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

Herein, we report a new Na-insertion electrode material, Na₂Ti₉O₁₉, as a potential candidate for Na-ion hybrid capacitors. We study the structural properties of nanostructured Na₂Ti₉O₁₉, synthesized by a hydrothermal technique, upon electrochemical cycling vs Na. Average and local structures of Na₂Ti₉O₁₉ 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 Na₂Ti₉O₁₉ 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 TiO₆ 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, Na₂Ti₉O₁₉ 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 Na₂Ti₉O₁₉ 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 Na₂Ti₉O₁₉ is a promising negative electrode for sodium-ion batteries and hybrid capacitors.