In this work, BiOCl nanosheets with and without surface bismuth vacancy (V-Bi-BiOCl and BiOCl) are prepared by a simple hydrothermal method. The formation of surface bismuth vacancy is confirmed mainly by XPS, EPR, PL, and UV-DRS. It is found that although BiOCl has a higher BET area (16 m(2)g(-1)) and a lower electrical resistance (2.62 Omega) than V-Bi-BiOCl (4 m(2)g(-1), 6.33 Omega), the specific capacitance (23.6 mF cm(-2)) of V-Bi-BiOCl is 3.4 times higher than that (6.9 mF cm(-2)) of BiOCl at 0.5 mA cm(-2). However, the improved capacitance has been mainly attributed to the formation of surface bismuth vacancies, which could provide excess electrochemically adsorption sites for Na (I) ions. This finding is obviously different from conventional viewpoint that the available electrochemically active surface and electrical conductance are usually regarded as the crucial factors for the electrochemical performance. Hence, this work provides a new idea to develop excellent electrode materials. (C) 2017 Elsevier Ltd. All rights reserved.