POLYMER ELECTROLYTE FOR A LITHIUM METAL POLYMER BATTERY HAVING IMPROVED PERFORMANCE

Abstract

A cross-linked copolymer is provided, including at least repeating units of poly(alkylene oxide) and at least repeating units of lithium polystyrene-sulfonyl(trifluoromethylsulfonyl)imide (PSTFSILi), as well as the use of such a cross-linked copolymer for preparing a solid polymer electrolyte, a solid polymer electrolyte having the cross-linked copolymer, and a battery, for example a lithium metal polymer (LMP) battery, including the solid polymer electrolyte.

Claims

1. A cross-linked copolymer, comprising: at least repeating units of lithium polystyrene-sulfonyl(trifluoromethylsulfonyl)imide (PSTFSILi) and at least repeating units of poly(alkylene oxide) selected from poly(ethylene oxide) units, poly(propylene oxide) units, poly(ethylene and propylene) oxide units, and one of the mixtures thereof, and in that said cross-linked copolymer is obtained by cross-linking a triblock copolymer of the BAB type, in which: an A block is a cross-linkable poly(alkylene oxide) capable of being obtained from: at least one monomer selected from ethylene glycol, propylene glycol and mixture thereof, or at least one poly(alkylene oxide) oligomer selected from poly(ethylene oxide)s, poly(propylene oxide)s, poly(ethylene and propylene) oxides, and one of the mixtures thereof, and at least one compound comprising at least one cross-linkable alkene or alkyne function, and each B block is an anionic polystyrene substituted by the anion of a lithium sulfonyl(trifluoromethylsulfonyl)imide (TFSILi) and corresponding to the following formula (I): ##STR00013## in which n denotes the number of lithium styrene-sulfonyl(trifluoromethylsulfonyl)imide for each of the B blocks.

2. The cross-linked copolymer according to claim 1, characterized in that it comprises from 10 to 50% by weight PSTFSILi with respect to the total weight of the cross-linked copolymer.

3. The cross-linked copolymer according to claim 1, characterized in that the A block has a number-average molecular weight less than or equal to 25 kDa.

4. The cross-linked copolymer according to claim 1, characterized in that the A block comprises a functional polymer having the following formula (II):
—[CoA-R.sup.1].sub.p—  (II) in which R.sup.1 is a substituent comprising at least one cross-linkable alkene or alkyne function; CoA is a poly(alkylene oxide) chain selected from the poly(ethylene oxide), poly(propylene oxide), poly(ethylene and propylene) oxide chains, and one of the mixtures thereof; and p is comprised between 10 and 50.

5. The cross-linked copolymer according to claim 4, characterized in that the functional polymer of formula (II) corresponds to the following formula (II-a): ##STR00014## in which y is comprised between 11 and 91.

6. The cross-linked copolymer according to claim 1, characterized in that the ratio of the number of moles of alkylene oxide to the number of moles of STFSILi (AO/Li) ranges from 7 to 65.

7. The cross-linked copolymer according to claim 1, characterized in that the compound comprising a cross-linkable alkene or alkyne function is selected from the compounds having the following formula (IV):
X—R′.sup.1—X′  (IV) in which R′.sup.1 is an alkyl group comprising at least one alkene or alkyne function, said alkyl group comprising from 4 to 10 carbon atoms; and X and X′, identical or different, are selected independently of one another, from the halogen, carboxylic acid, acyl chloride, ester and aldehyde functions.

8. The cross-linked copolymer according to claim 1, characterized in that the compound comprising a cross-linkable alkene or alkyne function is 3-chloro-2-chloro-1-propene.

9. The cross-linked copolymer according to claim 1, characterized in that the alkylene oxide oligomer has a molar mass ranging from 700 to 4000 g/mol.

10. A use of at least one cross-linked copolymer as defined in claim 1, for the preparation of a solid polymer electrolyte.

11. A solid polymer electrolyte, characterized in that it comprises at least one cross-linked copolymer as defined in claim 1, and at least one plasticizer.

