287 research outputs found
Recommended from our members
Distinct Surface and Bulk Thermal Behaviors of LiNi0.6Mn0.2Co0.2O2 Cathode Materials as a Function of State of Charge.
Understanding how structural and chemical transformations take place in particles under thermal conditions can inform designing thermally robust electrode materials. Such a study necessitates the use of diagnostic techniques that are capable of probing the transformations at multiple length scales and at different states of charge (SOC). In this study, the thermal behavior of LiNi0.6Mn0.2Co0.2O2 (NMC-622) was examined as a function of SOC, using an array of bulk and surface-sensitive techniques. In general, thermal stability decreases as lithium content is lowered and conversion in the bulk to progressively reduced metal oxides (spinels, rock salt) occurs as the temperature is raised. Hard X-ray absorption spectroscopy (XAS) and X-ray Raman spectroscopy (XRS) experiments, which probe the bulk, reveal that Ni and Co are eventually reduced when partially delithiated samples (regardless of the SOC) are heated, although Mn is not. Surface-sensitive synchrotron techniques, such as soft XAS and transmission X-ray microscopy (TXM), however, reveal that for 50% delithiated samples, apparent oxidation of nickel occurs at particle surfaces under some circumstances. This is partially compensated by reduction of cobalt but may also be a consequence of redistribution of lithium ions upon heating. TXM results indicate the movement of reduced nickel ions into particle interiors or oxidized nickel ions to the surface or both. These experiments illustrate the complexity of the thermal behavior of NMC cathode materials. The study also informs the importance of investigating the surface and bulk difference as a function of SOC when studying the thermal behaviors of battery materials
Recommended from our members
Effect of Liquid Electrolyte Soaking on the Interfacial Resistance of Li7La3Zr2O12 for All-Solid-State Lithium Batteries.
The impact of liquid electrolyte soaking on the interfacial resistance between the garnet-structured Li7La3Zr2O12 (LLZO) solid electrolyte and metallic lithium has been studied. Lithium carbonate (Li2CO3) formed by inadvertent exposure of LLZO to ambient conditions is generally known to increase interfacial impedance and decrease lithium wettability. Soaking LLZO powders and pellets in the electrolyte containing lithium tetrafluoroborate (LiBF4) shows a significantly reduced interfacial resistance and improved contact between lithium and LLZO. Raman spectroscopy, X-ray diffraction, and soft X-ray absorption spectroscopy reveal how Li2CO3 is continuously removed with increasing soaking time. On-line mass spectrometry and free energy calculations show how LiBF4 reacts with surface carbonate to form carbon dioxide. Using a very simple and scalable process that does not involve heat-treatment and expensive coating techniques, we show that the Li-LLZO interfacial resistance can be reduced by an order of magnitude
Probing the Surface of Nanodiamonds at Stanford Synchrotron Radiation Lightsource and San Jose State University
The nitrogen-vacancy center in diamond is a promising tool in oncology, electric field sensing, and quantum cryptography. High-pressure high-temperature (HPHT) nanodiamonds (NDs) are prime contenders for these fields because they host nitrogen-vacancy centers (NVCs) which are applicable towards cancer detection and electric and magnetic field sensing. However, to apply HPHT NDs to these fields, the surface must first be functionalized—a difficult process because of the inert nature of the surface. The project at hand focuses on surface modification of HPHT NDs with amines to allow for further bioconjugation of small molecules and plasmonic shells. This is done via liquid-phase chemistry and high-temperature gas-phase chemistry. To characterize the surface of aminated NDs, samples are probed using synchrotron radiation at the Stanford Synchrotron Radiation Lightsource (SSRL) alongside the transmission edge spectroscopy (TES) detector. Aminated NDs were characterized using X-ray photoelectric spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) at SSRL. X-ray spectra are suggestive of multiple nitrogen moieties on the surface of the aminated NDs. With verification of a homogeneously amine-terminated surface, the NDs are prepared for further functionalization which can be targeted to enhance the properties of the NVC charge states for applications in enhanced electric field and voltage sensing
Influence of Topology on the Ultrafast Carrier Dynamics in MoTe2
Transport properties of Weyl semimetals are intimately connected to the
underlying band structure. The signature of Weyl semimetals are linear
dispersing bands that touch, forming Weyl points that result in high charge
carrier mobilities. The layered transition metal dichalcogenide MoTe2,
undergoes a temperature dependent phase transition that directly converts the
trivial 1Tprime phase to the nontrivial Td phase, providing an opportunity to
understand how the formation of a Weyl point manifests in the ultrafast carrier
dynamics. In this study, we use resonant X-ray photoemission to monitor the
element specific evolution of excited carriers 1Tprime-MoTe2 and in the
vicinity of the Weyl point in Td-MoTe2. We find that the delocalization time of
1Tprime-MoTe2 is a factor 1.5 times faster than in Td-MoTe2. We argue that this
is a result of the change in the density of states and screening length, to a
higher carrier scattering rate in Td-MoTe2. Our study tracks the fate of
carriers in MoTe2 on sub-fs time-scales and with atomic site specificity
Efficacy of atmospheric pressure dielectric barrier discharge for inactivating airborne pathogens
Atmospheric pressure plasmas have gained attention in recent years for several environmental applications. This technology could potentially be used to deactivate airborne microorganisms, surface-bound microorganisms, and biofilms. In this work, the authors explore the efficacy of the atmospheric pressure dielectric barrier discharge (DBD) to inactivate airborne Staphylococcus epidermidis and Aspergillus niger that are opportunistic pathogens associated with nosocomial infections. This technology uses air as the source of gas and does not require any process gas such as helium, argon, nitrogen, or hydrogen. The effect of DBD was studied on aerosolized S. epidermidis and aerosolized A. niger spores via scanning electron microscopy (SEM). The morphology observed on the SEM micrographs showed deformations in the cellular structure of both microor- ganisms. Cell structure damage upon interaction with the DBD suggests leakage of vital cellular materials, which is a key mechanism for microbial inactivation. The chemical structure of the cell surface of S. epidermidis was also analyzed by near edge x-ray absorption fine structure spectros- copy before and after DBD exposure. Results from surface analysis revealed that reactive oxygen species from the DBD discharge contributed to alterations on the chemistry of the cell membrane/ cell wall of S. epidermidis
Recommended from our members
Ultrafast Coherent Control and Characterization of Surface Reactions using FELs
The microscopic understanding of reactions at surfaces requires an in-depth knowledge of the dynamics of elementary processes on an ultrafast timescale. This can be accomplished using an ultrafast excitation to initiate a chemical reaction and then probe the progression of the reaction with an ultrashort x-ray pulse from the FEL. There is a great potential to use atom-specific spectroscopy involving core levels to probe the chemical nature, structure and bonding of species on surfaces. The ultrashort electron pulse obtained in the linear accelerator to feed the X-ray FEL can also be used for generation of coherent synchrotron radiation in the low energy THz regime to be used as a pump. This radiation has an energy close to the thermal excitations of low-energy vibrational modes of molecules on surfaces and phonons in substrates. The coherent THz radiation will be an electric field pulse with a certain direction that can collectively manipulate atoms or molecules on surfaces. In this respect a chemical reaction can be initiated by collective atomic motion along a specific reaction coordinate. If the coherent THz radiation is generated from the same source as the X-ray FEL radiation, full-time synchronization for pump-probe experiments will be possible. The combination of THz and X-ray spectroscopy could be a unique opportunity for FEL facilities to conduct ultrafast chemistry studies at surfaces
- …