J. Korean Ceram. Soc. > Volume 56(3); 2019 > Article
Ko, Kang, Cheong, and Yoon: Recent Progress in Cathode Materials for Thermal Batteries

Abstract

Thermal batteries are reserve batteries with molten salts as an electrolyte, which activates at high temperature. Due to their excellent reliability, long shelf life, and mechanical robustness, thermal batteries are used in military applications. A high-performance cathode for thermal batteries should be considered in terms of its high capacity, high voltage, and high thermal stability. Research progress on cathode materials from the recent decade is reviewed in this article. The major directions of research were surface modification, compounding of existing materials, fabrication of thin film cathode, and development of new materials. In order to develop a high-performance cathode, a proper combination of these research directions is required while considering mass production and cost.

1. Introduction

Thermal batteries are reserve batteries that can be stored for a long period in an inactive state, which can then be activated to generate power when needed. Thermal batteries include molten salts as an electrolyte and employ an internal pyrotechnic source to bring the battery stack to the operating temperature. Thermal batteries are high-temperature power sources that typically operate between 350 and 550°C. Thermal batteries are used for many military applications, such as power sources for guided missiles and proximity fuses in ordnance devices because of their excellent mechanical robustness, reliability, and long shelf life1). The progress of research on thermal batteries until about 2006 is summarized in the review by Masset et al..1-5) This review provides an overview of molten salt electrolytes,2) cathode materials,3,4) and molten salt electrolytes5) of thermally activated batteries. As a representative thermal battery electrode, Li-Si is used as an anode and FeS2 (pyrite) is used as a cathode; LiCl-KCl is used as an electrolyte. Fig. 1 shows the basic structure of an Li-Si/LiCl-KCl/FeS2 thermal battery. So far, various studies have focused on optimizing each component in these systems.6-9)
Among them, studies focusing on the cathode are largely divided into materials and manufacturing methods. The most commonly used cathode material to date is FeS2. In addition to studies on optimizing electrode fabrication using FeS2,10-15) studies have been conducted on the use of materials such as CoS2,16-21) NiS2,22-24) NiCl2,25-28) CuVO,29-30) and others.31-33) Masset et al. cited the following key properties to be considered as cathode materials for thermal batteries:4)
• Redox potential: it should have a discharge potential that is compatible with the electrochemical window of the electrolyte in order to prevent oxidation.

• Ability to provide a fixed discharge plateau: it should undergo multiphase discharge and not intercalation.

• High thermal stability: thermal decomposition and associated possible chemical reactions caused by decomposition products should be minimized (e.g., S2 in the case of FeS2 reacting with the anode or pyrotechnic source in the battery).

• Electronically conductive: the resistance of the cathode should be minimized.

• Low solubility of the cathode materials in molten electrolytes: self-discharge reactions with attendant loss in capacity should be prevented.

• Low solubility of discharge products in the molten electrolytes: prevent self-discharge reactions.

• Stable towards moisture and/or oxygen: to prevent oxide production at the cathode surface.

• Ability to be wetted by electrolyte: this minimizes the contact resistance at the electrolyte (separator)/electrode interface.

• Good discharge kinetics (high exchange-current density): provides high rate capability.

• Reasonable costs.

• Being environmentally friendly is an additional desirable attribute.

The second area of focus in prior research is on the method for manufacturing the cathode electrode. Generally, the constituent materials are produced in the form of pellets with a thickness of several hundreds of micrometers using a powder compaction process. However, the formed pellets are thicker than the optimum thickness in terms of utilization of the electrode material because they must maintain a minimum mechanical strength. For these reasons, an electrolyte material is added to the electrode to compensate for the lower utilization of the electrode material. However, this approach makes the electrode becomes thicker and reduces the ratio of the active material in the electrode. When the thickness of the electrode pellet is greater than a certain level, electrode utilization decreases significantly, which may decrease the energy density of the thermal battery. To solve such a problem, it is essential to choose a process for thinning the electrode such that the electrode has sufficient mechanical strength with optimal electrochemical performance. Therefore, some studies focused on the use of a tape casting process to thin the cathode of a thermal battery.11-13)
In this review, the cathode materials of thermal batteries are reviewed in terms of material types and electrode manufacturing methods. The scope of this review is the past 10 years since the reviews published by Masset et al..1-5)