12. The solid polymer electrolyte according to claim 11, characterized in that the plasticizer is selected from the linear and cyclic carbonates; fluorinated carbonates; nitriles; lactones; liquid linear or cyclic polyethers; fluorinated polyethers; and one of the mixtures thereof.

13. The solid polymer electrolyte according to claim 11, characterized in that the plasticizer is a liquid linear or cyclic polyether of molar mass less than or equal to 10000 g.Math.mol.sup.−1, selected from: polyethylene glycols of formula H—[O—CH.sub.2—CH.sub.2].sub.q—OH, in which q is comprised between 1 and 13, glycol ethers of formula R.sup.7—[O—CH.sub.2—CH.sub.2].sub.q′—O—R.sup.7′, in which q′ is comprised between 1 and 13 and R.sup.7 and R.sup.7′, identical or different, are linear, branched or cyclic alkyl groups, ethers of formula R.sup.8—[CH.sub.2—O].sub.q″—R.sup.8′, in which q″ is comprised between 1 and 13, R.sup.8 and R.sup.8′, identical or different, are linear, branched or cyclic alkyl groups, the cyclic ethers, the cyclic polyethers; and one of the mixtures thereof.

14. The solid polymer electrolyte according to claim 11, characterized in that it comprises from 10 to 40% by weight of plasticizer, with respect to the total weight of the solid polymer electrolyte.

15. A battery comprising: a negative electrode comprising metallic lithium or an alloy of metallic lithium; a positive electrode, optionally supported by a current collector; a solid polymer electrolyte positioned between the positive electrode and the negative electrode; and the solid polymer electrolyte is as defined in claim 11.

16. The battery according to claim 15, characterized in that the composite positive electrode comprises: at least one positive electrode active material; at least one polymer binder; optionally at least one electron conductive agent; optionally at least one plasticizer; and said polymer binder being a BAB triblock copolymer having at least repeating units of lithium polystyrene-sulfonyl(trifluoromethylsulfonyl)imide (PSTFSILi) and at least repeating units of poly(alkylene oxide) selected from poly(ethylene oxide) units, poly(propylene oxide) units, poly(ethylene and propylene) oxide units, and one of the mixtures thereof, and in that said cross-linked copolymer is obtained by cross-linking a triblock copolymer of the BAB type, in which: an A block is a cross-linkable poly(alkylene oxide) capable of being obtained from: at least one monomer selected from ethylene glycol, propylene glycol and mixture thereof, or at least one poly(alkylene oxide) oligomer selected from poly(ethylene oxide)s, poly(propylene oxides)s, poly(ethylene and propylene) oxides, and one of the mixtures thereof, and at least one compound comprising at least one cross-linkable alkene or alkyne function, and each B block is an anionic polystyrene substituted by the anion of a lithium sulfonyl(trifluoromethylsulfonyl)imide (TFSILi) and corresponding to the following formula (I): ##STR00015## in which n denotes the number of lithium styrene-sulfonyl(trifluoromethylsulfonyl)imide for each of the B blocks.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0136] The attached drawings illustrate the invention:

[0137] FIG. 1 shows the Young's modulus (in MPa), as a function of the weight percent of PSTFSILi of materials according to the invention and not according to the invention.

[0138] FIG. 2 shows the variation in the ion conductivity (in S/cm) as a function of the ratio 1000/T, T being the temperature in Kelvin for materials according to the invention and not according to the invention.

[0139] FIG. 3 shows the glass transition temperature (in ° C.), as a function of the weight percent of PSTFSILi of materials according to the invention and not according to the invention.

[0140] FIG. 4 shows the melting temperature (in ° C.), as a function of the weight percent of PSTFSILi of materials according to the invention and not according to the invention.

[0141] FIG. 5 shows the voltage of a composite positive electrode according to the invention (in volts) as a function of the discharge capacity (in mAh) at 60° C. and at different regimes (from D/9.4 to D/0.9), the load always being C/9.4.

[0142] FIG. 6 shows the curve of the discharge capacity (in mAh) and the coulombic efficiency (in %), as a function of the number of cycles, at 60° C. and at different discharge regimes (from D/9.4 to D/0.9), the load always being C/9.4.

