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C: Energy Conversion and Storage; Energy and Charge Transport

Playing with the Redox Potentials in Ludwigite Oxyborates: FeBO and CuMBO (M = Fe, Mn and Cr) 3

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Jonas Sottmann, Lucie Nataf, Laura Chaix, Valerie Pralong, and Christine Martin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03734 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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The Journal of Physical Chemistry

Playing with the Redox Potentials in Ludwigite Oxyborates: Fe3BO5 and Cu2MBO5 (M = Fe, Mn and Cr) Jonas Sottmann◊,*, Lucie Nataf※, Laura Chaix‡, Valérie Pralong◊, Christine Martin◊ ◊

※

‡

Normandie Univ, ENSICAEN, UNICAEN, CNRS, CRISMAT, 14000 Caen, France Synchrotron Soleil, Saint-Aubin BP 48, 91192 GIF-SUR-YVETTE CEDEX, France

Laboratoire Léon Brillouin, UMR 12, LLB-Saclay, 91191 GIF-SUR-YVETTE Cedex, France

ABSTRACT Ludwigite oxyborates with general formula M22+M′+3BO5 (where M and M′ are metals) represent an interesting class of conversion-type electrode materials for lithium ion batteries. The homometallic Fe3BO5 shows a first lithiation capacity of 678 mAhg-1 (~6.5 Li). Very low voltage polarizations for conversion-type reactions (300 and 440 mV) are observed for the two reversible redox couples at ~1.3 and ~1.8 V which give rise to a stable capacity of 345 mAhg-1 between 0.75 and 3.0 V vs. Li/Li+. Ex-situ X-ray diffraction and operando X-ray absorption spectroscopy show that Fe3BO5 is almost completely converted to iron metal nanograins embedded in a lithia matrix during the initial lithiation and that subsequent cycling

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takes place between amorphous or nanocrystalline Fe-based phases. In Cu2MBO5 (M = Fe, Mn and Cr) the trivalent transition metals are found to be electrochemically active in addition to copper but at lower voltages causing a large spread in redox potentials. When limiting the voltage range to the Cu2+/Cu0 redox couple (at ~2.4 V) the best performance in terms of voltage polarization and reversible capacity is obtained for Cu2FeBO5. Annealing of Cu2FeBO5 in a reducing atmosphere at low temperatures (~250 °C) is identified as a means to improve the first cycle reversibility.

1. Introduction Lithium ion batteries (LIBs) have conquered the market for portable devices and the growing electric mobility sector. In the perspective of widespread use of clean but intermittent sources of electricity (wind and solar) LIB technology is currently implemented for stationary grid storage (e.g. ENERCON, Feldheim, Germany). Despite the fact that LIB technology is mature, intensive research is devoted to identify new eco-friendly, low cost and safe electrode materials with high energy densities for next generation LIBs. For this reason borates and oxyborates were explored for use in LIBs as cathodes, ionic conductors or anodes. Interesting electrochemical performances were found in this class of LIB materials. Higher specific capacities could be achieved by insertion in LiMBO3 borates (> 210 mAhg-1, with M = Fe, Mn, Co) than in the heavier commercialized cathode material LiFePO4 phosphate (170 mAhg-1). Even if the operation voltage (3.45 V for LiFePO4) is lowered by about 0.4 V by the BO3 for PO4 group replacement as a result of the inductive effect of the polyanions in homometallic compounds 1, the energy density is expected to be higher (586 vs. 660860 Wh kg−1, respectively) 2. Experimentally the theoretical capacity is almost reached by


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controlling particle size and surface poisoning by air for M = Fe, and the operation voltage is increased for M = Mn and Co with limited capacities 3. Specific capacity and operation voltage are optimised by combining another M cation with Fe

4-6

. Li7Mn(BO3)3 is another interesting

cathode host structure, made of MnO4 tetrahedra and BO3 triangles, from which three lithium can be extracted giving rise to a capacity of 280 mAhg-1 and a reversible capacity of 154 mAhg-1 in a voltage range of 4.7–1.7 V 7-8. The pyroborate MgMnB2O5 exhibits high reversible capacities of 250 mAhg-1 (1.4 Li) between 4.7 and 1.5 V when cycled vs. Li/Li+ resulting in an electrochemical ion-exchange of Mg and Li during the first cycle 9. Li3CuB3O7 and Li8Cu7B14O32 were proposed as materials for lithium insertion with possibly high ionic conductivity 10. High ionic conductivity was also expected for Li6CuB4O10

