Enhancement of Stability by Positive Disruptive Effect on Mn - Fe

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C: Energy Conversion and Storage; Energy and Charge Transport

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 Miguel Angel Oliver-Tolentino, Juvencio Vazquez-Samperio, Stephany Natasha ArellanoAhumada, Ariel Guzman-Vargas, Daniel Ramirez-Rosales, Jin An Wang, and Edilso Reguera J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05506 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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The Journal of Physical Chemistry

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 a

a,b

c

b

M. A. Oliver-Tolentino *, J. Vázquez-Samperio , S. N. Arellano-Ahumada , A. Guzmán-Vargas , D. Ramírezc b a Rosales , J. A. Wang , E. Reguera * a

Instituto Politécnico Nacional, Laboratorio Nacional de Conversión y Almacenamiento de Energía-CICATA, Calzada Legaría 694, Col. Irrigación, México D.F. 11500, Mexico. b

Instituto Politécnico Nacional, ESIQIE-Departamento de Ingeniería Química, Laboratorio de Investigación en Materiales Porosos, Catálisis Ambiental y Química Fina, UPALM Edif. 7 P.B. Zacatenco, GAM, México, D.F. 07738, Mexico c

Instituto Politécnico Nacional, ESFM-Departamento de Física, UPALM Edif. 9 Zacatenco, GAM, México, D.F. 07738, Mexico

Abstract 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]nvacancies 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 to the modification of electron delocalization on Fe (LS); which was caused by the external metals; mainly by Co.

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Introduction Research about energy storage has grown in recent years motivated by the need to ensure that the use of renewable energy will be economically and technologically viable, and environmentally friendly1. Rechargeable batteries have shown promising results because of their high storage capacities, especially lithium-ion batteries used in electronic devices. However, due to the high cost and low abundance of lithium, many studies have focused on using other ions, such as sodium, magnesium, calcium and zinc, to replace lithium in rechargeable batteries2,3. Interesting results about these electrochemical storage systems in aqueous and non-aqueous media have been reported.4 In particular, aqueous batteries have shown promising results due to the easier desolvation process of alkali ions and the lower viscosity in aqueous electrolyte solutions as compared to the organic one5. Different materials have been studied as electrodes for aqueous batteries that use sodium as an alkali ion; these include: rock salt NaxMnO26, the polyanionic compound Na2FeP2O77, and a NASICON-type structure specially doped with Ti8,9 and Prussian Blue Analogue (PBA)10-15. In particular, the easy diffusion of Na+ accompanied by solvation water within open framework cavities has been reported in the context of PBAs used as cathodes in aqueous batteries16. Both PBAs or hexacyanoferrates are crystalline open framework materials that show the general molecular formulae AxM[P(CN)6], where P and M are cations octahedrally coordinated to C and M to N, respectively. The number of cations in balance charge (A) depends on the oxidation state of P and M. Since Wessells reported the insertion of sodium in nickel and copper hexacyanoferrate for use in batteries17, different strategies for improving the electrochemical behavior of hexacyanoferrate as a cathode in aqueous sodium ion batteries have been reported; among these are: i) increasing electrolyte concentration, which increases the activity of Na+ ions14, to slightly improve working potential ii) using polyethilenglicol in an aqueous electrolyte solution to increase the operation potentials of batteries18, iii) adding surfactant to the electrolyte, which lead to molecules getting adsorbed on the electrode surface via electrostatic absorption, to efficiently suppress the evolution of hydrogen or oxygen19, iv) synthesising low-defect Prussian blue with vacancy-free [Fe(CN)6]n- to increase electrochemical stability by blocking active sites in the lattice where the coordination water is inhibited13, and v) synthesizing hexacyanoferrates with two external metals coordinated to N ends to promote an improvement in electrochemical properties; in NiCu hexacyanoferrate, it was associated with Tunable Reaction Potentials,20 and in NiCo Hexacyanoferrate, it was attributed to the modification of electron density21. However, these materials only exhibited a specific capacity near 60 and 80 mA h g-1, respectively. In this context, MnCo Hexacyanoferrate, which can reach a specific capacity near 120 mA h g-1, is a material with promising results. However, Pasta et al.22 reported the synthesis of NayCo1-xMnx[Fe(CN)6] with the purpose of decreasing the amount of Co in the framework to decrease electrode cost. Their results revealed the presence of i) low [Fe(CN)6]n- vacancies, ii) a spontaneous electron hopping from Fe to Mn during the electrochemical process, and iii) poor electrochemical stability associated with manganese due to the oxidization of this metallic ion to Mn3+ producing a localized strain attributed to contraction in Mn-N bond takes place and also the Mn3+ exhibited a Jahn Teller distortion. On the other hand, Kurihara et. al.23 reported that in the Mn1-yCoy[Fe(CN)6] system at