2. FeS2

The intrinsic difficulties of engineering a thermal battery based on Ca/CaCrO4 electrochemistry were obviated with the introduction of Li-alloy/FeS2 couples.1) The performance of the Ca/CaCrO4 electrochemical system tended to be somewhat unpredictable.1) FeS2 was readily obtained from processing pyrite, which is a plentiful, inexpensive material compared to chemically synthesized CaCrO4. In addition, FeS2 has higher electrical conductivity at elevated temperatures. This provides improved power and lifetimes over the Ca/CaCrO4 system.3,4) For these reasons, the Li-alloy/FeS2 system is still the most widely used electrode for thermal batteries. However, the performance improvement is limited due to the low thermal stability and low open circuit voltage compared to other materials, and studies have been conducted to supplement these findings.10-15)
Lee et al. studied the use of carbon black and carbon nanotubes on an FeS2 cathode in Li-Si/FeS2 thermal batteries.10) Discharge measurements and the calculated total polarization confirmed that the performance of the single cell improved when a carbonaceous material was added (Fig. 2(a, b)). In Fig. 2(c, d), Nyquist plots of data from the FeS2 cathode are shown for various contents of carbonaceous materials. The impedance of each sample increased as the content increased. The results from a pristine sample were not compared together; therefore, it is difficult to determine what level of impedance the 1 wt% sample represents. In addition, there is no information on how the impedance results were obtained when the content was more than 1 wt%, as well as the discharge and total polarization results when the content was greater than 0.1 wt%. Nevertheless, attempts were made to improve the performance of battery electrodes using carbonaceous materials,34-43) and the effect was confirmed in the FeS2 cathode of thermal batteries. However, additional validation is required for actual mass production.
Yoon et al. fabricated and evaluated a thin cathode using a tape casting method instead of conventional pellet type cathode.11) When producing a pellet with a powder compaction process, the thickness is minimized for mechanical strength, thus powder compaction is not an appropriate process from the perspective of optimizing the battery performance. In this study, tape casting was used to improve the utilization of cathode materials and the dispersion condition, binder type, and content of each material were determined. As shown in Fig. 3(a), a thin cathode with maximum content of FeS2 and a minimum amount of binder was prepared. Its mechanical strength was maintained even under severe deformation, as shown in Fig. 3(b), and its discharge rate was nearly twice that of the pellet type, as shown in the Fig. 3(c, d). Tape casting was used to overcome the limitation of a manufacturing process of an existing pellet and showed excellent performance.
Thin cathode production using tape casting was also studied from the perspective of the binder material. Jung et al. fabricated a high thermo-stable poly(imide-co-siloxane) (PIS) binder for thermal batteries and used it with an FeS2 thin cathode in tape casting.12) The PIS binder is stable up to 400°C and decomposes at 450°C (Fig. 4(a)). Thus, it cannot be used at 500°C, which is the operating temperature of normal thermal batteries, but shows a higher level compared to binder materials in typical secondary lithium batteries. 44-48) During discharge, FeS2 thin cathodes with PIS showed better performance than conventional pellet-type cathodes, and the total polarization was stable. The discharge capacity is lower than the results presented by Yoon et al..11) This may arise due to the difference in FeS2 particle size. The thermal battery is a special cell that operates at high temperature (500°C), thus the active electrode material must have excellent thermal stability up to 500°C. The thermal stability decreases when the particle size of FeS2 is reduced to a certain size or less, and the decomposition rate increases at 500°C, which decreases the discharge capacity.
Another approach to the use of binder materials in thin cathode manufacturing using tape casting is to use multi-walled carbon nanotubes (MWCNTs). Yoon et al. applied MWCNTs, which have excellent mechanical strength and conductivity, as an alternative to conventional organic binders for the thin cathode of thermal batteries.13) Carboneous materials have been extensively studied and used as additives for electrodes in rechargeable Li-ion batteries.34-43) Among them, some carboneous materials are used as current collectors and as electrode supports without adding other binder materials.34,49-54) Noda et al. showed that electrodes containing 99 wt% of active materials and 1 wt% CNTs can be produced.34) However, more research is required to obtain stable cycle performance without electrode degradation in rechargeable batteries, where charge/discharge cycles repeat. On the other hand, thermal batteries are the primary batteries with only one discharge, and are more suitable than rechargeable batteries in active material-CNT composites as the electrode. That is, the use of an FeS2-MWCNT composite is a universal method in battery research, but is a suitable and novel method for manufacturing a thin cathode for a thermal battery. Two studies11,12) related to thin cathode manufacturing with tape casting used organic binders and they were decomposed before reaching the operating temperature of 500°C. The decomposed organic binder material can generate gas that aggravates the performance of the battery, and the residual material can act as a resistor.55-60) To solve these problems, MWCNTs were used as a binder and conductor for the cathode electrode in this study. MWCNTs have better thermal stability than conventional organic binders and show a mass reduction rate of about 5% at 500°C. Fig. 5(a) shows the contents of each type of cathode. The FeS2-MWCNT composite cathode has a higher content of FeS2 than the other types. This is an important factor for increasing the energy density. As shown in Fig. 5(b, c), the FeS2-MWCNTs composite cathode exhibits higher capacity than the cathode with an organic binder of the same thickness. In this study, MWCNTs are added as a binder in a small amount compared to an organic binder and act as a conductor.
All three of the aforementioned studies11-13) focus on thin cathode manufacturing methods using tape casting. These are new attempts in terms of manufacturing methods to maximize the performance of thermal batteries based on the material properties of FeS2. That is, it is meaningful to maximize the utilization ratio of the electrode material, which is poor in the production of the conventional pellet-type material, by manufacturing the electrode with the optimum thickness form an electrochemical perspective. Thin cathode manufacturing is possible with tape casting or with any other method capable of producing films with an appropriate thickness, such as spraying61-65) or screen-printing. 66-70) Rather, the key to thin cathode manufacturing is high performance, stable, highly productive slurry manufacturing technology.71-74) Of course, thin cathode manufacturing technology is applicable not only to FeS2 but also to other cathode materials. For this purpose, it is necessary to study the binder, solvent, and dispersant suitable for each cathode material.

