Ultra‐stable calcium ion batteries with Prussian blue nanodisks

Calcium ion batteries, though more sustainable than lithium‐ion batteries, still face significant challenges, including the lack of highly rechargeable electrodes. Prepared Prussian blue nanodisk electrodes can significantly extend the longevity of Ca‐based cells. The method involves precipitating 20‐nm‐thick nanodisks of Prussian green from Fe(NO3)3 and K3Fe(CN)6, followed by sodiation with NaI. The Prussian‐blue‐based cathode with polyacrylic acid/polyaniline binder delivers an initial discharging capacity of 77.6 mAh g−1 at 0.1 A g−1 and retains 91.0% capacity after 700 cycles. Polyvinyl fluoride is detrimental to the Ca‐based cell, as its coulombic efficiency decreases from 94.8% (120th cycle) to 86.4% (400th cycle). With the same cathode, the Ca‐based cell is much less sensitive to high current densities than the Na‐based cell. This can be because only half the amount of cations is required to move in Ca‐based systems compared to Na‐based systems; thus, the charge‐transfer resistance is noticeably reduced in Ca‐based systems. A particular engineered form of Prussian blue nanodisks is prepared by using a proper amount of iron(III) nitrate, applying an optimum temperature, and sodiating with NaI. Developed with an optimized formulation of PAA/PANI binders, Prussian blue cathodes can extend the life of calcium‐ion batteries, exhibiting an impressive capacity retention of 91% for 700 cycles.