ELECTRODE MATERIAL OF FORMULA LiFe1-xCoxBO3 AND PRODUCTION METHOD THEREOF

Abstract

The present invention relates to an electrode material of formula LiFe.sub.1-xCo.sub.xBO.sub.3, where 0<x<1, and to a method of preparing the same comprising independently preparing an iron 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 LiFe.sub.1-xCo.sub.xBO.sub.3, wherein 0<x<1.

2. The electrode material of claim 1, wherein 0<x≦0.3.

3. A method of preparing an electrode material of formula LiFe.sub.1-xCo.sub.xBO.sub.3, with 0<x<1, comprising the steps of: a) preparing an iron borate from an iron compound and a boron compound by: a1) milling of a mixture of an iron 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 1,000° 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 iron 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 LiFe.sub.1-xCo.sub.xBO.sub.3, with 0<x<1.

4. The method of claim 3, wherein the iron compound is selected from the group consisting of: iron oxalate; iron carbonate; and iron 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 an iron borate from an iron compound and a boron compound, by thermal quenching under an inert atmosphere at a temperature in the range from 650 to 850° C. for a duration in the range from 5 to 30 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 30 minutes; c) preparing and milling a mixture containing the iron 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 LiFe.sub.1-xCo.sub.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

[0096] FIG. 1 corresponds to the diffractograms of the LiFe.sub.1-xCo.sub.xBO.sub.3 compounds when x=0; 0.5; and 1.

[0097] FIG. 2 corresponds to an enlarged view of the diffractograms of the LiFe.sub.1-xCo.sub.xBO.sub.3 compounds when x=0; 0.5; and 1.

[0098] FIG. 3 is an image obtained by scanning electron microscopy (SEM) of the LiFe.sub.0,5Co.sub.0,5BO.sub.3 compound according to a specific embodiment of the invention.

[0099] FIG. 4 corresponds to the C/20 galvanostatic cycling for the LiFe.sub.1-xCo.sub.xBO.sub.3 compounds (x=0.5 and 1).

[0100] FIG. 5 corresponds to the cyclic voltammogram at 0.05 mV/s for the LiFe.sub.0,5Co.sub.0,5BO.sub.3 compound.

EMBODIMENTS OF THE INVENTION

[0101] Compounds of formula LiFe.sub.1-xCo.sub.xBO.sub.3 (x=0; 0.5; and 1) have been prepared according to an embodiment of the method forming of object of the present invention.

[0102] 1/ Preparation of LiFe.sub.1-xCo.sub.xBO.sub.3 (x=0; 0.5; and 1)

[0103] The LiFe.sub.1-xCo.sub.xBO.sub.3 compound has been prepared according to the steps of: [0104] a) preparing an iron borate from an iron compound and a boron compound; [0105] b) preparing a cobalt borate from a cobalt compound and a boron compound; [0106] c) preparing and milling a mixture containing the iron borate, the cobalt borate, a precursor of lithium, and boric acid; [0107] d) thermally treating the mixture under an inert atmosphere; [0108] e) obtaining the material of formula LiFe.sub.1-xCo.sub.xBO.sub.3, with 0≦x≦1.

[0109] In this method, the iron borate, Fe.sub.2B.sub.2O.sub.5, and the cobalt borate, Co.sub.3B.sub.2O.sub.6, are synthe-sized separately.

[0110] Step a): Iron Borate Synthesis

[0111] The boron and iron compounds 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 then evaporated in air.

[0112] This mixture is then thermally treated in an alumina crucible under an inert atmosphere (argon) at 800° C. for 30 minutes with a temperature ramp of 5° C. per minute in rising mode and of 10° C. per minute in falling mode.

[0113] Step b): Cobalt Borate Synthesis

[0114] The boron and cobalt compounds are mixed in the same way as for the iron borate.

[0115] The mixture is then thermally treated in an alumina crucible in air at 800° C. for six hours with a temperature ramp of 5° C. per minute in rising mode and of 10° C. per minute in falling mode.

[0116] Step c): Preparation and Milling of a Mixture of Fe, Co, Li Compounds

[0117] The iron and cobalt borates are then mixed with the lithium salt and the boric acid. For this purpose, the compounds, in powder form, 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).

[0118] The cyclohexane is then evaporated in air.

