Secondary battery solid electrolyte composition and solid electrolyte prepared therefrom

Abstract

A solid electrolyte composition for a lithium secondary battery including a fluorine-based polymer having grafted thereon a unit comprising alkylene oxide group and a crosslinkable functional group. The polymer may be formed by a process including grafting a monomer on a fluorine-based polymer, where the monomer includes alkylene oxide group and a crosslinkable functional group. Also disclosed is a solid electrolyte for a secondary battery formed by thermally curing the composition. By graft copolymerizing a monomer including alkylene oxide group and a crosslinkable functional group on a fluorine-based polymer having high lithium ion conductivity, the solid electrolyte is capable of providing a solid electrolyte for a secondary battery having significantly enhanced solid electrolyte ion conductivity and electrochemical stability.

Claims

1. A solid electrolyte composition for a secondary battery comprising: a fluorine-based polymer having grafted thereon a unit comprising alkylene oxide group and a crosslinkable functional group, wherein the fluorine-based polymer is present in an amount of 0.2 parts by weight to 40 parts by weight with respect to 100 parts by weight of the composition.

2. The solid electrolyte composition for a secondary battery of claim 1, wherein the fluorine-based polymer comprises a structure of the following Chemical Formula 1: ##STR00005## wherein p, q and r are each independently an integer of 0≤p≤20,000, 1≤q≤22,000 and 0≤r≤15,000.

3. The solid electrolyte composition for a secondary battery of claim 1, wherein the grafted polymer comprises a structure of the following Chemical Formula 2: ##STR00006## wherein q, n, p, m and o are each independently an integer of 0≤q≤20,000, 1≤n≤22,000, 2≤p≤230, 1≤m≤200 and 2≤o≤50.

4. The solid electrolyte composition for a secondary battery of claim 1, wherein the alkylene oxide group is ethylene oxide or propylene oxide.

5. The solid electrolyte composition for a secondary battery of claim 1, wherein the crosslinkable functional group is one or more selected from the group consisting of a hydroxyl group, a carboxyl group and an isocyanate group.

6. The solid electrolyte composition for a secondary battery of claim 1, wherein the alkylene oxide group and the crosslinkable functional group have a molar ratio of 99.5:0.5 to 80:20.

7. The solid electrolyte composition for a secondary battery of claim 1, further comprising a multifunctional crosslinking agent having two or more functional groups capable of reacting with the crosslinkable functional group.

8. The solid electrolyte composition for a secondary battery of claim 7, wherein the multifunctional crosslinking agent is one or more selected from the group consisting of an isocyanate crosslinking agent, an epoxy crosslinking agent, an aziridine crosslinking agent and a metal chelate crosslinking agent.

9. The solid electrolyte composition for a secondary battery of claim 7, wherein the multifunctional crosslinking agent is comprised in 0.1 parts by weight to 6 parts by weight with respect to the whole 100 parts by weight of the electrolyte composition.

10. A solid electrolyte for a secondary battery formed by thermally curing the solid electrolyte composition for a secondary battery of claim 1.

11. The solid electrolyte for a secondary battery of claim 10, wherein the solid electrolyte has a thickness of 50 μm to 400 μm.

12. The solid electrolyte for a secondary battery of claim 10, further comprising a lithium salt in 30 parts by weight to 70 parts by weight with respect to 100 parts by weight of the electrolyte composition.

13. The solid electrolyte for a secondary battery of claim 12, wherein the lithium salt is one or more types selected from the group consisting of LiCl, LiBr, LiI, LiClO.sub.4, LiBF.sub.4, LiB.sub.10Cl.sub.10, LiPF.sub.6, LiCF.sub.3SO.sub.3, LiTFSI, LiCF.sub.3CO.sub.2, LiAsF.sub.6, LiSbF.sub.6, LiAlCl.sub.4, CH.sub.3SO.sub.3Li, (CF.sub.3SO.sub.2).sub.2NLi, LiN(SO.sub.2F).sub.2, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenylborate and lithium imide.

14. The solid electrolyte for a secondary battery of claim 10, which has ion conductivity of 1×10.sup.−6 S/cm to 4×10.sup.−5 S/cm.

15. A solid electrolyte composition for a secondary battery, comprising: a polymer, which is formed by a process comprising grafting a monomer on a fluorine-based polymer, wherein the monomer comprises alkylene oxide group and a crosslinkable functional group, wherein the polymer is present in an amount of 0.2 parts by weight to 40 parts by weight with respect to 100 parts by weight of the composition.