3. CoS2 and Bimetallic (Fe-Co) Disulfides

CoS2 has a lower solubility in molten electrolytes and much higher electronic conductivity, which permits a higher discharge rate. Most importantly, it has a much higher thermal stability and decomposes only above 650°C, which is ~ 100°C higher than for FeS2, facilitating its long-term application. However, the major disadvantage of CoS2 relative to FeS2 is its higher cost.4) Nevertheless, CoS2 generally has better properties than FeS2. For this reason, studies on thermal batteries using CoS2 cathodes have continued until recently.
Zhu et al. fabricated CoS2 thin cathodes using a screen-printing method and confirmed its feasibility.16) This study was conducted in the same context as the aforementioned FeS2 cathode fabricated as a thin film type using tape casting.11-13) That is, a method for optimizing the thickness in order to maximize utilization of the electrode material was used. A CoS2 cathode material and screen-printing method were used. Screen-printing is widely used in the textile and electronics industries, and it is a remarkably simple and reproducible production method. In other words, applying this method to thin cathode fabrication for thermal batteries is an appropriate choice. However, this study seems to have a problem in selecting the binder material. In this study, PVDF, which is widely used in the production of electrodes for lithium ion batteries, is used to prepare slurries for screen-printing. However, because the melting point of PVDF is about 160°C, it is not suitable for use in thermal batteries with operating temperatures of approximately 400 to 500°C. As shown in Fig. 6(a), a thin cathode was fabricated with a thickness of 50 μm. As shown in Fig. 6(b), the discharge capacity was more than double that of the pellet-type cathode, producing high voltage and low resistance. However, as described in the aforementioned studies using tape casting,11-13) it is a natural phenomenon that the utilization rate of the electrode increases to a certain point and the discharge capacity increases when the electrode is thin. In the case of the pellet-type electrode, the thickness of about 500 μm due to the limited mechanical strength, and lost electrode utilization is minimized by adding an electrolyte material. The key points of research for fabricating a thin cathode are 1) material selection (e.g., active material, binder, solvent, and dispersant) and 2) manufacturing method (e.g., tape casting, screen-printing, or spraying). With respect to these two issues, processing methods such as tape casting,75,76) screen-printing,77-82) and spraying83-89) have already been sufficiently investigated in other fields. Of course, appropriate equipment modifications are necessary depending on the application, but this is considered less important than material selection. Very high reliability is required because the thermal battery is a special battery that is primarily used for military applications. Therefore, additional studies on material selection are required to demonstrate the performance while being suitable for the application environment.
Liu et al. showed that CoS2/CNTs composites were prepared using a facile, simple hydrothermal growth process for CoS2 in the presence of CNTs, as a novel approach of synthesizing cathode composites for thermal battery fabrication. 17) The incorporation of CNTs in the synthesized CoS2 cathode materials effectively improved the electrical conductivity and discharge performance of the resulting cathode. In this study, single cells with a CoS2/CNTs cathode, an LiCl-KCl electrolyte, and an Li-Si anode were fabricated and evaluated. All components were fabricated into pellet-type materials using a powder compaction process. This study addresses the addition of CNTs to improve the electrochemical properties of pellet-type cathodes, as in the work of Lee et al..10) The difference between them is the type of active material and the method used to add CNTs. In this study, CoS2 was used as a cathode material and a CoS2/CNT composite was fabricated using a hydrothermal method. Hydrothermal growth was carried out at 180°C by adding CNTs and CoS2 raw materials to a CNT dispersion. There is no mention of the advantages of hydrothermal reacting raw materials and electrical additive over conventional mixing methods. Intuitively, when the hydrothermal method is used, the interfacial properties of CoS2 and CNTs improve and the electrical conductivity and discharge performance of the cathode are expected to increase.90-92) However, in this study, the content is not shown, and the goal is to compare CoS2/CNTs with CoS2. Fig. 7(a) shows that the thermal stability of CoS2/CNTs and CoS2 are similar. The low thermal stability of CoS2/CNTs is due to the addition of small amounts of CNTs. CoS2/CNTs with thermal stability above 600°C can perform better than FeS2 or FeS2/additive composites. The discharge evaluation results show that the CoS2/CNT cathode exhibited excellent performance in all aspects, such as discharge capacity and resistance. However, as in the case of FeS2, additional validation experiments are required based on these results.
