Nanostructured NaTiO for Hybrid Sodium-Ion Capacitors with Excellent Rate Capability.

Herein, we report a new Na-insertion electrode material, NaTiO, as a potential candidate for Na-ion hybrid capacitors. We study the structural properties of nanostructured NaTiO, synthesized by a hydrothermal technique, upon electrochemical cycling vs Na. Average and local structures of NaTiO are elucidated from neutron Rietveld refinement and pair distribution function (PDF), respectively, to investigate the initial discharge and charge events. Rietveld refinement reveals electrochemical cycling of NaTiO is driven by single-phase solid solution reaction during (de)sodiation without any major structural deterioration, keeping the average structure intact. Unit cell volume and lattice evolution on discharge process is inherently related to TiO distortion and Na ion perturbations, while the PDF reveals the deviation in the local structure after sodiation. Raman spectroscopy and X-ray photoelectron spectroscopy studies further corroborate the average and local structural behavior derived from neutron diffraction measurements. Also, NaTiO shows excellent Na-ion kinetics with a capacitve nature of 86% at 1.0 mV s, indicating that the material is a good anode candidate for a sodium-ion hybrid capacitor. A full cell hybrid Na-ion capacitor is fabricated by using NaTiO as anode and activated porous carbon as cathode, which exhibits excellent electrochemical properties, with a maximum energy density of 54 Wh kg and a maximum power density of 5 kW kg. Both structural insights and electrochemical investigation suggest that NaTiO is a promising negative electrode for sodium-ion batteries and hybrid capacitors.