[0119] Steps d) and e): Thermal Treatment and Obtaining of LiFe.sub.1-xCo.sub.xBO.sub.3

[0120] The mixture is then thermally treated in an alumina crucible under an inert atmosphere (argon) at 500° C. for 90 minutes with a temperature ramp of 5° C. per minute in rising mode and of 20° C. per minute in falling mode.

[0121] The compounds used and the respective quantities are mentioned in tables 1 and 2. The lithium and the boron are introduced in slight excess relative to the iron and/or to the cobalt.

TABLE-US-00001 TABLE 1 compounds implemented to prepare the iron and cobalt borates B.sub.2O.sub.3 (g) x FeC.sub.2O.sub.4•2H.sub.2O (g) CoC.sub.2O.sub.4•2H.sub.2O (g) step a) − step b) 0 3.38 0 0.66-0   0.5 3.38 7.32 0.66-1.39 1 0 7.32   0-1.39

TABLE-US-00002 TABLE 2 compounds used to prepare LiFe.sub.1−xCo.sub.xBO.sub.3 (x = 0; 0.5; and 1) x Fe.sub.2B.sub.2O.sub.5 (g) Co.sub.3B.sub.2O.sub.6 (g) Li.sub.2CO.sub.3 (g) boric acid (g) 0 0.87 0 0.32 0 0.5 0.44 0.40 0.32 0.09 1 0 1.50 0.59 0.32

[0122] 2/ Electrochemical Tests:

[0123] a) Preparation of the Positive Electrode

[0124] The active LiFe.sub.0,5Co.sub.0,5BO.sub.3 material is mixed by 85 wt. % with carbon of large specific surface area (Ketjenblack 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).

[0125] Then, the obtained product is mixed by 90 wt. % with polyvinylidene fluoride (10 wt. %) dissolved in N-methyl-2-pyrrolidone.

[0126] Finally, the mixture is spread on an aluminum foil (100-μm) and then dried at 60° C.

[0127] The electrode is then made of 76.5 wt. % of active material; 13.5 wt. % of carbon, and 10 wt. % of polyvinylidene fluoride (PVDF).

[0128] b) Mounting of the Accumulator

[0129] The formed electrode is introduced into a cell of “button cell” type at format 2032.

[0130] The negative electrode is made of metal lithium.

[0131] Two types of separators have been used: [0132] a polypropylene film (Celgard® 2400), and [0133] a polyolefin film (Viledon®).

[0134] The electrolyte used is made of ethylene carbonate, of propylene carbonate, of dimethyl carbonate, and of lithium hexafluorophosphate (LiPF.sub.6) (Powerlyte's Electrolyte LP100).

[0135] c) Galvanostatic Cycling

[0136] At room 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. FIG. 4 enables to compare the first cycle of a C/20 galvanostatic cycling between 1.5 and 4.7 V for the LiFe.sub.0,5Co.sub.0,5BO.sub.3 and LiCoBO.sub.3 compounds.

[0137] Further, FIG. 4 shows that the reversible capacity (that is, the capacity obtained in discharge mode) obtained at the first cycle for the LiFe.sub.0,5Co.sub.0,5BO.sub.3 compound is much greater than that obtained for LiCoBO.sub.3: 125 mAh/g vs. 64 mAh/g.

[0138] FIG. 5 corresponds to the cyclic voltammogram at 0.05 mV/s for the LiFe.sub.0,5Co.sub.0,5BO.sub.3 compound. This graph shows the redox potential of the Fe.sup.3+/Fe.sup.2+ and Co.sup.3+/Co.sup.2+ couples of the LiFe.sub.0,5Co.sub.0,5BO.sub.3 compound.

[0139] 3/ Characterization of the LiFe.sub.1-xCo.sub.xBO.sub.3 Compound:

[0140] FIGS. 1 and 2 correspond to the diffractograms of the LiFe.sub.1-xCo.sub.xBO.sub.3 compounds when x=0; 0.5; and 1. They clearly show that the monoclinic structure is kept for LiFe.sub.0,5Co.sub.0,5BO.sub.3.

[0141] FIG. 3 corresponds to an image obtained by scanning electron microscopy (SEM) of the LiFe.sub.0,5Co.sub.0,5BO.sub.3 compound according to the present invention.