Method for forming a Li-ion battery cell comprising an LNMO cathode material

Abstract

A process forms a Li-ion battery cell including an LNMO-based cathode material, an anode material, a separator and an electrolyte. The process successively includes charging the cell until the cell reaches a state of charge of 100%, storing the cell in the state of charge of 100% in an open circuit for a period of time of at least 48 hours, and removing gas generated during the charging and the storage.

Claims

1. A process for forming a Li-ion battery cell comprising an LNMO-based cathode material comprising an active material of formula LiNi.sub.xMn.sub.2-xO.sub.4 in which 0<x0.5, an anode material, a separator and an electrolyte, the process comprising the following successive steps: charging a Li-ion battery cell having a charge of less than 100% until the Li-ion battery cell reaches a state of charge of 100%; storing the Li-ion battery cell in the state of charge of 100% in an open circuit for a period of time of at least 48 hours; and removing gas generated during the charging and the storing, wherein, when a volume of the gas generated is below a threshold, the removing of the gas is followed by charging the Li-ion battery cell until the Li-ion battery cell reaches the state of charge of 100%, and then the Li-ion battery cell is discharged until it reaches a state of charge of between 20% and 50%.

2. The process as claimed in claim 1, wherein the storing is carried out at room temperature.

3. The process as claimed in claim 1, further comprising: discharging the Li-ion battery cell until the Li-ion battery cell reaches a state of charge of between 10% and 0%; and storing, after the discharging, the Li-ion battery cell in the state of charge of between 10% and 0% in an open circuit at room temperature.

4. The process as claimed in claim 1, wherein the charging, the storing, and the removing are repeated at least a second time.

5. The process as claimed in claim 1, wherein the period of time for the storing is at least equal to one week.

6. The process as claimed in claim 1, wherein the period of time for the storing is at least equal to two weeks.

7. The process as claimed in claim 1, wherein the period of time for the storing is at least equal to four weeks.

8. The process as claimed in claim 1, further comprising: measuring a volume of the gas generated during the storing.

9. The process as claimed in claim 1, further comprising, prior to the charging: activating the Li-ion battery cell by injecting the electrolyte into the Li-ion battery cell; and impregnating the electrolyte in an open circuit for 48 hours at room temperature.

10. The process as claimed in claim 1, wherein: the storing is carried out at a temperature of between 15 and 45 C.; and the period of time for the storing is less than or equal to two weeks.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram illustrating the various steps of the forming process according to the invention in one preferred embodiment;

(2) FIG. 2 is a graph showing the evolution of the volume of gas generated during periods of four weeks of storage in a state of charge of 100% and repeated three times;

(3) FIG. 3 is a histogram representing the evolution of the self-discharge after periods of four weeks of storage in a state of charge of 100%.

DETAILED DESCRIPTION

(4) In the description of the invention, the term based is synonymous with predominantly comprising.

(5) It is, moreover, specified that the expressions between . . . and . . . and from . . . to . . . used in the present description should be understood to include each of the limits mentioned.

(6) FIG. 1 illustrates one particular embodiment of the forming process according to the invention.

(7) As shown in FIG. 1, the cell is first of all activated by injection (100) of an electrolyte. A period of impregnation (200) in an open circuit at room temperature within the cell follows on for, for example, 48 hours. After having been charged (300) until it reaches a state of charge of 100%, the cell is then stored (400) in an open circuit for a period of at least 48 hours, for example for four weeks, at room temperature. During its storage, it generates gas and self-discharges due to parasitic reactions related to the instability of the electrolyte on contact with the electrode. With the accumulation of gas in the cell, the contact between the two electrodes is worsened. Optionally, a complete discharging (500) of the cell can then be carried out, followed by storage (600) in an open circuit at room temperature for, for example, 48 hours. The volume of gas generated during the storage (400) is then measured (700). If the volume is above a threshold, for example above 0.5 ml/g of LNMO-based material, the gas is removed (800) in order to ensure uniform contact between the two electrodes. The cell is then charged (300) once again until it reaches a state of charge of 100%, before being stored (400) once again in an open circuit at room temperature for, for example, four weeks.

