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Crystal structure and lithium insertion properties of orthorhombic Li2TiFe(PO4)3 and Li2TiCr(PO4)3
Solid State Sciences 6 (2004) 1113–1120 www.elsevier.com/locate/ssscie
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Crystal structure and lithium insertion properties of orthorhombic Li2 TiFe(PO4 )3 and Li2 TiCr(PO4 )3 Sébastien Patoux a,1 , Gwenaëlle Rousse b , Jean-Bernard Leriche a , Christian Masquelier a,∗ a Laboratoire de réactivité et de chimie des solides, CNRS UMR 6007, université de Picardie Jules Verne, 33, rue Saint-Leu, 80039 Amiens cedex, France b Laboratoire de physique des milieux condensés, université Pierre et Marie Curie, 4, place Jussieu, 75252 Paris cedex 05, France
Received 15 July 2004; accepted 28 July 2004 Available online 16 September 2004
Abstract The crystal structures of orthorhombic Li2 TiFe(PO4 )3 and Li2 TiCr(PO4 )3 were determined from Rietveld refinement of neutron powder diffraction data. Both compounds are isostructural in the P bna space group with a = 8.543(1) Å, b = 8.623(1) Å, c = 11.978(1) Å and a = 8.505(1) Å, b = 8.589(1) Å, c = 11.929(1) Å, respectively. Lithium ions are located in a single four-fold coordinated site Li(1). Electrochemical insertion of lithium proceeds through the reduction of Fe3+ into Fe2+ (∼ 2.8 V vs. Li+ /Li) and Ti4+ into Ti3+ (∼ 2.5 V vs. Li+ /Li) towards the new compositions Li3 TiCr(PO4 )3 and Li4 TiFe(PO4 )3 . In situ X-ray diffraction and potentiostatic intermittent titration technique indicate a solid solution mechanism for the reversible lithium insertion/extraction. Chemical reaction of Li2 TiFe(PO4 )3 with n-BuLi leads to orthorhombic Li4 TiFe(PO4 )3 . 2004 Elsevier SAS. All rights reserved. Keywords: Lithium; Phosphates; Anti-NASICON; Positive electrodes; Lithium batteries
1. Introduction An alternative class of positive electrode materials for lithium batteries based on three-dimensional opened-frameworks that reversibly insert lithium was proposed in the late 90’s [1,2]. These materials take advantage of a relatively high lithium ion mobility and benefit from the inductive effect of their polyanionic groups, which increase the operating voltage in comparison with simple oxides [3,4]. In this context, much attention was given to phosphates [5,6] and sulfates [7,8] of iron and vanadium. Good reversibility was demonstrated for the Fe3+ /Fe2+ couple in cheap and non-toxic iron-containing materials such as Fe2 (SO4 )3 (3.6 V vs. Li+ /Li) [8], LiFePO4 (3.45 V vs. Li+ /Li) [9] and Li3 Fe2 (PO4 )3 (2.8 V vs. Li+ /Li) [10]. * Corresponding author.
E-mail address: [email protected] (C. Masquelier). 1 Present address: Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA. 1293-2558/$ – see front matter 2004 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2004.07.028
Compositions of general formula Zx MM (XO4 )3 (Z = Li, Na; M & M a 3d element, X = P, As, S, Si, . . . ) may adopt two distinct crystallographic forms, denoted “A” (Sc2 (WO4 )3 -type) and “B” (NASICON-type), which differ in the way the [MM (XO4 )3 ] “lantern units” are connected. Depending on both the stable oxidation number of M or M in air (Mn+ = Nb5+ , Ti4+ , Fe3+ , for instance) and on the chemical nature of X (X = Si, P, S, W, Mo) and Z+ , a wide variety of compositions may be prepared for 0 x 4: Fe2 (SO4 )3 , NbTi(PO4)3 , LiTi2 (PO4 )3 , Li2 FeTi(PO4 )3 , Na3 Fe2 (PO4 )3 , Na4 Zr2 (SiO4 )3 , etc. Electrochemical insertion or extraction of lithium or sodium into/from these compositions lead to an even greater variety of materials, with unusual oxidation states and alkali ion distributions within the interstitial space (0 x 5). Recent progresses have been communicated recently on how to make them work efficiently at high current densities through “smart” conductive carbon coating at the surface of the particles [11–14]. A comparative electrochemical study for the A and B forms of Li3 Fe2 (PO4 )3 was recently communicated [10, 15]. Detailed structural descriptions are given in [16–18].
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LiTi2 (PO4 )3 adopts the B form only, which corresponds to the well-known NASICON structure inside which the Li ions are located in the six-fold coordinated site that is labeled M1 [19]. The lithium insertion in NASICON Li1+x Ti2 (PO4 )3 was firstly investigated by Delmas et al. in the late 80’s [20–22] and more recently through contributions from the same group [23,24] and from us [25]. In between the well-known LiTi2 (PO4 )3 and Li3 Fe2 (PO4 )3 compositions, the mixed Ti/Fe phosphate Li2 TiFe(PO4 )3 (TiIV , FeIII ) has been the object of only limited work so far, besides the first tentative of structural determination of orthorhombic A-Li2 FeTi(PO4 )3 by Catti [26] and preliminary electrochemical investigations on NASICON B-Li2+x FeTi(PO4 )3 by Goodenough’s group [5,27]. There was room then for detailed structural and electrochemical investigations of these systems that offer the possibility of understanding the relative stabilities of the [MM (PO4 )3 ] framework depending on the nature of the 3d element and on the value of x in Zx MM (PO4 )3 [28]. Our structural and electrochemical investigations of the NASICON forms B-Li2 TiM(PO4 )3 (M = Fe, Cr) were recently published [29]. In this paper, we first present the crystal structures of both A-Li2 TiM(PO4 )3 (M = Fe, Cr) solved by Rietveld refinement of neutron diffraction data and then their lithium insertion properties, investigated through slow electrochemical titration (GITT, PITT) and in situ X-ray diffraction. A fully lithiated A-Li4 TiFe(PO4 )3 composition was prepared by chemical reduction, and is compared with that of the in situ experiment.
2. Experimental A-Li2 TiM(PO4 )3 (M = Fe, Cr) were prepared by high temperature solid state reaction, starting from the respective oxides (TiO2 , Fe2 O3 and Cr2 O3 ), ammonium dihydrogen phosphate and lithium dihydrogen phosphate in stoichiometric proportions. Pure powders were obtained, after several intermediate thermal treatments and grindings, by prolonged final treatment at 1203 K. Phase purities and lattice parameter determinations were carefully monitored by X-ray diffraction on either a Philips PW1710 diffractometer (CuKα radiation, θ –2θ geometry, back monochromator) or a D8 Bruker diffractometer (Co-Kα radiation, θ –θ geometry, back monochromator). The experimental X-ray diffractograms with two possible unit cells (and corresponding space groups) are gathered in Fig. 1, as will be discussed later. The list of lattice parameters and space groups of the two compounds investigated is given in Table 1. The crystal structures were determined from neutron diffraction data recorded at 300 K on the high resolution powder diffractometer D1A at ILL, Grenoble, France. Data collection with high direct space −1 resolution (λ = 1.9104 Å, Qmax = 6.5 Å ) allowed a precise determination of the nuclear crystal structures. The program Fullprof [30] was used for structure refinements by
Fig. 1. X-ray diffraction patterns of A-Li2 TiM(PO4 )3 (M = Fe, Cr) with Bragg positions calculated in the space groups P bna and P bca.
