2.4. The study of the electrochemical performance of thin-film thermal batteries
The low χ and low ψ of LA136D contribute to manufacturing thin film cathodes with low volatility. Herein, we studied the LA136D thin film electrode electrochemical performance in both thermal battery single cells and stacks and verified the volatility of the electrode.Figure 5 (a) is the schematics of equipment used for thermal battery single-cell discharge. The single cell consists of a cathode, separator, and anode set between the two-heating plate. The temperature used to heat the single cell is 500 °C and the stress is 20 N cm−2. Figure 5 (b) are the discharge curves of single cells assembled by LA136D thin film cathode and traditional pellet cathode. It is shown that LA136D thin film cathode has a discharge capacity of 1360 As g−2 at 0.2 A cm−2. In contrast, the value is 1170 As g−2 for the pellet cathode. In addition, LA136D thin film cathode shows smaller polarization (~0.5 V) in comparison with the pellet electrode. Combined with the discharge capacity and voltage platform, LA136D thin film cathode shows a 22% energy density increase. Figure 5 (c) shows the pulse properties of single cells with different cathodes. It indicates that the internal resistance of the LA136D cell is only 17 mΩ upon 1 A cm−2 pulse test (The internal resistance is calculated according to R=U2−U1/I2−I1). In contrast, the internal resistance of the pellet cell is 75 mΩ, which is about four times higher than the LA136D cell. The superior discharge capacity and pulse performance indicate the superiority of the LA136D thin-film cathode to the traditional pressed-pellet thick cathode and prove that the LA136D binder is compatible with TBs cathode and molten electrolyte materials.
Furthermore, to verify the volatiles that produced in the thermal decomposition of LA136D do not bring safety risks to TBs, we studied the performance of the LA136D thin film cathode in the real hermetically sealed TBs stacks. Figure 5 (d) is the schematics of thermal battery stacks assembled in this study. The batteries include 40 single-cells in the series and 41 pyrotechnic pellets, the pyrotechnic pellets are positioned between cells and the two ends of the stacks. The thickness of the LA136D thin film cathode used in assembling thermal battery is 200 μm (100 μm electrode+100μm current collector). For comparison, a 300 μm+100 μm pellet cathode is also used to assemble the thermal battery stack. The diameter of both electrodes is 60 mm. The thermal design of the two thermal batteries is kept the same, namely, the highest temperature of the two thermal batteries is the same. The height of the thin film thermal battery and pellet thermal battery stack is 45.4 cm and 60.6 cm respectively. Thin-film thermal battery reduces about 25% height in comparison with pellet thermal battery. The height difference directly affects the activation time of the thermal battery. The activation time is extraordinarily important for military and emergency power sources because it directly relates to the response time in emergency accidents. Figure 5 (e) is the activation voltage curves of thermal batteries. The thin film thermal battery spends about 395 mS to reach 80 V working voltage, which is 38% faster than the pellet thermal battery. This means LA136D thin film cathode is more suitable to construct rapidly-activation thermal batteries.Figure 5 (f) are the 130 s pulse test results of the different thermal batteries. Consistent with single-cells, LA136D thin film cathode shows superior discharging capacity at big current density which indicates LA136D thin film is beneficial to construct high power thermal battery. When the TBs have a working time of approximately 100 seconds, the utilization efficiency of cathode materials of the thin-film cathode is three times higher than that of the pressed-pellet cathode.Figure 5 (g) is the schematics of the pressure and temperature test of the TBs. To reach the real-time temperature and pressure, a temperature and pressure sensor is put into the thermal battery. The temperature sensor is put on the stack’s side surface and the pressure sensor is put at the ends of the stacks. To examine the thermal battery in a rigorous state, thermal batteries are open circuits after activation without load.