NEGATIVE ELECTRODE ACTIVE MATERIAL BASED ON IRON AND LITHIUM HYDROXYSULFIDE

Abstract

A negative electrode active material for a lithium-ion battery has the following formula (I): Li1-xOHFe1+xS (I). x varies from 0.00 to 0.25, preferably from 0.05 to 0.20.

Claims

1-11. (canceled)

12. A negative electrode active material for a lithium-ion battery having the following formula (I):
Li1-xOHFe1+xS  (I), wherein x varies from 0.00 to 0.25.

13. The active material according to claim 12, wherein x varies from 0.05 to 0.20.

14. The active material according to claim 12, wherein x is equal to 0.05.

15. A method for making the active material according to claim 12, comprising: (a) adding iron and sulphur in an aqueous solution of LiOH at a concentration ranging from 2 to 6 mol/L; (b) heating the mixture obtained upon completion of (a) to a temperature ranging from 130 to 190° C.; (c) recovering said active material.

16. The method according to claim 15, wherein the concentration ranges from 4 to 6 mol/L.

17. A negative electrode comprising: at least one of the active material as defined in claim 12.

18. The negative electrode according to claim 17, wherein the negative electrode further comprises at least one conductive compound.

19. The negative electrode according to claim 17, wherein the conductive compound is chosen from metal particles, carbon, and the mixtures thereof.

20. The negative electrode according to claim 17, wherein the conductive compound is carbon.

21. The negative electrode according to claim 20, wherein the carbon is in the form of graphite, carbon black, carbon fibres, carbon nanowires, carbon nanotubes, carbon nanospheres, preferably carbon black.

22. The negative electrode according to claim 20, wherein the carbon is carbon black.

23. The negative electrode according to claim 17, wherein the content of active material varies from 50 to 97% by weight in relation to the total weight of the negative electrode.

24. The negative electrode according to claim 17, wherein the content of active material varies from 80 to 97% by weight.

25. The negative electrode according to claim 17, wherein the content of conductive compound varies from 3 to 30% by weight in relation to the total weight of the negative electrode.

26. The negative electrode according to claim 25, wherein the content of conductive compound varies from 3 to 20% by weight.

27. A lithium-ion battery comprising: at least one of the negative electrode as defined in claim 17.

Description

[0014] Other advantages and features of the invention will become clear from the detailed description and the appended drawings, in which:

[0015] FIG. 1 is a graph showing the potential of a half cell of a Li-ion battery as a function of the quantity of electrochemically inserted lithium, expressed as a capacity.

[0016] FIG. 2 is a graph showing the capacity of a half cell of a Li-ion battery as a function of the number of cycles.

[0017] FIG. 3 is a graph showing the potential of a half cell of a Li-ion battery as a function of the quantity of electrochemically inserted lithium, expressed as a capacity.

[0018] It is specified that the expression “from . . . to” used in this description of the invention must be understood as including each of the indicated endpoints.

[0019] The initial composition (after synthesis) of the negative electrode active material for a lithium-ion battery according to the invention satisfies the formula (I) as mentioned above.

[0020] Preferably, x is equal to 0.05. Consequently, the active electrode material satisfies the formula Li.sub.0.95OHFe.sub.1.05S.

[0021] Advantageously, the active material according to the invention occurs in the form of plates having lateral dimensions ranging from 0.5 to 3 μm and a thickness ranging from 90 to 110 nm.

[0022] The invention also relates to a method for preparing the active material according to the invention, comprising the following steps:

(a) adding iron and sulphur in an aqueous solution of LiOH at a concentration ranging from 2 to 6 mol/L, preferably from 4 to 6 mol/L;
(b) heating the mixture obtained upon completion of step (a) to a temperature ranging from 130 to 190° C.;
(c) recovering said active material.

[0023] Advantageously, the iron is in the form of an iron powder.

[0024] When the quantity of lithium exceeds the saturation concentration (about 5 mol/L at 20° C.), a portion of the hydroxide is solid.

[0025] According to a preferred embodiment, step (b) is followed by a cooling, filtration, and drying step. Then, after the drying, the resulting product can be ground.

[0026] The invention further relates to a negative electrode comprising the active material according to the invention.

[0027] In a preferred way, the negative electrode according to the invention further comprises at least one conductive compound. So, in this embodiment, the electrode according to the invention can comprise the active material according to the invention and at least one conductive compound.

[0028] According to a particular embodiment, the conductive compound is chosen from among the metal particles, carbon, and the mixtures thereof, preferably carbon.

[0029] The carbon can be in the form of graphite, carbon black, carbon fibres, carbon nanowires, carbon nanotubes, carbon nanospheres, preferably carbon black.

