Lithium Overpotential and Dendrite Formation

Lithium metal has a higher theoretical capacity compared to graphite, which is the most commonly used anode material in a lithium ion battery (LIB).^[1] Nevertheless, secondary lithium metal batteries have plenty of challenges to overcome, before they could replace the well established LIBs. One main challenge of these batteries is the lithium dendrite formation during cycling. In the presence of an appropriate organic electrolyte, lithium is kinetically stable against corrosion due to the formation of a solid electrolyte interphase (SEI).^[2] If the SEI is locally different in morphology and composition, the current density distribution on the lithium electrode during charging can be inhomogeneous. This leads to an inhomogeneous lithium plating process and results in a needle-like deposition, which is frequently called dendritic lithium deposition.^[3] Figure 1 depicts needle-like lithium deposition on the lithium electrode after cycling. During cycling, dendrites can grow from the anode to the cathode and can cause a short circuit or a thermal runaway.

Different types of lithium deposition morphologies are known: needle-like, mossy and granular lithium.^[4-5] The needle-like lithium (Figure 1b), with its high surface area, has most detrimental impact on cycling and safety performance of lithium metal cells. A novel method to monitor the changes of the surface morphology during cycling is the continuous observation of the development of lithium overpotential during cycling (Figure 1a).

In this work, we will explain how the overpotential occurs and how the lithium overpotentials correlate with the change of the surface structure. Furthermore, it is demonstrated that by increasing the surface area of the lithium metal electrodes and, in parallel, using a highly concentrated electrolyte, the formation of dendritic lithium can be reduced.

References

[1] von Sacken, U.; Nodwell, E.; Sundher, A.; Dahn, J. R., J. Power Sources 1995, 54(2), 240-245.

[2] Winter, M., Zeitschrift für Physikalische Chemie 2009, 223(10-11), 1395-1406.

[3] Aurbach, D.; Zinigrad, E.; Cohen, Y.; Teller, H., Solid State Ionics 2002, 148(3–4), 405-416.

[4] Gireaud, L.; Grugeon, S.; Laruelle, S.; Yrieix, B.; Tarascon, J. M., Electrochem. Commun. 2006, 8(10), 1639-1649.

[5] Sano, H.; Sakaebe, H.; Matsumoto, H., J. Power Sources 2011, 196(16), 6663-6669.

Figure 1 a) Overpotential of lithium during cycling, b) needle-like dendrites.