Electrochemical Mechanism and Effect of Carbon Nanotubes on the

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Electrochemical mechanism and effect of carbon nanotubes on the electrochemical performance of Fe1.19(PO4) (OH)0.57(H2O)0.43 cathode material for Li-ion batteries Abdelfattah Mahmoud, Claude Karegeya, Moulay-Tahar Sougrati, Jerome Bodart, Bénédicte Vertruyen, Rudi Cloots, Pierre-Emmanuel Lippens, and Frederic Boschini ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 14 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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ACS Applied Materials & Interfaces

Electrochemical Mechanism And Effect Of Carbon Nanotubes On The Electrochemical Performance Of Fe1.19(PO4)(OH)0.57(H2O)0.43 Cathode Material For Li-Ion Batteries Abdelfattah Mahmoud*1, Claude Karegeya1,2, Moulay Tahar Sougrati3, Jérôme Bodart1, Bénédicte Vertruyen1, Rudi Cloots1, Pierre-Emmanuel Lippens3, Frédéric Boschini1 1

GREENMAT, CESAM, Institute of Chemistry B6, University of Liège, 4000 Liège, Belgium

2

Faculty of Sciences, College of Education, University of Rwanda, 5039 Kigali, Rwanda

3

Institut Charles Gerhardt, UMR 5253 CNRS, Université de Montpellier, Place Eugène Bataillon, 34095 Montpellier cedex 5, France

Key words: Fe1.19(PO4)(OH)0.57(H2O)0.43/CNT composite, Hydrothermal synthesis, Cathode, Insertion reaction mechanisms, Operando XRD, Operando Mössbauer spectroscopy.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]/[email protected] Tel.: +32 4366 3543

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Abstract Hydrothermal synthesis route was used to synthesize iron (III) phosphate hydroxide hydratecarbon nanotube composites. Carbon nanotubes (CNT) were mixed in solution with Fe1.19(PO4)(OH)0.57(H2O)0.43 (FPHH) precursors for one-pot hydrothermal reaction leading to the FPHH/CNT composite. This produces a highly electronic conductive material to be used as cathode material for Li-ion battery. The galvanostatic cycling analysis shows that the material delivers a specific capacity of 160 mAh g-1 at 0.2 C (0.2 Li per f.u. in 1 hour), slightly decreasing with increasing current density. A high charge-discharge cyclability is observed, showing that capacity of 120 mAh g-1 at 1 C is maintained after 500 cycles. This may be attributed to the microspherical morphology of the particles and electronic percolation due to CNT, but also to the unusual insertion mechanism resulting from the peculiar structure of FPHH formed by chains of partially occupied FeO6 octahedra connected by PO4 tetrahedra. The mechanism of the first discharge-charge cycle was investigated by combining operando X-ray diffraction and 57Fe Mössbauer spectroscopy. FPHH undergoes a monophasic reaction with up to 10% volume changes based on Fe3+/Fe2+ redox process. However, the variations of the FPHH lattice parameters and the

57

Fe quadrupole splitting distributions during Li

insertion-deinsertion process show a two-step behavior. We propose that such mechanism could be due to the existence of different types of vacant sites in FPHH, including vacant “octahedral” sites (Fe vacancies) that improve diffusion of Li by connecting the onedimensional channels.


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1. Introduction The rechargeable Li-ion batteries dominate the currently used mobile storage systems thanks to their unrivalled electrochemical properties [1–4]. Current research on Li-ion batteries is focusing on the development of safe, non-toxic and cheap electrode materials with good electrochemical performance [5, 6]. Iron phosphate-based materials have attracted increasing attention due to their structure stability, environmental compatibility, low cost and promising electrochemical performance as electrode material for Li-ion batteries [7, 8]. Hydroxyl iron(III) phosphates have been proposed as electrode materials for Li-ion batteries due to their high structural stability, the presence of Fe3+ that can be reduced with the insertion of lithium and the existence of different vacant sites available for Li+ diffusion. The absence of lithium in the pristine material and the relatively low voltages make them obvious candidates for coupling with a lithium metal anode, considering that significant progress in this direction has been reported recently [9-11]. The Fe1.190.81(PO4)(OH)0.57(H2O)0.43 (FPHH) phase, where represents Fe vacancy, was previously obtained by hydrothermal synthesis [12]. The same method was also used with addition of carbon black in the solution to obtain FPHH/C composites [13]. The crystalline structure of FPHH was indexed in a tetragonal cell with the space group I41/amd. FPHH is formed by perpendicular chains of face-sharing iron octahedra developing along [100] and [010] directions and interconnected by PO4 units. Vacant sites are found along the same two directions forming channels for ion diffusion. In addition, the chemical composition of FPHH indicates that only 60% of iron sites are occupied and the remaining 40% octahedra could play the role of junctions between the channels, improving the ionic diffusion. The specific capacities for the first galvanostatic discharge were found to be 135 and 150 mAh g-1 at low current density for FPHH and FPHH/C, respectively. In both cases, the values are lower than the theoretical value of 178 mAh g-1 as expected for the full reduction of Fe3+ into Fe2+ but the effect of 10-20 wt% carbon 3 ACS Paragon Plus Environment


