Low-crystalline birnessite-MnO2 nanograins for high-performance supercapacitors

Abstract Crystallized transition metal oxides were widely used as the electrode materials for the electrochemical energy storage devices. However, their electrochemical performance including the capacitance and rate capability was greatly hindered due to ion penetrating long distances into the bulk regimes during the electrochemical process. Herein, low-crystalline birnessite-MnO2 nanograins are obtained, which can enhance the ion diffusion kinetics and electrochemical activity significantly due to the structural disorder and defects. The resultant electrode features a superior electrochemical performance with an areal capacitance up to 1154 mF/cm2 at 2 mA/cm2 and rate retention of 69 % when the current density increasing 10 times to 20 mA/cm2 with a commercial standard mass loading about 4.58 mg/cm2. And the cycle stability measurement shows no attenuation after 10000-cycle charge/discharge operation. Moreover, assembled asymmetric supercapacitor device (wide operation voltage of 2.3 V) delivers a maximum areal energy density of 0.36 mWh/cm2 with the areal power density of 5.57 mW/cm2. The electrochemical behavior studies and in-situ Raman reveal that the structural disorder in the low-crystalline nanograins greatly facilitate the reversible ion intercalation/deintercalation process during the electrochemical cycling process. Our results will provide an instructive route for the formulation of the design principle for the high-performance of transition metal oxides electrode material.