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Article 4

0.75

2

2

2

8

2

3+

2+

(NH) Fe(HO)[BPO]•0.25HO, a Fe /Fe Mixed Valence Cathode Material for Na Battery Exhibiting an Helical Structure Liang Tao, Gwenaelle Rousse, Moulay Tahar Sougrati, Jean-Noël Chotard, and Christian Masquelier J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5117596 • Publication Date (Web): 02 Feb 2015 Downloaded from http://pubs.acs.org on February 14, 2015

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Tao et al., Figure 1

The Journal of Physical Chemistry

(NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O

(a)

10μm NaFeII(H2O)2[BP2O8]·H2O

(b)

15μm

ACS Paragon Plus Environment


Heated at 300oC under Ar for 10 hours

Transmittance (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

Page 2 of 30 Tao et al., Figure 2

Heated at 110oC for 3 hours in Büchi oven

Pristine (NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O

N-H ʋ4 deformation

Wavenumber (cm-1)


Page 3 of 30

(a)


(NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O M(T)

χ (emu mol Fe-1Oe-1)

1/χ = f(T)

Θcw= -7.8K C = 4.06

10 kOe

µeff = 5.68µB

Temperature (K) NaFeII(H2O)2[BP2O8]·H2O

(b) M(T) χ (emu mol Fe-1Oe-1)

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


1/χ = f(T)

Θcw = -2.5K C = 3.78

10 kOe

µeff = 5.49µB

Temperature (K)



(a)

(NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O

Velocity (mm/s)

(b)

NaFeII(H2O)2[BP2O8]·H2O

Transmission

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


(mm/s) ACSVelocity Paragon Plus Environment

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(NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O RBragg = 3.00% Rf = 1.79% Rp = 0.79% Rwp = 1.11%

Intensity (arb. units)

(a)

Fe-Oaverage= ~2.14 Å P-Oaverage= ~1.50 Å B-Oaverage= ~1.46 Å

2θ (deg., CuKα) (b)

NaFe(H2O)2[BP2O8]·H2O RBragg = 7.95% Rf = 5.79% Rp = 1.04% Rwp = 1.50%


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Fe-Oaverage= ~2.18 Å P-Oaverage= ~1.55 Å B-Oaverage= ~1.47 Å

2θ (deg., Cu )

Kα ACS Paragon Plus Environment


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(NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O

(a)


NaFe(H2O)2[BP2O8]·H2O

(b)

65 helix


61 helix

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Fe-O4 2.097(4) Å Fe-O3 2.056(3) Å

Fe-O5 2.261(3) Å P-O3 1.504(3) Å

B-O1 1.463(5) Å

P-O2 1.506(4) Å

B-O2 1.459(4) Å P-O1 1.513(4) Å

P-O4 1.487(5) Å



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(a)

(b)


(NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O



Page 9 of 30

(a)


Voltage (V)

C/20 @55oC vs. Na+/Na

~0.80Na+ x in “NaxFe(H2O)2[BP2O8]·H2O”

(b) Fe3+/Fe2+ ~2.9V dx/dV

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


Voltage (V) ACS Paragon Plus Environment

(a) Voltage (V)

10 of 30 Tao et al.,Page Figure 10

Number of cycles

x in “Nax(NH4)0.75Fe (H2O)2[BP2O8]·0.25H2O”

(b)

Fe3+/Fe2+ ~2.9V

dx/dV

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

Capacity (mAh/g)


Voltage (V) vs. Na+/Na ACS Paragon Plus Environment


(NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O vs. Na+/Na

*


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* In situ cell 1.5 V Discharge

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Charge

4.0 V

1.5 V

2θ (deg. CuKα)



Charge

1.5 V Discharge

Charge

4.0 V

4.0 V


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

Discharge

1.5 V

2θ (deg. CuKα)


Tao et al., Figure Page 12 of12 30

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(NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O, a Fe3+/Fe2+ Mixed Valence Cathode Material for Na Battery Exhibiting an Helical Structure Liang Taoa,d, Gwenaëlle Rousseb,c,d, Moulay T. Sougratid,e, Jean-Noël Chotarda, Christian Masqueliera,d* a

Laboratoire de Réactivité et de Chimie du Solide (LRCS), CNRS UMR7314, Université de Picardie Jules Verne, 33 rue Saint Leu, 80039 Amiens, France b

c

d

FRE 3677, Chimie du Solide et Energie, Collège de France, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France

Sorbonne Universités - UPMC Univ Paris 6, 4 Place Jussieu, 75005 Paris, France.

