Pseudocapacitive behavior of amorphous Nb2O5 at low potentials

Supercapacitor has been demonstrated as one of the most attractive electrochemical energy storage devices for practical applications, especially for electric transportation systems, because of high power density, long cycle life, impressive conversion efficiency and desirable usage safety. However, energy densities of supercapacitors are typically low comparing with lithium-ion batteries, which impedes their broad applications. The development of advanced supercapacitors mainly concerns with increasing their energy densities. Being different from conventional double layer supercapacitors, pseudocapacitive materials can deliver much higher capacitances and energy densities. For example, some metal oxides can be used as electrode materials for stable charge and discharge at high rates on the basis on of reversible surface redox reactions, resulting in desired energy densities comparable to that of lithium-ion batteries. In this work, we study an amorphous alpha-Nb2O5, which is prepared using a facile hydrothermal method followed by a low temperature post-annealing process. The alpha-Nb2O5 is evaluated as a pseudocapacitive electrode material in the LiPF6-based organic electrolyte for lithium storage performance. The alpha-Nb2O5 demonstrates reversible Li+ intercalation/deintercalation behavior in a voltage range of 0.1-2.5 V (vs. Li/Li+), resulting in an impressive reversible lithium storage capacity higher than 250 mAh/g at 0.2 C (1 C = 400 mA/g), together with excellent cycling stability and rate capability up to 10 C for 10000 cycles. In addition, the cyclic voltammetry (CV) profiles obtained at a low scanning rate of 0.1 mV/s show unexpected square patterns in multiple cycles even at low potentials between 1.0 and 0.1 V (vs. Li/Li+), which is distinctly different from that of crystalline orthorhombic Nb2O5 electrode material. The CV analysis also indicates apparent intercalation capacity contribution of such alpha-Nb2O5 electrode material during lithium ion storage. All electrochemical experiments verify typical pseudocapacitive behavior for the amorphous alpha-Nb2O5 material in a wide voltage range of 0.1-2.5 V (vs. Li/Li+). The special pseudocapacitive property at low potentials enables the alpha-Nb2O5 to be used as an attractive anode in a hybrid supercapacitor coupled with the active carbon (AC) material as cathode with a wide voltage window. As a result, the assembled alpha-Nb2O5//AC hybrid supercapacitor full cell can be charged to a high voltage up to 4.5 V. According to the calculated energy density of the supercapacitor based on E=1/2CV(2), the high working voltage results in an impressive energy density of 178.5 Wh/kg for this hybrid supercapacitor cell on the basis of the mass of two active electrode materials. In addition, the full cell also shows outstanding high-rate performance, and our ongoing work will optimize associated structure to improve its cycling stability at harsh work conditions. This work offers a promising Nb2O5 material that is amorphous and showing pseudocapacitive behaviors for lithium ion storage at low potentials. Improved capacity and energy density have been achieved, together with desired cycling stability for prolonged cycles. Such an electrode material can be served as a feasible anode material for high-voltage and high-energy hybrid supercapacitor full cell when it is coupled with high-rate cathode materials.