Hierarchical nanosheet-constructed yolk-shell TiO₂ porous microspheres for lithium batteries with high capacity, superior rate and long cycle capability

A hierarchical nanosheet-constructed yolk-shell TiO<inf>2</inf> (NYTiO<inf>2</inf>) porous microsphere is synthesized through a well-designed, one-pot, template-free solvothermal alcoholysis process using tetraethylenepentamine (TEPA) as the structure directing reagent. Such a yolk-shell structure with a highly porous shell and dense mesoporous core is quite advantageous as an anode material for lithium ion batteries (LIBs). The outer, 2D nanosheet-based porous (15 nm) shell and the nanocrystal-based inner mesoporous (3 nm) core provide a stable, porous framework, effective grain boundaries and a short diffusion pathway for Li⁺ and electron transport, facilitating lithium insertion/extraction. The voids between the core and the shell can not only store the electrolyte due to capillary and facilitate charge transfer across the electrode/electrolyte interface but also buffer the volume change during the Li⁺ insertion/extraction. As a result, NYTiO<inf>2</inf> demonstrates excellent Li⁺ capacity with outstanding cycle performance and superior rate capability at different rates for >700 cycles, retaining a 225 mA h g^-1 reversible capacity after 100 cycles at 1 C. In particular, the reversible capacity can still be maintained at 113 mA h g^-1 after 100 cycles at 10 C. We also observe the formation of homogeneously distributed 5-10 nm Li<inf>2</inf>Ti<inf>2</inf>O<inf>4</inf> nanocrystallites on the surface of the nanosheets during the discharge-charge process. The synergy of the yolk-shell structure with dual mesopores in the shell and core and the Li<inf>2</inf>Ti<inf>2</inf>O<inf>4</inf> nanocrystallites endow the hierarchical NYTiO<inf>2</inf> with high reversible capacity, excellent rate capability and outstanding cycle performance.