Coulombic Efficiency Demystified
Let’s say you have a 1-liter bottle of water and you pour all of its contents into another empty 1-liter bottle and then back to the original bottle. A few drops of water spill as you do so. The loss is hardly noticeable - the drops may account for only 1% of the total, so you still have 99% of the water that you started with. This example essentially illustrates Coulombic efficiency - the ratio between the number of electrons (units of electrical charge) transferred from one electrode of a battery cell to the other during charge and the number transferred back during discharge. The difference between these two numbers typically reflects the fact that some lithium ions are lost during the charge/discharge process. The higher the Coulombic efficiency, the less capacity the battery loses in each charge/discharge cycle, and the longer its potential lifespan.
In our water bottle example, the equivalent Coulombic efficiency would be 99% — total (100%) minus percent lost (1%). 99% may sound like really good retention, but when you think about this happening many times in succession, the effect quickly adds up. For example, if you continue to pour the water back and forth between the two bottles 100 times, losing 1% of the water in each pour, you’ll end up with ~37% of the original volume, or 370 mL — and that loss will be very noticeable.
This is similar to what happens in batteries. Lithium ions in a battery are like the water in our analogy, with each bottle pour representing another battery charge/discharge cycle. As lithium ions move between the anode and cathode during charge and discharge, some are lost to side reactions. No physical system is ever perfectly efficient, and although 99% efficiency may sound excellent, the following chart demonstrates how the energy storage capability of a lithium-ion battery with a Coulombic efficiency of 99% decays dramatically after only a few dozen cycles.
One important caveat: to measure Coulombic efficiency accurately, the effects of any excess lithium must be excluded. QuantumScape’s technology does not require extra lithium foil on the anode, but some other lithium-metal technologies do. A cell that contains extra lithium-metal foil on the anode may temporarily mask capacity fade in cells with low Coulombic efficiency, and Coulombic efficiency may even appear to exceed 100% during early cycles. However, adding extra lithium foil does not solve the underlying problem, and carries penalties in the form of added materials cost and a more complex manufacturing process, which both make the cell more expensive.
Electric vehicle batteries must have excellent Coulombic efficiency, but that is not enough on its own. Battery cells can face a lot of challenges over a lifetime of hundreds of charge/discharge cycles, such as resistance growth or dendrite formation. A cell’s Coulombic efficiency represents the maximum limit of its cycle life performance, assuming these other problems don’t get in the way.
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