Low-strain binary hexacyanoferrate nanocuboids with concentration-gradient structure towards fast and durable energy storage

Concentration-gradient structured binary hexacyanoferrate nanocuboids (KFeMnHCF) with depressed internal stress and enhanced ion migration exhibit high reversible capacity, superior rate capability and high cycling stability as anode in AKIB. The exploration of low-strain and high-performance electrode is a crucial issue for aqueous potassium-ion battery (AKIB). Herein, a novel potassium mediated iron/manganese binary hexacyanoferrate nanocuboid, i.e., K x Fe y Mn 1− y [Fe(CN) 6 ]· n H 2 O (KFeMnHCF) nanocuboid, with the concentration-gradient (CG) structure is designed as a high-performance cathode for AKIB. Internal the CG-KFeMnHCF nanocuboids, the manganese content gradually decreases from the interior to the surface and the iron content changes reverse, resulting in the concentration-gradient structure. Both experimental and finite element simulation (FEA) results demonstrate the lower internal stress and better mechanical characteristics of CG structured nanocuboid than the homogenous structured one upon ion intercalation/deintercalation processes. Meanwhile, the electrochemical testing and theoretical calculation (DFT) results disclose the substitution of Fe to Mn in the KMnHCF crystal results in the enhanced electronic conductivity, potassium migration and electrochemical kinetics. Taken both advantages from the well-designed architecture and optimized crystal structure, the CG-KFeMnHCF achieves the superior rate capability and ultrahigh stability in aqueous potassium ion system. In particular, the CG-KFe 0.31 Mn 0.69 HCF achieves the best comprehensive properties among all the samples. The full AKIBs based on CG-KFe 0.31 Mn 0.69 HCF cathode achieves the high energy density (83 Wh kg −1 ), superior power density, high capacity retention (83%) over high-rate long-term cycles, good adaptation to a wide temperature range (−20 to 40 °C) and high reliability even under outside deformations. Therefore, this work not only provides a new clue to design the high-performance cathode, but also promotes the applications of AKIBs for diverse electronics and wide working environments.