black added during the hydrothermal synthesis was found to significantly improve the electrochemical performances by reducing the particle size, increasing the surface area and improving the electronic conductivity of the electrode material. This electrode material exhibits a good cycleability even at high current density with a capacity retention of 99% after 150 cycles [13]. In order to improve the homogenization of the electrode material while maintaining the particle morphology and the electronic percolation previously obtained with carbon black, we propose in the present work to include carbon nanotubes instead of carbon black in the precursor solution for hydrothermal synthesis, leading to FPHH/CNT composite. We show that such a procedure improves the electrochemical performance, providing a specific capacity for the first galvanostatic discharge close to the theoretical value and excellent cycle life. The knowledge of the reaction mechanisms during charge-discharge cycles is considered as one of the main keys to predict the needed changes in current materials and/or design the new materials suitable for Li-ion batteries [14–19]. Since the electrochemical mechanism of FPHH was not previously investigated, we carried out an in-depth analysis of reversible lithiation by combining operando XRD and Mössbauer spectroscopy techniques [20-23]. Operando XRD provides information about mechanisms involving crystalline phases during electrochemical measurements [24–29]. This technique has been used here to follow the phase evolution of FPHH and evaluate the values of the lattice parameters at each step of lithiation. However this technique is inefficient for characterizing local changes at the atomic scale during electrochemical reactions. In that case, elemental sensitive techniques may be useful methods and

57

Fe Mössbauer spectroscopy is of particular interest for iron-based electrode

materials [30–34].

The operando

57

Fe Mössbauer spectroscopy has been used here to

investigate the redox reactions and changes in Fe local structural environment. Such changes are especially complex for FPHH due to chemical (Fe/vacancies) and structural (position of


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Fe in FeO6) disorders and to the existence of OH- and H2O. The variations of the Mössbauer parameters along the first cycles provide interesting information about the Fe 3d electron populations (isomer shift), the charge anisotropy around Fe (quadrupole splitting), the Fe environment distribution (linewidth) and the relative contributions of Fe3+ and Fe2+ (subspectrum areas). The combined analysis of operando XRD and Mössbauer spectroscopy reveals the complexity of the mechanisms for such material affected by different types of disorders.

2. Experimental section

2.1 Synthesis Iron phosphate hydroxide hydrate FPHH/CNT composite was obtained by a one-pot hydrothermal synthesis [13]. All chemicals were of analytical grade and used as received without further purification. In a typical synthesis, 2.5 mmol of sodium dihydrogen phosphate (NaH2PO4.H2O ≥ 99.0%, Aldrich) and 2.5 mmol of iron oxalate phosphate (FeC2O4.2H2O, 99.0%, Aldrich) were dissolved in 60 ml of distilled water with 2.0 ml HNO3 (65%) in water. Then, 4.25 g of carbon nanotube suspension (MWCNT, 3%, Nanocyl) corresponding to 10 wt% of the other precursors (FeC2O4.2H2O+ NaH2PO4.2H2O) was added under vigorous stirring. The mixture was stirred for 15 min, transferred into a 125 ml teflon-lined stainless steel autoclave and placed in an electric oven at 220 °C for 6 hours. The product was then cooled down to room temperature, filtered on a Buchner funnel and washed with de-ionized water and ethanol several times. Finally, the product was dried in oven at 80 °C for 2 h under vacuum.



2.2 Characterizations The crystallographic structure and the purity of FPHH/CNT were characterized by powder Xray diffraction (XRD) with a Bruker D8 X-ray diffractometer using Cu Kα radiation. The cell parameters were refined by using the TOPAS software [35] using the fundamental parameters approach to model the instrumental contribution. The morphology and particle size were investigated by scanning electron microscopy (XL 30 FEG-ESEM, FEI) and transmission electron microscopy (STEM TECNAI, FEI) instruments. 57

