Subscriber access provided by United Arab Emirates University | Libraries Deanship
Article
Lithium Iron Methylene Diphosphonate: A Model Material for New Organic-Inorganic Hybrid Positive Electrode Materials for Li-Ion Batteries Sebastian Schmidt, Denis Sheptyakov, Jean-Claude Jumas, Marisa Medarde, Peter Benedek, Petr Novák, Sébastien Sallard, and Claire Villevieille Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02595 • Publication Date (Web): 06 Nov 2015 Downloaded from http://pubs.acs.org on November 11, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 9
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
Chemistry of Materials
Lithium Iron Methylene Diphosphonate: A Model Material for New Organic-Inorganic Hybrid Positive Electrode Materials for Li-Ion Batteries Sebastian Schmidt,a Denis Sheptyakov,b Jean-Claude Jumas,c Marisa Medarde,d Peter Benedek,a, † Petr Novák,a Sébastien Sallard,a and Claire Villevieillea a.
Paul Scherrer Institute, Electrochemical Energy Storage Section, CH-5232 Villigen PSI, Switzerland.
b.
Paul Scherrer Institute, Laboratory for Neutron Scattering and Imaging, CH-5232 Villigen PSI, Switzerland.
c.
Institute Charles Gerhardt, Université Montpellier 2, 34095 Montpellier Cedex 5, France.
d.
Paul Scherrer Institute, Laboratory for Scientific Developments and Novel Materials, CH-5232 Villigen PSI, Switzerland.
ABSTRACT: To increase the energy density of lithium ion batteries, new electrode materials with superior performance are ceaselessly being searched for. To combine the advantages of organic and inorganic materials, the creation of organicinorganic hybrid materials is a promising option for the future. In this work, we introduce hydrothermally synthesized lithium iron methylene diphosphonate (Li1.4Fe6.8[CH2(PO3)2]3[CH2(PO3)(PO3H)]·4H2O) as a model material for a new class of organic-inorganic hybrid materials for Li-ion battery positive electrodes. Hereby, we validate the concept of using diphosphonate ligands for hybrid Li-ion battery materials. Structure determination based on neutron and X-ray powder diffraction data indicates a monoclinic phase containing three different Fe positions, which is confirmed by Mössbauer spectroscopy. The material achieves a specific charge of 128 mAh/g upon galvanostatic cycling after 200 cycles at a theoretical value of 168 mAh/g. Operando XANES results confirm the reversible cycling of Fe ions between FeII and FeIII, and ex situ SQUID measurements indicate an exchange of 0.6 electron per Fe atom in the first cycle in agreement with the specific charge obtained for the 1st reduction. It proves the applicability of transition metal diphosphonates as positive electrode materials for Li-ion batteries.
Introduction Lithium-ion secondary batteries have become the major source of mobile electric power for a wide variety of applications, with the most prominent examples being all types of portable consumer electronics and power tools. Their performance in terms of energy density, temperature stability, and cycling stability is defined by the active materials and electrolytes used in the cells. Therefore, the search for novel materials and material classes that can improve battery properties is being ceaselessly undertaken. Presently, commercial positive electrode materials are lithiated transition metal oxides 1 (e.g. LiCoO2, 2 LiNixMnyCo1-x-yO2 3) or phosphates (e.g. LiFePO4 4). Academic research on positive electrode materials is still mainly focused on these types of inorganic compounds, i.e. transition metal oxides, phosphates, and silicates. 5 However, the scope for preparing new purely inorganic materials is rather narrow, because only a limited number of metals can exhibit the desired properties related to molar mass and achievable potential vs. Li+/Li. 6,7 Organic materials on the other hand provide a vast scope for pre-
