In situ electrochemical impedance spectroscopy monitoring of the high-temperature double-discharge mechanism of Nb12WO33 cathode material for long-life thermal batteries

As a primary energy storage device, the thermal battery offers advantages such as high specific energy and high-power density. However, developing new cathode materials with high specific capacity and thermal stability to meet the evolving needs of thermal batteries remains a significant challenge. Moreover, the high discharge temperatures of thermal batteries and the instability of the molten salt electrolyte system complicate the electrochemical in situ characterization of these systems. In this context, in situ electrochemical impedance spectroscopy (EIS) has become widely employed in electrochemistry and represents a promising technique for in situ monitoring of thermal battery systems. Niobium-tungsten oxides, which possess a Wadsley-Roth crystal shear structure, exhibit excellent rate capability and cyclic stability as anode materials for lithium-ion batteries. Among them, Nb₁₂WO₃₃ demonstrates remarkable lithium storage performance due to its unique 3D tunneling structure, which provides rapid de-intercalation channels for Li⁺ ions. Given its excellent thermal and electrochemical stability, this study proposes the use of Nb₁₂WO₃₃ as a cathode material for thermal batteries for the first time. Electrochemical impedance spectroscopy (EIS) at room temperature was employed to investigate the variations in the material's internal electronic conductivity impedance. The EIS Nyquist plots of the Nb₁₂WO₃₃ electrode reveal a distinctive phenomenon of three semicircles in the high- and mid-frequency regions within the operating potential range. This behavior is primarily attributed to the electron conduction within the Nb₁₂WO₃₃ electrode. The resistance associated with electronic conduction (R_E) exhibits a pattern of initial increase followed by a decrease. This phenomenon is explained by the valence transition of the Nb element from +5 to +4 occurring around 1.7 ​V. This step is more facile than the subsequent steps at 2.0 ​V and 1.2 ​V, resulting in the generation of a larger number of metastable electrons. Consequently, the internal channels become populated with electrons, leading to a significant increase in R_E. The thermal battery constructed with Nb₁₂WO₃₃ as the cathode material was discharged at 500 ​°C and a current density of 500 ​mA ​g⁻¹ (with a cut-off voltage of 1.5 ​V), achieving a high specific capacity of 436.8 ​mA ​h ​g⁻¹ and an average polarized internal resistance of 0.52 ​Ω during pulse discharge. Therefore, Nb₁₂WO₃₃ holds great potential as a cathode material for high-capacity, thermally stable thermal batteries. This study paves the way for the use of other niobium-tungsten oxides as cathode materials for thermal batteries and establishes a precedent for in situ EIS testing and analysis of thermal battery systems.