Réseau sur le Stockage Electrochimique de l’Energie (RS2E), CNRS FR 3459, France e

Institut de Charles Gerhardt, CNRS UMR5253, Université Montpelier 2, 34095 Montpelier, France

Corresponding author at [email protected]



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ABSTRACT

Borophosphates, previously identified as interesting non-linear optical, catalysts, molecular sieves and ion exchange materials have been disregarded so far for their electrochemical properties

as

electrode

materials

in

Li

or

Na

batteries.

We

have

prepared

(NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O and NaFe(H2O)2[BP2O8]•H2O via hydrothermal synthesis, determined their exact chemical formulas and crystal structures with magnetic susceptibility, Mössbauer spectroscopy, IR and XRD probes. Both borophosphates crystallize in a remarkable 6n screw axis helical structure. They were subsequently further investigated as Na/Na-ion battery cathodes for the first time. (NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O revealed interesting electrochemical responses, yielding a second discharge capacity of ~ 80 mAh/g within 1.5-4.0 V at C/50 rate, 55oC, via a solid-solution insertion mechanism as determined by in situ XRD measurements. Keyword: Borophosphates, Cathode, Na-ion Battery, Crystal Structure


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1 Introduction Lithium ion batteries have dominated the portable devices market and have been considered as one of the major candidates to power next generation electrical vehicles. Recent debates

1, 2

around the potential rise of Li cost and on its natural abundance made the Na-ion technology an alternative choice for large-scale energy storage applications, due to its similar chemistry, lower price and almost unlimited sodium resources. Many materials have been tested as cathodes for Na-ion batteries since 1970s, such as oxides , olivine phosphates

19-21

, fluorophosphates

3-14

, transition metal fluorides 15, 16 , alluaudites

22-26

and NASICON framework compounds

17, 18

27, 28

.

Despite all the efforts made for the past four decades, up to now the electrochemical performances of Na-ion batteries are still to be improved and one urging task of the optimization is to find suitable cathodes. As borophosphates contain both BO4 (low molecular weight for higher theoretical capacity) and PO4 groups (strong inductive effect for higher operating voltage), they may be interesting materials to evaluate.

Borophosphates have been intensively investigated by Kniep et al. since 1994

29-31

. Up to now,

over 100 borophosphates were prepared for their remarkable non-linear optical properties fluorescence effect

34

and their potential usage as catalysts

35, 36

32, 33

,

as well as corrosion protectors.

They crystallize in a large variety of crystal structures. It is noticed that the electrochemical properties of borophosphates (which should contain 3d transition metals (Mn, Fe, Co, Ni), possess low molecular weights and “open framework” structures), to the best of our knowledge, have never been investigated. The first compound which came to our mind is the Fe3+-containing FeIII(H2O)2[BP2O8]•H2O 37 that possesses a theoretical capacity of ~86.3 mAh/g. Interestingly, the isostructural Fe2+-containing Na-ion intercalated form NaFeII(H2O)2[BP2O8]•H2O was reported by Boy et al. soon after

38

, indicating that these family of compounds may possess possible

electrochemical activities vs. sodium. Hence the door was open for the exploration of borophosphates in Na or Na-ion batteries.

In this paper we report on the determination of the exact chemical composition of (NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O, parent to our target compound “FeIII(H2O)2[BP2O8]•H2O” by using magnetic susceptibility measurements in combination with

57

Fe Mössbauer and FTIR

spectroscopies, and X-Ray powder diffraction. The electrochemical properties were investigated and the mechanism of Na+ insertion / extraction was followed with the help of in situ X-ray diffraction.



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2. Synthesis and materials characterization (NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O: 7.90 g of H3PO4 (85% Aldrich), 1.76 g of H3BO3 (99.5% Alfa Aesar), 7.75 g of (NH4)2HPO4 (98% Aldrich) and 3.62 g of FeCl2 (99.5% Alfa Aesar) were added in sequence into a glass beaker containing ~30 ml distilled H2O. The obtained solution was magnetically stirred at 400 rpm, followed by a heat treatment at 80oC for ~10 minutes, prior to be transferred into two 25 ml Teflon containers and sealed within a stainless steel autoclave. The system was placed in a 180oC oven for 60 hours before the powder (that contains single crystals) was washed several times with distilled water, dried and collected for further characterizations.

NaFe(H2O)2[BP2O8]•H2O: 10 ml of distilled H2O, 1.5ml of H3PO4 (85% Aldrich), 0.695g of FeSO4•7H2O (99% Aldrich), 0.1 g of ascorbic acid (99% Aldrich) and 3.813 g of Na2B4O7•10H2O were added in sequence into a 25 ml Teflon container, stirred at 400 rpm for 10 minutes before being closed in a autoclave which was heated for 6 days in a 200oC oven. The products were washed 5 times by distilled hot water, filtrated and dried in a 70oC oven for 30 mins.

The morphology of the samples was examined with a FEI Quanta 200F field-emission scanning electron microscope (FESEM) equipped with an energy-dispersive X-ray (EDX) spectrometer and operated at 20 kV under high vacuum. These samples were coated with a thin layer of conductive Pt using a metalizer.