paring different compounds, as they can easily be modified by introducing different substituents and functional groups. Organic cathode materials with superior specific charge compared to inorganic materials cycling at reasonable potentials vs. Li+/Li were developed; however, they show stability issues as they gradually dissolve into the electrolyte upon cycling, or they are prone to undesired, irreversible oxidation reactions. 8,9 Therefore, the introduction of an interchangeable organic component into an inorganic material would provide the benefit of a stable inorganic part and the quasi-infinite composition possibilities offered by organic compounds. However, only a few examples of organic-inorganic hybrid active materials for Li-ion batteries have been reported in the literature so far. An organic-inorganic brannerite-type conversion material with bipyridine (bpy) ligands, [Mn(bpy)x(VO3)2, x = 0.5, 1], was described by Fernández de Luis et al.10 A negative electrode material cycling by an insertion mechanism was recently reported by Shen et al., consisting of Zn with imidazole and 2-aminobenzimidazole ligands. 11 To the best of our knowledge, only two classes of organicinorganic hybrid insertion materials for positive electrodes have been reported: (1) open-framework structures
ACS Paragon Plus Environment
Chemistry of Materials
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
with oxalate anions bridging VOHPO4 chains, [Li2(VO)2(HPO4)2(C2O4)] reported by Reddy et al., 12,13 (2) metal organic frameworks with benzyldicarbonyl (bdc) linkers, [Fe(III)(OH)1-xFx(bdc)]·H2O (x=0 or 0.2), reported by Férey et al. 14,15 As an extension of the concept, Férey et al. introduced cyclable benzoquinone molecules into the pores of the metal organic framework: thereby, the initial specific charge of the material could be strongly increased but at low cyclability owing to the extrusion and dissolution of the benzoquinone molecules upon cycling. 16 Recently, Awaga et al. introduced a new metal organic framework of Cu interconnected by anthraquinone dicarboxylate ligands where the organic and the inorganic components of the material were co-cycled without extrusion and dissolution of the anthraquinone. 17 However, they used only 10 wt-% active material and 70 wt-% conductive carbon to prepare their electrodes. Although these findings introduce and confirm the general concept of organic-inorganic hybrids as electrode materials for lithium ion batteries, the range of known hybrid materials suitable for Li-ion batteries is very narrow. Additionally, most of the materials have not been investigated for larger cycle numbers and have mainly been studied only to 50 cycles. Diphosphonates are versatile molecules to link different organic compounds to transition metal compounds. 18 They consist of two phosphonic acid groups connected by an organic linker. Various transition metal diphosphonates have been discovered and reported during the last two decades. 19-23 Phosphonates are also of high interest as organic linkers in the preparation of unconventional metal organic frameworks. 24 Recently, Pramanik et al. presented iron phosphonates as negative electrode materials for Li-ion batteries, which, judging from the electrochemical data, most probably undergo structural conversion upon cycling. 25 However, to the best of our knowledge, phosphonate-based hybrid materials have never been applied as positive electrode material so far. We expected them to be electrochemically active also at higher potentials by analogy with the lithium metal diphosphate (or pyrophosphate). 26,27 The possibility to add different organic groups or heteroatoms to the linker makes diphosphonates versatile candidates for testing the possible influence of different organic functional groups on the properties of organic-inorganic hybrid materials. An Fe(III) methylene diphosphonate named MIL-13 synthesized by a hydrothermal route was reported by Férey et al., but they did not report any electrochemical measurements. 28 Fe is the most frequently used transition metal for the development and investigation of new classes of polyanionic positive electrode materials, e.g. LiFePO4, 4,29 Li2FeP2O7, 26,27,29 LiFeBO3, 30,31 and Li2FeSiO4. 32-34 The aim of this study is to introduce lithium iron methylene diphosphonate as the first model material of a new class of versatile organic-inorganic hybrid positive electrode materials for Li-ion batteries based on metal diphosphonates. Methylene diphosphonate (or medronate) was chosen as it is the simplest diphosphonate, a CH2
Page 2 of 9
group linking two phosphonate groups. The reason for using iron as metal ion was its high cyclability, and high stability in other positive electrode materials.
