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.