Liu et al. also studied carbon-coated CoS2 that was fabricated using hydrothermal synthesis.18) In this work, a carbon-coated CoS2 composite was prepared using a facile onepot hydrothermal method with glucose as a carbon source. During the growth of CoS2, the glucose molecules were adsorbed and carbonized in situ on the surface of the as-synthesized CoS2, and the resulting carbon coating provided improved electrical conductivity and discharge performance to the composite. In addition to improving the electrochemical performance, the carbon coating also improves the stability of CoS2 in air. In this study, single cells composed of a carbon-coated CoS2 cathode, an LiCl-KCl electrolyte, and an Li-Si anode were fabricated and evaluated. All components were fabricated into pellet-type materials using a powder compaction process. The stability of the carbon-coated CoS2 and bare CoS2 in air is shown in the XRD and TGA data in Fig. 8(a, b). After 3 months, impure peaks were detected in bare CoS2, while the carbon-coated CoS2 remained pure. In addition, bare CoS2 decomposition occurred before 400°C after 3 months, while the carbon-coated CoS2 remained relatively stable. In the discharge evaluation results, the carbon-coated CoS2 cathode showed excellent performance compared to bare CoS2 in all aspects, such as discharge capacity and resistance. However, as in the case of CoS2/CNT composites, additional verification tests are required before beginning mass production.
Yu et al. used other approaches to improve the stability of CoS2 in air. In this work, Fe-doped CoS2 (CoXFe1−XS2) compounds were synthesized and evaluated as cathode materials for thermal batteries.19) Single cells composed of a CoXFe1−XS2 cathode, an LiCl-KCl electrolyte, and an Li-Si anode were fabricated and evaluated. All components were fabricated into pellet-type materials using a powder compaction process. To investigate the stability of CoXFe1−XS2 in air, the as-prepared samples were stored in air (25°C, 100% relative humidity) for 7 days. The XRD patterns Fig. 9(a) do not reveal any apparent impurities or secondary phases in the S-FeS2 sample, illustrating the stability of FeS2 in air. Obvious diffraction peaks corresponding to CoSO4·H2O can be seen as the content of Co increased further, especially for the S-CoS2. These results reveal that bimetallic disulfide has better stability in air than that of CoS2. As shown in Fig. 9(b, c), the bimetallic disulfide sample was superior to CoS2 in terms of its electrochemical performance, such as its discharge capacity and total polarization. As shown in Fig. 7(d), the discharge capacities of S-CoS2 and S-Co0.7Fe0.3S2 distinctly decrease due to formation of a large amount of CoSO4·H2O, while the S-Co0.3Fe0.7S2 still exhibits good discharge capacity. These results indicate that stability in air has a significant impact on the discharge performance of the disulfides. Bimetallic disulfides with suitable Fe content can exhibit stable discharge performance in air.
This approach was also studied by Zhu et al..20) This study shows that composite sulfide Fe0.5Co0.5S2 can be prepared through traditional high-temperature solid-state and liquid phase methods, and this composite can be used as a cathode material in thermal batteries.20) The goal of this study is to improve the poor thermal stability of FeS2 and the poor stability of CoS2 in air by applying Fe-Co bimetallic disulfide. In this study, single cells composed of a Fe0.5Co0.5S2 cathode, an LiCl-LiBr-LiF electrolyte, and an Li-Si anode were fabricated and evaluated. The cathode was made of thin film using screen-printing, and other components were fabricated into a pellet-type material using a powder compaction process. PVDF was used as a binder in thin-cathode manufacturing using screen-printing, as in the work of Zhu et al..16) In order to manufacture a thin cathode with high reliability, the application of a heat-resistant binder should be considered. Clearly, one can see in Fig. 10(a) that a liquid-phase-synthesized Fe0.5Co0.5S2 sample is purely single-phase while still maintaining a pyrite structure. All diffraction peaks shifted to lower angles than those observed from cubic FeS2. As shown in Fig. 10(b), Fe0.5Co0.5S2 has better thermal stability compared to FeS2. Better thermal stability points to a higher utilization rate for electrode materials, which can greatly increase the discharge performance. Fig. 10(c, d) shows the morphology of Fe0.5Co0.5S2 prepared using solid-state and liquid phase methods. The particle size of the liquid-phase-synthesized Fe0.5Co0.5S2 is much smaller, which is expected to have a significant impact on discharge performance. In other words, larger specific surface area can greatly decrease electrochemical polarization, thus significantly slowing down the working voltage drop and improving discharge capacity. The discharge evaluation results in Fig. 10(e, f) show this tendency.
Fe0.5Co0.5S2 cathodes have higher discharge capacity than an FeS2 cathode, which is due to the improved thermal stability provided by bimetallic Fe-Co. In addition, liquid phase synthesized Fe0.5Co0.5S2 has a higher discharge capacity than solid-state synthesized Fe0.5Co0.5S2. This is primarily due to the pure single-phase and small particle size of the aforementioned liquid phase synthesized Fe0.5Co0.5S2. New approaches to take advantage of each substance, not limited to conventional disulfide cathodes, are very encouraging.

