21Aug 2015

One of the serious consequences of fast charging lithium-ion batteries is the formation of lithium metal on the surface of the anode (the negative electrode when the battery is being charged). While the battery industry has invested significant effort to ensure the mechanical integrity of the battery and avoid unintended fires in case of mechanical damage, the formation of lithium plating during fast charging is a new challenge to battery vendors. Some battery manufacturers take it very seriously, whereas others tend to me more lax if not somewhat cavalier about its risks.

Let’s be clear about…Lithium metal plating inside the battery creates extremely hazardous conditions that may lead to fires or even exploding batteries.  Lithium plating leads to the formation of lithium metal deposits on the surface of the graphite (carbon) anode. These islands tend to grow over tend, both across the surface area and in the thickness forming dendritic-like structures. If they pierce (and they can) the separator — the porous plastic layer between the two electrodes — then an electrical short-circuit occurs leading to excessive heating and potential fires (in battery parlance, it is politely known as thermal runaway).

For the most part of the history of the lithium-ion battery, lithium plating was not a major concern. Well designed batteries ensured that they stayed away from the precursor conditions to lithium plating. Some battery manufacturers implemented additional safety measures — such as special surface coatings — that are intended to reduce the risk of a dendritic short-circuit. But with advent of high energy density cells and the rapid deployment of fast charging, the batteries are often operating near dangerous conditions. And some battery manufacturers seem to intentionally skirt the problem as it is not visible during daily operation — that is until a fire occurs and the damage is done.

The next photograph shows the anode surface of a dissected polymer lithium-ion cell — in fact, two identical cells, one charged at a slow charge rate (left side), and at a higher charge rate (right side). The cells were cycled 100 times before cut open and observed.

plating2

On the left side, the surface of the graphite anode is pristine. On the right side, bright stripes of lithium metal are apparent on the edges. That’s where lithium metal tends to start forming — the current density on the edges tend to be higher (concentrated electric field lines) thus presenting favorable conditions for the formation of lithium metal. Additionally, manufacturing defects are more likely to be present on the edges, also presenting “seeds” for plating.  As the cell is further cycled (and aged), the lithium plating propagates and covers more of the anode surface, creating increased risks of a catastrophic failure.