1. Introduction
Thermal batteries are reserve primary batteries designed to remain electrochemically inactive during storage and to become operational only when heat is supplied to the battery stack. An internal heat source raises the temperature sufficiently to melt a salt electrolyte, creating the ionic conductivity required for electrochemical discharge.[1,2] Depending on the electrolyte system and battery design, operating temperatures are typically several hundred degrees Celsius, with approximately 350 to 550 °C representing a commonly discussed range for modern systems.[5,6]
This architecture provides characteristics that are difficult to achieve simultaneously in conventional batteries. Because the electrolyte remains solid during storage, thermal batteries can have exceptionally long dormant storage lives. Once activated, molten salt electrolytes provide high ionic conductivity, allowing substantial current to be delivered over comparatively short operating periods.[1,2]
The same operating principle creates unusually demanding requirements for electrode materials. A cathode must provide useful electrochemical potential and capacity while remaining chemically and physically stable at temperatures at which many ordinary battery materials would decompose, dissolve, react with the electrolyte, or undergo rapid self discharge.
Transition metal sulfides proved particularly suitable for this environment. Among them, FeS₂ and CoS₂ became established thermal battery cathode materials, while NiS₂ has received sustained attention as an alternative with characteristics that in several respects fall between those of FeS₂ and CoS₂.[3,4,6]
Although these compounds are often grouped together as transition metal disulfides, they should not be regarded as interchangeable powders. Their different phase chemistry, electronic properties, and reaction pathways produce meaningful differences during high temperature electrochemical discharge.
2. The Function of the Cathode in a Thermal Battery
Before activation, the principal components of a thermal battery remain physically assembled, but the solid electrolyte provides very little useful ionic conductivity. Heating the battery above the melting point of the electrolyte transforms the salt into an ionically conductive liquid phase and allows lithium ions to move through the separator between the electrodes.[1,2]
In lithium based thermal battery systems, lithium is commonly supplied by a lithium alloy anode. During discharge, lithium is oxidized at the anode while the transition metal disulfide undergoes reduction at the cathode. Rather than accepting lithium through a single reversible intercalation process, the sulfide cathode generally passes through a succession of intermediate phases as sulfur is progressively converted to lithium sulfide and the transition metal is reduced.[3,4,8,9]
The cathode therefore influences more than theoretical cell capacity. Its electronic conductivity, reaction kinetics, intermediate phase conductivity, particle characteristics, thermal decomposition behavior, and interaction with the molten electrolyte can all contribute to the voltage profile and internal resistance of the operating cell.[2–4,8,9]
Figure 1. General operating principle of a thermally activated battery.
3. Requirements for a Thermal Battery Cathode Material
A useful thermal battery cathode must satisfy several requirements simultaneously.
High electrode potential is desirable because it increases cell voltage. High specific capacity increases the amount of electrical charge that can theoretically be obtained from a given mass of active material. The cathode must also possess adequate electronic conductivity or otherwise maintain sufficiently conductive pathways throughout discharge.[4,5]
Thermal stability is particularly important because the cathode must retain its intended chemical and structural characteristics during thermal activation and subsequent electrochemical discharge. The thermal behavior of transition metal disulfides is therefore a significant consideration in cathode material selection, with FeS₂, CoS₂, and NiS₂ providing different operating characteristics suitable for different thermal battery requirements.[3,4,7]
Compatibility with the molten electrolyte is also an important consideration. The electrolyte must provide sufficient ionic conductivity while maintaining suitable interfacial contact with the cathode throughout discharge. Electrolyte composition, melting point, ionic conductivity, wetting behavior, and interactions with electrode materials can therefore influence the performance of the complete electrochemical system.[2]
Finally, useful cathode behavior requires more than the correct chemical formula. Phase purity, elemental stoichiometry, particle size distribution, crystallite structure, surface condition, and impurities can influence both thermal and electrochemical behavior.[3,6,10,13,14]
These considerations help explain why FeS₂, CoS₂, and NiS₂ can behave differently despite their chemical similarity.