4. NiS2

The thermal stability of NiS2 is intermediate between that of FeS2 and CoS2, as shown in Fig. 11(a).4) The electrochemical performance of NiS2 in an LiCl-KCl eutectic electrolyte is compared to that of synthetic FeS2 and CoS2 in Fig. 11(b). As expected, the voltage response for the cell with the NiS2 cathode was intermediate between that of the CoS2 and FeS2 cells. However, the overall performance of the NiS2 cathode was similar to that of CoS2. Because the costs of NiS2 precursors are much less than those for CoS2, the similar electrochemical performance motivates the use of NiS2 for application where CoS2 might have been used.1-4) However, NiS2 does not provide a cost advantage over FeS2 and is not attractive enough to offset the high cost in terms of performance. Several studies that expand on this point have been published.
To maximize the performance of NiS2, Zhou et al. fabricated NiS2 as a nanostructure and evaluated its properties and performance.22) In this study, single cells composed of a nano-NiS2 cathode, an LiCl-LiBr-LiF electrolyte, and an Li- B anode were fabricated and evaluated. All components were fabricated into pellet-type materials using a powder compaction process. Nanostructured NiS2 powders were prepared by ball-milling for 10 to 40 h and subsequently heating at 400°C for 1 h using carbonyl nickel and sulfur as precursors. The theoretical capacity of NiS2 is about 870 mAh/g, but the discharge capacity in practical applications is much lower. The sluggish diffusivity of lithium ions in the NiS2 cathode and low surface area of the bulk electrodes are identified as the primary factors producing poor discharge performance.93) In addition, intermediate phase evolution with low conductivity can reduce the discharge performance. 94,95) In this study, the ball-milling time was defined as a control parameter to characterize the particle size of NiS2. As shown in Fig. 12(b), the surface area of the 40 h ball-milled NiS2 is the largest, which can increase the contact area of the electrode and the electrolyte, thereby improving the discharge performance. The discharge capacity of 40 h ball-milled NiS2 was largest at the discharge curves (0.1 A/cm2) shown in Fig. 12(c). The discharge curves (0.5 A/cm2) in Fig. 12(d) show the same tendency in the discharge capacity, but the voltage plateau becomes clearer. This is due to the intermediate phase generated during discharge, and it becomes clearer as the size of NiS2 particles decreases. That is, if the size of NiS2 particles is large, the reaction mechanism of NiS2 does not occur completely. Inferred from the results of this study, the negative influence of the intermediate phase (NiS) becomes stronger when the NiS2 particles are larger due to the relatively low conductivity of NiS,96) leading to a decrease in discharge performance. In other words, NiS2 produced rapid intermediate phase evolution, which increased the discharge capacity of the NiS2 cathode. This mechanism is shown in Fig. 12(a). This study is meaningful for improving the characteristics of the NiS2 cathode and for interpreting the performance improvement mechanism in various ways.
\$Zhou et al. studied carbon-modified nano-NiS2 based on the aforementioned nano-NiS2 results.23) In this study, single cells composed of a carbon-coated nano-NiS2/nano-NiS2 cathode, an LiCl-LiBr-LiF electrolyte, and an Li-B anode were fabricated and evaluated. All components were fabricated into a pellet-type material using a powder compaction process. Carbon modification is always seen as an effective method for improving the conductivity and thermal/structural stability of electrodes.97-102) In this study, NiS2 particles were coated with amorphous carbon. Then they accumulated into submicron particles and were connected/fixed by a carbon network. These effects are shown in Fig. 13(e, f), as shown in the TEM image and schematic diagram. The initial decomposition temperature of nano-NiS2 increased from 400 to 590°C after carbon modification. As shown in Fig. 13(b), the discharge performance improves as the thermal stability increases. In the case of carbon-coated nano-NiS2, the discharge capacity at high temperature shows a stable specific capacity up to 600°C near the initial decomposition temperature (Fig. 13(c)). Beyond 600°C, NiS2 rapidly decomposes and its specific capacity decreases. Carbon modification not only improves the conductivity of the cathode on its own, but it also inhibits the spread of the low conductivity product Li2S. This effect is reflected in the pulse discharge and resistance results (Fig. 13(d)).

5. NiCl2

Due to several advantages mentioned above, disulfide-based materials have been used as the cathode in thermal batteries for decades. However, it is difficult for disulfide materials to meet recent demands for high power thermal batteries due to their low thermal stability and low discharge voltage. As a result, research and development on cathode materials (e.g., NiCl2) other than disulfide-based materials have continued. NiCl2 has been considered as one promising cathode material for use in thermal batteries thanks to its high relative potential, high current discharge, high specific energy, and low cost. However, its high solubility in molten salt electrolytes causes thermal batteries to short circuit, and its low electronic conductivity results in long activation time, thus NiCl2 thermal batteries cannot provide electrochemical performance.25)
Zhou et al. conducted surface modification with a carbon coating to improve the performance of NiCl2-based batteries. 25) It was confirmed that modification using carbon coating has a positive effect on electrochemical performance of CoS218) and NiS2.23) In this study, the solubility of NiCl2 in the molten salt was evaluated using a contact angle measurement on the NiCl2 cathode, which could be used to analyze wetting of the LiCl-LiBr-LiF electrolyte. These results indicate that the carbon coating could inhibit NiCl2 from dissolving into the molten salt (Fig. 14(a)). Single cells composed of carbon-coated nano-NiCl2/pure NiCl2 cathodes, an LiCl-LiBr-LiF electrolyte, and an Li-B anode were fabricated and evaluated. All components were fabricated into pellet-type materials using a powder compaction process. The carbon-coated NiCl2 thermal batteries exhibited increased specific capacity and increased specific energy compared to that of pure NiCl2 thermal batteries.
Zhu et al. investigated an alternative to the conventional stainless steel substrate for NiCl2-based thermal batteries. 26) In this study, an NiCl2 cathode was fabricated by screen printing on stainless steel and nickel foam substrates. In addition, LiCl-LiBr-LiF electrolyte and Li-Si alloy anodes were fabricated into pellet-type materials using a powder compaction process. The shape of the NiCl2 cathode in Fig. 15(a, b) shows that NiCl2 penetrates the Ni foam. In Fig. 15(b), NiCl2 does not penetrate to the back of the Ni foam, but this could be improved by adjusting the viscosity of the NiCl2 slurry and the screen-printing processing conditions. Furthermore, additional research on Ni foam may be needed because the porosity of the Ni foam and the shape of the pore can greatly affect the performance of the cathode. The peak voltage of the NiCl2 cathode using an Ni foam substrate is about 2.55 V, which is much higher than that of sulfide cathode materials. In addition, the NiCl2 cathode with a Ni foam substrate has a specific capacity of 684.61 Asg−1 with a cut-off voltage of 1.5 V (stainless steel substrate: 299.39 Asg−1). This improvement can be attributed to the lower resistance of the NiCl2 cathode on the Ni foam substrate (Fig. 15(d)). It is significant that the results from this study can be used to improve the mass productivity of NiCl2 cathodes by using a metal foam substrate, which can greatly improve the electrochemical performance and screen printing process, making them suitable for mass production. This approach seems to be applicable to other cathode materials besides NiCl2, and it is important to consider the selection of suitable metal foams and shapes. In addition, because metal foam is expected to act as a support for the cathode material, it is also necessary to minimize the amount of binder material added to the slurry.

