Utilization of AFM for Observing Early-Onset Mechanisms of Lithium-Metal
Authors: Efaw Corey M. , Pawar Gorakh Machindranath , Davis Paul H , Liaw Boryann (Bor Yann) , Li Bin , Dufek Eric J. , Hurley Michael 
Published: July 2021

Lithium metal is seen as a “Holy Grail” material for battery technology, due to its high capacity and low electrochemical potential. However, the intrinsic plating/stripping mechanism of lithium metal, coupled with heterogeneous ionic pathways leads to treacherous dendrite growth and rapid consumption of active materials, diminishing cycle life. Variability in the mechanical and electrochemical properties of lithium’s surface initiates a cascading effect that ultimately ends in the failure mechanisms that plague lithium metal.

Simulations and ultra-resolved techniques (e.g., cryo-TEM) can provide fundamental understanding of certain phenomena but need to be closely coupled with realistic conditions to drive commercialization of lithium. Cell-level studies can provide clarity between different parameters, such as temperature, pressure, lithium preparation, electrolyte optimization, and cathode characteristics. However, cell-to-cell variability makes repeatability of results difficult, slowing the advancement in the readiness-level of novel cell designs. In this work, phenomena at the micro- to nanoscale were observed to better understand the behavior of lithium. At this scale, investigation of Li+ flux during deposition and dissolution is a crucial piece to solving the lithium-metal battery puzzle. This was accomplished with the use of different atomic force microscopy (AFM) techniques, coupled with other characterization tools to correlate to cell-level evaluation. The multi-functional capabilities of AFM may provide a bridge between atomistic/fundamental studies and cell-level testing, thanks in part to its non-destructive nature, high reproducibility, and ability to decipher the effect of various parameters on reaction mechanisms at the electrode-electrolyte interface.

Scanning Kelvin probe force microscopy (SKPFM), electrochemical AFM (EC-AFM) and quantitative nanomechanical mapping (QNM) were utilized to acquire spatially resolved properties for different use conditions and under different design parameters. SKPFM is a tool used to measure the surface potential, which directly relates to the work function. With this tool, local heterogeneities can be distinguished as a function of surface treatment or operation in a cell-level build. EC-AFM is utilized to observe mechanistic variations in-situ, detecting morphological changes caused by either lithiation/de-lithiation or solid-electrolyte interphase (SEI) breakdown/formation. Coupled with QNM, EC-AFM can also provide simultaneous topography and mechanical properties such as adhesion, modulus, and deformation.

These AFM tools can provide high-resolution insight into mechanistic phenomena such as dendrite/pit growth, “dead” Li formation, as well as SEI formation, furthering the understanding of the lithium-metal anode. The investigations accomplished with AFM advances the field of battery technologies, providing insight into the initiation and propagation of failure mechanisms, along with how to solve them.