Preparation With Hydrogen-driven Chemical Reaction And Electrochemical Properties Of Mg₂Si Anode Materials For Li-ion Batteries

With ever-increasing energy and environmental problem, Li-ion secondary batteries have a big potential for the practical application in the electric vehicles and the intelligent energy storage sectors. In recent years, magnesium silicide (Mg2Si) has been attracting more and more attention due to its favorable voltage profile, high specific capacity, high capacity, natural abundance, low cost, and environmental compatibility. Unfortunately, it is rather difficult to prepare pure Mg2Si by using conventional melting techniques due to the large discrepancy in the melting points of Mg and Si (differing by～760℃) and lack of solubility, as well as the low boiling temperature (1090℃) and the high reactivity of Mg.Based on the overall review on the research development of Mg2Si, this work developed a facile synthesis method of Mg2Si anode materials for Li-ion batteries by Hydrogen-driven Chemical Reaction (HDCR) as:2MgH2(s)+Si (s)â†'Mg2Si(s)+2H2(g)By applying MgH2instead of metallic Mg, the well crystallized Mg2Si with99%purity and homogeneous composition was readily obtained at temperature below500℃. Compared with the conventional methods, the newly developed approach in this work affords some advantages as it avoids the volatilization of Mg and the introduction of impurities, and it is facile, controllable, low-cost, and environment friendly with low cost, and high purity and yield. The as-prepared Mg2Si exhibited an improved Li-ion storage ability as anode material for Li-ion batteries. In contrast with the commercial Mg2Si anode material, the initial discharge specific capacity, charge specific capacity and the coulomb efficiency of the as-prepared Mg2Si increased by12.7%,52.8%, and34.4%respectively.For improving the electrochemical properties of Mg2Si prepared by HDCR, the effects of the pre-milling time and the synthesis temperature on the microstructure and electrochemical properties was systematically investigated. Results showed that the particle size of the pre-milled4h sample distributes uniformly, and its initial discharge specific capacity and coulomb efficiency are1095mAh/g and92%, respectively. After35charging/discharging cycles, the discharge capacity retention is about44%, which was a30%increase relative to the pristine sample. Further optimization of the reaction temperature indicated that Mg2Si prepared by the isothermal dehydrogenation at the temperature of-330℃exhibits the improved electrochemical properties. The initial discharge capacity of the as-prepared Mg2Si is1048mAh/g with a high coulombic efficiency of94%. After50cycles, the discharge capacity is about392mAh/g. The capacity retention was calculated to be about37%.Moreover, to further improve the cycling stability of Mg2Si, LiH was introduced into the starting mixture of MgH2and Si, and the ternary Li2MgSi alloy was successfully synthesized. The structure and electrochemical properties of the ternary Li2MgSi alloy were investigated. It was found that the particle size of the post-24milled Li2MgSi alloy is around400nm with uniform distribution. Electrochemical examinations showed that the capacity retention is as high as80%after35discharge-charge cycles, a1.4-fold increase with the binary Mg2Si sample.