New Amorphous Iron-Based Oxyfluorides as Cathode Materials for

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C: Energy Conversion and Storage; Energy and Charge Transport

New Amorphous Iron-Based Oxyfluorides as Cathode Materials for High-Capacity Lithium-Ion Batteries Kévin Lemoine, Leiting Zhang, Jean-Marc Grenèche, Annie Hémon-Ribaud, Marc Leblanc, amandine guiet, Cyrille Galven, Jean-Marie Tarascon, Vincent Maisonneuve, and Jerome Lhoste J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06055 • Publication Date (Web): 07 Aug 2019 Downloaded from pubs.acs.org on August 7, 2019

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The Journal of Physical Chemistry

New Amorphous Iron-based Oxyfluorides As Cathode Materials For High-capacity Lithium-ion Batteries Kévin Lemoine,† Leiting Zhang, ‡ Jean-Marc Grenèche,† Annie Hémon-Ribaud,† Marc Leblanc,† Amandine Guiet, † Cyrille Galven, † Jean-Marie Tarascon,‡, # Vincent Maisonneuve,† Jérôme Lhoste*,†

†

Institut des Molécules et Matériaux du Mans (IMMM) - UMR 6283 CNRS - Le Mans

Université, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France

‡

Collège de France, Chaire de Chimie du Solide et de l’Energie, UMR 8260 CNRS, 11 Place

Marcelin Berthelot, 75231 Paris, France

#

Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459, 80039

Amiens, France

ACS Paragon Plus Environment

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ABSTRACT: A novel two steps strategy to prepare amorphous oxyfluorides, containing both

divalent and trivalent 3d-metals, as cathode materials for lithium batteries is presented. The first step involves the preparation of hydrated fluorides M2+M3+2F8(H2O)2 (M2+ = Mn, Fe, Co, Ni, Cu; M3+ = V, Fe) by microwave-assisted solvothermal synthesis. Besides the

MnFe2F8(H2O)2 and CuFe2F8(H2O)2 phases, three new compounds Fe1.3V1.7F8(H2O)2, CoFe2F8(H2O)2 and NiFe2F8(H2O)2 which are isostructural with Fe3F8(H2O)2 have been unraveled. The second step consists in the decomposition of M2+M3+2F8(H2O)2 into amorphous oxyfluorides M2+M3+2F8-2xOx via suitable thermal treatments.. The amorphous materials show a greater electrochemical activity towards Li than their parent phases with

among them CuFe2F6O displaying the best performance as a cathode material with a first discharge capacity of 310 mAh.g-1. We show that such a large capacity results from

cumulative insertion and displacement reactions.


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1. Introduction With the sales boom in the electric vehicles coming soon and the digital nomads requiring

powerful electronic devices, researchers are looking for new active electrode materials to

push the lithium-ion batteries (LIBs) to the next level of performance for both capacity and sustainability-wise while enhancing safety.1,2,3 The first commercialized Li-ion battery, back to

1991, was consisting of LiCoO2 and carbonaceous materials as the positive and negative electrode, respectively. The practical utilization of LiCoO2 was for long limited to 50% of its theoretical capacity (290 mAh/g) because of a structural collapsing when attempting to tap a

greater capacity. To alleviate this limitation, chemists have partially replaced Co by Ni and Mn

or even Al so as to obtain capacities of , 200 mAh.g-1 for the layered transition metal oxides LiNi0.8Co0.15Al0.05O2 (NCA) and LiNixMnyCozO2 (NMC with x+y+z=1). Lastly, further substitution of the 3d metal by Li within the MO2 layers has led to the Li-rich layered oxide cathodes (e.g. Li1.2Ni0.13Mn0.54Co0.13O2) denoted Li-rich NMC which exhibit capacities exceeding 300 mAh.g-1 due to cumulative cationic and anionic redox processes.4 Safety-wise, the Li Metal-Polymer technology

(LMP) which operates at 70°C and uses a non-flammable organic solid polymer electrolyte is

also attractive, but suffers from limited energy density owing to the use of a positive electrode


4

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(LiFePO4) having both limited capacity (160 mAh.g-1) and moderate voltage (3.45 V vs. Li+/Li°).5,6,7

To increase the insertion redox voltage, fluoride based materials are a valuable alternative because of the high electronegativity of fluorine leading to high potentiels.8,9

