The principal inhibitor of fast charging lithium ion cells is the graphite negative electrode, where favorable conditions for lithium plating occur at high charge rates, causing accelerated degradation and safety concerns. The local response of graphite, both at the electrode and particle level, when exposed to fast charging conditions of around 6C is not well understood. Consequently, the conditions that lead to the onset of lithium plating, as well as the local dynamics of lithium plating and stripping, have also remained elusive. Here, we use high-speed (100 Hz) pencil-beam X-ray diffraction to repeatedly raster along the depth of a 101 μm thick graphite electrode in 3 μm steps during fast (up to 6C) charge and discharge conditions. Consecutive depth profiles from separator to current collector were each captured in 0.5 seconds, giving an unprecedented spatial and temporal description of the state of the electrode and graphite's staging dynamics during high rate conditions. The electrode is preferentially activated near the separator, and the non-uniformity increases with rate and is influenced by free-energy barriers between graphite's lithiation stages. The onset of lithium plating and stripping was quantified, occurring only within the first 15 μm from the separator. The presence of lithium plating changed the behavior of the underlying graphite, such as causing co-existence of LiC₆ and graphite in the fully discharged state. Finally, the staging behavior of graphite at different rates was quantified, revealing a high dependency on rate and drastic hysteresis between lithiation and delithiation.

Strong electrodes with good energy storage capabilities are necessary to accommodate the current needs for structural and flexible electronics. To this end, conjugated polymers such as polyaniline (PANI) have attracted much attention due to their exceptional energy storage performance. However, PANI is typically brittle and requires the use of substrates for structural support. Here, we report a strategy for developing free-standing structural supercapacitor and battery electrodes based on PANI. More specifically, aniline is polymerized in the presence of branched aramid nanofibers (BANFs) and single walled carbon nanotubes (SWCNTs). This results in a network morphology that allows for efficient load transfer and electron transport, leading to electrodes with capacity values up to 128 ± 5 mA h g⁻¹ (vs. a theoretical capacity of 147 mA h g⁻¹), Young's modulus of 4 ± 0.5 GPa, and tensile strength of 40 ± 4 MPa. Furthermore, the charge storage mechanism is investigated, in which both Faradaic and non-Faradaic contributions are observed. This work demonstrates an efficient strategy for designing structural electrodes based on conjugated polymers.

Conceptual and commercial examples of implantable sensors have been limited to a relatively small number of target analytes, with a strong focus on glucose monitoring. Recently, surface-enhanced Raman spectroscopy (SERS) pH sensors were demonstrated to track acid-producing enzymatic reactions targeting specific analytes. We show here that SERS pH tracking in the basic regime is also possible, and can be used to monitor urea concentration. To accomplish this, we developed a hydrogel consisting of polyelectrolyte multilayer microcapsules containing a SERS-sensitive pH reporter (4-mercapopyridine capped silver nanoparticles modified with bovine serum albumin). This pH sensing material exhibited a sensitive Raman scattering response to a wide range of pH from 6.5–9.7. By incorporating urease into the hydrogel matrix, the new sensor was capable of distinguishing urea concentrations of 0, 0.1, 1, and 10 mM. We also found that bovine serum albumin (BSA) prevented severe aggregation of the nanoparticle-based pH sensor, which improved sensing range and sensitivity. Furthermore, BSA safeguarded the pH sensor during the encapsulation procedure. Together, the combination of materials represents a novel approach to enabling optical sensing of reactions that generate pH changes in the basic range.

The impact of cell temperature is a relatively underexplored area within the burgeoning field of nonaqueous redox flow batteries (NAqRFBs). Here, we investigate the effect of elevated temperature on the performance of nonaqueous redox electrolytes and associated flow cells. Using a model compound, N-(2-(2-methoxyethoxy)-ethyl)phenothiazine (MEEPT), in a propylene-carbonate-based electrolyte, we experimentally measure the temperature dependence of relevant physicochemical properties (i.e., electrolyte conductivity, viscosity, diffusivity) and electrochemical characteristics (i.e., chemical and electrochemical reversibility) across a temperature range of 30 °C to 70 °C. We then perform flow cell studies, finding that while ohmic and mass transport resistances decrease significantly with increases in temperature for the MEEPT/MEEPT^+· redox couple, accessible electrolyte capacity gradually reduces at temperatures > 50 °C. Ex-situ, post-test characterization using microelectrode voltammetry suggests that this capacity fade is due to instability of the MEEPT radical cation. Finally, using MEEPT as a posolyte and a model viologen negolyte (bis(2-(2-methoxyethoxy)ethyl)viologen), we assemble a full cell and perform polarization analyses, observing a 2× increase in the peak power density when the operating temperature is increased from 30 °C to 70 °C. Broadly, this work highlights opportunities for systematic engineering of nonaqueous electrolytes and flow cells for higher power operation at elevated temperatures.

Because of their high specific and volumetric energy density, lithium-ion cells are popular in energy storage systems with strict mass and volume restrictions. Such applications include electric vehicles, power tools, consumer electronics, all-electric aircraft, as well as high-reliability military and crewed spacecraft. Next-generation rechargeable lithium cell designs are characterized by higher specific energy which further increases the severity of thermal runaway (TR) safety hazards. Even with carefully controlled high-quality manufacturing processes, TR can occur unpredictably due to latent defects inside the cells causing internal short circuits. This work elaborates on the lessons learned from cell and battery pack safety research for crewed spacecraft applications.

In the first, multi-institution study of its kind for flow batteries, several laboratories worldwide performed identical single-cell experiments. The findings show that procedural choices significantly alter commonly reported electrochemical metrics.