Separator for rechargeable lithium battery and rechargeable lithium battery including the same

1. A separator for a rechargeable lithium battery, the separator comprising:

a substrate, and

a heat-resistant porous layer on at least one side of the substrate, the heat-resistant porous layer including an imide-based copolymer,

wherein the imide-based copolymer includes a first repeating unit represented by Chemical Formula 1 and a second repeating unit represented by Chemical Formula 2:

wherein, in Chemical Formulae 1 and 2,

L 1 includes a C1 to C20 alkylene group, a C2 to C20 alkenylene group, a C2 to C20 alkynylene group, a C3 to C20 cycloalkylene group, a C3 to C20 cycloalkenylene group, a C4 to C20 cycloalkynylene group, or a C6 to C30 arylene group, in which at least one hydrogen of the alkylene group, alkenylene group, alkynylene group, cycloalkylene group, cycloalkenylene group, cycloalkynylene group, or arylene group is substituted by a fluorine atom,

L 2 includes a tetravalent aromatic group, a tetravalent aliphatic group, or a tetravalent alicyclic group, in which the aliphatic group and the alicyclic group each independently include a linking group of a single bond, —CO—, —O—, —SO 2 —, or —S— therein,

Q 1 and Q 2 each independently include a divalent aromatic group,

R and R′ each independently include a C1 to C20 alkyl group, a C2 to C20 alkenyl group, a C2 to C20 alkynyl group, a C3 to C20 cycloalkyl group, a C3 to C20 cycloalkenyl group, a C4 to C20 cycloalkynyl group, or a C6 to C30 aryl group,

a and a′ are each independently an integer of 0 to 3,

* indicates a bonding site to an adjacent atom, and

a ratio of m and n is about 1:9 to about 9:1.

2. The separator as claimed in claim 1 , wherein, in Chemical Formula 1, L 1 includes a C1 to C10 alkylene group substituted with at least one fluorine atom.

3. The separator as claimed in claim 1 , wherein, in Chemical Formula 2, L 2 is a tetravalent linking group represented by one of Chemical Formulae 3-1 to 3-7:

wherein, in Chemical Formulae 3-1 to 3-7,

X 1 is —CO—, —O—, —SO 2 —, or —S—,

X 2 and X 3 are each independently a single bond, —CO—, —O—, —SO 2 —, or —S—,

R 1 to R 9 each independently include a C1 to C20 alkyl group, a C2 to C20 alkenyl group, a C2 to C20 alkynyl group, a C3 to C20 cycloalkyl group, a C3 to C20 cycloalkenyl group, a C4 to C20 cycloalkynyl group, or a C6 to C30 aryl group,

* indicates a bonding site to an adjacent atom, and

a 1 to a 9 are each independently an integer of 0 to 3.

4. The separator as claimed in claim 1 , wherein, in Chemical Formula 2, L 2 is a tetravalent linking group represented by one of Chemical Formulae 4-1 to 4-5:

wherein, in Chemical Formula 4-5,

* indicates a bonding site to an adjacent atom, and

X 1 is —CO—, —O—, —SO 2 —, or —S—.

5. The separator as claimed in claim 1 , wherein, in Chemical Formulae 1 and 2, Q 1 and Q 2 are each independently a divalent linking group represented by one of Chemical Formulae 5-1 to 5-3:

wherein, in Chemical Formulae 5-1 to 5-3,

X 4 is a single bond, —CO—, —O—, —SO 2 —, —S—, a substituted or unsubstituted C1 to C20 alkylene group, a substituted or unsubstituted C2 to C20 alkenylene group, a substituted or unsubstituted C2 to C20 alkynylene group, a substituted or unsubstituted C3 to C20 cycloalkylene group, a substituted or unsubstituted C3 to C20 cycloalkenylene group, a substituted or unsubstituted C4 to C20 cycloalkynylene group, or a substituted or unsubstituted C6 to C30 arylene group,

R 10 to R 14 each independently include a C1 to C20 alkyl group, a C2 to C20 alkenyl group, a C2 to C20 alkynyl group, a C3 to C20 cycloalkyl group, a C3 to C20 cycloalkenyl group, a C4 to C20 cycloalkynyl group, or a C6 to C30 aryl group,

* indicates a bonding site to an adjacent atom, and

a 10 to a 14 are each independently an integer of 0 to 4.

6. The separator as claimed in claim 1 , wherein, in Chemical Formulae 1 and 2, Q 1 and Q 2 are each independently a divalent linking group represented by Chemical Formulae 6-1 or 6-2:

wherein, in Chemical Formula 6-2,

* indicates a bonding site to an adjacent atom, and

X 4 is a single bond, —CO—, —O—, —SO 2 —, —S—, or a substituted or unsubstituted C1 to C20 alkylene group.

7. The separator as claimed in claim 1 , wherein, in Chemical Formulae 1 and 2, a ratio of m and n is about 7:3 to about 9:1.

8. The separator as claimed in claim 1 , wherein the imide-based copolymer has a weight average molecular weight of about 10,000 to about 500,000.

9. The separator as claimed in claim 1 , wherein the imide-based copolymer has a decomposition initiation temperature of about 350° C. to about 700° C. based on a thermogravimetric analysis.

10. The separator as claimed in claim 1 , wherein:

the heat-resistant porous layer further includes an inorganic particle, and

the inorganic particle includes Al 2 O 3 , SiO 2 , B 2 O 3 , Ga 2 O 3 , TiO 2 , SnO 2 or a combination thereof.

11. The separator as claimed in claim 10 , wherein the inorganic particle is present in an amount of about 50 wt % to about 95 wt %, based on a total weight of the imide-based copolymer and the inorganic particle.

12. The separator as claimed in claim 10 , wherein the heat-resistant porous layer further includes polyvinylidene fluoride (PVdF), a polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) copolymer, polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-vinylacetate copolymer, polyethyleneoxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinyl alcohol, cyanoethyl cellulose, cyanoethylsucrose, pullulan, carboxylmethyl cellulose, an acrylonitrile-styrene-butadiene copolymer, or a combination thereof as a binder.

13. The separator as claimed in claim 12 , wherein the inorganic particle is present in an amount of about 75 wt % to about 95 wt %, based on a total weight of the imide-based copolymer, the inorganic particle, and the binder.

14. The separator as claimed in claim 1 , wherein a shrinkage ratio of the separator is less than about 5% when calculated according to Equation 1:

Shrinkage ratio (%)=[( L 0− L 1)/ L 0]×100  [Equation 1]

wherein, in Equation 1, L0 indicates an initial length of a separator and L1 indicates a length of a separator after being allowed to stand at 200° C. and for 1 hour.

15. A rechargeable lithium battery comprising the separator as claimed in claim 1 .