Moreover, iron-based fluorides offer positive attributes owing to their environmental friendliness and the abundance of Fe in the earth crust.10,11,12 Therefore, a drawback with “d-

metal based fluorides is nested in their limited electronically conductivity which must be

compensated by the use of carbon as an electrode additive, but to the expense of a lower electrode capacity.13 Nevertheless, under such conditions, capacities of nearly 150 mAh.g-1

are obtained for HTB-FeF3 0.33H2O/C nanocomposite (Hexagonal Tungsten Bronze).14 Another strategy to reduce the band gap (e.g, increase electronic conductivity) is to replace Fions by O2- or OH- ions leading to oxy/hydroxy/fluorides. For example, Duttine et al.

demonstrated that the lacunar oxyfluoride, FeF2.2O0.4

0.4

with HTB structure, synthesized from

the decomposition of FeF2.2(OH)0.8 0.33H2O, exhibits an enhanced electrochemical capacity with the increase of the O2-/F- anion ratio.15,16,17 Likewise, our previous work on iron

hydroxyfluorides prepared from the thermal decomposition of hydrated fluorides showed


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sustained capacities even after several cycles.18 Lastly, with the regain interest of F-based

electrodes, numerous oxyfluorides were considered as a midway approach to diminish the

band gap and slightly increasing the redox voltage while also offering the feasibility to reach high capacities (885 mAh.g-1 FeOF) via conversion reaction.19,20

New synthesis methods were developed to prepare fluorinated materials: they enlist either the heat treatment of FeSiF6.6H2O to obtain the non-stoichiometric oxyfluoride FeO0.7F1.3,19,21,22 , the alcoholysis of FeOF from FeF3.3H2O or FeF2.4H2O to obtain the rutile form of FeOF

23,24,25

or the high energy ball milling of LiF together with FeO to prepare a new

cubic variety of FeOF.26 Another interesting alternative route is the reaction of high-voltage

charged MnO with LiPF6 electrolyte to generate a Mn-O-F phase with a highly disordered Orich cubic-spinel-like core and a F-rich amorphous shell displaying great performances as cathode materials in LiBs.27,28 More recently, Fan et al. reported a Co0.1Fe0.9OF electrode, made from a concerted doping of cobalt and oxygen into iron fluoride, that shows a sustained reversible capacity of 350 mAh.g-1 at high rate, hence stressing further the importance of

having the proper 3d-metal.


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Beside composition, the crystalline character of an insetion compound is known to

affect its electrochemical performance. Along that line it is worh recalling that excellent

performances were obtained with amorphous FeOF nanococoons prepared by a plasma processing approach.31 Equally, the long studied V2O5 and FePO4 cathode materials were shown to display enhanced performances in their amorphous state,32,33,34 hence we decided

to further explore the morphology-electrochemical relationship in 3d-metal oxyfluorides. Toward this goal and as continuation of our previous study on Fe3F8(H2O)2,18 we herein report the synthesis of poorly-crystallized oxyfluorides via the thermal decomposition of the hydrated

fluorides CuFe2F8(H2O)2, MnFe2F8(H2O)2, CoFe2F8(H2O)2, NiFe2F8(H2O)2 and FeV2F8(H2O)2 rapidly obtained under mild hydrothermal conditions using a microwave oven at 160°C for 30

min. Crystalline structures and chemical compositions were investigated by X-ray diffraction

and

57Fe

Mössbauer spectrometry. Their thermal behavior was monitored by X-ray

thermodiffraction as well as thermal gravimetric analyses (TGA) coupled with mass

spectrometry. Surprisingly, the thermal decompositions led to amorphous oxyfluorides whose

formulations and morphologies were determined from TGA and electron microscopy (SEM

and TEM) measurements, respectively. Finally, the electrochemical lithium insertion


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properties demonstrate that all amorphous materials show better performances than pristine

crystallized fluorides with the Cu-based phase being the stellar as it displays a capacity of 310 mAh.g-1.

2. Methodology 2.1 Experimental Methods The hydrated fluorides M2+M3+2F8(H2O)2 (M2+ = Mn, Fe, Co, Ni, Cu; M3+ = Fe, V) were obtained by solvothermal reaction using a MARS-5 microwave Digestion System (CEM Corp.)

starting from chloride precursors (Alfa Aesar), 9.45 mL of absolute methanol (MeOH, 233 mmol, 24.7 mol.L-1, 99.8%, Sigma Aldrich) and 0.55 mL hydrofluoric acid solution (15 mmol, 27.6 mol.L-1, Riedel De Haen). The concentration [M2+] + [M3+] was fixed equal to 0.1 mol.L-1 (nM2+ = 0.333 mmol, nM3+ = 0.667 mmol) and the M2+/M3+/HF/MeOH ratio equal to 1/2/44/699. The mixtures were placed in Teflon autoclaves and heated at 160°C for 30 min with stirring.

