Fe2+ Redox Couple Approaching 4 V in Li2âx(Fe1âyMny)P2O7

Article pubs.acs.org/cm

Fe3+/Fe2+ Redox Couple Approaching 4 V in Li2−x(Fe1−yMny)P2O7 Pyrophosphate Cathodes Naoya Furuta,† Shin-ichi Nishimura,† Prabeer Barpanda, and Atsuo Yamada* Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan S Supporting Information *

ABSTRACT: Li-metal pyrophosphates have been recently reported as novel polyanionic cathode materials with competent electrochemical properties. The current study presents a detailed analysis of inherent electrochemical properties of mixed-metal pyrophosphates, Li2(Fe1−yMny)P2O7, synthesized by an optimized solid-state route. They form a complete solid solution assuming a monoclinic framework with space group P21/c. The electrochemical analysis of these single-phase pyrophosphates shows absence of activity associated with Mn, where near-theoretical redox activity associated with Fe metal center was realized around 3.5 V. We noticed a closer look revealed the gradual substitution of Mn into parent Li2FeP2O7 phase triggered a splitting of Fe3+/Fe2+ redox peak and partial upshifting in Fe3+/Fe2+ redox potentials nearing 4.0 V. Introduction of Mn into the pyrophosphate structure may stabilize the two distinct Fe3+/Fe2+ redox reactions by Fe ions in octahedral and trigonal-bipyramidal sites. Increase of the Gibb’s free energy at charged state by introducing Li+−Fe3+ and/or Li vacancy−Mn2+ pairs can be the root cause behind redox upshift. The underlying electrochemical behavior has been examined to assess these mixed-metal pyrophosphates for usage in Li-ion batteries. KEYWORDS: lithium ion battery, cathode material, pyrophosphate, polyanion compounds

■

INTRODUCTION With the growing demand of “energy storage” and “mobile energy supply” to empower modern electronics devices and possibly hybrid vehicles, Li-ion batteries have attracted considerable scientific attention over the last two decades.1 Building Li-ion batteries is a complex job, where “cathode” is the central component responsible for ∼48% of total cost. Naturally, the majority of battery research is focused on either improving the existing cathode materials or searching for newer ones.2 The first cathode to see commercial application was LiCoO2, which prompted the discovery of many other oxidebased cathode systems such as LiNiO 2 , LiMn 2 O 4 , LiMn1−xNixO4, and LiCo1/3Ni1/3Mn1/3O2.3−5 In a strategic shift from oxides, the inception of LiFePO46−8 triggered the investigation of “polyanionic compounds” leading to the discovery of many promising cathode systems like Li2MSiO4, Li2MPO4F, LiMBO3, and LiMSO4F.9−12 Nevertheless, considering the overall cost, safety, and electrochemical performance (∼160 mAh g−1 with a redox voltage of 3.43 V), olivineLiFePO4 is currently the most desirable polyanionic cathode for large-scale production.6,13 On the basis of a robust threedimensional PO43− framework, LiFePO4 offers good chemical/ thermal stability, operational safety, and good redox potential owing to the inductive effect.14 Moving over from LiFePO4, in an effort to design a novel PO4-based polyanionic cathode system, structural determination and electrochemical activity of Li2FeP2O7 (pyrophosphate), as a 3.5 V cathode system delivering 100 mAh g−1, that © 2012 American Chemical Society

is, close to 1-electron theoretical capacity, were first reported by Nishimura et al. in 2010,15 followed by Li2(MnxFe1−x)P2O7 (Zhou et al.)16 and Li2CoP2O7 (Kim et al.)17 in 2011. Scoring over LiFePO4, the pyrophosphate system offers several positive attributes such as reversible electrochemical activity without any particle-downsizing/carbon coating, a two-dimensional channel for Li-diffusion vs one-dimensional channel for LiFePO4, and the possibility of a 2-electron redox reaction. In addition, it can be economically prepared by conventional solid-state synthesis at 600 °C, without any further optimization. With the success of the Li2FeP2O7 system, we naturally got inspired to explore the Li 2MnP 2O 7 as well as their solid-solution phases Li2(Fe1−yMny)P2O7 (y = 0−1). The structure of the Li2MnP2O7 system was first analyzed by Adam et al. with no electrochemical investigation.18 Following, Zhou et al. have recently reported the synthesis and electrochemical properties of Li2(Fe1−yMny)P2O7, marked with high-polarization and low capacity in spite of using a large voltage window (1.5−4.8 V) that rapidly decayed with Mn content.16 It can be due to the comparatively large (>1 μm) and inhomogeneous particle size. Here, we report the optimized solid-state synthesis of a family of Li2(Fe1−yMny)P2O7 having fine and homogeneous particles. Their electrochemical analysis showed complete Fe3+/Fe2+ redox activity (well-defined at 3.5 V vs Li) with low polarization Received: October 29, 2011 Revised: February 29, 2012 Published: February 29, 2012 1055