Fe Mӧssbauer spectra were recorded in transmission mode by using a constant-acceleration

spectrometer with a

57

Co(Rh) source at room temperature. The samples were prepared with

about 30 mg of FPHH/CNT materials mixed with boron nitride. The spectrometer was calibrated at room temperature with the magnetically split sextet spectrum of a high-purity αFe foil as the reference absorber. The measurements were carried out in the velocity range of ± 4mm/s with optimal energy resolution. The Mössbauer spectra were fitted with Lorentzian curves using Fullham program to obtain the values of the isomer shift (δ), quadrupole splitting (∆), linewidth (Γ) and relative resonance areas (A) of the different spectral components. The quality of the fitting procedure was judged on the basis of minimizing the number of parameters and χ² values. The magnetic susceptibility of FPHH, χ, was measured with a Superconducting Quantum Interference Design (SQUID) magnetometer MPMS XL7. The sample was cooled to 2 K under a zero magnetic field. Then, a magnetic field of 100 mT was applied, and the susceptibility was recorded from 2 to 300 K (ZFC) and from 300 to 2 K (FC). The elemental analysis for the determination of carbon content was carried out in a Multi EA 4000 system. The high temperature sample oxidation in oxygen up to a combustion temperature of 1500 °C allowed the digestion of thermally very stable samples/compounds.


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Electrochemical measurements were conducted in two-electrode Swagelok cells, using Li metal (Aldrich) as anode material and 1 M LiPF6 in ethylene carbonate and dimethylcarbonate (1/1,v/v) as electrolyte solution. FPHH/CNT powder was mixed with carbon black (CB) and polyvinylidene fluoride (PVDF) in a weight ratio of 70:20:10 and ground together during 30 min. Pellets of 13 mm-diameter with a density of about 19 mg/cm2 were prepared by uniaxial pressing on a stainless steel grid. The current collectors were stainless steel and the separator between electrodes was a 25 µm monolayer polypropylene membrane. The Swagelok cells were assembled in an argon-filled glove box. The galvanostatic charge/discharge curves were measured using a multichannel Biologic potentiostat (VMP3) between 1.5 and 4.5 V vs. Li+/Li0 at different cycling rates C/n (1 Li per f.u. in n hours). Cyclic voltammetric measurements were carried out at a sweep rate of 0.1 mV/s in the potential range 2 - 4.5 V. To study the electrochemical reaction mechanisms of FPHH/CNT electrode material, both XRD and 57Fe Mössbauer spectroscopy were used in operando mode at room temperature. An electrochemical cell designed for operando measurements was used in reflection and transmission modes for XRD and Mössbauer experiments, respectively [36–38]. The XRD patterns were recorded every one hour and the Mössbauer spectra every two hours during the first cycle in galvanostatic regime at C/10 and C/20, respectively, which corresponds to the reaction of 0.1 Li per f.u.

3. Results and discussions 3.1. X-ray diffraction All the diffraction peaks of the FPHH/CNT composite can be indexed in a tetragonal cell with the space group I41/amd (Figure 1). There is no other crystalline phases, confirming the purity of the material. The lattice constants are a = 5.183 (2) Å and c = 13.06 (5) Å as previously reported for FPHH [12]. The carbon content in the FPHH/CNT composite was 7 ACS Paragon Plus Environment


measured to be around 10% CNT through elemental analysis. No peaks corresponding to carbon are visible in the XRD pattern, in agreement with previous reports by our group and by other authors on various electrode compounds/CNT composite materials. [39- 42].

Figure 1. X-ray diffraction pattern of FPHH/CNT composite material. The structure of FPHH was previously discussed and can be described as perpendicular chains of face-sharing FeO6 octahedra along [100] and [010] directions connected by PO4 tetrahedra [12]. Vacant sites form channels along these two directions that can be occupied by inserted lithium (Figure 2). The unit cell contains 8 MO6 octahedra (M = Fe , ) and 4 PO4 tetrahedra. By considering the composition of FPHH, this indicates that Fe and P occupy 59% and 100% of the available crystallographic sites, respectively. The partial occupation of the off center Fe octahedral sites leads to some disorder that could impact the diffusion of lithium. The iron oxidation state has been determined to be +3 by magnetic measurements (cf. 3.2) and Mössbauer spectroscopy (cf. 3.3). This result is in line


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with the charge neutrality for FPHH and agrees qualitatively with the structural data since the average Fe-O distance (2.04 Å) is closer to the sum of Fe3+ (0.65 Å) and O2- (1.36 Å) ionic radii than the sum of Fe2+ (0.78 Å) and O2- ionic radii.

c

Li MO6 PO4

O b

2a Figure 2. Representation of two unit cells of FPHH along the a-axis with MO6 octahedra (in violet) where M= Fe or vacancy, PO4 tetrahedra (in blue), oxygen atoms (in red) and channels for Li diffusion represented by green spheres.

