Synergistic Synthesis Regulation and Binder Optimization Unlock High-Rate and Durable Na_2-δMnHCF Cathodes for...

Prussian blue analogs (PBA) are promising sodium-ion battery (SIB) cathodes but are hindered by structural defects, lattice hydration, and unstable interfaces, which limit redox reversibility and cycling. This study combines synthesis control and binder engineering to address these issues. Using polyvinylpyrrolidone (PVP) and sodium citrate in N₂ coprecipitation with freeze-drying, Na_2-δMnHCF_PVP_SC_N₂_FD achieved higher crystallinity, fewer defects, and reduced excess and loosely bound interstitial water content. The optimized material exhibited a capacity of 92.9 mAh g^-1, outperforming vacuum-dried (65.5 mAh g^-1, +42%) and freeze-dried (80.2 mAh g^-1, +16%) samples. The overall water content decreased, and the capacity retention reached 83.8% after 100 cycles, which was higher than that of the nonoptimized electrode. In addition, when a poly(acrylic acid)-polyaniline (PAA:PANI = 1:2) hybrid binder was applied, the initial charging capacity increased compared to that of PAA alone. Capacities of 127.8 mAh g^-1 at 0.01 A g^-1 and 56.8 mAh g^-1 at 2 A g^-1 were achieved, and the capacity recovered to 88.6 mAh g^-1 when the current density returned to 0.05 A g^-1, corresponding to a retention of 82.34%. Although the recovery retention was lower than that of the PAA-only electrode, the PAA:PANI = 1:2 binder delivered the best overall rate performance by maintaining substantially higher capacities across the entire current density range. Long-term stability was also greatly improved, maintaining a capacity retention rate of 78.6 mAh g^-1 even after 500 cycles, which is 41% higher than that of PAA alone. Thus, the synergies of defect-minimized synthesis and conductive binder chemistry can convert Na_2-δMnHCF from a limited-performance PBA to a quantitatively validated high-performance cathode platform. This presents a path for next-generation sodium-ion cells with both durability and high-rate characteristics and provides a potentially generalizable framework for PBA design for sustainable large-scale energy storage.