Direct observation of SEI formation and lithiation in thin-film silicon electrodes via in-situ electrochemical atomic force microscopy

Silicon (Si) has been regarded as one of the most promising anode materials to fulfill the growing demand of high performance lithium-ion batteries based on its high specific capacity. However, Si is not yet capable of replacing the widely used graphite anode, due to solid electrolyte interphase (SEI) formation and extreme volume expansion during lithiation. In this work, advanced in-situ electrochemical Atomic Force Microscopy has been applied to track simultaneously the topographical evolution and mechanical properties of thin-film polycrystalline Si electrodes during SEI formation and initial lithiation. At first, a uniform flattening of the Si surface has been found, attributed to the SEI formation. This is followed by a non-uniform expansion of the individual particles upon lithiation. The experimental findings allow defining a detailed model, describing the SEI layer formation and lithiation process on polycrystalline silicon thin-film electrodes. Our results support further research investigations on this promising material.