Deposition of metallic Li on graphite anodes is a critical aging mechanism happening in Lithium-ion cells during charging at low temperatures, high C-rates and high states-of-charge [1,2]. Furthermore, Li deposition can also have negative effects on cell safety [3]. The cause for deposition of Li on anodes are negative anode potentials vs. (Li/Li⁺). Unfortunately, anode potentials are not accessible in commercial cells since they do not contain a reference electrode. In half cells such measurements are also not possible, since the interaction between anode and cathode is absent. Furthermore, the position of the reference electrode between anode and cathode is very important to measure anode potentials correctly [4] (Figure (a)).

In the present talk, we show that it is possible to reconstruct both, anodes (graphite) and cathodes (NMC/NCA) from commercial cells (16Ah pouch/3.25Ah 18650-type) into 3-electrode full cells with an additional Li reference electrode (Figure (a)). With these 3-electrode cells, it is possible to measure the anode potentials vs. (Li/Li⁺) for all important combinations of operating parameters: (i) temperature, (ii) charging C-rate, and (iii) end-of-charge voltage. Data evaluation results in parameter sets which can exclude Li deposition on anodes (Figure (b)). The data extracted from the 3-electrode full cells are in agreement with long-term cycling tests and with Post-Mortem analysis of the original commercial cells. Furthermore, the data from the 3-electrode full cells allow to extend the cycle life of the original commercial cells significantly.

References:

[1] J.C. Burns, D.A. Stevens, J.R. Dahn, J. Electrochem. Soc. 162 (2015) A959-A964.
[2] T. Waldmann, M. Kasper, M. Wohlfahrt-Mehrens, Electrochim. Acta 178 (2015) 525-532.
[3] M. Fleischhammer, T. Waldmann, G. Bisle, B.-I. Hogg, M. Wohlfahrt-Mehrens, J. Power Sources 274 (2015) 432–439.
[4] B.-I. Hogg and M. Wohlfahrt-Mehrens, in Meeting Abstracts, The Electrochem. Soc. (2015), 343 .