Due to the high structural flexibility and controllable thermal expansion, cubic double ReO₃-type negative thermal expansion (NTE) fluorides provide a solution for solving the prominent phenomenon of thermal expansion mismatch between materials. However, the expensive raw materials and complex synthesis steps limit its practical application. In this work, we have designed a more advantageous method for the synthesis of NTE material CaZrF₆, and it is expected to be generalized to the synthesis of other double ReO₃-fluorides. Intriguingly, a new orthorhombic phase CaZrF₆ has been synthesized via this method in a lower temperature. Unlike the strong isotropic NTE of the cubic phase CaZrF₆, the orthorhombic phase shows the strong anisotropic positive thermal expansion (PTE). The combined analysis of temperature-dependent X-ray diffraction (XRD), Raman spectra, and first-principles calculations shows that the low frequency phonon vibration mode with negative Grüneisen parameter in cubic CaZrF₆ are strongly correlated with the transverse thermal vibration of F atoms and dominates the NTE of the material.

Herein, a unique mesoporous heterostructure (average pore size: 15 nm) cobalt disulfide/carbon nanofibers (CoS₂/PCNFs) composite with excellent hydrophilicity (contact angle: 23.5°) is prepared using polyethylene glycol (PEG) as a pore-forming agent. The CoS₂/PCNF electrode exhibits excellent cycle stability (95.2% of initial specific capacitance at 10 A∙g⁻¹ after 8000 cycles), good rate performance (46.5% at 10 A∙g⁻¹), and high specific capacity (86.1 mAh∙g⁻¹ at 1 A∙g⁻¹, about 688.8 F∙g⁻¹ at 1 A∙g⁻¹). Density functional theory (DFT) simulation elucidates that CoS₂ tends to transfer substantial charges to CNF. As the center of positive charge, CoS₂ is more likely to capture negative ions in the electrolyte, thus accelerating the ion diffusion process. The excellent properties of the electrode material can not only accelerate the electrochemical reaction kinetics, but also provide abundant redox-active sites and a high Faradaic capacity for the entire electrode due to the synergistic contributions of CoS₂ nanoparticles, mesoporous heterostructure of PCNF, and admirable hydrophilicity of the composite material. A CoS₂/PCNF-0.25//AC (AC: activated carbon) asymmetric supercapacitor is assembled using CoS₂/PCNF-0.25 as the positive electrode and AC as the negative electrode, which possesses a high energy density (35.5 Wh∙kg⁻¹ at a power density of 824 W∙kg⁻¹) and superior cycling stability (maintaining over 98% of initial capacitance after 2000 cycles). In addition, the unique CoS₂/PCNF electrode is expected to be widely used in other electrochemical energy storage devices, such as lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, etc.

Herein, we prepare the unique hierarchical polypyrrole@cobalt sulfide (PPy-hs@CoS) hollow sphere-based nanofilms as interdigitated electrodes for flexible on-chip micro-supercapacitors (MSC). Benefiting from the excellent flexibility and high electrical conductivity of PPy-hs combined with the great electrochemical activity of CoS, such PPy-hs@CoS composite material can not only inhibit the volume expansion of PPy but also promote the diffusion of the electrolyte ions. The PPy-hs@CoS film-based electrode delivers a greatly improved specific capacitance and small resistance. Density functional theory calculations infer that OH⁻ prefers to bind to PPy on CoS@PPy and confirms the synergistic effect of each component for enhanced reaction kinetics. A quasi-solid-state on-chip flexible asymmetric MSC based on PPy-hs@CoS and activated carbon (AC) microelectrodes exhibits large areal-specific capacitance (131.9 mF/cm² at 0.3 mA/cm²), ultrahigh energy density (0.041 mWh/cm²@0.224 mW/cm² and 25.6 mWh/cm³@140.6 mW/cm³), and long cycle lifespan. We demonstrate the possibility to scale up the PPy-hs@CoS nanofilm microelectrode by arranging two of our asymmetric MSC in series and parallel connections, which respectively increase the output voltage and current. A self-charging system by connecting our asymmetric MSCs with a piece of commercial solar cells is developed as a potential possible mode for future highly durable and high-voltage integrated electronics.