In Situ Atomic Force Microscopy Studies of Silicon Lithiation and Delithiation in Liquid Electrolytes

Understanding the chemical and mechanical response of silicon (Si) to lithiation and delithiation in liquid, organic electrolytes is critical to successfully introducing Si into rechargeable lithium ion batteries.  Among Si materials, nanoscale Si, especially amorphous Si (a-Si), has demonstrated some of the best performance to date.  Nevertheless, an understanding of solid electrolyte interphase (SEI) growth and volume change hysteresis during cycling is limited.  To this end, in situ experiments are conducted using an atomic force microscope (AFM) to acquire both qualitative and quantitative data.  Using several electrolytes and electrolyte additives, the growth of SEI on thin film Si is analyzed.  Additionally, precision Si nanostructures are fabricated using electron beam lithography, and the SEI growth, volume changes, and mechanical properties of these structures are recorded.  The mechanical properties including elastic modulus and hardness are determined by indenting the nanopillar with a well-characterized AFM tip such that images of the pillar are acquired along with the numerical data of the indent.  The results demonstrate the Si suffers permanent degradation of mechanical properties on the first cycle despite appearing to physically remain intact.  The ability to determine the structural integrity of the Si before, during, and after lithiation in conventional, liquid electrolytes with and without additives is unique to AFM techniques which physically probe the sample.  Additionally, in situ AFM studies of nanoporous Si and nanoscale Si structures covered by atomic layer deposition (ALD) thin films are discussed.  The in situ AFM results presented in this work will help to guide experimental and computational modeling research by providing both qualitative information on how SEI on Si grows as well as numerical values for mechanical properties of Si as it is lithiated and delithiated.