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y>0.33, the spontaneous electron transfer between Mn and Fe is inhibited, which is associated with suppressed redox voltages of Mn and Co, probably because of the anomalous spin states of Co and Mn in PBA however, this phenomenon is not fully understood. For this reason, the present study is divided into two parts. In the first, we synthesize NaxCo1yMny[Fe(CN)6] at different Mn/Co ratios using citrate as a chelating agent to obtain a material without vacancies; electrochemical behavior during redox processes are correlated with electronic interaction between external metal sites in the framework through the interaction of spins using EPR spectroscopy. In the second part, to discuss the effect of the presence of vacancies on the electrochemical process, we synthesized a material without citrate, thus obtaining materials with low ferrocyanide vacancies. The results provide evidence of the disruption of metal-metal charge transfer between Mn and Fe in vacancy-free Mn-Co hexacyanoferrate. Experimental Synthesis of samples NaxCo1-yMny[Fe(CN)6] was obtained by co-precipitation method from the mixture of solution A (0.1 M of Na4Fe(CN)6), solution B (0.1 M of sodium citrate and 0.1 M of Mn(NO3)2, and/or Co(NO3)2) in the Y-type micro-mixer, using a peristaltic pump at 20 rpm. The solution was stirred for 12 h at room temperature; the resulting precipitate was washed with distilled water and ethanol several times and was finally dried under vacuum at 50 °C for 24 h. The samples were labelled CoHF, Co0.8Mn0.2HF, Co0.55Mn0.45HF, Co0.3Mn0.7HF and MnHF.

Physical Characterization The elemental composition of hexacyanoferrates was determined by optical emission spectrometry using inductively coupled plasma (OES-ICP), with a Perkin Elmer OPTIMA 8300 spectrophotometer. X-ray powder diffraction (XRD) patterns were collected with a Bruker D8 Advance diffractometer in the Bragg-Brentano configuration using CuKα radiation (λα = 1.5418 Å). The Raman spectra were obtained on a Thermo Scientific DXR spectrometer with a 532 nm laser. Electron Paramagnetic Resonance (EPR) spectra of powder samples were recorded using a Bruker Elexsys E-500-II EPR spectrometer operating at X-band frequency (9.4186 GHz), equipped with 100 kHz field modulation and phase sensitive detection to obtain the first derivative signal. EPR measurements were carried out at 300 K and at 77 K using a liquid-N2-immersion dewar. The insitu infrared was recorded with an FTIR Perkin Elmer spectrophotometer using a SP-02 spectroelectrochemical cell from Spectroelectrochemistry Partners; the cell was mounted on a Pike MIRacle ATR system. Mössbauer spectra were recorded at 77 K with a WissEl Elektronik GmbH MRG500 conventional constant acceleration spectrometer, equipped with a krypton proportional detector. The c-radiation source was 57Co of 925 MBq (25 mCi) within a rhodium matrix maintained at room temperature. Chemical isomer shift (IS) data are given relative to α-Fe. Absorption spectra were fitted by using the NORMOS program.



