Secondary batteries

Abstract

The present invention pertains to a secondary battery comprising at least one separator, said separator comprising at least one fluorinated polymer [polymer (F)], said polymer (F) comprising recurring units derived from vinylidene fluoride (VDF), hexafluoropropylene (HFP) and at least one (meth)acrylic monomer (MA) having formula (I) here below, wherein: —R.sub.1, R.sub.2 and R.sub.3, equal to or different from each other, are independently selected from a hydrogen atom and a C.sub.1-C.sub.3 hydrocarbon group, and —R.sub.OH is a C.sub.1-C.sub.5 hydrocarbon moiety comprising at least one hydroxyl group. ##STR00001##

Claims

1. A secondary battery comprising at least one separator, said separator comprising at least one fluorinated polymer [polymer (F)], said polymer (F) comprising recurring units derived from vinylidene fluoride (VDF), from 1.5% to 3.5% by moles of recurring units derived from hexafluoropropylene (HFP) and from 0.5% to 1.5% by moles of recurring units derived from hydroxyethyl acrylate (HEA) wherein the recurring units derived from vinylidene fluoride (VDF) are the complement to 100% by moles of total recurring units.

2. The secondary battery according to claim 1, said battery being an alkaline or alkaline-earth secondary battery.

3. The secondary battery according to claim 1, said battery being a Lithium-ion secondary battery.

4. The secondary battery according to claim 1, wherein the polymer (F) further comprises from 1% to 10% by moles of recurring units derived from at least one other hydrogenated comonomer [comonomer (H)] or at least one other fluorinated comonomer [comonomer (F)] or both.

5. The secondary battery according to claim 1, wherein polymer (F) comprises recurring units derived from vinylidene fluoride, 2.3% by moles of recurring units derived from hexafluoropropylene and 1.0% by moles of recurring units derived from hydroxyethyl acrylate wherein the recurring units derived from vinylidene fluoride (VDF) are the complement to 100% by moles of total recurring units.

6. The secondary battery according to claim 1, wherein the separator is a porous separator.

7. The secondary battery according to claim 6, wherein the porous separator is a microporous flat-sheet membrane or a non-woven cloth.

8. The secondary battery according to claim 1, wherein the separator is a dense separator.

9. The secondary battery according to claim 8, wherein the dense separator is a polymer electrolyte wherein at least one polymer (F) is swollen by a charge carrying medium.

Description

EXAMPLE 1

Preparation of VDF/HFP/HEA Polymer

(1) In a 4 lt. reactor equipped with an impeller running at a speed of 880 rpm were introduced in sequence 2455 g of demineralized water and 0.63 g of METHOCEL® K100 GR suspending agent.

(2) The reactor was vented and pressurized with nitrogen to 1 bar, then 8.55 g of a 75% by volume solution of t-amyl perpivalate initiator in isododecane were introduced into the reactor, followed by 107 g of HFP monomer and 947 g of VDF monomer. The reactor was then gradually heated to 52° C. to a final pressure of 110 bar. Temperature was maintained constant at 55° C. throughout the whole trial. Pressure was maintained constant at 110 bar throughout the whole trial by feeding a 19.96 g/l aqueous solution of HEA monomer to a total of 709 ml. After 510 minutes the polymerization run was stopped by degassing the suspension until reaching atmospheric pressure. The polymer so obtained was then recovered, washed with demineralised water and oven-dried at 50° C. (814 g). The polymer so obtained contained 2.3% by moles of HFP and 1.0% by moles of HEA, as determined by NMR.

COMPARATIVE EXAMPLE 1

Preparation of VDF/HFP/Acrylic Acid (AA) Polymer

(3) In a 4 lt. reactor equipped with an impeller running at a speed of 880 rpm were introduced in sequence 2460 g of demineralized water and 0.63 g of METHOCEL® K100 GR suspending agent.

(4) The reactor was vented and pressurized with nitrogen to 1 bar, then 9.98 g of a 75% by volume solution of t-amyl perpivalate initiator in isododecane and 5.35 g of diethyl carbonate were introduced into the reactor, followed by 0.5 g of acrylic acid (AA) monomer, 107 g of HFP monomer and 949 g of VDF monomer. The reactor was then gradually heated to 55° C. to a final pressure of 110 bar. Temperature was maintained constant at 55° C. throughout the whole trial. Pressure was maintained constant at 110 bar throughout the whole trial by feeding a 17.44 g/l aqueous solution of AA monomer to a total of 750 ml. After 516 minutes the polymerization run was stopped by degassing the suspension until reaching atmospheric pressure. The polymer so obtained was then recovered, washed with demineralised water and oven-dried at 50° C. (852 g).

(5) The polymer so obtained contained 2.5% by moles of HFP and 1.0% by moles of AA, as determined by NMR.

(6) Determination of Ionic Conductivity

(7) Films of polymers were dipped in an electrolyte solution of LiPF.sub.6 1M in ethylene carbonate/propylene carbonate (1:1 weight ratio) and stored at room temperature in a dry glove-box for 24 hours. The resulting polymer electrolyte was put between two stainless steel electrodes and sealed in a container.

