ABSTRACT
As the cell-level electrochemical performance benefits of nanoscale electrode materials continue to be identified and optimized in the laboratory, non-trivial technical and engineering challenges remain in order to successfully address practicality and safety issues for ultimate consumer relevance. The costeffective scale-up of nanopowders with desired tolerances of purity and physical properties, such as particle size distribution, is an engineering challenge. The low densities associated with nanopowders have to be weighed against higher gravimetric energy density of a candidate material. Uniform and reproducible electrode substrate fabrication is a technical challenge with high-surface-area nanopowders, and processing problems need to be practically addressed, especially when thicker electrode coatings and useful loadings are required for specific applications, e.g., for advanced high-energy lithium-ion batteries for future aerospace mission applications. Although a greater electrode-electrolyte contact area is desirable, the high reactivity of the high-surface-area nanomaterials toward cell electrolytes can introduce issues of undesirably thick SEI layers and side reactions, and uniformly coated nanomaterials are being investigated to mitigate this issue. Also, and equally important, both the environmental and the health risks associated
with the reactivity and physiological properties of nanomaterials are not well understood.In summary, nanostructured electroactive materials for the negative electrode for lithium-ion batteries can potentially afford the following performance advantages compared to microstructured materials: (1) Rapid charge-discharge performance characteristics as a result of high surface areas and the diminishing of slow solid-
state lithium ion diffusion mechanisms necessary within bulk particles. Faster rates are more probable due to faster reactions near the particle surface. (2) Improved cycle life and coulombic efficiency due to reduced stress-induced deformation and crystal lattice breakdown. (3) Increased specific capacity due to the large interfacial area for lithiation and de-lithiation and improved capacity retention. (4) Faster mass transport and reduced concentration polarization at higher charge/discharge rates due to shorter diffusion path lengths, which results in higher power capability. (5) Improved low-temperature cell performance with respect to the cell electrode component.Various nanomaterial morphologies and/or fabricated electrode structures are actively being explored and optimized to enhance anode and cell level performance with respect to energy density, cycle life, and safety. In addition to coated, uncoated, or immobilized nanoparticle and nanoalloy forms, nanotube structures, and nanostructured films and arrays possessing wire, rod, whisker, or columnar architectures are under development, and examples of such materials will be elucidated in the sections that follow.