3.2 Magnetic measurements Magnetic measurements were performed to determine the oxidation state of iron in FPHH. The inverse magnetic susceptibility of FPHH, χ-1, shows that the Néel temperature is of about 80 K in line with previously reported value [12] (Fig. 3). Below this temperature, the ZFC/FC curves show complex behavior probably due to antiferromagnetic interactions but the analysis



is out of the scope of the present study. It is worth noting that magnetic properties of the monoclinic phase also show complex interactions at low temperature [43, 44]. Above about 200 K, χ-1 is linear and can be fitted to the Curie-Weiss law. Two temperature ranges have been considered for the fitting procedure to evaluate the Curie constant C. The values obtained for the ranges 200-300 K and 250-300 K are C = 4.52(4) emu K mol(Fe)-1 and C = 4.43(5) emu K mol(Fe)-1, respectively, leading to close values for the effective magnetic moment for iron of µeff = 6.01(3) µB and 5.95(4) µB. Although a more accurate value of µB could be obtained by extending the temperature range these values are close to the spin-only value of high-spin Fe3+: µeff = 5.92 µB, which is a clear indication for the existence of Fe3+ oxidation state in FPHH.

180

χ-1 (mol(Fe) emu-1)

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Fe1.19PO4(OH)0.57(H2O)0.43

160 140 120 100 80 60 40 0

50

100

150

200

250

300

Temperature (K)

Figure 3. Temperature dependence of the inverse magnetic susceptibility of FPHH (red line) and linear fitting to the data between 250 and 300 K (blue line).

3.3 Mössbauer spectroscopy 57

Fe Mössbauer spectroscopy was used to ascertain the valence state and the local

environment of iron in FPHH/CNT. The

57

Fe Mössbauer spectrum recorded at room


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temperature consists of an asymmetric doublet reflecting isomer shift and quadrupole splitting distributions due to the existence of different iron environments (Figure 4). The experimental data were fitted with two doublets to take into account the asymmetry of the spectra that can be attributed to low (LQS) and high (HQS) quadrupole splittings, respectively. The spectrum does not exhibit any magnetic splitting, showing that FPHH/CNT is paramagnetic in agreement with magnetic measurements. This also indicates that no magnetic iron-based impurities are present such as hematite α-Fe2O3 or magnetite Fe3O4. The values of the isomer shifts of FPHH are similar for the two doublets: δ ≈ 0.40 mm/s, which is characteristic of high spin Fe3+ in FeO6-type octahedral sites in agreement with both XRD and magnetic measurements (Table 1). The two sub-spectra clearly differ by the values of the quadrupole splitting, ∆(Fe3+(LQS)) = 0.33 mm s-1 and ∆(Fe3+(HQS)) = 0.67 mm s-1 where LQS and HQS denote low and high quadrupole splitting, respectively. The relative areas of the LQS and HQS subspectra are A(LQS) = 33% and A(HQS) = 67%, respectively, reflecting an asymmetrical quadrupole splitting distribution in favor of the higher values. Such a distribution should be first related to the variations of the Fe-O bond lengths and O-Fe-O bond angles resulting from the Fe positional disorder within the FeO6 octahedra. The partial occupation (≈ 60%) of the Fe sites in the octahedral chains leads to different Fe secondnearest neighbor environments along the chains including in descending order -Fe-, Fe-Fe and Fe-Fe-Fe. In addition, the possible occupation of these octahedral vacancies by H atoms increases the number of asymmetrical Fe local environments that contribute to the HQS Mössbauer component.



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Figure 4. 57Fe Mӧssbauer spectrum recorded at room temperature for FPHH/CNT composite material. Table 1. Room temperature Mössbauer parameters of FPPH/CNT composite, δ is the isomer shift, referred to α-iron at 295 K, ∆ is the quadrupole splitting, Γ is the linewidth and A is the relative area. δ (mm/s) FPHH-10%CNT

Fe3+ (1) 3+

Fe (2)

∆ (mm/s)

Γ (mm/s)

A (%)

0.41 (1)

0.67 (3)

0.37 (2)

67 (1)

0.40 (1)

0.33 (3)

0.25 (2)

33 (1)

.