(8) The resistance of the polymer electrolyte was measured and the ionic conductivity ([σ]) was calculated using the following equation:

(9) $\left[\sigma \right]=\frac{d}{\left(R_b\times S\right)}$
wherein d is the thickness of the film, R.sub.b is the bulk resistance and S is the area of the stainless steel electrode.

(10) General Procedure for the Manufacture of Dense Separators on Pilot Scale

(11) Polymer powder was processed by extrusion in a LEISTRITZ LSM 30/34 twin-screw extruder, equipped with 6 temperature zones and a 4 mm-2 holes die.

(12) Temperature set points were set as detailed in Table 1 here below:

(13) TABLE-US-00001 TABLE 1 Feed zone T1 T2 T3 T4 T5 140° C. 145° C. 150° C. 155° C. 155° C. 165° C.

(14) Screws speed was set at 100 rpm with a feed rate of 15%. Extruded strands were cooled in a water bath, dried, calibrated and cut in a pelletizer.

(15) Dense separators were manufactured from the pellets so obtained by flat cast film extrusion.

(16) Flat Cast Film Extrusion

(17) For manufacturing dense separators, pellets were processed in a single screw Braebender extruder (screw speed=25 rpm) equipped with 5 temperature zones, set at 200° C., and a 0.5 mm×100 mm tape die. Upon exit from the die, the molten tape was rolled onto two subsequent chill rolls kept at a temperature of 60° C., whose speed was adapted so as to obtain a film thickness of about 20-25 μm.

(18) Thermal Aging

(19) Dense separators obtained as detailed hereinabove were placed in an oven at 150° C. for 15 to 42 hours. To verify thermal stability of the separator, its transmittance value at 450 nm was measured before and after thermal aging using a UV/VIS spectrophotometer.

(20) It has been found through observation of transmittance values at 450 nm that dense separators obtained from the VDF/HFP/HEA polymer prepared according to Example 1 of the invention show no degradation upon thermal aging at 150° C. for 15 to 42 hours (see Table 2 here below).

(21) On the other hand, dense separators obtained from the VDF/HFP/AA polymer prepared according to comparative Example 1 do not withstand sustained thermal aging at 150° C. as shown by a significant decrease in transmittance values after 15 to 42 hours (see Table 2 here below).

(22) In view of the above, dense separators obtained from the VDF/HFP/HEA polymer prepared according to Example 1 of the invention exhibit enhanced thermal stability properties as compared with dense separators obtained from the VDF/HFP/AA polymer prepared according to comparative Example 1.

(23) TABLE-US-00002 TABLE 2 Transmittance [%, 450 nm] before thermal after thermal after thermal aging aging 15 hours aging 42 hours Example 1 90.7% 90.9% 91.2% C. Example 1 91.3% 87.8% 86.6%

(24) Moreover, dense separators obtained from the VDF/HFP/HEA polymer prepared according to Example 1 of the invention are advantageously uniform in structure after thermal aging at 150° C. for 15 to 42 hours as compared with dense separators obtained from the VDF/HFP/AA polymer prepared according to comparative Example 1, which undergo thermal degradation accompanied by a yellowing of the materials so obtained.

(25) Further, dense separators obtained from the VDF/HFP/HEA polymer prepared according to Example 1 of the invention and from the VDF/HFP/AA polymer prepared according to comparative Example 1 were found to have advantageously both an ionic conductivity of about 10.sup.−5 S/cm as measured as detailed hereinabove.

(26) General Procedure for the Manufacture of Dense Separators on Industrial Scale

(27) Dense separators were also manufactured from pellets of polymers prepared according to Example 1 and comparative Example 1 by extrusion using a single screw extruder having a diameter of 45 mm, equipped with a film die having a lip length of 450 mm and a lip width of 0.55 mm and three calendering rolls.

(28) Temperature profile was set as detailed in Table 3 here below:

(29) TABLE-US-00003 TABLE 3 Zone 1 (hopper) 190° C. Zone 2 (barrel) 210° C. Zone 3 (barrel) 220° C. Zone 4 (head) 230° C. Rolls  75° C.

(30) Extrusion conditions were set as detailed in Table 4 here below:

(31) TABLE-US-00004 TABLE 4 Pressure [bar] 37 Melt temperature [° C.] 270 Throughput rate [Kg/h] 4.3 Screw speed [rpm] 10 Calendering rolls speed [m/min] 3.7

(32) It has been found that homogeneous large (450 mm) dense separators having a thickness of about 15 μm are advantageously obtained by extrusion of pellets of the VDF/HFP/HEA polymer prepared according to Example 1 of the invention following procedure as detailed hereinabove. On the other hand, the VDF/HFP/AA polymer prepared according to comparative Example 1 degraded during extrusion following procedure as detailed hereinabove so that no dense separator was obtained under this procedure.

(33) It has been thus demonstrated that secondary batteries complying with performance and safety requirements were successfully obtained according to the present invention using polymer (F) separators, said separators being advantageously endowed with outstanding thermal stability properties while retaining good ionic conductivity values.

(34) Also, separators having a length of up to 450 mm or more may be advantageously obtained by processing the polymer (F) according to the present invention, thus enabling manufacturing correspondingly large-sized secondary batteries.