The magnetotactic multicellular prokaryote (MMP), a motile aggregate of bacterial cells, is known to exhibit an unusual "ping-pong" motility in magnetic fields greater than the earth's field. This motility is characterized by rapid excursions, opposite the direction of an applied magnetic field, and slower returns along the direction of the magnetic field. We have carried out detailed observations of the time and spatial dependence of the ping-pong motility and find 1), the outward and return excursions exhibit a uniform deceleration and acceleration, respectively; 2), the probability per unit time of an MMP undergoing a ping-pong excursion increases monotonically with the field strength; and 3), the outward excursions exhibit a very unusual distance distribution which is dependent on the magnetic field strength. At any given field strength, a characteristic distance is observed, below which very few excursions occur. Beyond this distance, there is a rapid increase in the number of excursions with an exponentially decaying distribution. These observations cannot be explained by conventional magnetotaxis, i.e., a physical directing torque on the organism, and suggest a magnetoreceptive capability of the MMP.
In this work, the battery performance metrics of Coulombic efficiency (CE) and capacity retention (CR) are derived in terms of cycling current and side-reaction currents at each electrode. A cyclable lithium inventory (CLI) framework is developed to explain the fundamental differences between inventory-limited and site-limited cells. The otherwise unmeasurable oxidation and reduction side-reaction currents are expressed in terms of cycling current, CE, and CR for both limitation types. Important conclusions regarding cyclable lithium inventory, limitation type, and the interpretation of CE and CR are discussed. From the framework, CE depends on either the oxidation side-reaction current or the reduction side-reaction current, but not both. CE and CR depend on cell limitation type and are increased by faster cycling. Experiments are presented to support the framework and demonstrate that cells can transition between limitation types. These conclusions are presented in terms of lithium-ion batteries, but this framework may be extended to describe other battery systems.
Dissolution of transition metals (TMs) from lithium-ion battery cathodes under high-voltage conditions is a major issue affecting battery performance that is not well understood mechanistically. Here, this phenomenon is studied by chemically aging pristine and charged LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532) cathodes in the presence of different solutions. The solution composition was varied by 1) adding water to a standard electrolyte, 2) replacing LiPF 6 salt with lithium acetylacetonate (Li-acac), 3) and/or adding oxidatively unstable tris(2,2,2-trifluoroethyl) phosphite (TTFP) as an electrolyte additive. Our results demonstrate that while TM dissolution from pristine NMC532 cathodes is dominated by HF-attack, TM dissolution from charged NMC532 cathodes is affected by many other factors apart from HF-attack. We suggest that reduction of TMs due to chemical/electrochemical oxidation of the electrolyte at cathode/electrolyte interface, followed by formation of soluble TM-complexes with concomitant Li + intercalation into the cathode, is the dominant mechanism of TM-dissolution at high voltage.
A new class of electrolyte additives based on cyclic fluorinated phosphate esters was rationally designed and identified as being able to stabilize the surface of a LiNiMnCoO (NMC532) cathode when cycled at potentials higher than 4.6 V vs Li/Li. Cyclic fluorinated phosphates were designed to incorporate functionalities of various existing additives to maximize their utilization. The synthesis and characterization of these new additives are described and their electrochemical performance in a NMC532/graphite cell cycled between 4.6 and 3.0 V are investigated. With 1.0 wt % 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEOP) in the conventional electrolyte the NMC532/graphite cell exhibited much improved capacity retention compared to that without any additive. The additive is believed to form a passivation layer on the surface of the cathode via a sacrificial polymerization reaction as evidenced by X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonsance (NMR) analysis results. The rational pathway of a cathode-electrolyte-interface formation was proposed for this type of additive. Both experimental results and the mechanism hypothesis suggest the effectiveness of the additive stems from both the polymerizable cyclic ring and the electron-withdrawing fluorinated alkyl group in the phosphate molecular structure. The successful development of cyclic fluorinated phosphate additives demonstrated that this new functionality selection principle, by incorporating useful functionalities of various additives into one molecule, is an effective approach for the development of new additives.
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