KEYWORDS: Electrodes, System on a chip, Absorbance, Lithium, FT-IR spectroscopy, Reflectivity, Ions, Optical properties, Spectroscopy, Attenuated total reflectance
This study aims to expand the body of knowledge about the optical properties of battery cathode materials. Although
some studies have been conducted on the optical properties of Lithium Iron Phosphate (LiFePO4), to the authors’
knowledge, this is the first study of its kind on electrodes extracted from commercially available LiFePO4 batteries. The use of Vis/NIR and FTIR spectroscopy provides for a methodology to study the optical properties of LiFePO4 and may allow for the characterization of other properties such as particle size and the proportions of LiFePO4 versus FePO4 material. Knowledge of these properties is important for the development of a mechanism to measure the state-of charge (SOC) in lithium ion batteries. These properties are also important in a host of other applications including battery modeling and materials characterization.
Cylindrical LiFePO4 batteries (from A123 Systems Inc.) were acquired from the commercial market and charged to 10 different states between 30% and 80% of their nominal capacity using a constant-current, constant-voltage (CCCV)
cycling method. Visual inspection of the extracted electrodes shows that the LiFePO4/C-cathodes display subtle changes in color (shades of grey) with respect to SOC. Vis/NIR measurements support the visual observation of uniform intensity variations versus SOC. FTIR measurements show an absorbance signature that varies with SOC and is distinct from
results found in the literature for similar LiFePO4-based material systems, supporting the uniqueness of the absorbance fingerprint.
Arrays of metal nanoparticles, typically gold or silver, exhibit localized surface plasmon resonance, a phenomenon that has many applications, such as chemical and biological sensing. However, fabrication of metal nanoparticle arrays with high uniformity and repeatability, at a reasonable cost, is difficult. Nanosphere lithography (NSL) has been used before to produce inexpensive nanoparticle arrays through the use of monolayers of self-assembled microspheres as a deposition mask. However, control over the size and location of the arrays, as well as uniformity over large areas is poor, thus limiting its use to research purposes. In this paper, a new NSL method, called here geometrically confined NSL (GCNSL), is presented. In GCNSL, microsphere assembly is confined to geometric patterns defined in photoresist, allowing high-precision and large-scale nanoparticle patterning while still remaining low cost. Using this new method, it is demonstrated that 400 nm polystyrene microspheres can be assembled inside of large arrays of photoresist patterns. Results show that optimal microsphere assembly is achieved with long and narrow rectangular photoresist patterns. The combination of microsphere monolayers and photoresist patterns is then used as a deposition mask to produce silver nanoparticles at precise locations on the substrate with high uniformity, repeatability, and quality.
The localized surface plasmon resonance (LSPR) phenomenon that is characteristic of gold and silver nanoparticles has
applications in areas such as portable and remote chemical and biological sensing. However, fabrication of metal
nanoparticle arrays with high uniformity and repeatability, at a reasonable cost, is difficult. Nanosphere lithography
(NSL) has been used to produce inexpensive nanoparticle arrays, through the use of monolayers of self-assembled
microspheres as a deposition mask. However, lack of control over the location and size of the arrays, as well as poor
uniformity over large areas, limits its use to research purposes. Here, we present large-area fabrication of nanoparticle
arrays through both convective self-assembly NSL (CSANSL) and our new method, geometrically confined NSL
(GCNSL). In GCNSL, microsphere assembly is confined to geometric patterns defined in photoresist. We show that
400nm polystyrene microspheres can be assembled inside of large arrays of photoresist trenches from 4-20μm in width
and 500μm in length, with high uniformity, repeatability, and quality. Compared to CSANSL, GCNSL allows precise
patterning of nanoparticle arrays for use in practical LSPR sensing devices, while still remaining inexpensive.
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