<p>Nickel-manganese-layered double hydroxides (NiMn-LDHs) exhibit low intrinsic conductivity, nanosheet stacking, and insufficient structural stability, resulting in low energy density, which severely restricts their application in high-energy storage systems. The 3D nanoflower-like phosphate-intercalated NiMn-LDHs (P-NiMn-LDHs) were first synthesized via a one-step hydrothermal method, and then fabricated into working electrodes through a conventional slurry-coating process using nickel foam as the current collector. The present study systematically investigates the influences of phosphate intercalation on the morphology, crystal structure, electronic states, and electrochemical performance of NiMn-LDHs. The results indicate that phosphate intercalation maintains the layered framework while introducing local structural distortion of the host laminates, abundant defects, and loose porous ultrathin nanosheet assemblies, thereby increasing the specific surface area and electroactive sites. Electrochemical measurements demonstrate that the P-NiMn-LDHs electrode exhibits significantly higher specific capacitance and faster reaction kinetics compared to the pristine counterpart. It exhibits a capacity retention of 94.9% over 3000 cycles at 1 A g⁻¹, which is remarkably higher than the 88.9% of pristine NiMn-LDHs. This anion intercalation strategy provides a feasible approach for designing advanced high-performance electrode materials.</p> Graphical abstract <p>This study presents a high-performance phosphate-intercalated NiMn-layered double hydroxides (P-NiMn-LDHs) electrode. The 3D nanoflower-structured P-NiMn-LDHs active material is first synthesized via a facile one-step hydrothermal approach, and then fabricated into the working electrode through a conventional slurry-coating method using nickel foam as the current collector. The modified electrode leverages synergistic effects from local structural distortion of the host laminates and abundant defect engineering, hierarchical 3D nanoflower microstructure optimization, and electronic structure modulation of active metal sites. Electrochemical characterization reveals outstanding energy storage performance, including significantly enhanced specific capacitance, accelerated redox reaction kinetics, and an excellent capacity retention of 94.9% after 3,000 cycles at a current density of 1 A g-1, which is remarkably superior to the 88.9% retention of pristine NiMn-LDHs. First-principles calculations show a substantial enhancement in Na+ adsorption energy, elevated total density of states at the Fermi level, and optimized d-band centers of Ni/Mn active sites, providing atomic-level insights into the improved pseudocapacitive energy storage mechanisms. The P-NiMn-LDHs electrode also demonstrates excellent structural stability and low interfacial impedance, making it a promising candidate for high-performance supercapacitor energy storage devices.</p>