Experimental section Synthesis. The synthesis was performed via a hydrothermal process which is a variation of the synthetic path reported by Férey et al. for preparing the aforementioned Fe(III) methylene diphosphonate. 28 To a 0.15 M solution of tetraethyl methylene diphosphonate (98%, Alfa Aesar) in 23 mL H2O, 4 eq. LiOH·H2O (98%, Sigma Aldrich) and 1 eq. FeSO4·7H2O (99%, Sigma Aldrich) were added, and then, the pH was adjusted to 7.2 by using ascorbic acid (99%, Sigma Aldrich). The reaction mixture was bubbled with Ar for 15 min, placed in an autoclave with a Teflon inlet (45 mL), and heated under pressure for 7 d at 160 °C. After centrifugation and washing with H2O and ethanol, the product was collected with 60% yield. IR (attenuated total reflectance): ν = 3108 cm-1 (m), 1691 cm-1 (W), 1619 cm-1 (w), 1449 cm-1 (w), 1365 cm-1 (w), 1175 cm-1 (m), 1058 cm-1 (s), 982 cm-1 (s), 951 cm-1 (s), 803 cm-1 (s), 781 cm-1 (s), 732 cm-1 (m) (Figure S1). Structural and morphological investigation. X-ray powder diffraction (XRD) was performed on the X04SA beamline at the Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland, using a wavelength of 0.7089 Å. Neutron diffraction was carried out at the HPRT beamline at SINQ, Paul Scherrer Institut, Villigen, Switzerland, 35 at 1.6 K and 300 K using a wavelength of 1.8857 Å. Scanning electron microscopy (SEM) was performed on a Zeiss Ultra55 SmartSEM electron microscope with an EverhartThornley secondary electron detector. To mitigate charging effects, the powder sample was sputtered with a 5 nm layer of Au. For better visibility, the image contrast was increased by 25% after recording. Energy-dispersive X-ray spectroscopy of samples that were not sputtered was carried out using an Ametek EDAX detector using the Ametek EDAX Genesis “Spectrum” software. Infrared spectroscopy was performed on a Perkin Elmer 2000 infrared spectrometer. The SEM images were recorded for each powder sample using a Carl Zeiss UltraTM 55 (Germany) apparatus at 3 kV tension using the in-lens detector. Energy dispersive X-ray spectroscopy (EDX) measurements were also performed. Crystal structure determination. The diffraction pattern was indexed using Topas. 36 The structure determination was performed by FoX 37 in parallel tempering mode using the low-temperature neutron diffraction data and consecutive Rietveld refinements of both neutron and Xray diffraction data using FullProf. 38 Mössbauer spectroscopy. 57Fe Mössbauer spectra were recorded in transmission geometry in the constant acceleration mode, using equipment supplied by Ortec and Wissel and a 57Co(Rh) source with a nominal activity of 10 mCi. The velocity scale was calibrated by means of a
ACS Paragon Plus Environment
Page 3 of 9
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
Chemistry of Materials
room-temperature spectrum of α-Fe. Isomer shifts are given with respect to the room temperature spectrum of α-Fe. The hyperfine parameters, namely, isomer shift and quadrupole splitting were determined by fitting Lorentzian lines to the experimental data, using appropriate computer programs by W. Kündig 39 and by K. Rübenbauer and T. Birchall 40. Magnetic measurements. The DC magnetization measurements were carried out on a MPMS XL SQUID magnetometer (Quantum Design) under a vertical magnetic field of 1000 Oe. Cycled electrodes consisting of 50 wt-% active material and 50 wt-% carbon black were ground and introduced in gelatin capsules under Ar atmosphere in a glovebox and sealed with GE-Varnish. They were loaded with approximately 15 mg of material. They were mounted in plastic drinking straws and fixed on the MPMS RSO transport stick. All measurements were carried out from 10 to 315 K by heating after a degaussing and zero-field cooling procedure. Cell preparation and electrochemical measurements. Electrochemical studies were performed in in-house-built coin cell-like electrochemical cells assembled in an Arfilled glove box, with a Li metal counter electrode, a glass fiber separator, and a solution of 1 M LiPF6 in a 1:1 mixture of ethylene carbonate and dimethyl carbonate as the electrolyte. The working electrodes were prepared by doctor blading a slurry containing 50 wt-% lithium iron methylene diphosphonate, 40 wt-% Super C65 carbon black conductive additive, and 10 wt-% poly(vinylidene difluoride) (Kynar®) binder on an Al foil. Galvanostatic cycling was performed at 20 mA/g between 1.5 V and 4.5 V vs. Li+/Li with a 1 h potentiostatic step after each half cycle. This procedure will subsequently be described as constant current / constant potential (CC/CP) cycling. Operando XANES. Operando XANES was performed on the X05LA beamline at the Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland. Galvanostatic cycling during the recording of the spectra was performed in an in-house-developed coin cell recently reported by Bleith et al. 41. A 25 µm Kapton foil with a 30 nm Al coating (Goodfellow) was used as a conductive window material for the working electrode side. The working electrode was a self-standing film consisting of 45 wt-% lithium iron methylene diphosphonate, 35 wt-% Super C65, and 20 wt-% Kynar Flex® binder. During the measurement, the cell was cycled at 20 mA/g during lithiation and 14 mA/g during delithiation, and every 30 min, a XANES spectrum was recorded. The slower delithiation rate was selected to reduce the frequency of small internal short circuit events caused by Li dendrite formation, which was observed during preliminary tests with the cell at rates larger than 20 mA/g.
