Christopher J. Pelliccione

Surface Modification Approach to TiO2 Nanofluids with High Particle Concentration, Low Viscosity, and Electrochemical Activity

This study presents a new approach to the formulation of functional nanofluids with high solid loading and low viscosity while retaining the surface activity of nanoparticles, in particular, their electrochemical response. The proposed methodology can be applied to a variety of functional nanomaterials and enables exploration of nanofluids as a medium for industrial applications beyond heat transfer fluids, taking advantage of both liquid behavior and functionality of dispersed nanoparticles. The highest particle concentration achievable with pristine 25 nm titania (TiO2) nanoparticles in aqueous electrolytes (pH 11) is 20 wt %, which is limited by particle aggregation and high viscosity. We have developed a scalable one-step surface modification procedure for functionalizing those TiO2 nanoparticles with a monolayer coverage of propyl sulfonate groups, which provides steric and charge-based separation of particles in suspension. Stable nanofluids with TiO2 loadings up to 50 wt % and low viscosity are successfully prepared from surface-modified TiO2 nanoparticles in the same electrolytes. Viscosity and thermal conductivity of the resulting nanofluids are evaluated and compared to nanofluids prepared from pristine nanoparticles. Furthermore, it is demonstrated that the surface-modified titania nanoparticles retain more than 78% of their electrochemical response as compared to that of the pristine material. Potential applications of the proposed nanofluids include, but are not limited to, electrochemical energy storage and catalysis, including photo- and electrocatalysis.

Dispersion of Nanocrystalline Fe3O4 within Composite Electrodes: Insights on Battery-Related Electrochemistry

Aggregation of nanosized materials in composite lithium-ion-battery electrodes can be a significant factor influencing electrochemical behavior. In this study, aggregation was controlled in magnetite, Fe3O4, composite electrodes via oleic acid capping and subsequent dispersion in a carbon black matrix. A heat treatment process was effective in the removal of the oleic acid capping agent while preserving a high degree of Fe3O4 dispersion. Electrochemical testing showed that Fe3O4 dispersion is initially beneficial in delivering a higher functional capacity, in agreement with continuum model simulations. However, increased capacity fade upon extended cycling was observed for the dispersed Fe3O4 composites relative to the aggregated Fe3O4 composites. X-ray absorption spectroscopy measurements of electrodes post cycling indicated that the dispersed Fe3O4 electrodes are more oxidized in the discharged state, consistent with reduced reversibility compared with the aggregated sample. Higher charge-transfer resistance for the dispersed sample after cycling suggests increased surface-film formation on the dispersed, high-surface-area nanocrystalline Fe3O4 compared to the aggregated materials. This study provides insight into the specific effects of aggregation on electrochemistry through a multiscale view of mechanisms for magnetite composite electrodes.

Synthesis of cryptomelane type α-MnO2 (KxMn8O16) cathode materials with tunable K+ content: the role of tunnel cation concentration on electrochemistry

The role of tunnel cations in the electrochemistry of α-MnO2 materials has been long discussed and demands investigation as the electrochemistry of α-MnO2 materials is strongly dependent on the material specific properties (i.e. morphology, surface area, crystallite size, and chemical composition). Here, we systematically synthesized a series of α-MnO2 samples with differing K+ content but similar physicochemical and morphological properties allowing direct investigation of the role of tunnel cation (K+) on the lithium ion electrochemistry of α-MnO2 cathodes. The nanofibrous α-MnO2 materials have a chemical composition of KxMn8O16·yH2O, where 0 ≤ x ≤ 0.75 and 0.53 ≤ y ≤ 0.81. The α-MnO2 materials have similar morphology, crystallite size (17–19 nm), surface area (66–76 m2 g−1), and tunnel water content (0.53–0.81). The electrochemistry of the α-MnO2 materials was evaluated using cyclic voltammetry, galvanostatic cycling, and galvanostatic intermittent titration type tests. The α-MnO2 materials with 0 to 0.32 K+ content showed discharge curves with higher voltage, higher specific energies, and improved capacity retention compared to the 0.75 K+ containing α-MnO2 material. Fewer structural distortions were observed in lithiated samples with lower K+ content through modelling of X-ray absorption spectroscopy data indicating improved structural stability of those samples which positively impacted the electrochemistry.

Multi-Stage Structural Transformations in Zero-Strain Lithium Titanate Unveiled by in Situ X-ray Absorption Fingerprints

Zero-strain electrodes, such as spinel lithium titanate (Li4/3Ti5/3O4), are appealing for application in batteries due to their negligible volume change and extraordinary stability upon repeated charge/discharge cycles. On the other hand, this same property makes it challenging to probe their structural changes during the electrochemical reaction. Herein, we report in situ studies of lithiation-driven structural transformations in Li4/3Ti5/3O4 via a combination of X-ray absorption spectroscopy and ab initio calculations. Based on excellent agreement between computational and experimental spectra of Ti K-edge, we identified key spectral features as fingerprints for quantitative assessment of structural evolution at different length scales. Results from this study indicate that, despite the small variation in the crystal lattice during lithiation, pronounced structural transformations occur in Li4/3Ti5/3O4, both locally and globally, giving rise to a multi-stage kinetic process involving mixed quasi-solid solution/macroscopic two-phase transformations over a wide range of Li concentrations. This work highlights the unique capability of combining in situ core-level spectroscopy and first-principles calculations for probing Li-ion intercalation in zero-strain electrodes, which is crucial to designing high-performance electrode materials for long-life batteries.

Li/Ag2VO2PO4 batteries: the roles of composite electrode constituents on electrochemistry

In this study, we utilize silver vanadium phosphorous oxide, Ag2VO2PO4, as a model system to systematically study the impact of the constituents of a composite electrode, including polymeric and conductive additives, on electrochemistry. Notably, although highly resistive, this bimetallic cathode can be discharged as a pure electroactive material in the absence of a conductive additive as it generates an in situ conductive matrix via a reduction displacement reaction resulting in the formation of silver metal nanoparticles. Three different electrode compositions were investigated: Ag2VO2PO4 only, Ag2VO2PO4 with binder, and Ag2VO2PO4 with binder and carbon. Constant current discharge, pulse testing and impedance spectroscopy measurements were used to characterize the electrochemical properties of the electrodes as a function of depth of discharge. In situ EDXRD was used to spatially resolve the discharge progression within the cathode by following the formation of Ag0. Ex situ XRD and EXAFS modeling were used to quantify the amount of Ag0 formed. Results indicate that the metal center reduced (V5+ or Ag+) was highly dependent on composite composition (presence of PTFE, carbon), depth of discharge (Ag0 nanoparticle formation), and spatial location within the cathode. The addition of a binder was found to increase cell polarization, and the percolation network provided by the carbon in the presence of PTFE was further increased with reduction and formation of Ag0. This study provides insight into the factors controlling the electrochemistry of resistive active materials in composite electrodes.