[0143] FIG. 7 represents the power performances of two LMP batteries, according to the invention and not according to the invention.

EXAMPLES

[0144] The raw materials used in the examples are listed hereinafter: [0145] carbon black, Ketjenblack EC600JD, AkzoNobel, [0146] LiFePO4, Pulead, [0147] PVDF-co-HFP, Solvay, [0148] homo-PEO, Sumitomo Seika, [0149] LiTFSI, Solvay, [0150] current collector made from aluminium covered with a layer of carbon, Armor, [0151] sheet of metallic lithium, Blue Solutions, [0152] PEO oligomer, PEG2000, Sigma-Aldrich, [0153] 3-chloro-2-chloro-1-propene, [0154] TEGDME, Sigma-Aldrich, [0155] potassium hydroxide (KOH), [0156] tetrahydrofuran (THF), [0157] diethyl ether, [0158] acetonitrile, [0159] water, [0160] acryloyl chloride, [0161] triethylamine, [0162] dimethylformamide (DMF), [0163] 2-Hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone: photoinitiator Irgacure 2959, [0164] Nitroxide SG1 and alkoxyamine MAMA-SG1 having the following formulas:

##STR00009##

Unless otherwise indicated, all the materials were used as received from the manufacturers.

Example 1: Preparation of Cross-Linked Copolymers According to the First Subject of the Invention

[0165] Preparation of a Cross-Linkable Poly(Ethylene Oxide) Precursor of the A Block

[0166] Preparation of a Cross-Linkable Poly(Ethylene Oxide): Substep i-a) as Defined in the Invention

##STR00010##

[0167] 127.6 g of a PEO oligomer of 1.5 kDa and 9.5 g KOH are dissolved in 200 ml tetrahydrofuran (THF) at 40° C. When the solution is homogeneous, 10 g 3-chloro-2-chloro-1-propene dissolved in 20 ml THF are added to the solution of PEO oligomer prepared beforehand. The polycondensation reaction was implemented at 40° C. for 3 days. As the 3-chloro-2-chloro-1-propene is in deficit, the cross-linkable poly(ethylene oxide) obtained is terminated by hydroxyl functions.

[0168] The reaction medium was then cooled, centrifuged, then the supernatant was precipitated in diethyl ether. The purification is completed by ultrafiltration in order to eliminate the low molecular weight polymers and the salts which were not eliminated in the centrifugation step. Water is eliminated by rotary evaporation and the product is dried under vacuum.

[0169] Preparation of a Cross-Linkable Poly(Ethylene Oxide): Substep i-b) as Defined in the Invention

##STR00011##

[0170] 26.5 g cross-linkable poly(ethylene oxide) as prepared beforehand was allowed to react at ambient temperature for 15 hours with 7.4 g acryloyl chloride in the presence of 8 g triethylamine in 200 ml tetrahydrofuran. The terminal hydroxyl functions were thus functionalized by the acrylate functions. The residue obtained was precipitated in diethyl ether, filtered, then dried under vacuum.

[0171] Then, 2 g alkoxyamine of formula MAMA-SG1 was added to the diacrylate as obtained beforehand, at 80° C. in 50 ml ethanol under inert atmosphere. After 4 hours of reaction, the product obtained was precipitated in diethyl ether, filtered, then dried under vacuum.

[0172] Copolymerisation of Cross-Linkable Poly(Ethylene Oxide), Precursor of the A Block, with Lithium Styrene-Sulfonyl(Trifluoromethylsulfonyl)Imide (STFSILi) in Order to Form the BAB Triblock Copolymer: Step ii) as Defined in the Invention

##STR00012##

[0173] 5 g cross-linkable poly(ethylene oxide) containing an initiator as prepared beforehand in 30 ml DMF were allowed to react with 1 g lithium styrene-sulfonyl(trifluoromethylsulfonyl)imide (STFSILi) and 7 mg nitroxide SG1 under inert atmosphere at 120° C. At the end of 16 hours of reaction, approximately 80% of the monomer STFSILi had reacted.

