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

Abstract Low Coulombic efficiency and fast capacity fading are still the key bottlenecks for practical applications of Mg 2 Si as an anode material for Li-ion batteries. Herein, we report a first attempt to alleviate these problems by combining the Mg 2 Si anode with Li 12 Si 7 . A novel 7Mg 2 Si– x Li 12 Si 7 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 Li 12 Si 7 changes the lithiation/delithiation reaction process and significantly improves the cycling stability of the Mg 2 Si anode. The 7Mg 2 Si–Li 12 Si 7 (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 Mg 2 Si (19.5%), and also quite superior to those of any presently known bulk Mg 2 Si systems. The significantly improved cyclability is primarily attributed to the successful avoidance of Mg dissociation during the lithiation process because the added Li 12 Si 7 reacts with Mg 2 Si to convert it into Li 2 MgSi. This conversion effectively increases the Coulombic efficiency of 7Mg 2 Si– x Li 12 Si 7 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.