dx.doi.org/10.1021/cm2032465 | Chem. Mater. 2012, 24, 1055−1061

Chemistry of Materials

Article

and no Mn3+/Mn2+ activity below 4.5 V independent of their composition. The inactivity of Mn-species is no surprise as very low activity of Mn is common for many other polyanionic systems, presumably because of lower electronic conductivity of Mn compounds, Jahn−Teller distortion caused by Mn3+ ion, and a lack of isolated minority spin electron just below the Fermi level (d5 configuration in Mn2+ vis-à-vis d6 configuration in Fe2+). Although the inclusion of Mn did not improve the electrochemical capacity of Li2−x(Fe1−yMny)P2O7 pyrophosphates, we marked a large splitting and shifting in the redox potential of Fe3+/Fe2+ upon increasing Mn substitution. Similar observations have been reported in the case of the Li(Fe1−yMny)PO4 olivine system but with a much smaller extent of simple shifting without splitting.19 Here, we show the possibility of pushing the Fe3+/Fe2+ redox potential close to 4 V in a phosphate-based cathode system. We have discussed these inherent phenomena of redox peak splitting and upshifting, taking into account the phase stability in the pyrophosphate solid-solution phases.

■

polypropylene film as separator, and 1 M LiPF6 dissolved in a mixture of ethylene carbonate/diethyl carbonate (EC/DEC, 3/7 v/v) as electrolyte. These coin cells were subjected to galvanostatic cycling with a TOSCAT-3100 battery tester (Toyo system) (at 25 °C) in the voltage range 2−4.3 V (at C/20 rate), with constant voltage relaxation down to C/200 applied at 4.3 V.

■

RESULTS AND DISCUSSION Adam et al. elucidated the Li2MnP2O7 structure as a threedimensional [MnP2O7]∞ network resulting from interconnection of [Mn4P8O32]∞ undulating layers, thus accommodating Li cations at different tunnel sites.18 More specifically, the 3-d metal (Mn) forms MnO6 octahedra (Mn1) sharing edge with MnO5 trigonal-bipyramids (Mn2). The overall Mn2O9 units share corners with P2O7 (two corner sharing PO4 units) diphosphate units. Li cations were found to have four distinct sites with equal occupancy. Following this work, our group was the first to introduce Li2FeP2O7 as a novel cathode material for the lithium-ion battery system with its structural determination.15 Though strikingly similar, Li2FeP2O7 is not strictly isostructural with Li2MnP2O7 as illustrated in Figure 1. Both

EXPERIMENTAL SECTION

Materials Synthesis. The mixed-metal pyrophosphate [Li2(Fe1−yMny)P2O7, y = 0−1] samples were prepared by conventional solid-state synthesis using stoichiometric amounts of Li2CO3 (Wako, 99+%), FeC2O4·2H2O (JUNSEI, 99+%), MnC2O4 (Kojundo, 99+%), and (NH4)2HPO4 (Wako, 99+%). A calculated amount of precursors were thoroughly mixed by planetary ball-milling for 3 h (600 rpm) with optional addition of Ketjen black carbon additives. In all cases, Cr-hardened stainless steel (Cr-SS) milling media and container were used. Following, the precursor mixture was sintered at 600 °C for 12 h inside a tube furnace with steady argon flow. Upon cooling to ambient temperature, we obtained the desired pyrophosphate powders. Structural Characterization. Powder X-ray diffraction patterns were measured by using a Rigaku RINT-TTR III powder diffractometer equipped with Cu Kα radiation operating at 50 kV and 300 mA. A typical scan was performed in the 2θ range of 10−80° with intermittent steps of 0.03°. Full powder pattern matching and Rietveld refinement were performed using TOPAS-Academic V4.1 software. Mössbauer spectroscopy was performed using a Topologic System Inc. spectrometer equipped with a 57Co γ-ray source with due calibration done with an α-Fe foil as standard. Typically, ∼0.1 g powder samples were sealed in a Pb sample holder by polyethylene films, and the spectra were collected for ∼5 h in transmission mode. Xray absorption measurements were carried out at beamline 7C at Photon Factory, High Energy Accelerator Research Organization (KEK). A water-cooled Si(111) double-crystal monochromator was used for energy selection. The Fe and Mn K-edge absorption spectra were measured in transmission mode at room temperature. The intensities of transmitted X-rays were monitored by ionization chambers. The particle morphology was examined by SEM imaging of powder samples mounted on conducting carbon paste via a Hitachi S-4800 field-emission scanning electron microscope operating at 1−2 kV. TEM images and selected area diffraction patterns (SAED) on key samples were obtained with a JEOL JEM-2100 instrument operating at 200 kV. The powders were mixed in acetone, and a few drops were deposited on a holey carbon−copper grid for TEM study. Electrochemical Measurements. The working electrode was formulated by mixing 70 wt % of active materials (lithium mixed-metal pyrophosphates), 20 wt % of carbon black, and 10 wt % of polyvinylidene fluoride (PVdF) binder in a minimal amount of NMP (N-methylpyrrolidone) solvent. The slurry was cast on an Al film acting as current collector. After drying at 120 °C in vacuum overnight to remove all NMP, circular disks (with diameter = 18 mm) were punched out. The active material was ∼20 μm thick with a cathode loading of 3 mg/cm2. These cathode disks were used to assemble 2032-type coin cells with Li metal foil as anode,