After cooling, the solid products are filtered, washed with 2 mL of ethanol and dried in a

furnace under air at 100°C overnight. It must be noted that the synthesis of Fe2+Fe3+0.3V1.7F8(H2O)2 from FeCl2 4H2O and VCl3 leads to a partial oxidation of Fe2+ into Fe3+ ions due to the presence of water. NiFe2F8(H2O)2 and others M2+M3+2F8(H2O)2 samples were


8

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heated under ambient air during 1 h at 340 and 320°C (heating/cooling rate of 2°C/min) respectively, giving amorphous oxyfluorides with M2+M3+2F8-2xOx formulations (Table 1) which were established from thermal analyses (see the following section).

Table 1. Characteristics of M2+M3+2F8(H2O)2 and M2+M3+2F8-2xOx M2+M3+2F8(H2O)2 (crystallized pristine phase) Formulation

*

*Precusors

M2+M3+2F8-2xOx (amorphous phase)

Notation

Color

Fe1.3V1.7F8(H2O)2

FeV2

green

FeCl2 4H2O/VCl3

MnFe2F8(H2O)2

MnFe2

white

MnCl2/FeCl3

MnFe2F5.8O1.1

brown

CoFe2F8(H2O)2

CoFe2

Pink

CoCl2/FeCl3

CoFe2F6.6O0.7

brown

NiFe2F8(H2O)2

NiFe2

green

NiCl2/FeCl3

NiFe2F4.4O1.8

brown

CuFe2F8(H2O)2

CuFe2

Grey

CuCl2/FeCl3

CuFe2F6O

brown

M2+/M3+

Formulation **

-Fe2O3

Color black

The starting reactants (except FeCl2.4H2O) must be handled in the glovebox because of the moisture sensitivity

** During

the thermal treatment of Fe1.3V1.7F8(H2O)2 , the sublimation of VF3 above 300°C and hydrolysis leaves -

Fe2O3

After thermal treatment, all phases are immediately outgassed at ambient temperature under secondary vacuum (P < 10-5 bar) and stored in a glovebox. It must be noted that all sample

preparations for Mössbauer spectrometry, electrochemical characterizations were realized in

a glovebox.

2.2 Characterization Methods 2.2.1 Structural and Chemical Characterizations


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X-ray diffraction patterns were collected in the range 10° 4 25 4 150° on a Panalytical MPD-

PRO diffractometer equipped with a linear X’celerator detector with CuK6 (1.5406 Å) anode or

CoK6 (1.7890 Å) anode that avoids the X-ray fluorescence. Rietveld refinements were performed by using the Fullprof profile refinement program.38,39

Mössbauer measurements were performed in transmission geometry with a 925 MBq -

source of

57Co/Rh

mounted on a conventional constant acceleration drive. The samples with

5 mg Fe.cm-2 were prepared from a softly milled powder. Data were fitted using the MOSFIT program40 involving quadrupolar and/or magnetic components with lorentzian lines; the

isomer shift values are referred to that of

-Fe at RT. The velocity of the source was

calibrated using -Fe as the standard at RT.

SEM images of the powders were obtained using a JEOL JSM 6510 LV microscope with

acceleration voltages between 20 and 30 kV as a function of the analyzed samples.

Elementary quantitative microanalyses were performed using an Energy Dispersive X-ray

(EDX) OXFORD detector with Aztec software.

The TEM study was performed on a JEOL JEM 2100 HR electron microscope operating at

200 kV and equipped with a side entry ± 35° double-tilt specimen holder. The samples for


10

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transmission electron microscopy investigation were prepared by ultrasonic dispersion of the

raw powder in ethanol, deposition of a drop of the resulting suspension onto a holey carbon-

coated copper grid and finally air drying.

2.2.2 Thermal Analyses Mass Spectroscopy coupled Thermo Gravimetric Analysis- (TGA-MS) was performed using a

Netzch STA 449 F3 coupled with a QMS 403 C mass spectrometer. The thermoanalytical

curves were recorded together with the ion current curves in the multiple ions detection probe. A constant purge nitrogen gas flow of 80 mL.min-1 and a constant heating rate of 5°C.min-1

were applied.

The thermogravimetric (TGA) experiments were carried out with a thermoanalyzer SETARAM

TGA 92 with a heating rate of 5°C.min-1 from room temperature up to 900°C under dry air (Alphagaz, mixture of oxygen (20%) with nitrogen (80%), H2O < 3 ppm).

X-ray thermodiffraction (HT-XRD) was performed under dry air in an Anton Parr XRK 900

high temperature furnace with the diffractometer already described. The samples were heated from 40 to 600°C at a heating rate of 10°C.min-1. X-ray diffraction patterns were recorded in


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the 5-60° 2 theta range with a scan time of 10 min at 20°C intervals from room temperature to

400 and at 100°C intervals from 400 to 600°C.