using the Rietveld method [31]. For in situ X-ray diffraction, a Swagelok-type cell similar to the one described elsewhere [32] was mounted, horizontally, on the D8 Bruker diffractometer (Co-Kα radiation, θ –θ geometry, PSD counter). Electrochemical lithium insertion was undertaken in standard SwagelokTM cell configurations with a lithium foil as the negative electrode and the active materials (AM) as the positive electrode. The separator used was a Whatman GF/D borosilicate glass fibre sheet saturated with a 1 M LiPF6 electrolyte solution in EC:DMC (1:1 in weight). For optimal electrochemical activity, A-Li2 TiM(PO4 )3 powders were initially mixed with Super P carbon (MMM Carbon, Belgium) by ball-milling in a SPEX 8000 mixer that generates energetic shocks between powder particles (250 mg of AM + 50 mg of carbon, stainless steel ball of 5 g, 30 min). A “Mac-Pile” automatic cycling/data recording system (Biologic SA, Claix, France) operating either in galvanostatic intermittent titration technique (GITT) or in potentiostatic intermittent titration technique (PITT) was used for data collecting. Standard electrode investigation was conducted at a galvanostatic regime of C/10. GITT measurements consisted in 30 min charges or discharges at C/20 rate with open circuit periods of 30 min. PITT measurements were conducted using potential steps of 10 mV limited by a minimum current (imin ≈ 3.5 µA) equivalent to a C/300 regime. For in situ X-ray diffraction, GITT charge and discharge were conducted at a C/10 regime, interrupted every hour by relax periods during which each pattern was recorded.
3. Results and discussions 3.1. Crystal structures of orthorhombic A-Li2 TiM(PO4 )3 (M = Fe, Cr) Besides the initial crystal structure determination of monoclinic A-Fe2 (SO4 )3 (also called β-Fe2 (SO4 )3 ) by Christidis and Rentzeperis in 1975 [33], several isostruc-
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Table 1 Lattice constants and Rietveld refinements results from neutron diffraction (on D1A at ILL Grenoble, France) with λ = 1.9104 Å, for A-Li2 TiM(PO4 )3 (M = Fe, Cr) at 300 K Temperature (K) Neutronic Device (at ILL at Grenoble, France) Wavelength (Å) Space group (No.) a (Å) b (Å) c (Å) V (Å3 ) Z d (g cm−3 ) Melting point (K) Number of “independent” reflections Number of global refined parameters Number of profile refined parameters Number of intensity-dependent parameters Rp (%) Rwp (%) Rexp (%) χ2 RBragg (%) Rf (%)
Fig. 2. Schematic of the crystal structure of Li2 TiM(PO4 )3 (M = Fe, Cr) viewed along [100]P bna .
tural compounds have been reported such as β and β LiZr2 (PO4 )3 [34], Li2 Fe2 (MoO4 )3 [35], Li2.72Ti2 (PO4 )3 [36], Li3 In2 (PO4 )3 [37] and Li3 Fe2 (PO4 )3 [16,17], etc.). In the “A” form, the [MM (XO4 )3 ] lantern units are oriented along almost perpendicular directions ([012] and [01−2]) within the (b, c) plane of the P bna space group (Fig. 2). A-form unit-cells are indexed either in an orthorhombic (P bna, #60 or P bca, #61) or in a monoclinic (P 21 /n, #14) space group, depending on the ordering or not of lithium in the structure. Note that the presence of six different settings for each space group, added to 3 possible cell choices (for space group #14) explains the large diversity of space groups (extended Hermann–Mauguin symbols) and combi-
A-Li2 TiFe(PO4 )3
A-Li2 TiCr(PO4 )3
300 D1A 1.9104 P bna (60) 8.543(1) 8.623(1) 11.978(1) 882(1) 4 3.032 1380(10) 480 1 11 38 3.57 4.34 2.39 3.31 4.82 3.34
300 D1A 1.9104 P bna (60) 8.505(1) 8.589(1) 11.929(1) 871(1) 4 3.040 1670(10) 476 1 11 34 4.54 5.68 3.69 2.50 5.14 3.51
nations of cell parameters used by the different authors to describe such materials. The structure of γ -Li3 Fe2 (PO4 )3 [16] was used as a starting model for the Rietveld refinement of A-Li2 TiM(PO4 )3 (M = Fe, Cr). From full pattern matching refinements of A-Li2 TiFe(PO4 )3 and A-Li2 TiCr(PO4 )3 , we determined that the crystal system is orthorhombic with two possible space groups, as we did not observe any monoclinic distortion at room temperature. To confirm the presence of a single orthorhombic variety, both differential scanning calorimetry and differential thermal analysis were performed but did not reveal any phase transition in the temperature range 300– 700 K. Fig. 1 shows the X-ray diffraction patterns of the two compounds. The possible Bragg positions corresponding to the P bna (a = 8.54 Å, b = 8.62 Å and c = 11.98 Å) or P bca (a = 8.54 Å, b = 8.62 Å and c = 23.95 Å) space group descriptions are also indicated. The c cell parameter must be doubled in the case of the P bca description in order to refine satisfactorily all the diffraction peaks. However, contrary to the work of Catti [26] on A-Li2 TiFe(PO4 )3 , we did not observe any superlattice Bragg peak characteristic of ordering between Fe and Ti along c. This might be due to the temperature of our synthesis, 160 degrees lower than that of Catti’s, a few degrees below the melting point. We attempted as well to refine the structure of A-Li2 TiFe(PO4 )3 , either from X-ray or neutron diffraction data, by using the atomic fractional coordinates published by Catti (half of the Li positions non-refined), without any satisfactory result. The Rietveld refinements (Fig. 3) of the neutron powder diffraction patterns converged rapidly to satisfactory
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(a)
(b)
Fig. 3. Neutron diffraction profiles of the experimental (circles) and calculated (full lines) patterns at 300 K, for (a) Li2 TiFe(PO4 )3 and (b) Li2 TiCr(PO4 )3 . Table 2 Atomic coordinates of A-Li2 TiFe(PO4 )3 at 300 K. Thermal displacement parameters (Beq ) are defined as Beq = (8π 2 /3) i j Uij · ai∗ aj∗ · ai · aj Atom
Sites
x
y
z
Beq (Å2 )
Occ.
Li Fea Tia P(1) P(2) O(1) O(2) O(3) O(4) O(5) O(6)
8d 8d 8d 8d 4c 8d 8d 8d 8d 8d 8d
0.286(2) 0.252(1) 0.252(1) 0.1042(8) 0.038(1) 0.4252(8) 0.3258(8) 0.1572(9) 0.4397(8) 0.1446(8) 0.1619(8)
0.284(2) 0.467(1) 0.467(1) 0.1076(8) 0.25 0.1049(7) 0.4878(7) 0.0634(7) 0.3601(8) 0.1618(8) 0.2723(7)
0.317(1) 0.1112(9) 0.1112(9) 0.1479(6) 0.5 0.3438(5) 0.2727(5) 0.0294(5) 0.0704(5) 0.4191(6) 0.1773(5)
1.5(3) 1.0(1) 1.0(1) 0.8(1) 0.9(2) 1.0(1) 0.9(1) 1.2(1) 1.4(1) 1.2(1) 1.1(1)
1 0.5 0.5 1 1 1 1 1 1 1 1
a Constrained.