Figure 1. (Top) Schematic presentation of the crystal structure of Li2FeP2O7 and Li2MnP2O7 end members shown along the ac plane. The stacking of layers of atomic planes containing Fe/Mn and Li atoms is illustrated. (Bottom) The Li−Fe antisite mixing is highlighted in the case of Li2FeP2O7. Local atomic coordination among the LiO5 and MO5/MO6 (M = Fe/Mn) polyhedral units is depicted pointing to the Li−Fe antisite defect in the case of Li2FeP2O7.

these structures consist of stacking of two layers of metal ions (Fe or Mn) separated by a monolayer of Li atoms, forming a two-dimensional Li diffusion network. In the case of Li2FeP2O7, while the FeO6 octahedral site (Fe1) is fully occupied, the FeO5 bipyramidal site (Fe2) is occupied either by Fe or Li. Fe partially goes into the LiO5 bipyramidal site (Fe3), which shares an edge with Fe1 and Fe2 sites (Figure 1). It may arise from the similar ionic size between Li (73 pm-LiO4 and 90 pmLiO6) and Fe (77 pm-FeO4 and 92 pm-FeO6).20 Thus, Li2FeP2O7 can be seen as a disordered pyrophosphate, rich in Li−Fe antisite defects. We performed the solid-state synthesis of mixed-metal Li2(Fe1−yMny)P2O7 phases at 600 °C. The corresponding XRD powder diffraction patterns are given in Figure 2, all of which assumed a monoclinic crystal structure (P2 1 /c symmetry). The respective cell parameters of all compositions 1056



Article

Figure 2. (Left) Comparative powder X-ray diffraction patterns of mixed-metal pyrophosphates Li2(Fe1−yMny)P2O7 [y = 0−1]. The gradual peak upshift with higher Mn-content is a signature of lattice expansion. The simulated XRD pattern is also presented as a reference. (Right) TEM micrographs of representative pyrophosphate powders showing the formation of nanoparticles in each case. TEM images of Li2FeP2O7, Li2(Fe0.5Mn0.5)P2O7, and Li2MnP2O7 are given from top to bottom. The respective SAED patterns are shown in the inset.

are listed in Table 1. TEM study of selected pyrophosphate phases revealed the formation of small particles (100−500 nm) by conventional simple solid-state synthesis (Figure 2). Further, the SAED patterns showed lattice ordering with P21/c symmetry for all the samples. The similar XRD patterns affirm the formation of complete solid solution between two end members: Li2FeP2O7 and Li2MnP2O7. The increase in Fecontent was marked to upshift the XRD peaks owing to gradual lattice contraction, which is a signature of smaller Fe (77 pmFeO4 and 92 pm-FeO6) vs Mn radius (80 pm-MnO4 and 97 pm-MnO6).20 The variation of lattice parameters is plotted as a function of composition in Figure 3, showing almost identical values with previous reports16−18 for end members and steady change in all parameters unlike Zhou et al.’s report showing minimal change in lattice parameter “b” and abrupt change in all parameters at Fe ∼ 80% composition.16 This steady decrease in crystal parameters with increasing Fe content can be due to (i) the smaller ionic radii of Fe species,20 (ii) the increasing probability of Fe occupying Li-site, and (iii) the variation in

