Surface-induced changes in the conformation and glucan production of glucosyltransferase adsorbed on saliva-coated hydroxyapatite.

Glucosyltransferases (Gtfs) from S. mutans play critical roles in the development of virulent oral biofilms associated with dental caries disease. Gtfs adsorbed to the tooth surface produce glucans that promote local microbial colonization and provide an insoluble exopolysaccharides (EPS) matrix that facilitates biofilm initiation. Moreover, agents that inhibit the enzymatic activity of Gtfs in solution often have reduced or no effects on surface-adsorbed Gtfs. This study elucidated the mechanisms responsible for the differences in functionality that GtfB exhibits in solution vs surface-adsorbed. Upon adsorption to planar fused-quartz substrates, GtfB displayed a 37% loss of helices and 36% increase of ß-sheets, as determined by circular dichroism (CD) spectroscopy, and surface-induced conformational changes were more severe on substrates modified with CH3- and NH2-terminated self-assembled monolayers. GtfB also underwent substantial conformation changes when adsorbing to hydroxyapatite (HA) microspheres, likely due to electrostatic interactions between negatively charged GtfB and positively charged HA crystal faces. Conformational changes were lessened when HA surfaces were coated with saliva (sHA) prior to GtfB adsorption. Furthermore, GtfB remained highly active on sHA, as determined by in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, producing glucans that were structurally different than GtfB in solution and known to increase the accumulation and virulence of biofilms. Our data provide the first insight into the structural underpinnings governing Gtf conformation and enzymatic function that occur on tooth surfaces in vivo, which may lead to designing potent new inhibitors and improved strategies to combat the formation of pathogenic oral biofilms.

Structural and Morphological Analysis of the First Alloy/Dealloy of a Bulk Si-Li System at Elevated Temperature.

There have been tremendous improvements in the field of Si electrode materials, either by nanoscale or composite routes, and though silicon-containing carbon electrode materials have begun to penetrate the marketplace, the commercial capacities achieved by these cells still fall short of the promise of high capacity Si electrodes. Enabling a cheaper feedstock of Si in the bulk form would make this technology more accessible, though there are many challenges that must be overcome. Whereas other methods utilize nanomaterials and composites to overcome volume expansion and pulverization of a Si electrode, this study explores a thermal route to enable the use of carbon-free bulk Si. To accomplish this, a modified Swagelok cell has been constructed to accommodate high temperatures, corrosive molten salt electrolytes, and a molten lithium electrode to study lithiation of a bulk Si wafer at 250 °C. Scanning electron microscopy, X-ray diffraction, and microcomputed tomography were used to examine morphological and structural changes within the Si upon lithiation and delithiation. It was discovered that semiordered Li x Si phases were formed upon lithiation in molten LiTFSI electrolyte at 250 °C, and the higher temperature does not completely mitigate pulverization of the bulk Si electrode.

Accelerating Rate Calorimetry and Complementary Techniques to Characterize Battery Safety Hazards.

The hazards associated with lithium-based battery chemistries are well-documented due to their catastrophic nature. Risk is typically qualitatively assessed through an engineering risk matrix. Within the matrix, potentially hazardous events are categorized and ranked in terms of severity and probability to provide situational awareness to decision makers and stakeholders. The stochastic nature of battery failures, particularly the lithium-ion chemistry, makes the probability axis of a matrix difficult to properly assess. Fortunately, characterization tools exist, such as accelerated rate calorimetry (ARC), that characterize degrees of battery failure severity. ARC has been used extensively to characterize reactive chemicals but can provide a new application to induce battery failures under safe, controlled experimental conditions and quantify critical safety parameters. Due to the robust nature of the extended volume calorimeter, cells may be safely taken to failure due to a variety of abuses: thermal (simple heating of cell), electrochemical (overcharge), electrical (external short circuit), or physical (crush or nail penetration). This article describes the procedures to prepare and instrument a commercial lithium-ion battery cell for failure in an ARC to collect valuable safety data: onset of thermal runaway, endotherm associated with polymer separator melting, pressure release during thermal runaway, gaseous collection for analytical characterization, maximum temperature of complete reaction, and visual observation of decomposition processes using a high temperature borescope (venting and cell can breach). A thermal "heat-wait-seek" method is used to induce cell failure, in which the battery is heated incrementally to a set point, then the instrument identifies heat generation from the battery. As heat generates a temperature rise in the battery, the calorimeter temperature follows this temperature rise, maintaining an adiabatic condition. Therefore, the cell does not exchange heat with the external environment, so all heat generation from the battery under failure is captured.

