Investigation of lithium-air battery discharge product formed on carbon nanotube and nanofiber electrodes

Abstract

Carbon nanotubes have been actively investigated for integration in a wide variety of applications since their discovery over 20 years ago. Their myriad desirable material properties including exceptional mechanical strength, high thermal conductivities, large surface-to-volume ratios, and considerable electrical conductivities, which are attributable to a quantum mechanical ability to conduct electrons ballistically, have continued to motivate interest in this material system. While a variety of synthesis techniques exist, carbon nanotubes and nanofibers are most often conveniently synthesized using chemical vapor deposition (CVD), which involves their catalyzed growth from transition metal nanoparticles. Vertically-aligned nanotube and nanofiber carpets produced using CVD have been utilized in a variety of applications including those related to energy storage. Li-air (Li-O₂) batteries have received much interest recently because of their very high theoretical energy densities (3200 Wh/kgLi2O₂), which make them ideal candidates for energy storage devices for future fully-electric vehicles. During operation of a Li-air battery O₂ is reduced on the surface a porous air cathode, reacting with Li-ions to form lithium peroxide (Li₂O₂). Unlike the intercalation reactions of Li-ion batteries, discharge in a Li-air cell is analogous to an electrodeposition process involving the nucleation and growth of the depositing species on a foreign substrate. Carbon nanofiber electrodes were synthesized on porous substrates using a chemical vapor deposition process and then assembled into Li-O₂ cells. The large surface to volume ratio and low density of carbon nanofiber electrodes were found to yield a very high gravimetric energy density in Li-O₂ cells, approaching 75% of the theoretical energy density for Li₂O₂. Further, the carbon nanofiber electrodes were found to be excellent platforms for conducting ex situ electron microscopy investigations of the deposition Li₂O₂ phase, which was found to have unique disc and toroid morphologies. Subsequent studies were conducted using freestanding carpets of multi-walled CNT arrays, which were synthesized using a modified CVD process. The freestanding CNT arrays were used as a platform for studying the morphological evolution of Li₂O₂ discharge product as a function of rate and electrode capacity. SEM imaging investigations found that the Li₂O₂ particles underwent a shape evolution from discs to toroids as their size increased. TEM imaging and diffraction studies showed that the microscale Li₂O₂ particles are composed of stacks of thin Li₂O₂ crystallites and that splaying of the stacked crystallite array drives the observed disc to toroid transition. Modeling was performed to gain insights into the nucleation and growth processes involved during discharge in Li-O₂ cells. The modeling study suggests that poor electronic conductivity of the depositing phase limits the rate capability obtainable in Li-O₂ cells. Modeling can provide substantial insights into paths toward electrode optimization. Understanding the size and shape evolution of Li₂O₂ particles and engineering improved electrode architectures is critical to efficiently filling the electrode void volume during discharge thereby improving the volumetric energy density of Li-O₂ batteries.