Table 3 Atomic coordinates of A-Li2 TiCr(PO4 )3 at 300 K. Thermal displacement parameters (Beq ) are defined as Beq = (8π 2 /3) i j Uij · ai∗ aj∗ · ai · aj Atom
Sites
x
y
z
Beq (Å2 )
Occ.
Li Cra Tia P(l) P(2) O(1) O(2) O(3) O(4) O(5) O(6)
8d 8d 8d 8d 4c 8d 8d 8d 8d 8d 8d
0.286(3) 0.253 0.253 0.1048(9) 0.035(1) 0.4249(9) 0.3256(9) 0.1585(9) 0.4384(9) 0.1439(9) 0.1595(9)
0.287(3) 0.468 0.468 0.1068(9) 0.25 0.1048(8) 0.4887(9) 0.0617(9) 0.3582(9) 0.160(1) 0.2742(8)
0.319(2) 0.111 0.111 0.1473(7) 0.5 0.3437(7) 0.2718(6) 0.0290(7) 0.0718(6) 0.4197(7) 0.1763(6)
1.3(3) 0.7b 0.7b 0.5(1) 0.8(2) 1.1(1) 0.6(1) 1.4(1) 1.0(1) 1.1(1) 0.8(1)
1 0.5b 0.5b 1 1 1 1 1 1 1 1
a Constrained, but never refined simultaneously. b Not refined.
agreement factors and to thermal parameters with reasonable values. The refinements were performed in the P bna space group with a = 8.543(1) Å, b = 8.623(1) Å, c = 11.978(1) Å and a = 8.505(1) Å, b = 8.589(1) Å, c = 11.929(1) Å for A-Li2 TiFe(PO4 )3 and A-Li2 TiCr(PO4 )3 , respectively (Table 1). The asymmetric unit of the cells contains 3 PO4 tetrahedra (2 ∗ P(1)O4 + 1 ∗ P(2)O4 ) and 2 MO6 octahedra (M = Ti, Cr/Fe), into which the atoms of Ti and
Fe (similarly for Ti and Cr) are disordered. The atomic coordinates (Z = 4) are reported in Table 2 for A-Li2 TiFe(PO4 )3 and Table 3 for A-Li2 TiCr(PO4 )3 . In a first attempt, the two lithium ions were refined into the three crystallographic sites (Li(1), Li(2) and Li(3)) described previously for γ -A-Li3 Fe2 (PO4 )3 [16]. The occupancy factors obtained for Li(2) and Li(3) were close to 0 and negative, respectively, and their isotropic ther-
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Table 4 Selected bond lengths (Å) with standard deviation in parentheses. Average distances (X–O), predicted distances (predict.), bond valences (Vsum ) and distortion parameters (∆) of coordinate polyhedra are calculated using the Zachariasen formula A-Li2 TiFe(PO4 )3
A-Li2 TiCr(PO4 )3
Li: O(1) O(2) O(5) O(6) Li–O = 1.961(1), predict. = 1.979, ∆ = 83 × 10−3 and Vsum = 1.06(1)
1.974(8) 1.867(8) 2.015(8) 1.990(8)
Li: O(1) O(2) O(5) O(6) Li–O = 1.968(5), predict. = 1.979, ∆ = 122 × 10−3 and Vsum = 1.05(1)
1.983(9) 1.852(9) 2.025(9) 2.011(9)
Fe/Ti: O(1) O(2) O(3) O(4) O(6) O(6) Fe–O = 1.964(2), predict. = 2.016, ∆ = 82 × 10−3 and Vsum = 3.49(2) Ti–O = 1.964(2), predict. = 1.965, ∆ = 82 × 10−3 and Vsum = 4.06(2)
1.999(5) 2.045(5) 1.884(5) 1.914(6) 1.937(5) 2.005(5)
Cr/Ti: O(1) O(2) O(3) O(4) O(6) O(6) Cr–O = 1.948(1), predict. = 1.981, ∆ = 82 × 10−3 and Vsum = 3.32(1) Ti–O = 1.948(1), predict. = 1.965, ∆ = 82 × 10−3 and Vsum = 4.24(1)
1.989(2) 2.019(2) 1.875(3) 1.898(2) 1.907(3) 1.998(2)
P(1): O(1) O(2) O(3) O(6) P–O = 1.535(1), predict. = 1.521, ∆ = 2 × 10−3 and Vsum = 4.82(2)
1.533(3) 1.527(3) 1.538(3) 1.544(3)
P(1): O(1) O(2) O(3) O(6) P–O = 1.534(2), predict. = 1.521, ∆ = 5 × 10−3 and Vsum = 4.83(3)
1.534(4) 1.520(4) 1.532(3) 1.551(3)
P(2): 2∗O(4) 2∗O(5) P–O = 1.527(1), predict. = 1.521, ∆ = 0.4 × 10−3 and Vsum = 4.92(2)
1.524(3) 1.530(3)
P(2): 2∗O(4) 2∗O(5) P–O = 1.524(2), predict. = 1.521, ∆ = 12 × 10−3 and Vsum = 4.98(3)
1.507(3) 1.540(4)
mal displacements gave unreasonable values. Moreover, the distances, angles, bond-strengths and polyhedra distortions found in this case were unrealistic. We decided then to refine separately each of these sites and found out that only the Li(1) site (8d) was occupied, fully, as for Li2 Fe2 (MoO4)3 [35]. The lithium ions occupy then a single four-fold coordinated site, forming Li–O–M–O–Li–O infinite chains along [010] of edge-sharing LiO4 tetrahedra and MO6 octahedra. The refinement thus obtained conducted to very good factors of reliability (Table 1). All the thermal displacement factors are given as isotropic ones, in particular for the Li(1) site, as anisotropic refinements did not lead to any improvement of the structure. For A-Li2 TiFe(PO4 )3 , the atomic coordinates of Ti and Fe, and their Beq were constrained to be equal. For A-Li2 TiCr(PO4 )3 , we could not refine simultaneously the atomic coordinates of Ti and Cr as their scattering lengths are almost equal, but with opposite signs (bTi = −0.34 and bCr = +0.26). To overpass this problem, we refined the atomic coordinates of Cr and Ti one to one until their values became very close (equal at 3 significant numbers), after which they were constrained to be equal. The isotropic thermal displacements were then fixed to usual values for such a kind of structure. Table 4 presents selected bond lengths with the standard deviations in parentheses for both A-Li2 TiFe(PO4 )3 and A-Li2 TiCr(PO4 )3 . As
mentioned above, the correctness of the structure was confirmed by bond-valence calculations from the Zachariasen formula (Vi = j Sij = j exp{(r0 − rij )/0.37}), using the parameters r0 , characterizing a cation–anion pair [38]. The distortion parameters ∆ of a coordination polyhedron MON with an average M–O distance r, which is defined as ∆ = (1/N) n=1 to N {(rn − r)/r}2 , are also indicated in Table 4.
