Graphene mitigated fracture and interfacial delamination of silicon film anodes through modulating the stress generation and development.

ABSTRACT

Silicon film is an attractive anode candidate in lithium ion batteries due to its two-dimensional (2D) morphology that is beneficial to buffer the large volume expansion of traditional silicon anodes. Even so, the generation of stress during the lithiation/delithiation process can still lead to the cracking and delamination of the silicon film from the current collector, ultimately resulting in the fast failure of the electrode. Laying a graphene layer between the silicon film and the current collector has been demonstrated to alleviate the stress generated during the battery cycling, but its universal application in commercial silicon structures with other dimensionalities remains technically challenging. Putting graphene on top of a 2D silicon film is more feasible and has also been shown with enhanced cycling stability, but the underneath mechanical mechanisms remain unclear. Herein, using the combination of 2D graphene and 2D silicon films as a model material, we investigate the stress generation and diffusion mode during the battery cycling to disclose the mechanical and electrochemical optimization of a silicon anode experimentally and theoretically. As a result, the optimum thickness of the silicon film and the coated graphene layers are obtained, and it is found the in-plane cracking and out-of-plane delamination of the silicon film could be mitigated by coating graphene due to the slow transfer of the normal and shear stresses. This work provides some understanding of the electrochemically derived mechanical behaviors of the graphene-coated battery materials and guidelines for developing stable high-energy-density batteries.