Structural, Spectroscopic, and Electrical Features of Undoped and Mn

Jul 26, 2010 - The Mn2+ amount on the lithium site seems to be the main factor responsible for the conductivity decrease observed in doped samples...
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J. Phys. Chem. C 2010, 114, 13872–13878

Structural, Spectroscopic, and Electrical Features of Undoped and Mn-Doped LiTi2(PO4)3 Doretta Capsoni,† Marcella Bini,† Stefania Ferrari,† Piercarlo Mustarelli,† Vincenzo Massarotti,*,† Maria Cristina Mozzati,‡ and Alberto Spinella§ Department of Physical Chemistry “M. Rolla”, UniVersity of PaVia, Viale Taramelli 16, 27100 PaVia, Italy, CNISM - Department of Physics “A. Volta”, UniVersity of PaVia, Via Bassi 6, 27100 PaVia, Italy, and Centro Grandi Apparecchiature-UniNetLab, UniVersity of Palermo, Via F. Marini 14, 90128 Palermo, Italy ReceiVed: May 19, 2010

The study of the ionic conducting material LiTi2(PO4)3 and of its Mn-substituted derivate reveals that the Mn distribution is strictly related to the synthetic method. The results of the structural refinement of X-ray and neutron (ToF) powder diffraction data and of XPS analysis demonstrate that Mn2+ ions are located on the lithium octahedral site, while Mn3+ and Mn4+ ions occupy the titanium ones. The Mn2+ amount on the lithium site seems to be the main factor responsible for the conductivity decrease observed in doped samples. The EPR spectra evidence clustering effects of Mn on both Li and Ti sites and the presence of more insulated Mn2+ ions. The effect of the major Mn amount on Ti site in the case of the sol-gel synthesis is consistent with the different EPR line shape of the related sample. The neutron diffraction and 7Li MAS NMR results do reveal that lithium occupies the M1 site of the rhombohedral LiTi2(PO4)3 structure. 1. Introduction The NASICON type material LiTi2(PO4)3 is a versatile ionconducting compound that can find application as an intercalation electrode material for lithium batteries as well as a solid lithium electrolyte for sensors or other electrochemical devices.1-3 The ideal structure of this compound is rhombohedral (R-3c space group, S.G.) with a framework built up by Ti2(PO4)3 units in which TiO6 octahedra and PO4 tetrahedra share oxygen atoms. Two structural sites are disposable for Li+ ions: M1, which is surrounded by six oxygen atoms, and M2, which is characterized by an irregular eight- or ten-fold coordination. These sites are arranged in an alternating way along the conducting channels. From neutron diffraction studies, a preferred M1 occupation4,5 or a mixed M1/M26,7 is deduced for Li ions. Many cation substitutions on the Ti site are reported including Y, Cr/Mn, Al, Ga, In, Zr, and La: in all cases, the rhombohedral structure is maintained, as suggested from neutron or X-ray diffraction studies.8-18 Three structurally stable modifications are instead reported for Fe-substituted LiTi2(PO4)3 depending on the Fe amount.19 The electrochemical applications of LiTi2(PO4)3 require obtaining the material with the highest ionic conductivity, which can be improved by different routes, such as (i) partial Ti4+ substitution by trivalent ions like Al, Fe, Ga, Sc, and In,15,19 (ii) increase of the Li content in the unit cell,20,21 or (iii) creation of oxygen vacancies that promotes the formation of Ti3+.22 However, it has been verified that the experimental conductivity is also dependent on the sintering degree of the pellets, which can modify the sample porosity.20 The aim of our work is to investigate the relationships between the structural, electrical, and spectroscopic features of undoped and Mn-doped LiTi2(PO4)3, by varying both the Mn amount and the synthesis route. Samples of Li1-x/2MnxTi2-x/2(PO4)3 with 0 e x e * To whom correspondence should be addressed. Phone: +39-382987203. Fax: +39-382-987575. E-mail: [email protected]. † Department of Physical Chemistry “M. Rolla”, University of Pavia. ‡ CNISM - Department of Physics “A. Volta”, University of Pavia. § University of Palermo.

0.10 have been synthesized by means of solid state and sol-gel approaches. The Mn valence states and their distribution on the cationic sites have been investigated by using several experimental techniques, including X-ray (XRPD) and neutron powder diffraction (NPD) and structural refinement, electron paramagnetic resonance (EPR), X-ray photoemission spectroscopy (XPS), and 7Li and 31P NMR MAS spectroscopy. In addition, the conductivity data have been related to the different structural models obtained for the doped samples and, therefore, to the distribution of the Mn doping ions on the cationic sites. 2. Experimental Section The undoped LiTi2(PO4)3 sample (SS) was prepared via solid state synthesis from a stoichiometric mixture of Li2CO3 (Aldrich 99.6%), TiO2 (Merck, >99%), and NH4H2PO4 (Aldrich g99.99%). The mixture was treated in air 24 h at 473 K, 24 h at 873 K, and 24 h at 1173 K with intermediate grindings. Mn-doped samples with stoichiometry Li1-x/2MnxTi2-x/2(PO4)3 (x ) 0.04, 0.05, and 0.10) were prepared by the same solid state synthesis of the SS sample, by adding proper amounts of MnO. The formula is charge balanced on the hypothesis that Mn distributes in an equal amount on the Ti and Li sites as Mn3+ and Mn2+, respectively. In addition, a sample with x ) 0.02 was also prepared and studied only by EPR spectroscopy. In this case, the Mn amount is too low to discuss the Mn distribution by means of the other techniques applied in this study. In the following, this series of samples will be named as SSXX, with XX indicating the composition (decimal part of x). An undoped (SG) sample and an x ) 0.10 Mn-doped (SG10) sample were also prepared by a sol-gel synthesis devised by our group. Three solutions were prepared with concentrations in accordance to the reaction stoichiometry (Li:Ti:P 1:2:3): (1) CH3COOLi · 2H2O (Sigma g99%) in EtOH (1:1); (2) NH4H2PO4 in water; (3) titanium-isopropoxide (Fluka purum) mixed with acetylacetone (Sigma-Aldrich g99%) as a chelating agent in 1:1 molar ratio. Solutions 1 and 2 were slowly added to solution