Results and Discussion Structural and Morphological Characterization. The presence of iron and methylene diphosphonate in the final product is confirmed by Fourier transform infrared spectroscopy (FTIR) (Figure S1) and by energy-dispersive X-ray spectroscopy (EDX), no segregation between Fe and P signals has been observed in the powder, at least at the micrometer scale (data not shown). Lithium iron methylene diphosphonate crystallizes in a monoclinic (C2/c) phase, which was refined using synchrotron powder X-ray and neutron diffraction data (Figure 1, Figure 2, Table S1). It is closely related to the phase of Co2[CH2(PO3)2]·H2O reported by Lohse and Sevov in 1997 23. The stoichiometry obtained by crystal structure refinement is Li1.4Fe6.8[CH2(PO3)2]3[CH2(PO3)(PO3H)]·4H2O. The resulting crystal structure contains three different Fe sites. The Fe1 and Fe2 sites are located in two edge-sharing FeO6 distorted octahedra, forming an extended chain. The approximately half-occupied Fe3 site is tetrahedrally coordinated, sharing a corner with the Fe1-O6 octahedron, thereby linking the zigzagging chains of FeO6 octahedra. Inside pores with a typical diameter of approximately 4.1 Å viewed along the c-axis (not considering H), the lithium sites are “zigzagging” along the c-axis (Figure 2), providing space for one-dimensional lithium diffusion in and out of the structure upon cycling. However, the pores do not consist of planar rings but the O3P-CH2-PO3 units forming the pores are helically arranged along the c-axis without being directly interconnected. To measure the pore size, the structure was projected onto the AB-plane.
Figure 1. Rietveld refinement of the neutron diffraction pattern of lithium iron methylene diphosphonate measured at 300 K. The experimental pattern, calculated profile and difference curve are shown; the green lines below the graph show the calculated diffraction peak positions.
The determined crystal structure is confirmed by Mössbauer spectroscopy, which indicates the presence of three different Fe(II) sites, two of which are similarly coordinated and contribute approximately equally to the total amount of Fe. The third Fe(II) position has only half the
ACS Paragon Plus Environment
Chemistry of Materials
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 4 of 9
contribution, which is in good accordance with the approximate half-occupation of the tetrahedrally coordinated Fe3 site. A 22% contribution of Fe(III) was also found, but its peaks were rather broad, indicating either disordering or an amorphous structure (Figure 3).
Figure 2. Illustration of the monoclinic unit cell of lithium iron methylene diphosphonate (top) and the expanded crystal structure of lithium iron methylene diphosphonate plotted along the c-axis. The unit cell is indicated by the dotted frame.as refined from X-ray and neutron powder diffraction data. The red octahedra indicate FeO6, the orange tetrahedra indicate FeO4, and the yellow tetrahedra indicate PO3C.
Figure 3. Mössbauer spectrum of lithium iron methylene diphosphonate, the contribution of each individual Fe site to the fit is indicated by dashed/dotted lines.
Figure 4. Scanning electron microscopy images of lithium iron methylene diphosphonate at different magnification, as overview (top) and enlarged image (bottom).
