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 Li4-x/2Ti5-x/2EuxO12@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 Li4-x/2Ti5-x/2EuxO12@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 Li4-x/2Ti5-x/2EuxO12@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+.