Atomic-scale manipulation of electrode surface to construct extremely stable high-performance sodium ion capacitor

A state-of-the-art approach has been performed to stabilize the surface of P2-type (Na0.66Mn0.54Ni0.13Co0.13O2 (NMNC)) material through the atomic layer deposition (ALD) of Al2O3 (10 ALD cycles) to fabricate a 3 V sodium ion capacitor (NIC) with ultrahigh rate stability. This is the first known report of stabilizing the NIC electrode surface by a metal oxide coating using ALD. The capacitor constructed with an Al2O3-coated NMNC (NMNC-Al) cathode and a commercial activated carbon (CAC) anode in an organic electrolyte delivers a discharge capacitance of 68 F g(-1) at 0.35 A g(-1) current density and exhibits extremely high electrochemical stability of similar to 98% of its initial value after 10,000 cycles. In contrast, the capacitor containing a pristine NMNC electrode displays a capacity retention of 78%. The NMNC-Al/CAC cell also has an energy density of 63 Wh kg(-1) at a power density of 6.6 kWh kg(-1). The capacitance, energy, and power densities obtained from the NMNC-Al/CAC cell are the best-reported values for sodium-based capacitors and outperforms well-established lithium ion capacitors. The electrochemical impedance spectroscopy study reveals that the sluggish reaction kinetics of the NMNC electrode at high current density is successfully overcome by coating an ultrathin Al2O3 layer by ALD on its surface.