Mg2si Anode with Greatly Enhanced Cycle Life
Authors: Zhan Hui , Men Fang , Zhong Hai 
Published: February 2020

Lithium-ion batteries (LIBs) with silicon anode have a potential application in electric vehicles and energy storage systems owing to the high capacity of silicon, however, the volume change and thus-resulted fast capacity decay or electrode failure greatly restricts its practical use. Using Si-M composite instead of pure Si has been proved to be an efficient strategy to minimize the capacity fade. Among these Si composites, magnesium silicide (Mg2Si) attracts much attention due to the natural abundance of Mg and cost concern. More importantly, Mg and Si can accommodate Li respectively which brings a higher specific capacity than other Si-M composite. Despite the above advantages, similar to pure Si anode, Mg2Si anode also suffers from the large volume change during the cycling. In this work, we propose using fluoroethylene carbonate (FEC) or vinylene carbonate (VC) additive to enhance the cycling life of Mg2Si anode and greatly improved cycling stability has been achieved on Mg2Si anode.
Experiment
Mg2Si (Adamas Beta, 98%) and acetylene black (ACB) powder were vacuum dried at 80°C for 12h and then ball milled for 3h to get Mg2Si/ACB mixture (mass ratio of 5:3). The electrode contained 80% above mixture and 20% PVDF binder. 1M LiPF6 in EC and DMC mixture solvent (volume ratio of 1:1) was used as the electrolyte. The cyclic performance was evaluated on LAND CT2001A system between 0.01 and 2.0V with a current of 100mA/g. The impedance spectra were collected on Autolab electrochemical working station. The surface information of the cycled electrode was obtained through SEM observation and XPS measurement.
Results and discussions
Figure 1 shows the cycling performances of Mg2Si in bare electrolyte or the electrolyte with VC or FEC addition. Rapid capacity fading was found on Mg2Si in bare electrolyte, such as lower than 300 mAh/g after several cycles. In comparison, in FEC-added electrolyte, Mg2Si still released 400 mAh/g capacity after 100 cycles. In VC-added electrolyte, similar improvement was also observed, but not as notable as in FEC-added electrolyte.
SEM observation supplied the SEI information of the Mg2Si electrodes cycled in electrolyte with or without additives. In bare electrolyte, the thinnest and most porous SEI layer was found on Mg2Si. While in FEC or VC-added electrolyte, the SEI layer was much thicker and denser, and VC addition led to the most compact SEI layer on Mg2Si anode. EIS impedance further confirmed this difference by the biggest diameter shown by Mg2Si anode in VC-added electrolyte. To understand the reason of the different cycling behavior of Mg2Si, XPS analysis was conducted. Given the fixed amount of LiPF6 in all tested electrolytes, the F-P bond could be used as a reference, and the amount of Li-F could be determined in a semi-quantitative model. The results revealed the highest content of LiF in the SEI formed on Mg2Si in FEC-added electrolyte and the lowest content of LiF in the SEI formed in bare electrolyte. Combined with previous research and some additional experiments, we think FEC could promote the LiF accumulation in the outer layer rather than the inner layer of SEI and reduce the LiF content in the bulk Mg2Si anode, and this eventually brings some protection effect on Mg2Si.
Conclusions
FEC addition in electrolyte greatly enhanced the cycling stability of Mg2Si, and around 400mAh/g capacity could be obtained after 100cycles. Detailed analysis revealed the protection effect of FEC on Mg2Si anode.

Figure 1