Experimental and Theoretical Investigation on the Origin of the High Intercalation Voltage of K2Zn3[Fe(CN)6]2 Cathode

Abstract

K2Zn3[Fe(CN)6]2 was investigated as a cathode for K-ion batteries and compared to its sodium analog. The hexacyanometallate using K+ as intercalating ion shows interesting electrochemical properties: i) The redox processes occur at higher potentials, ii) From the two redox processes, the one occurring at higher potentials displays most of the specific capacity while the insertion of sodium occurs mostly at lower potentials, iii) the specific capacity of both materials is similar and iv) although the hysteresis at higher potentials is higher with K+ than with Na+, the occurrence of deintercalation at higher potentials presents undoubtedly more advantages. In the attempt to explain this behavior, electronic structure calculations show that a transition from insulating to conducting behavior occurs as the system changes from discharged to charged states. The electronic density distributions of the hosts are similar independently if K+ or Na+ ions are intercalated. It is found that the inclusion of dispersion interactions is essential to match the intercalation voltages experimentally observed. Thus, dispersion forces are very important to understand the interactions of the ion with the host and should be considered when designing cathodes for K-ion batteries using hexacyanometallate.