Electrochemical Characterization Working electrodes were prepared by stirring 80 wt % of hexacyanoferrate powder, 10 wt % of amorphous carbon (Timcal SuperP Li), and 10 wt % polyvinylidenedifluoride (Aldrich) in N-methyl2-pyrrolidone, then the mixture was coated on a carbon plate (Fuel Cell Grade) and dried at 40 °C in vacuum for 24 h. The amount of hexacyanoferrate deposited was 5 mg cm-2. The electrochemical data were recorded with a Bio-logic potentiostat−galvanostat SP300 using a three electrodes cell, where Ag/AgCl (1M KCl) electrode was used as a reference electrode. A large, partially charged hexacyanoferrate on a carbon electrode was employed as counter electrode, which acts as reversible ion sinks. All electrochemical experiments were carried out in a 1 M NaNO3 solution. Results and discussion Structural Characterization The chemical composition of every sample calculated with OES-ICP and its respective values are shown in Table 1. All samples showed a high amount of sodium and Mn, or Co/Fe ratios close to 1, which indicates that the structure has a low amount of vacancies of [Fe(CN)]4-. The X-ray patterns (Fig. S1 in supporting information) reveal peak-splitting of the cubic crystalline phase at 2θ= 24.5°, 38.8°, 49.5° and 55.93° corresponding to the (220), (420), (440) and (620) planes, respectively. These results indicate the crystallization of materials in a monoclinic lattice with a P21/n space group, as previously reported24. This result was verified by Le Bail fitting method (Fig. 1). The splitting is due a to high amount of sodium in the structure, produced using the citrate-chelating method, which decreases kinetic reaction in the formation of hexacyanoferrate13. Na+ ions are located asymmetrically at the N-coordinated corners with smaller Na−N distances, thus inducing a distorƟon on an elementary cell (inset in Fig 1) associated with the cooperative displacement of (NaOH2)+ groups in an alternating cubic [111] direction24. The lattice parameter a decrease as the amount of cobalt increases (see Table 1). This could be associated with the fact that the atomic radius of Co2+ (0.885 Å) is smaller than that of Mn2+ (0.970 Å). The C≡N- group has the ability to act as an σ-donor by donating electrons to the metal coordinated to the N end. This electron subtraction occurs through the 5σ orbital, which has a certain antibonding character. The metal coordinated to C end exhibited π bonding interaction, which involves the t2g electrons of the metal with the π and π* orbitals of the ligand25. This phenomenon allows the oxidation state of internal and external metals in the cyano complex to be sensed by infrared (Figure S2) and Raman spectroscopy (Fig. 2). The FTIR spectrum of every material reveals a band ca. 2070 cm-1 assigned to M2+-CN-FeII links. The substitution of Mn2+ (Z/r2= 3.287) by Co (Z/r2= 3.652) in the structure promotes the increase of ν(CN) stretching vibration; due to the polarizing power (Z/r2) of cobalt, the charge subtraction at the N end through the 5σ orbital was increased. The splitting in the ν(CN) vibration in MnHF and the asymmetric signal observed in Co0.55Mn0.45HF and Co0.8Mn0.2HF, can be associated to not all C≡N- bridges being equivalent due to the decrease of local symmetry by the improvement of Na+framework interaction, as it has been reported for zinc hexacyanoferrate in dehydrated form26. Fe


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cation in haxacyanoferrate materials presented local Oh point group symmetry with an inversion center and two stretching modes A1g and Eg27. The vibration mode A1g appears near to 2128 cm-1, whereas at 2095 cm-1, the, Eg mode verifies the presence of M2+-CN-FeII links in all samples28,29. The band near to 2080 cm-1 can be attributed to T1u vibration, which indicates a localized structural distortion promoting deviations from ideal Oh to D4h symmetry27. This band decreased inversely to cobalt’s amount in relation to Mn due to the high polarizing power of cobalt, which inhibits symmetry distortion in the framework. The presence of bands at a lower Raman shift is attributed to the band T1u being able to split into two contributions, as it has been reported30. To determine the effect of symmetry distortion of external metal on iron coordinated to C in [a/the] cyanide group, 57 we carried out Fe Mössbauer analyses. Spectra at room temperature are shown in Figure S3. The low value of an isomer shift (IS≈ -0.19 mm/s) is assigned to FeII-C in low spin (LS) with electronic configuration t2g6 eg0; the singlet is due to the t2g levels, which are full of d electrons (S=0). Additionally, quadrupole splitting caused by any electric field gradient was detected (QS)27. However, the results of this paper show quadrupole splitting QS≈ 0.19 mm/s, which can be interpreted as an anisotropic charge involving an important distortion in the octahedral environment of the [Fe(CN)6]4- block31. The replacement of cobalt by manganese in the framework (Table 1) decreased the IS value related to the higher polarizing power of cobalt. The charge subtraction over N ends therefore increased, and therefore increased the π-back donation from the iron atom towards the CN ligand. The decrease of QS can be attributed to a lower cell distortion. These results agree with the Raman experiments discussed above. The spin interaction between manganese and cobalt in the structure can be analyzed by EPR spectroscopy (Fig 3); due to the iron in the open framework material having S=0, it would not show influence on the EPR response. All EPR spectra of MnHF and CoMnHF compounds showed a singlet signal with an average line width of 20 gauss, with the exception of CoHF compound, which was EPR-silent. The Mn coordinated to N exhibited electronic configuration in high spin t2g3 eg2 with S=5/2,32such that, in EPR experiments, three Kramer’s doublets ± 5/2, ± 3/2 and ± 1/2 were expected. However, at room temperature (Fig. 3A), the MnHF compound shows a broad singlet EPR signal with g= 2.023. Here, the degeneracy of Kramer´s doublets was removed had been removed through the applied magnetic field and EPR spectra coming from transitions between energy levels of these doublets. The resonance at g around 2 arises from the transition between energy levels of S= ± 1/2 Kramer’s doublet. The high possibility of only the transition of the ground doublet (± 1/2) appearing is because the exited doublets (± 5/2 and ± 3/2) are occupied. If this is so, the lifetime of the excited states is generally so short because of the relaxation to the lattice and transitions between them being too broad to be observable. Therefore, in practice, resonance is almost always restricted to the ground doublet25. The shape and line width of these signals imply the presence of dipole coupling and exchange interaction. This behavior could be attributed to ferromagnetic interaction, which indicates that two magnetic orbitals are orthogonal, that ground state of the system has parallel electron spins33. In contrast, the CoHF compound did not show any EPR signal, which is associated with the antiferromagnetic ordering of Co2+ spin (S= 3/2). This behavior in nature is a consequence of the Pauli principle, where two non-orthogonal magnetic orbitals lead to antiparallel spin ordering33.