10

. However, a high activation

energy for Li+ migration (Ea = 1.07 eV) as well as a poor electronic/ionic conductivity at room temperature (~10-8 and 10-13 S cm-1, respectively) and a limited stability window (3.5-2.0 V) were observed

11

. On the other hand a high temperature structural form of Li6CuB4O10 shows a

high ionic conductivity (~10-2 S cm-1, Ea = 0.52 eV above 370 °C). Various metal borates and oxyborates are explored as potential anode materials relying on a conversion rather than an insertion mechanism. Iron borates and oxyborates (calcite-type FeBO3 and norbergite-type Fe3BO6 with BO3 triangles and BO4 tetrahedra, respectively) are electrochemically active when cycled between 0.9 V or below and 3.0 V vs. Li/Li+, and specific capacities close to the theoretical ones are reported for the initial lithiation with partial reversibility and good cycling stability

12-16

. FeBO3 and Fe3BO6 follow a direct Fe3+/Fe0

multiphase reduction during the initial lithiation and reversible redox reactions occur between Fe0 and Fe2+/3+ in an amorphous or nanocrystalline matrix upon subsequent cycling operation voltage can be lowered by the V for Fe substitution in FeBO3

17

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. The

and Cr in Fe3BO6 18.


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Warwickite-type mixed valence Mn2BO4 (with BO3 triangles) prepared by hydro-/solvothermal route shows interesting performance in terms of reversible capacity and capacity retention Kotoite-type M3B2O6 (M = Co, Ni, Cu)

20-22

19

.

shows high first lithiation capacities however with

limited reversibility. Indeed, after the initial lithiation, the transition metals are likely found to be dispersed in a lithia matrix (made of Li2O and Li3BO3) and upon subsequent delithiation the metallic copper is only partially reoxidized in Cu+ with a concomitant decomposition of Li2O 20. Recently, bismuth oxyborate Bi4B2O9 was identified as a conversion material with a low voltage polarization of only 300 mV

23

. While the relatively low sustained capacity (140 mAhg-1) at

intermediate voltages (1.7-3.5 V) has to be improved for commercial interest, the low voltage polarization shows some promise for conversion-type metal borates and oxyborates for use in LIB electrodes. The ludwigite Cu2FeBO5 oxyborate retained our attention due to its original behaviour

24

: Its

annealing at moderate temperatures in reducing atmosphere leads to extrusion of copper and formation of a new oxyborate Fe3BO5 accompanied by a release of oxygen. In oxidizing conditions the exchange between Cu and Fe in the ludwigite framework is partially reversible and accompanied by a capture of oxygen. Moreover, upon electrochemical reduction of Cu2FeBO5 in a LIB at room temperature, a conversion reaction was also found during the initial lithiation resulting in a collapse of the crystalline structure and giving rise to a specific capacity of 400 mAhg-1 (4 Li)

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. During subsequent cycling the process is partially reversible with a

capacity of 180 mAhg-1 at C/20 in the voltage range of 1.5–4.7 V vs. Li /Li+ although no recrystallization of Cu2FeBO5 was evidenced. The electrochemical activity was attributed to the Cu2+/Cu0 redox couple, as in this voltage range no electrochemical activity of Fe3BO5 was observed.


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Cu2FeBO5 and Fe3BO5 oxyborates share the ludwigite framework with general formula M22+M′3+BO5 (in which M and M′ may be various cations like Co, Al, Fe, Cu, etc.)

26-27

. The

framework illustrated in Fig. 1 is described by distorted (M/M’)O6 octahedra that share edges and corners to form zigzag walls. These walls delimit triangular tunnels occupied by boron which is in triangular BO3 coordination with short B-O distances (about 1.4 Å).

