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.