Li-ion and Na-ion transportation and storage properties in various sized TiO₂ spheres with hierarchical pores and high tap density

Titanium oxide (TiO₂) has attracted great interest as a promising anode material for lithium (Li) ion batteries (LIBs) and sodium (Na) ion batteries (SIBs). However, the key factors that dictate the Li-ion and Na-ion storage and transportation in TiO₂ remain unclear. Herein, we report a facile hydrolysis route to crafting a variety of high tap-density TiO₂ spheres with controllable size and hierarchical pores. The Li-ion and Na-ion storage properties based on these TiO₂ spheres were systematically investigated. The pore distribution and the size of TiO₂ spheres were found to exert profound influence on the Li-ion and Na-ion storage and transportation. The Li-ion storage and transportation in dense TiO₂ spheres was dependent mainly upon the micropore distribution and volume and independent of the size of spheres. In contrast, the excellent Na-ion storage and transportation in TiO₂ spheres was enabled by the loose structure with a large macroscopic pore volume and shortened Na-ion diffusion length. High tap-density TiO₂ spheres (1.06 g cm^-3) with superior Li-ion and Na-ion storage properties were produced, exhibiting a Li-ion storage specific capacity of 189 mA h g^-1 at 1C and a high capacity retention of 88.1% after 100 cycles, and a Na-ion storage specific capacity of 184 mA h g^-1 at 1C and capacity retention of 90.5% after 200 cycles. The ability to understand the critical factors controlling the Li-ion and Na-ion storage in high tap-density TiO₂ spheres enables their implementation for practical applications in Li-ion and Na-ion batteries.