What are Thermocells?

Thermal battery is also called molten salt battery, heat activated reserve battery. The electrolyte is a non-conductive solid during storage, and an electric ignition head or a firing pin mechanism is used to ignite the heating agent inside it to make the electrolyte melt and become an ion conductor and be activated. It is a primary storage battery that uses molten salt as an electrolyte and uses an automatic activation mechanism to ignite a heat source to melt and activate the electrolyte. Thermal batteries include: magnesium / vanadium pentoxide thermal batteries, calcium / lead sulfate thermal batteries, LAN sulfide thermal batteries, lithium alloy-iron sulfide thermal batteries.

Thermal battery is a high-temperature energy source, with an operating temperature of 350 ° C ~ 550 ° C. In addition to the anode and cathode, batteries are often separated by an ion-conducting molten electrolyte. A small metal oxide or ceramic powder is added to the electrolyte as a binder, such as MgO, SiO ₂ , and BN. They play a role in fixing the electrolyte. The deformation flow characteristics of the electrolyte layer are affected by factors such as temperature, pressure, composition, and binder content. Before the electrolyte melts, the battery is emotional and can be stored for a long time. Once the ignition is activated, it can discharge at high power and the operating current can reach several amps per square centimeter for several seconds. It can also be discharged at low power for a long time for 1 h or Longer. Thermal batteries use some alkaline metals or alkaline earth metals with small atomic weight, chemical properties, and negative electrode potentials as anode materials (such as lithium, sodium, calcium, etc.). They have high specific power and specific energy. Therefore, thermal batteries Ideal as a military power supply ^[1]

The National Laboratory of the United States has developed a thermal battery with a life of about 5 minutes and applied it to nuclear weapons. The battery is manufactured using a "cup-lid" process. Each single battery is sealed in a metal cup, which is connected to each other by a metal strip. The glass fiber cloth impregnated with the electrolyte is separated, and the activation heat source is a so-called "hot paper"-a mixed powder of zirconium and BaCrO ₄ mixed with ceramic fibers is processed into a thin sheet. This material has poor stability and is quite sensitive to static electricity and vibration. In the early 1960s, the emergence of sheet technology greatly promoted the development and application of thermal batteries. All battery components are sheet-shaped. The electrolyte is fixed by kaolin and then converted to calcined SiO _{2. The} sheet-shaped heat source is mainly composed of iron powder and The composition of KCLO _{4 is} safer and more stable than "hot paper". Only a small amount of gas is generated after ignition. The working temperature of the thermal battery can be controlled by adjusting the ratio of iron to KCLO ₄ . The emergence of sheet technology and the application of high-efficiency thermal insulation materials have greatly improved the specific power and specific energy of thermal batteries, especially the battery life ^[1]

Thermal battery is a reserve type high temperature molten salt primary battery, which is mainly used for the power supply of missiles, rockets and emergency electronic equipment. With the development of this battery technology, non-military equipment may also be applied. The early thermal battery cathode materials were WO ₃ , V ₂ O ₅ , Pb ₂ SO ₄ and the like, and the anode materials were Ca and Mg. The Ca ₂ CaCrO ₄ series developed in the 1960s was better than modern thermoelectric Li (M) ₂ FeS ₂ The energy and specific power are very low, and there are side chemical reactions, and the heat is released to cause runaway. The origin of modern thermal batteries has benefited from the development of high-temperature batteries. In 1968, the US Agung laboratory carried out research on Li ₂ S high temperature thermal batteries. The open circuit voltage of the Li ₂ S electrochemical system was 2.23 V and the theoretical specific energy was 2602 Wh / kg. Since the two active materials Li and S are molten in the working state Liquid, difficult to store and control, this series was forced to give up in the early 1970s. In 1973, the solid (-) LiAl and metal sulfide electrodes replaced the Li and S electrodes. The metal sulfides were Cu, Fe, Ni, Cr, and Co. The iron sulfide was selected because it has a better comparison with other metal sulfides. High specific energy and low price. In order to obtain higher output voltage and capacity, FeS _{2 was used} instead of FeS in the mid-1980s. At the same time, LiSi alloys were also used as negative electrodes for thermal batteries. In recent years, thermal batteries have developed rapidly. For thermal batteries with different performance, new anode lithium alloy active materials LiSiMg alloy and LiB alloy need to be developed, and new cathode active materials LVO, etc. ^[2]

Theoretically, FeS _{2 has a} medium potential, affects the specific characteristics of the battery, and has poor thermal stability and severe weight loss at 450 , so the improvement of the positive electrode material for thermal batteries is necessary. The potential of Cr ₂ O ₃ in chromium compounds is low, and CrO ₃ , Cr ₂ O ₅ , and CrO ₂ lack thermal stability, so they are not suitable for the positive electrode of a thermal battery. CaCrO ₄ has a long history as a positive electrode for thermal batteries. Other chromates such as Li ₂ Cr ₂ O ₇ and K ₂ Cr ₂ O ₇ are also commonly used as positive electrodes. In particular, Li ₂ Cr ₂ O _{7 has a} small molecular weight and the highest electrochemical capacity. , More people's attention. Because the melting point of lithium chromate is about 516 lower, it is often necessary to add a binder so that the active material molten salt does not flow during the electrode operation, and the shape remains unchanged. Li ₂ Cr ₂ O ₇₂ LiAl has an open-circuit voltage of 3.0 V and a discharge cell voltage of 2.5 to 2.6 V at 100 mA / cm², which is basically equivalent to lithium vanadium oxide (LVO). Among the manganese compounds, ₂ MnO ₂ is quite common as other battery cathode materials such as zinc manganese and lithium manganese batteries. It has also been studied and tested as the positive electrode of thermal batteries. ₂ MnO ₂₂ LiAl has an open circuit voltage of 3.1V, 100mA / cm² The discharge cell voltage is 3.0V ^[2] .