In this work, various nickel cobalt double hydroxide nanoflowers with different Ni/Co ratios (denoted as NixCo(1-x)(OH)(2)), assembled by filmy nanoflakes, are prepared via a facile template-free hydrothermal process. The sizes of these nanoflowers are easily tuned by the Ni/Co ratio in the precursors. The formation mechanism of flower-like nickel cobalt hydroxide, based on the synergistic effect of ammonia complexation, Ni/Co ratios, precipitators and solvents in the template-free hydrothermal system, is demonstrated in detail. As the battery materials, the as prepared flower-like nickel cobalt double hydroxides exhibit excellent specific capacities and high rate performance. Ni0.28Co0.72(OH)(2) displays the highest capacity of 206.7 mA hg(-1) at 1 mV s(-1) and 174.3 mA hg(-1) at 1 Ag-1, respectively. The capacity retention of Ni0.28Co0.72(OH)(2) is 59.1% (from 206.7 to 122.2 mA hg(-1)) at the potential scan rate from 1 to 25 mV s(-1). Due to the high rate performance corresponding to high power energy, Ni0.28Co0.72(OH)(2) is used as positive material to assemble the hybrid device (asymmetric supercapacitor) with activated carbon as negative material. The as-prepared asymmetric supercapacitor exhibits 19.4 Wh kg(-1) at 80.5 W kg(-1), and even 20.6 Wh kg(-1) at 3.93 kW kg(-1). (C) 2015 Elsevier Ltd. All rights reserved.