4. Electrochemical and chemical insertion of lithium into Li2 TiM(PO4 )3 (M = Fe, Cr) As plotted in Fig. 4, the electrochemical insertion of lithium into Li2 TiM(PO4 )3 (M = Fe, Cr) proceeds as a smooth decrease of the cell voltage between 3.2 and 2.0 V vs. Li+ /Li (Fig. 4). These data show an excellent reversibility of such positive electrode materials that operate on the Fe3+/2+ and Ti4+/3+ redox couples. As reported by Morin [39] for Li3 Cr2 (PO4 )3 , Cr3+ remains inactive towards reduction vs. Li in Li2+x TiCr(PO4 )3 . At C/20 regime, the cells reached almost their theoretical capacities (131 mAh g−1 for Li2 TiFe(PO4 )3 /Li4 TiFe(PO4 )3 ) that correspond to the complete reduction of Ti4+ and Fe3+ into Ti3+ and Fe2+ , respectively.
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As chromium is inactive, the GITT data of Li2+x TiCr(PO4 )3 locates the reduction of Ti4+ to Ti3+ at values comprised in between 2.60 and 2.25 V. Accurate examination of this data reveals the presence of two distinct “S-type shape” curves which are separated by an inflection point at x ∼ 0.5. This is more obviously seen in the −∂x/∂V vs. potential plot in the insert of Fig. 4. Two distinct redox values were found at ∼ 2.50 (0 x 0.5) and ∼ 2.35 (0.5 x 1). The existence of these two distinct potentials might be the
Fig. 4. Potential-composition curves vs. Li+ /Li of A-Li2 TiFe(PO4 )3 and A-Li2 TiCr(PO4 )3 , performed in GITT mode (regime of C/20 for 30 min and relax periods of 30 min). The derivative curves (−∂x/∂V ) are deduced from PITT measurements with a limiting current corresponding to a C/300 regime.
consequence of the progressive filling of two vacant lithium sites Li(2) and Li(3) towards a probable similar distribution as in the γ form of A-Li3 Fe2 (PO4 )3 [16,18]: Li(1) fully occupied, Li(2) and Li(3) occupied at 1/4th in the new composition Li3 TiCr(PO4 )3 . In the case of A-Li2+x TiFe(PO4 )3 , at least two signals, corresponding to the redox couples Fe3+/2+ (∼ 2.8 V vs. Li+ /Li, 0 x 1) and Ti4+/3+ (∼ 2.5 V vs. Li+ /Li, 1 x 2), were separated in agreement with usual values reported in the literature [40]. However, potential intermittent titration technique (PITT) reveals (Fig. 5) solid solution insertion/extraction processes all along the 0 x 2 com-
Fig. 5. Potentiometric titration curve (PITT) of A-Li2+x TiFe(PO4 )3 in the 3.4–2.0 V range vs. Li+ /Li (imin = 3.5 µA).
Fig. 6. X-ray diffraction patterns collected in situ during lithium insertion into Li2+x TiFe(PO4 )3 .
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Fig. 8. X-ray diffraction patterns at 300 K of Li4 TiFe(PO4 )3 prepared by either chemical lithiation or electrochemical reduction. The Bragg positions are calculated in the space group P bna with a = 8.523 Å, b = 8.728 Å and c = 12.177 Å.
Acknowledgements
Fig. 7. Refined cell dimensions of A-Li2+x TiFe(PO4 )3 (opened symbols) and A-Li2+y TiCr(PO4 )3 (filled symbols) (0 x 2 and 0 y 1) as a function of lithium inserted. The cross symbols indicate the values obtained for chemically-lithiated Li4 TiFe(PO4 )3 .
The authors express their sincere thanks to J.M. Tarascon, M. Morcrette, M. Nelson and C. Delacourt for fruitful discussions and constructive critics of the manuscript.
References positional range for Li2+x √TiFe(PO4 )3 . The shapes of the current decays (i = f (1/ t )) denote kinetic limitation of the electron scattering inside the materials. Such a singlephase domain surprisingly contrasts with Li1+x Ti2 (PO4 )3 and Li3+x Fe2 (PO4 )3 , for which two-phase lithium insertion/extraction reactions were systematically observed. The solid solution lithium insertion/extraction mechanism is further supported by the in situ X-ray diffraction data of Fig. 6 collected during lithium insertion all along 0 x 2 for Li2+x TiFe(PO4 )3 . Continuous shifts (with a small slope change around x = 1 though) of the diffraction peaks towards smaller values of 2θ are observed, which, after full pattern refinements in the P bna space group, translates into global unit cell volume variations
V /V of +1.81% for Li2+x TiCr(PO4 )3 and of +2.76% for Li2+x TiFe(PO4 )3 (Fig. 7) when Ti4+ and/or Fe3+ are reduced to Ti3+ Fe2+ . We confirmed the crystallographic features of the fully lithiated phase Li4 TiFe(PO4 )3 by preparing this composition from a classical chemical lithiation reaction involving n-BuLi as the reducing agent in hexane solution. After immersion for 7 days under magnetic stirring, the powder was washed 4 times, dried under vacuum for several hours and kept in an argon-filled dry box. The resulting powder was placed within the in situ cell for XRD data collection (Fig. 8). The refined lattice parameters agree very satisfactorily with those deduced from the in situ experiment at the following values: a = 8.523(6) Å, b = 8.728(6) Å and c = 12.177(9) Å in the P bna space group.
[1] A.K. Padhi, PhD thesis, University of Texas at Austin, USA, 1997. [2] K.S. Nanjundaswamy, A.K. Padhi, J.B. Goodenough, S. Okada, H. Ohtsuka, H. Arai, J. Yamaki, Solid State Ionics 92 (1996) 1–10. [3] A. Manthiram, J.B. Goodenough, J. Solid State Chem. 71 (1987) 349– 360. [4] A. Manthiram, J.B. Goodenough, J. Power Sources 26 (1989) 403– 408. [5] A.K. Padhi, K.S. Nanjundaswamy, C. Masquelier, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 2581–2586. [6] A.K. Padhi, K.S. Nanjundaswamy, C. Masquelier, S. Okada, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1609–1613. [7] A.K. Padhi, V. Manivannan, J.B. Goodenough, J. Electrochem. Soc. 145 (1998) 1518–1520. [8] S. Okada, K.S. Nanjundaswamy, A. Manthiram, J.B. Goodenough, H. Ohtsuka, H. Arai, J. Yamaki, in: 36th Power Sources Conference, June 6–9, 1994. [9] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188–1194. [10] C. Masquelier, A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Solid State Chem. 135 (1998) 228–234. [11] N. Ravet, Y. Chouinard, J.F. Magnan, S. Besner, M. Gaulthier, M. Armand, J. Power Sources 97–98 (2001) 503–507. [12] H. Huang, S.-C. Yin, L.F. Nazar, Electrochem. Solid State Lett. 4 (2001) 170–172. [13] H. Huang, S.C. Yin, T. Kerr, N. Taylor, L.F. Nazar, Adv. Mater. 14 (21) (2002) 1525–1528. [14] M.Y. Saïdi, J. Barker, H. Huang, J.L. Swoyer, G. Adamson, Electrochem. Solid State Lett. 5 (7) (2002) A149–A151; M.Y. Saïdi, J. Barker, H. Huang, J.L. Swoyer, G. Adamson, J. Power Sources 119–121 (2003) 266–272. [15] M. Morcrette, C. Wurm, C. Masquelier, Solid State Sci. 4 (2002) 239– 246. [16] A.B. Bykov, A.P. Chirkin, L.N. Demyanets, S.N. Doronin, E.A. Genkina, A.K. Ivanov-shits, I.P. Kondratyuf, B.A. Maksimov, O.K.
