Elemental Distribution Analysis of LiFePO4/Graphite Cells Studied with Glow Discharge Optical Emission Spectroscopy (GD-OES)

Abstract

LiFePO4/graphite cells with an aging state and several capacity fading levels were studied using glow discharge – optical emission spectroscopy (GD-OES). Depth profiles were obtained from the surface to the collector in positive and negative electrodes. Results show that Li is distributed homogeneously in terms of depth in LiFePO4 and graphite electrodes for cells with 85–100% capacity retention. For a cell that deteriorated to 44% capacity retention after only 50 cycles, the Li amount was apparently slightly lower toward the current collector in the positive electrode. Deposits formed on the graphite electrode surface (SEI) comprise Li, H, C, F, O, and P, with a large amount of Li for the cell with sudden deterioration. The deposition amount increases with progression of capacity fading, suggesting that deposits on the negative electrode are related to cell deterioration. The deposition surface shows Fe, implying that Fe dissolved and migrated from the positive electrode during charge–discharge cycles at 45°C. The Li intensities obtained from GD-OES spectra of all cells were correlated with Li contents obtained using induced coupled plasma – optical emission spectroscopy (ICP-OES) and atomic absorption spectroscopy (AAS), which enable us to ascertain the Li contents of the electrodes quantitatively.