Much progress has recently been made in the development of active materials, electrode morphologies and electrolytes for lithium ion batteries. Well-defined studies on size effects of the three-dimensional (3D) electrode architecture, however, remain to be rare due to the lack of suitable material platforms where the critical length scales (such as pore size and thickness of the active material) can be freely and deterministically adjusted over a wide range without affecting the overall 3D morphology of the electrode. Here, we report on a systematic study on length scale effects on the electrochemical performance of model 3D np-Au/TiO2 core/shell electrodes. Bulk nanoporous gold provides deterministic control over the pore size and is used as a monolithic metallic scaffold and current collector. Extremely uniform and conformal TiO2 films of controlled thickness were deposited on the current collector by employing atomic layer deposition (ALD). Our experiments demonstrate profound performance improvements by matching the Li(+) diffusivity in the electrolyte and the solid state through adjusting pore size and thickness of the active coating which, for 200 μm thick porous electrodes, requires the presence of 100 nm pores. Decreasing the thickness of the TiO2 coating generally improves the power performance of the electrode by reducing the Li(+) diffusion pathway, enhancing the Li(+) solid solubility, and minimizing the voltage drop across the electrode/electrolyte interface. With the use of the optimized electrode morphology, supercapacitor-like power performance with lithium-ion-battery energy densities was realized. Our results provide the much-needed fundamental insight for the rational design of the 3D architecture of lithium ion battery electrodes with improved power performance.

Safe, reliable materials with fast charging kinetics are required to increase the power density of batteries in electric vehicles. One potential avenue for improving kinetics involves disturbing the electrode crystalline structure to alter diffusion properties. However, it remains controversial whether amorphization universally benefits intercalation kinetics, and the specific enhancement mechanisms with respect to the crystalline counterpart are often unclear. In this work, we systematically explore the effects of amorphization on Li + intercalation kinetics using variable-thickness TiO 2 films derived from atomic layer deposition. The amorphous films exhibit an order-of-magnitude faster Li + diffusivity and >0.3 eV reduction in the effective Li + migration barrier with respect to the crystalline anatase phase, resulting in superior high-rate capacity. To investigate the origin of this improvement, we perform a detailed analysis of the energy landscape, migration barriers, and diffusion rates in validated models of amorphous TiO 2 using multiscale simulations. The range of site energies produced by the intrinsic structural disorder of amorphous TiO 2 is found to generate low-barrier pathways for Li + migration that penetrate some distance into the material, resulting in defined regions with faster diffusion behavior. We propose that the formation of these fast ion transport "highways" improves accessibility to interior sites, leading to significantly improved overall rate performance in the amorphous films. In addition to confirming the viability of amorphous TiO 2 films as an alternative to crystalline layered materials for high-rate-performance energy storage, this work outlines a strategy for determining the conditions under which such performance might be realized in other similar materials.

Many application-relevant properties of nanoporous metals critically depend on their multiscale architecture. For example, the intrinsically high step-edge density of curved surfaces at the nanoscale provides highly reactive sites for catalysis, whereas the macroscale pore and grain morphology determines the macroscopic properties, such as mass transport, electrical conductivity, or mechanical properties. In this work, we systematically study the effects of alloy composition and dealloying conditions on the multiscale morphology of nanoporous copper (np-Cu) made from various commercial Zn-Cu precursor alloys. Using a combination of X-ray diffraction, electron backscatter diffraction, and focused ion beam cross-sectional analysis, our results reveal that the macroscopic grain structure of the starting alloy surprisingly survives the dealloying process, despite a change in crystal structure from body-centered cubic (Zn-Cu starting alloy) to face-centered cubic (Cu). The nanoscale structure can be controlled by the acid used for dealloying with HCl leading to a larger and more faceted ligament morphology compared to that of HPO. Anhydrous ethanol dehydrogenation was used as a probe reaction to test the effect of the nanoscale ligament morphology on the apparent activation energy of the reaction.