Large lithium storage in highly crystalline TiNb2O7 nanoparticles synthesized by a hydrothermal method as anodes for lithium-ion batteries

Lithium storage in hydrothermally synthesized TiNb2O7 (H-TNO) nanoparticles has been investigated by electrochemical and structural analysis and compared with that in solid-state synthesized TiNb2O7 (S-TNO) particles. The H-TNO was characterized to be highly crystallized primary particles having a single-crystal-like structure with a uniform face orientation. Cyclic voltammogram and charge-discharge potential characteristics of the H-TNO showed that additional lithium storage occurred for x > 4.0 in LixTiNb2O7 between 0.9 and 0.75 vs. Li/Li+. The H-TNO used as the anodes for lithium-ion batteries had highly reversible capacities of 341 mAh g−1 and 1480 mAh cm−3, performance superior to that in the case of S-TNO. Ex-situ X-ray absorption fine structure (XAFS) analysis exhibited simultaneous changes in the charge compensation in Ti and Nb with an increase of Li+ ion amounts in H-TNO for x > 4.0. Apparent chemical diffusion coefficient (Dapp) values of lithium in the H-TNO for x > 3.5 were one order of magnitude larger than those in the S-TNO from the results of galvanostatic intermittent titration technique (GITT) analysis. The additional lithium storage in H-TNO was attributed to the enhancement of lithium diffusivity in the highly crystallized primary particles with the short lithium diffusion length. The H-TNO exhibited high capacity, high-rate charge capability, and long-cycle-life performance.