Figure 1: Illustration of the ludwigite framework corresponding to the general formula M22+M′3+BO5 showing MO6/M’O6 octahedra (coloured polyhedra) and BO3 triangles (orange). The red balls correspond to oxygen atoms. Visualization by VESTA 28. In this work we investigated the electrochemical activity of Fe3BO5 between 0.75 and 3.5 V and studied the reaction mechanism by ex-situ X-ray diffraction (XRD) and operando Fe K edge X-ray absorption near edge spectroscopy (XANES). We further access the iron redox activity in


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Cu2FeBO5 by enlarging the potential window towards lower potentials compared to

25

. Having

identified the active redox couples in Fe3BO5 and Cu2FeBO5 ludwigites, Cu2MnBO5 and Cu2CrBO5 were also characterized to test the influence of the trivalent transition metal on the battery performance, in particular on the Cu2+/Cu0 redox activity. Finally, we studied the impact of

annealing

of

Cu2FeBO5

in

reducing

atmosphere

- yielding

a

composite

of

Cu2FeBO5/Fe3BO5/Cu in micro/nanostructural state - 24 showing improved reversibility and voltage polarization.

2. Experimental Fe3BO5 was synthesized at 950 °C for 48 h in an evacuated quartz ampoule, from a mixture of 0.666 Fe : 1.167 Fe2O3 : 0.5 B2O3 pressed in the shape of bars, in order to stabilize divalent iron. Cu2MBO5 (with M = Fe, Mn or Cr) were prepared in air, starting from a mixture of 2 CuO : 0.5 M2O3 : 0.5 B2O3, heated at 900 °C during 24 h. The quality of the samples was checked at room temperature by XRD using a PANalytical diffractometer, working with Co radiations. The patterns are all characteristic of well crystallized samples belonging to the ludwigite class of materials. For the electrochemical characterization Swagelok cells were assembled in an argon filled glove box (M. Braun) with H2O and O2 levels less than 0.1 ppm. The working electrode composition was 70 wt% of active material (Fe3BO5 or Cu2MBO5), 20 wt% of conductive carbon black (acetylene black) and 10 wt% polytetrafluoroethylene (PTFE) binder. The mixture was rolled into a thin film with a mass loading of active material of about 5 mg cm-2. The working electrode was separated from the lithium foil as counter electrode by electrolyte soaked glass fibre. The electrolyte was LiPF6 (1 M) dissolved in EC (ethylene carbonate) and EMC


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(ethyl methyl carbonate) with a volume ratio of 3:7. The electrochemical studies were carried out using a VMP II potentiostat/galvanostat (Biologic SA). The cells were left at open circuit for at least 2 h to make sure the electrolyte was fully soaked into the electrode before galvanostatic cycling at a current rate of C/20 (30 mAg-1, if not otherwise specified) or cyclic voltammetry (CV) at a voltage sweep rate of 10 mV/h between 0.75 and 3.5 V. For ex-situ XRD the cycled batteries were disassembled in the glove box where the electrode powder was transferred on a PANalytical Si low background sample holder with Kapton foil to perform the XRD measurement under inert conditions. The ex-situ XRD patterns were background subtracted using a blank measurement of the sample holder with Kapton foil. Operando XANES was performed at ODE beamline, at Synchrotron Soleil

29

. The

electrochemical cycling of Fe3BO5 for the operando characterization was performed in the OsloSNBL cell

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, i.e. a Swagelok-type electrochemical cell with Kapton windows. The first

galvanostatic lithiation was followed operando over ~10 h. Fe K edge XANES was collected every 6 min in transmission mode using a bent crystal as polychromator with a focal area of 25 x 25 µm² at FWHM and an exposure time of 6 sec. The data were collected in an energy range of 7090 to 7280 eV with an energy resolution of 0.5 eV. The energy was calibrated using an Fe foil. FeO, Fe2O3 and Fe foil were used as reference materials. The XANES data were analysed using ATHENA

31

for absorption edge determination, spectrum normalization and

linear combination fitting of the normalized XANES data in an energy range of -25 to 70 eV around the absorption edge.

