Super high-rate, long cycle life of europium-modified, carbon-coated, hierarchical mesoporous lithium-titanate anode materials for lithium ion batteries

Europium-modified, carbon-coated, hierarchical mesoporous Li₄Ti₅O₁₂ microspheres were prepared via the co-precipitation method. X-ray diffraction (XRD) and Raman analyses revealed that europium ions were doped into 16d Li⁺/Ti⁴⁺ sites of Li₄Ti₅O₁₂. Microscopic observations reveal that primary nanoparticles of Li_4-x/2Ti_5-x/2Eu_xO₁₂@C (x = 0.004) are assembled into hierarchical mesoporous microspheres, with an average particle size of about 473.4 nm and a uniform particle size distribution. X-ray photoelectron spectroscopy demonstrated that partial Ti⁴⁺ is reduced to Ti³⁺ induced by carbon coating and double-valence state of europium (Eu²⁺/Eu³⁺) doping into the Li₄Ti₅O₁₂. The samples exhibit excellent electrochemical properties including fast lithium storage performance, outstanding cycle stability and high rate capability. The highest initial discharge capacity of Li_4-x/2Ti_5-x/2Eu_xO₁₂@C (x = 0.004) reached 198.7 mA h g^-1 and the discharge capacity still maintained 173.4 mA h g^-1 at 5C after 1000 cycles. Even cycled at 100C, the discharge capacity of Li_4-x/2Ti_5-x/2Eu_xO₁₂@C (x = 0.004) maintained 92.1 mA h g^-1. The excellent electrochemical performance can be attributed to the hierarchical mesoporous structure combined with modified strategies including europium doping and carbon coating, which not only improved the lithium-ion diffusion coefficient, but also increased the electronic conductivity. Moreover, the electrical conductivity between the Li₄Ti₅O₁₂ particles was enhanced by carbon coating and the bulk electronic conductivity of Li₄Ti₅O₁₂ was also improved by the presence of Ti³⁺. © 2016 The Royal Society of Chemistry.