Gary M. Koenig

Growth mechanism of Ni0.3Mn0.7CO3 precursor for high capacity Li-ion battery cathodes

Transition metal carbonate (Ni0.3Mn0.7CO3) was co-precipitated as the precursor for Li- and Mn-enriched composite materials used as advanced cathodes for lithium-ion batteries. The optimal pH range for synthesis of Ni0.3Mn0.7CO3 in a continuous stirred tank reactor (CSTR) at the pilot scale was predicted by taking into account the chemical equilibriums between the products and reactants. The nucleation and growth of precursor particles were investigated during the CSTR process by monitoring particle size distributions, particle morphologies, chemical compositions, and structures with time. It was found that in the early stage of co-precipitation both the particle size distribution and the chemical composition were not homogeneous; a lead time of about 5 hours under our experiment conditions was necessary to achieve the uniformity in particle shape and chemical composition. The latter was not altered during extended times of co-precipitation; however, a continuous growth of particles resulted in relatively large particles (D50 > 30 μm). The electrochemical performance of the final lithiated cathode materials is reported.

Chemoresponsive assemblies of microparticles at liquid crystalline interfaces

Assemblies formed by solid particles at interfaces have been widely studied because they serve as models of molecular phenomena, including molecular self-assembly. Solid particles adsorbed at interfaces also provide a means of stabilizing liquid–liquid emulsions and synthesizing materials with tunable mechanical, optical, or electronic properties. Whereas many past studies have investigated colloids at interfaces of isotropic liquids, recently, new types of intercolloidal interactions have been unmasked at interfaces of liquid crystals (LCs): The long-range ordering of the LCs, as well as defects within the LCs, mediates intercolloidal interactions with symmetries that differ from those observed with isotropic liquids. Herein, we report the decoration of interfaces formed between aqueous phases and nematic LCs with prescribed densities of solid, micrometer-sized particles. The microparticles assemble into chains with controlled interparticle spacing, consistent with the dipolar symmetry of the defects observed to form about each microparticle. Addition of a molecular surfactant to the aqueous phase results in a continuous ordering transition in the LC, which triggers reorganization of the microparticles, first by increasing the spacing between microparticles within chains and ultimately by forming two-dimensional arrays with local hexagonal symmetry. The ordering transition of the microparticles is reversible and is driven by surfactant-induced changes in the symmetry of the topological defects induced by the microparticles. These results demonstrate that the orderings of solid microparticles and molecular adsorbates are strongly coupled at the interfaces of LCs and that LCs offer the basis of methods for reversible, chemosensitive control of the interfacial organization of solid microparticles.

Single nanoparticle tracking reveals influence of chemical functionality of nanoparticles on local ordering of liquid crystals and nanoparticle diffusion coefficients.

This letter reports that darkfield microscopy can be used to track the trajectories of chemically functionalized gold nanoparticles in nematic liquid crystals (LCs), thus leading to measurements of the diffusion coefficients of the nanoparticles in the LCs. These measurements reveal that the diffusion coefficients of the nanoparticles dispersed in the LC are strongly dependent on the surface chemistry of the nanoparticles. Because the changes in surface chemistry are measured to have negligible influence on the diffusion coefficients of the same nanoparticles dispersed in isotropic solvents, we conclude that surface chemistry-induced changes in the local order of LCs underlie the behavior of the diffusion coefficients of the nanoparticles in the LC. Surface chemistry-dependent ordering of the LCs near the surfaces of the nanoparticles was also found to influence diffusion coefficients measured when the LC was heated above the bulk nematic-to-isotropic transition temperature. These experimental measurements are placed into the context of past theoretical predictions regarding the impact of local ordering of LCs on diffusion coefficients. The results that emerge from this study provide important insights into the mobility of nanoparticles in LCs and suggest new approaches based on measurements of nanoparticle dynamics that can yield information on the ordering of LCs near nanoparticles.

Characterization of the reversible interaction of pairs of nanoparticles dispersed in nematic liquid crystals.

Observations of reversible interactions between pairs of chemically functionalized nanoparticles dispersed in nematic liquid crystals (LCs) are reported. In contrast to the irreversible association of microparticles in nematic LCs, by using gold nanoparticles and darkfield microscopy, particle tracking reveals the pairwise interactions of the nanoparticles in nematic LCs to be long-ranged and reversible. The measured range and strength of the pairwise interaction of the nanoparticles in the LCs was found to differ substantially from past theoretical predictions of nanoparticle interactions in LCs. The observation of reversible interactions between nanoparticles in LCs suggests that nematic LCs may provide new routes to spontaneous formation of ordered nanoparticle arrays.

