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
posted on 2018-08-22, 00:00authored byM. A. Oliver-Tolentino, J. Vázquez-Samperio, S. N. Arellano-Ahumada, A. Guzmán-Vargas, D. Ramírez-Rosales, J. A. Wang, E. Reguera
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.