[0030] In particular, the negative electrode according to the invention advantageously comprises SuperC65® carbon black marketed by Timcal.

[0031] In a preferred way, the content of active material according to the invention varies from 50 to 97% by weight, preferably from 70 to 97% by weight, more preferably from 80 to 97% by weight in relation to the total weight of the negative electrode.

[0032] Advantageously, the content of conductive compound varies from 3 to 30% by weight, preferably from 3 to 20% by weight, in relation to the total weight of the negative electrode.

[0033] According to an especially preferred embodiment, the content of active material varies from 70 to 97% by weight and the content of conductive compound varies from 3 to 30% by weight, in relation to the total weight of the negative electrode.

[0034] The present invention can also relate to a Li-ion battery cell comprising a negative electrode comprising the active material according to the invention, a positive electrode, a separator, and a electrolyte.

[0035] Preferably, the battery cell comprises a localized separator between the electrodes serving as an electrical insulator. Several materials may be used as separators. The separators are generally composed of porous polymers, preferably polyethylene and/or polypropylene. They can also be made of glass microfibres.

[0036] Advantageously, the separator used is a separator made of glass microfibres, CAT No. 1823-070® marketed by Whatman.

[0037] Preferably, said electrolyte is a liquid.

[0038] This electrolyte may comprise one or more lithium salts and one or more solvents.

[0039] The lithium salt or salts generally comprise inert anions. Appropriate lithium salts can be chosen from lithium bis[(trifluoromethyl)sulfonyl]imide (LiN(CF.sub.3SO.sub.2).sub.2), lithium trifluoromethane sulfonate (LiCF.sub.3SO.sub.3), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxolato) borate (LiDFOB), lithium bis(perfluoroethylsulfonyl)imide (LiN(CF.sub.3CF.sub.2SO.sub.2).sub.2), LiClO.sub.4, LiAsF.sub.6, LiPF.sub.6, LiBF.sub.4, LiI, LiCH.sub.3SO.sub.3, LiB(C.sub.2O.sub.4).sub.2, LiN(R.sub.FSO.sub.2).sub.2, LiC(R.sub.FSO.sub.2).sub.3, wherein R.sub.F is a group chosen from a fluorine atom and a perfluoroalkyl group comprising one to eight carbon atoms.

[0040] The lithium salt or salts are preferably dissolved in one or more solvents chosen from the polar aprotic solvents, for example ethylene carbonate (noted “EC”), propylene carbonate, dimethyl carbonate, diethyl carbonate (noted “DEC”), methyl and ethyl carbonate, 1,3-dioxolane, and dimethoxyethane, as well as all mixtures of these various solvents.

[0041] Advantageously, the electrolyte comprises an equivolume mixture of ethylene carbonate and diethyl carbonate with the lithium salt LiPF.sub.6 at 1M.

[0042] One object of the present invention is also a Li-ion battery comprising at least one negative electrode according to the invention.

[0043] The present invention also relates to the use of a material with the following formula (I):

Li.sub.1−xOHFe.sub.1+xS  (I),

wherein x varies from 0.00 to 0.25, preferably from 0.05 to 0.20, as a negative electrode active material for a lithium-ion battery.

[0044] In a particularly preferred way, x is equal to 0.05.

[0045] The present invention is illustrated in a non-limiting way by the following examples.

EXAMPLES

I. Preparation of Electrochemical Half-Cells

[0046] 1. Synthesis of the Active Materials

Synthesis of the Active Material Li.SUB.0.95.OHFe.SUB.1.05.S

[0047] 1.117 g of iron powder and 0.64 g of sulphur are added to 30 ml a LiOH solution (equivalent to 6 mol/L in concentration). This combination is placed in a 40-mL autoclave and is heated without stirring to 160° C. for 14 h in a closed enclosure withstanding pressure. After cooling, a black precipitate is observed and is then filtered and dried for 4 h at 90° C. in a dry nitrogen atmosphere. After this drying, the product is ground and a fine powder is obtained (the yield is about 80% by weight).

[0048] 2. Preparation of the Active Material Electrodes (Negative Electrodes)

[0049] From material, two active material electrodes were prepared called EN-A and EN-B, respectively.

[0050] 2.1 Preparation of Electrode EN-A

[0051] The active material electrodes is made by mixing 80% by weight of active material of formula Li.sub.0.95OHFe.sub.1.05S and 20% by weight of SuperC65® carbon black, and the mixture is then ground by hand in an agate mortar.

[0052] 2.2 Preparation of Electrode EN-B

[0053] The negative electrode is made of only the active material of formula Li.sub.0.95OHFe.sub.1.05S.

