As MEMS devices for biomedical and other applications continue to develop and decrease in dimensions, the demand for power supplies with the appropriate size and energy density continues to grow. Although energy density is an important factor, one of the most crucial factors is the ability to fabricate cells in a variety of shapes so to enable the greatest design flexibility when fabricating a device. Recently our group has introduced an electrochemically self formed battery to grant a path towards the greatest flexibility. In short, a nanocomposite of an alkali halide such as lithium iodide is placed between current collectors and polarized thereby creating a lithium anode and polyiodide cathode in-situ. As with primary lithium-iodine cells the transport within the cathode is a complex mechanism involving the Li+, I-, and e- all within the polyiodide network. After our recent work on in-situ EIS evaluation of the technology, we have launched on an effort to greater understand the limiting transport mechanisms in the positive electrode as a function of polyiodide network development.
An in-depth characterization study was performed on the LiI-I2-PVP-H20 at various molar ratios to understand the structural and conductivity changes that take place during formation of the cell A combination of AC impedance and DC polarization studies were used for the impedance characterization in conjunction with blocking electrode methodology for separating the conductivity into its electronic and ionic portions. Also, FTIR and Raman were used to structurally characterize the samples for both the polyiodide formation and the interaction between the polyiodides and polyvinylpyrrolidone (PVP). Being non conjugated, PVP was chosen as it does not intrinsically contribute to the conductivity of the composite but does induce the formation of polyiodide species. As different molar ratio composites are prepared, the concentration of different polyiodide species (I3-, I5-, In-) within the composite change and affect the overall conductivity. A 3-dimensional plot of composite conductivity reveals a high electronic conductivity ridge for samples containing either LiI anhydrous or monohydrate at a constant I2 to PVP ratio. These 3-dimensional plots also allow us to correlate represent in an ex-situ format the electronic and ionic conductivity of the cathode/electrolyte at various depths of discharge.