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[28] S. Patoux, PhD thesis, Université de Picardie Jules Verne at Amiens, France, 2003. [29] S. Patoux, G. Rousse, J.-B. Leriche, C. Masquelier, Chem. Mater. 15 (2003) 2084–2093. [30] J. Rodriguez-Carvajal, Physica B 192 (1993) 55, WinPLOTR software downloadable at: http://www-llb.cea.fr/fullweb/winplotr/winplotr. htm. [31] H.M. Rietveld, J. Appl. Crystallogr. 2 (1969) 65. [32] S. Patoux, C. Masquelier, Chem. Mater. 14 (2002) 2334–2341. [33] P.C. Christidis, P.J. Rentzeperis, Z. Kristallogr. 141 (1975) 233–245. [34] M. Catti, N. Morgante, R.M. Ibberson, J. Solid State Chem. 152 (2000) 340–347. [35] C.C. Torardi, E. Prince, Matter. Res. Bull. 21 (1986) 719–726. [36] S. Wang, S.-J. Hwu, Chem. Mater. 4 (1992) 589–595; S. Wang, S.-J. Hwu, J. Solid State Chem. 90 (1991) 377–381. [37] D. Tran Qui, S. Hamdoune, Acta Crystallogr. C 43 (1987) 397– 399. [38] I.D. Brown, D. Altermatt, Acta Crystallogr. B 41 (1985) 244. [39] E. Morin, PhD thesis, Université Pierre et Marie Curie at Paris VI, France, 1998. [40] C. Masquelier, S. Patoux, C. Wurm, M. Morcrette, in: A. Nazri, G. Pistoia (Eds.), Lithium Batteries, Science & Technology, Kluwer Academic, 2004, pp. 445–477.
UP section
Crystal structure and lithium insertion properties of orthorhombic Li2 TiFe(PO4 )3 and Li2 TiCr(PO4 )3 Sébastien Patoux a,1 , Gwenaëlle Rousse b , Jean-Bernard Leriche a , Christian Masquelier a,∗ a Laboratoire de réactivité et de chimie des solides, CNRS UMR 6007, université de Picardie Jules Verne, 33, rue Saint-Leu, 80039 Amiens cedex, France b Laboratoire de physique des milieux condensés, université Pierre et Marie Curie, 4, place Jussieu, 75252 Paris cedex 05, France
Received 15 July 2004; accepted 28 July 2004 Available online 16 September 2004
Abstract The crystal structures of orthorhombic Li2 TiFe(PO4 )3 and Li2 TiCr(PO4 )3 were determined from Rietveld refinement of neutron powder diffraction data. Both compounds are isostructural in the P bna space group with a = 8.543(1) Å, b = 8.623(1) Å, c = 11.978(1) Å and a = 8.505(1) Å, b = 8.589(1) Å, c = 11.929(1) Å, respectively. Lithium ions are located in a single four-fold coordinated site Li(1). Electrochemical insertion of lithium proceeds through the reduction of Fe3+ into Fe2+ (∼ 2.8 V vs. Li+ /Li) and Ti4+ into Ti3+ (∼ 2.5 V vs. Li+ /Li) towards the new compositions Li3 TiCr(PO4 )3 and Li4 TiFe(PO4 )3 . In situ X-ray diffraction and potentiostatic intermittent titration technique indicate a solid solution mechanism for the reversible lithium insertion/extraction. Chemical reaction of Li2 TiFe(PO4 )3 with n-BuLi leads to orthorhombic Li4 TiFe(PO4 )3 . 2004 Elsevier SAS. All rights reserved. Keywords: Lithium; Phosphates; Anti-NASICON; Positive electrodes; Lithium batteries
1. Introduction An alternative class of positive electrode materials for lithium batteries based on three-dimensional opened-frameworks that reversibly insert lithium was proposed in the late 90’s [1,2]. These materials take advantage of a relatively high lithium ion mobility and benefit from the inductive effect of their polyanionic groups, which increase the operating voltage in comparison with simple oxides [3,4]. In this context, much attention was given to phosphates [5,6] and sulfates [7,8] of iron and vanadium. Good reversibility was demonstrated for the Fe3+ /Fe2+ couple in cheap and non-toxic iron-containing materials such as Fe2 (SO4 )3 (3.6 V vs. Li+ /Li) [8], LiFePO4 (3.45 V vs. Li+ /Li) [9] and Li3 Fe2 (PO4 )3 (2.8 V vs. Li+ /Li) [10]. * Corresponding author.
E-mail address: [email protected] (C. Masquelier). 1 Present address: Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA. 1293-2558/$ – see front matter 2004 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2004.07.028
Compositions of general formula Zx MM (XO4 )3 (Z = Li, Na; M & M a 3d element, X = P, As, S, Si, . . . ) may adopt two distinct crystallographic forms, denoted “A” (Sc2 (WO4 )3 -type) and “B” (NASICON-type), which differ in the way the [MM (XO4 )3 ] “lantern units” are connected. Depending on both the stable oxidation number of M or M in air (Mn+ = Nb5+ , Ti4+ , Fe3+ , for instance) and on the chemical nature of X (X = Si, P, S, W, Mo) and Z+ , a wide variety of compositions may be prepared for 0 x 4: Fe2 (SO4 )3 , NbTi(PO4)3 , LiTi2 (PO4 )3 , Li2 FeTi(PO4 )3 , Na3 Fe2 (PO4 )3 , Na4 Zr2 (SiO4 )3 , etc. Electrochemical insertion or extraction of lithium or sodium into/from these compositions lead to an even greater variety of materials, with unusual oxidation states and alkali ion distributions within the interstitial space (0 x 5). Recent progresses have been communicated recently on how to make them work efficiently at high current densities through “smart” conductive carbon coating at the surface of the particles [11–14]. A comparative electrochemical study for the A and B forms of Li3 Fe2 (PO4 )3 was recently communicated [10, 15]. Detailed structural descriptions are given in [16–18].
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LiTi2 (PO4 )3 adopts the B form only, which corresponds to the well-known NASICON structure inside which the Li ions are located in the six-fold coordinated site that is labeled M1 [19]. The lithium insertion in NASICON Li1+x Ti2 (PO4 )3 was firstly investigated by Delmas et al. in the late 80’s [20–22] and more recently through contributions from the same group [23,24] and from us [25]. In between the well-known LiTi2 (PO4 )3 and Li3 Fe2 (PO4 )3 compositions, the mixed Ti/Fe phosphate Li2 TiFe(PO4 )3 (TiIV , FeIII ) has been the object of only limited work so far, besides the first tentative of structural determination of orthorhombic A-Li2 FeTi(PO4 )3 by Catti [26] and preliminary electrochemical investigations on NASICON B-Li2+x FeTi(PO4 )3 by Goodenough’s group [5,27]. There was room then for detailed structural and electrochemical investigations of these systems that offer the possibility of understanding the relative stabilities of the [MM (PO4 )3 ] framework depending on the nature of the 3d element and on the value of x in Zx MM (PO4 )3 [28]. Our structural and electrochemical investigations of the NASICON forms B-Li2 TiM(PO4 )3 (M = Fe, Cr) were recently published [29]. In this paper, we first present the crystal structures of both A-Li2 TiM(PO4 )3 (M = Fe, Cr) solved by Rietveld refinement of neutron diffraction data and then their lithium insertion properties, investigated through slow electrochemical titration (GITT, PITT) and in situ X-ray diffraction. A fully lithiated A-Li4 TiFe(PO4 )3 composition was prepared by chemical reduction, and is compared with that of the in situ experiment.