2.2.3 Electrochemical Characterizations Electrochemical reaction with lithium was evaluated using a Swagelok Li metal cell. Electrodes were composed of 80% of active materials and 20% of carbon-SP (super pure) as the conductive agent; they were ball-milled using a SPEX Miller 800 at 875 cycles/min for 15 minutes. The area of the electrode was 1 cm2 with a loading mass of 8 mg. The electrolyte was the commercially LP30 (1M LiPF6). The cells were cycled between 2 and 4 V versus Li+/Li, current density was 10 mA.g-1.

3. Results and Discussion 3.1 Hydrated Fluorides Synthesis and characterization of pristine M2+M3+2F8(H2O)2

The synthesis of M2+Fe2F8(H2O)2 (M2+= Mn, Cu) under supercritical hydrothermal conditions developed during the 90’s was revisited thanks to a mild solvothermal medium assited by

microwave heating. This new straightforward synthetic approach for the preparation of

hydrated fluorides was extended to three new compounds: Fe1.3V1.7F8(H2O)2, CoFe2F8(H2O)2 and NiFe2F8(H2O)2. The X-ray powder diffraction patterns of these new phases, isostructural to Fe3F8(H2O)2, are indexed in the monoclinic system with the C2/m space group (Figure 1a


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and Figure 2a). Copper Cu2+ Jahn-Teller effect implies that CuFe2F8(H2O)2 is indexed in the monoclinic system with the C2/c space group (Figure 1b).37

(a)

(b)

C2/m

C2/c

[Fe3+F6]

[Fe3+F6]

[M2+2+FF44(H (H22O) O)22]] [Fe

HTB layer

distorded HTB layer

2+ Fe [Cu F4(H2O)2]

Fe

Figure 1. [100] projections of Fe3F8(H2O)2 (a) and CuFe2F8(H2O)2 (b). Rietveld refinements were performed and good fits were obtained for all phases (Figure S1).

Pure phases were obtained for all compositions except for CoFe2F8(H2O)2 which presents very a small amount of Co2+Fe3+F5·7H2O impurity. Substitutions occur at both M2+ and M3+ sites for Fe1.3V1.7F8(H2O)2 and NiFe2F8(H2O)2; the site occupancies are introduced taking into account the results of

57Fe

Mossbauer spectrometry. The details of the structure

determinations are given together with the X-ray atomic coordinates, interatomic distances

and bond valence calculations in Tables S1, S2 and S3. The unit cell volume variation is

consistent with the sum r of ionic radii: r = rM2+ + rM3+ (Figure 2b).


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(a)

380

CuFe2

Unit cell volume (Å3)

C2/c

(b) NiFe2 CoFe2

C2/m

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

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Fe1.3V1.7 Fe3 MnFe2

MnFe2

375

Fe1.3V1.7

370

Fe3

CuFe2 CoFe2

365

NiFe2 360 1.96 1.98 2.00 2.02 2.04 2.06 2.08 2.10 2.12 2.14

10

20

30

40

50

60

2

r (Å)

Figure 2. XRD patterns of M2+M3+2F8(H2O)2 phases at room temperature (a) and variation of the unit cell volumes with r = rM2+ + rM3+ (b).

Mössbauer studies Mössbauer experiments were systematically carried out for M2+M3+2F8(H2O)2 phases at 300 and 77 K. The spectra are compared in Figure 3 and the corresponding refined values of

hyperfine parameters are listed in Table 2. All 300 K Mössbauer spectra exhibit pure

quadrupolar hyperfine structures which can be described by means of one or two components. Indeed, it is concluded from the values of isomer shift to the presence of a priori single high spin Fe3+ species in the Ni, Co and Mn based systems.


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-4

-2

0

V (mm/s)

2

4 -12

-6

0

V (mm/s)

6

12

1.00 0.99

0.96 0.92

0.96

CuFe2

0.88

0.93 1.00

0.84 0.96

0.99

NiFe2

1.00

0.98 0.97

-10

-5

0

5

10

1.00

v

0.96

0.98

CoFe2

0.92

0.96

1.00

1.00

0.96

0.98

MnFe2

0.92 0.88

Relative transmission

0.88

Relative 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 59 60


0.96 0.94

1.00

1.00 0.99

Fe FeV 1.32V1.7

0.99

0.98 0.97

0.98

-4

-2

0

2

V (mm/s)

4 -12

-6

0

6

12

V (mm/s)

Figure 3. 57Fe Mössbauer spectra of M2+M3+2F8(H2O)2 phases at 300 K (left) and 77 K (right). Blue and red lines represent the major and minor Fe3+ components for M2+ = Cu, Ni, Co and Fe3+ with Fe2+ components for Fe1.3V1.7 (see Table 2).

