Understanding Protection Mechanisms of Graphene-Encapsulated Silicon Anodes with Operando Raman Spectroscopy.

Carbon-coated silicon micro- and nanostructures have been widely used as composite anodes for lithium-ion batteries combining the benefits of high theoretical capacity of Si and better conductivity of carbon. To optimize structures that allow the Si volume expansion without losing the electrical connection, a detailed carbon protection mechanism is desired. We fabricate a network of interconnected sandwich branches with a silicon thin film encapsulated between a porous 3-dimensional graphene foam and graphene drapes (so-called a graphene ensemble). This prototype binder-free anode, of great mechanical strength and composed of only silicon and few-layer graphene, provides distinct signals under operando Raman spectroscopy. During electrochemical cycles, the graphene G peak shows variation of peak position and intensity, while the 2D peak experiences a negligible shift from limited deformation. Silicon displays excellent structural reversibility under the sandwich protection, validating the functions of graphenic carbon coating. This specific graphene ensemble can also serve as an experimental scaffold for mechanical and chemical analysis of many active materials.