There has been a recent push to develop powerful and safe batteries with enhanced application needs and safety requirements. A new thermoelectric hybrid battery (TEHB) system has been proposed to utilize the battery waste heat, FeS2 as an example, to further reproduce electricity and decrease temperatures as the battery is discharging and charging. Therefore, the thermoelectric properties of FeS2 as battery electrode are studied in order to enhance its thermoelectric performance and develop a system to utilize battery heat efficiently and securely. Here, FeS2 thermoelectric material is firstly prepared with hydrothermal method, followed by second-phase Fe7S8 with higher electrical conductivity which was introduced through newly designed annealing process. The results showed that the electrical conductivity of FeS2 that was annealed at 873K reached 136Scm−1 at 300K and 302Scm−1 at 648K. These were two orders of magnitude higher than the other synthesized samples. When Fe7S8 nano-crystals were introduced, there was a decrease in lattice thermal conductivity and an increase in Seebeck coefficient. These results are an important step in improving the battery efficiency by converting the unavoidable heat that was generated by battery into electricity.