the LiMn.sub.0,7Co.sub.0,3BO.sub.3 compound obtained according to two embodiments of the invention.

[0113] FIG. 10 corresponds to the diffractogram of the counter-example of the synthesis of the LiMn.sub.0,7Co.sub.0,3BO.sub.3 compound.

[0114] FIG. 11 corresponds to the voltammograms of the LiMnBO.sub.3, LiMn.sub.0,5Co.sub.0,5BO.sub.3, and LiCoBO.sub.3 compounds.

EMBODIMENTS OF THE INVENTION

[0115] Compounds of formula LiMn.sub.xCo.sub.1-xBO.sub.3 have been prepared according to two embodiments of the invention (methods A and B) and according to a method corresponding to a counter-example (method C).

[0116] 1/ Method A: Preparation of the LiMn.sub.xCo.sub.1-xBO.sub.3 Compound (x=0; 0.3; 0.5; 0.7; 1)

[0117] The LiMn.sub.xCo.sub.1-xBO.sub.3 compound has been prepared according to the steps of: [0118] a) preparing a manganese borate from a manganese compound and a boron compound; [0119] b) preparing a cobalt borate from a cobalt compound and a boron compound; [0120] c) preparing and milling a mixture containing the manganese borate, the cobalt borate, a precursor of lithium, and boric acid; [0121] d) thermally treating the mixture under an inert atmosphere; [0122] e) obtaining the material of formula LiMn.sub.xCo.sub.1-xBO.sub.3, with 0<x<1.

[0123] Step a):

[0124] In a ball mill, 6.76 g of MnC.sub.2O.sub.4.2H.sub.2O and 1.32 g of B.sub.2O.sub.3 are dispersed in cyclohexane. The mixture is milled at a speed of 500 revolutions/minute for 5 hours. The used mill is a planetary mill (Retsch) containing 10 stainless steel balls. The cyclohexane is then evaporated in air.

[0125] The milled mixture (manganese/boron) is thermally treated at 700° C. for 6 hours (5° C./min heating speed) under an inert atmosphere.

[0126] The manganese borate thus obtained is then gradually cooled with a cooling temperature equal to 10° C./minute down to 25° C.

[0127] Step b):

[0128] Concurrently, in another ball mill, 7.32 g of CoC.sub.2O.sub.4.2H.sub.2O and 1.39 g of B.sub.2O.sub.3 are dispersed in cyclohexane. The mixture is milled at a speed of 500 revolutions/minute for 5 hours. The mill used is a planetary mill (Retsch) containing 10 stainless steel balls. The cyclohexane is then evaporated in air.

[0129] The milled mixture (cobalt/boron) is thermally treated at 800° C. for 6 hours (heating speed equal to 5° C./min) under an oxidizing atmosphere. The cobalt borate thus obtained is then gradually cooled with a cooling temperature equal to 10° C./minute down to 25° C.

[0130] Step c):

[0131] 0.86 g of the obtained manganese borate and 0.80 g of the obtained cobalt borate are mixed with 0.63 g of Li.sub.2CO.sub.3, and 0.17 g of H.sub.3BO.sub.3 in a ball mill. The mixture is dispersed in cyclohexane and then milled at a speed of 500 revolutions/minute for 5 hours. The used mill is a planetary mill (Retsch) containing 10 stainless steel balls. The cyclohexane is then evaporated in air.

[0132] Steps d) and e):

[0133] The milled mixture resulting from step c) is thermally treated under argon, at 500° C. for 6 hours (heating speed equal to 5° C./min).

[0134] The LiMn.sub.0,5Co.sub.0,5BO.sub.3 compound thus obtained is then gradually cooled with a cooling temperature equal to 10° C./minute down to 25° C.

[0135] By adapting the quantities of components used, the LiMn.sub.xCo.sub.1-xBO.sub.3 materials, with x=0; 0.3; 0.7; 1 have been prepared in the same way.