Copper vanadium oxide provides high voltage, high capacity, and high thermal stability. V2O5 and CuO were previously evaluated for use as possible cathodes in thermal batteries.29) In addition, the electrochemistry of Cu5V2O10 was reported by Sakurai et al.103) and Eguchi et al..104)
A series of the CuO-V2O5 oxides, such as CuV2O6, Cu2V2O7, and Cu5V2O10 were synthesized and evaluated as possible candidate cathode materials in high voltage thermal batteries.29) In this study, single cells composed of a copper vanadium oxide cathode, an LiCl-KCl electrolyte, and an Li-Al anode were fabricated and evaluated. All components were fabricated into pellet-type materials using a powder compaction process. In the Fig. 16(a), Cu2V2O7 showed lower thermal stability than CuV2O6, but the weight loss was negligible (0.085%) up to 600°C. This is much smaller than the weight loss observed with sulfide cathode materials. A 3.4 V voltage plateau at a current density of 100 mA/g and a 2.5 V voltage plateau at a current density of 310 mA/g were observed during discharge at 525°C. This is a much higher value than was observed with FeS2, which has a voltage plateau ranging from 1.5 to 2 V under similar conditions, and is a characteristic that can contribute to increasing the energy density.
Dai et al. studied Cu3V2O8 and Cu5V2O10 thin-film cathodes for thermal batteries.30) In this study, single cells composed of a copper vanadium oxide cathode, a high-voltage compatible electrolyte, and an Li-Si anode were fabricated and evaluated. All components were fabricated into pellet-type materials using a powder compaction process. The cathode was also fabricated as a thin film using tape casting in order to facilitate a suitable performance comparison. The high-voltage compatible electrolyte consists of LiPO3, Li2SO4, Li2CO3, and LiF. Copper vanadium oxide has higher voltage than sulfide materials; therefore, it is necessary to study the molten salt electrolyte within a suitable voltage window.105) In this study, the final constant stable open circuit voltage (OCV) is 2.971 V at 500°C. The discharge profile of the Cu5V2O10 thin films contains multiple plateaus, similar to what was observed with a pressed film (Fig. 17(a)). The discharge profile of Cu3V2O8 shows a much more obscure phase change than that of Cu5V2O10. To determine the cause of these differences, an analysis of the discharge reaction for each substance would be necessary. The specific capacity of a thin film cathode is higher than that of a pressed cathode, regardless of the cathode materials.
Copper vanadium oxide has excellent properties, such as high voltage and high thermal stability, but studies on this material remain lacking. Applying the functional coating or manufacturing method mentioned above and optimizing the material properties were may significantly contribute to the development of thermal batteries with high energy density.

7. Other Materials

In addition to the aforementioned materials, other cathode materials for thermal batteries have been studied recently. Irvine et al. investigated ZrS3 as a possible cathode material for use in thermal batteries.31 ZrS3 has a pseudo one-dimensional structure (Fig. 18(a)). In this study, single cells composed of a ZrS3 cathode, an LiCl-KCl electrolyte, and an Li-Si anode were fabricated and evaluated. All components were fabricated into pellet-type materials using a powder compaction process. A single 1.7 V voltage plateau at a current density of 11 mA/cm2 was observed during discharge at 500°C with a capacity of 357 mAh/g. However, these results are inferior to existing sulfide materials, such as FeS2, CoS2 and NiS2. One thing to note is the high thermal stability of 700°C. This study is an early attempt to apply ZrS3 as the cathode in thermal batteries, thus additional work should focus on improving the performance of these devices.
Irvine et al. investigated CoNi2S4 as a candidate cathode material for use in thermal batteries.32) CoNi2S4 adopts an inverse spinel structure, as shown in Fig. 18(d). In this study, single cells composed of a CoNi2S4 cathode, an LiCl-KCl electrolyte, and an Li-Si anode were fabricated and evaluated. All components were fabricated into pellet-type materials using a powder compaction process. The CoNi2S4 cell exhibits two voltage plateaus at 500°C, one at 1.75 V and the second at 1.5 V. The CoNi2S4 cell has a specific capacity of 318 mAh/g from 2.58 to 1.25 V OCV. However, these results are inferior to those mentioned above. CoNi2S4 shows the thermal stability of 550°C, which is lower than that of FeS2; hence, its application scope becomes even narrower.
It is encouraging that the development of new cathode materials for thermal batteries is ongoing. However, future studies should focus on improving the capacity, thermal stability, and voltage currently required in industry.

