Direct Observation of Li Dendrite Growth through Operando electrochemical (S)TEM

The high demand for new energy storage materials has created a need for experimental techniques that can provide real-time information on the dynamic structural changes/processes that occur locally at the electrode/electrolyte interface during battery operation. In this regard, in-situ electrochemical stages for (scanning) transmission electron microscopes ((S)TEM) enable the fabrication of a “nano-battery” to study the fundamentals of electrochemical processes under operando conditions with the high spatial and temporal resolution of an electron microscope.  Here, we describe quantitative operando observations using an in-situ liquid electrochemical (S)TEM cell to study lithium dendrite formation (results were obtained from a range of electrodes/electrolytes combinations including a Pt microelectrode and LiPF₆ in PC electrolyte).  Images in the STEM usually have mass thickness contrast, meaning that something thicker, heavier/more dense appears darker in the bright field image and lighter in the dark field image.  As Li metal is lighter and less dense than the surrounding electrolyte, the formation of dendrites appears to be light in the bright field image and dark in the dark field image – the contrast is effectively “reversed”.  Such contrast is unique to Li metal and provides a very straightforward way to identify Li metal (and therefore dendrites) during the nano-battery operation in the microscope. From the individual STEM images (that are combined to form video rate movie of the dynamic process), the amount of Li deposited/incorporated into the electrolyte during the charge/discharge cycle can be directly quantified and correlated with the standard ex-situbulk scale cyclic voltammetry measurements, thereby providing a direct nanoscale view of the whole electrochemical process.  Results show that the amount/morphology of Li deposited changes after the first cycle due to the differences in initial electroactive surface area of the bare Pt electrode and the surface roughness of Pt electrode after Li dendrite deposition in subsequent cycles.  Furthermore, this morphology and the amount of “dead” Li can be controlled by application of various types of additives that can drastically suppress the growth or change the morphology of the Li dendrites.  These enables for direct link between the STEM images and the electrochemical behavior can be extended to any combination of next generation battery systems (Na, Mg, Zn, Al etc in any aqueous or none-aqueous electrolytes) to provide significant insights into the electrochemistry of conventional battery systems on the nanometer scale.

Acknowledgments

This work was supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the US Department of Energy, Office of Science, and Basic Energy Sciences. The research is also part of the Chemical Imaging Initiative at Pacific Northwest National Laboratory under Contract DE-AC05-76RL01830 operated for DOE by Battelle. A portion of the research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.