4. Iron Disulfide, FeS₂
Iron disulfide has one of the longest histories among modern thermal battery cathode materials. In its cubic pyrite form, FeS₂ combines relatively high electrochemical capacity with abundant constituent elements and an extensive base of manufacturing and electrochemical experience.[3,6]
FeS₂ is also unusual among thermal battery cathodes because suitable material can be obtained either from natural pyrite or by chemical synthesis. Natural pyrite generally requires processing, purification, grinding, and classification before use. Synthetic FeS₂ provides another route in which phase composition and physical characteristics can be deliberately controlled during manufacture.[3,6,10,13]
Work performed specifically on thermal battery cathodes has shown that synthetic FeS₂ can perform comparably to, and under some tested conditions better than, natural pyrite. Guidotti and colleagues reported that synthetic material prepared by an aqueous route avoided some of the pronounced polarization behavior observed with natural pyrite in their single cell testing, particularly at lower operating temperature.[10] This does not establish that synthetic FeS₂ is universally superior to natural pyrite because performance depends strongly on the characteristics of the particular materials and cell conditions. It does demonstrate that the source and physical structure of FeS₂ can materially affect thermal battery behavior.
4.1 Thermal Stability
The thermal behavior of FeS₂ has been extensively investigated because of its established use in thermally activated batteries. Pyrite exhibits useful electrochemical activity at the elevated temperatures required for molten salt electrolyte operation, making its thermal characteristics an important consideration in cathode selection and battery design.
Reviews commonly identify temperatures around 550 °C as an important region for the onset of FeS₂ thermal decomposition, although the observed behavior depends on material characteristics and experimental conditions.[3,6,7]
Understanding the relationship between temperature, phase stability, and electrochemical behavior allows battery developers to select materials and operating conditions appropriate for the intended application.
4.2 Electrochemical Phase Evolution
FeS₂ discharge is not a one step conversion directly from FeS₂ to metallic iron.
The classic thermal battery literature describes an initial lithium containing intermediate commonly identified as Li₃Fe₂S₄, historically called the Z phase. Additional lithium rich iron sulfide phases appear as discharge progresses, followed eventually by formation of Li₂S and reduced iron containing products.[3]
The formation of intermediate phases contributes to the characteristic voltage profile of FeS₂ during discharge. Masset and Guidotti examined the relationship between these phase transformations, electronic transport, and cathode impedance, demonstrating the importance of understanding the evolving crystal structure and electrochemical behavior of FeS₂.[3]
Particle size is another important factor influencing discharge behavior. Choi and colleagues investigated the electrochemical characteristics of FeS₂ powders with different particle sizes and demonstrated that particle refinement can affect discharge capacity, resistance, and thermal behavior under different operating conditions.[14] These findings illustrate the importance of controlling powder characteristics according to the requirements of the intended thermal battery application.
4.3 Natural and Synthetic FeS₂
The distinction between synthetic FeS₂ and natural pyrite deserves particular attention in cathode material qualification.
Natural pyrite is a geological material. Its trace elemental composition, secondary mineral content, grain structure, and particle characteristics can depend on the geological source and subsequent processing. Shao Horn and Horn found substantial differences in impurities, secondary phases, particle distributions, and internal grain structure between the natural and synthetic FeS₂ materials examined in their comparative study.[13]
Synthetic FeS₂ provides an alternative to naturally occurring pyrite in which chemical composition, phase development, and physical characteristics can be controlled through the manufacturing process. This approach allows manufacturers to target specific material properties and apply analytical characterization throughout production. X-ray diffraction, elemental analysis, thermal analysis, and particle characterization provide complementary methods for evaluating the resulting material.[10,13,14]
For applications requiring reproducible cathode behavior, the relevant consideration is therefore not simply whether the FeS₂ is natural or synthetic, but whether the material consistently meets the required specifications for phase composition, stoichiometry, impurity profile, particle characteristics, and thermal behavior.
5. Cobalt Disulfide, CoS₂
Cobalt disulfide is another major transition metal sulfide cathode and is particularly associated with applications requiring greater thermal stability and strong high temperature discharge performance.
Like FeS₂, CoS₂ adopts a cubic pyrite type structure. Its electrochemical and transport properties, however, differ significantly. CoS₂ is commonly described as having metal like electronic conductivity and can maintain useful performance at temperatures above the practical stability range of FeS₂.[4,6,7]
Published reviews commonly place its thermal stability at approximately 650 °C under relevant test conditions, giving it an important advantage where elevated operating temperature or longer thermal exposure would accelerate FeS₂ degradation.[4,6]
CoS₂ has consequently been investigated and used as an alternative to FeS₂ for demanding thermal battery applications.[1,4]
5.1 Discharge Mechanism
The discharge mechanism of CoS₂ provides an instructive example of why thermal battery cathode chemistry continues to evolve even for materials that have been studied for decades.
