Polymeric Binder for Fused Salts Electrolytes Based Batteries

Abstract

The present invention relates to polymeric binders of formula: [(—CH2CF2—)x′(—CF2CF2—)y′[—CH2CH(R)—]z′]m wherein: x′+y′+z′=1, only one x′, y′ or z′ could be simultaneously equal to zero; R is an alkyl radical CnH2n+1— with 0≦n≦8, 10≦m≦106.

Description

BRIEF DESCRIPTION OF THE FIGURES

In drawings which illustrates embodiments of the present invention;

FIG. 1 is a comparative Power Ragone of coin cells battery for the binder PVDF-TFE-PP and PVDF-HFP of diethylmethylsulfonium-TFSI+1 m LiTFSI at 60° C.,

FIG. 2 illustrates examples of the normalized cycling stability of coin cells batteries mounted with an anode prepared with a nanosize Li₄Ti₅O₁₂ material using the PVDF-TFE-PP and PVDF-HFP binder. The examples were performed using the molten salt ethylmethylimidazolium-TFSI+1 m LiTFSI at 25° C.,

FIG. 3A represents an electron microscopy photograph which illustrates the characterization dispersion with nanotitanate PVDF-TFE-PP binder,

FIG. 3B is a larger view of the photograph of FIG. 3A,

FIG. 3C represents an electron microscopy photograph which illustrates the characterization dispersion with nanotitanate PVDF-HFP (i.e. PVDF) binder,

FIG. 3D is a larger view of the photograph of FIG. 3C, and;

FIG. 4 illustrates examples of the normalized cycling stability of coin cells batteries mounted with an anode prepared with a micro size Li₄Ti₅O₁₂ material using the PVDF-TFE-PP and PVDF-HFP binder. The examples are for the molten salt Ethylmethylimidazolium-TFSI+1 m LiTFSI at 25° C.

DETAILED DESCRIPTION OF THE INVENTION

In view to qualify the binder, the onium has particularly been chosen from N,N′-alkyl-imidazolium, tetraalkylammonium and trialkylsulfonium, with alkyls substituents particularly containing 1 to 3 carbon atoms and such as counter anion of the onium is (FSO₂)₂N⁻ or/and (CF₃SO₂)₂N⁻, and wherein metallic salt is (FSO₂)₂NLi or/and (CF₃SO₂)₂NLi.

First of all, a film of PVDF-HFP copolymer Poly(vinylidene fluoride-co-hexafluoropropylene), produced by Solvay (Solef® 20810/1001) and a film of Poly(tetrafluoroethylene-co-vinylidene fluoride-co-polypropylene), named PVDF-TFE-PP, obtained from Aldrich (56% wt TFE and 27% wt VDF) have been respectively placed in a solution of widely used N-methyl-N′-ethyl-imidazolium·TFSI. After 24 hours at 80° C., the PVDF-HFP copolymer present an uptake of ionic liquids>20% wt while the PVDF-TFE-PP present almost no uptake of solvent. This insolubility in the imidazolium based ionic liquids is an important property for a binder and a strong argument in favor of the described binders.

In view to evaluate the influence of the binder on power characteristic of the battery with a Ragone plot, one with a LiCoO₂ cathode (2.5 C/cm²) using PVDF-HFP, such as disclosed herein, and a Li₄Ti₅O₁₂ anode (2 C/cm²) using PVDF-HFP, as disclosed herein, was compared with an equivalent battery using PVDF-TFE-PP binder instead of PVDF-HFP. The battery was assembled with a paper separator and used diethyl-methyl-sulfonium·TFSI with 1 M LiTFSI as the electrolyte.

It appears as described in the following Ragone plot that the power capability of the battery at 60° C. is strongly improved with PVDF-TFE-PP binder, especially considering that the capacity of both cathode at C/20 rate and 25° C. are equivalent.

A general procedure to prepare anode and cathode electrodes is provide in example 2 and 3 with PVDF-HFP copolymer, those electrodes was used as reference electrodes to qualify alternative binders.

EXAMPLE 1

Film of PVDF-HFP copolymer Poly(vinylidene fluoride-co-hexafluoropropylene), produced by Solvay (Solef® 20810/1001) and a film of Poly(tetrafluoroethylene-co-vinylidene fluoride-co-polypropylene), named PVDF-TFE-PP, obtained from Aldrich (56% wt TFE and 27% wt VDF) have been respectively placed in a solution of 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. After 24 hours at 80° C., the PVDF-HFP copolymer present an uptake of ionic liquids>20% wt while the PVDF-TFE-PP present negligible (<2% wt) uptake of solvent. This insolubility in ionic liquids is an important property for a binder and a strong argument in favor of the described binders.

