Towards 3D Lithium-Ion Full Cells Using Honeycomb-Patterned Carbon Nanotube Forests
Authors: Church Richard Bertram , Hart Anastasios John 
Published: September 2023

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. Yet, LiB performance can be improved by redesigning the cell geometry to move from planar to 3D designs. 3D designs enable the use of thick electrodes to increase the cell level energy density by minimizing the volume and mass contributions of inactive components, such as current collectors or separators. The rationale for 3D full cell designs lies in the relationship between electrode geometry and the respective energy and power densities. When compared to a planar design, 3D architectures allow for a higher surface area for the same volume and footprint of cell. As a result, a given volume of active material can be distributed with a lower electrode feature thickness. Compared to a planar electrode of a given energy density, the lower thickness contributes to a quadratic improvement in power density which decouples the inherent tradeoff between the energy density and power density that is experienced by planar cells. This suggests that a high aspect ratio 3D cell would give greater power per footprint area while retaining high areal energy density. To meet these criteria, the materials used in fabricating 3D cells must be processable in a manner enabling precise control over geometry, layer conformality and porosity.
We are developing thick 3D “honeycomb” full cells by starting with patterned, vertically aligned carbon nanotubes (VA-CNTs) as current collectors. CNTs are widely known to have high thermal and electrical conductivities, and to be mechanically durable; properties which make them an ideal scaffold for 3D cell development. VA-CNTs (“forests”) form by self-organization of CNTs during chemical vapor deposition (CVD) on substrates using common hydrocarbon sources (e.g., C2H2, C2H4). However, well-established CNT growth techniques utilize rigid non-conductive substrates, such as silicon wafers, which necessitates a transfer to a suitable substrate for electrochemical applications. Recently, we translated insights from CNT growth on silicon wafer substrates to grow CNT forests on thin metal foils (Cu) suitable for electrode fabrication. We did this by optimizing the moisture level within the CVD furnace, and using a thin-film stack that forms high-density nanoparticles upon annealing while preventing the poisoning of the Fe catalyst by diffusion of Cu. The resulting CNT forests can reach thicknesses over 350 μm and can be grown from patterned catalyst films having regular hole arrays with feature sizes as small as ~5 μm.
Thick composite electrodes are then created by coating CNT forests with Si thin films by CVD. The inherent nanoporosity of the CNT forests (>95%) allows for precursor diffusion into the entirety of the forest cross-section and conformal coating of the individual CNTs. The CVD process is tuned to create dense electrodes with tailored Si loading that can range from ~10 to ~90 at.%. Half-cells using monolithic and honeycomb patterned Si-CNT electrodes (~250 μm tall), Li-metal foil, and a liquid electrolyte/separator combination have been cycled over a range of current densities, demonstrating the electronic connection between the deposited Si and Cu foil via the aligned CNTs. At low current densities and high Si loadings these honeycomb electrodes can exhibit large gravimetric (~1500 mAh/gSi) and areal (~12 mAh/cm2) capacities, with honeycomb Si-CNT composites exhibiting reduced capacity fading when compared to non-patterned electrodes.
We continue by investigating these Si-CNT composites as a template for a 3D full cell design. In literature, 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 the high aspect ratio CNT forests. These copolymer films are doped with lithium salts to exhibit ionic conductivities on the order of ~10-6 to 10-5 S/cm, which is among the highest conductivities ever exhibited by conformal electrolyte technologies. To complete a full cell design, a slurry-based cathode will be infiltrated into the iCVD coated Si-CNT composite electrodes, from which we aim to assess the cycling behavior and compare the areal energy and power densities of non-patterned and honeycomb architectures.