Quantifying silicon anode restructuring during calendar aging of lithium-ion batteries by plasma focused ion beam tomography and chemical mapping

Abstract

To design high-performance lithium-ion batteries with Si anodes, it is critical to understand the underlying physical mechanisms that cause performance loss during aging. To quantify calendar aging losses, we conducted electrochemical tests on 3-electrode cells with a NMC811 cathode, a Si anode and a Li-metal reference electrode. As expected, these cells showed capacity decay and impedance rise over 8-months of testing. The impedance rise could be mainly attributed to the NMC811 cathode, except at low cell voltages wherein the Si anode is highly delithiated. Additional half-cell cycling tests with electrodes harvested from these aged cells revealed that a loss of lithium inventory (LLI) and a loss of active material (LAM) in the Si anode caused the loss in full-cell capacity. To elucidate loss mechanisms, we developed a technique that combined plasma focused ion beam (PFIB) tomography and energy dispersive X-ray spectroscopy (EDS)-based segmentation to visualize and quantify chemical and morphological properties of the electrode microstructure. These results revealed that aging led to increases in electrode thickness, electrode fragmentation, and quantity of solid electrolyte interphase (SEI). In addition, we used cryo-scanning transmission electron microscopy (STEM) imaging and electron energy loss spectroscopy (EELS) measurements to examine sub-nanometer changes in the Si particles on aging: the results showed increasing particle fragmentation and SEI formation on the newly created surfaces. This work provides a key insight into Si anode aging mechanisms through the development of a microstructure characterization technique that can resolve the various components within an electrode with the highest resolution to date.