Figure 3. Variation of lattice parameters in the mixed-metal pyrophosphates: Li2(Fe1−yMny)P2O7 [y = 0−1]. Higher Mn-content gradually increases all lattice parameters owing to its larger ionic size, retaining the same structure.

symmetry of FeO6 octahedra and FeO5 tirgonal-bipyramid units. The Fe site features of Li2(Fe1−yMny)P2O7 pyrophosphate phases were probed further by Mössbauer spectroscopy. The collected Mössbauer spectra and corresponding parameters are summarized in Figure 4 and Table 2, respectively. In all compositions, Mössbauer spectra affirm the presence of Fe2+ with high-spin species, with minimal (∼5%) Fe3+ impurities. Generally, nonresolved broad spectra were observed, which were successfully fitted using two distinct Fe2+ sites. While the outer doublet was assigned to the FeO6 (or MO6) octahedral site (site 1), the inner doublet was ascribed to the combination of FeO5 (or MO5/LiO5) sites (site 2). It was difficult to

Table 1. Lattice Parameters and Cell Volume of Li2(Fe1−yMny)P2O7 Solid Solution Phases with Monoclinic (P21/c) Symmetry materials

a, Å

b, Å

c, Å

β, deg

V, Å3

Li2FeP2O7 Li2(Fe0.75Mn0.25)P2O7 Li2(Fe0.50Mn0.50)P2O7 Li2(Fe0.25Mn0.75)P2O7 Li2MnP2O7

11.0262(3) 11.0528(2) 11.0849(3) 11.1232(2) 11.1584(2)

9.7558(2) 9.7739(2) 9.7879(2) 9.8034(2) 9.8116(1)

9.8128(2) 9.8361(2) 9.8586(2) 9.8798(1) 9.8948(1)

101.5666(13) 101.8770(15) 102.1290(17) 102.3383(15) 102.4802(13)

1034.12(4) 1039.84(4) 1045.76(4) 1052.47(4) 1057.71(3)

1057



Article

Next, the electrochemical performance of these isostructural mixed-metal pyrophosphates was studied. As shown in Figure 5, Li2FeP2O7 led to reversible capacity of 100 mAh g−1 involving a 3.5 V Fe3+/Fe2+ redox activity, close to the 1electron theoretical capacity.15−17 Interestingly, the first charging voltage profile shows slightly higher redox activity than subsequent charging modules, a phenomenon similar to that observed in the case of Li2FeSiO4, where Li−Fe site exchange occurs upon first charging.21 Our ex situ X-ray diffraction study showed significant amount of irreversible disorder is induced during the first charging of Li2−xFeP2O7, similar to the case of Li2−xFeSiO4. Details on this structural disorder will be reported elsewhere.22 Despite several attempts, we were unable to trigger a 2-electron reaction, as it is predicted to occur at high voltage of 5.2 V.16 The energy density of Li2FeP2O7 system can be improved by Mn substitution with the possibility of Mn3+/Mn2+ redox activity around 4.5 V. However, gradual substitution of Mn resulted in a steady decrease in capacity as Mn remained electrochemically inactive16 with the sole contribution coming from existing Fe (Figure 5). Irrespective of composition, we were able to achieve near complete Fe3+/Fe2+ redox activity centered at 3.5 V with excellent cycling stability below 4.2 V. Despite the inactivity of Mn-species in Li2(Fe1−yMny)P2O7, the presence of Mn modifies the underlying electrochemical mechanism. The differential plot of galvanostatic cycling (dQ/ dV) brings light to two major changes: (i) the “splitting” of Feredox peak at first charging that becomes stable against cycling and gets wider with higher Mn content and (ii) the systematic “upshifting” of Fe-redox peaks with Mn substitution as shown in Figure 6. Lets first consider the stabilized Fe-redox peak “splitting” by Mn substitution. It is reasonable to consider that the redox split in the first charge of Li2−xFeP2O7 is due to the two crystallographic environments of Fe, as they are very different; one is in octahedra and the other is in trigonalbipyramid. Structural disorder induced in the first charging may widen and average the two potentials, and the difference disappeared in the subsequent cycles. By introducing Mn, the disorder upon charging would be suppressed and hence the potential split is retained after the second cycle by two possible reasons: (i) small amount of lithium extraction by inactive Mn and (ii) large difference of ionic radius of Li+ and Mn2+ making cation mixing more difficult.20 Actually, the voltage difference upon first and subsequent charge processes is getting smaller as the Mn content increases, providing additional evidence of the

Figure 4. Mössbauer spectra of Li2(Fe1−yMny)P2O7 compounds for y = 0−0.75. All these spectra can be satisfactorily fitted with two doublets corresponding to different Fe sites in pyrophosphate framework.

distinguish between the Fe signals from FeO5 and LiO5 polyhedra. Overall, the Li2(Fe1−yMny)P2O7 compounds were structurally very similar, with near equal and random distribution of 3d-metals (Fe or Mn) in MO6 (site 1) and MO5 (site 2). We could not detect any preferential occupation of Fe/Mn in any particular site (Table 2).