8. Conclusions

Thermal batteries are primarily limited to military applications because of their specificity. This closed and specific propensity is reflected in research activities, and the number of published studies is very small compared to studies on general Li ion batteries. Nevertheless, over the past decade, much valuable research has been conducted on cathode materials for thermal batteries. Table 1 summarizes the main results of recent studies. Today, electronics industries in all fields are rapidly developing, and demand for power sources with high energy density is increasing. The military requires a high-performance power source that can power advanced weapons systems. In other words, future thermal batteries require high energy density and high reliability against external stimuli.
High capacity, high voltage, and high thermal stability are the three most important considerations for the development of high performance thermal batteries. There are four major approaches to improve these.
1. Surface modification: Coating with carbon or other materials to improve the thermal stability or electrical conductivity of conventional cathode materials.

2. Alloy and compound: Synthesis of alloy or compound materials to improve the thermal stability or specific capacity of conventional cathode materials.

3. Manufacturing method: Manufacturing of a thin film cathode to overcome the limitations of pellet-type materials formed via powder compaction.

4. New materials

It would be possible to develop a cathode for high performance thermal batteries by appropriately combining the above considerations.

Acknowledgments

This research was supported by ‘Agency for Defense Development’ (2019-0060) as a collaborative core technology research project.
Fig. 1
Cross-sectional view of an Li-Si/FeS2 thermal battery. Reprinted with permission from Ref. [1]. Copyright 2006 Elsevier B. V.
Fig. 2
(a) Single cell discharge results and (b) total polarization values of pristine, 0.1 wt.% CB, and 0.1 wt.% MWCNT samples. (c, d) Nyquist plots for CB-added and MWCNT-added samples. Reprinted with permission from Ref. [10]. Copyright 2013 The Korean Society of Industrial and Engineering Chemistry.
Fig. 3
(a) The contents of substances contained in the thin cathode and pellet type cathode. (b) Photograph of a thin cathode prepared using tape casting. (c) Discharge graphs of the thin cathode and pellet-type cathode. (d) Discharge results of the thin cathode and pellet-type cathode. Reprinted with permission from Ref. [11]. Copyright 2017 Elsevier Ltd and Techna Group S.r.l.
Fig. 4
(a) Schematic view of the FeS2/poly(imide-co-siloxane) composite cathode. (b, c) Discharge capacity and total polarization of the FeS2/PIS thin cathode and pellet type cathode. (d) TGA data. (e, f) SEM images of the thin cathode and pellet-type cathode after 1 h treatment at 450°C. Reprinted with permission from Ref. [12]. Copyright: This is an open access article distributed under the Creative Commons Attribution License.
Fig. 5
(a) Content of substances contained in the cathodes. (b) Discharge graphs of the cathodes. (c) Discharge results of the cathodes. Reprinted with permission from Ref. [13]. Copyright 2017 The American Ceramic Society.
Fig. 6
(a) Photograph and SEM images of a thin film cathode prepared using screen-printing. (b) Discharge curves and (c) resistance changes of the thin cathode and pellet cathode. Reprinted with permission from Ref. [16]. Copyright 2017 Elsevier B. V.
Fig. 7
(a) Thermal stability of CoS2 and CoS2/CNTs. (b) SEM image of CoS2/CNTs. (c) Specific discharge capacity, (d) energy, (e) pulse power, and (f) polarization data from CoS2 and CoS2/CNTs. Reprinted with permission from Ref. [17]. Copyright 2016 Elsevier Ltd.
Fig. 8
(a) XRD patterns of fresh products and products that were stored for 3 months. (b) TG curves of CoS2 and C@CoS2 after being stored in dry air for 3 months. (c) Discharge capacity, (d) energy, (e) pulse power, and (f) polarization data from CoS2 and C@CoS2. Reprinted with permission from Ref. [18]. Copyright 2017 published by Elsevier Ltd.
Fig. 9
(a) XRD patterns of S-CoXFe1−XS2 (samples stored in air). (b) Discharge performance and (c) resistance of the CoXFe1−XS2 cell. (d) Discharge performance of the S-CoXFe1−XS2 cell. Reprinted with permission from Ref. [19]. Copyright: This is an open access article distributed under the Creative Commons Attribution License.
Fig. 10
