Direct-Write Lithiation of Silicon Using a Focused Ion Beam of Li

Electrochemical processes that govern the performance of lithium ion batteries involve numerous parallel reactions and interfacial phenomena that complicate the microscopic understanding of these systems. To study the behavior of ion transport and reaction in these applications, we report the use of a focused ion beam of Li⁺ to locally insert controlled quantities of lithium with high spatial resolution into electrochemically relevant materials in vacuo. To benchmark the technique, we present results on direct-write lithiation of 35 nm thick crystalline silicon membranes using a 2 keV beam of Li⁺ at doses up to 10¹⁸ cm^-2 (10⁴ nm^-2). We confirm quantitative sub-microm control of lithium insertion and characterize the concomitant morphological, structural, and functional changes of the system using a combination of electron and scanning probe microscopy. We observe saturation of interstitial lithium in the silicon membrane at ≈10% dopant number density and spillover of excess lithium onto the membrane's surface. The implanted Li⁺ is demonstrated to remain electrochemically active. This technique will enable controlled studies and improve understanding of Li⁺ ion interaction with local defect structures and interfaces in electrode and solid-electrolyte materials.