3.4 Morphological properties The images obtained by scanning electron microscopy (SEM) show that FPHH/CNT composites are formed by FPHH spherical particles with relatively regular size distribution (3 - 5 µm) while CNTs are observed at the particle surface (Figures 5a and 5b). The carbon nanotubes agglomerate together and form layers around FPHH particles. A rather good 12 ACS Paragon Plus Environment

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dispersion and homogeneity of carbon nanotubes at the surface and between FPHH particles is observed (see Figure S1), which is very suitable for fabrication of the thick-film electrode. The carbon nanotubes interlace the particle forming a conductive network between particles which should enhance the electronic conductivity [41, 42]. The images obtained by transmission electron microscopy (TEM) show the existence of smaller particles of about 250-500 nm diameter that are trapped in carbon nanotube matrix (Figure 5c). This favors FPHH particle interconnections that contribute to facilitating the accessibility of more active sites and shortening diffusion paths of lithium ions. The TEM image of FPHH/CNT composite at higher magnification clearly indicates that CNTs appear at FPHH particles surface (Figure 5d). Such framework is expected to facilitate charge transfer for FPHH particles and improve electronic conductivity.

Figure 5. (a), (b) SEM and (c), (d) TEM micrographs of FPHH/CNT composite material.

3.5 Electrochemical performances The cyclic voltammogram curves recorded for FPHH/CNT composite at a scan rate of 0.1 mV s−1 show the existence of a main reduction peak at 2.38 V and an oxidation peak at 2.83 V that 13 ACS Paragon Plus Environment


can be both attributed to the Fe3+/Fe2+ redox couple (Figure 6a). The rather large broadening can be associated to the S-shape of the voltage curves vs specific capacity that suggests Li insertion monophasic reaction (Figure 6b). The rate capability helps to examine a possible kinetic limitation of lithium-ion transfer in the electrode. Figure 7a illustrates the specific capacity as a function of the cycle number for FPHH/CNT composite at different current rates. The electrode presents a stable cycling behavior at all current densities. The average discharge capacities are 180, 160, 145, 126, 110 and 60 mAh g-1 at the current densities of 0.1 C, 0.2 C, 0.25 C, 0.5 C, 1 C and 4 C, respectively. The variations of the specific capacity as a function of the C-rate show that the capacity retention slightly decreases with increasing current density (Figure 7b). The capacity fade is less than 13% when the rate capability is doubled. The galvanostatic cycling at 1 C displayed in Figure 7c shows that the material exhibits reversible capacity during more than 500 charge-discharge cycles. Such high capacity retention may be attributed to the nano/microspherical particle morphology and the electronic percolation facilitated by CNT but also to the Li insertion mechanism described in the next section.

Figure 6. (a) Cyclic voltammogram curves of FPHH/CNT composite at a scan speed of 0.1 mV s−1 and (b) voltage profile curves of first discharge-charge cycles at different C-rates.


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Figure 7. Electrochemical performances of FPHH/CNT composite: (a) evolution of the discharge and charge specific capacities on cycling at increasing rates from 0.1 C to 4 C, (b) evolution of the specific capacity with C-rate and (c) variations of the specific capacity of the discharge and coulombic efficiency with cycle number at 1 C rate.

3.5 Charge-discharge reaction mechanisms The electrochemical measurements show that about 1.2 Li per mole can be reversibly inserted and deinserted in FPHH/CNT at 0.1 C (Figure 5b). This corresponds to the transfer of 1 electron per Fe3+/Fe2+ redox couple in Fe1.19(PO4)(OH)0.57(H2O)0.43. Operando XRD and 57Fe Mössbauer spectroscopy were used to confirm this change in iron oxidation state and obtain more details about the reaction mechanism during the first discharge-charge cycle. 3.5.1 Operando X-ray diffraction The structural changes accompanying Li insertion/deinsertion in FPHH/CNT composite was were characterized by using the operando XRD technique with the specific Swagelok



electrochemical cell in reflection mode. The reflections belonging to the electrochemical cell do not shift during cycling and are marked in Figure S2 of Supplementary Information. The FPHH/CNT electrode material was discharged to 2.0 V and then charged to 4.5 V at C/10, which corresponds to the first Li insertion/deinsertion cycle. Each XRD pattern was recorded in the 2θ range from 25.5 to 45° for 1 h, which reflects the reaction of 0.1 Li per f.u. The corresponding voltage profile vs specific capacity is similar to that obtained for the electrochemical tests at the same C-rate (Figure 8a). The analysis of the XRD patterns clearly shows that the peaks gradually shifts to lower angles during the discharge and then they move towards higher angles during the charge; no new Bragg peaks occur (Fig. 8b). The peak shifts correspond to the variations of the lattice parameters with the amount of inserted Li+ (Figure 8c). Both a and c lattice constants increase by about 3 % during discharge (Li insertion) and decrease during charge, resulting in similar variations of the cell volume V up to 10% (Figure 8d) [45]. The increase of a (and b) by about 0.15 Å could be correlated to both the increase of the ionic radius from Fe3+ (0.65 Å) to Fe2+ (0.78 Å) and the cationic repulsion due to the insertion of Li+. It is worth noticing that, although the increase of the lattice parameters is progressive during the discharge, there is a change in the slope/inflection point at about 0.6 Li resulting in a stronger increase of c and V in the second part of the discharge than in the first part. This trend is reversible, showing stronger decrease of c and V in the first part of charge than in the second part. The changes are less significant for a (and b) but the existence of an inflection point is also observed at about 0.6 Li. Finally, the XRD pattern of the material discharged to 2.0 V shows broadened peaks compared to the pristine material suggesting higher structural disorder. Thus, the analysis of the operando XRD patterns shows the existence of a solid solution domain in the 0 < x < 1.19 composition range and the nonlinear variations of the lattice parameters suggest more complex mechanism than simple random occupation of vacant sites by Li+ ions. 16 ACS Paragon Plus Environment

