Characterization of Lithium Thin Film Batteries by Electrochemical Impedance Spectroscopy
In this work thin film batteries composed of a stack of several active layers comprising a platinum current collector, a LiCoO2 positive electrode, a LiPON vitreous solid electrolyte and a lithium negative electrode, were characterized by EIS. These measurements were performed at different states-of-charge between 3.6V and 4.2V (figure 1) and at different temperatures in the frequency range from 1 MHz to 50 mHz. It was found that the Warburg diffusion impedance decreases as the potential is increased, which is related with the growth of the diffusion path into the LiCoO2 during his delithiation. Then, 3 R//C contributions are observed (A, B, C) but only one (C) in the low frequency region depends on potential step. Iriyama and al.  showed that it corresponds to the charge transfer step at the LiPON/LiCoO2 interface. The contribution (A) is the most obvious to assign and corresponds to the bulk solid electrolyte. Measurements on Pt/LiPON/Pt blocking electrode cells allowed confirming this assignation and to determine intrinsic electrolyte parameters. The determination of LiPON resistance and capacitance by fitting impedance spectra, the knowledge of layer thickness and active surface allow us to calculated ionic conductivity and dielectric constant of LiPON respectively around 2.10-6 S.cm-1and 20. The activation energy of ionic conductivity in LiPON was found to be close to 0.56 eV. The origin of contribution B is more difficult to clarify. This latter is not important after the first charge of the microbattery (figure 2), but increases consistently after few months of storage at 100% SOC. Actually, the electrochemical deposition of lithium during the charge is likely to lead to a change in the geometry of the Li/LiPON interface and in its chemical composition (pollution through the voids formed at the interface) inducing an increase of interfacial resistance. In previous studies, Bates and al.  showed that a little part of transferred lithium no longer participates to the reversible capacity of the microbattery. We show here that a specific cycle can restore the interface quality then restoring a lower internal resistance.
AC impedance was used to understand microbattery behavior without disassembling them. This technique allowed to confirm the presence of three R//C contributions and to follow their evolutions with potential or storage conditions. Analysis of Pt/LiPON/Pt cells confirmed that electrolyte is the major contribution to the whole impedance and allowed to extract intrinsic parameter values of LiPON. Futures studies will focus on the interfacial degradations mechanisms and modeling to improve the description of microbatteries behavior.
 Y. Iriyama, T. Kako, C. Yada, T. Abe and Z. Ogumi, Solid State Ionics, 176, 2371 (2005).