(8) The process can be repeated in this way as long as the volume of gas generated is above the threshold.

(9) On the other hand, if, during the measuring step (700), the volume of gas generated is below the threshold, a final removal (900) of gas followed by charging (1000) of the cell until it reaches a state of charge of 100% are carried out. The cell is then formed and ready to be used. If it must be stored or transported, a discharge can then be applied until it reaches a state of charge of between 20% and 50%.

(10) In fact, the process consists here in storing the cell in an open circuit in the charge state until the gas-generating process has slowed to acceptable speeds for the intended application. This gas-generating process speed depends essentially on the packaging of a cell. Thus, if the volume of gas generated is below the threshold, the speed of this process is judged to be sufficiently low to allow the use of a cell in cycling.

(11) The process according to the invention for forming a Li-ion battery cell comprising an LNMO-based cathode material, an anode material, a separator and an electrolyte is such that said LNMO-based cathode material comprises an active material of formula LiNi.sub.xMn.sub.2-xO.sub.4, in which 0<x0.5.

(12) In addition to the active material, the LNMO-based cathode material may also comprise carbon fibers. For example, these are vapor grown carbon fibers (VGCFs) sold by the company Showa Denko. Other types of appropriate carbon fibers may be carbon nanotubes, doped nanotubes (optionally doped with graphite), carbon nanofibers, doped nanofibers (optionally doped with graphite), single-walled carbon nanotubes or multi-walled carbon nanotubes. The methods of synthesis relating to these materials can include arc discharge, laser ablation, a plasma torch and chemical vapor decomposition.

(13) The LNMO-based cathode material may also comprise one or more binder(s).

(14) For example, the binder(s) can be chosen from polybutadiene/styrene latexes and organic polymers, and preferably from polybutadiene/styrene latexes, polyesters, polyethers, polymer derivatives of methyl methacrylate, polymer derivatives of acrylonitrile, carboxymethylcellulose and its derivatives, polyvinyl acetates or polyacrylate acetate, polyvinylidene fluorides, and mixtures thereof.

(15) The process according to the invention for forming a Li-ion battery cell comprising an LNMO-based cathode material, an anode material, a separator and an electrolyte is such that said cell comprises a graphite-based or LTO-based anode material.

(16) The graphite carbon can be chosen from synthetic graphite carbons and natural graphite carbons, starting from natural precursors, followed by purification and/or a post-treatment. Other carbon-based active materials can be used, such as pyrolytic carbon, amorphous carbon, active carbon, coke, coal pitch and graphene. Mixtures of graphite with one or more of these materials are possible.

(17) The LTO-based anode material comprises, for example, an active material of formula Li.sub.4Ti.sub.5O.sub.12 (LTO).

(18) The graphite-based or LTO-based anode material can also comprise one or more binders as for the cathode.

(19) The binders described above for the cathode can be used for the anode.

(20) A separator is preferably placed between the electrodes of the Li-ion battery cell. It acts as an electrical insulator. Several materials can be used as separators. The separators are generally composed of porous polymers, preferably of polyethylene and/or polypropylene.

(21) The process according to the invention for forming a Li-ion battery cell comprising an LNMO-based cathode material, an anode material, a separator and an electrolyte is such that said battery comprises an electrolyte, which is preferably liquid.

(22) The most common lithium salt is an inorganic salt, namely lithium hexafluorophosphate (LiPF.sub.6). Other inorganic salts are appropriate and can be chosen from LiClO.sub.4, LiAsF.sub.6, LiBF.sub.4 or LiI. Organic salts are also appropriate and can be chosen from lithium bis[(trifluoromethyl)sulfonyl]imide (LiN(CF.sub.3SO.sub.2).sub.2), lithium trifluoromethanesulfonate (LiCF.sub.3SO.sub.3), lithium bis(oxalato)borate (LiBOB), lithium fluoro(oxolato)borate (LiFOB), lithium difluoro(oxolato)borate (LiDFOB), lithium bis(perfluoroethylsulfonyl)imide (LiN(CF.sub.3CF.sub.2SO.sub.2).sub.2), LiCH.sub.3SO.sub.3, LiR.sub.FSOSR.sub.F, LiN(R.sub.FSO.sub.2).sub.2 and LiC(R.sub.FSO.sub.2).sub.3, R.sub.F being a group chosen from a fluorine atom and a perfluoroalkyl group comprising between one and eight carbon atoms.

