Research on lithium batteries has shown that plating (deposition) of metallic Li is an important battery degradation and failure mechanism and a safety concern of Li-ion batteries because it can lead to short circuits and uncontrollably energetic chemical reactions.
When the charging current is very high, as might occur during regenerative braking, the transport rate of Li+ ions to the graphite negative electrode exceeds the rate that Li+ can be inserted (intercalated) into the graphite. Under these conditions, Li+ may deposit as metallic Li, which can lead to a short circuit, degrading the battery’s life and durability. Here is an MCMB electrode, observed in situ, operando, as it is being plated with Li in the experimental system described in “Direct in situ observation and numerical simulation of non-shrinnking core behavior in MCMB graphite,” Journal of the Electrochem Soc 159, 1501 (2012) Red particles are Stage 2 graphite; gold particles are Stage 1 graphite. Note that the colors have been enhanced with Photoshop. (This video takes some time, but it’s worth it.) From the video, we can count nuclei and determine the Li particle growth rate.
Lithium deposition (plating) as a mechanism for degradation and failure is especially likely at low temperatures, because the “proper” alternative, chemical insertion into the graphite electrode, becomes too slow, and Li plating becomes a viable alternative. However, whether or not Li plating occurs may also depend on the battery’s previous history, state of charge (SOC), and chemistry. (We note that the State of Charge is determined by the amount of Li present—when all of the available Li is in the negative electrode, the cell is fully charged; when it is in the positive electrode, the cell is fully discharged.) Many battery systems now operate simply by fixing a single, global maximum charging rate, because the onset of deposition depends on so many factors—a far-from-optimal solution. Additional Lithium battery research is required to understand this degradation mode.
The video shows that
(1) Li particles appear to nucleate readily but sparsely, and they grow slowly
(2) Li particles seem to avoid forming on the most lithiated (gold) graphite particles
(3) A Li particle can sit on a stage 2 (red) graphite particle for many hours without lithiating the particle to stage 1 (gold), suggesting that many of these MCMB particles are sufficiently defective to prevent lithiation to the theoretical limit.
Visible region is approximately 1 mm from bottom to top, and the video represents about 1 day.