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The fluorine gradient-doped LiNi0.5Mn1.5O4 spinels are synthesized by a facile one-step method and the effect of heat treatment on their structure, morphology, and electrochemical performance are investigated. The results show that introduction of fluorine leads to a larger lattice parameter and particle size, and the formation of F-enriched surface. Whereas at 400 °C, the fluorine gradient-doped LiNi0.5Mn1.5O4 sample exhibits an improved long-term cycling stability and high rate performance, due to the suppression of the reaction between electrolyte and cathode, resulting in a decrease in the total resistance and the formation of a thin, uniform and smooth film on the surface. As a result of in situ XRD with charged pristine and the fluorine gradient-doped samples, the similar thermal-decomposition pathways from the charged spinel to the final NiMn2O4-type spinel structure with a small amount of NiMnO3 and α-Mn2O3 are observed. In addition, the disappearance temperature of the charged spinel structures is at about 280 °C for the fluorine gradient-doped sample, exhibiting an improved thermal stability of high voltage cathode material. These results show that fluorine gradient-doped LiNi0.5Mn1.5O4 sample is a promising positive electrode material for high performance lithium ion batteries.

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.

A series of Al-doped Li1+xNi0.5AlxMn1.5-xO4 cathode materials for lithium ion batteries are successfully synthesized via a co-precipitation method. Raman spectroscopy studies reveal that the cation-disordered tends to increase with the content of Al. The data of X-ray photoelectron spectroscopy (XPS) depth profiles reveal that the inert Al3+ ions segregates preferentially to the surface during the synthesis process. The results of the electrochemical tests suggest that the substitution of a small amount of Al has the ability to improve the rate capability of LiNi0.5Mn1.5O4 spinel with conventional electrolytes. Especially, the Al-doped (x = 0.06) sample delivers a long cycle-life at high rate (20 C). The enhanced performance is attributed to the formation of Al-enriched surface, providing a more stable interface with the electrolyte at high voltage (∼4.7 V), along with the stabilization of the spinel structure with a disordering of the cations and improved Li-ion diffusion, based on results of the potentiostatic intermittent titration technique.