Andrew Jay Leenheer

Lithium Electrodeposition Dynamics in Aprotic Electrolyte Observed in Situ via Transmission Electron Microscopy

Electrodeposited metallic lithium is an ideal negative battery electrode, but nonuniform microstructure evolution during cycling leads to degradation and safety issues. A better understanding of the Li plating and stripping processes is needed to enable practical Li-metal batteries. Here we use a custom microfabricated, sealed liquid cell for in situ scanning transmission electron microscopy (STEM) to image the first few cycles of lithium electrodeposition/dissolution in liquid aprotic electrolyte at submicron resolution. Cycling at current densities from 1 to 25 mA/cm(2) leads to variations in grain structure, with higher current densities giving a more needle-like, higher surface area deposit. The effect of the electron beam was explored, and it was found that, even with minimal beam exposure, beam-induced surface film formation could alter the Li microstructure. The electrochemical dissolution was seen to initiate from isolated points on grains rather than uniformly across the Li surface, due to the stabilizing solid electrolyte interphase surface film. We discuss the implications for operando STEM liquid-cell imaging and Li-battery applications.

A Sealed Liquid Cell for In Situ Transmission Electron Microscopy of Controlled Electrochemical Processes

A microfabricated liquid cell that permits imaging and controlling of electrochemical processes in a transmission electron microscope (TEM) has been developed, and its capabilities are demonstrated. The liquid cell comprises two silicon chips with suspended electron-transparent silicon nitride membranes that encapsulate and hermetically seal a thin (~100 nm) liquid layer in the TEM high-vacuum environment. Up to 10 integrated electrodes with selectively exposed areas allow multiple experiments to be performed on the same chip, and the electrode geometry has been designed to facilitate the assembly of nanostructures or nanoscale patterning of thin-film materials on the electrodes. We demonstrate the cell operation by picoampere-level electrochemical control and imaging of copper electrodeposition. A wide variety of materials and electrolytes may be studied with this cell design, and the relatively small (~1 μm2) exposed electrode areas enable quantitative electrochemical control at low currents.

Phase Boundary Propagation in Li-Alloying Battery Electrodes Revealed by Liquid-Cell Transmission Electron Microscopy

Battery cycle life is directly influenced by the microstructural changes occurring in the electrodes during charge and discharge cycles. Here, we image in situ the nanoscale phase evolution in negative electrode materials for Li-ion batteries using a fully enclosed liquid cell in a transmission electron microscope (TEM) to reveal early degradation that is not evident in the charge-discharge curves. To compare the electrochemical phase transformation behavior between three model materials, thin films of amorphous Si, crystalline Al, and crystalline Au were lithiated and delithiated at controlled rates while immersed in a commercial liquid electrolyte. This method allowed for the direct observation of lithiation mechanisms in nanoscale negative electrodes, revealing that a simplistic model of a surface-to-interior lithiation front is insufficient. For the crystalline films, a lithiation front spread laterally from a few initial nucleation points, with continued grain nucleation along the growing interface. The intermediate lithiated phases were identified using electron diffraction, and high-resolution postmortem imaging revealed the details of the final microstructure. Our results show that electrochemically induced solid-solid phase transformations can lead to highly concentrated stresses at the laterally propagating phase boundary which should be considered for future designs of nanostructured electrodes for Li-ion batteries.

Design of a Heated Liquid Cell for in-situ Transmission Electron Microscopy

Microfabricated, silicon-based chips developed for advanced capabilities in sample holders have recently led to a broad expansion of in-situ transmission electron microscopy (TEM) experiments involving materials’ responses to increased temperature, electrical bias, or mechanical stress. By including freestanding, electron-transparent silicon nitride membranes, a thin environmental chamber for gases or liquids can be created in the TEM allowing new insights into the nanoscale processes involved in electrochemical, catalytic, or biological systems. Heated gas environments in the TEM have been previously demonstrated with microfabricated chips [1-2], but little work has been done with heated liquid environments. With control over the thermal environment, systems that activate at increased temperature (e.g. nanoparticle growth, protein denaturation, corrosion) or systems that degrade with temperature cycling (e.g. battery materials) can be studied.

Understanding Reaction Mechanisms in Electrochemistry and Corrosion: Liquid-Cell S/TEM

Electrochemical and corrosion studies have greatly benefited from using liquid-cell S/TEM techniques for providing real-time information on the nanoscale mechanisms occurring at solid-liquid interfaces [1]. Within a liquid environment, nanoscale electrodes and metals undergo reactions with the solution creating surface layers and films at the interface. In batteries, these interface layers are known as solidelectrolyte interfaces (SEI). In corrosion experiments, the surface layers are known as scale materials. These systems have related domination of the surface film composition and structure impacting the overall behavior of the electrode and the rate of reaction during corrosion. Therefore, to better understand these material systems and determinant mechanisms, we are investigating using real-time imaging and spectroscopy to characterize these interfaces for initial structural identification, in-situ monitoring of interfacial processes, and post-mortem analysis of electrode/material surfaces.

Controlled Electrochemical Li Cycling in a TEM to Observe Li Morphology Evolution

To meet the increased demand for high power energy storage for grid and transportation applications, a stable highly efficient Li metal battery electrode is being investigated. The system is dependent on the formation of a solid electrolyte interphase that is capable of suppressing Li dendrite formation, which is the limiting characteristic that prevents application of high capacity Li metal anodes in current lithium ion batteries (LIBs). The SEI layer that is formed between the electrolyte and the Li metal electrode is dependent on the breakdown of the Li containing electrolyte. We investigated lithium bis(fluorosulfonyl)imide (LiFSI) in dimethoxyethane (DME) for suppressed Li dendrite formation [1]. This electrolyte is targeted for stable stripping of lithium at current densities up to 10 mA/cm 2 and Coulombic efficiencies above 99.1%. The morphological evolution of the Li deposition and stripping on copper electrodes was monitored using quantitative in situ scanning transmission electron microscopy (STEM) in a custom fabricated electrochemical cell.