Virus-Based Metal Nanofoams As Scalable Three-Dimensional Current Collectors
Authors: Ransil Alan , Belcher Angela M 
Published: February 2020

In this work, we demonstrate the use of M13 virus to produce metal nanofoams that can be used as current collectors in lithium ion battery electrodes. Metal foam current collectors have been demonstrated to promote rapid electronic conduction in battery electrodes while maintaining a high active material mass fraction. An ideal three dimensional metal current collector comprises a nanofoam, with pore size on the order of one lithium diffusion distance through the active material on the timescale of one (dis)charge. In such a structure, the high specific surface area of a metal nanofoam allows deposition of large quantities of active material, while a continuous metal network embedded within the electrode provides an electronically conductive pathway. However, unlike macroscopic metal foams, metal nanofoams are not widely available because robust and scalable synthetic routes for their production are difficult to achieve. Our lab has taken steps toward this by producing metal nanofoams using M13 virus as a biological template, and showing that these materials can be used as 3D current collectors. M13 is a filamentous virus that has been used as a template for nanomaterials for both electronic and non-electronic applications. Its utility arises from its nanoscale dimensions, as well as the fact that its surface chemistry and morphological properties can be controlled genetically. To fabricate metal nanofoams, we treat the M13 virus capsid as a high aspect ratio nanoparticle and use it to assemble a template. The process uses amino acid residues at three positions on the pVIII major capsid protein: the lysine residue located at position 14 (L14), a glutamic acid fusion to the protein’s N terminus (EEAE), and a tyrosine to methionine point mutation at site 21 (Y21M). The first step in nanofoam fabrication uses glutaraldehyde to crosslink a concentrated virus solution, making use of the lysine residues exposed on the virus capsid. This facile crosslinking process results in a hydrogel. These hydrogels can be formed on a wide range of substrates, and freestanding hydrogels can be produced. The virus used for this process is genetically modified with an EEAE amino acid fusion that increases its negative surface charge. That surface charge is used to bind a palladium-based catalyst, which is then used for electroless deposition of nickel onto the hydrogel. The resulting open-cell nanofoam comprises struts that are templated onto the filamentous virus. As such, modifying the morphology of the virus particles was expected to affect the foam morphology. Because virus genes encode the amino acid sequence of the proteins forming the virus capsid, this can be done genetically. We demonstrate this control using the Y21M mutation, a point mutation shown to increase the persistence length of the virus. Using a Y21M variant as a genetically programmable control, we are able to lower the relative density of our foams. We also demonstrate control over foam morphology using virus concentration, and by controlling immersion time in the electroless bath. Lithium ion battery active materials were deposited on the nanofoam surface using electrodeposition. This technique is employed to deposit a controllable thickness of manganese oxide onto the surface of our biologically derived metal nanofoams. The resulting foams are then assembled into half cells. Using pulsed electrodeposition, the active material thickness was controlled by varying the number of pulses. By depositing thin active materials, we demonstrate that it is possible to produce ‘capacitor-like’ biotemplated electrodes with excellent rate capability. Thicker active material layers are used to produce more energy-dense electrodes with lower rate capability. Beyond the use of virus to synthesize three-dimensional current collectors for coin cells, we demonstrate that the biological templating method allows scale-up of the nanofoams to produce larger batteries. As the foams are based on self-assembling virus nanoparticles, it is possible to obtain large quantities of the template through biological amplification. Unlike a process employing extensive top-down fabrication techniques, genetically programmable templates provide a facile route toward large-format electrodes with control over nanoscale morphology. Through amplification of the virus and scale-up of the nanofoam production process, we demonstrate 4”x3” pouch cells. These electrodes are shown to be flexible, allowing compatibility with a wide range of systems. This demonstrates our biological templating method as an advance toward practical metal nanofoam engineering materials.
Figure: an M13-templated nickel nanofoam before active material deposition.

Figure 1