The electrons occupying the orbitals t2g in FeII are partially delocalized on the neighboring (Co/Mn) as has been reported for Prussian Blue34. In MnHF, which contains Mn2+ (t2g3 eg2), the t2g and eg orbitals are both exactly half full; the t2g electron with a fraction of spin from FeII can interact with electrons in eg orbitals of Mn, thus promoting a spin order parallel to each other. Whereas for CoHF with Co2+ (t2g5 eg2), the interaction occurs in the t2g orbital provoking an antiparallel order to each other. Whereas, as Co concentration decreases, the EPR spectra intensity increases until obtaining a maximum intensity for MnHF. However, this increase is not proportional to Co concentration. More specifically, the EPR signals of Co0.3Mn0.7HF and Co0.55Mn0.45HF exhibit practically the same intensity (see Fig S4), which suggests that spin alignment is modified by Mn-Co interaction. To investigate other mechanisms that may be involved in the evolution of the EPR spectra with different Co:Mn ratios, the CoMnHF compounds were measured at 77 K (Fig 3B). EPR behavior for CoHF is similar to that observed at 300 K, while the EPR signal decreased for MnHF. This fact can be attributed to cell contraction when the sample is cooled, which shortens the distance between Mn coordinated to N ends, modifies the spin alignment, and produces a ferrimagnetic interaction. A similar phenomenon is observed for the Co0.3Mn0.7HF sample. These results are ascribed to the presence of a high amount of manganese, which exhibited a high lattice parameter (a), as discussed above. Whereas in the material with a low amount of manganese, such as Co0.8Mn0.2HF, the EPR signal was increased, which is indicative of some Co spins of the framework ordering in a parallel way to those of Mn spins. This spin ordering is probably due to the smaller size in the lattice parameter, such that the distance between the external metals does not change significantly when the sample is cooled, thus avoiding an alignment of spins that cause a ferrimagnetic interaction as revealed by MnHF and Co0.3Mn0.7HF samples. The parallel alignment between the Co and the Mn spins for materials with a low amount of Mn is confirmed by the EPR spectrum of Co0.55Mn0.45HF, which was increased 1.73 times with respect to its own intensity at 300 K. This signal increase implies that an extra number of paramagnetic entities contribute to the EPR signal, which suggests that electron partial delocalization in the Co2+-NC-FeII-CN-Mn2+ chain is modulated towards Co sites due to their high charge subtraction ability of the iron modifying the spin orientation.