3. Results and discussion 3.1. Fe3BO5


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Fe3BO5 shows a first lithiation capacity of 678 mAhg-1 (~6.5 Li) and a reversible capacity of 345 mAhg-1 in the voltage range of 0.75-3.0 V vs. Li/Li+ (Fig. 2a). The initial lithiation capacity is close to the theoretical one (728 mAhg-1 or 7 Li) corresponding to a full conversion of Fe2.33+ to Fe0. The reaction is initiated at ~1.1 V for which a plateau ranging over most of the compositional range (x) is observed. This plateau is observed at significantly lower voltage than for Fe3BO6 (~1.4 V) but only slightly lower voltage than for FeBO3 (~1.2 V)

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which may be

related to the presence of different anionic groups, i.e. BO3 triangles and BO4 tetrahedra. All these materials deliver capacities close to the theoretical one during the initial lithiation (720 and 870 mAhg-1 for FeBO3 and Fe3BO6, respectively) 14. In terms of reversible capacity and cycling stability Fe3BO5 is comparable to FeBO3 (~330 mAhg-1), while Fe3BO6 shows higher reversible capacity (~600 mAhg-1) but with lower cycling stability over the first five cycles. The reversible cycling of Fe3BO5 involving three lithium is characteristic of a solid solution type redox process showing two redox processes at 1.28 and 1.79 V, as reported on differential capacity plot (Fig. 2b). The voltage polarization of 330 and 400 mV for the two reversible redox reactions in Fe3BO5 oxyborate is lower compared to other Fe based conversion materials - oxides in particular showing voltage polarizations of more than 500 mV

13-14, 32

. In analogy to kotoite-type M3B2O6

(M = Co, Ni, Cu) reversible cycling is expected to occur in form of a conversion-type reaction between amorphous or nanocrystalline iron based phases in a lithia matrix (made of Li2O and Li3BO3) 20. The BO3 polyanion in Fe3BO5 seems to lower voltage polarization compared to iron oxides 13, 32 as similar values were reported for other borates and oxyborates 14, 20, 23. The reason for this relatively small voltage polarization is not fully understood and may, according to 23, be


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due to different reaction paths for lithiation and delithiation, linked to different mobilities of ionic species (here Li, O, BO3 and Fe).

Figure 2. (a) Galvanostatic charge-discharge curves of Fe3BO5 at C/20 and (b) corresponding differential capacity plot of the first reversible cycle.

To understand the electrochemical process involved in the initial delithiation and reversible cycling, ex-situ XRD analysis was performed to identify the fully reduced and re-oxidized phases. As shown at several stages of the first electrochemical cycle (Fig. 3) the crystalline structure of Fe3BO5 is gradually lost during the initial conversion reaction to form amorphous


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and/or nanocrystalline Fe giving rise to the broad feature at 2 θ ≅ 52 °. The theoretical capacity for a full conversion reaction (7 Li) is not achieved (inset) meaning that other amorphous Febased phases may also be present. Fe3BO5 does not recrystallize during subsequent delithiation. The same set of three low intensity Bragg peaks (highlighted with * in Fig. 3) appear at intermediate stages of both lithiation and delithiation, that was not observed for Cu2FeBO5 FeBO3 and Fe3BO6

14

25

,

. Although only a few small broad peaks are not sufficient to identify the

phase(s), the analysis points rather towards Li-B-O than Fe-based phases. This would also fit with the observation of a lithia matrix consisting of a mixture of lithium oxide (Li2O) and lithium orthoborate (Li3BO3) embedding metal nanograins after conversion of kotoite-type oxyborate M3B2O6 (with M = Co, Ni, and Cu) with Li 20.

Figure 3. Ex-situ XRD (Co radiations) patterns collected at several stages of the first galvanostatic cycle corresponding to circles marked on the V(x) curve of the inset. The peaks labelled with * are observed at intermediate stages of cycling. The peak labelled T is due to the PTFE binder.


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Operando Fe K edge XANES was acquired during the initial lithiation of Fe3BO5 (Fig. 4a). The shape of the XANES spectrum of the pristine Fe3BO5 is gradually transformed to one approaching that of metallic iron. The mixed valence of Fe3BO5 with an Fe2+/Fe3+ ratio of 66:33 was confirmed by linear combination fitting of the normalized XANES spectrum using FeO to Fe2O3 leading to a ratio of 7:3 (Fig. S1a). After the initial lithiation the electrode is composed of metallic Fe and Fe3BO5 in a 9:1 ratio (Fig. S1b) which is in close agreement with the ~10 % lower specific capacity than the theoretical one (~6 of 7 Li) which is required to reduce all Fe2+/3+ in Fe3BO5 to Fe0. Only two principal components, i.e. Fe3BO5 and Fe0, are required for linear combination fitting of the entire set of operando XANES spectra measured during the initial lithiation (Fig. 4b-c). This observation is consistent with the ex-situ XRD results showing two Fe-based phases during the initial conversion reaction giving rise to the voltage plateau at ~1.1 V. It is further consistent with the proposed Li-B-O intermediate crystallized phase(s).