Scanning electron microscopy (SEM) shows submicrometric platelets with a thickness of approximately 40 – 50 nm (Figure 4) and a length of up to ≈ 1 µm. However, the platelets tend to stack to form larger micrometer-scale structures. Electrochemistry. Constant current / constant potential (CC/CP) cycling of lithium iron methylene diphosphonate exhibited an initial specific charge of 97 mAh/g upon discharge. It slightly decreased to 85 mAh/g after 10 cycles, corresponding to 50% of the theoretical specific charge of the material, i.e. 168 mAh/g (Figure 5). The two offset values at the 158th cycle were caused by a temporary disruption of the measurement due to a power shutdown. The lower specific charge of lithium iron methylene diphosphonate compared to the theoretical value can be explained by the results of the operando X-ray absorption spectroscopy near the Fe K-edge (XANES), which indicates that a certain portion of Fe is electrochemically inactive and remains in Fe(II) oxidation state in the fully oxidized material (Figure 6). This may be explained by the lower lithium content in the pristine material, the inactivity of one of the three different Fe sites, or the inactivity of a certain portion of the material in the electrode, e.g. stacked particles lacking contact with the conductive carbon additive. Due to the remaining Fe(II) at 4.5 V vs. Li+/Li, the edge jump position only slightly shifts and the varying of the Fe(III) content had to be additionally ana-
ACS Paragon Plus Environment
Page 5 of 9
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
Chemistry of Materials
lyzed by comparing the relative white line intensities (Figure S4, Figure S5). The influence of the Fe(II) and Fe(III) edge positions can be seen with a derivative representation of the XANES spectra. The two maxima representing the E0 position (i.e. the zeroes in the 2nd derivative) of Fe(II) and Fe(III) remain in both samples, however their relative intensity obviously changes between the sample at 1.5 V vs. Li+/Li and the one at 4.5 V vs. Li+/Li (Figure S7). This result provides a qualitative proof of the redox activity of Fe.
!
"#$
(Curie-Weiss law)
(Equation 2)
The calculated value of Δpeff between the oxidized and the reduced sample is approximately 0.65 µB, which we attribute to an oxidation state change of roughly 0.6 e- per Fe. The SQUID data are in agreement with the XANES results (Figures 6 and S4-S6) on the electrochemical activity of the Fe during the cycling of the lithium iron diphosphonate. It is interesting to note that the fully oxidized material deviates from linearity already at 100 K, whereas the fully reduced sample was near-linear until 10 K. Although all samples remain paramagnetic down to the lowest temperature investigated, this indicates the existence of magnetic correlations in the oxidized sample below 100 K. The difference in linearity is most probably caused by a modification of the exchange paths during the oxidation and reduction processes. To have comparable fits, both linear fits were performed with values measured between 110 – 315 K.
Figure 5. Specific charge vs. cycle number plot (top) and CC/CP profile (bottom) of lithium iron methylene diphosphonate cycled at 20 mA/g with a 1 h potentiostatic step after each half-cycle.
To semi-quantitatively describe the change in Fe oxidation state, oxidized and reduced electrodes were investigated by SQUID magnetometry (Figure S7). The effective magneton numbers peff were calculated by equation 1 from the Curie constants Cp, which were extracted by a linear fit of the inverse molar magnetic susceptibility against the temperature according to the Curie-Weiss law (equation 2). kB corresponds to the Boltzmann constant, NA to the Avogadro number, µB to the Bohr magneton, χ to the magnetic susceptibility, and θp to the Weiss constant.
≅ 2.83
(Equation 1)
Figure 6. Top: XANES spectra in fluorescence mode from the + operando measurement of fully oxidized (4.5 V vs. Li /Li) + and fully reduced (1.5 V vs. Li /Li) lithium iron methylene diphosphonate. XANES spectra of FeSO4 and Fe2(SO4)3 are shown by dotted lines as Fe(II) and Fe(III) references. Bottom: Contour plot of the operando measurement using data normalized to pre-edge line = 0, post-edge line = 1.
To investigate the influence of the cycling on the crystal structure of the material, ex situ X-ray diffraction was performed. By Rietveld refinement of the corresponding diffraction patterns, it was found that the material shows
ACS Paragon Plus Environment
Chemistry of Materials
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
only small changes in the lattice parameters (Table S2, Figure S8). This proves that the structure of the material remains stable upon cycling. The specific charge of 85 mAh/g remained fairly unchanged until the 60th cycle; thereafter, it gradually increased to reach 128 mAh/g after 200 cycles, corresponding to 76% of the theoretical specific charge. This increase accompanies a slight decrease in the average overpotential (Figure S9). To explain this specific charge increase while no significant change in the CC/CP profile is visible, we suppose a mecha42 nism similar to that described by Wang et al., where during electrochemical cycling of ammonolized MoO3 nanobelts, there was an initial decrease in specific charge, followed first by a stable interval, and then by an increase in their specific charge. Wang et al. attributed the specific charge increase to a dislocation of the nanobelt aggregates. We suppose that the de-stacking of platelets in the electrode during the cycling would increase the surface area available for Li insertion in comparison to that available in the pristine material and would provide a larger contact area between the active material, conductive carbon additive, and electrolyte. Such de-stacking could be observed by post mortem SEM investigations, showing also that even after de-stacking, the single platelets are still well-contacted with the conductive additive. Therefore, a possible adverse effect, i.e. specific charge fading, originating from a disconnection of the platelets by the de-stacking can be ruled out (Figure S10). In Figure 5, it can be observed that the potential profile of the CC/CP cycles is dominated by a strong hysteresis; the main + oxidation occurs above 3.5 V vs. Li /Li, while the main reduc+ tion occurs below 3.2 V vs. Li /Li.
