Thin-film batteries attract increasing interest in e.g. medicine and biotechnologies where miniaturized batteries are required or toxic liquid electrolytes must be avoided. Another focus lies on their use as model systems. Thereby, special attention has to be paid to the thermodynamic behavior during electrochemical cycling.
In-situ thermodynamic investigation is performed by the recently developed technique “Thin-Film Calorimetry” (TFC). It is based on high-temperature stable piezoelectric resonators which serve simultaneously as the substrate for the films of interest and as highly precise temperature sensor. Deviations from their undisturbed course of the temperature dependent resonance frequency can be assigned to generation or consumption of heat caused by e.g. phase transformations. Thin-film all-solid-state electrochemical cells are deposited on top of electroded langasite resonators via ion beam sputtering. In this work, they are composed of the thin-film sequence:
Cu (300 nm) | Fe-doped Li3PO4 (85 nm) | LiPON (1500 nm) | Ag (300 nm)
Although not containing classical cathode/anode materials, this film sequence is working as a secondary battery. During first lithiation cycle the applied electrical potential results in the formation of a reaction layer at the interface between LiPON and silver, serving as a cathode. Simultaneously, lithium is plated at the counter electrode, serving as the anode of the galvanic cell.
Schemes of the TFC system as well as of the electrochemical cell are shown in Fig. 1.
In-situ thin-film calorimetry and cyclic voltammetry are performed simultaneously at isothermal temperature steps from room temperature up to 100 °C in an argon atmosphere. Thereby, the potential is swept between 0.5 V and 4 V using rates of 1–10 mV/s. The combined results are shown in Fig. 2. The cell shows a lithiation peak at 2.6 V as well as a delithiation peak at 1.4 V. The in-situ TFC measurements indicate that the cell cools down by about 1 K during one cycle despite heating up due to Joule heat as would be expected. The point with the highest temperature shift is near the maximum state of charge. One possible explanation for this can be changes in entropy of the battery electrodes. The effects contributing to this behavior will be discussed in more detail.
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