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Figure 4. (a) Evolution of XANES spectra as a function of the reacted lithium (x) with Fe3BO5 during the initial lithiation. (b) Voltage profile of Fe3BO5 during initial lithiation plotted along with the (c) evolution of weight fractions of Fe3BO5 and Fe vs. x in ‘LixFe3BO5’. The weight fractions were determined from linear combination fitting of the normalized XANES spectra. Due to the lack of XANES data during subsequent delithiation, the reversible redox potentials of Fe3BO5 were compared to those of FeO, Fe2O3 and FeBO3 in Table 1. The two reversible redox couples of Fe3BO5 determined by CV at average potentials of 1.25 and 1.73 V are found to be close to those previously reported for FeO and/or Fe2O3 and FeBO3, respectively. This may indicate that similar phases as for FeO and/or Fe2O3 and FeBO3 are involved in the reversible cycling of the electrode starting from nanograins of Fe0 embedded in a lithia matrix.


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Table 1. Comparison of the redox potentials of iron oxides and borates.

Fe3BO5

Average oxidation state

Initial lithiation (V)

+2.33

0.92

Reversible Average delithiation/lithiation potential (V) (V) 1.47 / 1.02

1.25

1.97 / 1.48

1.73

Reference

This work, Fig. 5a

FeO

+2

0.70

1.83 / 0.84

1.34

32

Fe2O3

+3

0.81

1.59 / 0.97

1.28

13

FeBO3

+3

0.97

2.12 / 1.51

1.81

13

Potentials were obtained from cyclic voltammograms at the maximum of the redox peaks.

3.2. Cu2MBO5 (M = Fe, Mn and Cr) Fe3BO5 being electrochemically active, the iron redox activity in Cu2FeBO5 was reinvestigated here while the previous study of Cu2FeBO5 25 focussed on the copper redox activity. In addition, the influence of the trivalent transition metal on the battery performance was explored in the series Cu2MBO5, with M = Fe, Mn or Cr. In order to access the iron redox activity in Cu2FeBO5 the electrode has to be cycled over a lower voltage range than in the previous report where the voltage window was limited to the Cu2+/Cu0 redox couple (1.5-4.7 V vs. Li/Li+)

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. In the voltage range 0.75-3.5 V vs. Li/Li+

Cu2FeBO5 shows a first lithiation capacity of 615 mAhg-1 (~6 Li) and a reversible capacity of 303 mAhg-1 (Fig. S2) compared to 400 mAhg-1 (~4 Li) and a reversible capacity of 180 mAhg-1 when cycled between 1.5 and 4.7 V at C/20

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. The specific capacities are comparable to those

achieved for Fe3BO5 when cycled in the same voltage range. The cyclic voltammograms of Cu2FeBO5 and Fe3BO5 are compared in Fig. 5a-c. During the initial lithiation two redox


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potentials are identified at 1.65 and 1.05 V for Cu2FeBO5 (Fig. 5b and Table 2). The former was attributed to the conversion reaction involving Cu2+/Cu0 redox couple

25

while the latter is

ascribed to the Fe3+/Fe0 redox couple. The iron redox couple is observed at higher potential than for Fe3BO5 (1.05 vs. 0.92 V, Tables 1 and 2) and a sloped voltage profile is observed instead of the plateau for Fe3BO5 for x > 4 (Fig. S2). This is likely related to the amorphous nature of the electrode after the electrochemical grinding involved in the first conversion reaction of Cu2FeBO5 (x < 4)

25

. In addition to the reversible Cu2+/Cu0 redox couple at ~2.44 V both

reversible iron related redox couples of Fe3BO5 at ~1.7 and ~1.3 V are also observed for Cu2FeBO5 (Fig. 5c).

Figure 5. Cyclic voltammograms as a function of the cycle number (Z) of (a) Fe3BO5, (b) Cu2FeBO5 and (c) comparison of Fe3BO5 and Cu2FeBO5 for Z = 2, (d) Cu2CrBO5, (e) Cu2MnBO5 and (f) comparison of Cu2MBO5 (M = Fe, Mn and Cr) for Z = 2. The redox potentials of Fe3BO5 and Cu2MBO5 are reported in Table 1 and Table 2, respectively.