Table 2. Refined Mössbauer Spectrum Parameters of Li2(Fe1−yMny)P2O7 Solid Solution Phasesa y in Li2(Fe1−yMny)P2O7 0

0.25

0.5

0.75

a

Fe2+ Fe2+ Fe3+ Fe2+ Fe2+ Fe3+ Fe2+ Fe2+ Fe3+ Fe2+ Fe2+ Fe3+

(1) (2) (3) (1) (2) (3) (1) (2) (3) (1) (2) (3)

Isomer shift δ, mm·s−1

quadrupole splitting ΔEQ, mm·s−1

fraction, %

line width Γ, mm·s−1

1.2088(18) 1.2255(15) 0.58(2) 1.2161(16) 1.2311(14) 0.47(2) 1.2225(14) 1.2400(12) 0.476(18) 1.2323(18) 1.2438(13) 0.501(16)

2.091(8) 2.506(6) 0.39(3) 2.142(7) 2.530(6) 0.36(3) 2.190(7) 2.520(6) 0.41(3) 2.243(12) 2.486(10) 0.44(3)

44(2) 51(3) 4.6(5) 45(3) 51(3) 4.4(5) 43(3) 52(3) 4.8(5) 41(7) 54(8) 5.0(7)

0.354(4) 0.354(4) 0.354(4) 0.350(4) 0.350(4) 0.350(4) 0.341(4) 0.341(4) 0.341(4) 0.354(4) 0.354(4) 0.354(4)

Normalized χ2 for each fitting were 1.05, 1.02, 1.03 and 1.09 for y = 0, 0.25, 0.5 and 0.75 respectively. 1058



Article

Figure 6. Differential galvanostatic profiles (dQ/dV) of Li2(Fe1−yMny)P2O7 cathodes as a function of Mn content are given showing differences in the first and subsequent charging segments. The inclusion of Mn leads to a split in the Fe3+/Fe2+ redox peak along with an upshift to higher voltage. The extent of the split of the Fe-redox potential during the second charging segment becomes wider with higher Mn content and saturates around Mn ∼ 50% composition. Mn substitution leads to (partial) upshifting of the second Fe redox peak close to 4 V.

Figure 5. Galvanostatic cycling profiles of Li2(Fe1−yMny)P2O7 compounds, cycled between 2 to 4.3 V at a rate of C/20 (1 Li in 20 h) in coin cell settings. In each case, complete Fe3+/Fe2+ redox activity is realized, whereas Mn remains absolutely inactive. The inset graph shows the comparison between theoretical capacity (dashed line) associated to Fe-species and the observed capacity in different Li2−x(Fe1−yMny)P2O7 composition.

upshifting roots in the overall lattice volume expansion in the Li2(Fe1−yMny)P2O7 phases, which increases from 1033 Å3 (y = 0%) to 1063 Å3 (y = 100%) (Table 1). As with all phosphopolyanionic compounds, the P2O7 units form a rigid 3-D framework in pyrophosphate compounds. Hence the volume expansion mostly arises due to the increase in bond lengths of the (Fe, Mn)O polyhedra (here MO6 and MO5 units, M = Fe/Mn).25 Such modification enhances the ionicity of the transition metal center, hence lowering the Fe 3dO 2p antibonding state. This increase in ionicity results in higher Feredox potential corroborative with an inductive effect.25 Such intuitive considerations have recently been verified more precisely by first principle calculation on similar phenomena of multicomponent olivine with the direct charge density/shift calculation around the transition metals.26,27 This redox upshifting can be further explained considering its energetics. As Mn is electrochemically inactive, the delithiation of Li2(Fe1−yMny)P2O7 phases forms intermediate compositions with oxidized Fe3+ and unreacted Mn2+ surrounding the

