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What is coulombic efficiency and why is it important to EVs?

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Coulombic efficiency (CE) is the ratio of the number of electrons transferred out of a battery to the number of electrons transferred back into the battery over a full charge cycle. It’s sometimes called Faradaic efficiency or current efficiency. The higher the CE, the less capacity a battery loses over a complete charge cycle and the longer its potential lifespan. CE is not equally applicable to all lithium (Li) battery chemistries. It can be a useful parameter for comparing various Li-ion chemistries but is less useful with Li-metal batteries (LIBs).

This FAQ looks at how CE is related to capacity retention, at typical CE values for Li-ion batteries, and how it’s affected by factors like charge current rate, state of charge (SoC), internal resistance, temperature, and aging, and concludes with a review of why CE is less applicable to LIBs.

In a perfect battery, CE would be 100%. But in real Li-ions, side reactions occur between the electrolyte and electrodes during charging and discharging, resulting in the loss of some lithium ions. The side reactions can be chemical or electrochemical, and it’s important to eliminate the sources of all side reactions as much as possible. Another source of lost capacity and lower CE can be the decomposition of the electrolyte, resulting in electrochemically inactive material.

The batteries in electric vehicles (EVs) need to have a very high CE. For example, a CE of 99% may sound high, but after 100 cycles, that battery would only retain 37% of its original capacity. For a long battery life and a consistent driving range, EV batteries need CEs of 99.98% or better (Figure 1).

Figure 1. Li-ion batteries need a CE of about 99.98% to provide the performance expected by EV owners. (Image: QuantumScape)

The CE of a battery can be improved with high-quality materials and manufacturing, but it’s also subject to several operating and use conditions: 

  • Aging can be a catch-all term. It can refer to calendar time but is strongly correlated with use conditions like too many deep discharges, overcharging, operating at too high or too low a temperature, etc. 
  • Fast charging is desirable, but there can be a penalty in terms of CE. Fast charging can result in increased battery temperatures and higher rates of side reactions in the cells. Both can reduce CE. Those effects can materially affect Li-ion CE at charge rates as low as 0.8 C.
  • State of charge (SoC) can also affect CE. Charging cells with too high a SoC can reduce CE. In addition, discharging Li-ions to a low depth of discharge (DoD) can also reduce CE.
  • Li-ion cells with a low internal resistance can be more efficiently charged and discharged and tend to have higher CEs.
  • Temperature is a critical factor. Most Li-ion cells should be charged and discharged between 0 and 45 °C. Outside that temperature range, performance suffers, and too high or too low a temperature can result in permanent damage. 

Li metal cells and CE
The definition of CE for Li-ions doesn’t transfer to LIBs. LIBs have a different structure with a Li metal anode, and Li is also stored in the cathode, resulting in ‘excess’ Li metal availability. The ‘excess’ Li means that a high CE doesn’t necessarily correlate with cycle life. As a result, the calculated theoretically expected CE can be higher or lower than the actual CE for LIBs.

The type of electrolyte used greatly impacts the CE of LIBs. For example, two LIBs were tested with different electrolytes. One battery had an average CE of 99.69%, while the other had an average CE of 99.76%. The battery with the lower CE used an electrolyte that contributed to high cycle life and was stable for over 180 testing cycles. The battery with the higher CE but an inferior electrolyte lasted only 50 cycles (Figure 2).

Figure 2. These LIBs use different electrolytes and the LIB on the left has a real CE larger than its calculated CE while the LIB on the right has a real CE lower than its calculated CE. (Image: Nature Energy)

Summary
CE is an important aspect of Li-ion performance and a good indicator of battery lifetime. It can be affected by several external factors like temperature, DoD, SoC, and aging. But it does not apply to LIBs. 

References

Images

  • Figure 1, QuantumScape, one-quarter down the page
  • Figure 2, Nature Energy, Page 3, Figures 1c & 1d

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