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The performance of a liquid metal battery can be significantly enhanced by lowering the cell operating temperature through alloying of the cell components. The effect of alloying pure tin metal, a cathode material for liquid metal batteries, on electrochemical properties is investigated by preparing a Li|Sn-Bi (Sn:Bi = 56:44 at%) battery cell. The Li|Sn-Bi cell achieves mean voltages of 0.856 and 0.683 V during charging and discharging, respectively, at 100 mA cm−2, which are 0.039 and 0.067 V higher than those of the Li|Sn cell. The Li|Sn-Bi cell exhibits a high rate capability owing to the low charge-transfer resistance between the liquid electrodes and liquid electrolytes. The charge and discharge capacity losses are 6.5% and 18.6%, respectively, at 1000 mA cm−2 compared with those at 100 mA cm−2. The Li|Sn-Bi cell can be discharged even at 1000 mA cm−2 owing to an increase in the mean discharge voltage by the alloying of tin with bismuth. The Li|Sn-Bi cell also exhibits improved cycle performance, with a capacity retention of 94.2% after 1000 cycles and a capacity decay of 0.0065% per cycle. This is attributed to the lack of formation of intermetallic byproducts between Bi and the stainless steel case and suppression of the side reaction between Sn and stainless steel by bismuth in the Sn-Bi lamellar microstructure. Thus, using a Sn-Bi alloy as the positive material significantly improves battery performance, making the Li|Sn-Bi cell a good candidate for stationary storage applications.

Thermal batteries are primary batteries, used in military applications, and must be designed to meet mission-specific requirements. This mission-specific nature makes the design of thermal batteries a pain-stacking task as no specific guideline or concrete modeling technology is yet provided on optimizing heat, weight, energy density, and other important criteria when the requirements are given. In this study, we present an optimization methodology that can facilitate the weight-optimized design of a thermal battery by modeling the design processes and optimizing accordingly. Using this methodology, a 25 cell Li-Fe thermal battery was optimized with specific performance requirements as a case study. The performance of the thermal battery is highly dependent on heat and thus it is important to accurately predict the temperature during the optimization process. The multi-physics model can obtain accurate temperature prediction, but it takes a very long time to calculate and is difficult to apply to the optimization problem. Therefore, a deep neural network (DNN) model trained from a small number of multi-physics simulation data is adopted to overcome this issue. As a result, the DNN model showed an accurate prediction result with a maximum error of 1.5 °C, which is less than 1 % error. The DNN model significantly improved the time-efficiency by reducing the calculation time per solution of the multi-physics model, from 10 min to 0.01 s. The results indicate that our methodology can effectively guide the optimal design of complex thermal battery systems.

High-power, large-capacity reserve thermal batteries are useful power sources for the actuators of guided weapons and propulsion for the underwater vehicles. However, unexpected thermal runaway and explosions can occur due to the thermal design error. Preventing such accidents is crucial and arduous challenge in developing high capacity, reliable thermal batteries. To do so, the heat balance between the heat source and the electrode/solid-electrolyte components must be carefully considered when designing a thermal battery. Herein, the optimal discharge temperatures and heat balances of thermal batteries are obtained through the discharge experiment of unit cells with Li(Si) and pure-Li anodes (LAN): (Li(Si): 500 °C/100 cal g⁻1, LAN: 550 °C/110 cal g⁻1). The thermal properties of two types of thermal batteries (150 V, 80 cells, 1 kWh) with Li(Si) and LAN are also analyzed using COMSOL Multiphysics software. Furthermore, to compare the performance, two high-power, large-capacity thermal batteries with the same heat-source energy (110 cal g⁻1) are fabricated and discharged at 40 A over 600 s. The experimentally evaluated temperatures of the cathodes, electrolytes, heat sources, and other components are found to be consistent with the simulation results suggesting that this study provides a complete and accurate computational methods to design high capacity thermal batteries.

Thermal batteries are primary power sources for military applications requiring high reliability, robustness and long storage life. Conventional electrodes for thermal batteries are prepared by compacting powder mixtures into pellets. Separator is composed of halide mixture, such as LiCl-KCl eutectic salt, blended with MgO to immobilize the molten salt. In order to increase the power density and energy density, the resistance of electrolyte should be reduced because the resistance of electrolyte is predominant in thermal batteries. In this study, wetting behaviors and impregnation weight of molten salts as well as the micro structures of ceramic felt were investigated to be applicable to thin electrolyte. Discharge performances of single cell with the ceramic separator impregnated by molten salt were evaluated also. Zirconia felt with high porosity and large pore outperformed alumina felt in wetting characteristics and molten salt impregnation as well as discharge performances. Based on the results of this study, ceramic felt separator impregnated with molten salt have revealed as an alternative of conventional thick MgO based separator with no conspicuous sign of thermal runaway by short circuit.

Thermal batteries have been used for many military applications such as missiles and ordnance devices. As the thermal battery is composed of anode, cathode, MgO based-molten salt electrolyte and additional heat source, its specific energy density is lower than those of commercial batteries. By means of substituting MgO based-electrolyte to increase energy density of thermal batteries, we have indentified various ionic solid electrolytes such as LLT(La0.5Li0.5TiO3), LSTZ(Li3/8Sr7/16Ta3/4Zr1/4O3) and LLZ(Li7La3Zr2O12). As a ruslt, we found out that the LLZ with additivies, Ca and Nb, presents not only excellent ionic conductivity but also stable discharge characteristics at high temperature(≈ 300oC). Additionally, we have verified that the LLZ solid electrolyte is suitable for the full scale of thermal batteries with high energy density.