Sodium–lithium co-doping driven enhancement of structure, superconductivity, and vortex pinning in Bi-2212 superconductors

Abstract This work examines the structural, electrical, and magnetic behaviour of Bi-2212 superconductors co-doped with sodium and lithium, prepared by the conventional solid-state reaction method. Bulk ceramic samples with fixed sodium substitution at the calcium site (y = 0.05) and varying lithium content at the copper site (x = 0.00–0.20) were produced and analysed using X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), DC resistivity, and magnetic techniques. XRD analysis confirmed the dominance of the Bi-2212 phase with minor Bi-2201 and Bi-2223 traces, while Li incorporation promoted the growth of ( 00 l ) planes and induced a slight monotonic expansion of the lattice parameters and crystallite size. SEM observations revealed that Na–Li co-doping enhanced grain growth and reduced porosity, leading to improved intergranular connectivity. EDS analysis confirmed the preservation of the Bi-2212 phase composition and revealed a slight decrease in oxygen concentration in Li-doped samples, indicating oxygen deficiency, in agreement with previous reports and the observed c-axis elongation from XRD analysis. Resistivity measurements indicated that optimal doping (Na05–Li10) resulted in the lowest residual resistivity (1.59 mΩ·cm) and the highest superconducting transition temperatures ( T c onset = 85.4 K, T c offset = 75.28 K). Magnetoresistance data analysed within the thermally activated flux flow (TAFF) framework showed increased vortex activation energies and improved flux pinning with co-doping. Both the irreversibility field H irr (0) and the upper critical field H c2 (0) were significantly enhanced, reaching 25.94 T and 48.55 T, respectively, for the Na05–Li10 composition. Magnetic measurements showed enhanced remanent magnetization, increased critical current density ( J c ), and stronger flux pinning force, consistent with improved vortex immobilization in the Na–Li co-doped Bi-2212 system. These results indicate that sodium–lithium co-doping significantly enhances the microstructural and superconducting characteristics of Bi-2212, thereby extending its suitability for high-field applications.