Ultrafine manganese hexacyanoferrate with low defects regulated by potassium polyacrylate for high-performance aqueous Zn-ion batteries

Manganese hexacyanoferrate (MnHCF) is an ideal candidate for aqueous Zn-ion batteries (AZIBs) because of their high capacity and high operating voltage. Adding surfactant during the preparation process is an effective way to reduce the [Fe(CN)6] vacancies and water, leading to a stable structure and excellent electrochemical performance. However, the size of MnHCF becomes large (> 100 nm) after adding surfactant, resulting in the sluggish kinetics of Zn-ion transport and low utilization rate of active materials. Herein, ultrafine MnHCF (15 nm) (MnHCF-15) with few vacancies and low water content has been fabricated by introducing a typical surfactant of potassium polyacrylate (PAA-K). Moreover, PAA-K can also precisely regulate the size of MnHCF, which exhibit a series of nano-sizes of 15, 50, 80, and 120 nm. The MnHCF-15 cathode exhibited a high capacity of 139.2 mA h g-1 at 0.05 A g-1 with a high operating voltage of 1.75 V. The excellent cycling stability of 92.5 %, 91.9 %, and 84.5 % capacity retention at 0.1, 0.2, and 0.5 A g-1 for 1000 cycles were also achieved because of the ultrafine size and the low defect of MnHCF-15. Ex-situ XRD and XPS were carried out to probe Zn-ion storage mechanism, indicating a reversible electrochemical reaction of the MnHCF-15 cathode. Impressively, the soft-packed AZIBs with high electrochemical performance highly reveal the practical application of the MnHCF-15. The results of this work offer a promising cathode material and lead to a new understanding on the nano-size effect of PBAs in AZIBs.