The magnetic susceptibility properties were characterized by using either a SQUID 5S or a SQUID XL magnetometer (Quantum Design), in zero-field-cooled (ZFC) and field-cooled (FC) modes, under applied magnetic fields of 10 kOe from 2 K to 350 K. Powder samples of roughly 20~30 mg were placed into gel caps for the measurement.

Mössbauer spectra were recorded in order to probe the Fe environments. The absorber was prepared from grinding (NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O or NaFe(H2O)2[BP2O8]•H2O powders (30 mg) with 60 mg of BN. The spectra were recorded at room temperature in transmission geometry for 72 hours using a 0.55 Gbq source Co57Rh in a constant acceleration mode.

Infra-red spectra were recorded on light grey transparent KBr/borophosphates pellets by using a Nicolet Avatar 370DTGS spectrometer. These “KBr” pellets were prepared from careful grinding


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of 2~3 mg of borophosphate powder together with ~ 200 mg of KBr and followed by a cold pressing of ~ 10 MPa.

X-ray powder diffraction (XRD) patterns were collected using a Bruker D8 laboratory diffractometer equipped with CuKα radiation in the 2θ range 10°-110°, for 8 sec./ 0.01°. All diffraction patterns were refined using the Rietveld method suite

as implemented in the FullProf

40

. Operando XRD patterns were recorded within the same machine. The in-house

designed stainless steel in situ cell Na

39

anode,

NaClO4/PC

41

as

was assembled in an Ar-filled glove box, using a metallic the

liquid

electrolyte

and

~30

mg

of

(NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O/C as a cathode, similar to the assembling of a Swagelok-type cell. A thin Al foil (3µm) was used behind the beryllium window to prevent from its possible oxidation at high voltages. The diffraction peak of Al (at 2θCuKα = 38.6o) was used as a position reference for successive experiments. Typically the cell was cycled at C/50 rate meanwhile XRD patterns were collected in operando every 2 hours for a 10o-40o 2θ range.

Electrochemical tests were conducted vs. Na metal in coin-type configuration cells on Macpile or VMP system (Biologic S.A., Claix, France). Prior to being used as cathodes, the (NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O powders were ball-milled with 20% in mass of Carbon SP for 20 min. The as-prepared active material was magnetically stirred with a PVDF-HFP binder in the weight ratio of 100:11 in acetone at a speed of 350 rpm for 12 hours inside an Ar-filled glove box. The slurry was then cast on an Al foil, prior to be dried in a Buchi oven at 110oC for 2 hours. The foil was then punched into electrodes of ~16 mm in diameter, with typical mass loadings of active material of ~2 mg on each electrode. A Whatman GF/D borosilicate glass fiber separator was chosen, saturated with 1M NaClO4 in propylene carbonate (PC) electrolyte. The coin cells were assembled in an argon-filled glove box, using Na metal as the negative electrode. Galvanostactic charge-discharge tests were carried out at C/50 rate, 1.5-4.0 V, 55oC.

3. Results and Discussions 3.1 The chemical formula of (NH4)xFe(H2O)2[BP2O8]•(1-x)H2O Yilmaz et al. were the first to report on the preparation of “FeIII(H2O)2[BP2O8]•H2O” from FeCl2 and (NH4)2HPO4 precursors. Shortly after, Huang et al.

42

37

, starting

repeated the experiment

and seemingly obtained a kind of ammonium borophosphate, as both of the ICP-AES and FT-IR



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Page 18 of 30

measurements indicated the existence of N-H functional groups in the as prepared material, so that the original reported formula became doubtful. This first indication was further supported by an X-ray diffraction structural analysis conducted on a single crystal, revealing that the “Fe(H2O)2[BP2O8]•H2O” was actually a Fe2+/Fe3+ mixed valence ammonium borophosphate. It was therefore important to determine the precise chemical formula of our as prepared material prior to its electrochemical characterization, through magnetic susceptibility measurements, Mössbauer spectroscopy and single crystal X-Ray diffraction. Figure 1 shows Scanning Electron Microscopy

images

of

(NH4)xFe(H2O)2[BP2O8]•(1-x)H2O

(transparent

grey-purple)

and

NaFe(H2O)2[BP2O8]•H2O (transparent light green) single crystals. They both display bi-pyramidal shapes of ~40 µm in their largest dimension, of hexagonal symmetry. FTIR spectroscopy was used to investigate and characterize the presence of NH4+ groups within (NH4)xFe(H2O)2[BP2O8]•(1-x)H2O at room temperature and after heat treatment at 110°C or 300°C. The three recorded spectra are presented in Figure 2, revealing similar absorption bands, particularly the one at around 1450 cm-1 which corresponds to the

