A facile and selective hydrothermal synthesis is performed to control the crystalline phases of cobalt hydroxides into α-Co(OH)2 and β-Co(OH)2: α-Co(OH)2, consisting of both octahedral and tetrahedral Co sites, is produced without ionic liquids, whereas β-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. α-Co(OH)2 reveals flake-like structure, whereas β-Co(OH)2 exhibits nanorod-like network structure. The interlayer spacing of α-Co(OH)2 is 8.24 Å, which is larger than 4.63 Å of β-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 α-Co(OH)2 interlayers, which shows the specific capacitance of 613 F g─1 less than 1066 F g─1 of β-Co(OH)2 at 2 A g─1. When the current density increases up to 20 A g─1, the capacitance retention of β-Co(OH)2 is 80%, greater than 70% of α-Co(OH)2. Configuring β-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.