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In this work we show for the first time that a continuous plasma process can synthesize materials from bulk industrial powders to produce hierarchical structures for energy storage applications. The plasma production process's unique advantages are that it is fast, inexpensive, and scalable due to its high energy density that enables low-cost precursors. The synthesized hierarchical material is comprised of iron oxide and aluminum oxide aggregate particles and carbon nanotubes grown in situ from the iron particles. New aerosol-based methods were used for the first time on a battery material to characterize aggregate and primary particle morphologies, while showing good agreement with observations from TEM measurements. As an anode for lithium ion batteries, a reversible capacity of 870 mA h g-1 based on metal oxide mass was observed and the material showed good recovery from high rate cycling. The high rate of material synthesis (∼10 s residence time) enables this plasma hierarchical material synthesis platform to be optimized as a means for energetic material production for the global energy storage material supply chain.

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Anisotropic battery electrodes that allow enhanced diffusion through the thickness of the electrode can be engineered to improve rate performance but direct measurement of 3D diffusion in these non-transparent nanoscale pores is extremely challenging. To address this, we used 1H and 7Li pulsed field gradient (PFG) NMR to measure anisotropic diffusion in a model porous silicon substrate. We show that NMR provides resolution for solvent (here, H2O, DMSO, and the battery electrolyte LIPF6:DC:EMC) in- and outside of the pores in the Si substrate. Exchange between ions/molecules in- and outside the pores is measured with 1H 2D exchange spectroscopy (EXSY). When the diffusivity of in-pore NMR peak is analyzed, the results are in reasonable agreement with the pore dimensions measured with electron microscopy. Better agreement is obtained for pore diameters; for pore length measurements, exchange between the pore and ex-pore solvent should be accounted for. These results suggest that PFG-NMR can serve as a non-destructive characterisation method for both in- and ex-situ analysis of materials ranging from complex battery and supercapacitor electrodes to catalyst supports and tissue scaffolds.

Technological advances in membrane technology, catalysis, and electrochemical energy storage require the fabrication of controlled pore structures at ever smaller length scales. It is therefore important to develop processes allowing for the fabrication of materials with controlled submicron porous structures. We propose a combination of colloidal lithography and chemical vapor deposition of carbon nanotubes to create continuous straight pores with diameters down to 100 nm in structures with thicknesses of more than 300 μm. These structures offer unique features, including continuous and parallel pores with aspect ratios in excess of 3000, a low pore tortuosity, good electrical conductivity, and electrochemical stability. We demonstrate that these structures can be used in Li-ion batteries by coating the carbon nanotubes with Si as an active anode material.

Li-O2 batteries offer a high theoretical discharge capacity due to the formation of light discharged species such as Li2O2 which fill the porous positive electrode. However, in practice it is challenging to reach the theoretical capacity and completely utilize the full electrode pore volume during discharge. With the formation of discharge products, the porous medium evolves, and the porosity and tortuosity factor of the positive electrode are altered through shrinkage and clogging of pores. A pore shrinks as solid discharge products accumulate, the pore clogging when it is filled (or when access is blocked). In this study, we investigate the structural evolution of the positive electrode through a combination of experimental and computational techniques. Pulsed-field Gradient Nuclear Magnetic Resonance results show that the electrode tortuosity factor changes much faster than suggested by the Bruggeman relation (an equation that empirically links the tortuosity factor to the porosity), and that the electrolyte solvent affects the tortuosity factor evolution. The latter is ascribed to the different abilities of solvents to dissolve reaction intermediates, which leads to different discharge product particle sizes: on discharging using 0.5 M LiTFSI in dimethoxyethane, the tortuosity factor increases much faster than for discharging in 0.5 M LiTFSI in tetraglyme. The correlation between discharge product size and tortuosity factor is studied using a pore network model which shows that larger discharge products generate more pore clogging. The Knudsen diffusion effect, where collisions of diffusing molecules with pore walls reduce the effective diffusion coefficients, is investigated using a kinetic Monte Carlo model and is found to have an insignificant impact on the effective diffusion coefficient for molecules in pores with diameters above 5 nm, i.e., most of the pores present in the materials investigated here. As a consequence, pore clogging is thought to be the main origin of tortuosity factor evolution.