[0174] The lithium styrene-sulfonyl(trifluoromethylsulfonyl)imide (STFSILi) can be prepared as described in international patent application WO 2013/034848 A1.

[0175] The copolymer obtained was precipitated in diethyl ether, then purified by dialysis in water (cut-off threshold at 3 kg.Math.mol.sup.−1) before drying by lyophilization.

[0176] A BAB triblock copolymer CP-1 according to the invention was obtained comprising 13.1% by weight PSTFSILi with respect to the total weight of the copolymer. This copolymer has a ratio EO/Li of 48.4.

[0177] Cross-Linking and Shaping of the Copolymer

[0178] 100 mg of copolymer as prepared beforehand was placed in solution in 5 ml of an acetonitrile/water mixture (5/1 by volume). 0.9 mg of UV photoinitiator (2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone) was introduced into the solution. The resulting solution was then poured into a polypropylene Petri dish (6 cm diameter) placed on a flat surface. The major part of the solvent was evaporated in an oven at 40° C. for 24 hours, then at 60° C. for 12 hours. Then the polymer film obtained was cross-linked under a mercury UV lamp sold under the trade name P300 MT Power supply by Fusion UV System Inc for 30 seconds at 15 mW/cm.sup.2 under ambient atmosphere. The film obtained was dried in the glove box to form a cross-linked copolymer CP.sub.r-1 according to the invention, comprising 13.1% by weight of PSTFSILi, with respect to the total weight of the copolymer, in a ratio EO/Li of 48.4.

[0179] Obtaining Other Copolymers

[0180] By modifying the quantity of the STFSILi monomer, other copolymers, cross-linked CP.sub.r-2 and CP.sub.r-3; and non-cross-linked CP-2 and CP-3 were obtained.

[0181] Table 1 below lists the composition of the copolymers obtained:

TABLE-US-00001 TABLE 1 Copolymer % PSTFSILi EO/Li ratio cross-linking CP-1 13.1 48.4 no CP.sub.r-1 13.1 48.4 yes CP-2 22 25.9 no CP.sub.r-2 22 25.9 yes CP-3 28.4 18.4 no CP.sub.r-3 28.4 18.4 yes

Example 2: Preparation of Solid Polymer Electrolytes According to the Third Subject of the Invention

[0182] The cross-linked polymers CP.sub.r-1, CP.sub.r-2 and CP.sub.r-3 in the form of films were quenched in TEGDME as plasticizer for 1 hour in a dry room (dewpoint at −45° C.), then the films obtained were recovered, and the excess plasticizer was removed with a Kimtech paper tissue. The films were weighed before and after absorption of the plasticizer, which makes it possible to deduce the percentage of plasticizer in the films. The average thickness of the films ranges from 30 to 60 μm.

[0183] Table 2 below shows the composition of the solid polymer electrolytes obtained:

TABLE-US-00002 TABLE 2 Solid polymer % weight electrolyte TEGDME Copolymer EO/Li ratio E-1a 14 CP.sub.r-1 59 E-1b 12.3 CP.sub.r-1 57 E-2a 22.8 CP.sub.r-2 40 E-2b 17.4 CP.sub.r-2 35 E-2c 14.5 CP.sub.r-2 33 E-3a 23.4 CP.sub.r-3 30 E-3b 21.3 CP.sub.r-3 28 E-3c 16 CP.sub.r-3 24 E-3d 18.7 CP.sub.r-3 26

Example 3: Physico-Chemical Characteristics

[0184] Young's Modulus

[0185] The Young's modulus (modulus of elasticity) was calculated based on tensile stress vs elongation curves obtained by means of a dynamic mechanical analyzer sold under the trade name of Dynamic Mechanical Analyzer DMA Q800 by the company TA Instruments, at 50° C., with a dry air stream.