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Table 2. Comparison of the redox potentials of Cu2MBO5 (M = Fe, Mn and Cr). Initial Reversible Average lithiation delithiation/lithiation potential (V) (V) (V)

Voltage polarization (V)

1.65

2.80 / 2.07

2.44

0.73

1.94 / 1.53

1.74

0.41

1.52 / 0.99

1.26

0.53

1.55

2.66 / 2.18

2.42

0.48

1.09

1.57 / 1.11

1.34

0.46

1.32

2.95 / 2.26

2.61

0.69

Cu2FeBO5 1.05

Cu2MnBO5 Cu2CrBO5

Potentials were obtained from cyclic voltammograms shown in Fig. 5 at the maximum of the redox peaks.

Iron being electrochemically active in Cu2FeBO5 the electrochemical response of other trivalent transition metals in the series Cu2MBO5, with M = Fe, Mn or Cr was compared. During the initial lithiation two redox potentials are identified for Cu2FeBO5 and Cu2MnBO5 while only one reaction is observed for Cu2CrBO5 (Fig. 5b, d-e and Table 2). The first reaction at 1.65, 1.55 and 1.32 V for Cu2MBO5 with M = Fe, Mn and Cr, respectively, corresponds to the compositional range x < 4 in Fig. S3 and is related to the conversion reaction acting on the Cu2+/Cu0 redox couple. Similar overpotentials are required to initiate the conversion reaction of Cu2FeBO5 and Cu2MnBO5 while additional ~300 mV are required for Cu2CrBO5. The redox reaction of Cu2MnBO5 at a potential of ~1.09 V during the initial lithiation is ascribed to manganese in analogy to the iron redox activity observed in Cu2FeBO5 at ~1.05 V. No redox activity of chromium is observed in the tested voltage range (0.75-3.5 V) since it is active at lower voltages (Fig. S4) as for Cr3BO6 18. The reversible Cu2+/Cu0 redox reactions take place at 2.80/2.07 V for Cu2FeBO5, at 2.66/2.18 V for Cu2MnBO5 and at 2.95/2.26 V for Cu2CrBO5


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during reversible delithiation/lithiation. The potential and voltage polarization increase following Cu2MnBO5 < Cu2FeBO5 < Cu2CrBO5. While the voltage polarization of the Cu2+/Cu0 redox couple could be slightly improved in Cu2MnBO5, Cu2FeBO5 presents the largest reversible capacity values especially when the voltage range is limited to the Cu2+/Cu0 redox couple (Fig. S3 and S5). It may thus worth playing on the Fe/Mn ratio in Cu2Fe1-xMnxBO5 to lower the voltage polarization while maintaining high reversible capacities.

3.3. Cu extruded Cu2FeBO5 samples As annealing of Cu2FeBO5 in a reducing atmosphere leads to the extrusion of copper and the formation of Fe3BO5 in micro/nanostructural state 24, this process should result in improved electronic conductivity by in-situ formation of copper particles inside the micrometric electrode particles and the initiation of reactions should enhance further electrochemical conversion. For studying the impact on voltage polarization and reversibility several samples were prepared by treatment at 250, 350 and 450 °C for 1 h in Ar/H2 (10%), corresponding to different Cu2FeBO5, Cu and Fe3BO5 amounts as observed by XRD (Fig. 6a). At 250 °C the extrusion of Cu barely started, increasing temperature the fractions of Cu and Fe3BO5 increase at the expense of Cu2FeBO5. At 450 °C Cu2FeBO5 is no more evidenced in the XRD pattern. The so obtained composites were characterized electrochemically (Fig. 6b-f). Galvanostatic cycling between 0.75 and 3.5 V at C/20 results in first lithiation and delithiation capacities of 606 and 346 mAhg-1 for the 250 °C composite, 461 and 283 mAhg-1 for the 350 °C composite and 308 and 209 mAhg-1 for the 450 °C composite, respectively. While the overall capacity values decrease with increasing annealing temperatures (Fig. 6b), the Coulombic efficiency is improved (57 %, 61 % and 68 % for 250, 350 and 450 °C, respectively, Fig. 6c). The Coulombic