Earlier descriptions proposed a sequence in which CoS₂ was progressively reduced through Co₃S₄ and Co₉S₈ before ultimately reaching metallic cobalt.[4]
Later in situ diffraction experiments produced a somewhat different picture. Payne and colleagues simultaneously followed electrochemical discharge and phase evolution in a working CoS₂ thermal battery cell. Under those experimental conditions they found no evidence for Co₃S₄. Instead, the observed sequence involved CoS₂ converting to a CoS related phase, followed by Co₉S₈ and finally cobalt.[9]
This distinction is scientifically important. It demonstrates that simplified reaction schemes do not necessarily capture the complete phase behavior of a cathode under actual discharge conditions.
For a materials supplier, it also illustrates why phase purity of the initial CoS₂ matters. The starting cathode enters a complex series of electrochemical and solid state transformations in which both phase composition and transport properties can affect the resulting discharge.
5.2 Conductivity and Rate Performance
The relatively high electronic conductivity of CoS₂ is an important characteristic supporting its use as a thermal battery cathode. Combined with its thermal stability and electrochemical properties, this makes CoS₂ particularly attractive for applications requiring substantial current delivery at elevated operating temperatures.[4,6,7]
Research continues to investigate methods for improving the electrochemical performance of CoS₂ cathodes through particle engineering, surface modification, and control of interfacial transport. Yang and colleagues demonstrated that heterostructure engineering could improve discharge capacity under demanding operating conditions, illustrating the continuing potential for materials development within this established cathode system.[11]
These investigations reinforce the importance of understanding how the physical and chemical characteristics of CoS₂ influence its behavior during high temperature electrochemical discharge.
6. Nickel Disulfide, NiS₂
Nickel disulfide has long been considered an attractive alternative thermal battery cathode. NiS₂ crystallizes in the cubic pyrite structure, also known mineralogically as vaesite, and its thermal and electrochemical properties are often described as intermediate between FeS₂ and CoS₂.[4,6]
Its potential advantage is not simply that it occupies a midpoint between two established materials. NiS₂ possesses high theoretical electrochemical capacity and can provide substantial discharge capacity when reaction kinetics and cathode structure allow sufficient active material utilization.
6.1 Direct Observation of NiS₂ Discharge
In situ diffraction has provided unusually detailed information about NiS₂ cathode chemistry.
Payne and colleagues collected powder neutron diffraction and electrochemical data simultaneously during discharge of a working thermal battery cell. Four nickel containing phases were observed during the experiment, NiS₂, NiS, Ni₃S₂, and metallic Ni.[8]
Earlier mechanisms had proposed Ni₇S₆ as an intermediate. The in situ study did not observe that phase and instead proposed a mechanism based on sequential evolution through NiS and Ni₃S₂ before formation of nickel.[8]
As with CoS₂, the result demonstrates the value of observing phase evolution under actual discharge conditions rather than inferring the mechanism solely from equilibrium phase diagrams or post discharge analysis.
6.2 Particle Size and Thermal Stability
Particle size and crystallite dimensions are important characteristics influencing the electrochemical behavior of NiS₂. Research has demonstrated that controlling these properties can improve reaction kinetics and active material utilization, making particle engineering an important area of ongoing cathode development.[6]
The relationship between particle characteristics and thermal behavior is also significant. Studies of NiS₂ demonstrate that changes in particle dimensions can influence both electrochemical performance and decomposition characteristics, emphasizing the importance of selecting powder properties appropriate for the intended operating conditions.[6]
A 2025 study by Liu and colleagues illustrates the continuing interest in controlling NiS₂ powder characteristics. Their synthesized material had an average particle size of 2.87 µm and a reported decomposition temperature of 598 °C. Under their specific electrochemical test conditions, the material delivered approximately 694 mAh g⁻¹ at 500 °C and 100 mA cm⁻².[12]
These findings demonstrate the importance of synthesis methods, phase composition, particle characteristics, and electrode construction in determining the thermal and electrochemical properties of NiS₂.
