Growth of conformal graphene cages on micrometre-sized silicon particles as stable battery anodes

Nanostructuring has been shown to be fruitful in addressing the problems of high-capacity Si anodes. However, issues with the high cost and poor Coulombic efficiencies of nanostructured Si still need to be resolved. Si microparticles are a low-cost alternative but, unlike Si nanoparticles, suffer from unavoidable particle fracture during electrochemical cycling, thus making stable cycling in a real battery impractical. Here we introduce a method to encapsulate Si microparticles (similar to 1-3 mu m) using conformally synthesized cages of multilayered graphene. The graphene cage acts as a mechanically strong and flexible buffer during deep galvanostatic cycling, allowing the microparticles to expand and fracture within the cage while retaining electrical connectivity on both the particle and electrode level. Furthermore, the chemically inert graphene cage forms a stable solid electrolyte interface, minimizing irreversible consumption of lithium ions and rapidly increasing the Coulombic efficiency in the early cycles. We show that even in a full-cell electrochemical test, for which the requirements of stable cycling are stringent, stable cycling (100 cycles; 90% capacity retention) is achieved with the graphene-caged Si microparticles.