ORNL microscopy directly images lithium dendrite formation in batteries; new technique for studying electrochemical processes
Scientists at the Department of Energy’s Oak Ridge National Laboratory have captured the first real-time nanoscale images of lithium dendrite structures known to degrade lithium-ion batteries. A paper describing their study, which probed the mechanisms of solid electrolyte interphase (SEI) formation and dendrite growth in a standard organic battery electrolyte (LiPF6 in EC:DMC), is published in the ACS journal Nano Letters.
They combined quantitative electrochemical measurement and STEM (scanning transmission electron microscopy), or in situ ec-S/TEM, to estimate the density of the evolving SEI and to identify Li-containing phases formed in the liquid cell. They reported that the SEI is approximately twice as dense as the electrolyte as determined from imaging and electron scattering theory. They also observed site-specific locations where Li nucleates and grows on the surface and edge of the glassy carbon electrode. The ORNL team’s electron microscopy could help researchers address long-standing issues related to battery performance and safety.
Li metal is an attractive anode material for rechargeable batteries owing to its high theoretical capacity (3860 mAh g−1) and highly negative reduction potential (−3.04 V vs standard hydrogen electrode). However, two critical issues have limited the practical use of Li in rechargeable batteries: its incompatibility with most liquid electrolytes resulting in the formation of a relatively unstable solid electrolyte interphase (SEI) and its propensity to form dendrites that can short-circuit the battery.
An unstable SEI can result in continuous “cracking” and “healing” of the layer due to volumetric expansion and contraction, which will result in a continual irreversible capacity loss as more and more Li-ions are required to repassivate the anode surface during SEI repair. Li-ion transport through inorganic and organic components can result in chaotic transport pathways. It has been reported that, while inorganic components of the SEI may be more electrochemically stable than the organic phases, they may also significantly decrease lithium transport. This in turn may facilitate increased site-specific lithium deposition and lead to enhanced Li dendrite nucleation and growth.—Sacci et al.
The researchers studied dendrite formation by using a miniature electrochemical cell that mimics the liquid conditions inside a lithium-ion battery. Placing the liquid cell in a scanning transmission electron microscope and applying voltage to the cell allowed the researchers to watch as lithium deposits—which start as a nanometer-size seed—grew into dendritic structures.
It gives us a nanoscopic view of how dendrites nucleate and grow. We can visualize the whole process on a glassy carbon microelectrode and observe where the dendrites prefer to nucleate and also track morphological changes during growth.—ORNL’s Raymond Unocic, in situ microscopy team leader
In addition to imaging the structures at high-resolution, the team’s microscopy technique gathered precise measurements of the cell’s electrochemical performance.
This technique allows us to follow subtle nano-sized structural and chemical changes that occur and more importantly, correlate that to the measured performance of a battery.—Robert Sacci, ORNL postdoctoral researcher and lead author
This real-time analysis in a liquid environment sets the ORNL team’s approach apart from other characterization methods.
Usually when you run a battery over many charge-discharge cycles, you typically wait until things start failing and at that point you perform a root-cause failure analysis. Then you see there’s a dendrite—but so what? Now that we can see exactly how the dendrites are forming using our technique, we can be proactive and devise strategies for inhibiting or reducing these phenomena.—Raymond Unocic
The ORNL team believes scientists who are experimenting with different ways to tackle the dendrite problem, such as liquid additives or stronger separators, will benefit from its research.
… the interplay between both the quantitative electrochemical measurement and quantitative image analysis makes in situ ec-S/TEM a powerful and informative technique for studying electrochemical processes.—Sacci et al.
This research was supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by DOE’s Office of Science. The study also used resources at Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL.
Robert L. Sacci, Jennifer M. Black, Nina Balke, Nancy J. Dudney, Karren L. More, and Raymond R. Unocic (2015) “Nanoscale Imaging of Fundamental Li Battery Chemistry: Solid-Electrolyte Interphase Formation and Preferential Growth of Lithium Metal Nanoclusters” Nano Letters doi: 10.1021/nl5048626