[0054] 3. Assembling the Electrochemical Half-Cells

[0055] Two electrochemical half-cells were then prepared comprising the negative electrodes EN-A and EN-B, respectively. The half-cells were called DC-A and DC-B, respectively.

[0056] Assembling the electrochemical half-cells is done in a glove box with the help of a device consisting of a 12-mm-diameter Swagelok® fitting. Each half-cell comprises a separator, a positive electrode, and an electrolyte.

[0057] 3.1 Assembling Half-Cell DC-A

Active Material Electrode

[0058] A weight of 25 mg of electrode EN-A, in the form of a powder, is then spread over a stainless steel piston placed in electrochemical half-cell DC-A.

Separator

[0059] Two layers of glass microfibre separator, CAT No. 1823-070®, are used to avoid any short-circuiting between the positive electrode and the negative electrode during the charge and discharge cycles. These separators are cut out according to a diameter of 12 mm and a thickness of 1 mm, and are deposited on the powder constituting the negative electrode.

Counter-Electrode Made of Lithium

[0060] Pellets measuring 12 mm in diameter are cut out from a sheet of lithium metal. The resulting pellet is then made to adhere to a stainless steel current collector by pressure. This collector is then deposited on the separating membrane in the cell.

Electrolyte

[0061] The electrolyte used comprises a commercial solution composed of LiPF.sub.6 dissolved at a concentration of 1 mol/L in an equivolume mixture of ethylene carbonate and diethyl carbonate. A second piston is put into the cell to ensure a seal and allow for electrical contact with the active material electrode.

[0062] 3.2 Assembling Half-Cell DC-B

Active Material Electrode

[0063] A weight of 25 mg of electrode EN-B, in the form of a powder, is then spread over a stainless steel piston placed in electrochemical half-cell DC-B.

[0064] The separator, lithium counter-electrode, and electrolyte are identical to those used in half-cell DC-A.

II. Electrochemical Tests

[0065] 1. Half-Cell DC-A

[0066] Galvanostatic cycling is conducted using a BioLogic cycler with a cycling regimen of C/10, with C corresponding to the current for inserting one lithium per iron into the material in one hour.

[0067] The voltage terminals were defined as from 3.0 to 1.0 V with respect to the reference Li/Li.sup.+ electrode. Three charge and discharge cycles were performed, as shown in FIG. 1. Indeed, FIG. 1 is a graph showing the potential of half-cell A as a function of the quantity of inserted lithium expressed as a capacity in relation to the mass of active material of which the electrode is composed (mAh/g).

[0068] In this FIG. 1, curve A1 corresponds to the first charge and discharge cycle. Curve A2 corresponds to the second charge and discharge cycle. Curve A3 corresponds to the third charge and discharge cycle.

[0069] Consequently, the observation is that the first discharge cycle reaches an irreversible capacity of 200 mAh/g, whereas the following cycles lead to a reversible capacity of 240 mAh/g.

[0070] Furthermore, the capacity of half-cell A according to the invention, as a function of the number of cycles, was also evaluated, as shown by curve B in FIG. 2.

[0071] After 20 charge and discharge cycles, the capacity is about 250 mAh/g. Consequently, the capacity of half-cell A according to the invention is much greater than that of a half-cell having a negative electrode comprising the oxide LTO. Indeed, the oxide LTO has a theoretical capacity of 175 mAh/g.

[0072] In addition, half-cell A holds up well to cycling.

[0073] The average potential determined as the potential at half capacity is 1.7V vs Li/Li.sup.+ and is very close to the potential of the oxide LTO.

[0074] Consequently, the active material according to the invention makes it possible to obtain an improved capacity.

[0075] 2. Half-Cell DC-B

[0076] Galvanostatic cycling is conducted using a BioLogic cycler with a cycling regimen of C/10, with C corresponding to the current for inserting one lithium per iron into the material in one hour.

[0077] The voltage terminals were defined as from 3.0 to 1.0 V with respect to the reference Li/Li.sup.+ electrode. Three charge and discharge cycles were performed, as shown in FIG. 3. Indeed, FIG. 3 is a graph showing the potential of half-cell B as a function of the capacity.

[0078] In this FIG. 3, curve C1 corresponds to the first charge and discharge cycle. Curve C2 corresponds to the second charge and discharge cycle. Curve C3 corresponds to the third charge and discharge cycle.

[0079] Consequently, the observation is that the capacity is 140 mAh/g after three cycles.

[0080] Consequently, the capacity of half-cell B according to the invention remains high. Indeed, despite the absence of a conductive compound, the capacity loss is only about 45% compared to that of half-cell A.

[0081] So the active material according to the invention is a conductive material. In other words, conductivity is improved thanks to the active material of the invention.