Nanostructured Na₂Ti₉O₁₉ for Hybrid Sodium-Ion Capacitors with Excellent Rate Capability

Herein, we report a new Na-insertion electrode material, Na 2 Ti 9 O 19, as a potential candidate for Na-ion hybrid capacitors. We study the structural properties of nanostructured Na 2 Ti 9 O 19, synthesized by a hydrothermal technique, upon electrochemical cycling vs Na. Average and local structures of Na 2 Ti 9 O 19 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 Na 2 Ti 9 O 19 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 6 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, Na 2 Ti 9 O 19 shows excellent Na-ion kinetics with a capacitve nature of 86% at 1.0 mV s –1, 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 Na 2 Ti 9 O 19 as anode and activated porous carbon as cathode, which exhibits excellent electrochemical properties, with a maximum energy density of 54 Wh kg –1 and a maximum power density of 5 kW kg –1 . Both structural insights and electrochemical investigation suggest that Na 2 Ti 9 O 19 is a promising negative electrode for sodium-ion batteries and hybrid capacitors.