Description

Preparation Example 1 Preparation of PVDF-co-(PCTFE-g-(PEGMA-co-HEMA) Graft Copolymerized Using ATRP Method (A1)

(1) In a 1000 ml flask, 10 g of P(VDF-co-CTFE) having a weight average molecular weight (hereinafter, Mw) of 600,000 as a fluorine-based polymer, and 116 g of PEGMA and 3.35 g of HEMA as monomers to polymerize were introduced to 450 ml of an acetone solvent, and the result was stirred for 1 hour under a nitrogen condition.

(2) After that, 0.00266 g of CuCl.sub.2 as an ATRP reaction catalyst, 0.0091 g of TPMA as a ligand, and 0.245 g of Tin(II) 2-ethylhexanoate (Sn(EH).sub.2) as an initiator were introduced to the flask, and an ATRP reaction was progressed by stirring the result for 30 hours under a nitrogen condition.

(3) After completing the reaction, the produced polymer was immersed in an ether solvent 3 times to remove monomers that did not participate in the reaction. The finally obtained polymer was dried for 1 week under a vacuum condition to obtain a gel-type PVDF-co-(PCTFE-g-(PEGMA-co-HEMA) polymer.

Preparation Example 2 Preparation of PVDF-co-(PCTFE-g-(PEGMA-co-HEMA) Graft Copolymerized Using ATRP Method (A2)

(4) In a 1000 ml flask, 10 g of P(VDF-co-CTFE) having a weight average molecular weight (hereinafter, Mw) of 600,000 as a fluorine-based polymer, and 54 g of PEGMA and 1.5 g of HEMA as monomers to polymerize were introduced to 300 ml of an acetone solvent, and the result was stirred for 1 hour under a nitrogen condition.

(5) After that, 0.002 g of CuCl.sub.2 as an ATRP reaction catalyst, 0.0051 g of TPMA as a ligand, and 0.231 g of Sn(EH).sub.2 as an initiator were introduced to the flask, and an ATRP reaction was progressed by stirring the result for 30 hours under a nitrogen condition.

(6) After completing the reaction, the produced polymer was immersed in an ether solvent 3 times to remove monomers that did not participate in the reaction. The finally obtained polymer was dried for 1 week under a vacuum condition to obtain a gel-type PVDF-co-(PCTFE-g-(PEGMA-co-HEMA) polymer.

Comparative Preparation Example 1 Preparation of Polymer Using P(VDF-co-CTFE) Alone (B1)

(7) A P(VDF-co-CTFE) polymer having a weight average molecular weight (hereinafter, Mw) of 600,000 was prepared alone without graft copolymerizing PEGMA and HEMA, the monomers in Preparation Examples 1 and 2.

Comparative Preparation Example 2 Preparation of Polymer without Using P(VDF-co-CTFE) (B2)

(8) A polymer having a weight average molecular weight (hereinafter, Mw) of 230,000 was prepared by, in Preparation Examples 1 and 2, polymerizing PEGMA and HEMA in a molar ratio of 9:1 without using P(VDF-co-CTFE) as a main chain.

(9) Preparation Examples 1 and 2, and Comparative Preparation Examples 1 and 2 are shown in the following Table 1.

Experimental Example—Measurements of Glass Transition Temperature and H.SUB.Tm

(10) Measurement Device: DSC discovery 250 (TA instruments)

(11) Measurement Condition: 20° C. to 100° C. (1st cycle), −90° C. to 200° C. (2nd cycle), 10° C./min, N.sub.2 atm

(12) 10 mg of each of the polymers prepared in Preparation Examples 1 and 2, and Comparative Preparation Examples 1 and 2 was taken and placed in the DSC sample pan, and injected into the cell of the device. After measuring under the above-described temperature condition, an inflection point of a part where the slope changes were taken from the graph of temperature and heat capacity, and this was measured as a glass transition temperature (T.sub.g). In the graph of temperature and heat capacity, another endothermic peak appeared after the glass transition temperature, and this point was Tm (melting point) and the width of the peak at which the Tm appeared was measured as H.sub.Tm. Having a large H.sub.Tm means requiring large energy for crystals to melt, and having a larger H.sub.Tm means the polymer having higher crystallinity.

(13) TABLE-US-00001 TABLE 1 Content P(VDF-co- of CTFE): Fluorine- PEGMA: Based Glass HEMA polymer Transition (Molar in Temperature H.sub.Tm Polymer Ratio) Polymer Mw (PDI) (Tg, ° C.) (J/g) A1 1:13.5:1.5  10% 1,800,00 −64  0.58 0 (6.7) A2 1:6.3:0.7  25% 1,010,00 −58  4.28 0 (5.7) B1 1:0:0 100%   600,000 −25 16.18 (—) B2 0:9:1  0%   230,000 −73 — (3.2) (PDI: polydispersity index)

Example—Preparation of Solid Electrolyte

(14) A solution obtained by dissolving 5 g of the polymer PVDF-co-(PCTFE-g-(PELMA-co-HEMA) prepared in each of Preparation Examples 1 and 2, trifunctional toluene diisocyanate as a multifunctional crosslinking agent and LiTFSI as a lithium salt while varying a content thereof as in the following Table 2 in 50 ml of a tetrahydrofuran (hereinafter, THF) solvent was stirred for 6 hours to prepare a homogeneous solution. The solution was casted on a Teflon plate with a size of 2 cm×2 cm, and the result was dried for 6 hours at room temperature in a dry room, and then heated for 1 hour at a temperature of 120° C. to progress a thermal curing reaction. After that, the solid film was removed from the Teflon plate using a knife to obtain a solid electrolyte for a secondary battery.