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efficiencies are also higher than for Cu2FeBO5 and Fe3BO5 both showing 52 %. During the initial lithiation, the redox potentials observed at 1.7 and 1.2 V are attributed to the copper and iron related redox couples, respectively, involved in the initial conversion of Cu2FeBO5 and Fe3BO5 (Fig. 6d-f). The relative peak intensities in the differential capacity plots (Fig. 6d-f) and the relative length of the voltage plateaus (Fig. 6c) change according to the Cu2FeBO5 and Fe3BO5 ratios in the composites confirming once again the previous assignment of the redox couples. This also holds for the reversible redox pairs at 2.2/2.6 V, 1.6/1.9 V and 1.1/1.4 V. With increasing annealing temperature the redox pair at 2.2/2.6 V vanishes, i.e. for the composite annealed at 450 °C the redox activity of copper is fully suppressed. This may be due to the smaller surface area of the larger Cu particles in this 450 °C annealed sample limiting the conversion reaction taking place at the interfaces compared to the nanocrystalline Cu obtained during electrochemical cycling of Cu2FeBO5. The presence of Cu metal particles does not seem to affect the voltage polarization but improves the Coulombic efficiency to the detriment of the specific capacity. Annealing at low temperatures may thus offer a good compromise between capacity and Coulombic efficiency.


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Figure 6. (a) XRD patterns (Co radiations) of Cu2FeBO5 annealed at 250, 350 and 450 °C in Ar/H2 atmosphere, (b) corresponding voltage profiles as a function of the specific capacity for the so formed composites as well as (c) normalized capacities which are also compared to pristine Cu2FeBO5 and Fe3BO. The corresponding differential capacity plots are shown for the composites prepared at (d) 250, (e) 350 and (f) 450 °C.

4. Conclusions The homometallic Fe3BO5 shows a first lithiation capacity of 678 mAhg-1 (~6.5 Li) due to an almost complete conversion of Fe2.33+ to metallic Fe resulting in an amorphous or nanocrystalline matrix which is maintained during subsequent cycling. Two reversible redox couples with low voltage polarizations for conversion-type reactions (300 and 440 mV) are observed at ~1.3 and ~1.8 V giving rise to a stable capacity of 345 mAhg-1 between 0.75 and 3.0 V vs. Li/Li+. The trivalent transition metals in the series Cu2MBO5 (M = Fe, Mn or Cr) were found to be


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electrochemically active in addition to copper but at lower voltages causing a large spread in redox potentials. The electrochemical activity of M in Cu2MBO5 opens perspectives on other homometallic ludwigite oxyborates operating at attractive voltages for anodes. When limiting the voltage range to the Cu2+/Cu0 redox couple (at ~2.4 V) the best performance in terms of voltage polarization and reversible capacity was obtained for Cu2FeBO5. Annealing of Cu2FeBO5 in a reducing atmosphere at low temperatures (~250 °C) was identified as a means to improve the first cycle reversibility. This approach applied to Cu-substituted homometallic ludwigite oxyborates (e.g. Fe3+Fe2+2-xCu2+xBO5) is expected to result in improved battery performance.

ASSOCIATED CONTENT Supporting Information. Results of the linear combination fitting of the normalized XANES spectra of pristine Fe3BO5 and at the end of the first lithiation of Fe3BO5. Galvanostatic chargedischarge curves of Cu2MBO5 (M = Fe, Mn and Cr) for different voltage ranges and at different C-rates. AUTHOR INFORMATION Corresponding Author * Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors gratefully acknowledge F. Damay (Laboratoire Léon Brillouin) and F. Baudelet (Synchrotron Soleil) for fruitful discussions. The authors thank S. Dufourd, S. Eichendorff and S. Gascoin (CRISMAT) and Jean Coquet (Synchrotron Soleil) for technical help. This work was partly supported by funding from ANR-16-CE08-0007-02, BORA-BORA.

ABBREVIATIONS LIBs, lithium ion batteries; XRD, X-ray diffraction; XANES, X-ray absorption near edge spectroscopy; PTFE, polytetrafluoroethylene; EC, ethylene carbonate; EMC, ethyl methyl carbonate; CV, cyclic voltammetry.

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Playing with the Redox Potentials in Ludwigite ... - ACS Publications

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