EXAMPLE 2

The following example described the general electrode preparation procedure. 85 gr of LiCoO₂ (approx. 5 μm diameter) and 10 gr of carbon black (CPChem, Shawinigan Black®) were thoroughly mixed in an agate crusher with the equivalent of 5 gr Poly(vinylidene fluoride-co-hexafluoropropylene), produced by Solvay (Solef® 20810/1001), dissolved in NMP at 4% wt concentration. 125 ml of NMP were also added to adjust the viscosity of the solution for coating. After crushing up to obtain a dispersed mixture, characterized with a Gardco®) fineness of grind gages, this past was coated on a 20 μm dual side coated conductive aluminum (Intellicoat, Product Code 2651), with a Gardco® universal blade applicator of 7 mils gate clearance. After evaporation of the solvent in air, the cathode electrode (85% wt LiCoO₂, 10% wt carbon and 50% wt binder) was dried under vacuum at 60° C. during 24 hours and store under Helium in a glove box. The film has a thickness of ≈47 μm and a porosity of 152%. This electrode has a 2 C/cm² reversible capacity. Depending on the composition of the coating mixture, clearance of the blade, it is possible to obtain electrode with a thickness comprise between 10 and 100 μm and porosity comprise between 100 and 300%. Porosity is adjusted if necessary by lamination or compression on a carver press.

EXAMPLE 3

An anode of 30-50 nm lithium titanate spinel Li₄Ti₅O₂ (Altair Nanomaterials Inc.) was prepared with the same composition (85% wt Li₄Ti₅O₁₂, 10% wt carbon and 5% wt binder) as in example 2. The past was coated on a 20 μm dual side coated conductive aluminum (Intellicoat, Product Code 2651), with a 12 mils gate clearance of the blade applicator. After drying as in example 1, a film of 50 μm and 209% porosity was obtained. This film has a 2.5 C/cm² reversible capacity. Depending on the composition of the coating mixture, clearance of the blade, it is possible to obtain electrode with a thickness comprise between 10 and 100 μm and porosity comprise between 100 and 300%. Porosity is adjusted if necessary by lamination or compression on a carver press.

EXAMPLE 4

Two coins cells batteries were assembled, first one with a LiCoO₂ cathode (≈2 C/cm²) using PVDF-HFP, such as disclosed in example 2, and a Li₄Ti₅O₁₂ anode (≈2.5 C/cm²) using PVDF-HFP, as disclosed in example 3, and a second equivalent one using PVDF-TFE-PP binder, as described in example 1, instead of PVDF-HFP. The two batteries were assembled with a 20 μm porous paper separator (alkylated cellulose) previously soaked in an electrolyte solution composed of diethyl(methyl)sulfonium bis(trifluoromethyl-sulfonyl)imide ionic liquid containing 1 Mol/kg LiTFSI. Those batteries tests at 25° C. in slow scan voltammetry (C/20) between 1.5 and 2.6 V vs Li⁺/Li⁰ presents similar capacities.

The coin cells was charged at a rate of C/3 and maintained at 2.6 Volts for 2.5 hours. The discharge rate in stability test was 1 C.

It appears as described in FIG. 1 Ragone plot that the power capability of the battery at 60° C. is improved with PVDF-TFE-PP binder, especially considering that the capacity of both cathode at C/20 rate and 25° C. are equivalent.

EXAMPLE 5

In view to evaluate the influence of the binder on power characteristic of the battery with a Ragone plot, one with a LiCoO₂ cathode (2.5 C/cm²) using PVDF-HFP, such as disclosed in example 2, and a Li₄Ti₅O₁₂ anode (2 C/cm²) using PVDF-HFP, as disclosed in example 3, was compared with an equivalent battery using PVDF-TFE-PP binder instead of PVDF-HFP. he battery was assembled with a paper separator and used diethyl-methyl-sulfonium-TFSI with 1 M LITFSI as the electrolyte.

It appears as described in the Ragone plot of FIG. 1 that the power capability of the battery at 60° C. is strongly improved with PVDF-TFE-PP binder, especially considering that the capacity of both cathode at C/20 rate and 25° C. are equivalent.

EXAMPLE 6

Two coins cells batteries were assembled, first one with a LiCoO₂ cathode (≈2 C/cm²) using PVDF-HFP, such as disclosed in example 2, and a Li₄Ti₅O₁₂ anode (≈2.5 C/cm²) using PVDF-HFP, as disclosed in example 3, and a second equivalent one using PVDF-TFE-PP binder, as described in example 1, instead of PVDF-HFP. The two batteries were assembled with a 20 μm porous paper separator (alkylated cellulose) previously soaked in an electrolyte solution composed of ethyl(methyl)imidazolium bis(trifluoromethylsulfonyl)-imide ionic liquid containing 1 Mol/kg LiTFSI. Those batteries tests were performed at 25° C. in slow scan voltammetry (C/20) between 1.5 and 2.6 V vs Li⁺/Li⁰ and presents similar capacities.