Two quadrupolar components attributed to ferric species are necessary to well describe the

300 K spectrum of the Ni based sample while a mixed valence iron state is observed for the V based sample. At 77 K, except for Ni2+Fe3+2F8(H2O)2 which remains partially paramagnetic, the Fe3+ subnetwork is magnetically ordered but different situations have to be distinguished:

they are discussed in the light of the X-ray study, the thermal experiments and taking into account the values of ionic radii of Co2+, Ni2+, Cu2+ and Fe3+, 0.75 Å, 0.69 Å, 0.73 Å, and 0.65

Å, respectively, favoring or disfavoring cationic inversion. ACS Paragon Plus Environment

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Table 2. Refined values of hyperfine parameters of M2+M3+2F8(H2O)2 phases at 300 K and 77 K mm/s ± 0.02

EQ mm/s ± 0.02

Bhf T ±2

± 0.02

Fe3+

0.46

0.58

-

0.39

Fe2+

-

0.61

Fe3+

0.57

0.40

-

0.39

Fe2+

-

0.61

1 doublet

Fe3+

0.46

0.49

1 sextuplet

Fe3+

0.58

-0.34

51

0.92

1 doublet

Fe3+

0.51

0.67

-

0.08

Fe3+

0.46

0.64

-

0.73

Fe3+

0.46

0.31

-

0.27

1 sextuplet

Fe3+

0.73

1 doublet

Fe3+

0.52

0.54

-

0.27

300 K

1 doublet

Fe3+

0.46

0.52

-

77 K

1 sextuplet

Fe3+

0.58

-0.43

50

1 doublet

Fe3+

0.44

0.61

-

1 sextuplet

Fe3+

0.57

-0.42

57

0.96

1 doublet

Fe3+

0.51

0.67

-

0.04

Compound

$

Fe1.3V1.7F8(H2O)2 300 K

2 doublets

77 K

2 doublets

CoFe2F8(H2O)2 300 K 77 K NiFe2F8(H2O)2 300 K 77 K

2 doublets

MnFe2F8(H2O)2

CuFe2F8(H2O)2 300 K 77 K

, EQ and 2 are given in mm.s-1, Bhf in T

For instance, the Mössbauer spectrum for MnFe2F8(H2O)2 is very well defined and the Zeeman sextet is composed of narrow lorentzian lines suggesting a good crystalline state.

Moreover, the hyperfine data are in perfect agreement with those previously estimated in the case of MnFe2F8(H2O)2 synthesized by hydrothermal route.36


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For NiFe2F8(H2O)2, a significant quadrupolar component (27 %) is observed at the centrum of the 77K Mössbauer spectrum. Consequently, the fitting procedure was revised for

the corresponding 300 K Mössbauer spectrum: a second quadrupolar component was

considered in spite of the lack of resolution of the quadrupolar spectra that does not allow for

an easy description into two components. As all the Bragg peaks of the X-ray pattern of

NiFeF8(H2O)2 are unambiguously attributed, preventing from the presence of any crystalline additional phase, the small second quadrupolar doublet is attributed to a partial cationic

inversion which is perfectly confirmed from X-ray diffraction: better reliabilities of the Rietveld refinement are obtained assuming this Ni2+-Fe3+ cationic inversion (Table S4).

In the case of Co-containing compound, a small quadrupolar component appears at

77K in addition to a sextet with narrow lorentzian lines (see Figure 3) it can be attributed to the small amount of Co2+Fe3+F5·7H2O impurity previously observed from the X-ray pattern.

On the contrary, no impurity is detected from the X-ray pattern of CuFe2F8(H2O)2 sample but a minor quadrupolar feature is detected on the 77K Mössbauer pattern which has

to be attributed to some Fe-based impurity either in amorphous or nanocrystalline form.


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Finally, it is important to note that for Fe2+Fe3+0.3V3+1.7F8(H2O)2, the 77 K spectrum consists of two paramagnetic doublets attributed to both iron valences with a ratio Fe3+/Fe2+ ~ 1.5. Due to

the complexity of amorphous phase spectra, a more thorough and complete Mössbauer study

coupled with analyzes of major instruments such as a pair distribution fonction (PDF) will be

the subject of a future article.

