Enhanced stability and rate performance of zinc-doped cobalt hexacyanoferrate (CoZnHCF) by the limited crystal growth and reduced distortion

Cobalt hexacyanoferrate (CoHCF) is a potential cathode for aqueous Na-ion batteries due to its high theoretical specific capacity (170 mAh g(-1)); however, its lower rate capability and cyclability limit its applications. Structural distortion at a weak N-coordinated crystal field during cycling disintegrates Co, yielding an irreversible reaction. Different Zn amounts ranging 0-1 were added to the Co site to suppress the structural irreversibility of CoHCF, yielding Co1-xZnxHCF powder; this Zn (x <= 0.09) addition reduced the powder's dimension because the lower four coordination of Zn-N, not the six coordination of Co-N, limits the powder growth. Simultaneously, a small lattice parameter and interaxial angle (similar to 90 degrees) are obtained, implying that a narrower Co1-xZnxHCF inner structure is formed to accommodate Na ions. Moreover, the electronic conductivity of Co1-xZnxHCF gradually increased within 0-0.09 range. A smaller particle size with a high surface area leads to a near-surface-limited redox process, similar to a capacitive reaction. Both the surface-limited reaction and electronic conductivity enhances the reversibility due to the smaller charge transfer resistance at the electrode/electrolyte interface caused by Zn addition. Replacing redox-active Co with non-active Zn amount of 0.07 (Co0.93Zn0.07HCF) slightly reduces the specific capacity from 127 to 119 mAh g(-1) at 0.1 A g(-1) due to the shrunken Co charging sites. Rate performance is enhanced by compromising the capacity and reduced distortion, resulting in 81% retention at a 20 times-faster charging rate. Notably, the Co0.93Zn0.07HCF sample exhibited the good stability while preserving 74% of the initial capacity at 0.5 A g(-1) after 200 cycles. (c) 2022 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences Published by Elsevier B.V. All rights reserved.