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Figure 8. (a) Voltage curve and operando XRD patterns of FPHH/CNT electrode material collected at a rate of C/10 during the first discharge-charge cycle every 1 hour in the voltage range from 2.0 to 4.5 V, (b) enlarged view of 103 and 112 Bragg peaks, (c) variations of a and c lattice constants and (d) cell volume V as a function of the number of inserted lithium during the first discharge-charge cycle.



0.32

Charge

Discharge

0.30 0.28 0.26

FWHMsample(°)

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0.24 0.22 0.20 0.18 0.16 0.14 0.12

0

0.4

0.8

1.2

0.8

0.4

x in LixFPHH/CNT Figure 9. Evolution of the sample contribution to the full width at half maximum of the 103 Bragg peak as a function of lithium amount during the first discharge and first charge. Data points in the grey-shaded zones are not reported because the profile refinement is unreliable due to the superposition of the Bragg peak with a reflection of the in situ electrochemical cell. The operando XRD patterns show variations of the peak linewidth during the first cycle as clearly observed for the (103) Bragg peak (Figure 8b). To evaluate these variations, the sample contribution to the reflection profiles was determined by modeling the instrumental contribution using the fundamental parameters approach to X-ray line-profile fitting [35]. The full-width at half maximum of the sample contribution (FWHMsample) to the (103) Bragg peak does not change significantly for the insertion of up to about 0.5 Li and then increases until the end of the lithiation step (Figure 9). It further increases at the beginning of the delithiation (removal of the first 0.2 Li, at least) before decreasing back to a slightly higher value than in the pristine material. This evolution could be related to increase of disorder due to Li insertion in FPHH. The occupation of vacant sites by inserted lithium induces local structural 18 ACS Paragon Plus Environment

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relaxations and disorder. The observed change in the slope of FWHMsample at 0.5 Li could reflect the occupation of different types of vacant sites as discussed in the next section. The decrease of FWHMsample during the charge indicates that disorder decreases with the deinsertion of Li and shows the reversibility of this structural phenomenon. 3.5.2 Operando Mössbauer spectroscopy To obtain more details about the electrochemical mechanisms, changes in the Fe oxidation state and local structural environments were investigated by using operando

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Fe Mössbauer

spectroscopy with the specific Swagelok electrochemical cell in transmission mode. The FPHH/CNT electrode material was discharged to 2.0 V and then charged to 4.5 V at C/20, which corresponds to the first Li insertion/deinsertion cycle (Figure 10a). The Mössbauer data were recorded continuously while constant current was imposed (galvanostatic condition) during the first cycle, except at the beginning and at the end of the discharge where the open circuit condition was used, showing fast open circuit voltage relaxation (OCV). Each spectrum was recorded during 2 h, which corresponds to the reaction of 0.1 Li per f.u. except of course for the two OCV spectra. The first spectrum (OCV) is similar to that of FPHH, confirming there is no effect of the electrode formulation or the Swagelok cell, and the experimental data were fitted to two doublets (Figure 10b). At the end of discharge, the spectrum is formed by two broad peaks reflecting the distribution of Mössbauer parameters. Following the same strategy as the pristine material, the experimental data were also fitted to two doublets. The values of the isomer shift obtained for these two doublets are similar and the average value = 1.1 mm s-1 is characteristic of high spin Fe2+, confirming the full reduction of Fe3+ into Fe2+ due to Li insertion. The values of the quadrupole splitting are different for the two doublets: ∆(Fe2+(LQS)) = 2.5 mm s-1 and ∆(Fe2+(HQS)) = 3 mm s-1. Their high values mainly reflect the strong anisotropy of the Fe 3d electron distribution caused by the existence of the additional Fe 3d↓ electron compared to Fe3+ and is typical of 19 ACS Paragon Plus Environment


