Revisiting NaTi₂(PO₄)₃/nanocarbon composites prepared using nanocarbons with different dimensions for high-rate sodium-ion batteries: The surface properties of nanocarbons

In this study, we intend to revisit oxide/nanocarbon composites for a systematic study of oxide particle size, chemical bonding between oxide and carbon, electrical conductivity and ion transport in the composites on the electrochemical properties of NaTi₂(PO₄)₃@nanocarbon microsphere composites prepared using zero-dimensional carbon black, one-dimensional carbon nanotubes, and two-dimensional graphene as anode materials for high-rate sodium-ion batteries. In the solution-based synthesis of the composites, oxide precursor nanoparticles deposited on nanocarbons are converted into final oxide nanoparticles through heat treatment. We demonstrate that growth of the NaTi₂(PO₄)₃ particles in the NaTi₂(PO₄)₃@nanocarbon composites occurs during heat treatment when the concentration of oxygen functional groups per unit specific area of nanocarbons is high. Growth of oxide precursor nanoparticles is observed for carbon black with a high concentration of oxygen functional groups during heat treatment owing to the proximity between precursor particles. On the other hand, growth of precursor nanoparticles is effectively prevented for carbon nanotubes and graphene with a low concentration of oxygen functional groups. Rate capability increases in the order of NaTi₂(PO₄)₃@carbon black < NaTi₂(PO₄)₃@graphene < NaTi₂(PO₄)₃@carbon nanotubes mainly due to the smaller sizes of oxide particles and more efficient Na⁺ transport across carbon nanotubes compared to other nanocarbons.