O3-type NaNi_0.4Fe_0.2Mn_0.4O₂ cathodes suffer from harmful phase transitions and large volume strains, leading to sluggish Na⁺ kinetics and poor cyclability. To address these issues, we propose dual-site doping via a high-entropy strategy, O3-Na_0.96Ca_0.02Ni_0.27Fe_0.2Mn_0.35Al_0.02Cu_0.1Ti_0.05O₂ (NCNFMACTO) cathode. Based on the Rietveld refinement of X-ray diffraction (XRD) data and DFT calculations, it is inferred that Ca²⁺ doping occupies the sodium sites, generating an “anchoring effect” that stabilizes the transition metal layer during the desodiation process. Concurrently, doping the transition-metal layer with multi-principal elements (Al, Cu, Ti) induces a lattice-regulating effect. This reinforces TM-O covalency and impedes slab gliding, thereby suppressing detrimental phase evolution and minimizing volume change. As a result, the material undergoes a reversible O3-P3-O3 phase transition with only 1.1% volumetric strain, significantly enhancing Na⁺ diffusion kinetics. The doped cathode exhibits remarkable cycling stability (91% capacity retention after 200 cycles at 1C and 80.1% capacity retention after 1000 cycles at 5C) and excellent air stability. Moreover, the NCNFMACTO//HC full cell maintains 85.2% capacity retention over 400 cycles at 1C. The full cell demonstrates an excellent energy density of 264.87 Wh·kg^–1 at 44.42 W·kg^–1, fully demonstrating its feasibility. This work provides key insights for designing high-stability sodium-layered oxide cathodes.

Recently, more and more supercapacitors (SCs) have been developed as AC line filter capacitors, which are generally named AC line filter electrochemical capacitors (FECs). Compared to traditional bulky aluminum electrolytic capacitors (AECs), FECs have higher capacity and lower space occupancy, which makes them a strong competitor. However, different from the common SCs for energy storage, it is necessary to consider the frequency response of the SCs for AC line filtering, where the contradiction between frequency response and specific capacitance is a challenge. The researchers have proposed different solutions from the perspective of materials, morphology, and configuration for this challenge. Based on the above background, in this review, we briefly introduce the principle and parameters of AC line filter electrochemical capacitors. We systematically summarize the state-of-the-art progresses of FECs and discuss their possible application and development in the future. The development of FECs can greatly promote the planarization, integration, and miniaturization of filter capacitors, and provide a new solution for the utilization of green and unstable energy.