TABLE-US-00001 Masses used for step c) for the LiMn.sub.xCo.sub.1−xBO.sub.3 materials lithium x Mn borate Co borate carbonate boric acid 0.3 0.51 g 1.10 g 0.63 g 0.23 g 0.5 0.86 g 0.80 g 0.63 g 0.17 g 0.7 1.22 g 0.48 g 0.63 g 0.10 g

[0136] 2/ Method B: Synthesis of LiMn.sub.0,7Co.sub.0,3BO.sub.3 by Thermal Quenching

[0137] Method B comprises the same steps as method A but decreases the duration of the thermal treatments. In this case, it is a thermal quenching.

[0138] Step a):

[0139] In a ball mill, 6.76 g of MnC.sub.2O.sub.4.2H.sub.2O and 1.32 g of B.sub.2O.sub.3 are dispersed in cyclohexane. The mixture is milled at a speed of 500 revolutions/minute for 5 hours. The used mill is a planetary mill (Retsch) containing 10 stainless steel balls. The cyclohexane is then evaporated in air.

[0140] The milled mixture (manganese/boron) is thermally treated at 700° C. for 15 minutes under an inert atmosphere (air quenching).

[0141] Step b):

[0142] Concurrently, in another ball mill, 7.32 g of CoC.sub.2O.sub.4.2H.sub.2O and 1.39 g of B.sub.2O.sub.3 are dispersed in cyclohexane. The mixture is milled at a speed of 500 revolutions/minute for 5 hours. The used mill is a planetary mill (Retsch) containing 10 stainless steel balls. The cyclohexane is then evaporated in air.

[0143] The milled mixture (cobalt/boron) is thermally treated at 800° C. for 15 minutes under an oxidizing atmosphere.

[0144] Step c):

[0145] 1.22 g of the obtained manganese borate and 0.48 g of the obtained cobalt borate are dispersed in cyclohexane with 0.63 g of Li.sub.2CO.sub.3 and 0.10 g of H.sub.3BO.sub.3 in a ball mill. The mixture is milled at a speed of 500 revolutions/minute for 5 hours. The used mill is a planetary mill (Retsch) containing 10 stainless steel balls. The cyclohexane is then evaporated in air.

[0146] Steps d) and e):

[0147] The milled mixture resulting from step c) is thermally treated under argon, at 500° C. for 1 hour and 15 minutes (without undergoing the temperature rise and with an air quenching).

[0148] 3/ Electrochemical Tests [0149] a) Preparation of the positive electrode

[0150] The active LiMn.sub.xCo.sub.1-xBO.sub.3 material is mixed by 85 wt. % with a carbon of large specific surface area (Ketjen black JD600) (15 wt. %) for 4 hours at 500 revolutions per minute in a 50-mL bowl containing 10 stainless steel balls by means of a planetary mill (Retsch).

[0151] Then, the obtained product is mixed by 90 wt. % with polyvinylidene fluoride (10 wt. %) dissolved in N-methyl-2-pyrrolidone.

[0152] Finally, the mixture is spread on an aluminum foil (100-micrometer thickness) and then dried at 60° C.

[0153] The electrode is then made of 76.5 wt. % of active material; 13.5 wt. % of carbon, and 10 wt. % of polyvinylidene fluoride (PVDF). [0154] b) Mounting of the accumulator

[0155] The positive electrode thus formed is introduced into a cell of “button cell” type at format 2032. The negative electrode is made of metal lithium.

[0156] Two types of separators are used: one made of a polypropylene film (Celgard® 2400) and the other made of polyolefin (Viledon®).

[0157] The electrolyte used is made of ethylene carbonate, of propylene carbonate, of dimethyl carbonate, and of lithium hexafluorophosphate (LiPF.sub.6) (Electrolyte LP100). [0158] c) Galvanostatic cycling

[0159] At ambient temperature, a current is imposed to the system to obtain a C/20 rate, that is, the extraction/insertion of a lithium ion within 20 hours.

[0160] FIG. 8 illustrates the specific capacity vs. the voltage for a positive electrode according to method A or according to method B.

