Symmetric Sodium-Ion Capacitor Based on NaMnO Nanorods for Low-Cost and High-Performance Energy Storage.

Batteries and electrochemical capacitors play very important roles in the portable electronic devices and electric vehicles and have shown promising potential for large-scale energy storage applications. However, batteries or capacitors alone cannot meet the energy and power density requirements because rechargeable batteries have a poor power property, whereas supercapacitors offer limited capacity. Here, a novel symmetric sodium-ion capacitor (NIC) is developed based on low-cost NaMnO nanorods. The NaMnO with unique nanoarchitectures and iso-oriented feature offers shortened diffusion path lengths for both electronic and Na transport and reduces the stress associated with Na insertion and extraction. Benefiting from these merits, the symmetric device achieves a high power density of 2432.7 W kg, an improved energy density of 27.9 Wh kg, and a capacitance retention of 85.2% over 5000 cycles. Particularly, the symmetric NIC based on NaMnO permits repeatedly reverse-polarity characteristics, thus simplifying energy management system and greatly enhancing the safety under abuse condition. This cost-effective, high-safety, and high-performance symmetric NIC can balance the energy and power density between batteries and capacitors and serve as an electric power source for future low-maintenance large-scale energy storage systems.