2. Experimental A-Li2 TiM(PO4 )3 (M = Fe, Cr) were prepared by high temperature solid state reaction, starting from the respective oxides (TiO2 , Fe2 O3 and Cr2 O3 ), ammonium dihydrogen phosphate and lithium dihydrogen phosphate in stoichiometric proportions. Pure powders were obtained, after several intermediate thermal treatments and grindings, by prolonged final treatment at 1203 K. Phase purities and lattice parameter determinations were carefully monitored by X-ray diffraction on either a Philips PW1710 diffractometer (CuKα radiation, θ –2θ geometry, back monochromator) or a D8 Bruker diffractometer (Co-Kα radiation, θ –θ geometry, back monochromator). The experimental X-ray diffractograms with two possible unit cells (and corresponding space groups) are gathered in Fig. 1, as will be discussed later. The list of lattice parameters and space groups of the two compounds investigated is given in Table 1. The crystal structures were determined from neutron diffraction data recorded at 300 K on the high resolution powder diffractometer D1A at ILL, Grenoble, France. Data collection with high direct space −1 resolution (λ = 1.9104 Å, Qmax = 6.5 Å ) allowed a precise determination of the nuclear crystal structures. The program Fullprof [30] was used for structure refinements by
Fig. 1. X-ray diffraction patterns of A-Li2 TiM(PO4 )3 (M = Fe, Cr) with Bragg positions calculated in the space groups P bna and P bca.
using the Rietveld method [31]. For in situ X-ray diffraction, a Swagelok-type cell similar to the one described elsewhere [32] was mounted, horizontally, on the D8 Bruker diffractometer (Co-Kα radiation, θ –θ geometry, PSD counter). Electrochemical lithium insertion was undertaken in standard SwagelokTM cell configurations with a lithium foil as the negative electrode and the active materials (AM) as the positive electrode. The separator used was a Whatman GF/D borosilicate glass fibre sheet saturated with a 1 M LiPF6 electrolyte solution in EC:DMC (1:1 in weight). For optimal electrochemical activity, A-Li2 TiM(PO4 )3 powders were initially mixed with Super P carbon (MMM Carbon, Belgium) by ball-milling in a SPEX 8000 mixer that generates energetic shocks between powder particles (250 mg of AM + 50 mg of carbon, stainless steel ball of 5 g, 30 min). A “Mac-Pile” automatic cycling/data recording system (Biologic SA, Claix, France) operating either in galvanostatic intermittent titration technique (GITT) or in potentiostatic intermittent titration technique (PITT) was used for data collecting. Standard electrode investigation was conducted at a galvanostatic regime of C/10. GITT measurements consisted in 30 min charges or discharges at C/20 rate with open circuit periods of 30 min. PITT measurements were conducted using potential steps of 10 mV limited by a minimum current (imin ≈ 3.5 µA) equivalent to a C/300 regime. For in situ X-ray diffraction, GITT charge and discharge were conducted at a C/10 regime, interrupted every hour by relax periods during which each pattern was recorded.
3. Results and discussions 3.1. Crystal structures of orthorhombic A-Li2 TiM(PO4 )3 (M = Fe, Cr) Besides the initial crystal structure determination of monoclinic A-Fe2 (SO4 )3 (also called β-Fe2 (SO4 )3 ) by Christidis and Rentzeperis in 1975 [33], several isostruc-
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Table 1 Lattice constants and Rietveld refinements results from neutron diffraction (on D1A at ILL Grenoble, France) with λ = 1.9104 Å, for A-Li2 TiM(PO4 )3 (M = Fe, Cr) at 300 K Temperature (K) Neutronic Device (at ILL at Grenoble, France) Wavelength (Å) Space group (No.) a (Å) b (Å) c (Å) V (Å3 ) Z d (g cm−3 ) Melting point (K) Number of “independent” reflections Number of global refined parameters Number of profile refined parameters Number of intensity-dependent parameters Rp (%) Rwp (%) Rexp (%) χ2 RBragg (%) Rf (%)
Fig. 2. Schematic of the crystal structure of Li2 TiM(PO4 )3 (M = Fe, Cr) viewed along [100]P bna .
tural compounds have been reported such as β and β LiZr2 (PO4 )3 [34], Li2 Fe2 (MoO4 )3 [35], Li2.72Ti2 (PO4 )3 [36], Li3 In2 (PO4 )3 [37] and Li3 Fe2 (PO4 )3 [16,17], etc.). In the “A” form, the [MM (XO4 )3 ] lantern units are oriented along almost perpendicular directions ([012] and [01−2]) within the (b, c) plane of the P bna space group (Fig. 2). A-form unit-cells are indexed either in an orthorhombic (P bna, #60 or P bca, #61) or in a monoclinic (P 21 /n, #14) space group, depending on the ordering or not of lithium in the structure. Note that the presence of six different settings for each space group, added to 3 possible cell choices (for space group #14) explains the large diversity of space groups (extended Hermann–Mauguin symbols) and combi-
A-Li2 TiFe(PO4 )3
A-Li2 TiCr(PO4 )3
300 D1A 1.9104 P bna (60) 8.543(1) 8.623(1) 11.978(1) 882(1) 4 3.032 1380(10) 480 1 11 38 3.57 4.34 2.39 3.31 4.82 3.34
300 D1A 1.9104 P bna (60) 8.505(1) 8.589(1) 11.929(1) 871(1) 4 3.040 1670(10) 476 1 11 34 4.54 5.68 3.69 2.50 5.14 3.51
nations of cell parameters used by the different authors to describe such materials. The structure of γ -Li3 Fe2 (PO4 )3 [16] was used as a starting model for the Rietveld refinement of A-Li2 TiM(PO4 )3 (M = Fe, Cr). From full pattern matching refinements of A-Li2 TiFe(PO4 )3 and A-Li2 TiCr(PO4 )3 , we determined that the crystal system is orthorhombic with two possible space groups, as we did not observe any monoclinic distortion at room temperature. To confirm the presence of a single orthorhombic variety, both differential scanning calorimetry and differential thermal analysis were performed but did not reveal any phase transition in the temperature range 300– 700 K. Fig. 1 shows the X-ray diffraction patterns of the two compounds. The possible Bragg positions corresponding to the P bna (a = 8.54 Å, b = 8.62 Å and c = 11.98 Å) or P bca (a = 8.54 Å, b = 8.62 Å and c = 23.95 Å) space group descriptions are also indicated. The c cell parameter must be doubled in the case of the P bca description in order to refine satisfactorily all the diffraction peaks. However, contrary to the work of Catti [26] on A-Li2 TiFe(PO4 )3 , we did not observe any superlattice Bragg peak characteristic of ordering between Fe and Ti along c. This might be due to the temperature of our synthesis, 160 degrees lower than that of Catti’s, a few degrees below the melting point. We attempted as well to refine the structure of A-Li2 TiFe(PO4 )3 , either from X-ray or neutron diffraction data, by using the atomic fractional coordinates published by Catti (half of the Li positions non-refined), without any satisfactory result. The Rietveld refinements (Fig. 3) of the neutron powder diffraction patterns converged rapidly to satisfactory
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(a)
(b)
Fig. 3. Neutron diffraction profiles of the experimental (circles) and calculated (full lines) patterns at 300 K, for (a) Li2 TiFe(PO4 )3 and (b) Li2 TiCr(PO4 )3 . Table 2 Atomic coordinates of A-Li2 TiFe(PO4 )3 at 300 K. Thermal displacement parameters (Beq ) are defined as Beq = (8π 2 /3) i j Uij · ai∗ aj∗ · ai · aj Atom
Sites
x
y
z
Beq (Å2 )
Occ.
