Figure 5. The electrochemical performance of thermal batteries with LA136D thin film electrode. (a) The schematics of the equipment used for thermal batteries single-cell discharge, the heating temperature is 500 °C and the compress is controlled at 20 N cm−2; (b) the comparison of the discharge specific capacity and energy density between LA136D thin film cathode and pellet cathode in single-cell configuration, the thickness of electrochemical materials is 100 μm and 300 μm respectivly for thin-film and pellet cathodes, the diameter of the cathode is 60 mm, the cathode materials and molten electrolyte used in this experiment are Fe0.5Co0.5S2 and LiCl-KCl; (c) the comparison of pulse discharge performance of thermal batteries with LA136D thin film cathode and pellet cathode; (d) the comparision of the stacks height of thermal batteries with LA136D thin film cathode and pellet cathode; (e) the comparison of activation time of thermal batteries with different cathodes; (f) the comparison of pulse discharge performance of thermal battery stacks with different cathodes; (g) the illustration of the equipment and method used to detect the internal temperature and pressure of the thermal batteries, the results are shown in (h) and (i) respectively.
Figure 5 (h) and (i) are the temperature and pressure curves of thermal batteries after activation. Due to the smaller height, the temperature increase in the thin film thermal battery is faster than the pellet. The P is directly related to temperature and the amount of gases in binder thermal decomposition. The pressure increase in the thin-film thermal battery is faster than the pellet due to the faster rate of temperature increase. The gases produced by binder decomposition also contribute to the fast increase of gas pressure in thin-film thermal batteries. The highest relative pressure after activation is 0.16 MPa and 0.21 MPa respectively for the thin-film and pellet thermal batteries which indicates LA136D only produces 0.05 MPa gases in the thermal battery. The internal gas pressure of the LA136D thermal battery is less than the safety threshold (0.3 MPa) which we simulate in the previous, indicating that the LA136D does not lead to any distortion of TBs bulk. Therefore, we have shown LA136D will not bring safety risks to the thermal battery stacks and will not detriment the electrochemical performance due to the minim addition.
Excepting for the gas pressure increment, the gas released in binder thermal decomposition may deteriorate the electrode integrity, as shown in Figures 6 (a) and (b) . A binder with a low gas production rate in thermal decomposition is beneficial to maintain the mechanical integrity electrode while a binder with a high gas production rate is bad for the electrode’s mechanical integrity. To verify the mechanical integrity of the LA136D thin film electrode in TBs operation, we disassembled the thermal battery stack after discharge.Figure 6 (c) is the digital picture of the LA136D thin film cathode after discharge. It can be seen that the thin film electrode maintains well integrity even after discharging at high temperatures, and without any powdered materials drop. The cross-section of the discharged single cell also shows a regular morphology (Figure 6 (d) ). These results prove that LA136 is capable to maintain mechanical integrity in TBs operation. In contrast, a thin-film cathode prepared by PVDF binder shows many cracks after discharge.
Figure 6 (e) is the comparison of the reported binder used in fabricating thermal battery thin film cathodes in the aspects ofχ and ψ . It can be found LA136D have the lowest χand ψ . Such properties enable LA136D to fabricate thin-film cathodes with low volatility and high electrochemical performance.