German researchers are declaring victory in solving the mystery of lithium plating by observing the phenomenon during the lithium-ion battery charging process.
The cathode in lithium-ion batteries comprises a lithium metal oxide while the standard material for battery anodes is graphite (carbon) with a layered structure. During the charging process, the lithium ions are stored in these layers.
However, occasionally lithium ions form metallic lithium instead of intercalating into the anode, as desired. The lithium deposits onto the anode, resulting in a drop in performance and in extreme cases to short circuits; metallic lithium is also highly inflammable.
Hitherto, this metallic lithium deposition, known as lithium plating, has been difficult to observe without destroying the battery. But researchers at Technical University Munich (TUM) have managed to observe the process with a non-destructive investigation.
Using neutron beams, the scientists Dr. Veronika Zinth at the Neutron Source Heinz Maier-Leibnitz (FRM II) and Christian von Lüders at the Department of Electrical Energy Storage Technology were able to observe the processes inside of batteries without cutting them open.
“In contrast to other methods, with neutron diffraction we can make more precise statements about when and how strongly lithium plating takes place,” said Veronika Zinth.
Using the material research diffractometer STRESS-SPEC at FRM II, the researchers installed a battery, in both charging and discharging states, into the neutron beam.
The incident neutron beam is diffracted according to Bragg’s law and collected in a detector. Using these signals, the researchers were able to indirectly deduce how much metallic lithium had formed.
Initial results of the study, published in the Journal of Power Sources under the title ‘Lithium plating in lithium-ion batteries at sub-ambient temperatures investigated by in situ neutron diffraction’, showed that the faster the charging process, the more metallic lithium is formed. At minus 20 degrees Celsius, up to 19% of the lithium ions normally involved in the charging and discharging process take on the metallic form.
During a 20-hour resting phase following a fast recharge, some of the metallic lithium reacts with the graphite, intercalating between the graphite layers as lithium ions. This was effectively a delayed, slow charging process, albeit only a part of the lithium plating is reversible.
The study also showed low temperatures encourage the formation of metallic lithium.
TUM is planning further experiments to shed more light on the lithium plating mechanism and how quickly batteries can be charged before lithium plating sets in. The results may help answer the question of how the phenomenon might be averted altogether.
