A facile and selective hydrothermal synthesis is performed to control the crystalline phases of cobalt hydroxides into alpha-Co(OH)(2) and beta-Co(OH)(2): alpha-Co(OH)(2), consisting of both octahedral and tetrahedral Co sites, is produced without ionic liquids, whereas beta-Co(OH)(2), containing octahedral Co sites, is synthesized in the presence of ionic liquids. The ionic liquids play significant role as co-solvent and template to tune the morphology of Co(OH)(2). alpha-Co(OH)(2) reveals flake-like structure, whereas beta-Co(OH)(2) exhibits nanorod-like network structure. The interlayer spacing of alpha-Co(OH)(2) is 8.24 angstrom, which is larger than 4.63 angstrom of beta-Co(OH)(2) due to the expansion of interlayer by the precursor Cl- anions. The presence of Cl- anions hinders the insertion of hydroxide ion into alpha-Co(OH)(2) interlayers, which shows the specific capacitance of 613 F g(-1) less than 1066 F g(-1) of beta-Co(OH)(2) at 2 A g(-1). When the current density increases up to 20 A g(-1), the capacitance retention of beta-Co(OH)(2) is 80%, greater than 70% of alpha-Co(OH)(2). Configuring beta-Co(OH)(2) and reduced graphene oxide as positive and negative electrodes, asymmetric supercapacitor delivers the maximum energy and power densities of 20.05 W h kg(-1) and 13.40 kW kg(-1) with the capacitance retention of 93% over 10,000 cycles.