Electrical characterization of all-solid-state thin film batteries

All-solid-state thin film micro-batteries comprised of a lithium anode, lithium phosphorus oxy-nitride (LiPON) solid electrolyte and Li x CoO 2 cathode were evaluated at different temperatures from −50 to 80 °C for electrical behavior and impedance raise. The cell dimensions were ∼2 cm long, ∼1.5 cm wide and ∼15 μm thick. The rated capacity of the cells was about 400 μAh. The cells were cycled (charge/discharge) at room temperature over 100 times at a 0.25 C rate. The charge and discharge cut-off voltages were 4.2 and 3.0 V, respectively. The cells did not show any capacity decay over 100 cycles. The measured capacity was 400 μAh. The coulombic efficiency was 1, which suggests that the cell reaction is free from any parasitic side reactions and the lithium intercalation and de-intercalation reaction is completely and totally reversible. These cells also have good high-rate performance at room temperature. For example, these cells discharged at a 2.5 C rate delivered ∼90% of the capacity at a 0.25 C rate. However, the delivered capacities even at a 0.25 C rate at 80 and −50 °C were much lower than the room temperature capacity. Cells soaked at −50 °C were not damaged permanently as seen by the near normal behavior when returned to room temperature. However, cells heated to 80 °C were permanently damaged as seen by the lack of normal performance back at room temperature. Cell impedance was measured before and after cycling at different temperatures. The high-frequency resistance (generally ascribed to the electrolyte and other resistances in series with the electrolyte resistance) decreased with decreasing temperature. However, the interfacial resistance increased significantly with decreasing temperature. Further, the electrolyte resistance accounted for ∼2% of the total cell resistance. The cycled cells showed higher impedance than the uncycled cells.