We describe the first direct in-situ, operando measurements of Li transport in an operating cell. The architecture of the experiment, useful in viewing battery failure in-situ, is described in S. J. Harris, A. Timmons, D. R. Baker, C. Monroe, “Direct in situ measurements of Li transport in Li-ion battery negative electrodes,” Chem. Phys. Letters 485, 265 (2010).
The electrochemical (current-voltage) data can be successfully modeled with a diffusion equation that contains no material or microstructural information. However, this does not mean that we can extract a meaningful diffusion coefficient from the data, since the data are also consistent with more complex transport mechanisms.
Our approach is to place 1 cm**2 positive and negative electrodes next to each other. This experimental arrangement has several advantages for performing studies on Li transport compared to a more conventional face-to-face arrangement
- By placing the electrodes “face up” we can take advantage of graphite’s color changes upon lithiation to measure in-situ, operando spatial profiles of intercalated Li.
- The side-by-side arrangement leads to large concentration gradients. While undesirable in a conventional cell, large gradients are useful when the goal is to make transport measurements.
- Our arrangement, in effect, converts the problem of measuring Li transport perpendicular to the current collector through a distance of about 0.1 mm, to the problem of measuring Li transport parallel to the current collector through a distance of about 10 mm, making the measurement easier.
Our technique can also provide data for studying Li plating and Li dendrite growth, both of which can cause lithium battery degradation and failure
The video shows the initially grey electrode that contains minimal Li.
We first see a blue color (dilute stage 2, LiC18) washes over electrode, starting from the edge nearest the lithium metal and moving away from the edge.
This is followed by red (stage 2, LiC12) and then gold (stage 1, LiC6) bands, where the red-blue and gold-red boundaries also start at the electrode edge and move away from that edge. The gold region continues to expand into the red region until most of the visible electrode is gold.
Visible region is approximately 1 mm from bottom to top, and the entire video represents about 1 day of charging.