3.2 Amorphous Oxyfluorides

Evidence of amorphous phases In a subsequent step, appropriate thermal treatments under dry air of the hydrated fluorides M2+M3+2F8(H2O) lead to their decompositions into amorphous oxyfluorides formulated M2+M3+2F8-2xOx except for the Fe1.3V1.7 combination. Indeed, HT-XRD under dry air shows that Fe1.3V1.7F8(H2O)2 is stable up to 220°C and decomposes into a crystalline phase between 240 and 280°C with a structure close to that of HTB-FeF3 (Figure 4a). Above 280°C, this last phase transforms into an unknown intermediate phase and finally, hemathite ( -Fe2O3) appears above 400°C. In the residue, no vanadium phase is identified due to the sublimation

of VF3 above 300°C as confirmed by TGA analysis (Figure 4b). As a consequence, no


19


Fen+/V3+ stable oxyfluoride can be obtained from the thermal decomposition of

Fe1.3V1.7F8(H2O)2 under dry air.

(a)

(b) Fe1.3V1.7F8(H2O)2 0

T(°C) -20

TG (wt. %)

600 500 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 80 40

HTB phase

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 20 of 39

1.7 VF3 Exp = 70.4 % Theo = 70.0 %

-40

-60

0.65 Fe2O3

-80

10

20

30

40

50

60

100

200

300

400

500

600

700

800

Temperature (°C)

Figure 4. Thermal evolution under dry air of the X-ray diffractograms (a) and thermogravimetric curve (b) of Fe1.3V1.7F8(H2O)2.

HT-XRD under dry air of the other hydrated fluorides M2+M3+2F8(H2O)2, stable up to 240300°C, show systematically an amorphous domain between two crystallized domains (Figure

5). In the first domain, above 180°C, the diffraction peak positions of the hydrated phases shift

and their intensity decreases due to the elimination of water and/or fluoride species leading to

amorphous phases (Figure S2). In the second domain, amorphous phases are only observed

for MnFe2 and NiFe2 (Figure S3) and a larger region for CoFe2 and CuFe2 (Figure S3 and Figure 5). In the last domain, the amorphous phases crystallize into hematite type M3+2O3 or


20

Page 21 of 39 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


spinel type M2+M3+2O4 structures. It must be noted that the oxidation of Mn2+ in Mn3+ ions occurs during the hydrolysis process.

CuFe2

sample holder *

*

T ( °C) 600 550 500 450 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40

20

30

40

2

50

60

°

Figure 5. Thermal evolution of the X-ray diffractograms under dry air of CuFe2F8(H2O)2.

The amorphous phases are stabilized during the thermal decomposition of M2+M3+2F8(H2O)2 compounds under air for 1 h at 340°C for NiFe2F8(H2O)2 and 320°C for the other compositions. As observed by scanning electron microscopy (SEM), microsized particles (Ø ~

3-5 µm) are obtained for the hydrated fluorides (Figure 6a for CuFe2F8(H2O)2 and Figure S4 for other compositions). After calcination, the decomposition of the crystalline materials into

amorphous phases causes, for all phases, a significant decrease of the particle size down to

a diameter of a few hundred nanometers (Figure 6b for CuFe2F8(H2O)2 and Figure S4 for the ACS Paragon Plus Environment

21



Page 22 of 39

Page 23 of 39 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




higher than expected for a single dehydration process ( 10 wt%). The concomitant loss of

water and HF molecules is then assumed, it is related to an hydrolysis reaction that takes

place simultaneously with the dehydration process without any influence of the nature of the

gas atmosphere. On the basis of the experimental weight losses, we hypothesized the

stabilization of oxyfluorinated compounds according to reaction (1). The resulting formulas are

listed in Table 2. M2+Fe3+2F8(H2O)2

M2+Fe3+2F8-2xOx + 2xHF + (2-x)H2O

(1)

(b) m/z=17 m/z=18 m/z=19

CuFe2F8(H2O)2

TG (wt. %)

0

-5

OH+ -10

exp 16.0% F+

CuFe2F6O

-15

Ion current (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 24 of 39

H2O+ -20 100

200

300

400

500

600

Temperature (°C)

Figure 8. TGA-MS analyses under N2 of CuFe2F8(H2O)2.

Table 3. Determination of the composition of the amorphous phases from reaction (1) Pristine phase

Experimental weight loss

x

Amorphous formula

MnFe2F8(H2O)2

17.2%

1.1

MnFe2F5.8O1.1

CoFe2F8(H2O)2

16.2%

0.7

CoFe2F6.6O0.7


24

Page 25 of 39 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


NiFe2F8(H2O)2

21.0%

1.8

NiFe2F4.4O1.8

CuFe2F8(H2O)2

16.0%

1.0

CuFe2F6O

Furthermore, the synthesis of the amorphous oxyfluorides is independent of the atmosphere

as proved by comparison with the TGA under air (Figure S8). The hydrolysis is completely

achieved above 500°C by small water amounts of the flowing gas. The corresponding spineltype oxides are obtained for Co, Ni and Cu according to reaction (2) and a Mn3+/Fe+3

hematite-type oxide according to reaction (3):

