I.A) Rechargeable Lithium-ion Batteries
Faculty: Lamartine Meda (Xavier); Post-doc: Anantharamulu Navulla (Xavier);
Objective. The focus of this project is to develop new electrode materials based on polyatomic phosphate (PO43-) anions with improved ionic conductivity and thermal stability relative to oxide-based electrodes with comparable voltage.
Description of the Research Performed. A New LiSICON (Lithium Super Ionic Conductor) type material, LiNi4(PO4)3 has been synthesized by the Pechini type sol-gel method from stoichiometric amounts of LiNO3, Ni(NO3)3•6H2O, and NH4H2PO4. The final material was obtained at 750°C/5h. No phase changes were observed with increase in annealing temperature and time up to 900°C/24h except for an increase in crystallite size.
The x-ray diffraction pattern (Fig.1a) is similar to that of mononoclinic LiSICON/NaSICON systems. Therefore, assuming monoclinic lattice, lattice parameters are derived by using the POWD software. Fig. 1b shows the crystallite size is about 150-200 nm for samples annealed at 750°C/5h. The FT-IR spectrum (not shown) exhibits strong absorptions below 1,500 cm–1, which correspond to the “PO4” unit. Fig. 1c shows the complex impedance (Cole–Cole
) plots of real (Zreal) as a function of imaginary (Zimag) parts of impedance of LiNi4(PO4)3. The Cole–Cole
plot is characteristic of ionic conducting nature of the sample. The calculated ionic conductivity from the impedance plot is 3.5 x 10-6 S cm-1. Fig. 1d shows the cyclic voltammetry (CV) between 1.5 and 3.5 V. The CV profile demonstrates the electrochemical reversibility of the material and exhibits a redox reaction that corresponds to Ni2+/Ni3+.
Future Plans. 1. Obtain the electrochemical properties of this material such as capacity and cycle life. We would like to prepare the polymorphs of LiM4(PO4)3 where M = Mn, Fe.
I.B) Lithium Ion Batteries: Investigations of Tungsten Oxide Nanowires as Electrodes
Faculty: Lamartine Meda (Xavier); Post-doc: Anantharamulu Navulla (Xavier); Undergraduate Students at Xavier: Aaron Dangerfield (sophomore), Milana Jones (sophomore), and Christian White (Junior)
Objective. To synthesize new electrode nanocomposites based oxide
Description of the Research Performed. Tungsten oxide nanowires were synthesized via a simple thermal evaporation method at 650°C and ~ 10-3 Torr using WO3 powder without any catalysts or carrier gas. Gold coated silicon (Au/Si) substrates were positioned in the end and the precursor was in the middle of a quartz tube. As-grown nanowires were immediately characterized in a split, flat-cell (MTI Corp.) using metallic lithium foil as the counter and reference electrodes, Celgard as the separator, and the WO3-x nanowires as the cathode. One molar LiPF6 in a 1:1 mixture of EC and DMC was used as the organic electrolyte for control experiments.
Our observations revealed irreversible loss in current for both the nanowires (diameter 40~60 nm) and bulk (~1.5 mm) WO3-x when cycled below 1 V vs Li/Li+. The most significant results on the electrochemical characterization is the instability of bulk crystalline WO3 at low potentials. The instability of the WO3-x nanowires could be due to corrosion of the electrode in the electrolyte, structural transformation, and bad adhesion of the nanowires to the substrate after a few cycles.
Future plans: Repeat the experiments up to 5 cycles using galvanostatic charge-discharged method to study more in depth the electrochemical properties of WO3-x nanowires relative to the bulk properties. Explore the growth of WO3-x on stainless substrates for electrochemical studies.