Determining Real Molecules in Operating Batteries

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In a rechargeable lithium-ion battery powering an electric vehicle, laptop computer, or cell phone, lithium ions entering the electrode leave behind solvent molecules that accumulate. For the first time, scientists revealed the structural and chemical evolution of molecules at an electrode surface in an operating battery (Nano Letters, "In situ mass spectrometric determination of molecular structural evolution at the solid electrolyte interphase in lithium-ion batteries"). They did so using a combination of in situ liquid secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM).

Scientists used a combination of techniques to reveal unprecedented insights into the changes that occur at the molecular level in an operating battery. The far right image shows the chemical changes in an operating battery during the charging process: positively charged lithium ions (red dots) are attracted by the negatively charged copper (Cu) electrode, while negative ions (green dots) migrate toward a positive electrode (outside of the image). The electrochemical reaction creates a layer—referred to as the solid electrolyte interphase (SEI) layer—near the copper electrode that is enriched in solvent molecules (blue). The chemistry of the SEI layer is critical to battery performance. (The techniques used in this research were in situ liquid secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM).)

This new technique found that, upon charging, the distribution of ions in the electrolytes is altered and becomes inhomogeneous around the electrode. The result? A layer near the negative electrode depleted in lithium ions and salts but enriched in solvent molecules. This layer may contribute to reduced battery performance.

Using this new technique could lead to new insights about the detailed molecular-level structure at electrode-electrolyte interfaces as well as how solid electrolyte interphase (SEI) reactions could be initiated in a battery. This layer affects lithium-ion transport and battery performance. Such information is critical to improve the performance of the device.

For the first time, researchers led by the Joint Center for Energy Storage Research have directly observed structural and chemical information at the molecular level in an operating lithium-ion battery. This first-of-a-kind capability combines in situ liquid secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM).

Scientists observed that, upon charging, positive lithium ions moved toward the negative electrode, while the negatively charged ions moved toward the positive electrode. Lithium ions were reduced and deposited on the negative electrode. The loss of lithium ions and migration of negative ions to the other electrode leads to solvent molecule enrichment at the interaction layer near the negative electrode. This enriched solvent layer has lower ionic conductivity and contributes to the reduction in battery performance.

Also, the researchers found that upon charging and discharging, lithium deposits formed irreversibly on the electrode (that is, did not dissolve upon reversing of the battery), further reducing the performance of the battery.

This new powerful technique can be extended to probe other electrochemical devices for gaining insights into how the devices fade and fail, and ultimately guide strategies to improve device performance.