7. Comparing FeS₂, CoS₂, and NiS₂
No single ranking adequately describes these three cathode materials.
| Characteristic | FeS₂ | CoS₂ | NiS₂ |
|---|---|---|---|
| Structural family | Cubic pyrite | Cubic pyrite | Cubic pyrite, vaesite |
| Representative thermal stability | Decomposition becomes important around 550 °C | Commonly reported near 650 °C | Generally intermediate, with 598 °C reported for one recent synthesized material |
| Electronic and discharge behavior | Established electrochemical activity and multistage discharge characteristics | Relatively high, often described as metal like conductivity | Thermal and discharge behavior generally intermediate between FeS₂ and CoS₂ |
| Major strength | Established chemistry, high capacity, extensive operating history | Thermal stability, conductivity, high rate capability | High capacity potential and attractive intermediate properties |
| Important discharge intermediates | Lithium iron sulfide phases | CoS and Co₉S₈ observed in recent in situ work | NiS and Ni₃S₂ observed in in situ work |
Reported decomposition temperatures depend on material preparation and test conditions and should therefore be regarded as representative rather than universal values.[3,4,6,7,12,14] Electrochemical performance depends additionally on current density, anode chemistry, electrolyte composition, electrode construction, and cutoff voltage.
The appropriate cathode material therefore depends on the required operating envelope.
Where extensive historical qualification and high specific capacity are priorities, FeS₂ remains important. Where thermal stability and conductivity under demanding high temperature conditions dominate, CoS₂ offers substantial advantages. NiS₂ presents a different combination of properties and remains attractive where high capacity and intermediate thermal behavior can be combined with sufficient active material utilization.
8. Why Cathode Powder Characteristics Matter
Comparisons based solely on chemical formula obscure an important point. A thermal battery does not operate on an abstract compound called FeS₂, CoS₂, or NiS₂. It operates on a real powder possessing a particular phase composition, particle distribution, surface condition, impurity profile, and thermal history.
Experimental studies of natural and synthetic FeS₂, particle size effects in FeS₂, engineered CoS₂, and synthesized NiS₂ demonstrate that these physical and chemical characteristics can influence high temperature electrochemical behavior.[10–14]
Figure 2. Conceptual relationship between cathode powder characteristics and high temperature electrochemical behavior. Individual relationships depend on cathode chemistry and operating conditions.
8.1 Phase Purity and Stoichiometry
Phase purity and elemental stoichiometry are important characteristics of transition metal disulfide cathode materials. The crystalline phases present in a powder, together with its overall chemical composition, determine the starting material available for electrochemical reaction.
For FeS₂, CoS₂, and NiS₂, control of the intended crystalline phase and metal to sulfur ratio is therefore an important consideration in material manufacturing and qualification.
X ray diffraction provides information about crystalline phase composition, while elemental analysis offers complementary information about bulk stoichiometry and trace elemental content. Used together, these techniques support the characterization of cathode powders according to their intended material specifications.
8.2 Particle Size and Morphology
Particle size influences several competing processes.
Smaller particles generally provide shorter diffusion distances and greater surface area, which can accelerate electrochemical reaction. Increased surface area can also increase chemical reactivity and, in some materials, reduce thermal stability.[6,14]
Conversely, longer transport distances in coarse particles can limit active material utilization under demanding discharge conditions.
The ideal particle distribution is therefore application dependent rather than simply as small as possible. The literature on FeS₂ and NiS₂ demonstrates that changes in particle and crystallite dimensions can alter discharge behavior and thermal stability.[6,12,14]
8.3 Impurity Profile
A nominal purity value such as 99 percent or 99.9 percent is useful but incomplete.
Different impurity species can have different effects. Natural and synthetic FeS₂ samples, for example, have been shown to contain different concentrations and forms of metallic impurities, oxides, sulfides, and other secondary phases.[13]
The relevant question is therefore not merely how much impurity is present, but what the impurities are and whether they are incorporated into the primary phase or present as separate secondary phases. Their significance will depend on whether they influence phase stability, conductivity, decomposition, or electrochemical reaction under the intended operating conditions.
8.4 Lot Consistency
Thermal batteries are often selected for applications in which predictable behavior after extended storage is particularly important.[1,5]
From a manufacturing and qualification perspective, reproducibility is therefore an important additional consideration. If particle distribution, stoichiometry, secondary phase content, or impurity levels vary significantly between production lots, downstream cathode processing and cell behavior may also become less reproducible.
