Haste makes waste, as the saying goes. Such a maxim may be especially true of batteries, thanks to a new study that seeks to identify the reasons that cause the performance of fast charged lithium-ion batteries to degrade in electric vehicles.
In new research from the U.S. Department of Energy’s (DOE) Argonne National Laboratory, scientists have found interesting chemical behavior of one of the battery’s two terminals as the battery is charged and discharged.
Lithium-ion batteries contain both a positively charged cathode and a negatively charged anode, which are separated by a material called an electrolyte that moves lithium ions between them. The anode in these batteries is typically made out of graphite—the same material found in many pencils.
In lithium-ion batteries, however, the graphite is assembled out of small particles. Inside these particles, the lithium ions can insert themselves 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 cause terminal damage – no pun intended – to a battery.
“Plating is one of the main causes of impaired battery performance during fast charging,” said Argonne battery scientist Daniel Abraham, an author of the study. “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.”
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,” he said.
“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,” Abraham said.
“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.”
A paper based on the study, “Increased disorder at graphite particle edges revealed by multilength scale characterization of anodes from fast charged lithium-ion cells,” appeared in the October 8 issue of the Journal of the Electrochemical Society.
Lithium (Li)-ion batteries with graphite anodes and Li metal oxide cathodes are the dominant commercial battery chemistry for electric vehicles (EVs) (1). However, their cycle lifetime and operational stability still demand further improvements (2–5). During long-term cycling, Li-ion batteries undergo irreversible capacity decay due to decreased utilization of anode/cathode active materials, metallic Li plating, electrolyte dry-out, impedance build-up, or excessive heat generation (6–9). Some of these issues also lead to battery shorting and thermal runaway (10, 11).
To enable mass adoption of EVs, increasing efforts have been made to realize the fast charging of Li-ion batteries (12). Under this condition, all of the detrimental factors mentioned above are aggravated (6, 7, 13), further compromising the battery cycling life and safety. As a result, a clear understanding of the failure mechanisms of Li-ion batteries is crucial for their future development.
Plating of metallic Li on graphite anodes is a major cause of the capacity decay of Li-ion batteries (6, 7, 12, 14–17). Significant amounts of solid electrolyte interphase (SEI) and dead Li form and remain inactive, leading to an accelerated loss of Li inventory. It is generally believed that the slow kinetics of Li ion intercalation into graphite causes metallic Li plating (14).
Three-electrode measurements (18–25) showed that the potential of graphite anodes shifted negatively under increased charging rates and finally dropped below 0 V vs. Li0/Li+, reaching Li-plating conditions. However, Li-plating phenomenon on graphite anodes is still not fully understood. Firstly, the actual onset potential of Li plating is still unclear, which is not necessarily below 0 V vs. Li0/Li+ (18).
Furthermore, few studies explained why Li plated on graphite in spatially inhomogeneous patterns (7, 14, 17). Most importantly, in some reports, Li plates even under a moderate charging rate below 1.5 C (6, 7). Under these conditions, three-electrode measurements indicate that the anode potential does not drop below 0 V vs. Li0/Li+ (18). Kinetic arguments alone are not sufficient to resolve these problems, so we hypothesize that previously neglected thermodynamic factors may also play crucial roles in Li plating.
It is well-known that the equilibrium electrode potential of a redox reaction shifts with temperature (26–35). Exothermic reactions and joule heating during cycling raise the temperature of batteries (10), which can also build up an internal temperature gradient.
Simulations (7, 36–42) and experimental studies (41, 43–49) showed intensified heating under increased cycling rates, and temperature differences of 2 K to nearly 30 K within the batteries (10). This spatial variation in temperature leads to a heterogeneous distribution of the equilibrium potential for both Li plating and graphite intercalation on the anode, which could make Li plating thermodynamically favorable at certain locations.
In this paper, we discover that temperature heterogeneities within Li-ion batteries can cause Li plating by shifting its equilibrium electrode potential. We first introduce a method to quantify the temperature dependence of the equilibrium potential for both Li plating and graphite intercalation.
Then, we correlate the shift of the equilibrium potential to Li plating using a Li-graphite coin cell with an intentionally created heterogeneous temperature distribution and explain the observation with thermal and electrochemical simulations. Finally, the effects under fast charging conditions are examined. The data explicitly show that metallic Li can plate above 0 V vs. Li0/Li+ (room temperature) on a graphite anode.
The temperature dependence of the equilibrium potential likely participates in the capacity decay of commercial Li-ion batteries, which can be increasingly severe during fast charging conditions. This research brings insights into a key failure mechanism of Li-ion batteries, highlights the importance of maintaining homogeneous temperature within batteries, and will inspire future development of Li-ion batteries with improved safety and cycle lifetime.
reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7703581/
More information: Saran Pidaparthy et al, Increased Disorder at Graphite Particle Edges Revealed by Multi-length Scale Characterization of Anodes from Fast-Charged Lithium-Ion Cells, Journal of The Electrochemical Society (2021). DOI: 10.1149/1945-7111/ac2a7f