Optical microscopy coupled with electrochemistry has revealed the onset of lithium plating on graphite electrodes, a side reaction that prevents the fast charging of lithium-ion batteries.Safe and efficient fast charging of lithium-ion batteries is one of the biggest challenges facing electric vehicles. During this process, unwanted side reactions can take place inside the battery, particularly lithium plating, compromising operation. Using in-situ optical microscopy, researchers from Massachusetts Institute of Technology led by Martin Z. Bazant have revealed how lithium plating occurs on graphite particle anodes [Gao et al., Joule 5 (2021) 393-414, https://doi.org/10.1016/j.joule.2020.12.020].
“We were hoping to find the true reason for the onset of lithium plating,” explains Tao Gao, first author of the study, now at the University of Utah. “We [wanted] to understand why and how lithium plating occurs in a graphite anode, because this critical side reaction determines a battery’s charging performance, durability, and safety.”
During charging, lithium ions are extracted from the cathode and move through the electrolyte to the anode, where they are reduced. Ideally, lithium ions are inserted into the graphite anode (intercalation) but can also be reduced to metallic lithium under certain conditions. Until now, it was generally believed that this plating occurs when the voltage of the graphite anode drops below 0 V. Avoiding lithium plating is essential, as even small amounts of the metal in a battery affect the performance, durability and stability. Loss of lithium ions results in lower storage capacity, internal resistance can increase, hindering ion transport, and metallic structures known as dendrites can lead to short circuits and thermal runaway.
Using a model system of pyrolytic graphite, the researchers were able to study the onset on lithium plating in detail using optical microscopy. Different phases of graphite have unique colors, so the researchers were able to track the process of lithium intercalation, phase separation, and plating.
“We observed lithium insertion on a single graphite particle, its phase transformation and plating using in-situ optical microscopy,” says Gao. “Based on this, we developed a physics-based model that can predict the onset of lithium plating in lithium-ion batteries.”
The researchers believe that lithium plating occurs on saturated surfaces, where the edge planes of graphite are fully filled so further lithium-ion insertion is blocked. According to their observations, voltage does not play a significant role in the lithium plating process.
“The proposed new mechanism based on our results… successfully resolves the conflicts between the thermodynamic criteria and many experiment results,” says Gao. “It provides a physics-consistent picture that can explain the onset of lithium plating.”
A better understanding of the onset of lithium plating will help in designing battery systems that avoid or mitigate the problem. Ultimately, improved battery design could enable fast charging without limiting durability and safety.