The coin cells was charged at a rate of C/3 and maintained at 2.6 Volts for 2.5 hours. The discharge rate in stability test was 1 C.

It appears as described in FIG. 2 that batteries made from a nanotitanate (e.g., Li₄Ti₅O₁₂ (≈30 to 50 nm)) and the PVDF-TFE-PP has a long-term cycling stability improved over a similar battery comprising the PVDF-HFP.

EXAMPLE 7

Two soft cells batteries were assembled, first one with a LiCoO₂ cathode (≈2 C/cm²) using PVDF-HFP, such as disclosed in example 2, and a Li₄Ti₅O₁₂ anode (≈2.5 C/cm²) using PVDF-HFP, as disclosed in example 4, and a second equivalent one using PVDF-TFE-PP binder, as described in example 1, instead of PVDF-HFP. The two batteries were assembled with a 20 μm porous paper separator (alkylated cellulose) previously soaked in an electrolyte solution composed of ethyl(methyl)imidazolium bis(fluoromethylsulfonyl)-imide ionic liquid containing 1 Mol/kg LiTFSI. Those batteries tests were performed at 25° C. in slow scan voltametry (C/20) between 1.5 and 2.6 V vs Li⁺/Li⁰ and presents similar capacities.

The coin cells was charged at a rate of C/3 and maintained at 2.6 Volts for 2.5 hours. The discharge rate in stability test was 1 C.

It appears as described in FIG. 4 that batteries made from a microtitanate (e.g., Li₄Ti₅O₁₂≈1 to 30 μm) and the PVDF-TFE-PP has a long-term cycling stability improved over a similar battery comprising the PVDF-HFP.

FIGS. 3A to 3D represent electron microscopy photographs which illustrates the dispersion with nanotitanate PVDF-TFE-PP binder and PVDF-HFP (i.e, PVDF). These photographs indicate that PVDF-TFE-PP has a better dispersion than PVDF-HFP. Therefore an anode using terpolymer of [(—CH₂CF₂—)_x′(—CF₂CF₂—)_y′[—CH₂CH(R)—]_z′]_m as sold by Aldrich under the reference 455,458-3 (CAS 54675-89-7) present an improved dispersion of active material relatively to a PVDF based electrodes.

Claims

1. An electrode material comprising: one electroactive compound;one carbonaceous conductivity enhancer; andone polymeric binder of formula [(—CH2CF2—)x′(—CF2CF2—)y′[—CH2CH(R)—]z′]m wherein: x′+y′+z′=1, provided that only one x′, y′ or z′ could be simultaneously equal to zero.R is an alkyl radical CnH2n+1— with 0≦n≦8, and10≦m≦106.

2. The electrode material of claim 1 wherein x′, y′ and z′ are comprised between 0.05 and 0.95.

3. The electrode material of claim 2 wherein (—CF2CF2—) account for 45-65% wt, (—CF2CH2—) account for 15-35% wt, [—CH2CH(R)—] account for 5-25% wt and R is H or CH3.

4. The electrode material of claim 1 wherein x′ or y′ or z′ equal zero.

5. The electrode material of claim 4 wherein x′ or y′=0 and z′ is comprised between 0.05 and 0.95.

6. The electrode material of claim 5 wherein R is H or CH3 and [—CH2CH(R)—] account for 10-90% wt.

7. The electrode material of claim 4 wherein z′=0 and x′ is comprised between 0.05 and 0.95.

8. The electrode material of claim 7 wherein R is H or CH3 and (—CF2CF2—) account for 10-90% wt.

9. The electrode material of claim 1 wherein the electroactive compound inserts and releases lithium cation at potential≦2 Volts vs Li+/Li0.

10. The electrode material of claim 9 wherein the electroactive compound is an oxide comprising a titanium spinel Li4x+3yTi5−xO12 wherein 0≦x, y≦1, or an oxide Li[Ti1.67Li0.33−yMy]O4 wherein 0≦y≦0.33 and wherein M=Mg and/or Al in which the M cations are partially replaced by one or more suitable monovalent, divalent, trivalent or tetravalent metal M′ cations to provide an electrode Li[Ti1.67Li0.33−yMy−zM′z]O4 in which z<y, or a double nitride of a transition metal and lithium comprising Li3−xCo2N wherein 0≦x≦1 or having a structure of the antifluorite type comprising Li3FeN2 or Li7MnN4, or MoO2, or WO2, or mixtures thereof.

11. The electrode material of claim 1 wherein the electroactive compound inserts and releases lithium cation at potential≧2 Volts vs Li+/Li0.