ʋ4 N-H deformation 43,

indicating the presence of NH4+ ions in all three samples. It is noted that even at elevated temperatures (green and blue curve; 110oC is the temperature used to dry the electrode before battery assembling, 300oC is the temperature used to test the reversible dehydration), N-H groups still remain in the structure. The temperature dependence of the magnetic susceptibility of the as prepared [BP2O8]∞ borophosphates is presented in Figure 3 a & b, revealing complete paramagnetic behaviors in agreement with the literature

37, 42

. The data were fitted using a Curie-Weiss law in the whole

temperature range, following the formula: 𝜒 =

𝐶

𝑇−Θ𝑐𝑐

. Negative Curie temperatures of −7.8 K for

(NH4)xFe(H2O)2[BP2O8]•(1-x)H2O and −2.5 K for NaFe(H2O)2[BP2O8]·H2O indicate the presence of weak antiferromagnetic interactions between transition metal atoms. The effective magnetic moment of Fe2+ (d6) ions in NaFe(H2O)2[BP2O8]·H2O was calculated to be 5.49(1) µB, in

extremely good agreement with the theoretical value for Fe2+ ions in a weak field octahedral environment (~5.48 µB). The as calculated effective magnetic moment of 5.49(1) µB reveals that Fe2+ ions are in high spin ground state and probably have large orbital moment contributions. On the other hand, for (NH4)xFe(H2O)2[BP2O8]•(1-x)H2O, the effective magnetic moment of Fe3+ (d5) ions was calculated as 5.68(1) µB, lower than the theoretical value of ~5.92 µB obtained from the


Page 19 of 30

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Fe3+ spin-only

contributions in octahedron environment (Fe3+ d5 is isotropic in such

coordination). Interestingly, the theoretical magnetic moment that Fe2+ (d6) ions carry in a weak field octahedron environment is ~ 5.48 µB, which is smaller than the as found value. Huang et al. 42

pointed out that it was quite unlikely that a pure Fe3+ borophosphate had been obtained

through the method Yilmaz proposed

37

. The as revealed magnetic moment of our sample

(~5.68 µB) is comparable to the report of Huang (~5.69 µB), which indicates that the so called “Fe(H2O)2[BP2O8]·H2O” we prepared is probably a mixed valence ammonium-containing iron borophosphate : (NH4)xFe(H2O)2[BP2O8]•(1-x)H2O. Mössbauer spectroscopy is one of the most sensitive probes to detect slight differences within the Fe environment, appropriately used here to determine the exact chemical composition of the as prepared target “Fe(H2O)2[BP2O8]·H2O” (pure Fe3+ vs. mixed Fe2+ and Fe3+). Figure 4a depicts the asymmetrical-shaped Mössbauer spectrum recorded on the as prepared (NH4)xFe(H2O)2[BP2O8]•(1-x)H2O powder, indicating that multiple Fe environments coexist in the structure. After careful examination of the recorded spectrum, the best fit was obtained by using 4 components (doublets): three kinds of Fe2+ contributions (red, blue and orange) accounting for ~74(1)% (molar ratio) of the total iron environments with an average isomer shift of ~1.21 mm/s (indicative of octahedral sites

44

) and one component for Fe3+ ions (green doublet, accounts for

~26(1)% of total iron) in octahedral site. These 3 Fe2+ doublets differ by their quadrupole splitting (QS) from 2.5 to 1.37 mm/s, indicating rather different iron environments. It is noticed that our previous work

45, 46

revealed similar coexistence of multiple Fe2+ environments in the LiFeBO3

structure, which may shed some light on the origins of those 3 Fe2+ doublets. Hence, (NH4)xFe(H2O)2[BP2O8]•(1-x)H2O contains large amounts of Fe2+ (~74(1)% (molar) of total Fe environments) instead of a pure Fe3+ environment. In agreement with Huang

42

, (NH4)+ ions

partially substitute H2O within the 61 (65) helical channel and therefore lead to charge neutralization for the global formula (NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O (x = 0.75). Similarly,

the

as-prepared


was

characterized

by

Mössbauer

spectroscopy (Figure 4b). Unlike (NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O, this compound is a theoretically pure Fe2+ polyanionic framework, in agreement with the fitting of the Mössbauer spectrum using 2 Fe2+ environments that account for 97% (molar) of total Fe environments. The minor Fe3+ contribution may come from a surface oxidation or from very small amounts of amorphous Fe3+-containing impurities invisible from X-ray diffraction. Details of both fittings are enclosed in Table 1a and Table 1b.



