Electron Paramagnetic Resonance at Hunter College Makia Hughes John F. Kennedy High School Physics Department, Hunter College of Solid State NMR/EPR Lab  EPR measurements of Lithiated Silver Vanadium Oxide  Dr. John Flowers, Professor of Chemistry at Medgar Evers College of CUNY

Our Goal To use EPR to examine and confirm the mechanisms of discharge in a Sulfur Vanadium Oxide (SVO) battery in order to develop higher quality batteries.

What is EPR? Electron Paramagnetic Resonance also known as Electron Spin Resonance is a spectroscopic technique which detect species that have unpaired electrons. A surprisingly large number of materials have unpaired electrons. (examples: free radicals and many transition metal ions) EPR alone yields inconvertible evidence of free radicals and has the unique power of identifying the paramagnetic species. Emblem of the Society of Free Radical Research

EPR Instruments The EPR Spectrometer consists of many different components. Microwave Bridge: Electromagnetic Radiation Source Microwave Cavity: Amplifies weak signals from the sample Magnet: Splits the electronic spin energy levels Console: Contains signal processing and control electronics and a units for acquiring a spectrum Computer: Used to analyze data as well as coordinating all the units for acquiring a spectrum.

How does EPR Work? Like a proton, an electron has a spin, which gives it a magnetic property known as a magnetic moment. When an external magnetic field is supplied, the paramagnetic electrons can either orient in a direction parallel or antiparallel to the direction of the magnetic field . This creates two distinct energy levels for the unpaired electrons and measurements are taken as they are driven between the two levels.

EPR Spectroscopy EPR spectroscopy is the measurement and interpretation of the energy differences between the atomic or molecular states. These measurements are obtained because the relationship between the energy differences and the absorption of electro-magnetic radiation. To acquire a spectrum, the frequency of the electromagnetic radiation is changed and the amount of radiation which passes through the sample with a detector is measured to observe the spectroscopic absorptions

EPR Results 4 Samples Tested Supports our Hypothesis 1) Pure SVO- Gives EPR signal due to the paramagnetism of SVO; High Q value 2) Li0.72Ag2V4O11 Weak or No EPR signal; Lower Q value 3) Li2.13Ag2V4O11 Weak or No EPR signal; Lower Q value 4) Li5.59Ag2V4O11 Definite EPR Signal; Lower Q value Supports our Hypothesis