It’s exciting in science when a conventional view is overthrown, as this opens the way for both new perspectives on old data and new experiments to design to test new theories. A great example of this challenging of convention appears in a Nature Materials article this week that provides a refreshing look at how charging rates affect battery electrodes. The results should provide a new way to interpret electrochemical data and give a new research path to explore to increase cycling lifetimes of most types of batteries.
We need batteries in a renewable energy economy because we need to be able to store energy easily that we want to use at a later time. Solar energy only arrives when the sun is out. Wind energy is only available when it’s windy. We can rely on these fairly well during certain times of days, but batteries will be necessary to store this energy for use during the night, or on still days, etc.
But this battery technology needs to get better. They need to be able to hold more charge at lower mass and weight, definitely, but they also need to be able to charge and discharge more quickly. Imagine going to an ‘electric station’ to plug in your car and wait for four hours to recharge your battery! Not happening…but the problem is that conventional wisdom (read: previous interpretations of data) have indicated that fast charging/discharging will ruin the life cycle of the battery.
The logic goes like this: charging and discharging deintercalate and intercalate the battery electrodes with an ion, typically lithium (Li). This leads to a swelling and shrinking of the electrode, since they’re taking on more mass with volume, that can lead to fractures and cracks in the electrodes, degrading its ability to perform. Previous data suggested that the rate at which you intercalate, known as the charging/discharging rate, affects this process, such that faster rates lead to worse damage to the electrodes. This is believed to occur because faster rates lead to greater phase separation in the electrode. Only certain phases are active sites for Li intercalation, and therefore those areas get a much higher current density. Such intense ‘hot spots’ can lead to much greater damage and pretty much ruin long cycling lifetimes.
Or so goes convention. Now, Li et al describe lithium iron phosphate (LiFePO4) electrodes that show very different behavior. Their team took a battery using LiFePO4 electrodes and charged and discharged it thousands of times. However, they stopped the process at varying times throughout the process in each case, essentially freezing the structure midway through the process, and sent the electrodes to a lab to use x-ray microscopy to study the structure. The results essentially give a time-resolved window into electrode structural changes during charging, looking at phase separation and active regions as a function of charging rate.
The results paint a new picture. Instead of current density increasing dramatically with fast charging due to hot spots, they find that current density is basically uniform across charging rates. Even slower charging rates show hot spots. This is because higher charging rates increase the active areas, essentially accommodating the higher current by creating larger active areas that reduce the number of dense hot spots in the electrode.
The relationship between charging rate and phase separation is a crucial one, as this dictates how quickly one can charge a battery and still get the expected performance. These new results indicate that fast charging could be possible if used with structures that respond to this higher rate by establishing larger areas of active intercalation processes. This flies in the face of previous notions, but opens up a new research path to explore which electrodes exhibit this behavior and how to take advantage of it for faster charging, longer living batteries!
Li Y, El Gabaly F, Ferguson TR, Smith RB, Bartelt NC, Sugar JD, Fenton KR, Cogswell DA, Kilcoyne AL, Tyliszczak T, Bazant MZ, & Chueh WC (2014). Current-induced transition from particle-by-particle to concurrent intercalation in phase-separating battery electrodes. Nature materials PMID: 25218062