(23) The lithium salt(s) is (are) preferably dissolved in one or more solvents chosen from aprotic polar solvents, for example ethylene carbonate (denoted EC), propylene carbonate, dimethyl carbonate (denoted DMC) and ethyl methyl carbonate (denoted EMC).

EXAMPLE

1. Process for Forming a Li-ion Battery Cell

(24) The cell is first of all activated by injection (100) of an electrolyte. A period of impregnation (200) at room temperature (23 C.) within the cell follows on for 48 hours. After having been charged (300) until it reaches a state of charge of 100%, the cell is stored (400) in an open circuit at room temperature for four weeks.

(25) Complete discharge (500) of the cell is then carried out, followed by storage (600) in an open circuit at room temperature for 12 hours. The volume of gas generated is measured (700) at the end of this first period of four weeks of storage, called first storage. Since it is too high (1.4 ml according to curve A in FIG. 2), removal (800) of the gas is carried out. The cell is charged (300) once again until it reaches a state of charge of 100%, before being stored (400) once again in an open circuit at room temperature for four weeks. This is the second period of four weeks of storage, called second storage.

(26) The cell is completely discharged (500) and then stored (600) in an open circuit at room temperature for 12 hours. The volume of gas generated is again measured (700). The cell degases further, but to a lesser extent than during the first storage (0.6 ml according to curve B in FIG. 2). This observation is a sign of gradual stabilization of the cathode/electrolyte interface.

(27) Since the volume of gas generated is still too high, a further removal (800) of the gas is carried out and the formation process is relaunched. At the end of the third period of storage, called third storage, followed by complete discharge (500) and storage (600) in an open circuit at room temperature for 12 hours, the volume of gas generated is measured (700) (0.2 ml according to curve C in FIG. 2). Since it is judged to be sufficiently low, final removal (900) of the gas is carried out, followed by charging (1000) of the cell until it reaches a state of charge of 100%. The cell is then formed and ready to be employed for use.

2. Cell Evaluation and Result

2.1 Result Regarding Degassing

(28) FIG. 2 shows, as mentioned above, that the Li-ion battery cell degasses to an increasingly lesser extent with the number of repetitions of the forming process according to the invention. Thus, after a first storage (400) in a state of charge of 100% for four weeks at room temperature, curve A indicates that the cell generates 1.4 ml of gas.

(29) Since this volume of gas is too high, the forming process according to the invention is relaunched.

(30) After the second storage of four weeks, curve B indicates that the cell generates 0.6 ml of gas. Since this volume of gas is again judged to be too high, the various steps of the forming process according to the invention are repeated.

(31) After the third storage of four weeks, curve C indicates that the cell generates only 0.2 ml.

(32) The cell is consequently formed and ready to be used.

2.2 Result Regarding Self-Discharge

(33) FIG. 3 shows that the self-discharge is increasingly weak with the number of repetitions of the forming process according to the invention. Thus, after a first storage (400), the self-discharge of the Li-ion battery cell is evaluated at 29%.

(34) The forming process according to the invention is repeated and the cell is again stored for four weeks at room temperature. After this second storage, the self-discharge is evaluated at 18%.

(35) The forming process according to the invention is again repeated and, after the third storage, the self-discharge is evaluated at only 8%.

(36) All of the results that FIGS. 2 and 3 disclose suggest that the cathode/electrolyte interface is increasingly stable.

(37) The initial objective of developing a process for specifically forming a Li-ion battery cell comprising an LNMO-based cathode material making it possible to considerably reduce the degassing and the self-discharge is thus achieved.