suppressed structural disorder upon first charge by Mn substitution. The possibility of Mn3+/Mn2+ redox activity below 4 V can be ruled out as firmly evidenced by the Mössbauer and XANES spectra as discussed in the later section. Indeed, ab initio computational study predicts Mn involves redox activity at very high voltage of 4.5 V (Mn3+/Mn2+) and 5.3 V (Mn4+/Mn3+) in the pyrophosphate structure.16 The second major observation is the gradual “upshifting” of the redox peaks with higher Mn content in the pyrophosphates. Post-splitting, both the Fe-redox peaks experience continuous upshift with higher Mn content as seen in Figure 6. Similar effect of Mn on Fe-peak upshift as high as 0.1 V has been reported in the case of the olivine system Li(Fe1−yMny)PO4.19,24 One explanation for this Mn-induced Fe-redox 1059



Article

redox activity to form ∼100% Fe3+ species (at 4.5 V). This twostep Fe3+/Fe2+ redox activity involving voltage approaching 4 V is fully reversible. It is the highest ever reported Fe3+/Fe2+ redox potential (3.9 V) in any phosphate-based cathode. Intuitively, the inherent high-voltage can be understood by the strong Fe3+ - Fe3+ repulsion in the unique edge-sharing geometry of octahedra and trigonal bipyramid, destabilizing the charged state and raising the voltage according to, Voltage = E(Li) + E(charged) − E(discharged), ignoring TS and PV terms. One may still question if the 3.9 V redox peak arises due to Mnspecies present in the composition. To clear this doubt, we performed an ex situ XANES study on the Li2(Fe0.5Mn0.5)P2O7 cathode at different states of charging/discharging (2−4.5 V). As shown in Figure 8, there is absolutely no change in the K-

remaining Li+ ions. It results in stronger electrostatic repulsion between Li+ and Fe3+ (or, lithium vacancy and Mn2+), thus developing destabilization and higher Gibbs free energy per Li atom.23 This can increase the energy and redox potential of Fespecies as observed for both Fe-redox peaks in Figure 6. It is interesting to note a smaller increment in the lower peak and abrupt increase in the higher peak closing to 4 V. This observation of Fe3+/Fe2+ redox activity close to 4 V was further verified by ex situ Mössbauer spectroscopy. The active electrode mixture containing Li2(Fe0.5Mn0.5)P2O7 was pressed into pellets to use as cathode in assembling coin cells. These cells were charged/discharged to set different potentials (4.5 V, 3.7 V, and 2 V). Finally, the coin cells were dismantled to recover the electrode pellets and perform Mössbauer analysis. The resulting spectra and parameters are shown in Figure 7 and

Figure 8. Ex situ XANES spectra of Li2(Fe0.5Mn0.5)P2O7 obtained for the Mn K-edge and Fe K-edge at fully charged state (4.5 V), partially discharged state (3.7 V), and fully discharged state (2.0 V). They confirm there is absolutely no change in the oxidation state of Mn and visible change in oxidation state of Fe during electrochemical cycling of cathode in the voltage range of 2−4.5 V. Thus, the binary Fe−Mn pyrophosphate prepared in this study involves only Fe-redox activity.

Figure 7. Ex situ Mössbauer spectra of the Li2(Fe0.5Mn0.5)P2O7 pellet at different states of electrochemical cycling: (a) fully charged at 4.5 V, (b) partially discharged at 3.7 V; and (c) fully discharged at 2.0 V. Both the redox peaks centered around 3.6 and 3.9 V involve Fe3+/Fe2+ redox activity. The Fe-species are completely reversible upon discharging as evident by similar Mössbauer spectra of the initial cathode and fully discharged cathode.

edge of Mn with insertion/extraction of Li into the cathode. While Mn remains inert, we observed a visible change in the Kedge spectrum of Fe during electrochemical cycling, owing to its varying oxidation state. It confirms inactivity of Mn acting as a spectator ion in Li2−x(Fe1−yMny)P2O7, with net activity solely arising from Fe-redox inching close to 4 V. While it may come as a surprise, a particular structure can develop Fe3+/Fe2+ redox potentials as high as 4 V as shown by a recent study on a Mnsubstituted Li(Fe1−yMny)SO4F fluorosulfate electrode crystallizing in a triplite structure.28 Coincidentally, Li(Fe1−yMny)SO4F also stabilizes into a monoclinic framework (space group,