In Operando Detection of the Onset and Mapping of Lithium Plating Regimes during Fast Charging of Lithium-Ion Batteries.

Existing in operando methods for detection of plated lithium can only detect the presence of plating after the charge is complete and irreversible damage has already occurred. In this work, the characteristic potential minimum on the graphite electrode during high rate lithiation is proposed and assessed as an in operando technique for detecting the onset of lithium plating. While other studies have shown that rapid self-heating of a cell can cause this type of "voltage overshoot", we confirm through temperature-controlled coin cell experiments that such a voltage profile can also be caused by the occurrence of severe lithium plating. In cells which demonstrated voltage overshoot, macroscopically observable lithium plating films were present on the graphite electrodes upon disassembly, resulting in very poor single-cycle Coulombic efficiency. The significance of this voltage characteristic is confirmed through direct observation of the onset of lithium plating in an in situ optical microscopy cell. We observe that the growth of large metallic lithium deposits within the porous electrode structure can cause swelling and cracking of the graphite electrode, suggesting loss of active material due to mechanical electrode degradation as an important consequence of severe lithium plating.

Lithium-ion Battery Thermal Safety by Early Internal Detection, Prediction and Prevention.

Temperature rise in Lithium-ion batteries (LIBs) due to solid electrolyte interfaces breakdown, uncontrollable exothermic reactions in electrodes and Joule heating can result in the catastrophic failures such as thermal runaway, which is calling for reliable real-time electrode temperature monitoring. Here, we present a customized LIB setup developed for early detection of electrode temperature rise during simulated thermal runaway tests incorporating a modern additive manufacturing-supported resistance temperature detector (RTD). An advanced RTD is embedded in a 3D printed polymeric substrate and placed behind the electrode current collector of CR2032 coin cells that can sustain harsh electrochemical operational environments (acidic electrolyte without Redox, short-circuiting, leakage etc.) without participating in electrochemical reactions. The internal RTD measured an average 5.8 °C higher temperature inside the cells than the external RTD with almost 10 times faster detection ability, prohibiting thermal runaway events without interfering in the LIBs' operation. A temperature prediction model is developed to forecast battery surface temperature rise stemming from measured internal and external RTD temperature signatures.

Modulation of Lithium Plating in Li-Ion Batteries with External Thermal Gradient.

Li-ion battery safety is often threatened by undesirable lithium metal electrodeposition or dendrite growth, during charging. The unpredictable and complex onset of widely ranging lithium morphologies limits reproducibility, making prevention and detection strategies difficult to assess. This work blends the fundamentals of classical metallurgical dendrite growth with traditional Li-ion battery charging, to prove the ability to modulate lithium metal deposition through an applied interelectrode thermal gradient. With NMC (nickel-manganese-cobalt) cathode warmed to 40 °C and graphite anode cooled to 0 °C, irreversible lithium plating is observed within 10 cycles, and complete cell deactivation within 20 cycles. The stages of failure over these first 20 cycles are assessed with electrochemical impedance spectroscopy. This work provides a technique for accelerated aging and the reliable study of lithium deposition in Li-ion batteries.

Albumin conformational change and aggregation induced by nanostructured apatites.

Biomaterials with nanostructured surfaces influence cellular response in a significantly different, and often beneficial, manner compared to materials with coarser features. Hydroxyapatite [HA, Ca10(PO4)6(OH)2] and strontium-apatite [Sr10(PO4)6(OH)2] microspheres that present nanotopographies similar to biological apatites were incubated in albumin solutions, at physiological conditions (40 mg ml-1; 37 °C), for up to 72 h. Electronic and vibrational circular dichroism spectroscopies revealed spectral signatures characteristic of stacked ß-sheet regions in higher ordered structures (e.g., fibrils). The presence of stacked ß-sheets was further evidenced by thioflavin T staining. The sequestration of interfacial Ca atoms by pyrophosphate ions (P2O74-), prior to albumin adsorption, prevented stacked ß-sheet formation on hydroxyapatite. These results suggest that the charge and/or spatial arrangement of Ca atoms direct stacked ß-sheet formation during bovine serum albumin adsorption. Stacked ß-sheet spectral features were also observed after incubating HA in fetal bovine serum, highlighting that this phenomena could direct cellular response to these biomaterials in vivo.

