In this talk, we examine ways that solution processed nanostructured materials can be used to address both fundamental and practical issues relevant to next generation electrochemical energy storage. Materials with appropriate porous architectures can be synthesized using a range of solution phase methods, including polymer templating, nanoparticle assembly, and selective dealloying of mixed metal precursors. We begin with nanoporous materials for application in fast charging energy storage systems referred to as pseudocapacitors. Here the goal is both to produce practical new fast charging material, as well as to define the nature of pseudocapacitance in nanostructured materials and to establishing design rules for synthesizing future generations of high rate capability materials. We find that in optimized nanoporous materials, the nanoscale structure and porosity can produce a very desirable combination of electrical connectivity, electrolyte access to the interior of the material, ample surface redox sites, and very short solid-state diffusion lengths for lithium ions, all of which facilitate fast charge and discharge. Perhaps more importantly, many nanoscale materials appear to show suppression of the intercalation induced phase transitions that can cause kinetic limitations in bulk materials. The result is a new family of fast charging nanostructured energy storage materials. In this talk, we will specifically explore anode materials based on nanoporous MoS₂, MoO₂, and Nb₂O₅, and nanoporous cathode materials based on LiMn₂O₄ (LMO) and LiVPO4F (LVPF). A combination of electrochemical kinetics and operando X-ray diffraction allows us to correlate nanometer scale architecture, the speed of charge and discharge, and the presence or absence of intercalation induced phase transitions to show that an optimize nanoporous structure is the key to producing new fast charging energy storage materials.

Similar porous architectures can also be used to increase stability and cycle life in high capacity alloy-type anode materials that are being considered as replacements for graphite in traditional lithium ion batteries. Unfortunately, alloy-type anodes suffer from short lifetimes due to large volume changes during lithiation or sodiation. One solution is to use nanoscale porosity to help accommodate those large volume changes. Here we specifically focus on nanoporous Sn and nanoporous SbSn, both of which are synthesized by selective etching, and both of which can be alloyed with either lithium or sodium to produce very high capacity anodes. Using transmission X-ray microscopy (TXM), we are able to directly image changes in both individual grains, and in the pore structure itself upon cycling. We find that porous materials expand much less than bulk materials, because the pores help accommodate the strain. More importantly, we find that porous alloys like nanoporous SbSn are more stable than pure metals because the two components, which alloy at different potentials, help stabilize the pores system so that the alloy can expand into the pores with causing them to break or fracture. This results in significantly improved cycle stability. Taken together, these two families of materials emphasize the key role that can be played by nanoscale porosity in optimizing the properties of next generation energy storage materials.

References

1. K. Lesel, J.B. Cook, Y. Yan, T.C. Lin, S.H. Tolbert, “Using Nanoscale Domain Size to Control Charge Storage Kinetics in Pseudocapacitive Nanoporous LiMn₂O₄ Powders.” ACS Energy Lett. 2017, 2, 2293-2298.
2. B. Cook, T.C. Lin, E. Detsi, J. Nelson Weker, S.H. Tolbert “Using X-ray Microscopy To Understand How Nanoporous Materials Can Be Used To Reduce The Large Volume Change In Alloy Anodes.” Nano Lett., 2017, 17, 870−877.
3. B. Cook, E. Detsi, Y. Liu, Y.-L. Liang, H.-S. Kim, X. Petrissans, B. Dunn, S.H. Tolbert, “Nanoporous Tin with a Granular Hierarchical Ligament Morphology as a Highly Stable Li-Ion Battery Anode.” ACS Appl. Mater. Interfaces, 2017, 9, 293−303.
4. B. Cook, H.-S. Kim, T.C. Lin, C.-H. Lai, B. Dunn, S.H. Tolbert, “Pseudocapacitive Charge Storage in Thick Composite MoS₂ Nanocrystal Based Electrodes. Adv. Energy Mater. 2016, 1601283.