Comparative Example—Preparation of Solid Electrolyte

(15) A solution obtained by dissolving 5 g of the polymer prepared in each of Comparative Preparation Examples 1 and 2, trifunctional toluene diisocyanate as a multifunctional crosslinking agent and LiTFSI as a lithium salt while varying a content thereof as in the following Table 2 in 50 ml of a THF solvent was stirred for 6 hours to prepare a homogeneous solution. The solution was casted on a Teflon plate with a size of 2 cm×2 cm, and the result was dried for 6 hours at room temperature in a dry room, and then heated for 1 hour at a temperature of 120° C. to progress a thermal curing reaction. After that, the solid film was removed from the Teflon plate using a knife to obtain a solid electrolyte for a secondary battery.

(16) Examples 1 to 5 and Comparative Examples 1 to 6 are shown in the following Table 2.

(17) TABLE-US-00002 TABLE 2 Content of Content of Multifunctional LiTFSI Crosslinking Polymer (wt %) Agent.sup.A Example 1 A1 20 1:0.5 Example 2 A1 30 1:0.5 Example 3 A1 40 1:0.5 Example 4 A1 50 1:0.5 Example 5 A2 40 1:0.5 Comparative B1 30 Example 1 Comparative B1 40 — Example 2 Comparative B2 30 1:0.5 Example 3 Comparative B2 20 1:1   Example 4 Comparative B2 30 1:1   Example 5 Comparative B2 40 1:1   Example 6 (A: crosslinkable functional group in polymer:crosslinkable functional group in multifunctional crosslinking agent (molar ratio))

Experimental Example—Measurement on Ion Conductivity of Electrolyte

(18) Ion conductivity of the solid electrolyte prepared in each of Examples 1 to 5 and Comparative Examples 1 to 6 was obtained using the following Mathematical Formula 1 after measuring the impedance.

(19) For the measurement, a film sample of the solid electrolyte having certain width and thickness was prepared. A SUS substrate having excellent electron conductivity was brought into contact with both surfaces of the plate-shaped sample as an ion blocking electrode, and then an alternating current voltage was applied through the electrodes on both surfaces of the sample. Herein, an amplitude range was set to a measurement frequency of 0.1 Hz to 10 MHz as the applied condition. Resistance of the bulk electrolyte was obtained from an intersection point (R.sub.b) where a half-circle or a straight line of the measured impedance trajectory meets a real number axis, and ion conductivity of the polymer solid electrolyte membrane was calculated from the sample width and thickness. The results are shown in the following Table 3.

(20) $\begin{array}{cc}\sigma \left(S.Math.{\mathrm{cm}}^{-1}\right)=\frac{1}{R_b}\frac{t}{A}& \left[\mathrm{Mathematical}\mathrm{Equation}1\right]\end{array}$

(21) σ: ion conductivity

(22) R.sub.b: intersection point between impedance trajectory and real number axis

(23) A: sample width

(24) t: sample thickness

(25) TABLE-US-00003 TABLE 3 Film Formation Ion Conductivity (S/cm) Example 1 O 2.7 × 10.sup.−17 Example 2 O 1.9 × 10.sup.−6  Example 3 O 3.2 × 10.sup.−5  Example 4 O 4.5 × 10.sup.−5  Example 5 O 2.4 × 10.sup.−5  Comparative O 8.5 × 10.sup.−7  Example 1 Comparative O 2.1 × 10.sup.−6  Example 2 Comparative X 2.7 × 10.sup.−5  Example 3 Comparative O 3.5 × 10.sup.−6  Example 4 Comparative O 6.7 × 10.sup.−6  Example 5 Comparative O 9.8 × 10.sup.−17 Example 6

(26) As shown in Table 3, ion conductivity of the solid electrolyte for a secondary battery comprising a polymer in which a monomer comprising alkylene oxide group and a crosslinkable functional group is grafted on a fluorine-based polymer was measured to be higher compared to the comparative examples with no grafting, and it was seen that ion conductivity was enhanced. In Comparative Example 3, it was seen that the electrolyte membrane according to the present invention was not formed although ion conductivity was high.