Silicon anodes are a promising choice to replace graphite anodes in lithium ion batteries for electric vehicle applications because they have a near 10-fold increase in theoretical energy density. Unfortunately, the solid electrolyte interphase (SEI) on silicon has been found to degrade both mechanically and chemically. The strain due to SEI formation versus the strain in the electrode due to lithiation and delithiation is not well understood. This work used in situ moiré microscopy to make in-plane strain measurements with an emphasis on the observed strain during SEI formation. A novel method was used to create smooth, 50 nm silicon thin films on 2 µm thick copper foil electrodes. A sample grating was patterned on the backside of the electrodes for use with an in-house moiré microscope (see figure). The reference grating was incorporated into the microscope and imaged onto the sample to form moiré patterns which were then used to calculate in-plane strain. Strain measurements were obtained in situ with constant current cycling and cyclic voltammetry. Special care was taken to try and deconvolute the different sources of strain. The changes in strain due to variations in electrolyte additives and the introduction of electrolyte were explored. Figure 1