We report in situ measurements of initial solid electrolyte interphase (SEI) evolution on silicon negative electrode and also the lithiation-induced silicon volume changes in lithium ion half-cells. Thin film amorphous silicon (a-Si) electrodes are designed to enable unambiguous measurement of SEI thickness evolution and silicon thickness evolution. The thin film electrode samples are assembled into a custom-designed electrochemical cell integrated with an atomic force microscope (AFM) to facilitate real time measurements. The films are subjected to two potentiostatic lithiation and delithiation cycles by applying a series of constant potential values against lithium metal, which serves as the counter and reference electrode. The thickness changes are measured after a state of equilibrium is reached at each potential value. Experiments are carried out with two electrolytes – 1.2M lithium hexafluoro-phosphate (LiPF₆) in ethylene carbonate (EC) and 1.2M LiPF₆ in propylene carbonate (PC) – to investigate the influence of electrolyte compositions on SEI formation and electrochemical performance. The results reveal that the predominant SEI thickness evolution occurs during the first cycle, and reaches steady values of approximately 17nm with EC electrolyte and 10nm with PC electrolyte. Despite the thinner SEI layer, capacity loss due to SEI formation is larger with PC electrolyte compared to that of EC electrolyte. Si volume change shows approximately 21% of irreversible volume expansion during the first cycle and no appreciable irreversible volume expansion was observed in the subsequent cycle. In addition, silicon volume expansion shows an approximate linear relationship with the state of charge. Volume change and capacity of silicon films show hysteresis as functions of equilibrium potential. We seek to explain such hysteretic behavior in terms of the stress-potential coupling in silicon, which shows both qualitative and quantitative agreement. Beyond the specific materials systems considered here, the in situ AFM based experimental technique developed in this work has broader applications to investigate the role of electrolyte composition in the SEI formation kinetics.