Extraordinary Performance of Carbon-Coated Anatase TiO₂ as Sodium-Ion Anode

The synthesis of in situ polymer-functionalized anatase TiO₂ particles using an anchoring block copolymer with hydroxamate as coordinating species is reported, which yields nanoparticles (≈11 nm) in multigram scale. Thermal annealing converts the polymer brushes into a uniform and homogeneous carbon coating as proven by high resolution transmission electron microscopy and Raman spectroscopy. The strong impact of particle size as well as carbon coating on the electrochemical performance of anatase TiO₂ is demonstrated. Downsizing the particles leads to higher reversible uptake/release of sodium cations per formula unit TiO₂ (e.g., 0.72 eq. Na⁺ (11 nm) vs only 0.56 eq. Na⁺ (40 nm)) while the carbon coating improves rate performance. The combination of small particle size and homogeneous carbon coating allows for the excellent electrochemical performance of anatase TiO₂ at high (134 mAh g^-1 at 10 C (3.35 A g^-1)) and low (≈227 mAh g^-1 at 0.1 C) current rates, high cycling stability (full capacity retention between 2nd and 300th cycle at 1 C) and improved coulombic efficiency (≈99.8%). The extraordinary electrochemical performance of carbon-coated TiO₂ nanoparticles, obtained via in situ polymer functionalization, is reported. The enhanced performance as Na-ion anodes can be ascribed to the synergetic effect of the small particle size (≈11 nm) and homogeneous carbon coating as confirmed via comparison with uncoated particles of comparable size (≈15 nm), and larger (≈40 nm) particles with carbon coating.