Researchers from the US Department of Energy’s Brookhaven National Laboratory and Stony Brook University have combined in situ EDXRD with ex situ XRD and XAS measurements to visualize the formation of the conductive silver matrix within an Ag2VP2O8 electrode used in a specialized medical battery. From this, they were able to elucidate a rate-dependent discharge mechanism: that by using lower current densities early in the discharge of a multifunctional bimetallic cathode–containing cell, it is possible preferentially to form metallic silver that is more evenly distributed, resulting in the opportunity for more complete cathode use and higher functional capacity. The work (Kirshenbaum et al.) appears in the journal Science.
Although silver compounds may be too expensive for applications other than medical ones, observed Nancy J. Dudney and Juchuan Li from Oak Ridge National Laboratory in a Perspective in the same issue of Science, the study is “an exciting step toward understanding how optimized battery electrode architectures can maximize the energy per unit volume and weight.”
Armed with this insight into battery cathode discharge processes, we can target new materials designed to address critical battery issues associated with power and efficiency.—study coauthor Esther Takeuchi, a SUNY Distinguished Professor at Stony Brook University and Chief Scientist in Brookhaven Lab’s Basic Energy Sciences Directorate
This approach can be extended to other bimetallic and polyanionic electrode materials to probe their discharge mechanisms and spatially resolve changes as a function of usage profile, providing the insight needed to optimize these materials for their use in the next generation of batteries.—Kirshenbaum et al.
The silver vanadium phosphaste cathode in this battery consists of relatively dense thick pellets without binder or conductive additives. When Li+ ions and electrons move into the silver vanadium phosphate particles, V4+ is reduced to V3+ and Ag+ is reduced to metallic silver; under the right conditions, the silver forms an effective electronic path throughout the electrode, enhancing the insertion of Li+ into the cathode lattice and hence increasing the amount of accessible energy in the battery.
Such reduction displacement reactions (conversion reactions) occur with a wide range of binary and bimetallic oxide, fluoride, and sulfide compounds; these materials have potentially very high energy densities that may yield rechargeable and low-cost battery materials, Dudney and Li observed.
In the study, the scientists used bright x-ray beams at Brookhaven Lab’s National Synchrotron Light Source (NSLS)—a DOE Office of Science User Facility—to probe lithium batteries with silver vanadium diphosphate (Ag2VP2O8) electrodes. This promising cathode material, which may be useful in implantable medical devices, exhibits the high stability, high voltage, and spontaneous matrix formation central to the research.
As these single-use batteries discharge, the lithium ions stored in the anode travel to the cathode, displacing silver ions along the way. The displaced silver then combines with free electrons and unused cathode material to form the conductive silver metal matrix, acting as a conduit for the otherwise impeded electron flow.
To visualize the cathode processes within the battery and watch the silver network take shape, we needed a very precise system with high-intensity x-rays capable of penetrating a steel battery casing. So we turned to NSLS.—coauthor and Stony Brook University Research Associate Professor Amy Marschilok
Energy dispersive x-ray diffraction (EDXRD) at NSLS provided this real-time—in situ—visualization data. In EDXRD, intense beams of x-rays passed through the sample, losing energy as the battery structure bent the beams. Each set of detected beam angles, like time-lapse images, revealed the shifting chemistry as a function of battery discharge. Once the data was collected, Brookhaven Lab postdoctoral researcher and study coauthor Kevin Kirshenbaum led the data analysis effort.
In most batteries, the speed of lithium-ion diffusion determines the rate of discharge, a key factor in overall performance and efficiency. The material closest to the lithium anode would ordinarily discharge first, as the ions have a shorter distance to travel. In a surprising discovery, the researchers found that the material farthest from the anode and nearest the coin cell surface discharged first in the battery.
This is because the non-discharged cathode material is a very poor electric conductor, so the resistance for lithium ion diffusion is less than for electron flow. This highlights a uniquely efficient aspect of in situ silver matrix formation: The silver matrix forms primarily where needed, which is more efficient than using conductive additives.—coauthor and SUNY Distinguished Teaching Professor Kenneth Takeuchi
The in situ diffraction data was combined with two techniques applied after operation: x-ray absorption spectroscopy (XAS) and angle-resolved x-ray diffraction (XRD).
Spectroscopy can reveal exact chemistry because each element absorbs and emits light uniquely, but the x-rays used for XAS cannot penetrate the battery casing. So after each step in the discharge, the researchers removed the cathode and ground it into a powder to measure the average elemental composition. Chia-Ying Lee of the University at Buffalo prepared the reduced cathode materials for the initial ex situ measurements.
These techniques provide complementary data: the in situ diffraction shows where the silver is formed within the cathode, while the spectroscopy shows more precisely how much silver was formed, Esther Takeuchi explained.
…bimetallic polyanionic materials (crystalline or amorphous) containing Cu or Fe have promise as active electrode materials with widespread application. To further improve access to full capacity, future, thicker electrodes could also include gradients in morphology spanning the thickness of the electrode and the distance from the electrode terminal. Using the battery chemistry itself to drive the formation of the electrode structure, as shown by Kirshenbaum et al., is an elegant approach toward such an optimized structure.—Dudney and Li
Kevin Kirshenbaum, David C. Bock, Chia-Ying Lee, Zhong Zhong, Kenneth J. Takeuchi, Amy C. Marschilok, and Esther S. Takeuchi (2015) “In situ visualization of Li/Ag2VP2O8 batteries revealing rate-dependent discharge mechanism,” Science 347 (6218), 149-154. doi: 10.1126/science.1257289
Nancy J. Dudney and Juchuan Li (2015) “Using all energy in a battery” Science 347 (6218), 131-132. doi: 10.1126/science.aaa2870