high spin Fe2+ [46]. The difference between the two values reflects the asymmetrical distribution of the quadrupole splitting and it is worth noticing that the ratio between the ratio of the relative areas of the two subspectra A(HQS)/A(LQS) ≈ 2 is similar to that found for Fe3+ in the pristine material. The Mössbauer spectrum obtained at the end of charge is similar to that of the pristine material and is characteristic of Fe3+ high spin state in FPHH, showing the reversibility of the lithiation-delithiation mechanism. All the intermediate spectra were fitted to two doublets for Fe3+ and two doublets for Fe2+ in line with the fitting procedures used at the beginning and at the end of discharge, respectively (Figure 10c). This approach allows to decompose the Mössbauer spectra in two main components reflecting the asymmetrical distributions of the Mössbauer parameters and to follow their evolutions during the first cycle. An example of the 4-doublets fitting procedure is shown for the spectrum obtained after the insertion of 0.3 Li during the first discharge (Figure 10b).


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Figure 10. Operando

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Fe Mössbauer spectra of FPHH/CNT for the first discharge–charge

cycle in galvanostatic regime (C/20) at room temperature: (a) voltage profile, (b) selected Mössbauer spectra and fitted doublets at different stages of lithiation, (c) fitted Mössbauer spectra obtained during the first discharge and charge. The total area of the spectra do not vary significantly during the first cycle indicating that recoilless fractions of Fe3+ and Fe2+ species are almost identical. Thus, their relative contributions to the spectra obtained by adding the relative areas of the corresponding two doublets give the relative amounts of Fe3+ and Fe2+ in lithiated FPHH. The relative amount of Fe3+ linearly decreases during the discharge while that of Fe2+ linearly increases and these trends are reversed during the charge (Figure 11). This means that progressive reduction of Fe3+ into Fe2+ is the only reaction associated to Li insertion and this mechanism is reversible



for deinsertion leading to the oxidation of Fe2+ into Fe3+. These results quantitatively confirm the reversible monophasic reaction: Fe3+1.19(PO4)(OH)0.57(H2O)0.43 + 1.19 Li+ + 1.19 e- = Li1.19Fe2+1.19(PO4)(OH)0.57(H2O)0.43

100

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(1)

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Relative area (%)

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Fe2+ 60

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Figure 11. Variations of the relative contributions of Fe3+ (open circles) and Fe2+ (squares) subspectra to the

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Fe Mössbauer spectra of FPHH/CNT for the first discharge–charge cycle

in galvanostatic regime (C/20) at room temperature.

The variations of the Mössbauer parameters of the two doublets of Fe3+ and Fe2+ in FPHH during the first cycle as a function of the number of Li per f.u. are shown in Figure 12 and 13, respectively.


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0.6

Fe3+(HQS) Fe3+(LQS)

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0.8 0.6 0.4 0.2

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Figure 12. Variations of the Mössbauer parameters for the two doublets Fe3+(HQS) and Fe3+(LQS) for the first discharge–charge cycle of FPHH/CNT in galvanostatic regime (C/20) at room temperature: (a) isomer shift δ, (b) full width at half maximum Γ and (c) quadrupole splitting ∆.



1.3

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Figure 13. Variations of the Mössbauer parameters for the two doublets Fe2+(HQS) and Fe2+(LQS) for the first discharge–charge cycle of FPHH/CNT in galvanostatic regime (C/20) at room temperature: (a) isomer shift δ, (b) full width at half maximum Γ and (c) quadrupole splitting ∆. The LQS and HQS components can be associated to two types of Fe environments with low and high charge anisotropies, respectively. Note that large error bars are found for the Fe3+ 24 ACS Paragon Plus Environment

