Effect of Carbon Coating on the Self-Discharge Behaviors of Niobium Oxide in Sodium Ion Capacitors

Sodium-ion energy storage devices have been researched on a large scale due to their abundant resources and low costs. The sodium-ion capacitors (SICs) combine the advantages of batteries and supercapacitors. However, the inevitable self-discharge phenomenon limits certain applications. Here, we report a composite negative material for SICs, which is T-Nb2O5 nanoparticles (represented by T-Nb2O5@C) embedded in a carbon matrix. T-Nb2O5@C shows a better rate performance than T-Nb2O5, which is attributed to its fast ion transport in the electrode. Most importantly, the blocking effect on electrons is increased due to the addition of the carbon layer, which greatly inhibits the self-discharge behavior of the device. SICs with T-Nb2O5@C negative and activated carbon (AC) positive exhibit excellent energy density and power density. The voltage of T-Nb2O5@C//AC from 4.0 to 1.5 V takes 500 h. The low leakage current of T-Nb2O5@C//AC is 0.016 mA. It provides a strategy for suppressing electrode self-discharge in high-power energy storage devices.