Li Fea Tia P(1) P(2) O(1) O(2) O(3) O(4) O(5) O(6)
8d 8d 8d 8d 4c 8d 8d 8d 8d 8d 8d
0.286(2) 0.252(1) 0.252(1) 0.1042(8) 0.038(1) 0.4252(8) 0.3258(8) 0.1572(9) 0.4397(8) 0.1446(8) 0.1619(8)
0.284(2) 0.467(1) 0.467(1) 0.1076(8) 0.25 0.1049(7) 0.4878(7) 0.0634(7) 0.3601(8) 0.1618(8) 0.2723(7)
0.317(1) 0.1112(9) 0.1112(9) 0.1479(6) 0.5 0.3438(5) 0.2727(5) 0.0294(5) 0.0704(5) 0.4191(6) 0.1773(5)
1.5(3) 1.0(1) 1.0(1) 0.8(1) 0.9(2) 1.0(1) 0.9(1) 1.2(1) 1.4(1) 1.2(1) 1.1(1)
1 0.5 0.5 1 1 1 1 1 1 1 1
a Constrained.
Table 3 Atomic coordinates of A-Li2 TiCr(PO4 )3 at 300 K. Thermal displacement parameters (Beq ) are defined as Beq = (8π 2 /3) i j Uij · ai∗ aj∗ · ai · aj Atom
Sites
x
y
z
Beq (Å2 )
Occ.
Li Cra Tia P(l) P(2) O(1) O(2) O(3) O(4) O(5) O(6)
8d 8d 8d 8d 4c 8d 8d 8d 8d 8d 8d
0.286(3) 0.253 0.253 0.1048(9) 0.035(1) 0.4249(9) 0.3256(9) 0.1585(9) 0.4384(9) 0.1439(9) 0.1595(9)
0.287(3) 0.468 0.468 0.1068(9) 0.25 0.1048(8) 0.4887(9) 0.0617(9) 0.3582(9) 0.160(1) 0.2742(8)
0.319(2) 0.111 0.111 0.1473(7) 0.5 0.3437(7) 0.2718(6) 0.0290(7) 0.0718(6) 0.4197(7) 0.1763(6)
1.3(3) 0.7b 0.7b 0.5(1) 0.8(2) 1.1(1) 0.6(1) 1.4(1) 1.0(1) 1.1(1) 0.8(1)
1 0.5b 0.5b 1 1 1 1 1 1 1 1
a Constrained, but never refined simultaneously. b Not refined.
agreement factors and to thermal parameters with reasonable values. The refinements were performed in the P bna space group with a = 8.543(1) Å, b = 8.623(1) Å, c = 11.978(1) Å and a = 8.505(1) Å, b = 8.589(1) Å, c = 11.929(1) Å for A-Li2 TiFe(PO4 )3 and A-Li2 TiCr(PO4 )3 , respectively (Table 1). The asymmetric unit of the cells contains 3 PO4 tetrahedra (2 ∗ P(1)O4 + 1 ∗ P(2)O4 ) and 2 MO6 octahedra (M = Ti, Cr/Fe), into which the atoms of Ti and
Fe (similarly for Ti and Cr) are disordered. The atomic coordinates (Z = 4) are reported in Table 2 for A-Li2 TiFe(PO4 )3 and Table 3 for A-Li2 TiCr(PO4 )3 . In a first attempt, the two lithium ions were refined into the three crystallographic sites (Li(1), Li(2) and Li(3)) described previously for γ -A-Li3 Fe2 (PO4 )3 [16]. The occupancy factors obtained for Li(2) and Li(3) were close to 0 and negative, respectively, and their isotropic ther-
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Table 4 Selected bond lengths (Å) with standard deviation in parentheses. Average distances (X–O), predicted distances (predict.), bond valences (Vsum ) and distortion parameters (∆) of coordinate polyhedra are calculated using the Zachariasen formula A-Li2 TiFe(PO4 )3
A-Li2 TiCr(PO4 )3
Li: O(1) O(2) O(5) O(6) Li–O = 1.961(1), predict. = 1.979, ∆ = 83 × 10−3 and Vsum = 1.06(1)
1.974(8) 1.867(8) 2.015(8) 1.990(8)
Li: O(1) O(2) O(5) O(6) Li–O = 1.968(5), predict. = 1.979, ∆ = 122 × 10−3 and Vsum = 1.05(1)
1.983(9) 1.852(9) 2.025(9) 2.011(9)
Fe/Ti: O(1) O(2) O(3) O(4) O(6) O(6) Fe–O = 1.964(2), predict. = 2.016, ∆ = 82 × 10−3 and Vsum = 3.49(2) Ti–O = 1.964(2), predict. = 1.965, ∆ = 82 × 10−3 and Vsum = 4.06(2)
1.999(5) 2.045(5) 1.884(5) 1.914(6) 1.937(5) 2.005(5)
Cr/Ti: O(1) O(2) O(3) O(4) O(6) O(6) Cr–O = 1.948(1), predict. = 1.981, ∆ = 82 × 10−3 and Vsum = 3.32(1) Ti–O = 1.948(1), predict. = 1.965, ∆ = 82 × 10−3 and Vsum = 4.24(1)
1.989(2) 2.019(2) 1.875(3) 1.898(2) 1.907(3) 1.998(2)
P(1): O(1) O(2) O(3) O(6) P–O = 1.535(1), predict. = 1.521, ∆ = 2 × 10−3 and Vsum = 4.82(2)
1.533(3) 1.527(3) 1.538(3) 1.544(3)
P(1): O(1) O(2) O(3) O(6) P–O = 1.534(2), predict. = 1.521, ∆ = 5 × 10−3 and Vsum = 4.83(3)
1.534(4) 1.520(4) 1.532(3) 1.551(3)
P(2): 2∗O(4) 2∗O(5) P–O = 1.527(1), predict. = 1.521, ∆ = 0.4 × 10−3 and Vsum = 4.92(2)
1.524(3) 1.530(3)
P(2): 2∗O(4) 2∗O(5) P–O = 1.524(2), predict. = 1.521, ∆ = 12 × 10−3 and Vsum = 4.98(3)
1.507(3) 1.540(4)
mal displacements gave unreasonable values. Moreover, the distances, angles, bond-strengths and polyhedra distortions found in this case were unrealistic. We decided then to refine separately each of these sites and found out that only the Li(1) site (8d) was occupied, fully, as for Li2 Fe2 (MoO4)3 [35]. The lithium ions occupy then a single four-fold coordinated site, forming Li–O–M–O–Li–O infinite chains along [010] of edge-sharing LiO4 tetrahedra and MO6 octahedra. The refinement thus obtained conducted to very good factors of reliability (Table 1). All the thermal displacement factors are given as isotropic ones, in particular for the Li(1) site, as anisotropic refinements did not lead to any improvement of the structure. For A-Li2 TiFe(PO4 )3 , the atomic coordinates of Ti and Fe, and their Beq were constrained to be equal. For A-Li2 TiCr(PO4 )3 , we could not refine simultaneously the atomic coordinates of Ti and Cr as their scattering lengths are almost equal, but with opposite signs (bTi = −0.34 and bCr = +0.26). To overpass this problem, we refined the atomic coordinates of Cr and Ti one to one until their values became very close (equal at 3 significant numbers), after which they were constrained to be equal. The isotropic thermal displacements were then fixed to usual values for such a kind of structure. Table 4 presents selected bond lengths with the standard deviations in parentheses for both A-Li2 TiFe(PO4 )3 and A-Li2 TiCr(PO4 )3 . As
mentioned above, the correctness of the structure was confirmed by bond-valence calculations from the Zachariasen formula (Vi = j Sij = j exp{(r0 − rij )/0.37}), using the parameters r0 , characterizing a cation–anion pair [38]. The distortion parameters ∆ of a coordination polyhedron MON with an average M–O distance r, which is defined as ∆ = (1/N) n=1 to N {(rn − r)/r}2 , are also indicated in Table 4.