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components of Li-rich FPHH and for the Fe2+ components of Li-poor FPHH due their small contributions to the Mössbauer spectra. The values of the isomer shift show some random variations of ±0.02 and ±0.03 mm s-1 for the Fe3+ and Fe2+ components, respectively, that could be attributed to inaccuracies for low intensity peaks that show poor signal-to-noise ratio (Figures 12a and 13a). Thus, the experimental data do not allow to determine some clear trends in the variations of the isomer shift and one can consider that their values for Fe3+ and Fe2+ remain close to those of the pristine and fully lithiated materials, respectively. This indicates there is no significant change in the Fe 3d electron configurations of Fe3+ and Fe2+ during lithiation-delithiation process. The line broadenings also show random variations but of ±0.1 mm s-1 for both Fe3+ and Fe2+ if we exclude spurious strong variations due to small contributions of the subspectra (Figures 12b and 13b). The average value for Fe3+: = 0.36(5) mm s-1 indicates a rather sharp distribution of the quadrupole splittings while for Fe2+: = 0.50(8) mm s-1 reflects a more broadened distribution. The difference between the values of ∆(Fe3+(HQS)) and ∆(Fe3+(LQS)) in lithiated FPHH does not change noticeably during the first cycle and is of about 0.3 mm s-1 (Figure 12c). In addition, there is no noticeable changes in the shape of the quadrupole splitting distribution as shows by the almost constant ratio between the areas of the subspectra: A(HQS)/A(LQS) ≈ 2. However, both LQS and HQS decrease by about 0.1 mm s-1 for the insertion of 0.4 Li and then increase until the end of discharge, this behavior being reversible during the charge. This can be related to changes in the charge anisotropy around Fe3+, although these ions are not directly affected by Li insertion. The difference between the values of ∆(Fe2+(HQS)) and ∆(Fe2+(LQS)) in lithiated FPHH is of about 0.5 mm s-1, in line with the observed broader Fe2+ Mössbauer peaks compared to the Fe3+ peaks (Figure 13c). The variations of ∆(Fe2+(HQS)) show a minimun for the fully lithiated FPHH while maximum values are found for Li-poor FPHH. The values of ∆(Fe2+(LQS)) are more randomly distributed and do not show any clear trend. 25 ACS Paragon Plus Environment


The variations of the quadrupole splittings of Fe3+ and Fe2+ combined with those of the c lattice parameters obtained by XRD during the first discharge suggest a two-step (or multistep) insertion mechanism probably due to the progressive and selective occupation of vacant sites during lithiation. A similar behavior is observed for the deinsertion of lithium during the charge, showing the reversibility of this mechanism. The existence of different types of disorders in FPHH makes difficult an accurate interpretation of the experimental data. However, we can distinguish the vacant sites within the octahedral O6 units (vacant octahedral sites) and those within the [100] and [010] channels (vacant channel sites). Because 60% of octahedral sites are occupied by Fe atoms, there are at most 0.8 vacant octahedral sites per f.u. This is a maximum value since these vacant sites can be occupied by H atoms, but this is not enough for the full reduction of Fe3+ into Fe2+ that requires 1.2 vacant sites per f.u. This means that only the vacant channel sites or more probably both vacant sites should be occupied by Li. This suggests that observed two-step variations of some experimental data during discharge could be related to the successive occupations of vacant octahedral and channel sites due to different energies.

4. Conclusions FPHH/CNT composite was obtained by the hydrothermal synthesis route. Electrochemical measurements show this electrode material exhibits an excellent cyclability with an averaged voltage of 2.6 V vs Li+/Li0. This can be attributed to the stable and open structure of FPHH formed by chains of (Fe/)O6 octahedra developing in [100] and [010] directions and connected by PO4 tetrahedra, but also to the particle morphology achieved by forming FPHH/CNT composite in which CNT improves the electronic percolation of the composite. The typical FPHH/CNT sphere size is in the range of 0.25-4 µm. The combined results obtained by operando XRD and

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Fe Mössbauer spectroscopy reveals a rather complex 26

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mechanism resulting from the combination of several characteristics of the FPHH structure: partial occupation (60%) of the octahedral sites by Fe, positional disorder of Fe within the FeO6 octahedra and existence of O-H bonds. The analysis of the results shows that the insertion mechanism is a monophasic reaction with 10% volume variations associated to the Fe3+/Fe2+ redox reaction. However, the observed variations of the lattice constants and of the Fe3+/Fe2+ quadrupole splitting distributions during the first discharge-charge cycle shows multi-step mechanism that could be due to selective occupation by Li+ ions of different types of vacant sites such as those found in FeO6 octahedra and channels. The existence of the octahedral vacant sites could improve the Li+ diffusion through FPHH by interconnecting the diffusion channels but additional experimental or theoretical studies are required for a more accurate analysis.

ACKNOWLEDGMENT The authors are grateful to University of Liège and FRS-FNRS for equipment grants. Part of this work was supported by the Walloon Region under the “PE PlanMarshall2.vert” program (BATWAL – 1318146). C. Karegeya acknowledges the abroad study leave of the University of Rwanda. A. Mahmoud is grateful to the Walloon region for a Beware Fellowship Academia 2015-1, RESIBAT n° 1510399. The authors are grateful to Corine Reibel for magnetic measurements. ASSOCIATED CONTENT Supporting Information available: [SEM image of the FPHH/CNT composite material with low magnification (Figure S1) and the evolution of XRD patterns recorded during the first cycle of FPHH/CNT composite material (Figure S2)]



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Parameters of LiFePO4 and FePO4, J. Phys. Chem. C. 2016, 120, 28375-28389.



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