Li–Si-alloy-assisted improvement in the intrinsic cyclability of Mg2Si as an anode material for Li-ion batteries

Low Coulombic efficiency and fast capacity fading are still the key bottlenecks for practical applications of Mg2Si as an anode material for Li-ion batteries. Herein, we report a first attempt to alleviate these problems by combining the Mg2Si anode with Li12Si7. A novel 7Mg2Si–xLi12Si7 composite with x = 0, 0.25, 0.5, 0.8, 1.0, 1.5 and 2 is designed and prepared. The results show that the presence of Li12Si7 changes the lithiation/delithiation reaction process and significantly improves the cycling stability of the Mg2Si anode. The 7Mg2Si–Li12Si7 (molar ratio: 7:1) composite shows the best cycling stability because it maintains 72.6% of the maximum Li-extraction capacity (814 mAh g−1) after 50 cycles, which is more than a 3-fold increase compared with that of pristine Mg2Si (19.5%), and also quite superior to those of any presently known bulk Mg2Si systems. The significantly improved cyclability is primarily attributed to the successful avoidance of Mg dissociation during the lithiation process because the added Li12Si7 reacts with Mg2Si to convert it into Li2MgSi. This conversion effectively increases the Coulombic efficiency of 7Mg2Si–xLi12Si7 upon cycling and improves the reversibility of Li-storage, consequently inducing a significant improvement in the cycling stability. The finding of this study provides new, valuable information to further improve the cycling life of bulk Si-based anode materials for next-generation Li-ion batteries.

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