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Page 20 of 30

Rietveld refinements of the (NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O structure were conducted from XRD patterns, starting from the models proposed in literature

37, 42, 47

with NH4+ ions being fully

part of the structure as mentioned above, resulting in satisfactory global agreement factors: RBragg =3.00%, Rf =1.79%, Rwp =0.79%, Rp =1.11% (S.G. P6522; a= 9.4693(1) Å, c= 15.6935(1) Å V= 1218.66(1) Å3) (depicted in Figure 5a, Table 2a and Table S1). Similarly, Rietveld refinements of the NaFe(H2O)2[BP2O8]·H2O structure were conducted from XRD data, starting from the models proposed in literature

37, 38, 42, 47

resulting in good global agreement factors:

RBragg =7.95%, Rf =5.79%, Rwp =1.04%, Rp =1.50% (S.G. P6122; a= 9.4920(1) Å, c= 15.9171(1) Å V= 1241.97(2) Å3) (see Figure 5b and Table 2b ). It is noted that the cell parameters and the unit cell volume of the Na analogue are slightly larger. It is noted as well that the absolute configuration of structures crystallizing in the enantiomeric space groups P6122 and P6522 is quite challenging to determine. To this end, single crystal data collection was undertaken. The refined Flack parameter was close to 0 (0.031(15)) for the P6122 description while in the P6522 space group, it is close to 1 (0.96(3)). Therefore, the P6122 space group was adopted. In the P6522 description, the RBragg value was found to be much larger (13.9 %). As illustrated in Figure 5 , Figure 6 and Figure S2, NaFe(H2O)2[BP2O8]·H2O and (NH4)0.75Fe(H2O)2 [BP2O8]•0.25H2O contain [BP2O8]∞ infinite helices which are wound around a left-handed or a right-handed 6-fold screw axis (61 or 65) built of corner-shared BO4 and PO4 tetrahedra. The BO4 units share all their oxygen atoms with four neighboring PO4 tetrahedra (Figure 7), whereas PO4 groups are only connected with two BO4 units (sharing the other two oxygen atoms with two FeO6 octahedra). In other words, the crystal structure of (NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O is built on [BP2O8]∞ spiral ribbons wound around a 65 screw axis along the [001] direction, connected via FeO6 octahedra. It is noted that all the individual ammonium/water molecules are located inside the helical channel (Figure 8a), while the FeO6 octahedra connect two neighboring [BP2O8]∞ spiral ribbons via O(2) and O(4), leaving two O(5) bonded to hydrogen atoms. The Fe-O distances within the FeO6 octahedra range from 2.06 to 2.26 Å, therefore forming slightly distorted octahedra (Δ = 17.1×10-4). On the other hand, the P-O distances range from 1.49 to 1.51 Å and B-O distances of ~1.46 Å suggest that both phosphorus and boron atoms are in rather regular tetrahedral coordination. Besides, calculated bond valence sums using the Zachariasen formula for Fe, B, P, O(1), O(2), O(3) and O(4) were respectively found as 2.07(1), 3.13(2), 5.45(3)

2.10(2), 2.13(2), 1.78(2) and 1.80(2) in

reasonable agreement with the expected values.


Page 21 of 30

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

The


crystal

structure

of


is

very

similar

to

that

of

II

(NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O. The Fe-O distances within Fe O6 octahedra range from 2.10 to 2.21 Å, while the P-O distances (1.51-1.56 Å) and B-O distances (1.46-1.47 Å) are reasonable for both atoms in tetrahedral coordination (Figure 5b). In this structure, Na+ ions are also forming spiral ribbons around the 61 screw axis providing a possible Na+ diffusion path. Together with the [BP2O8]∞ spiral ribbons, this leads to a double helix configuration (Figure 6b). It is noted that as for the (NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O, the resulting channel accommodates molecular H2O (Figure 8b). 3.2 Electrochemical Properties of the as Prepared Borophosphates (Na+ and Li+ insertion) As mentioned in the crystal structure section of the as prepared borophosphates, the [BP2O8]∞ based family of borophosphates possess a 61 (65) helical channel along the c-axis allowing a possible diffusion path of alkali ions (Na+ and Li+) and individual H2O molecules or mixed ammonium/water molecules (Figure 8a and b). This rarely-seen structure may give rise to interesting electrochemical properties, although this family of compounds, to the best of our knowledge, has never been reported to be electrochemically reactive vs. Li+ or Na+.