Electrochemical Evaluation The cyclic voltammetry profile of each material at 1 mVs-1 in 1M NaNO3 solution is shown in Fig. 4A. CoHF exhibited two faradaic processes, which agrees with recent reports.21,22 The first at formal potential (Ef = 0.35 V vs Ag/AgCl) is attributed to an electrochemical process of the Co3+/Co2+ redox couple. This is accompanied by a change in electronic configuration, from high (HS: t2g5 eg2) to low spin (LS: t2g6 eg0); the redox potential associated to the FeIII/FeII redox couple occurred at Ef = 0.87 V vs Ag/AgCl. The increment in formal potential for Fe (LS) with respect to other hexacyanoferrates17 is due to the high polarizing power of Co3+ formed in the first redox process, increasing charge subtraction on N ends, and promoting an increment in the π back bonding interaction on the iron atom and on the cyanide ions. This last decreased the absolute energy of the fully filled t2g6 orbitals (FeII in low spin configuration), favoring the 2+ oxidation state of the iron centers35. The anodic peak at 0.43 V vs Ag/AgCl is associated with Na-ion desertion


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along the [111] direction during the oxidation process, which induces a structural change in the unit cell from monoclinic to cubic22. The peak to peak separation for FeIII/FeII redox couple is ∆Ep= 60 mV, indicating a good electrochemical reversibility, while ∆Ep= 100 mV is observed in the Co3+/Co2+ redox couple. This semi-reversible process is due to the stability of Co3+ LS configuration (t2g6 eg0), where a transfer of charge density from t2g to eg orbitals should take place. The semireversibility can be verified by the electrochemical profile observed between 0.55-0.75 V/ vs Ag/AgCl, which has been attributed to the gradual rearrangement of the electronic structure during the Co3+ (LS)/Co2+(HS) redox process13. Any change was observed in the electrochemical profile after 50 cycles. During the anodic sweep, MnHF showed a faradaic process at 0.52 V vs Ag/AgCl (thin line), which was attributed to an oxidation from FeII (t2g6 eg0) to FeIII (t2g5 eg0) due to spontaneous electron hopping from Mn to Fe through the cyanide linker, as reported22. This behavior could be explained by the metal to metal charge transfer mechanism, as reported for manganese PBA36. The structural changes in the unit cell during sodium extraction take place at 0.58 V vs Ag/AgCl. The anodic peak at 1.1 V vs Ag/AgCl may be attributed to with the oxidation process of Mn2+ (t2g3 eg2) to Mn3+ (t2g3 eg1) (see Figure 4A). The presence of iron 3+ in the structure reduces the electron density around [the/a] metal coordinated to nitrogen, which increases potential when manganese oxidation takes place. The absence of [a cathodic peak associated with manganese reduction, as well as a faradaic process in the second cycle (thick line), indicates the poor stability of MnHF in aqueous media during the redox process, which is verified because the solution turns yellowish. This fact suggests that a fraction of the [Fe(CN)6]3- complex anion is partially decomposed. As discussed prior, the external metal substitution of manganese by cobalt into the Co0.3Mn0.7HF, Co0.55Mn0.45HF and Co0.8Mn0.2HF samples showed a Mn2+-NC-FeII-CN-Co2+ chain, which modulates spin delocalization around FeII (LS). The metal substitution modifies electron density along the chain, which affects electrochemical behavior. In general, the electrochemical profiles exhibited two faradaic processes, at low potential 0.7 V vs Ag/AgCl for Co0.8Mn0.2HF, any well-defined peak was detected in the oxidation process 1, indicating an increment in rearrangement of the electronic structure due to the presence of Co3+ (LS). Whereas, electrochemical reversibility in redox process 2 decreased (∆Ep= 88 mV), suggesting that Fe (LS) did not solely participate in the faradaic process due to manganese’s contribution. This can be verified in Co0.3Mn0.7HF, where the faradaic process at high potential is irreversible by the presence of Mn3+ formed during the oxidation process, which increases the hybridization between the Mn and N orbitals and decreases stability. On the other hand, the Co0.55Mn0.45HF showed better electrochemical reversibility in every redox process, which indicates fast reaction kinetics (which decreases energy loss in the battery). The charge/discharge experiments at 1C (60 mA g-1) for CoHF, Co0.8Mn0.2HF and Co0.55Mn0.45HF are shown in Figure 4B; minor current losses after several cycles of 50, 45, and 40 (thick line), respectively, were observed. The CoHF material exhibited complex galvanostatic behavior associated with the spin transition in cobalt during the redox process; voltage discharge began at 0.8 V vs Ag/AgCl. On the other hand, the CoMnHF compound showed two principal and defined plateaus at 0.3-0.58 V vs Ag/AgCl and 0.9-1.0 V vs Ag/AgCl. The second process showed a higher slope than the first. This can be