Internal Morphologies of Cycled Li-Metal Electrodes Investigated by Nano-Scale Resolution X-ray Computed Tomography.

While some commercially available primary batteries have lithium metal anodes, there has yet to be a commercially viable secondary battery with this type of electrode. Research prototypes of these cells typically exhibit a limited cycle life before dendrites form and cause internal cell shorting, an occurrence that is more pronounced during high-rate cycling. To better understand the effects of high-rate cycling that can lead to cell failure, we use ex situ nanoscale-resolution X-ray computed tomography (nano-CT) with the aid of Zernike phase contrast to image the internal morphologies of lithium metal electrodes on copper wire current collectors that have been cycled at low and high current densities. The Li that is deposited on a Cu wire and then stripped and deposited at low current density appears uniform in morphology. Those cycled at high current density undergo short voltage transients to >3 V during Li-stripping from the electrode, during which electrolyte oxidation and Cu dissolution from the current collector may occur. The effect of temperature is also explored with separate cycling experiments performed at 5 and 33 °C. The resulting morphologies are nonuniform films filled with voids that are semispherical in shape with diameters ranging from hundreds of nanometers to tens of micrometers, where the void size distributions are temperature-dependent. Low-temperature cycling elicits a high proportion of submicrometer voids, while the higher-temperature sample morphology is dominated by voids larger than 2 µm. In evaluating these morphologies, we consider the importance of nonidealities during extreme charging, such as electrolyte decomposition. We conclude that nano-CT is an effective tool for resolving features and aggressive cycling-induced anomalies in Li films in the range of 100 nm to 100 µm.

Enhanced Lithiation Cycle Stability of ALD-Coated Confined a-Si Microstructures Determined Using In Situ AFM.

Microfabricated amorphous silicon (a-Si) pits ∼4 µm in diameter and 100 nm thick were fabricated to be partially confined in a nickel (Ni) current collector. Corresponding unconfined pillars were also fabricated. The samples were coated with 1.5, 3, or 6 nm of Al2O3 ALD. These samples were tested in electrolytes of 3:7 by weight ethylene carbonate:ethyl methyl carbonate (EC:EMC) with 1.2 M LiPF6 salt with and without 2% fluoroethylene carbonate (FEC) and in a pure FEC electrolyte with 10 wt % LiPF6. The samples were imaged with an atomic force microscope during electrochemical cycling to evaluate morphology evolution and solid electrolyte interphase (SEI) formation. The partially confined a-Si structures had superior cycle efficiency relative to the unconfined a-Si pillars. Additionally, samples with 3 nm of ALD achieved higher charge capacity and enhanced cycle life compared to samples without ALD, demonstrated thinner SEI formation, and after 10 cycles at a 1 C rate remained mostly intact and had actually decreased in diameter. Finally, the samples with 3 nm of ALD had better capacity retention in the baseline 3:7 EC:EMC than in either of the FEC containing electrolytes.

Controlled Growth of Nanostructured Biotemplates with Cobalt and Nitrogen Codoping as a Binderless Lithium-Ion Battery Anode.

Biomass can serve as a sustainable template for the synthesis of carbon materials but is limited by the intrinsic properties of the precursor organism. In this study we demonstrate that the properties of a fungal biotemplate can be tuned during cultivation, establishing a new electrode manufacturing process and ultimately improving the electrochemical performance of the biomass-derived electrode. More specifically, the carbon/nitrogen ratio of Neurospora crassa mycelia mats was shifted by 5-fold while generating cobalt nanoparticles into the hyphal structure originating from macroconidia spores. This shift was achieved through nitrate limitation and equal molar concentrations of Mg2+ and Co2+ in the growth media. The resulting mycelia mat was converted via a high-temperature pyrolysis process (800 °C) to produce a freestanding cobalt and nitrogen codoped electrode material with no postmodification. Ultimately, nitrogen doping resulted in one of the highest recorded specific reversible capacity for a freestanding biomass-derived lithium-ion anode (400 mAh g-1 at C/10). We observed an additional improvement in capacity to 425 mAh g-1 with the incorporation of 3 wt % Co. Our results show how shaping the chemical characteristics of an electrode during the growth of the biotemplate allows for sustainable carbon-based material manufacturing from a living (self-assembled) material.