Over‐Stoichiometric NbO₂ Nanoparticles for a High Energy and Power Density Lithium Microbattery

Abstract Effective utilization of active materials in microbatteries can be enhanced by rational design of the electrodes. There is an increasing trend of using 3 D electrodes that are coated in nanosized active materials to boost both energy and power densities. This article focuses on the fabrication of 3 D electrodes based on monolithic carbon foams coated in over‐stoichiometric NbO₂ nanoparticles. The electrodes exhibit remarkable energy and power densities at various current densities when tested in lithium microbatteries. An areal capacity of around 0.7 mAh cm⁻² and energy density up to 45 mWh cm⁻³ have been achieved. More than half of the areal capacity can be accessed at a current density of about 11 mA cm⁻², with the corresponding energy and power densities being 21 mWh cm⁻³ and 1349 mW cm⁻³. These values are comparable to those of microsupercapacitors containing carbon and MnO₂ nanomaterials. Furthermore, the electrochemical reversibility improves progressively upon cycling along with substantial increase in the charge transfer kinetics of the electrode. Based on impedance analyses almost a fourfold decrease in the charge transfer resistance has been observed over 25 cycles. Such enhancement of the electronic properties of NbO₂ can account for the high electrochemical rate performance of the 3 D electrodes.