The most common effects of lithium plating include:  

• Reduced capacity,
• Faster aging, and
• Increased internal resistance, which causes slower acceleration in EVs and longer charging time.

Now, let’s dig into the details. Plating causes temporary metallic lithium deposits on the anode surface. Over time the deposits can be reused during a subsequent discharge or will diffuse into the anode again. All the lithium that is recovered this way is called reversible plating and has no further effect on the cell.  

Depending on the severity and morphology of the plating, some plated lithium can react with the electrolyte or damage the anode surface. This is called irreversible lithium plating. Once it starts, it becomes self-reinforcing.  

Under normal operation, the surface of the anode is covered by an insulating layer composed of degenerated electrolyte called the surface electrolyte interphase (SEI). This layer is formed in the early life of a battery and will keep growing slowly over time. The occurrence of plating can damage this layer and force a re-formation of the layer, consuming electrolyte and lithium. Due to the consumption of lithium during this unwanted reaction, plating reduces the amount of active lithium inventory in the cell, leading to a loss of capacity.  

Additionally, metallic lithium can lose contact with the anode surface and be completely covered by insulating SEI, rendering it inert and leading to further capacity loss. Capacity loss caused by plating is much larger than the loss caused by normal battery cycling aging.

This newly formed SEI layer can also have a different morphology, being much thicker than the original layer and containing cavities that were created by the deposited metallic lithium. This inhibits ion flow through the SEI and increases the internal resistance of the cell. An increase in internal resistance reduces the battery's performance by reducing the maximum available power. In EVs, an increase in internal resistance would cause slower acceleration and longer charging time.  

And that’s not all! The accumulation of lithium reaction products in the cell is also linked to the sudden death or knee point effect. Here the cell’s aging rate is suddenly and drastically increased, with the cell losing its remaining capacity within a few cycles. For the first life of the battery, the planned end of life of a cell is at around 70 – 80% remaining usable capacity; for second-life applications, a knee point effect can be detrimental.

Apart from the significant capacity loss and increased internal resistance, plating also compromises battery safety. Lithium ions will find the shortest way to deposit, so they prefer locations that already have plating. This can lead to a localized growth that will eventually result in what’s called a lithium dendrite on the anode surface. Dendrites look a little bit like narrow trees with sharp branches.  

In the worst case, dendrites can pierce through the separator between the anode and cathode, causing a short circuit in the cell. Short circuits cause rapid cell self-discharge and can even lead to a thermal runaway, resulting in a battery fire.

If plating is detected during the charging process, it can be prevented. But most existing methods can only detect a plating event after it occurred. These methods include the observation of the discharge curve for a stripping plateau. This is a plateau in the cell voltage. It happens because the metallic lithium deposits on the anode surface create a higher discharge voltage.  

A similar approach can be utilized for a relaxation phase after a plating event, where the voltage will first relax towards the metallic lithium potential on the surface and, over time, as some of the lithium diffuses back into the anode, cause a transition to the normal graphite anode potential.

There are also experimental methods to detect plating during charging with additional external equipment, such as very precise measurements of the changes in cell temperature or measurements of the cell thickness. These methods also require expensive additions to the battery management systems.  

There are some promising results in research that try to utilize the change of internal resistance during a plating event. These methods could be used in the near future to detect plating directly during the charging sequence and warn the user or shut down the charge procedure if plating is detected.

Until then, in most real-world applications, the only signals recorded are the cell voltage, current, and temperature measurements. However, the temperature is not usually measured at each cell, so it cannot be utilized to detect plating on an individual cell. The current is a controlled parameter that could only be implemented to prevent plating because the threshold at which current plating occurs is dependent on the temperature.  So, in practice, the most reliable indicator to detect the onset of plating is in the cell voltage.  

How cloud-based battery analytics help detect plating

With the use of battery safety analytics, continuous safety monitoring can recognize early signs of a failure and unsafe behavior that could be caused by lithium plating. ACCURE's cloud-based battery analytics solutions also have access to larger data sets that help in understanding the plating indicators and inform the development of new algorithms and the application of existing analytic methods to prevent plating before it can happen. Various use cases can provide different insights, helping make the methods more robust and reliable.  

The more we know about batteries and their behavior in the real world, the better we can manage them to increase safety and reliability, and extend lifetimes.

Learn more about battery safety and types of battery faults in the Battery Safety White Paper.