A commercial Li4Ti5O12 material was modified by NH4F using a facile and dry method at a low temperature in air. X-ray diffraction reveals that the fluorination did not change the bulk structure of Li4Ti5O12. X-ray photoelectron spectroscopy demonstrates that LiF was formed at the surface and Ti4+ was partially changed into Ti3+. Microscopic images show that some nanoislands were formed on the surface, which enlarged the surface area. Consequently, the NH4F-modified Li4Ti5O12 material exhibited significantly enhanced capacities and rate capabilities, even at low temperatures. The discharge capacity was increased from 149 to 167 mA h g-1 at 1 C, and the capacity retention was increased from 17.8 to 52.0% at 15 C. The capacity retention of NH4F-modified Li4Ti5O12 was greater than that of Li4Ti5O12 at each low-temperature point. Additionally, the introduction of F can protect the Li4Ti5O12 material from side reactions with the electrolyte and the atmosphere, enhancing the surface stability and reducing the release of gaseous products. It is believed that the NH4F-modified Li4Ti5O12 with enhanced electrochemical performance is a promising anode material for lithium ion batteries. Furthermore, this facile surface fluorination strategy is amenable to large-scale production.
Keywords: Li4Ti5O12; NH4F; anode material; lithium ion batteries; surface fluorination.