Hydrothermal Synthesis of Nickel Disulfide and Its Application in Thermal Battery

Thermal batteries work at 450-550°C with molten salts as electrolytes, which have high power density and reliability. They are widely applied in military weapons^[1] as activating power source. At present, pyrite from natural minerals are the main cathode materials for thermal batteries due to its low cost, fine compatibility with molten electrolytes and stable discharging stages^[2]. However, when the battery discharging at large current density (>0.5Acm^-2), the discharging time and discharging voltages drop greatly, which will enhance the difficult of the designability of the thermal battery.

  Compared with FeS₂, NiS₂ has been less investigated in spite of its smaller electrical resistivity^[3] and higher decomposition temperature ^[4]. Hydrothermal method is an efficient way to synthesize NiS₂ with high purity and small particle sizes. Compared with other routes such as solvothermal method^[5], solid-state reaction^[6] or ultrasonic spray pyrolysis^[7]method, this method features simple operation and low cost for scaling up.

  In this work, cubic phase Vaesite has successfully prepared by hydrothermal method via the reactants of NiSO₄, S and FeSO₄. Fig. 1 shows  that the NiS₂ particles reuniting together and the average diameter of the synthetic NiS₂ is about 2.8μm according to the laser particle analysis. Fig. 2 shows that the synthetic NiS₂ is cubic phase vaesite. Fig. 3 shows the polarization cures of the synthetic NiS₂ and natural FeS₂ single cell thermal batteries. Before 1000As g^-1, the resistance of the NiS₂ thermal battery is stable and 0.08Ω smaller than that of the natural FeS₂ one; after 1000 As g^-1, the resistance of the NiS₂ thermal battery rising rapidly and at 1300As g^-1. The resistance of the NiS₂ thermal battery is larger than that of the FeS₂ one. The resistivity of NiS₂ is smaller than that of the FeS₂^[3] which will enhance the electrical conductivity of the cathode of the thermal battery. The larger specific surface area contacting with the electrolytes will reduce the contacting resistance and the concentration polarization between the interface of the NiS₂ and the electrolytes. As a result, the resistance of the NiS₂ thermal battery is smaller than that of the FeS₂ thermal battery. When discharging at 0.5 A cm^-2 constant current density, the voltage between AB region of the NiS₂ thermal battery is 120 mv higher than that of the FeS₂ one due to its larger surface contacting with the molten electrolytes. Selecting 1.5 V as cutoff voltage, the specific capacity and power density of the NiS₂ thermal battery is 1174 As g^-1 and 0.58 Wh g^-1 respectively, which are 1.1 and 1.3 times larger than those of the FeS₂ thermal batteries.Fig.5 show the pulse current discharging of the thermal batteries. Compared with FeS₂, the voltage drops of the NiS₂ thermal batteries were 0.05V and 0.08V smaller when 0.5 A cm^-2 and 1 A cm^-2 were loaded (Fig.4). The small voltage drop and the high discharging voltage is beneficial to improve the reliability and designability of the thermal battery. The NiS₂ single cell thermal battery shows better electrochemical performance than that of the natural FeS₂ at the condition of large and pulse current density discharging, which offer a better solution for the high power density thermal battery technology. the electrochemical performance of the packed NiS₂thermal batteries should be further investigated.

 Figure Captions 

Fig.1a , NiS₂ SEM images; b, particle distributions of the NiS₂

Fig.2. XRD patterns of (a)NiS₂(b)Natural FeS₂

Fig.3 Polarization of the thermal cells: a, NiS₂; b, FeS₂. Constant current density: 0.5Acm^-2; Pulse current density (0.3s):1Acm^-2

Fig.4 Discharge curves of NiS₂ and FeS₂ thermal batteries under 500°C at 0.5 A cm^-2

 Fig.5 Pulse current discharging curves of the thermal cells: a, NiS₂; b, FeS₂. Constant current density: 0.2 A cm^-2. AB region pulse current density: 0.5 A cm^-2 ; EF region pulse current density: 1 A cm^-2

References

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[7]Q.F. Shen, X.F. Zhou, S.Y. Wang. Journal of NanjingNormal University, 8, 59, (2008)