High-entropy structures in layered compounds, especially transitional metal dichalcogenides (TMDCs), have powered the field with disordered and versatile chemical compositions, showing great potential in various functional applications, including energy storage and catalysis. However, the reported high-entropy phases are mainly 1T phases, 2H phases are rare, and approximately 3R phases are still lacking. Here, phase engineering of high-entropy TMDCs is achieved by tuning the chemical composition of (Mo_0.5W_0.5)_1−x(Nb_0.5Ta_0.5)_xSe_2+δ, 0 ≤ x < 1, and −0.1 ≤ δ ≤ 0.3. A phase diagram is constructed to guide the synthesis of pure 2H/3R phases over a wide composition/entropy range. The increase in VB-group element content and Se overdose facilitated the formation of 3R phases, whereas the opposite occurred for 2H phases. Thermodynamic first-principles calculations evaluate the stability of phases in different polytypes and compositions, matching well with the composition-dependent crystalline habits. Moreover, the optimized thermoelectric performance, with a figure of merit (zT = 0.36@723 K) in 2H phase of x = 0.2, is attributed to the low thermal conductivity (κ) caused by the high-entropy effect, which is one of the highest among (Mo/W)Se₂-based materials. Our work enriches high-entropy TMDCs with versatile polytypes, expanding their potential uses for various fields.

Advancements in power electronics necessitate dielectric polymer films capable of operating at high temperatures and possessing high energy density. Although significant strides have been achieved by integrating inorganic fillers into high-temperature polymer matrices, the inherently low dielectric constants of these matrices have tempered the magnitude of success. In this work, we report an innovative nanocomposite based on sulfonylated polyimide (SPI), distinguished by the incorporation of sulfonyl groups within the SPI backbone and the inclusion of wide bandgap hafnium dioxide (HfO₂) nanofillers. The nanocomposite has demonstrated notable enhancements in thermal stability, dielectric properties, and capacitive performance at elevated temperatures. Detailed simulations at both molecular and mesoscopic levels have elucidated the mechanisms behind these improvements, which could be attributed to confined segmental motion, an optimized electronic band structure, and a diminished incidence of dielectric breakdown ascribed to the presence of sulfonyl groups. Remarkably, the SPI-HfO₂ nanocomposite demonstrates a high charge-discharge efficiency of 95.7% at an elevated temperature of 150 °C and an applied electric field of 200 MV/m. Furthermore, it achieves a maximum discharged energy density of 2.71 J/cm³, signalling its substantial potential for energy storage applications under extreme conditions.