Graphene confined intermetallic magnesium silicide nanocrystals with highly exposed (2 2 0) facets for anisotropic lithium storage

Intermetallic Magnesium silicide (Mg₂Si) is regarded as a promising electrode material for lithium-ion batteries (LIBs), by virtue of its desirable electrochemical activity, high theoretical capacity, suitable voltage profiles, and lightweight. Nevertheless, its practical application is still hindered by poor electrochemical kinetics and rapid capacity fading. Herein, high-purity intermetallic Mg₂Si nanocrystals (NCs) encapsulated by graphene-layer matrix (Mg₂Si@G) are designed for advanced lithium storage. The graphene-confined Mg₂Si NCs, featuring with high-purity, highly exposed (2 2 0) facets and nanopores, are fabricated via a facile hydrogen-driven silicification and subsequent freeze-drying process. Combined DFT calculations and experimental studies illustrate an anisotropic lithium storage of Mg₂Si, exhibiting rapid Li-ions migration path along exposed (2 2 0) facets and highly reversible solid solution behavior. Benefiting from the desirable structure features and interactions, Mg₂Si@G ensures a spatially confined (de)lithiation with high electrochemical activity and fast electronic/ionic transport kinetics, leading to largely enhanced lithium storage performance. The resulting Mg₂Si@G electrode delivers a high capacity (100th capacity of 831 mAh g⁻¹ at 100 mA g⁻¹), outstanding rate capability and long-term cycle stability (3000th capacity of 578 mAh g⁻¹ at 2 A g⁻¹). This work presents a new perspective towards the rational development of well-performing Mg₂Si materials for lithium storage.