The initial cycling of a LIB, regardless of anode material, results in the formation of a solid electrolyte interphase (SEI) layer between the electrolyte and the anode. This passivating layer, consisting of decomposition products from the electrolyte solution, is critical to the reliability and performance of the battery. The SEI must be both electronically insulating and ionically conductive: permeable to lithium ions yet resistant to electron flow. The resistance can be inhomogeneous along the anode plane or perpendicular to it. To our knowledge, no characterization technique has been capable of measuring spatial variation in SEI resistivity.
In order to measure SEI resistivity with nanometer-scale resolution, our group has utilized scanning spreading resistance microscopy (SSRM) with a specialized atomic force microscopy (AFM) system installed in an argon glove box to minimize sample exposure to oxygen and water. The setup employs a doped diamond coated silicon probe that exhibits high wear resistance and low electronic resistance. Through application of a sample-probe bias voltage while utilizing variable forces on the probe in AFM contact mode, the technique allows for the measurement of SEI resistivity laterally and vertically. The vertical depth of measurement is controllable by the applied probe force (as well as raster scan line density) and is thus capable of milling away SEI material to measure resistivity at defined depths. Subsequent AFM height measurement allows for measured resistance to be associated with the depth of measurement. The electronic system attached to the system measures current through the probe-sample with a logarithm-scale amplifier, which can be directly converted to resistance or resistivity.
Our measurements of resistance vs. depth for SEIs formed on single-crystal silicon wafers  after a single cycle of lithiation/delithiation demonstrate strong trends for decrease in resistance as the probe penetrates deeper levels of the SEI. Such plots provide an interesting basis for the study of SEIs formed under different cycling conditions with distinct electrolyte solutions.
Figure: (a) Schematic diagram of electronic setup of the described system, (b) resistance map at a depth of 43 nm on a SEI formed on a single-crystal silicon wafer , with a single cycle of lithiation (0.01 V for 5 hours) and delithiation (1.0 V for 5 hours) in a battery cell containing ethylene carbonate (EC) and diethyl carbonate(DEC) electrolytes, (c) topography map displaying the SEI depth of resistance mapping.