M2+ = Co,Ni,Cu:

Mn2+:

M2+Fe3+2F8-2xOx + (4-x)H2O

Mn2+Fe3+2F5.8O1.1 + 3.4H2O

M2+Fe3+2O4 + (8-2x)HF (2)

3/2Mn3+2/3Fe3+4/3O3 + 5.8HF + 1/2H2

(3)

3.2 Electrochemical Investigations The lithium insertion properties were investigated to examine the impact of the

cationic/anionic nature and of the loss of water molecules. The cells were cycled below 4 V with a discharge cut off limited to 2 V vs Li+/Li (Figure 9). The M2+M3+2F8(H2O)2 materials and the corresponding amorphous phases were tested. Despite a theoretical capacity of

150


25


mAh.g-1, MnFe2, NiFe2, MnFe2 pristine phases reveal poor capacity with

Page 26 of 39

0.1Li+ inserted.

This is most likely related to the presence of structural water molecules blocking the lithium

diffusion into/through the structure. On the other hand, the first discharge of CuFe2F8(H2O)2 leads to a capacity of 110 mAh.g-1 corresponding to 1.5Li+. Even with the presence of structural water molecules, Cu2+ remains electrochemically active at 2.8 V. At the first charge, only 49% of Li+ is extracted giving a capacity of 43 mAh.g-1. Such a behavior was previoulsy

observed in CuF2 with an important reversibility loss

80% after the first discharge of and

consequently, a poor capacity retention.41,42

Cycling of the amorphous MFe2F8-2xOx shows much better performances than pristine materials. MnFe2F5.8O1.1 and CoFe2F6.6O0.7 show nearly alike capacities at the first discharge (86 and 66 mAh.g-1, respectively). NiFe2F4.4O1.8 delivers 101 mAh.g-1 in the first discharge and maintains at 64 mAh.g-1 after ten cycles, demonstrating a 63% capacity retention.

Regarding capacity retention, Mn phase shows the best performance, although far to be spectacular, since it can retain 80% of the reversible capacity (68 mAh.g-1) after 10 cycles

while in terms of voltage hysteresis, it is the CoFe2F6.6O0.7 that shows among all the others the lowest polarization (< 0.5 V). Lastly, the lowest redox voltage for NiFe2F4.4O1.8 as compared to


26

Page 27 of 39

CoFe2F6.6O0.7 and MnFe2F5.8O1.1, is in agreement with its highest oxygen content in this series of amorphous oxyfluorides.

Voltage (V vs Li+/Li)

(a) MnFe2F5.8O1.1 4.0

MnFe2F5.8O1.1

MnFe2F8(H2O)2

3.5

10

3.0

2.0


x (Li)

20

(b)

40

60

80

Capacity (mAh/g)

100

CoFe2F6.6O0.7 4.0 3.5

CoFe2F8(H2O)2

CoFe2F6.6O0.7

10

5–2 - 1

3.0 2.5 2.0 0.1


5–2 - 1

2.5

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.2

0.3

0.4

x (Li)

0.5

0.6

0.7

(c)

10

20

30

40

50

60

70

Capacity (mAh/g) NiFe2F4.4O1.8

4.0 3.5 3.0

NiFe2F8(H2O)2

NiFe2F4.4O1.8

10

5–2 - 1

2.5 2.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

x (Li)


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


(d)

20

40

60

80

100

Capacity (mAh/g) CuFe2F6O

4.0 3.5 3.0

CuFe2F6O

10

5–2 - 1

CuFe2F8(H2O)2

2.5 2.0 0.5

1.0

1.5

2.0

x (Li)

2.5

3.0

3.5

50

100

150

200

250

300

Capacity (mAh/g)


27


Page 28 of 39

Figure 9. First discharge and charge voltammetric curves vs Li+/Li for hydrated fluorides M2+M3+2F8(H2O)2 and after the thermal treatment giving M2+M3+2F8-2xOx phases (left). Cyclic voltammetry curves for amorphous oxyfluorides (right). The current density used is 10 mA.g-1.

