Direct Observations of Graphite Anode Lithiation and Internal Short Circuits by Li Metal Deposition using Cross-Sectional in situ Optical Microscopy in Li-Ion Full Cells

The anode is the rate-limiting compound during fast-charging of Li-ion batteries, due to the reduced mass transport through the electrode.¹ Lithiation gradients within the anode build-up, leading to a higher state-of-charge at the surface of the anode, which favors Li metal deposition at these spots.² Since graphite anodes undergo a characteristic color change from gray (unlithiated graphite) to blue (LiC₁₈) to red (LiC₁₂) to gold (LiC₆) during lithiation, lithiation gradients can be observed by eye sight.³ We introduce a novel cross-sectional in situ optical microscopy set-up, which allows direct observation of lithiation gradients in graphite electrodes and Li metal deposition in real type Li-ion full cells.⁴ Simultaneous collection of the image and electrochemical data allows correlation of observed events on the cross-section, e.g. thickness changes, Li dendrite growth and internal short circuit, with the electrochemical responds. Validation of our unique set-up is performed by comparing the electrochemical responds with coin full cells and estimation of known constants: we estimate solid-state diffusion coefficients for LiC₁₂ and LiC₆ in the order of 10^-10
cm² s^-1 and find reversible anode thickness changes of 4-13%.⁴ Additionally, lithiation fronts across the anode are analyzed and are found to be in the order of thousands µm² min^-1 for both LiC₁₂
and LiC₆ in different graphite electrodes.^4,5 Li metal deposition with and without internal short circuit (ISC) is investigated.⁵
Without ISC, Li metal deposition starts during the constant-current charging step and re-intercalates into the anode during the constant-voltage charging step.⁵ Due to the absence of an anode and a separator overhang, ISCs caused by single Li-dendrites are observed.⁵ From these observations, we can draw conclusions on the self-healing of ISC caused by Li dendrites.

Acknowledgement

The authors would like to acknowledge the Federal Ministry for Economic Affairs and Climate Action (BMWK) for financial support of the Structur.e project (03ETE018E), the German Federal Ministry of Education and Research (BMBF) for funding of the project CharLiSiKo (03XP0333A) and the TEESMAT project (Horizon 2020 E.U. Framework Program, Grant Agreement N° 814106, http://www.teesmat.eu).

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