As illustrated in Figure 9a, NaFe(H2O)2[BP2O8]·H2O shows high electrochemical relativity vs. Na in the first three cycles (~0.8Na+ removal per formula unit upon reduction). At the 2nd cycle, it could deliver a specific capacity of ~66 mAh/g (0.8 Na+) at 55oC from 4.0 V to 2.0 V vs. Na+/Na. The derivative curve of the 2nd cycle (Figure 9b) reveals that the Fe3+/Fe2+ redox couple of NaFe(H2O)2[BP2O8]·H2O is located at ~ 2.9 V vs. Na+/Na. Unfortunately, the delivered capacity drops drastically for the following cycles. At the 10th cycle, NaFe(H2O)2[BP2O8]·H2O is almost electrochemically inactive vs. Na. The fast decay of the delivered capacities may result from side reactions upon oxidation and/or degradation with the electrolyte. As indicated by Figure 8a, the 1st charge (Na+ ions removal from the cathode) is almost 3 times longer than that of the 1st discharge. This highly irreversible side reaction may cause the electrolyte degradation which is detrimental for battery cycling. In fact, the electrolytes for Na-ion battery are still under optimizations 48, and NaClO4/PC may not be the best choice for NaFe(H2O)2[BP2O8]·H2O. When (NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O is used as a Na battery cathode (started firstly from discharge), it demonstrates high electrochemical reactivity vs. Na at 55oC. The pristine material contains both Fe2+ and Fe3+ valence states and therefore (NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O can



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Page 22 of 30

accommodate ~0.75 sodium ions per formula unit during the first discharge until 1.5 V vs. Na+/Na. In the following discharge (Figure 10a), it can reversible deliver a specific capacity of 80 mAh/g (~0.9 Na+ insertion per formula unit) from 4.0 V to 1.5 V vs. Na+/Na with a discharge plateau (the same as NaFe(H2O)2[BP2O8]• H2O) located at ~2.9 V vs. Na+/Na (Figure 10b), which is 0.2 V lower than the operating voltage of the same material assembled in Li batteries. When the cells were charged / discharged up to 40 cycles, the retained capacity gradually faded to 60% of the 2nd discharge (~50 mAh/g). One reason of the poor capacity retention may refer to the electrolyte issue

48

mentioned previously. Additional experiments were conducted using

NaPF6-based electrolytes with similar outputs. In situ X-Ray diffraction experiments during charge (extraction of ~1.10 Na+ per formula unit, including irreversible side reactions) and subsequent discharge (0.70 Na+ per formula unit) of (NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O at room temperature were conducted and are displayed in Figure 11. The in situ experiments suggest

that the sodium

ions

insertion into

(NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O occurs via a solid solution reaction. As illustrated in Figure 12 and Figure S1, when the Na/(NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O

cell was being charged, a

gradual shift in the position of the peaks towards higher 2θ angles was identified, as the cell parameters a and c decreased from 9.513(1) Å and 15.795(3) Å to 9.385(1) Å and 15.600(2) Å respectively (corresponding to a unit cell volume contraction of ~ 3.9%). Similarly on discharge, a gradual shift in the position of the peaks towards lower 2θ angles was observed with the cell parameters a and c back to 9.497(1) Å and 15.820(3) Å (corresponding to a unit cell volume expansion of ~ 3.7%).

4. Conclusions We have successfully prepared high-purity NaFe(H2O)2[BP2O8]•H2O and (NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O. The so called FeIII(H2O)2[BP2O8]•H2O is in fact an ammonium mixed FeII/FeIII borophosphate, as verified by magnetic susceptibility, IR, Mössbauer and X-ray diffraction probes. The crystal structures of both borophosphates are built on the [BP2O8]∞ spiral ribbons wound around a 65 (61) screw axis along the [001] direction, connected via MO6 octahedrons. These “unique” structures gave rise to interesting electrochemical properties (although limited in terms

of

theoretical

gravimetric

capacity)

vs.

sodium

ions,

particularly

for

(NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O. It can reversibly deliver a specific capacity of ~ 80 mAh/g (0.9 Na+ insertion per formula unit) at 55oC between 4.0 V and 1.5V with a discharge plateau


Page 23 of 30

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located at ~2.9 V vs. Na+/Na, through a solid solution mechanism of Na+ insertion / extraction into the material. Further investigations, particularly on the electrolyte side, are highly recommended.

During the course of our study, Ceder et al.

49, 50

reported on a new family of polyanionic

frameworks: the carbonophosphates with general chemical composition AxM(CO3)(PO4) (A = Li+/Na+; M = Mg, Mn, Fe, Co, Ni, Cu). Among them, Li3Fe(CO3)(PO4) demonstrates interesting electrochemical activity. These discoveries together with our work on borophosphates may pave the way for “mixed” polyanionic frameworks to be used as possible Li / Na battery cathodes or as ionic conductors.

Acknowledgements We acknowledge the Centre National de la Recherche Scientifique (CNRS) and UMICORE (Belgium) for the PhD Grant of LT. We also appreciate fruitful discussions with Dr. Robert Dominko from National Institute of Chemistry, Ljubljana, Slovenia. GR acknowledges the low temperature platform from UPMC for easy access and help with magnetization measurements.

