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

Abstract

A hierarchical nanosheet-constructed yolk–shell TiO₂ (NYTiO₂) 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, NYTiO₂ 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 Li₂Ti₂O₄ 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 Li₂Ti₂O₄ nanocrystallites endow the hierarchical NYTiO₂ with high reversible capacity, excellent rate capability and outstanding cycle performance.