As we all know, lithium-ion (Li-ion) battery technology has not only given more autonomy to our electronic devices, such as smartphones, but has also given a great boost to the development of electric vehicles. These batteries have been optimized so that they can deliver a high energy density safely for a long lifetime, however one of the aspects that still limits their use is the speed of charging, especially to improve the acceptance of electric vehicles, they must be ready for use in a short period of time.
Fast charging of Li-ion batteries is necessary for the massification of electric transport. Source: edited powerpoint image, original from publicdomainvectors.
In general, a Li-ion battery takes about an hour to charge, during which time it is safely fully charged, with minimal degradation of the cell components. This is perhaps not very attractive to many people, compared to the time it takes to fill up the fuel tank at a conventional gas station, so attempts have been made to develop lithium-ion batteries that can be fully charged in a few minutes, but this results in accelerated degradation of the cell, causing irreversible damage due to the loss of mobility of the lithium ions.
A Li-ion battery is a device composed of two or more electrochemical cells designed to store and deliver electrical energy, which uses a lithium salt to produce the ions responsible for generating the chemical reaction between the cathode and anode responsible for producing the flow of electrons. In modern high-energy batteries, such as those used in electric cars, the positive electrode, or cathode, is composed of a layered oxide of a transition metal such as Ni, Co and Mo while the anode, or negative electrode, is composed of graphite.
General diagram of a Li-ion cell. Source: image created in powerpoint.
Now, during cell charging, the lithium ions are extracted from the cathode and transported to the anode, where they are intercalated between the planes of the graphite particles, what has been detected when trying to subject batteries of this type to fast charges, is that there is an enrichment of salts at the cathode and a depletion of salts at the anode, which leads to an uneven distribution of lithium ions increasing the thickness of the electrode. This condition results in a reduction of ion conductivity in the electrolytic medium and polarization of both electrodes, leading to irreversible damage to the cell.
In this regard, new research at Argonne National Laboratory has discovered an interesting chemical behavior of one of the two battery terminals when the battery is charged and discharged in response to a U.S. Department of Energy (DOE)-funded initiative to identify the chemical processes that limit fast charging of Li-ion batteries in order to develop the technology needed to complete charging a battery in 10 minutes or less. (DOE) funded initiative, the purpose of which is to identify the chemical processes that limit the fast charging of Li-ion batteries with the goal of developing the technology needed to complete the charging of a battery in 10 minutes or less.
According to the authors of the study, they claim to have observed that the anode coating is one of the consequences that cause the deterioration of batteries undergoing fast charging. As mentioned above, during battery charging, lithium ions move through the electrolyte to the graphite anode, which is inserted between the atomic planes in a process called intercalation, a process in which, if done correctly, batteries can be charged and discharged.
However, during fast charging this process is not carried out properly, and instead of intercalation occurring, the lithium ions move in a disorderly manner and tend to accumulate on the surface of the anode, causing a coating effect, which results in damage to the battery. But in addition to the coating of the anode, the researchers point out that the insertion of reaction products into the pores of the electrode also occurs, which also causes the electrode to expand and deteriorate.
During fast charging, lithium ions accumulate on the surface of the graphite electrode. Source: image created in powerpoint.
Basically, the fast charging process produces an irreversible deformation of the graphite atomic lattice, and the insertion of other products in the spaces of this lattice, with which, the lithium ions do not find a space to insert themselves, and instead, they begin to coat the electrode producing a kind of plating.
This research is important as it provides an explanation as to why Li-ion batteries cannot be fast-charged, but at the same time it opens the way to find solutions to this problem, as according to the researchers, the key is now to focus on finding ways to avoid deformation of the graphite atomic lattice during fast charging, or to find a way for the lithium ions to insert themselves into this lattice in an organized way during the process, and thus avoid loss of battery performance.
Hopefully, this type of research will continue to show us the aspects that need to be improved in order to obtain fast-charging batteries that will allow greater adoption of electric vehicles.
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