Supporting Information Paragraph Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org



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Page 24 of 30

Figure Captions Figure 1: SEM image of borophosphates single crystals prepared through hydrothermal method with a hexagonal bi-pyramidal shape: (a) “FeIII(H2O)2[BP2O8]·H2O” ((NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O), (b) NaFeII(H2O)2[BP2O8]·H2O. Figure 2: FT-IR spectra of “Fe(H2O)2[BP2O8]·H2O” (NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O), pristine sample (red curve), sample heated at 110oC for three hours (green curve) and sample heated at 300oC for 10 hours (blue curve); the positions of the ʋ4 N-H deformation vibrations are marked with a shadowed bar. Figure 3: Temperature dependence of the magnetic susceptibility (χ) of borophosphates, (a) “FeIII(H2O)2[BP2O8]·H2O” ‘(NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O), (b) NaFeII(H2O)2[BP2O8]·H2O, measured under field-cooled (FC) conditions with a field of 10 kOe between 350 and 2 K. A Curie-Weiss fit was performed as shown as inset I (1/χ vs. T). Magnetization (M) is plot as a function of the magnetic field (H) in the inset as well. Figure 4: Mossbauer Spectra recorded at 293K. The spectrum of (a) “FeIII(H2O)2[BP2O8]·H2O” ((NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O) is fitted with three Fe2+ environments and one Fe3+ environment, while the spectrum of (b) NaFeII(H2O)2[BP2O8]·H2O is fitted with two Fe2+ and one Fe3+ environments. Figure 5: (a) Rietveld refinement of (NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O (CuKα) Rbragg=3.0% Rf=1.79% Rp=0.79% Rwp=1.11%. The black dots correspond to observed data, red line is the calculated fit and the blue line is the difference between calculation and observation. The vertical green sticks represent the expected positions of Bragg peaks (first row: (NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O, second row: minor impurity (~1.0 wt%) of Fe7(PO4)2(HPO4)4.) (b) Rietveld refinement of NaFe(H2O)2[BP2O8]·0.25H2O (CuKα) Rbragg=7.95% Rf=5.79% Rp=1.04% Rwp=1.50%. The black dots correspond to observed data, red line is the calculated fit and the blue line is the difference between calculation and observation. The vertical green sticks represent the expected positions of Bragg peaks. Figure 6: Anionic partial structures ([BP2O8]∞) of (a) (NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O and (b) NaFe(H2O)2[BP2O8]·H2O built of four-membered rings of tetrahedra in which BO4 (in green) and PO4 (in purple) alternate and forming a spiral ribbon (61 or 65 helix) Figure 7: (NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O: the local environment of the FeO6 octahedron and M-O distances within FeO6, PO4 and BO4 polyhedra. Figure 8: View along the 61 and 65 screw axis of a) (NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O and b) NaFe(H2O)2[BP2O8]·H2O. Mixed Ammonium/water (grey/red spheres) and water molecules (red ellipsoids) are located inside the channels formed by the [BP2O8]∞ spiral ribbons. Note that hydrogen atoms are not shown here. Figure 9: Electrochemical properties of of NaFe(H2O)2[BP2O8]·H2O prepared via hydrothermal synthesis, cycled at 55oC vs. Na+/Na (assembled in coin type cell; a); derivative curve (b).


Page 25 of 30

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


Figure 10: (a) Electrochemical responses of (NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O prepared via hydrothermal synthesis, cycled at 55oC vs. Na+/Na (assembled in coin type cell); capacity retention up to cycle #40 (inset); (b) derivative curve of the second cycle. Figure 11: In situ XRD patterns recorded during battery operation with diffractometer using the CuKα radiation. The cell was cycled at C/50 rate for the first charge and the second discharge while XRD patterns were collected every 2 hours for a 10o-40o 2θ range. Figure 12: Selected 2θ regions of XRD patterns showing the respective highly reversible shifts of the reflection peaks. (NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O at 1.5 V (red curve) and (NH4)0.75Fe(H2O)2[BP2O8]·0. 25H2O at 4.0 V (blue curve)



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 26 of 30

Table 1.a Mössbauer fitting parameters of “Fe(H2O)2[BP2O8]·H2O” (NH4)0.75Fe(H2O)2[BP2O8]·0.25H2O) at 293 K. IS, QS and LW are the isomer shift, the quadrupole splitting and the linewidth respectively. IS is quoted with respect to a-Fe standard at room temperature.