The first oxidation is highly irreversible; therefore, the initial coulombic efficiency is low (36%). However, it increases quickly to 86% after 10 cycles and to 97% after 50 cycles. Then, it remains constant around this value, with an average coulombic efficiency of 97.6(5)% after 50 cycles. The different potential profile for the first cycle approaches the stable shape directly at the second oxidation, and the potential hysteresis is slightly reduced between the 2nd and the 10th cycle. For the potential superior to 4.3-4.4 V vs. Li+/Li, a contribution to the irreversible specific charge coming from electrolyte decomposition is probable. But the extent of this irreversible specific charge suggests a contribution of (an) additional process(es). One possibility is an activation of the material during the first few cycles. There is a slight change of the unit cell parameters, but no additional phase could be detected by ex situ X-ray diffraction at 4.5 V vs. Li+/Li after the first delithiation (Table S2, Figure S11). Additionally, the diphosphonate signals are still visible in ex situ FTIR after 40 cycles, though working with low relative amounts of material and high carbon background in the electrodes. Therefore, we assume that the activation does not proceed by the decomposition of the methylene diphosphonate (Figure S12). Further research is dedicated to the determination of the mechanisms occurring during the first delithiation, using bulk and surface sensitive analysis techniques.
Page 6 of 9
Conclusion Lithium iron methylene diphosphonate has been successfully synthesized and applied as positive electrode material for Li-ion batteries. The determined crystal structure, ex situ XRD, and operando XANES results indicate that the cycling mechanism proceeds by a 1D Li+ insertion with little change in the lattice parameters. The specific charge can reach a value of 128 mAh/g and no fading after 200 cycles with a coulombic efficiency of 97.6(5) % has been observed. On the basis of these findings, future works are suggested to focus on the influence of the organic part of the diphosphonate on the cycling behavior of electrode materials, by using different transition metal ions, and different diphosphonates, a wide variety of which are known and an interesting selection is even commercially available. The goal of our further research is to determine the influence of additional heteroatoms and rigid groups on the structure and thereby on the cycling behaviour
ASSOCIATED CONTENT Supporting Information. Additional tables and figures on the electrochemical performance, structural characterization, magnetic measurements, IR spectroscopy and XANES. This material is available free of charge via the Internet at http://pubs.acs.org.”
AUTHOR INFORMATION Corresponding Author * Email:
[email protected] Present Addresses † ETH Zürich, Professur Materialien & Komponenten, ETZ H 65, Gloriastrasse 35, 8092 Zürich, Switzerland.
Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.
Funding Sources Swiss National Science Foundation, project no. 200021_146224.
ACKNOWLEDGMENT The authors would like to express their sincere gratitude to the Swiss National Science Foundation (SNF) for funding (project no. 200021_146224). Further, the authors are grateful to Dr. Nicola Casati from the MS-powder beamline (X04SA) at SLS, and Dr. Daniel Grolimund, Dr. Vallerie Samson, and Beat Meyer from the microXAS (X05LA) beamline at SLS for their support in performing the synchrotron experiments. Dr. Liliana Viciu from the Kovalenko group at ETH Zürich is gratefully acknowledged for her valuable hints on SQUID magnetometry experimental procedures, as well as Mr. Mickaël Morin from LDM, PSI, for his help during the experiments. This work is based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institut, Villigen, Switzerland.
ACS Paragon Plus Environment
Page 7 of 9
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
Chemistry of Materials
REFERENCES 1. Wagner, R.; Preschitschek, N.; Passerini, S.; Leker, J.; Winter, M., Current research trends and prospects among the various materials and designs used in lithium-based batteries. J Appl Electrochem 2013, 43, 481-496. 2. Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B., LixCoO2 (0
