Electrochemical Properties Of The Li_xAl_ySi_z Alloys Prepared By Hydrogen-Driven Chemical Reaction For Li-ion Batteries

The decreasing oil reserves, the exorbitant fluctuations in oil prices in recent years and the evolution of mobile portable devices have demanded an extensive research into Li-ion secondary batteries. Ternary Li-Al-Si alloys have been attracting attention due to their high theoretical capacity (～1000 mAh/g), moderate charge/discharge voltage plateau, alleviated volume effect, environmental benignity and low cost, and are thus regarded as promising substitutes for anodic materials of Lithium ion batteries. Unfortunately, it is rather difficult to prepare the Li-Al-Si alloys by conventional melting techniques due to the large difference in the melting points of Li, Al and Si (～1260℃) and the evaporation of metal Li. In this work, a first attempt was carried out to synthesize the ternary Li-Al-Si alloys by a facile method of hydrogen-driven chemical reaction (HDCR) technique in this paper. By heating a LiH-Al-Si mixture at various molar ratios, the Li-Al-Si alloys were successfully synthesized. And then, the structure and electrochemical Li storage properties of the resulting products were systematically investigated and discussed. Finally, high-energy ball milling (HEBM) treatment was applied to further improve electrochemical properties of the as-prepared samples.Three Li-Al-Si alloys including LiAlSi, Li5AlSi2 and Li7Al3Si4 were synthesized by heating the mixtures of LiH, Al with Si. It was found that the purity and electrochemical properties of LiAlSi and Li5AlSi2 alloys prepared by HDCR were greatly improved. The as-prepared LiAlSi, Li5AlSi2 and Li7Al3Si4 delivered the maximum charge capacity of 1051,932 and 845 mAh/g, respectively, and the maximum discharge capacity of 1074,1080 and 997 mAh/g. After 30 cycles, the charge capacity was reduced to 795,446 and 448 mAh/g, while the discharge capacity was reduced to 801,456 and 445 mAh/g, respectively. The corresponding discharge capacity retention was approximately 75%,42% and 45%, respectively.The electrochemical Li storage process of LiAlSi anode was a two-step reaction, which effectively reduced the volume change and improved cycling stability of the electrodes upon cycling. However, the main electrochemical Li storage process of Li5AlSi2 anode was a one-step reaction. The formation of two-phase region was responsible for the large volume change caused by the lattice mismatch, which induces the rapid capacity fading during the subsequent cycles. In addition, a two-step Li insertion/extraction reaction was observed for Li7Al3Si4 alloy.Further HEBM treatment improved the electrochemical performance of the as-prepared Li-Al-Si samples by tailoring the phase constituent and particle morphology. After ball milling for 24,8 and 12 h, respectively, the as-prepared LiAlSi, Li5AlSi2 and Li7Al3Si4 sample displayed optimal electrochemical property. The maximum charge capacities were increased to 1227,1075 and 850 mAh/g, respectively, and the maximum discharge capacities were increased to 1255,1236 and 968 mAh/g. After 30 cycles, the charge capacities were 932,636 and 551 mAh/g, while the discharge capacity was reduced to 936、637 and 561 mAh/g, respectively. The corresponding discharge capacity retentions were calculated to be approximately 75,52,49 and 58%. The regular particle morphology and reduced particle size for the post-milled samples can effectively alleviate the volume effect upon cycling, which is responsible for improved electrochemical cycling stability.