Site Population(%)

IS (mm/s)

QS (mm/s)

LW (mm/s)

Fe2+a

1.16(1)

2.50(2)

0.32(2)

13(3)

Fe2+b

1.22(1)

1.37(2)

0.32(2)

28(2)

Fe2+c

1.23(1)

1.74(2)

0.32(2)

33(1)

Fe3+

0.44(1)

0.54(1)

0.34(2)

26(2)

Table 1.b Mössbauer fitting parameters of NaFe(H2O)2[BP2O8]·H2O at 293 K IS (mm/s)

QS (mm/s)

LW (mm/s)

Fe2+a

1.19(2)

2.05(2)

0.32(2)

74(2)

Fe2+b

1.23(3)

1.97(2)

0.31(2)

23(2)

Fe2+

0.42(1)

0.53(3)

0.34(2)


Site Population(%)

3(1)

Page 27 of 30

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


Table 2a Crystallographic data and global agreement factors of (NH4)0.75Fe(H2O)2[BP2O8]•0.25H2O derived from Rietveld refinements on room temperature XRD pattern (CuKα). Cell Parameters

S.O.F.

x

y

Z

Biso(Å2)*

Fe

Wyckoff site 6b

1.0

0.5535(1)

0.1071(1)

¼

1.01

B

6b

1.0

0.8493(6)

0.6986(6)

¼

0.91

P

12c

1.0

0.3835(2)

0.1672(2)

0.4140(1)

0.83

c = 15.6935(1)Å

O1

12c

1.0

0.4067(4)

0.1779(4)

0.5096(2)

1.08

V = 1218.66(1) Å3

O2

12c

1.0

0.2145(4)

0.0232(5)

0.4027(2)

1.21

O3

12c

1.0

0.5116(4)

0.1372(4)

0.3754(2)

1.42

O4

12c

1.0

0.3867(4)

0.3163(4)

0.3816(2)

1.40

O5

12c

1.0

0.2956(4)

0.4847(4)

0.1203(2)

2.09

O6

6b

0.25

0.0898(9)

0.1795(5)

¼

8.58

N

6b

0.75

0.0898(9)

0.1795(5)

¼

8.58

P6522 a = 9.4693(1)Å

Atom

RBragg=3.00% Rf=1.79% Rp=0.79% Rwp=1.11% χ2=3.23

*the thermal motion of the atoms are directly taken from 37 and 42. Table 2b Crystallographic data and global agreement factors of NaFe(H2O)2[BP2O8]•H2O derived from Rietveld refinements on room temperature XRD pattern (CuKα LRCS France). Cell Parameters

Atom

Wyckoff site

S.O.F.

x

y

Z

Biso(Å2)*

Fe

6b

1.0

0.5493(2)

0.0987(3)

¼

1.06

P6122

B

6b

1.0

0.8373(11)

0.6747(23)

¼

0.89

a = 9.4920(1)Å

P

12c

1.0

0.3881(2)

0.1633(2)

0.4129(1)

1.45

c = 15.9171(1)Å

O1

12c

1.0

0.4244(5)

0.1838(4)

0.5129(2)

1.59

V = 1241.97(2) Å3

O2

12c

1.0

0.2066(6)

0.0140(4)

0.4023(3)

1.83

O3

12c

1.0

0.6193(4)

0.1401(4)

0.1173(2)

1.09

O4

12c

1.0

0.3775(5)

0.3074(5)

0.3793(2)

2.37

O5

12c

1.0

0.2984(4)

0.4957(4)

0.1079(2)

2.34

O6

6a

1.0

0.1217(8)

0

0

6.48

Na

6b

1.0

0.1888(5)

0.3777(9)

¼

3.10

RBragg=7.95% Rf=5.79% Rp=1.04% Rwp=1.50% χ2=5.05 *the thermal motion of the atoms are directly taken from 38 and referred to our single crystal X-ray diffraction results.



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Page 28 of 30

References 1. Wadia, C.; Albertus, P.; Srinivasan, V., Resource Constraints on the Battery Energy Storage Potential for Grid and Transportation Applications. J. Power Sources 2011, 196, 1593-1598. 2. Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; Carretero-Gonzalez, J.; Rojo, T., Na-Ion Batteries, Recent Advances and Present Challenges to Become Low Cost Energy Storage Systems. Energ. Environ. Sci. 2012, 5, 5884-5901. 3. Braconnier, J.-J.; Delmas, C.; Fouassier, C.; Hagenmuller, P., Comportement Electrochimique Des Phases Naxcoo2. Mater. Res. Bull. 1980, 15, 1797-1804. 4. Kikkawa, S.; Miyazaki, S.; Koizumi, M., Electrochemical Aspects of the Deintercalation of Layered Amo2 Compounds. J. Power Sources 1985, 14, 231-234. 5. Tarascon, J. M.; Hull, G. W., Sodium Intercalation into the Layer Oxides Naxmo2o4. Solid State Ionics 1986, 22, 85-96. 6. Fouassier, C.; Matejka, G.; Reau, J.-M.; Hagenmuller, P., Sur De Nouveaux Bronzes Oxygénés De Formule Naxcoo2 (0

Fe2+ Mixed Valence ... - ACS Publications

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