associated with sodium insertion/desertion of process two, which takes place in a solid solution state in a cubic cell; whereas the sodium insertion/desertion in the first process occurs by the presence of two-monoclinic and cubic phases, as reported for similar framework materials37. The increase in manganese content increases discharge voltage at 0.95 and 1.0 V/SCE for Co0.8Mn0.2HF and Co0.55Mn0.45HF, respectively. The specific capacity of materials here studied is in the following order: 118.81 mAh g-1 (CoHF) > 113.84 mAh g-1 (Co0.8Mn0.2HF) > 112.82 mAh g-1 (Co0.55Mn0.45HF). Based on our results, the material with the best electrochemical properties is the Co0.55Mn0.45HF system, which exhibits higher discharge potential and greater stability than other materials with manganese, according to the results of the cyclic voltamperometry. This is associated with charge density subtraction in the Co2+-NC-FeII-CN-Mn2+ chain, towards the cobalt due to its polarizing power; promoting a decrease in distance between the external metals, modifying the electron delocalization between orbitals t2g of Fe and eg orbitals of Mn, which can modulate the kinetics electron transfer to and from eg orbitals.

Effect of vacancies on the metal-metal charge transfer. A new synthesis was made with the aim of understanding the influence of vacancies on material behavior, electronic and electrochemical properties [of? Subject/noun missing]. Considering the better electrochemical properties shown by Co0.55Mn0.45HF (free of vacancies) a similar compound was synthesized using sodium chloride instead of sodium citrate. A solution 1M of sodium chloride was used for the new synthesis and the solid obtained was labeled as n-CoMnHF. Its chemical composition was Na1.65Mn0.50Co0.50[Fe(CN)6]0.87, revealing the presence of crystalline defects by [Fe(CN)6]4- vacancies. XRD peak-splitting, which is characteristic of monoclinic phases, was not clearly observed for n-CoMnHF (Fig. 5A) due to the presence of a lower amount of sodium with respect to Co0.55Mn0.45HF. A important magnetic difference between them was observed from the EPR spectra (Figure 5B). The Co0.55Mn0.45HF compound has a strong and a wide singlet EPR signal that implies a ferromagnetic order as discussed above. However, the n-CoMnHF compound showed a weak singlet signal with a hyperfine splitting superimposed on it, which is characteristic of Mn2+ with an electronic spin of S=1/2 and a nuclear spin of I=5/2 (see inset in Fig 5B). The hyperfine splitting implies that the distance between Mn and Co ions was increased, weakening dipole-dipole and exchange interactions. This could be attributed with the presence of defects in the frameworks by [FeII(CN)6]4- vacancies produced during synthesis. These vacancies promote the completion of coordination spheres with water molecules by some Mn and Co sites. The deficiency of cyanide ligands inhibits electron delocalization in the iron center with the external metals. The EPR spectrum of n-CoMnHF compound at 77 K (Figure not shown) reveals a singlet with a hyperfine splitting superimposed on it. The increase in the EPR signal intensity at 77 K compared to the signal intensity of the same compound at 300 K implies typical paramagnetic behavior38. The electrochemical experiments (Figure 5C) exhibited a lower specific capacity (87 mAh g-1) than that found in Co0.55Mn0.45HF (112.82 mAh g-1), attributed to the presence of ferrocyanide vacancies39,40 and lower discharge voltage (0.9 V Ag/AgCl) due to the low presence of Mn3+ species; whereas Figure 5D showed high electrochemical stability of Co0.55Mn0.45HF with a fraction capacity retention of 80 % while n-CoMnHF exhibited a value of 60 % after 100 charge/discharge


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cycles at 1C; these results showed that the improvement in the stability of hexacyanoferrates without vacancies can be associated with electronic interaction between external metals through electron delocalization, which is inhibited by the presence of coordinated water, and not only to the blockage of active sites, as previously reported41.