The most interesting result is found with CuFe2F6O. While only Cu2+ participates in CuFe2F8(H2O)2, both Fe3+ and Cu2+ contribute to the reduction process in the corresponding amorphous phase. A first plateau at 3.2 V for 2Li+ can be attributed to the redox process of both Fe3+ cations ; it is followed by a second plateau at 2.2 V for 1.5Li+ related to the conversion reaction of Cu2+ in metal copper. It results a total capacity at the first discharge of 310 mAh.g-1 (3.5Li+) for the amorphous CuFe2F6O against 110 mAh.g-1 (1.5Li+) for the corresponding pristine phase. The benefit of such a copper-iron combination was previously observed by Wang et al. with a better electrochemical performance for solid solution CuyFe1yF2

than for pure copper phase CuF2.45 The author obtained low hysteresis for CuyFe1-yF2 by

classic galvanostatic cycling:

0.43 V for Fe3+/Fe2+ and

0.48 V for Cu2+/Cu from the second

cycle. The derivative of capacity (dQ/dV) versus V reveals

0.25 V for Fe3+/Fe2+ and

0.60 V

for Cu2+/Cu in CuFe2F6O (Figure S9). The discrepancy in voltage hysteresis is related to different particle sizes together with various anionic environments for metal cations and the

introduction more covalent O into an insulating F-network enhances the overall conductivity. ACS Paragon Plus Environment

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Among all the investigated phases, amorphous CuFe2F6O displays the highest capacity after 145 mAh.g-1 capacity after ten cycles, hence suggesting the benefits of coupled insertion and

displacement reactions, with the latter be specific to coined cations such as Cu and Ag.

Conclusion

Suitable thermal treatments under air of the metal mixed hydrated fluorides M2+Fe3+2F8(H2O)2 (M = Mn, Co, Ni, Cu) result in the elaboration of four new amorphous oxyfluorides M2+Fe3+2F82xOx.

All pristine fluorides were solvothermally prepared from metal chlorides in HF medium

under microwave heating. Their structures were determined by powder X-ray diffraction and 57Fe

Mössbauer spectrometry. The formulation of the amorphous oxyfluorides were obtained

from thermal analyses (TGA, TGA-MS, HT-DRX and SEM-EDS). TEM study revealed that the

thermal treatment induces an amorphization together with a decrease of the particle size; the

cluster of particles become porous. The electrochemical investigations demonstrate that the

amorphous oxyfluorides present better lithium insertion properties than pristine fluorides. CuFe2F6O shows the best performances with a high first discharge capacity of 310 mAh.g-1 related to intercalation and conversion mechanisms coming from a successive reduction process of Fe3+ into Fe2+ followed by Cu2+ into Cu. This work proves that the oxyfluorides are ACS Paragon Plus Environment

29


Page 30 of 39

good candidates as cathode materials and that the amorphous state combined with mixed

metal is one of the routes to access high-performance active electrode materials for lithium-

ion batteries.


30

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Associated content

Crystal structure data and atomic coordinates, thermal analyses (thermogravimetry and mass

spectrometer), thermal evolution of the X-ray diffractograms under dry air, XRD diffractograms

of CuFe2F8(H2O)2 before and after thermal decomposition under air, SEM images before and after calcination under air of M2+M3+2F8(H2O)2 (M2+ = Mn, Fe, Co, Ni, Cu; M3+ = Fe, V). TEM micrographs at different magnifications with Selected Area Electron Diffraction and EDS-SEM elemental mapping images of amorphous oxyfluorides. Derivative of capacity (dQ/dV) versus

V of CuFe2F6O.

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

Author information Corresponding Author: E-mail: [email protected] (JL)

ORCID numbers Kévin Lemoine (KL): 0000-0003-0401-6416 Leiting Zhang (LZ): 0000-0003-4057-7106 Jean-Marc Grenèche (J-MG): 0000-0001-7309-8633 Annie Hémon-Ribaud (AHR): 0000-0003-4845-5971


31


Page 32 of 39

Marc Leblanc (ML): 0000-0001-7958-0359 Amandine Guiet (AG): 0000-0001-7590-1119 Cyrille Galven (CG): Jean-Marie Tarascon (J-MT): 0000-0002-7059-6845 Vincent Maisonneuve (VM): 0000-0003-0570-953X Jérôme Lhoste (JL): 0000-0002-4570-6459

Author Contributions

KL prepared the samples under the guidance of JL, VM and ML. KL realized and interpreted

thermal experiments under the guidance of JL. KL and AHR collected the XRD diffraction

data which were analyzed by KL, VM and AHR. CR collected ATG-SM data. AG did the

microscopy characterizations. KL performed the electrochemistry characterizations under the

guidance of LZ and J-MT. J-MG collected and analyzed Mössbauer spectroscopy data. KL,

JL and ML wrote the manuscript, which all authors have read and approved.

Acknowledgments


32

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Thanks are due to the French Research Ministry for the doctoral grant of KL. The authors

greatly acknowledge the platforms “Diffusion et Diffraction des Rayons-X” and “Microscopy” of

IMMM.


33


Page 34 of 39

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