Porous film, separator including the same, electrochemical device including separator, and method of preparing porous film

A porous film comprising cellulose nanofibers impregnated with a carbonate-based solvent-containing electrolyte solution, wherein the porous film has a reaction heat of 150 J/g or less at a temperature ranging from about 30° C. to about 300° C., the reaction heat being measured by differential scanning calorimetry (DSC), wherein the porous film comprises first cellulose nanofibers comprising paper mulberry pulp cellulose nanofibers, and second cellulose nanofibers that are different from the first cellulose nanofibers and comprise microbial or bacterial cellulose nanofibers, wherein the second cellulose nanofibers comprise carboxyl group-containing cellulose nanofibers,

• wherein the carboxyl group is bound to a carbon atom of a pyranose ring of the second cellulose nanofibers and is represented by Formula 1 or 2 below:
• —R1—O—R2—COOM Formula 1
• —O—R2—COOM Formula 2,
• wherein R1 and R2 are each independently a substituted or unsubstituted C1-C10 alkylene group, and M is hydrogen or an alkali metal,
• and wherein the porous film comprises about 10 parts by weight to about 300 parts by weight of the second cellulose nanofibers per 100 parts by weight of the first cellulose nanofibers.