A new study by researchers from Argonne National Laboratory and the University of Illinois Urbana-Champaign seeking to identify the reasons that cause the performance of fast-charged lithium-ion batteries to degrade in EVs has found interesting chemical behavior of the anode as the battery is charged and discharged. An open-access paper on the study is published in the Journal of The Electrochemical Society.
Fast charging of LIB cells can degrade their energy and power performance characteristics. Here we show representative data from as-prepared graphite electrodes (Gr-AP) and from electrodes extracted from (discharged) cells that were previously charged at rates up to 6 C (Gr-LP). Electrochemical measurements show that the harvested electrodes have lower capacities than the Gr-AP electrode and display a sloping lithiation profile, rather than a staged profile, even at low rates.
… The implication of our study is that even a small number of high-rate cycles appears to be sufficient to induce significant and permanent disorder in graphite domains that are closest to the electrolyte, be it at particle surfaces or the inner pores. Such disorder could affect the lithiation kinetics, preventing the graphite particles from accepting charge and favoring the occurrence of Li plating. Moreover, we show that these structural changes are highly non-uniform across the various pores within a same particle, which could either be a cause or consequence of reaction heterogeneities commonly observed at high rates. Awareness and mitigation of the structural evolution at the graphite/electrolyte interfaces is crucial to the development of LIBs that can endure repeated fast-charging and yet reliably deliver high-performance over the 10 + year lifespan of electric vehicles.
The anode in Li-ion batteries is typically made out of graphite assembled out of small particles; lithium ions can insert themselves into the anode material in a process called intercalation. When intercalation happens properly, the battery can successfully charge and discharge.
When a battery is charged too quickly, however, intercalation becomes a trickier business. Instead of smoothly getting into the graphite, the lithium ions tend to aggregate on top of the anode’s surface, resulting in a plating effect that can damage the battery.
Plating is one of the main causes of impaired battery performance during fast charging. As we charged the battery quickly, we found that in addition to the plating on the anode surface there was a build up of reaction products inside the electrode pores.
As a result, the anode itself undergoes some degree of irreversible expansion, impairing battery performance.
Using a technique called scanning electron nanodiffraction, Abraham and his colleagues from the University of Illinois Urbana-Champaign observed another notable change to the graphite particles. At the atomic level, the lattice of graphite atoms at the particle edges becomes distorted because of the repeated fast charging, hindering the intercalation process.
Basically, what we see is that the atomic network in the graphite becomes warped, and this prevents lithium ions from finding their ‘home’ inside the particles—instead, they plate on the particles. The faster we charge our battery, the more atomically disordered the anode will become, which will ultimately prevent the lithium ions from being able to move back and forth.
The key is to find ways to either prevent this loss of organization or to somehow modify the graphite particles so that the lithium ions can intercalate more efficiently.
Saran Pidaparthy et al (2021) “Increased Disorder at Graphite Particle Edges Revealed by Multi-length Scale Characterization of Anodes from Fast-Charged Lithium-Ion Cells” J. Electrochem. Soc. 168 100509 doi: 10.1149/1945-7111/ac2a7f