Structure and enhanced electrochemical performance of the CaF2-modified Li4Ti5O12 anode material

The surface of pure Li 4 Ti 5 O 12 was modified with CaF 2 via a chemical co-precipitation method followed by calcination at 400   °C for 5   h in order to study the reaction mechanism of the fluoride modification process. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were used to confirm the structure and morphology of the material. After the modification, the structure of Li 4 Ti 5 O 12 was unchanged, as F − ions reacted with Ca 2   + ions to form CaF 2 instead of reacting with Li 4 Ti 5 O 12 . A consecutive CaF 2 coating layer was not formed on the surface of Li 4 Ti 5 O 12 ; instead, many microscale CaF 2 crystals stacked on the Li 4 Ti 5 O 12 particles. The stacked CaF 2 crystals could not only reduce electrode polarization, but also partially suppress reductive decomposition, resulting in the formation of a thinner SEI film, thereby reducing resistance of the SEI film ( R sei ) and the charge-transfer resistance ( R ct ), both of which were beneficial to the electrochemical performance of Li 4 Ti 5 O 12 . The 2   wt% CaF 2 -modified Li 4 Ti 5 O 12 exhibited a superior rate capacity, with discharge capacities between 0 and 3   V of 227.8, 215.8, 207.6, 190.6, 180.2, and 157.6   mAh   g −   1 at rates of 0.2, 0.5, 1, 3, 5, and 10 C (1 C   =   250   mAh   g −   1 ), respectively. Moreover, the 2   wt% CaF 2 -modified Li 4 Ti 5 O 12 showed a discharge capacity retention of 96.7% after 125   cycles at a rate of 3 C, higher than that of pure Li 4 Ti 5 O 12 (94.4%), which was due to the fact that CaF 2 modification enhanced the capability of the material to resist HF attack during the discharge/charge processes.