Electrochemical Mechanism In order to elucidate whether the disruptive effect of spontaneous charge transfers between Mn and Fe depend only on the Mn/Co ratio, as was reported beforeref, in situ infrared spectroscopy was carried out during the charge/discharge process in Co0.55Mn0.45HF and n-CoMnHF, and the results are shown in Figure 6. The sample n-CoMnHF (Fig. 6A) exhibited a minimum of %T near 2065 cm-1 , indicating that the metal in the oxidation state of 2+ is coordinated to the cyanide linker through C and N. During sodium desertion, an IR vibration at 2150 cm-1 is observed in the first redox process to around 0.45 vs Ag/AgCl, indicating the presence of Fe(III) LS and suggesting that metal-metal charge transfer between Fe and Mn takes place at 0.35 V vs Ag/AgCl, as discussed above. An evident IR band associated to changes in metal oxidation states coordinated to N appeared ca. 2100 cm-1. On the other hand, in Co0.55Mn0.45HF (Fig. 6B), the band attributed to the presence of metal coordinated to N ends in oxidation state 3+ (2100 cm-1) is observed in the first faradaic process during sodium desertion, whereas the IR band at 2150 cm-1 , attributed to FeIII(LS), appears until 0.75 V vs Ag/AgCl, where the second faradaic process starts. The features discussed above suggest that the interaction between cobalt and manganese, through spin delocalization of the Fe center, disrupts electron transfer from Fe to Mn. The mechanism proposed for Co0.55Mn0.45HF can be explained by the charge density distribution of the Co-NC-Fe-CN-Mn chain because, during sodium desertion process, the oxidation of Co2+(HS) to Co3+(LS) and the oxidation of Mn2+(HS) to Mn3+(HS) occur simultaneously. Co3+ increases the subtraction charge over FeII (LS) through cyanide and decreases the electron density around Mn. This fact modulates the electron delocalization towards the cobalt site, decreasing the capacity to add one electron on the eg manganese orbitals, which inhibits the charge transfer from Fe to Mn. On the other hand, the [Fe(CN)6]n- vacancies in n-CoMnHF favors the presence of coordinated water to the external metal, as in the chains OH2-Mn-NC-Fe and OH2-Co-NC-Fe, and additionally decrease the interaction with the metal coordinated to nitrogen, inducing electron hopping from Fe to Mn. Conclusions The high sodium content within the hexacyanoferrate framework produces cell distortion and some changes in octahedral symmetry. Higher distortion was exhibited in MnHF and it decreased by the incorporation of Co, which possesses high polarizing power, thus modulating the electron delocalization of Fe (LS) through the cyanide linker in the Mn-NC-Fe-CN-Co chain. The Co0.55Mn0.45HF sample, free of vacancies, showed the change from an antiparallel spin order of Co to a parallel spin order caused by the presence of Mn. The metal to metal charge transfer between Mn and Fe was disrupted in a vacancy-free framework, due to the decrease in the addition capacity of one electron on eg manganese orbitals during redox process; this was attributed to the interaction between Co and Mn through the cyanide linker, which improves the electrochemical



properties of redox stability and discharge voltage. Co0.55Mn0.45HF material, free of vacancies, exhibited a fraction capacity retention of 80 % while n-CoMnHF exhibited a value of 60 % after 100 cycles of charge/discharge. Conflicts of interest The authors declare that they have no conflict of interest.

Supporting Information. XRD patterns of Cox Mn1-xHF samples; Infrared spectrum; Mössbauer spectrum and EPR signal vs manganese amount with respect to cobalt content in hexacyanoferrates. Author Information Corresponding Author [email protected] [email protected] ORCID iD Oliver-Tolentino, Miguel Angel: 0000-0001-8454-0837 Vazquez-Samperio, Juvencio: 0000-0001-6060-9393 Arellano-Ahumada, Stephany: 0000-0003-4719-9013 Guzman-Vargas, Ariel: 0000-0002-2213-1242 Ramirez-Rosales, Daniel: 0000-0001-7815-2029 Wang, Jin An: 0000-0002-7007-8212 Reguera, Edilso: 0000-0002-4452-9091

Acknowledgements The authors are grateful to CONACYT (project 225115) for the acquisition of the EPR spectrometer and INFRA 2014-225161; Particularly, M. Oliver-Tolentino is grateful to project CONACYT-255354 for their support in the participation of international meetings. References 1. Pan, H.; Hu, Y.-S.; Chen, L. Room-Temperature Stationary Sodium-ion Batteries for LargeScale Electric Energy Storage. Energy Environ. Sci. 2013, 6, 2338-2360. 2. Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research Development on Sodium-Ion Batteries. Chem. Rev. 2014, 114, 11636-11682.


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FIGURE CAPTION

Figure 1. Powder X-ray Diffraction and Le Bail profile fitting (red line) for sample Co0.55Mn0.45HF, inset: Crystalline structure, where Na+ cations (white spheres), oxygen from water (red spheres). The framework negative charge is accumulated on the N atom (blue sphere) coordinated to Co/Mn (green spheres). C atom (black sphere) is coordinated to Fe atom (yellow sphere).

