Surface-controlled sodium-ion storage mechanism of Li4Ti5O12 anode

Sodium-ion batteries (SIBs) are the promising candidate in grid systems owing to the wide distribution and abundance of sodium resources. However, the charge storage mechanism and size-effect of the active materials for SIBs remain largely unexplored. Herein, we synchronously investigate the electrochemical properties and structural evolutions of Li4Ti5O12 (LTO) anode for Li+ and Na+ storage, along with the nano-size effect on their charge storage behaviors. Through detailed kinetic analysis, it is found the Li+ storage in LTO is diffusion-controlled in bulk and shows increased surface-controlled contributions when the size is reduced to 18 nm. In comparison, for the Na+ storage, the domination step is surface-controlled in all the sizes of LTO from 260 to 18 nm, with over 55% contribution even at the low sweep rate of 0.1 mV s−1. The limited near-surface reaction region and low diffusion kinetics determine the surface-controlled Na+ storage behaviors and the specific capacities. The suitable size of LTO-18 nm anode displays a high capacity of ∼140 mAh g−1 (at 0.05 A g−1, ∼0.29C), high-rate capability (42 mAh g−1 at ∼57C), and long-term cycling stability (over 1200 cycles). Our finding provides insights into a precise design of nanomaterials for high-power sodium-ion storage devices.