Heat‐Resistant Carbon‐Coated Potassium Magnesium Hexacyanoferrate Nanoplates for High‐Performance Potassium‐Ion Batteries

Abstract

Metal hexacyanoferrates (HCFs) are regarded as promising cathode materials for potassium‐ion batteries (PIBs) on account of their low cost and high energy density. However, the difficult‐to‐remove [Fe(CN)6] vacancies and crystal water lead to structural instability and capacity deterioration as well as the stereotype of poor thermostability of conventional HCFs. Herein, we report (100) face‐oriented potassium magnesium hexacyanoferrate (KMgHCF) nanoplates with low [Fe(CN)6] vacancies and high crystallinity, enabling thermostability up to 550 °C, high‐temperature carbon coating and crystal water elimination. The as‐obtained KMgHCF/C nanoplates exhibit superior potassium storage properties, including a large reversible capacity of 84.6 mAh g−1, a high voltage plateau of 3.87 V, excellent long‐term cycling performance over 15000 cycles and high rate capability at 5 A g−1. The unprecedented cycling stability of KMgHCF/C is attributed to the synergistic effect of a highly reversible two‐phase reaction, low [Fe(CN)6] vacancies and no crystal water, a specially exposed steady (100) surface, and a protective carbon coating. This work provides a new material selection and modification strategy for the practical application of HCFs in PIBs.