| Thermal batteries are widely used as power sources for advanced weapons,such as missiles and torpedoes,due to their advantage of high output power,long shelf life,and high reliability.As discharge voltage lower than 2 V,traditional transition metal sulfide cathode materials,such as FeS2 and CoS2,can no longer meet the increasing performance requirements for thermal batteries of weapons,which greatly limits the development of thermal battery technology.Cathode materials with high electrode potential have become one of the research focuses in the fields of thermal battery.NiCl2 with decomposition temperature above 800℃,theoretical voltage of 2.64 V(v.s.Li)and theoretical capacity of 413 mAh/g,attracts much attention.However,when discharging in the conventional LiCl-KCl and LiF-LiCl-LiBr molten salt electrolytes,NiCl2 suffers from the phenomenon of low discharge capacity and voltage instability,which is hard to meet the requirement of practical application.In order to improve the discharge performance of NiCl2 thermal batteries,this paper mainly studied the compatibility and discharge performance of NiCl2 cathode material in conventional LiF-LiCl-LiBr molten salt electrolyte and Br-free LiF-LiCl-Li2SO4 molten salt electrolyte.Firstly,using LiF-LiCl-LiBr molten salt as electrolyte,the constant current and pulse discharge characteristics of anhydrous NiCl2 were studied.To find the cause for the phenomenon of low discharge capacity and voltage instability,the compatibility between NiCl2 and LiF-LiCl-LiBr molten salt electrolyte was further investigated.During constant current discharge,discharge capacity and stability of discharge voltage of NiCl2 single cells are greatly reduced with the increase of current density.The pulse discharge test shows that the total polarization resistance of the single cell fluctuates during the discharge process.Thermal analysis and XRD results show that there is a chemical reaction between NiCl2 and LiBr in LiF-LiCl-LiBr molten salt electrolyte at high temperature,and EDX line-scanning test result shows that the reaction will cause change in the composition of molten salt in the cathode layer.The addition of separator powder(EB)to the cathode layer can significantly improve the stability of the discharge voltage.Secondly,LiF-LiCl-Li2SO4 eutectic molten salt was used as the electrolyte compatible with NiCl2 to avoid the chemical reaction between NiCl2 and molten salt electrolyte at high temperature,and the melting point and electrical conductivity of LiF-LiCl-Li2SO4 molten salt electrolyte were studied.Constant current discharge performance of NiCl2 in LiF-LiCl-Li2SO4 molten salt electrolyte was also evaluated.Thermal analysis and XRD results show that NiCl2 has good compatibility with LiF-LiCl-Li2SO4 molten salt electrolyte at high temperature.In the constant current discharge test,the LiSi/LiF-LiCl-Li2SO4/NiCl2 single cell has a relatively stable discharge voltage.Under the current density of 500 mA/cm2 and cut-off voltage of 1.2 V,the single cell has a stable discharge platform around 2 V and a specific capacity of 377 mAh/g.Energy density and power density of LiSi/LiF-LiCl-Li2SO4/NiCl2 single cell are higher than those using the LiF-LiCl-LiBr molten salt electrolyte.Finally,the electrochemical performance of NiCl2-NiF2 composite cathode material in LiF-LiCl-Li2SO4 molten salt electrolyte was studied.The NiCl2-NiF2 composite cathode materials can alleviate the voltage drop phenomenon of LiSi/LiF-LiCl-Li2SO4/NiCl2 single cell.Meanwhile,the addition of NiF2 can reduce the polarization resistance of the single cell during the middle and late stages of discharge,and increase the discharge voltage of the single cell at large current density. |
