Silicone microspheres are exceedingly difficult to make. Here, polydimethylsiloxane microspheres (≈1 μm diameter) are synthesized using ultrasonic spray pyrolysis, the first demonstration of a scalable synthetic procedure for crosslinked silicone microspheres. This continuous, aerosol process is also used to directly produce fluorescent, magnetic, and copolymeric derivatives; the potential biomedical applications of these microspheres are explored.
Rechargeable lithium-ion batteries have been widely commercialized in portable electronics. However, adoption of lithium-ion batteries in more demanding applications such as electric vehicles and renewable energy storage has been hindered by capacity loss and poor performance. Although there has been enormous progress on the advancement of high capacity anode materials such as silicon, the current stage of the cathode performance is a limiting factor for the development of higher performance Li-ion batteries. Lithium iron phosphate, LiFePO4, (LFP) is a promising candidate material because it is inexpensive, environmentally friendly and has a high theoretical capacity (170 mAh g-1). Unfortunately, LFP suffers from mechanical degredations and surface instabilities that lead to capacity fade. In this work, we investigate in situ stress and strain generation in LFP composite electrodes during battery charging – discharging. Repeated lithiation and delithiation cause continuous stress and strain evolution in the electrode due to lithium intercalation and the interaction between electrode and electrolyte species. Electrodes are constrained on substrate for stress measurements, whereas free-standing electrodes are fabricated for strain measurements. Digital image correlation (DIC) is utilized to measure in situ strain generation, whereas beam curvature technique is employed to monitor in situ stress evolution in the electrode. Electrodes are cycled between 2.6 to 4.4 V at different scan rates in various electrolytes. Strain and stress derivatives are calculated with respect to applied potential. The derivatives provide remarkable information about the evolution of surface stress and structural changes in the electrode due to phase transformations. Electrochemical stiffness is calculated by combining stress and strain. Changes in stiffness during electrochemical cycling reveal underlying mechanisms governing stress and strain evolution in the LFP electrode. Acknowledgement This work was supported as part of the Center for Electrochemical Energy Science (CEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.”
The capacity of most battery insertion reactions is limited by the number of open lithium sites in electrode’s crystal structure, usually reaching one per metal atom. Conversion reactions, where the metal oxygen bond is broken to form clusters of Li2O and metal particles, can achieve far higher lithium stoichiometry. However, this process is much less reversible than intercalation and occurs at significantly lower potentials than theoretically expected, which has limited their commercial appeal. To pinpoint the underlying connection between structural changes and the observed overpotentials for lithiation, we have studied conversion reactions in MOx (M = Ni, Fe, and Cr) thin films using the sub-nm interfacial sensitivity of in situ x-ray reflectivity. These studies show that conversion begins at elevated potentials (near 2 V) at the surface before propagating into the bulk. Heterostructures incorporating metal interlayers can also help define the direction and extent of the phase separation. We will discuss the possible origins of this effect and strategies to improve the kinetics and scalability of these model electrodes. This work was supported as part of the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science. Figure 1
A multivalent successor to lithium ion batteries will likely incorporate a metal anode. However, many electrodeposition processes are prone to energy losses from side reactions and overpotentials for plating or stripping. To develop a link between the kinetics of crystal growth/dissolution with electrochemical data, we have followed electrodeposition of Mg and Zn with operando x-ray diffraction using a variety of electrolytes and current collectors. In some cases we find that alloying or passivation competes with metal deposition and can dramatically alter the morphology of the deposited metal, as seen by texture analysis of the diffraction.
Mechanical deformation in battery electrodes incites degradation and capacity loss by causing cracks in the active material, electrical isolation of particles from the current collector, and additional surface area available for electrolyte decomposition. Surface (stress) and bulk (strain) mechanical changes occur asynchronously, and increasing our understanding of their individual contributions to deformation can aid in the design of more robust and longer-lasting batteries. One method with which to compare the complex interplay between stress and strain is electrochemical stiffness. Electrochemical stiffness is the voltage-dependent ratio of the stress derivative to strain derivative. It offers insight into the rate at which competing surface and bulk processes are occurring during electrochemical cycling. In this talk we discuss electrochemical stiffness measurements obtained for graphite, LiMn2O4 (LMO) and LiFePO4 (LFP), anode and cathode materials chosen because of their wide-spread commercial use and differing intercalation mechanisms and crystal structures. Electrochemical stiffness measurements for LMO reveals asynchronous stress and strain behavior. An electrochemical stiffness peak shows that stress dominates prior to LMO delithiation, which suggests the existence of surface morphological changes as delithiation begins. Likewise, the stress derivative with respect to potential for LFP also shows an increased rate of stress change just prior to delithiation. The origin of asynchronous behavior in stress and strain measurements will be discussed.
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