Figure 2. Raman spectra of hexacyanoferrates for: a) CoHF, b) Co0.8Mn0.2HF, c) Co0.55Mn0.45HF, d) Co0.3Mn0.7HF and e) MnHF.

Figure 3. Electronic paramagnetic resonance spectra at: A) room temperature and B) 77 K. for a) CoHF, b) Co0.8Mn0.2HF, c) Co0.55Mn0.45HF, d) Co0.3Mn0.7HF and e) MnHF. Figure 4. A) Cyclic Voltammetry at 1 mV s-1, where thin line is 1st cycle and tick line is 2nd cycle for MnHF (e), 20th cycle for Co0.3Mn0.7HF (d), 40th cycle for Co0.55Mn0.45HF (c) and 45th cycle for Co0.8Mn0.2HF (b) and 50th for CoHF (a); B) Galvanostatic experiment at 1C, for a) CoHF, b) Co0.8Mn0.2HF, c) Co0.55Mn0.45HF. Figure 5. Comparison between Co0.55Mn0.45HF (free of vacancies) and n-CoMnHF (with vacancies) A) X-Ray Diffraction, B) EPR spectra at room temperature, C) Galvanostatic experiments at 1C, D) fraction capacity retention vs cycle number at 1C. Figure 6. Contour maps of in-situ infrared experiments during charge process in: A) n-CoMnHF and B) Co0.55Mn0.45HF, where R is Co or Mn.


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Table 1. Sample composition, a lattice parameter, from Mössbauer spectra isomer shift and quadrupole splitting values.

Figure 1.



Figure 2.

T1u

A1g

Eg

a

b

Intensity/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 22

c

d

e 2040

2060

2080

2100

2120

2140 -1

Raman Shift/cm


2160

Page 17 of 22

Figure 3.

2000

3000

A

Intensity/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c d e

3000

4000

5000

B

a b

2000

4000

5000

Magnetic Field/G



Figure 4.

A

a

200 III

3+

II

1.4

b

1.2

Fe /Fe

2+

Co /Co

Charge

B

b

c

150

d

150

E/V vs Ag/AgCl

-1

150

i/mA g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 22

1.0

Process 2

0.8 c

0.6

Process 1

0.4 0.2

a

Discharge 3+

III

II

Fe /Fe

2+

Mn /Mn

0.0

0

10 20 30 40 50 60 70 80 90 100 110 120 -1

Specific Capacity/mAh g

100 e

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 E/V vs Ag/AgCl


Page 19 of 22

Figure 5.

B

n-CoMnHF

Co 0.55Mn0.45HF

Intensity/a.u.

Intensity/a.u.

A

n-CoMnHF

Co0.55Mn0.45HF 2000

10

15

20

25

30

35

40

45

50

55

60

5000

1.0

0.6 Co0.55Mn 0.45HF

0.2

Fraction Capcity Retention

n-CoMnHF

0.8

0.0

4000

1.1

C

1.0

0.4

3000

Magnetic Field/G

1.4 1.2

3000 4000 5000 Magnetic Field/G

2000

2θ/degree

E/V vs Ag/AgCl

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 0.7 0.6

n-CoMnHF

0.5 0.4 0.3 0.2

C-rate: 1C

0.1 0.0

0 10 20 30 40 50 60 70 80 90 100110120 -1 Specific Capacity/mAh g

Co0.55Mn0.45HF

0.9

0

20


D 40

60

Cycle Number

80

100


Figure 6.


Page 20 of 22

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Table 1.

Sample

Formula

a (Å)

IS (mm/s)

QS (mm/s)

CoHF

Na1.84 Co[Fe(CN)6]0.96

10.37

-0.1894

0.1841

Co0.8 Mn0.2HF

Na1.88 Co0.8Mn0.2[Fe(CN)6]0.97

10.41

-0.1897

0.1872

Co0.55Mn0.45HF

Na1.88 Co0.55Mn0.45[Fe(CN)6 ]0.97

10.46

-0.1903

0.1932

Co0.3 Mn0.7HF

Na1.88 Co0.3Mn0.7[Fe(CN)6]0.97

10.51

-0.1908

0.1952

MnHF

Na1.92 Mn[Fe(CN)6 ]0.98

10.57

-0.1911

0.1972




Page 22 of 22

Enhancement of Stability by Positive Disruptive Effect on Mn - Fe

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