Using Localized Surface Plasmon Resonances to Probe the Nanoscopic Origins of Adsorbate-Driven Ordering Transitions of Liquid Crystals in Contact with Chemically Functionalized Gold Nanodots

We report that localized surface plasmon resonances (LSPRs) of gold nanodots immersed under liquid crystals (LCs) can be used to characterize adsorbate-induced ordering transitions of the LCs on the surfaces of the nanodots. The nanoscopic changes in ordering of the LCs, as measured by LSPR, were shown to give rise to macroscopic ordering transitions of the LCs that were observed by polarized light microscopy. The results reported herein suggest that (i) LCs may be useful for enhancing the sensitivity of LSPR-based detection of binding events and (ii) that LSPR measurements of gold nanodots provide a means to characterize the nanoscopic origins of macroscopic, adsorbate-induced LC ordering transitions.

Flow induced deformation of defects around nanoparticles and nanodroplets suspended in liquid crystals

A three-dimensional molecular theory is used to describe the effect of flow on the defects that arise around nanoparticles and nanodroplets suspended in a nematic liquid crystal. It is observed that flow displaces the Saturn ring line defect that forms around a nanoparticle at equilibrium in the upstream direction; it is eventually closed by the flow and becomes a Hedgehog point defect. In contrast, the Saturn ring that forms around a nanodroplet is slightly displaced in the downstream direction. Experimental measurements of defects around nanoparticles have not been reported in the literature. In the absence of experiments, the validity of theoretical predictions is assessed through a direct comparison to results of many-body molecular dynamics simulations of a coarse grain liquid crystal model. Theoretical predictions and molecular simulations are in quantitative agreement, thereby lending credibility to the predictions presented in this work and suggesting that flow can be used to manipulate defect structure and aggregation of nanoparticles in nematic liquid crystals.

Effects of anchoring strength on the diffusivity of nanoparticles in model liquid-crystalline fluids

The diffusivity of a nanoparticle suspended in a liquid crystal is investigated in the limit of nematic ordering and under isotropic conditions. Molecular simulations are performed with the liquid-crystalline solvent represented at the level of Gay–Berne mesogens in the canonical (N,V,T) ensemble. The mesogen–colloid interaction strength is varied to induce anchoring that ranges from parallel to perpendicular. Mean square displacements, orientational correlation functions, and relative colloidal diffusivities are reported for different types of mesogenic anchoring on the nanoparticle. The Gay–Berne parametrization is contextualized with respect to experimental observations, and a specific set of parameters is found to reproduce the characteristic ratio of mesogenic diffusivities observed in recent experiments. The results presented in this work provide a means to determine anchoring strength at small length scales, and the parameterizations provided in this work could serve as a starting point to interpret experimental data for nanoparticle suspensions in liquid-crystals at a molecular level.

Compositional control of precipitate precursors for lithium-ion battery active materials: role of solution equilibrium and precipitation rate

Multicomponent transition metal oxides are among the most successful lithium-ion battery cathode materials, and many previous reports have described the sensitivity of final electrochemical performance of the active materials to the detailed composition and processing. Coprecipitation of a precursor template is a popular, scalable route to synthesize these transition metal oxide cathode materials because of the homogeneous mixing of the transition metals within the particles, and the morphology control provided by the precursors. However, the deviation of the precursor composition from feed conditions is a challenge that has generally not been reported in previous studies. Using a target final material of the high voltage spinel LiMn1.5Ni0.5O4 as an example, we show in this study that the compositional deviation caused by coprecipitation can be significant under certain conditions, impacting the calcined final material structure and electrochemical properties. The study herein provides insights into the role of solution equilibrium and rate of precipitation of the transition metals during precipitate formation on precursor, and thus final active material, composition. Such knowledge is necessary to rationally predict and tune multicomponent battery precursor compositions synthesized via coprecipitation with high levels of accuracy.

Iron Phosphate Polyanion Compounds as Anodes for Aqueous Lithium-Ion Batteries

Aqueous lithium-ion batteries offer the possibility of the gravimetric capacity, round trip efficiency, and high cycle life of lithium-ion battery materials coupled with the cost and safety advantages of aqueous electrolytes. A common challenge in these systems is the narrow voltage window of aqueous electrolytes and achieving suitable stability of the lithium-ion materials in the electrolyte. This paper will describe synthesis and characterization of iron phosphate compounds as anodes for aqueous lithium-ion batteries. While these materials are at or near the lower range of the stability window of water, oxygen in the electrolyte and subsequent oxygen reduction was the primary challenge in operation of these cells.