(a) XRD patterns on Fe0.5Co0.5S2 synthesized using two different methods. (b) Thermal analysis curves. SEM images of (c) S-Fe0.5Co0.5S2 and (d) L-Fe0.5Co0.5S2. (e) Discharge curves for single cells. (f) Pulsed discharge curves and resistance change curves of single cells. Reprinted with permission from Ref. [20]. Copyright 2018 Elsevier B. V.
Fig. 11
(a) TGA traces for synthetic FeS2, CoS2, and NiS2 in argon. (b) Discharge of Li-Si/LiBr-KBr-LiCl/MS2 cells at 500°C. Reprinted with permission from Ref. [4]. Copyright 2007 Elsevier B. V.
Fig. 12
(a) The effect of nano-NiS2 in a thermal battery. (b) N2 adsorption-desorption isotherms (inset: BET surface area). Discharge performance of NiS2 with different ball-milling times at current densities of (c) 0.1 A/cm2 and (d) 0.5 A/cm2. (e) pulsed discharge performance. (f) Resistance variation. Reprinted with permission from Ref. [22]. Copyright 2017 Elsevier B. V.
Fig. 13
(a) TG data from nano-NiS2 before and after carbon modification. (b-c) Specific capacities of NiS2 and NiS2/C at 0.1 A/cm2. (d) Pulse discharge and resistances. (e) TEM images of discharge products. (f) Schematic diagram showing carbon modification. Reprinted with permission from Ref. [23]. Copyright 2018 The Royal Society of Chemistry.
Fig. 14
(a) Contact angle test. (b-d) Comparison of the electrochemical performance of single-cell thermal batteries with carbon-coated NiCl2 and pure NiCl2 cathodes at 480°C and at various current densities. Reprinted with permission from Ref. [25]. Copyright 2017 Elsevier B. V.
Fig. 15
(a) Photographs of the NiCl2 cathode. (b) Cross-sectional SEM images of the NiCl2 cathode. (c) Discharge curves and (d) resistance changes of single cells with NiCl2 cathodes. Reprinted with permission from Ref. [26]. Copyright 2017 Elsevier B. V.
Fig. 16
(a) TGA curves of CuV2O6 and Cu2V2O7. Discharge curves of the (b) CuV2O6 and (c) Cu2V2O7 cathodes. Reprinted with permission from Ref. [29]. Copyright 2012 Elsevier B. V.
Fig. 17
(a) Discharge profile for an LiSi/Cu5V2O10 cell. (b) Polarization of an LiSi/Cu5V2O10 cell. (c) Discharge profile of an LiSi/Cu3V2O8 cell. (d) Polarization of an LiSi/Cu3V2O8 cell. Reprinted with permission from Ref. [30]. Copyright 2011 The Electrochemical Society.
Fig. 18
(a) Crystal structure of ZrS3. (b) SEM image of a ZrS3 cathode. (c) Discharge performance of an LiSi/ZrS3 cell. Reprinted with permission from Ref. [31]. Copyright: This is an open access article distributed under the Creative Commons Attribution License. (d) Crystal structure of CoNi2S4. (e) SEM image of CoNi2S4. (f) Discharge performance of an LiSi/CoNi2S4 cell. Reprinted with permission from Ref. [32]. Copyright: This is an open access article distributed under the Creative Commons Attribution License.
Table 1
Summary of Recent Studies on Cathode Materials for Thermal Batteries
Cathode Anode Electrolyte Cathode type/Thickness (μm) Specific capacity(A ·sec/g) Discharge temperature (°C) Cut-off (V) OCV (V) Voltage plateau (V) Thermal stability (°C) Ref.
1 FeS2 Li-Si LiCl-KCl Pellet/ - - 500 1.56 1.9 1.75/ 1.5/ 1.1 580 [10]
2 FeS2 Li-Si LiCl-KCl Thin film/ 100 1934 500 1.3 2.0 1.75/ 1.4 580 [11]
3 FeS2 Li-Si LiCl-KCl Thin film/ 100 1283 500 1.3 1.9 1.8/ 1.4 580 [12]
4 FeS2 Li-Si LiCl-KCl Thin film/ 50 2305 500 1.3 2.0 1.85/ 1.4 580 [13]
5 CoS2 Li-Si LiCl-LiBr-LiF Thin film/ 50 2092 500 1.25 1.99 1.9/ 1.76 650 [16]
6 CoS2 Li-Si LiCl-KCl Pellet/ - 791.28 500 1.7 1.84 1.77/ 1.3 650 [17]
7 CoS2 Li-Si LiCl-KCl Pellet/ - 848.88 500 1.7 1.84 1.78/ 1.3 650 [18]
8 Co0.3Fe0.7S2 Li-Si LiCl-KCl Pellet/ - 2304 525 1.0 2 1.85/ 1.3 620 [19]
9 Fe0.5Co0.5S2 Li-Si LiCl-LiBr-LiF Thin film/ 50 1872.89 - 1.25 2.04 2 620 [20]
10 NiS2 Li-B LiCl-LiBr-LiF Pellet/ - 2858.4 500 0.5 2.2 1.98/ 1.8/ 1.4 400 [22]
11 NiS2 Li-B LiCl-LiBr-LiF Pellet/ - 2196 500 1.4 2.0 2.0/ 1.6 590 [23]
12 NiCl2 Li-B LiCl-LiBr-LiF Pellet/ - 2127.6 480 0.8 2.35 2.3/ 1.5 - [25]
13 NiCl2 Li-Si LiCl-LiBr-LiF Nickel foam/400 684.61 - 1.5 2.55 - - [26]
14 CuV2O6 Li-Al LiCl-KCl Pellet/ 1000 220 525 2.0 3.5 - 600 [29]
15 Cu2V2O7 Li-Al LiCl-KCl Pellet/ 1000 125 525 2.0 3.5 - 600 [29]
16 Cu5V2O10 Li-Si LiPO3, LiSO4, Li2CO3, LiF Thin film/ 80 1044 500 2.0 2.971 2.5/ 1.75/ 1.5 - [30]
17 Cu3V2O8 Li-Si LiPO3, LiSO4, Li2CO3, LiF Thin film/ 80 990 500 2.0 2.971 2.4 - [30]
18 ZrS3 Li-Si LiCl-KCl Pellet/ - 1285.2 500 1.0 1.8 1.7 700 [31]
19 CoNi2S4 Li-Si LiCl-KCl Pellet/ - 1144.8 500 1.25 2.58 - 500 [32]

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