We report progress in developing Li⁺ focused ion beams (FIB) as a novel probe for exploring nanoscale electrochemistry in battery-relevant materials. Electrochemical processes involve numerous parallel reactions that complicate our microscopic understanding of these systems, and new tools are needed to probe ionic transport at the level of grain boundaries, interfaces, and single defects. Here, we demonstrate the use of Li⁺ FIB to direct-write controlled quantities of Li into 35 nm thick crystalline silicon membranes in vacuo at doses from 10¹⁵ cm^-2 to 10¹⁸ cm^-2. The concomitant morphological, structural, and functional changes in the implanted silicon are characterized using a combination of electron and scanning probe microscopy. We observe saturation of interstitial lithium in μm-scale implanted regions as well as spill-over of excess lithium on the membrane’s surface. Prospects for future work exploring transport across grain boundaries and implantation at higher resolution are discussed.