If you are operating or designing lithium-ion batteries for use at temperatures less than 5 C, there are concerns about lithium plating during charge. Lithium plating causes battery damage resulting in reduced life and capacity, higher resistance and decreased safety. The primary factors influencing the possibility of plating are temperature, charge rate and voltage and the design of the battery. All lithium-ion batteries are susceptible, some more than others.
Jolson has developed custom testing based on published methods to detect lithium plating.
Background:
Most lithium-ion batteries use graphitic carbon as the negative active material. During charge, lithium ions in the electrolyte intercalate into the interplanar regions of the graphite. On discharge, the lithium de-intercalates. The intercalation rate of the lithium into the graphite is limited by a number of factors. If the charge is done too fast, rather than intercalate the lithium ions will react (reduce) at the surface of the graphite to form elemental lithium, this is what is termed lithium plating. At colder temperatures, the intercalation becomes even more hindered, so the risk of lithium plating increases dramatically. It would be beneficial to users that charge batteries at cold temperatures to understand the temperatures where plating is likely, so they can take steps to either reduce charge rate or warm the batteries.
The plating of elemental lithium is a problem to avoid for several reasons. Although most of the plated lithium will discharge back to lithium ions, some of it reacts with the electrolyte to form additional solid electrolyte interface (SEI) material. This additional SEI is not desired for many reasons, as follows:
- The SEI incorporates lithium ion into its structure thus removing them from the active material. Loss of lithium to the SEI is a primary mode of cell degradation. Lithium-ion batteries are designed to have excess lithium, however, as lithium is lost to the SEI the lithium content will eventaully limit the capacity of the battery. The more lithium that is lost, the lower the capacity of the battery – a direct relationship.
- The SEI forms a resistive layer on the graphite. Although the SEI will slowly grow over the life of the battery naturally, plating will greatly increase this. The increasing cell resistance results in lower battery energy efficiency and greater heat generation.
- The SEI layer is the least thermally stable material in the battery and at temperatures above ~60° C, it will begin to decompose. As it decomposes, it generates more heat and more SEI is formed leading to the initiation of a thermal runaway. Thus, increased SEI leads to decreased safety of the cells.
- The formation of SEI is also known to generate gases from the decomposition of the electrolyte. These gases increase the internal pressure of the cell. For small cylindrical cells, when the pressures increase to the point of triggering the current interrupt device (CID) then the cell has failed (benignly). Flat batteries, often known as ‘lithium polymer’ will inflate which removes cell compression and leads to loss of performance or failure.
The bottom line is that for best performance, life and safety, lithium plating must be avoided.