Enhancement of Stability by Positive Disruptive Effect on Mn–Fe Charge Transfer in Vacancy-Free Mn–Co Hexacyanoferrate Through a Charge/Discharge Process in Aqueous Na-Ion Batteries

Several materials have been studied as electrodes for aqueous batteries that use sodium as alkali ion; these include Prussian blue analogue or hexacyanoferrates. The inhibition or disruption on metal–metal charge transfer plays an important role for improving electrochemical stability of the material. The stability improvement is achieved when two external metals are coordinated to N ends in the Na-rich hexacyanoferrates. Additionally, the presence of vacancies in the material is another important factor that influences its stability. In this study, NaxCo1–yMny[Fe(CN)6] has been synthesized at different Mn/Co ratios by precipitation using citrate as a chelating agent to obtain a material without vacancies. Its electrochemical behavior during redox processes and the correlation with the electronic interaction between external metal sites in the framework through the interaction of spins have been studied too. To discuss the effect of the presence of [Fe(CN)6]n- vacancies on the electrochemical process, we synthesized a material without citrate for obtaining materials with low ferrocyanide vacancies. The vacancy-free Co0.55Mn0.45HF versus n-CoMnHF, were compared in this work. These studies reveal that manganese hexacyanoferrate is unstable. The partial substitution of Co by Mn modifies the metals spin ordering and consequently, the interaction between metals coordinated to N in the cyanide linker. Such partial substitution, with a Mn/Co ratio of 1:1 (Co0.55Mn0.45HF), improves the electrochemical stability and enhances the discharged potential as well. On the other hand, when vacancies are present, the n-CoMnHF compound showed a decrease in its crystallinity as well as in its external metal interaction. Both changes may be due to the presence of coordinated water, which modifies electrochemical performance. A spontaneous hopping from Mn to Fe during oxidation in n-CoMnHF was detected, but this phenomenon was disrupted in Co0.55Mn0.45HF. Such charge transfer inhibition was associated with the modification of electron delocalization on Fe (LS); which was caused by the external metals; mainly by Co.