[0186] The attached FIG. 1 shows the Young's modulus (in MPa) as a function of the weight percent of PSTFSILi in the cross-linked copolymer (in %) (curve with blank circles, copolymers CPr-1, CP.sub.r-2 and CP.sub.r-3), in the non-cross-linked copolymer (curve with solid circles, copolymers CP-1, CP-2 and CP-3), and in a mixture of cross-linked copolymer and TEGDME plasticizer (curve with solid squares, electrolytes E-1b, E-2b and E-3b). FIG. 1 shows firstly that the cross-linking of the A block in the BAB triblock copolymer has a significant impact on the Young's modulus, since an increase by a factor of 3 to 5 is obtained for one and the same weight percent of PSTFSILi (e.g. 0.46 MPa to 1.7 MPa for the copolymer comprising 28.4% by weight PSTFSILi). Furthermore, this cross-linking makes it possible to obtain sufficient mechanical strength to be able to combine the cross-linked copolymer according to the invention with a plasticizer. In particular, the plasticizing increases the ion conductivity, while guaranteeing a fully acceptable Young's modulus. Thus, a good ion conductivity/mechanical strength compromise is obtained at 50° C. for the solid polymer electrolytes E-1b, E-2b and E-3b. Finally, it is possible to modulate the mechanical strength of the copolymer as a function of the weight percent of PSTFSILi in the cross-linked copolymer.

[0187] Ion Conductivity

[0188] The ion conductivity was calculated according to the following formula:

[00001]$\sigma =\frac{l}{S*R_{\mathrm{el}}}$

[0189] wherein S and I are respectively the surface area and the thickness of the solid polymer electrolyte or of the copolymer. R.sub.ei is the resistance of the solid polymer electrolyte or of the copolymer determined at high frequency by impedance spectroscopy (VMP300, Bio-Logic) on a Li/solid polymer electrolyte or copolymer/Li symmetrical cell. The temperature is set between 10 and 80° C. by means of a climatic enclosure.

[0190] The attached FIG. 2 shows the variation in the ion conductivity (in S/cm) as a function of the ratio 1000/T, T being the temperature in Kelvin, for the non-cross-linked copolymers CP-2 (curve with solid circles) and CP-3 (curve with solid triangles) for the cross-linked copolymers CP.sub.r-2 (curve with blank circles), and CP.sub.r-3 (curve with blank triangles) and for the solid polymer electrolytes E-2c (curve with solid squares) and E-3c (curve with blank squares).

[0191] FIG. 2 shows that the copolymers without plasticizer have a conductivity of 4 to 8*10.sup.−6 S/cm at 60° C., which is too low for use in a battery, in particular under a high regime and high base weight of the positive electrode (e.g. >0.8 mAh/cm.sup.2). Plasticizing by a low quantity of plasticizer makes it possible to achieve a conductivity of 1.3*10.sup.−5 S/cm, without compromising the mechanical stability of the cross-linked copolymer.

[0192] FIGS. 3 and 4 show respectively the glass transition temperature in ° C. (FIG. 3) and the melting temperature in ° C. (FIG. 4) as a function of the weight percent of PSTFSILi, for the cross-linked copolymer (curve with blank circles, copolymers CP.sub.r-1, CP.sub.r-2 and CP.sub.r-3), for the non-cross-linked copolymer (curve with solid circles, copolymers CP-1, CP-2 and CP-3), and for the solid polymer electrolyte (curve with solid squares, electrolytes E-1a, E-2a and E-3a).

[0193] The glass transition and melting temperatures were obtained by measuring the thermodynamic properties by DSC using an appliance sold under the trade name DSC3 by Mettler-Toledo. The measurements were carried out with the following parameters: 10° C./min between −110° C. and 130° C.

[0194] FIGS. 3 and 4 show respectively a marked drop in the glass transition temperature and the melting temperature for the solid polymer electrolyte, with respect to those of the cross-linked and non-cross-linked copolymers. Such temperatures are adapted to obtain an electrolyte having good ion conduction at low temperature (polymer less crystalline), and capable of being implemented in a battery at a lower temperature.

Example 4: Electrochemical Characterizations

[0195] 4.1 Preparation of a Composite Positive Electrode

[0196] A composite positive electrode in the form of a film was prepared as follows: a mixture of 46.3 g LiFePO.sub.4, 1.2 g carbon black, 17.5 g copolymer CP-3, 6.5 g deionized water was introduced into a Brabender Plastograph. Mixing was carried out at 60° C. at 80 rpm.