Material qualification should consequently consider not only whether an individual sample performs acceptably, but whether the manufacturing process can repeatedly produce material within the required chemical and physical ranges.
9. Characterization of Transition Metal Disulfide Cathodes
No single analytical method adequately characterizes a thermal battery cathode powder.
X ray diffraction identifies crystalline phases and can detect undesired lower sulfides, elemental phases, or other crystalline contaminants.
Elemental analysis determines whether the metal to sulfur ratio is consistent with the intended compound and can quantify trace metallic impurities where appropriate.
Particle size analysis characterizes the distribution of particle dimensions, which can influence mixing, pressing, reaction kinetics, and active material utilization.
Scanning electron microscopy provides information about particle morphology, agglomeration, surface texture, and microstructural differences that are not apparent from bulk particle size measurements alone.
Thermogravimetric analysis and differential thermal analysis or differential scanning calorimetry can identify decomposition behavior, mass loss, and thermal transitions relevant to the anticipated operating environment.
The thermal battery literature makes extensive use of these complementary methods because cathode performance arises from the interaction of chemistry, structure, and thermal behavior rather than from elemental composition alone.[3,4,6,8–10,12–14]
10. Continuing Development of Disulfide Cathodes
FeS₂, CoS₂, and NiS₂ are mature materials, but they are not scientifically static.
Recent research continues to investigate particle refinement, surface modification, carbon containing composites, heterostructures, elemental substitution, and alternative synthesis methods intended to improve conductivity, reduce polarization, increase active material utilization, or modify thermal stability.[5–7,11,12]
These studies also illustrate an important limitation of optimizing one property in isolation.
These investigations demonstrate the continuing opportunities for improving the performance of established disulfide cathode materials. Advances in particle engineering, surface modification, and synthesis methods provide additional approaches to controlling the physical and chemical characteristics that influence electrochemical behavior.
Continued development of thermal battery cathodes therefore involves an increasingly detailed understanding of the relationship between composition, particle characteristics, thermal properties, and electrochemical kinetics. These relationships provide a foundation for developing and qualifying materials with properties suited to specific thermal battery requirements.
11. Outlook
Transition metal disulfides are likely to remain important materials in thermal battery technology because they offer a combination of capacity, thermal stability, and electrochemical behavior that is difficult to reproduce with a single alternative material class.
The direction of future development is likely to place increasing emphasis on engineered powder properties.
For FeS₂, continued development includes the production and characterization of synthetic materials with controlled purity, stoichiometry, grain structure, particle distribution, and thermal properties.
For CoS₂, research continues to investigate its high temperature electrochemical behavior, rate capability, electronic transport characteristics, and the relationship between material structure and cathode performance.
For NiS₂, advances in synthesis methods and particle engineering provide opportunities to further understand and control its electrochemical capacity, reaction kinetics, and thermal characteristics.
Across all three materials, improved understanding of phase evolution during actual discharge will help connect measurable powder characteristics with cell level behavior. Modern in situ diffraction studies of CoS₂ and NiS₂ demonstrate how much additional information can be obtained when electrochemical response and crystal structure are observed simultaneously.[8,9]
As thermal battery performance requirements continue to develop, cathode powders will increasingly need to be treated as engineered functional materials rather than simply high purity chemical compounds.
12. Conclusion
Iron disulfide, cobalt disulfide, and nickel disulfide are established transition metal disulfide cathode materials whose distinct thermal, structural, and electrochemical characteristics make them important components of thermal battery technology.
FeS₂ offers high electrochemical capacity, extensive historical use, and the availability of synthetically manufactured material with controlled chemical and physical characteristics. CoS₂ combines relatively high electronic conductivity with substantial thermal stability and established high temperature discharge performance. NiS₂ provides high theoretical capacity and an additional combination of thermal and electrochemical properties relevant to thermal battery applications.
Research into these materials continues to advance understanding of their crystal structures, discharge mechanisms, thermal behavior, and the influence of particle characteristics on electrochemical performance.
For manufacturers and battery developers, these findings underscore the importance of controlling the chemical and physical properties of cathode powders. Phase composition, stoichiometry, particle size distribution, impurity profiles, and thermal characteristics all contribute to the properties of the resulting electrode material.
As thermal battery technology continues to develop, the availability of well characterized transition metal disulfide powders with consistent and reproducible properties will remain an important consideration in cathode material development and qualification.