Table 3, respectively. Comparing the spectra at 2 V, 3.7 V, and 4.5 V, we observe that the first redox peak centered at 3.6 V involves the majority of the Fe3+/Fe2+ reaction. Thus, the Mössbauer spectrum at an intermediate state of 3.7 V consists of signals both from Fe2+ and from Fe3+ species, where the fraction of Fe3+ and Fe2+ signals is consistent with the charges consumed for electrochemical reaction. However, the redox peak around 3.9 V leads to the completion of the Fe3+/Fe2+

Table 3. Refined Mössbauer Spectrum Parameters of Li2−x(Fe0.5Mn0.5)P2O7 (y = 0.5) Cathode at Different States of Dischargea a (4.5 V)

b (3.7 V) c (2.0 V) a

2+

Fe Fe2+ Fe3+ Fe3+ Fe2+ Fe2+ Fe2+

(1) (2) (3) (1) (2) (1) (2)

isomer shift δ, mm·s−1

quadrupole splitting ΔEQ, mm·s−1

fraction, %

line width Γ, mm·s−1

0.375(3) 0.446(3) 1.25(4) 0.445(3) 1.205(4) 1.231(2) 1.2344(18)

0.991(10) 0.470(7) 2.43(8) 0.463(4) 2.641(7) 2.146(11) 2.521(8)

40(3) 55(3) 4.3(8) 60.9(1.1) 39.1(1.3) 43(3) 57(4)

0.374(7) 0.374(7) 0.374(7) 0.394(6) 0.394(6) 0.320(5) 0.320(5)

Normalized χ2 for each fitting were 1.10, 1.30 and 1.31 for a, b and c respectively. 1060



Article

C2/c) with edge-sharing MOx polyhedral units, having random distribution of constituent Fe and Mn species. So far, pristine Li2FeP2O7 stands out as the phosphate-based compound with the highest Fe-redox potential (3.5 V vs Li). Here, we showed Mn substitution can alter its local structure during electrochemical reaction, hence increasing the Fe3+/Fe2+-redox potential close to 4 V. To date, this is the highest possible redox potential in any (PO4)-based polyanionic cathode. Following the recently reported Fe-based 3.9 V fluorosulphate (LiFe1−yMySO4F) triplite cathodes,28−30 pyrophosphate cathodes deliver yet another example of the possibility of designing and achieving an Fe-based compound with very high redox potential. The striking point is the Fe3+/Fe2+ redox activity could be raised to 3.9 V despite the absence of more electronegative SO42− and F− anions.

the University of Tokyo. XANES experiments were performed by KEK-PF User Program No. 2011G063. The crystal structures were illustrated using the computer program VESTA.31

■

■

CONCLUSION To summarize, mixed-metal pyrophosphate Li2(Fe1−yMny)P2O7 (y = 0−1) compounds were successfully synthesized by conventional solid-state synthesis. Showing a gradual change in lattice parameters, they form a complete series of solid solution assuming into a monoclinic 3D framework with P21/c symmetry. Independent of composition, we could achieve near theoretical electrochemical activity related to Fe species centered around 3.5 V without any further electrode optimization. But, no electrochemical activity associated with Mn substituent was observed. However, careful electrochemical experiments on Li2(Fe1−yMny)P2O7 led to the observation of Mn-induced (i) splitting and (ii) up-shifting of Fe-redox peak. As a result, for the first time, we showed the observation of the Fe3+/Fe2+ redox potential approaching 4 V in a phosphate− polyanionic compound. This Mn-induced twin phenomenon of Fe-redox peak “splitting” and “upshifting” has been explained considering the enhanced structural integrity against first charging and modification of (Fe, Mn)O bond length and iconicity, as well as local development of higher Gibbs free energy. The future development in electrode optimization and high-voltage electrolyte can realize practical applications of these pyrophosphate compounds in Li-ion batteries.

■

ASSOCIATED CONTENT

S Supporting Information *

Figure showing the historical overview of the positions of Fe3+/ Fe2+ redox couples for iron oxyanionic compounds. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