4. Electrochemical and chemical insertion of lithium into Li2 TiM(PO4 )3 (M = Fe, Cr) As plotted in Fig. 4, the electrochemical insertion of lithium into Li2 TiM(PO4 )3 (M = Fe, Cr) proceeds as a smooth decrease of the cell voltage between 3.2 and 2.0 V vs. Li+ /Li (Fig. 4). These data show an excellent reversibility of such positive electrode materials that operate on the Fe3+/2+ and Ti4+/3+ redox couples. As reported by Morin [39] for Li3 Cr2 (PO4 )3 , Cr3+ remains inactive towards reduction vs. Li in Li2+x TiCr(PO4 )3 . At C/20 regime, the cells reached almost their theoretical capacities (131 mAh g−1 for Li2 TiFe(PO4 )3 /Li4 TiFe(PO4 )3 ) that correspond to the complete reduction of Ti4+ and Fe3+ into Ti3+ and Fe2+ , respectively.
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As chromium is inactive, the GITT data of Li2+x TiCr(PO4 )3 locates the reduction of Ti4+ to Ti3+ at values comprised in between 2.60 and 2.25 V. Accurate examination of this data reveals the presence of two distinct “S-type shape” curves which are separated by an inflection point at x ∼ 0.5. This is more obviously seen in the −∂x/∂V vs. potential plot in the insert of Fig. 4. Two distinct redox values were found at ∼ 2.50 (0 x 0.5) and ∼ 2.35 (0.5 x 1). The existence of these two distinct potentials might be the
Fig. 4. Potential-composition curves vs. Li+ /Li of A-Li2 TiFe(PO4 )3 and A-Li2 TiCr(PO4 )3 , performed in GITT mode (regime of C/20 for 30 min and relax periods of 30 min). The derivative curves (−∂x/∂V ) are deduced from PITT measurements with a limiting current corresponding to a C/300 regime.
consequence of the progressive filling of two vacant lithium sites Li(2) and Li(3) towards a probable similar distribution as in the γ form of A-Li3 Fe2 (PO4 )3 [16,18]: Li(1) fully occupied, Li(2) and Li(3) occupied at 1/4th in the new composition Li3 TiCr(PO4 )3 . In the case of A-Li2+x TiFe(PO4 )3 , at least two signals, corresponding to the redox couples Fe3+/2+ (∼ 2.8 V vs. Li+ /Li, 0 x 1) and Ti4+/3+ (∼ 2.5 V vs. Li+ /Li, 1 x 2), were separated in agreement with usual values reported in the literature [40]. However, potential intermittent titration technique (PITT) reveals (Fig. 5) solid solution insertion/extraction processes all along the 0 x 2 com-
Fig. 5. Potentiometric titration curve (PITT) of A-Li2+x TiFe(PO4 )3 in the 3.4–2.0 V range vs. Li+ /Li (imin = 3.5 µA).
Fig. 6. X-ray diffraction patterns collected in situ during lithium insertion into Li2+x TiFe(PO4 )3 .
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Fig. 8. X-ray diffraction patterns at 300 K of Li4 TiFe(PO4 )3 prepared by either chemical lithiation or electrochemical reduction. The Bragg positions are calculated in the space group P bna with a = 8.523 Å, b = 8.728 Å and c = 12.177 Å.
Acknowledgements
Fig. 7. Refined cell dimensions of A-Li2+x TiFe(PO4 )3 (opened symbols) and A-Li2+y TiCr(PO4 )3 (filled symbols) (0 x 2 and 0 y 1) as a function of lithium inserted. The cross symbols indicate the values obtained for chemically-lithiated Li4 TiFe(PO4 )3 .
The authors express their sincere thanks to J.M. Tarascon, M. Morcrette, M. Nelson and C. Delacourt for fruitful discussions and constructive critics of the manuscript.
References positional range for Li2+x √TiFe(PO4 )3 . The shapes of the current decays (i = f (1/ t )) denote kinetic limitation of the electron scattering inside the materials. Such a singlephase domain surprisingly contrasts with Li1+x Ti2 (PO4 )3 and Li3+x Fe2 (PO4 )3 , for which two-phase lithium insertion/extraction reactions were systematically observed. The solid solution lithium insertion/extraction mechanism is further supported by the in situ X-ray diffraction data of Fig. 6 collected during lithium insertion all along 0 x 2 for Li2+x TiFe(PO4 )3 . Continuous shifts (with a small slope change around x = 1 though) of the diffraction peaks towards smaller values of 2θ are observed, which, after full pattern refinements in the P bna space group, translates into global unit cell volume variations
V /V of +1.81% for Li2+x TiCr(PO4 )3 and of +2.76% for Li2+x TiFe(PO4 )3 (Fig. 7) when Ti4+ and/or Fe3+ are reduced to Ti3+ Fe2+ . We confirmed the crystallographic features of the fully lithiated phase Li4 TiFe(PO4 )3 by preparing this composition from a classical chemical lithiation reaction involving n-BuLi as the reducing agent in hexane solution. After immersion for 7 days under magnetic stirring, the powder was washed 4 times, dried under vacuum for several hours and kept in an argon-filled dry box. The resulting powder was placed within the in situ cell for XRD data collection (Fig. 8). The refined lattice parameters agree very satisfactorily with those deduced from the in situ experiment at the following values: a = 8.523(6) Å, b = 8.728(6) Å and c = 12.177(9) Å in the P bna space group.
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