This study aims to investigate the effect of physical modification of FeS2 powder cathode materials of thermal batteries and conductive carbon addition in the electrochemical properties of the thermal batteries. The physical modification of FeS2 powder was performed by pulverizing FeS2 powder and the conductive carbon addition was performed by adding carbon black(CB) and multi-walled carbon nanotube(MWCNT) with various amounts from 0.1 to 1 wt.%, compared to the amount of pure FeS2. The microstructure and thermal stability of the prepared cathode
materials are also investigated to study how the changes of the cathode materials affected the electrochemical properties of the prepared thermal batteries.
The average particle size and distribution of FeS2 powder are changed with the ball-milling time. After 170 hour ball-milling, the average particle size was drastically decreased from 98.4 ㎛ to 1.46 ㎛ with the narrowest size distribution with a single peak and fine particles of FeS2 were obtained. With the unit cell discharge test at 450℃, the electrochemical capacity of 1.46 ㎛ pyrite-cell is significantly increased, compared to that of 98.4 ㎛ pyrite-cell. The internal resistance is also decreased. The result is attributed to the increase of active reaction area of pyrite by ball milling. However, with unit cell discharge test at 500℃, the 1.46 ㎛ pyrite cell shows lower internal resistance than 98.4 ㎛ pyrite cell only in the Z-phase region (FeS2 → Li3Fe2S4). After that, the 1.46 ㎛ pyrite cell shows rapidly decreasing capacity and internal resistance in X-phase region (Li3Fe2S4 → LiFe2S4) compared to the 98.4 ㎛ pyrite cell. It can be concluded that at higher temperature, thermally unstable pulverized pyrite is thermally decomposed as well as self-discharged simultaneously causing higher resistance and lower capacity at 500℃ than that of the 98.4 ㎛ pyrite cell.
Among the samples prepared with various amounts of CB or MWCNT, the 1 wt.% CB-added sample exhibits the highest electrochemical properties. Within the range of 0.1~0.5 wt.%, the MWCNT-added samples show better electrochemical properties, such as charge transfer resistance and exchange current density, than the CB-added samples. However, in the range of 0.5~1 wt.%, the CB-added samples exhibit better electrochemical properties than the MWCNT-added samples. These results suggest that the improvement in the electrochemical performance of thermal batteries can be achieved by the addition of the conductive carbonaceous materials to pyrite electrode.