[0161] 4/ Characterization of the LiMn.sub.xCo.sub.1-xBO.sub.3 Compound

[0162] FIG. 3 corresponds to an image obtained by scanning electron microscopy (SEM) of the LiMn.sub.0,5Co.sub.0,5BO.sub.3 compound (x=0.5) obtained according to method A.

[0163] FIGS. 1 and 2 show the diffractograms (X rays) of the compounds according to the invention (LiMn.sub.xCo.sub.1-xBO.sub.3 with x=0.3; 0.5; 0.7 obtained according to method A) as compared with the compound only containing manganese (x=1), LiMnBO.sub.3 and the compound only containing cobalt (x=0), LiCoBO.sub.3 obtained according to method A.

[0164] The diffractograms show the conservation of the structure of the material when x varies between 0 and 1. The evolution of the lattice parameters of the LiMn.sub.xCo.sub.1-xBO.sub.3 material varies linearly with the insertion of cobalt into the material.

[0165] FIG. 4 corresponds to the graph of the reversible specific capacity of the LiMn.sub.xCo.sub.1-xBO.sub.3 compound (x=0; 0.3; 0.5; 0.7; and 1) obtained according to method A according to the number of cycles.

[0166] The partial substitution of manganese with cobalt in the LiMn.sub.xCo.sub.1-xBO.sub.3 material enables to increase the reaction potential of the material. Indeed, the average discharge potentials of materials LiMnBO.sub.3, LiMn.sub.0,5Co.sub.0,5BO.sub.3 and LiCoBO.sub.3 respectively are 2.8 V; 3 V, and 3.1 V (FIG. 11).

[0167] FIGS. 5 and 6 show that method B (quenching, FIG. 6) provides particles/agglomerates of smaller size than method A (longer thermal treatment, FIG. 5).

[0168] FIG. 7 corresponds to the diffractogram of the LiMn.sub.0,7Co.sub.0,3BO.sub.3 compound obtained according to method A or method B.

[0169] Method B provides particles/agglomerates having a smaller size, but also a greater reversible specific capacity at the 1.sup.st cycle (54 vs. 70 mAh/g) and a better cycling stability, the initial reversible capacity being kept over 10 cycles (FIG. 9).

[0170] Methods A and B correspond to two embodiments of the present invention. They enable to separately synthesize manganese and cobalt borates such as M.sub.3B.sub.2O.sub.6 (M=Mn or Co). Such a multiple-step synthesis enables to stabilize the cobalt in the II+ oxidation state during the forming of the mixed LiMn.sub.xCo.sub.1-xBO.sub.3 compounds.

[0171] 5/ Method C: Counter-Example: Synthesis of LiMn.sub.0,7Co.sub.0,3BO.sub.3 in one Step

[0172] In this method, MnC.sub.2O.sub.4.2H.sub.2O and Co(OH).sub.2 are in stoichiometric proportion to obtain the LiMn.sub.0,7Co.sub.0,3BO.sub.3 compound. The LiOH.H.sub.2O and H.sub.3BO.sub.3 precursors are slightly in excess.

[0173] These precursors are dispersed in cyclohexane and mixed for five hours at 500 revolutions per minute in a 50-ml bowl containing 10 stainless steel balls by means of a planetary mill (Retsch). The cyclohexane is evaporated in air.

[0174] A thermal treatment is then carried out in an alumina crucible under argon at 500° C. for 40 minutes.

[0175] The diffractogram of FIG. 10 shows that the LiMn.sub.0,7Co.sub.0,3BO.sub.3 compound has not been obtained, but that the two polymorphous versions of LiMnBO.sub.3, as well as metal cobalt, are present.

[0176] Accordingly, method C does not enable to obtain the LiMn.sub.xCo.sub.1-xBO.sub.3 compound, given that the cobalt in the II+ oxidation state is reduced into metal cobalt during the thermal treatment. This method does not enable to partially substitute manganese with cobalt.