12. The electrode material of claim 11 wherein the electroactive compound is a double oxide of cobalt and lithium optionally partially substituted of general formula Li1aCo1−x+yNixAlyO2 wherein 0<x+y<1; 0<y<0.3; 0<a<1, or LiyN1−x−zCoxAlzO2 wherein 0≦x+y≦1 and 0≦y≦1, or a manganese spinel Li2Mn2−xMxO4 wherein M is Cr, Al, V, Ni; 0≦x≦0.5, or a double phosphate of the Olivine or Nasicon structure comprising Li1−aFe1−xMnxPO4 and Li1−x+2aFe2P1−xSixO4 wherein 0<x, a<1, or LiCoPO4 wherein Co is substituted by one or more suitable metal cation, or LiNiO2 wherein Ni is substituted by one or more suitable metal cation, or a mixtures thereof.

13. The electrode material of claim 9 wherein the electroactive compounds has a mean diameter size of between 10 nm to 30 μm.

14. The electrode material of claim 1 wherein the carbonaceous conductivity enhancer is carbon black or graphite in powder or fiber form, or a mixture thereof.

15. The electrode material of claim 14 wherein the conductivity enhancer has a mean diameter of between 10 nm and 30 μm.

16. The electrode material of claim 9 wherein the electroactive material accounts for 45 to 95% wt, the carbonaceous carbon additive accounts for 3 to 30% wt and the polymeric binder accounts for 3 to 30% wt.

17. The electrode material of claim 16 wherein the porosity of the electrode is between 30 and 300%.

18. The electrode material of claim 17 wherein the porosity is adjusted by further lamination process.

19. The electrode material of claim 1 wherein the electrode is prepared by coating technology from a suspension of components in a solvent, or a mixture of solvent, in which the polymeric binder is soluble.

20. The electrode material of claim 19 wherein the electrode material is coated on a current collector especially aluminum.

21. An electrochemical generator having at least one electrode material from claim 1.

22. The electrochemical generator of claim 21 having one positive electrode according to claim 11, one negative electrode according to claim 9, and one separator placed between the two electrodes and wherein both porous electrodes and separator are filled by an organic ionic liquids electrolyte, wherein said electrolyte is an electrolytic combination of: at least one ionic compound having one cation of the onium type with at least one heteroatom comprising N, O, S or P bearing a positive charge and the anion including, in whole or in part, at least one imide ion choose from (FSO2)2N−and (CF3SO2)2N−, or a mixtures thereof; andat least one other component comprising a metallic salt and eventually an aprotic co-solvent with a boiling point>150° C.

23. The electrochemical generator of claim 22 wherein the separator is a porous polymer matrix or a gel formed between a polymer and the organic ionic liquids electrolyte.

24. The electrochemical generator of claim 22 wherein the onium is choose from ammonium (R4N+), phosphonium (R4P+), oxonium (R3O+), sulfonium (R3S+), guanidinium [(R2N)3C+], amidinium [(R2N)2C+R′], imidazolium [(RN)2(CR′)3], pyrazolium [(RN)2(CR′)3], pyrolidinium [(R2N(CR′)3] or a mixture thereof, and wherein: R, R2, R3, and R4 and are independently choose from:a linear, branched or cyclic alkyl, alkenyl, oxaalkyl, oxaalkenyl, azaalkyl, azaalkenyl, thiaalkyl, thiaalkenyl, or dialkylazo group comprising from 1 to 18 atoms;a cyclic or heterocyclic aliphatic radical of from 4 to 26 carbon atoms optionally comprising at least one lateral chain comprising one or more heteroatoms;an aryl, arylalkyl, alkylaryl and alkenylaryl group of from 5 to 26 carbon atoms optionally comprising one or more heteroatoms in the aromatic nucleus;groups comprising aromatic or heterocyclic nuclei, condensed or not, optionally comprising one or more atoms of nitrogen, oxygen, oxygen, sulfur or phosphorus;and wherein two adjacent groups R can form a cycle or a heterocycle of from 4 to 9 carbon atoms, and wherein one or more R groups on the same cation can be part of polymeric chain;and wherein R′ is H or R as defined above.

25. The electrochemical generator of claim 22 wherein the metallic salt is LiN(FSO2)2 or LiN(CF3SO2)2.

26. A polymeric binder of formula [(—CH2CF2—)x′(—CF2CF2—)y′[—CH2CH(R)—]z′]m wherein: x′+y′+z′=1, provided that only one x′, y′ or z′ could be simultaneously equal to zero,R is an alkyl radical CnH2n+1 with 0≦n≦8,10≦m≦106,wherein polymeric binder swelling in an organic ionic liquids is less than 5%.

27. The polymeric binder of claim 26, its swelling in an organic ionic liquids is less than 2%.

28. The polymeric binder of claim 26 wherein the polymer is such as (—CF2CF2) account for 45-65% wt, (—CF2CH2—) account for 15-35% wt, [—CH2CH(R)—] account for 5-25% wt and R is H or CH3.