ABSTRACT
Aliovalent substitutions in layered transition-metal cathode materials has been demonstrated to improve the energy densities of lithium ion batteries, with the mechanisms underlying such effects incompletely understood. Performance enhancement associated with Ti substitution of Co in the cathode material Li1(NixMnxCo1-2x)O2 were investigated using density functional theory calculations, including Hubbard-U corrections. An examination of the structural and electronic modifications revealed that Ti substitution reduces the structural distortions occurring during delithiation due to the larger cation radius of Ti(4+) relative to Co(3+) and the presence of an electron polaron on Mn cations induced by aliovalent Ti substitution. The structural differences were found to correlate with a decrease in the lithium intercalation voltage at lower lithium concentrations, which is consistent with quasi-equilibrium voltages obtained by integrating data from stepped potential experiments. Further, Ti is found to suppress the formation of a secondary rock salt phase at high voltage. Our results provide insights into how selective substitutions can enhance the performance of cathodes, maximizing the energy density and lifetime of current Li ion batteries.
ABSTRACT
Intercalation compounds such as transition metal oxides or phosphates are the most commonly used electrode materials in Li-ion and Na-ion batteries. During insertion or removal of alkali metal ions, the redox states of transition metals in the compounds change and structural transformations such as phase transitions and/or lattice parameter increases or decreases occur. These behaviors in turn determine important characteristics of the batteries such as the potential profiles, rate capabilities, and cycle lives. The extremely bright and tunable x-rays produced by synchrotron radiation allow rapid acquisition of high-resolution data that provide information about these processes. Transformations in the bulk materials, such as phase transitions, can be directly observed using X-ray diffraction (XRD), while X-ray absorption spectroscopy (XAS) gives information about the local electronic and geometric structures (e.g. changes in redox states and bond lengths). In situ experiments carried out on operating cells are particularly useful because they allow direct correlation between the electrochemical and structural properties of the materials. These experiments are time-consuming and can be challenging to design due to the reactivity and air-sensitivity of the alkali metal anodes used in the half-cell configurations, and/or the possibility of signal interference from other cell components and hardware. For these reasons, it is appropriate to carry out ex situ experiments (e.g. on electrodes harvested from partially charged or cycled cells) in some cases. Here, we present detailed protocols for the preparation of both ex situ and in situ samples for experiments involving synchrotron radiation and demonstrate how these experiments are done.
Subject(s)
Electric Power Supplies , Electrodes , Lithium/chemistry , Sodium/chemistry , Cations, Monovalent/chemistry , Synchrotrons , X-Ray Absorption Spectroscopy/methods , X-Ray Diffraction/methodsABSTRACT
A basic approach was optimized for the synthesis of highly selective and sensitive in situ mesoporous (MCM) type imprinted silica polymers for the detection of dipicolinic acid (DPA) using europium as a reporter. DPA is a ubiquitous biochemical marker available during the germination event of endospore-forming bacteria such as Bacillus . Additionally, an MCM-MIP (molecularly imprinted polymeric phenomena) detector and a companion MCM-non-surface-MIP detector were synthesized using europium reporters for the sensing of DPA under optimized laboratory conditions. Our results showed that the in situ molecular imprinting process enabled rapid, selective detection of DPA with high sensitivity compared to MCM-MIP (imprinted for DPA; no DPA present), MCM-Non-MIP (no imprint present), and MCM-SR-MIP (imprinted with DPA present) detectors. The lower detection limit observed for DPA concentration is 5.49 × 10(-10) mol dm(-3) for MCM-MIP. The performance of the sensor in high-salt-water conditions, under photo-bleaching, and its reusability were also evaluated. The synthesized in situ MCM-MIP material should permit the detection of DPA for field assays related to suspect bacterial sporulation events.
Subject(s)
Biosensing Techniques/methods , Europium/chemistry , Nanoparticles/chemistry , Picolinic Acids/chemistry , Silicon Dioxide/chemistry , Spores, Bacterial/isolation & purification , Bacillus megaterium/physiology , Equipment Reuse , Hydrogen-Ion Concentration , Magnesium Chloride/chemistry , Microscopy, Electron , Molecular Imprinting , Photobleaching , Picolinic Acids/analysis , Polymers , Sensitivity and Specificity , Sodium Chloride/chemistry , X-Ray DiffractionABSTRACT
An in situ mesopourous surface imprinted polymeric (SIP) sensor was synthesized for a highly sensitive, selective, and kinetically faster detection of the high-vapor-pressure nerve gas surrogate methyl salicylate (MES). Visual detection occurred on the filtrate thin films at 25 pM. Other nerve gas surrogates, TP, DMP, DMMP, PMP, and 1,4-thioxane, were tested and showed a decrease in sensitivity compared to MES. In addition, 2,6-dipicolinic acid (DPA), a biological indicator, was also investigated and showed a decrease in sensitivity compared to MES. Finally, the detection plateau was reached at 40 s and at 1.5 x 10(-4) M from pH 6-11.
