In this work CoNi 2 S 4 was investigated as a candidate cathode material for Li thermal batteries. The CoNi 2 S 4 was synthesized by a solid state reaction at 550 • C in a sealed quartz tube. Neutron powder diffraction was utilized to confirm normal spinel structure up to 200 • C, however, there was cation disorder above this temperature. The electrochemical properties of the batteries were investigated at 500 • C by galvanostatic discharge to elucidate the mechanism and the products NiS, Co 3 S 4 and Co 9 S 8 of the discharge mechanism were confirmed using powder X-ray diffraction. CoNi 2 S 4 exhibits two voltage plateaus vs Li 13 Si 4 at 500 • C, one at 1.75 V and the second at 1.50 V. CoNi 2 S 4 has an overall capacity of 318 mA h g −1 from OCV 2.58 V to 1.25 V vs Li 13 Si 4 which is comparable to that of the well-known metal disulfides. Today the human society and technology progress in electric vehicles, medical equipments, military applications and space technology require power systems with high energy, safety and long shelf -life characteristics. There are different kind of power systems such as supercapacitors, fuel cells and thermal batteries for this goal. Thermal batteries are electrochemical devices that offer a direct conversion of chemical energy to electrical energy by an electrochemical oxidationreduction reaction at high temperature (>300• C) utilizing a molten salt electrolyte. Thermal batteries are primary batteries which find use in a number of applications as they are known for their long shelf life and ability to be discharged at particularly high rates when compared to other types of batteries.1 A key component of thermal batteries is the halide salt electrolyte that is a solid at ambient temperatures which gives the cells their long life but melts in the 300-500• C region. 2 This solid -liquid phase transition leads to the electrolyte becoming ionically conductive and allowing the ions to transfer between the anode and the cathode. Thermal batteries typically use a lithium alloy as the anode, a halide salt eutectic as the electrolyte, an insulating porous material as the separator and a transition metal sulfide as the cathode. The Li 13 Si 4 alloy is often used as the anode as the lithium diffusion in silicon (10 −8 cm −2 s −1 ) is greater than in other alloys 3 and has moisture stability as well as it staying solid at the operating temperature. The discharge reaction of Li 13 Si 4 to Li 7 Si 3 at a potential of 0.157 V against Li metal at 415• C corresponds to a capacity of 485 mA h g −1 .4The electrolyte that has been used in this work is the lithium chloride -potassium chloride eutectic which has a melting point of around 354• C and requires a minimum of 35 wt% MgO as a separator. 5 The most common transition metal disulfides to be used as cathodes are FeS 2 , NiS 2 or CoS 2 and all of these materials exhibit a potential of ∼ 1.70 V vs Li 13 Si 4 at the beginning of their discharge but also have further electrochemical transitions to complete the reduction to the transition metal. 6 T...