Thesis Defense: Richard Church
Towards Thick Battery Electrodes and Interdigitated Cell Architectures Via Micro-structured Carbon Nanotube Forests
The growing demand for electric vehicles and portable electronics has created a significant interest in the scalability, recyclability, and economics of both traditional and emerging battery technologies. Although lithium-ion batteries (LiB) are approaching their theoretical energy density they will remain a widespread and promising technology as alternative (e.g., Li-metal) chemistries and solid-state architectures are still in relatively early stages of commercial scale-up. In the meantime, LiB performance can be improved by redesigning the cell geometry to incorporate thick 3D electrodes. Using thick electrodes increases the cell level energy density by minimizing the volume and mass contributions of inactive components, including the current collectors and separator. However, batteries with thick planar electrodes suffer from capacity limitations due to increased mechanical fatigue, Li-ion diffusion distances, and tortuosity. 3D electrode designs compensate for this weakness by providing micro-scale channels within the electrode to enable rapid charge transport and accommodate active material expansion. To meet these criteria, the materials used in 3D electrodes must be mechanically robust, electrically conductive, and processable in a manner enabling precise control over geometry and porosity.
In this thesis we first develop thick 3D “honeycomb” battery electrodes using patterned, vertically aligned carbon nanotubes (VA-CNTs) on metal foils as current collectors. We translate insights from CNT growth on silicon wafer substrates to grow CNT forests over 250 μm on thin metal foils (Cu) that are suitable for electrode fabrication. Thick electrodes are then created by coating CNT forests with Si thin films by low pressure chemical vapor deposition. Half-cells using monolithic and honeycomb patterned Si-CNT electrodes were cycled over a range of current densities, demonstrating the electronic connection between the deposited Si and Cu foil via the aligned CNTs. The honeycomb electrodes exhibit large gravimetric (~1750 mAh/gSi) and areal (~20 mAh/cm2) capacities, and exhibit reduced capacity fading when compared to non-patterned electrodes.
Next, the Si-CNT composites are investigated as a template for a 3D full cell design, Geometrically, compared to a planar electrode of a given energy density, the decreased diffusion distance of a 3D cell results in an improvement in power density, thereby decoupling the inherent tradeoff between the energy density and power density that is experienced by planar cells. The difficulty of producing 3D full cells comes from the need to produce high conformality electrolyte films that are pinhole free and which demonstrate sufficient ionic conductivity. To address this issue, we utilize an initiated chemical vapor deposition (iCVD) process to deposit conformal poly(hydroxyethyl methacrylate-co-ethylene glycol diacrylate) thin films on to the patterned Si-CNT composites. Doping these copolymer films with lithium salts results in ionic conductivities on the order of ~10-5 S/cm, which is among the highest conductivities exhibited by conformal electrolyte technologies. To complete a full battery cell, a slurry-based cathode is infiltrated into the iCVD coated Si-CNT composite electrode. These cells are then soaked in a liquid electrolyte and cycled to demonstrate the first-time use of an iCVD polymer electrolyte in a full cell and a proof-of-concept CNT-based 3D full cell. Lastly, a 2D finite element simulation is presented to predict the theoretical energy and power densities of idealized interdigitated CNT-based full cells.
Anastasios John Hart, Department Head, Professor of Mechanical Engineering, MIT; Director, Laboratory for Manufacturing and Productivity; Director, Center for Advanced Production Technologies