REFERENCES

(1) Cheng, F.; Liang, J.; Tao, Z.; Chen, J. Adv. Mater. 2011, 23, 1695. (2) Tarascon, J. M.; Recham, N.; Armand, M.; Chotard, J. N.; Barpanda, P.; Walker, W.; Dupont, L. Chem. Mater. 2010, 22, 724. (3) Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. Mater. Res. Bull. 1980, 15, 783. (4) Guyomard, D.; Tarascon, J. M. Adv. Mater. 1994, 6, 408. (5) Ohzuku, T.; Makimura, Y. Chem. Lett. 2001, 30, 642. (6) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1188. (7) Yamada, A.; Chung, S. C.; Hinokuma, K. J. Electrochem. Soc. 2001, 148, A224. (8) Yamada, A.; Chung, S. C. J. Electrochem. Soc. 2001, 148, A960. (9) Nyten, A.; Abouimrane, A.; Armand, M.; Gustafsson, T.; Thomas, J. O. Electrochem. Commun. 2005, 7, 156. (10) Ramesh, T. N.; Lee, K. T.; Ellis, B. E.; Nazar, L. F. Electrochem. Solid-State Lett. 2010, 13, A43. (11) Yamada, A.; Iwane, I.; Harada, Y.; Nishimura, S.; Koyama, Y.; Tanaka, I. Adv. Mater. 2010, 22, 3583. (12) Recham, N.; Chotard, J. N.; Dupont, L.; Delacourt, C.; Walker, W.; Armand, M.; Tarascon, J. M. Nat. Mater. 2010, 9, 68. (13) Yamada, A.; Kudo, Y.; Liu, K. Y. J. Electrochem. Soc. 2001, 148, A1153. (14) Padhi, A. K.; Nanjundaswamy, K. S.; Masquelier, C.; Okada, S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1609. (15) Nishimura, S.; Nakamura, M.; Natsui, R.; Yamada, A. J. Am. Chem. Soc. 2010, 132, 13596. (16) Zhou, H.; Upreti, S.; Chernova, N. A.; Hautier, G.; Ceder, G.; Whittingham, M. S. Chem. Mater. 2011, 23, 293. (17) Kim, H.; Lee, S.; Park, Y.-U.; Kim, H.; Kim, J.; Jeon, S.; Kang, K. Chem. Mater. 2011, 23, 3930. (18) Adam, L.; Guesdon, A.; Raveau, B. J. Solid State Chem. 2008, 181, 3110. (19) Kobayashi, G.; Yamada, A.; Nishimura, S.; Kanno, R.; Kobayashi, Y.; Seki, S.; Ohno, Y.; Miyashiro, H. J. Power Sources 2009, 189, 397. (20) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (21) Nishimura, S.; Hayase, S.; Kanno, R.; Yashima, M.; Nakayama, N.; Yamada, A. J. Am. Chem. Soc. 2008, 130, 13212. (22) Shimizu, D.; Nishimura, S.; Barpanda, P.; Kurita, T.; Yamada, A. Manuscript in preparation. (23) Padhi, A. K.; Nanjundaswamy, K. S.; Masquelier, C.; Okada, S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1609. (24) Malik, R.; Zhou, F.; Ceder, G. Phys. Rev. B 2009, 79, 214201. (25) Yamada, A.; Takei, Y.; Koizumi, H.; Sonoyama, N.; Kanno, R.; Itoh, K.; Yonemura, T.; Kamiyama, T. Chem. Mater. 2006, 18, 804. (26) Seo, D.-H.; Gwon, H.; Kim, S.-W.; Kim, J.; Kang, K. Chem. Mater. 2010, 22, 518. (27) Gwon, H.; Seo, D.-H.; Kim, S.-W.; Kim, J.; Kang, K. Adv. Funct. Mater. 2009, 19, 3285. (28) Barpanda, P.; Ati, M.; Melot, B. C.; Rousse, G.; Chotard, J. N.; Doublet, M. L.; Sougrati, M. T.; Corr, S. A.; Jumas, J. C.; Tarascon, J. M. Nat. Mater. 2011, 10, 772. (29) Ati, M.; Melot, B. C.; Rousse., G; Chotard, J. N.; Barpanda, P.; Tarascon, J. M. Angew. Chem., Int. Ed. 2011, 50, 10574. (30) Ati, M.; Melot, B. C.; Chotard, J. N.; Rousse, G.; Reynaud, M.; Tarascon, J. M. Electrochem. Commun. 2011, 13, 1280. (31) Momma, K.; Izumi, F. J. Appl. Ctystallogr. 2011, 13, 1272.

AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Author Contributions †

Equally contributed to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the scientific assistance by Yamada Lab members and the financial support from the Cabinet Office, Government of Japan, the Funding Program for WorldLeading Innovative R&D on Science and Technology, and Mitsubishi Motor Company. P.B. is grateful to the Japan Society for the Promotion of Sciences for a JSPS Fellowship at 1061


Fe2+ Redox Couple Approaching 4 V in Li2âx(Fe1âyMny)P2O7

Recommend Documents

Fe2+ Redox Couple Approaching 4 V in Li2âx(Fe1âyMny)P2O7