Endnotes and Works Cited
[1] Guidotti, R. A., and Masset, P. J. “Thermally activated (“thermal”) battery technology: Part I: An overview.” Journal of Power Sources 161, no. 2, 1443–1449, 2006. doi:10.1016/j.jpowsour.2006.06.013.
[2] Masset, P. J., and Guidotti, R. A. “Thermal activated (thermal) battery technology: Part II. Molten salt electrolytes.” Journal of Power Sources 164, no. 1, 397–414, 2007. doi:10.1016/j.jpowsour.2006.10.080.
[3] Masset, P. J., and Guidotti, R. A. “Thermal activated (“thermal”) battery technology: Part IIIa: FeS₂ cathode material.” Journal of Power Sources 177, no. 2, 595–609, 2008. doi:10.1016/j.jpowsour.2007.11.017.
[4] Masset, P. J., and Guidotti, R. A. “Thermal activated (“thermal”) battery technology. Part IIIb. Sulfur and oxide based cathode materials.” Journal of Power Sources 178, no. 1, 456–466, 2008. doi:10.1016/j.jpowsour.2007.11.073.
[5] Ko, J., Kang, S. H., Cheong, H. W., and Yoon, Y. S. “Recent Progress in Cathode Materials for Thermal Batteries.” Journal of the Korean Ceramic Society 56, no. 3, 233–255, 2019. doi:10.4191/kcers.2019.56.3.05.
[6] Li, R., Guo, W., and Qian, Y. “Recent Developments of Cathode Materials for Thermal Batteries.” Frontiers in Chemistry 10, 832972, 2022. doi:10.3389/fchem.2022.832972.
[7] Muthu, P., Rajagopal, S., Saju, D., Kesavan, V., Dellus, A., Sadhasivam, L., and Chandrasekaran, N. “Review of Transition Metal Chalcogenides and Halides as Electrode Materials for Thermal Batteries and Secondary Energy Storage Systems.” ACS Omega 9, no. 7, 7357–7374, 2024. doi:10.1021/acsomega.3c08809.
[8] Payne, J. L., Percival, J. D., Giagloglou, K., Crouch, C. J., Carins, G. M., Smith, R. I., Comrie, R., Gover, R. K. B., and Irvine, J. T. S. “In Situ Thermal Battery Discharge using NiS₂ as a Cathode Material.” ChemElectroChem 4, no. 8, 1916–1923, 2017. doi:10.1002/celc.201700095.
[9] Payne, J. L., Percival, J. D., Giagloglou, K., Crouch, C., Carins, G. M., Smith, R. I., Gover, R. K. B., and Irvine, J. T. S. “In situ thermal battery discharge using CoS₂ as a cathode material.” Journal of The Electrochemical Society 166, no. 12, A2660–A2664, 2019. doi:10.1149/2.1431912jes.
[10] Guidotti, R. A., Reinhardt, F. W., Dai, J., and Reisner, D. E. “Preparation and Characterization of Synthetic Pyrite for Thermal Battery Cathodes.” Proceedings of the 40th Power Sources Conference, p. 339, 2002.
[11] Yang, P., Zhang, X., Cao, Y., Xie, Y., Wang, C., Li, X., and Cui, Y. “Heterostructure-promoted rate performance of CoS₂ in thermal activated batteries.” Applied Energy 361, 122916, 2024. doi:10.1016/j.apenergy.2024.122916.
[12] Liu, Y., Yao, B., Zhu, J., Yang, W., Zhou, L., and Fu, L. “Synthesis and discharge performance of NiS₂ by a simple sintering technique from NiSO₄·6H₂O used in thermal battery.” Materials Letters 391, 138468, 2025. doi:10.1016/j.matlet.2025.138468.
[13] Shao Horn, Y., and Horn, Q. C. “Chemical, structural and electrochemical comparison of natural and synthetic FeS₂ pyrite in lithium cells.” Electrochimica Acta 46, no. 17, 2613–2621, 2001. doi:10.1016/S0013-4686(01)00465-0.
[14] Choi, Y., Yu, H. R., Cheong, H. W., Cho, S., and Lee, Y. S. “Effects of Pyrite (FeS₂) Particle Sizes on Electrochemical Characteristics of Thermal Batteries.” Applied Chemistry for Engineering 25, no. 2, 161–166, 2014. doi:10.14478/ace.2013.1123.