[0197] The paste thus obtained was then calendered at 60° C. on a current collector made from aluminium coated with carbon. The film obtained was dried for 10 minutes at 100° C. before use.

[0198] The composite positive electrode obtained comprises 71.2% by weight of LFP active material, 26.9% by weight of copolymer CP-3 and 1.9% by weight of carbon black. It has a thickness of approximately 45 μm. The base weight obtained is 1.37 mAh/cm.sup.2.

[0199] An LMP accumulator was prepared by assembly under controlled atmosphere (dewpoint −50° C.) of: [0200] a film of solid polymer electrolyte E-3d as prepared beforehand, of thickness 58.8 μm, [0201] a sheet of metallic lithium of thickness approximately 50 μm, and [0202] a positive electrode as prepared beforehand.

[0203] To this end, the sheet of lithium and the film of solid polymer electrolyte are calendered at 70° C. and 5 bars to ensure good Li/electrolyte contact, then finally the composite positive electrode is calendered on the Li/electrolyte assembly in order to form the accumulator. The electrolyte film is placed between the metallic lithium film and the composite positive electrode film. A conductive wire is connected to the lithium and another conductive wire is connected to the current collector of the composite positive electrode.

[0204] The accumulator obtained, having a structure of sandwich type, is enclosed under vacuum in a pouch (known as a “coffee bag”) to be tested under uncontrolled atmosphere.

[0205] An accumulator under a pressure of 1 bar and a surface area of 2.8 cm.sup.2 was obtained.

[0206] During the operation of the accumulator, the TEGDME contained in the solid polymer electrolyte migrates at least partially into the composite positive electrode, in particular until equilibrium is reached between the quantity of TEGDME in the composite positive electrode, on the one hand, and in the solid polymer electrolyte, on the other hand.

[0207] FIG. 5 shows the voltage of the composite positive electrode in volts as a function of the discharge capacity (in mAh) at 60° C. and at different regimes (from D/9.4 to D/0.9), the load always being C/9.4. D represents the nominal capacity in mAh, and D/n, a discharge current corresponding to obtaining the capacity D in n hours. The polarization is proportional to the current density applied, which is typical for the single-ion type polymers, as the ion transport is ensured by migration only. Thus, the capacity values obtained are highly dependent on the low-voltage cutoff terminal.

[0208] FIG. 6 shows the curve of the discharge capacity (in mAh) and the coulombic efficiency (in %), as a function of the number of cycles, at 60° C. and at different discharge regimes (from D/9.4 to D/0.9), the charge always being C/9.4. Very good cycling strength is obtained over more than 60 cycles, associated with a faradic yield of 98.4%.

[0209] FIG. 7 shows the comparison of the power performances of two LMP batteries: [0210] a first LMP battery (curve with blank diamonds) operating at 60° C. comprising a solid polymer electrolyte E-3d and a composite positive electrode as defined above, and [0211] a second LMP battery currently used in industry (curve with solid circles) operating at 80° C. comprising a solid polymer electrolyte including 48% by weight of homo-PEO, 12% by weight of LiTFSi lithium salt, and 40% by weight of PVdF-co-HFP, and a positive electrode containing 68% by weight of LFP active material, 24% by weight of homo-PEO, 6% by weight of LiTFSi lithium salt and 2% by weight of carbon black and having a thickness of approximately 60 μm and a base weight of 1.5 mAh/cm.sup.2.

[0212] FIG. 7 shows the discharge capacity normalized by the nominal capacity (D/D.sub.0) as a function of the discharge regime (D/n) for the aforementioned batteries.

[0213] The results obtained are remarkable, taking account of the thickness of the solid polymer electrolyte (58.8 μm), the very high base weight of the electrode (1.37 mAh/cm.sup.2), for a composite positive electrode initially not plasticized. They show that the solid polymer electrolytes of the invention have better performance than that of the commercial electrolytes at high regimes and equal at low cycling regimes.