10.1021/jp104571a  2010 American Chemical Society Published on Web 07/26/2010

Undoped and Mn-Doped LiTi2(PO4)3 3, and the final metal ion/H2O molar ratio was 1:25. For the Mn-doped sample, a proper amount of (CH3COO)2Mn · 4H2O (Aldrich +99%) was added to the solution. The mixture was stirred at room temperature until the gelation occurred. The sample was treated at 333 K for 2 days in a muffle. The obtained powder was heated in air at 623 K for 2 h, at 673 K for 3 h, and at 873 K for 4 h. Room temperature (r.t.) XRPD measurements were performed in air on a Bruker D5005 diffractometer with Cu KR radiation, a Ni filter, and a position sensitive detector (PSD). Rietveld structural and profile refinement was carried out by means of the TOPAS V3.0 program.23 NPD measurements at r.t. were performed on the SS10 sample using the Time-of-Flight (GEM) at the ISIS Spallation Pulsed Source, Rutherford Appleton Laboratory (Chilton, U.K.). The sample was placed in a vanadium can. The Rietveld refinement was performed by using the TOPAS V3.0 program.23 The occupancy factors of the dopant ions are varied with a suitable constraint: the Mn ions can occupy both the lithium (6b) and titanium (12c) cationic sites, fixing their sum to the Mn total content. The Ti amount is fixed to the stoichiometric value. EPR measurements were performed at about 9.4 GHz at r.t. with a Bruker spectrometer. Particular care was paid in determining the sample mass and position in the resonant cavity to compare the relative signal intensities (areas) of the samples. The 7Li and 31P MAS NMR measurements were performed at r.t. and 155.5 and 161.9 MHz, respectively, on an Avance II spectrometer (Bruker) based on a 9.4 T magnet. MAS spectra were acquired with a 4 mm probe head (Bruker), equipped with cylindrical zirconia rotors and a boron nitride stator. The samples were spun at 10-13 kHz, and the data were averaged over 8-256 acquisitions using a single-pulse sequence. 7Li spectra were acquired with a 90° pulse of 3 µs and a recycle time of 10 s. The spectra were referenced to 1.0 M LiCl in H2O. 31P spectra were acquired with a 90° pulse of 6 µs and a recycle time of 8 s. The spectra were referenced to H3PO4 85% aqueous solution. The spectra were best-fitted with the programs WSOLIDS.24 The IS measurements were performed by means of a Frequency Response Analyzer (FRA) Solartron 1170 apparatus in the frequency range 1-105 Hz, with the experimental setting described elsewhere.25 Conductivity measurements were carried out as a function of temperature on disk shaped samples sintered at the final synthesis temperature, sputtered with Pt to deposit the electrodes, and inserted in a polythermal electrochemical cell supplied by a guard circuit to minimize the noises. The measurements were performed in air, after a pellets pretreatment at 400 K overnight. The XPS measurements were performed with a PHI Versa Probe 5000 (Physical Electronics) spectrometer on pelletized samples with a pass energy of 23.5 eV using the Al KR as the exciting source of the X-ray. The spectra were fitted with the Multipak software, and the internal correction of the binding energy was done using the adventitious C1s peak with an energy of 284.6 eV. The SEM measurements were performed with a Zeiss EVOMA10-HR microscope on Au sputtered samples. 3. Results 3.1. XRPD and NPD. All of the samples show the peaks expected for the trigonal R3jc LiTi2(PO4)3 phase (JCPDS card no. 35-0754), suggesting that the Mn insertion in the cationic framework does not cause any structural change. In the SS04

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13873 TABLE 1: Lattice Parameters, Discrepancy Factors, and Goodness of Fit Obtained by the Rietveld Refinement sample

a/Å, c/Å

Rwp

Gof

SS SS04 SS05 SS10 XRPD NPD SG SG10

8.5079(1), 20.8531(4) 8.5115(1), 20.8492(4) 8.5104(1), 20.8498(4)

6.76 8.59 8.40

2.10 2.42 1.83

8.5117(1), 8.5145(3), 8.5107(2), 8.5113(3),

7.72 4.26 6.94 5.51

2.27 1.40 1.62 1.59

20.8521(4) 20.8564(9) 20.8688(10) 20.8852(11)

TABLE 2: Cation Distribution Obtained by the Rietveld Refinement sample

refined stoichiometry

SS04 SS05 SS10 XRPD NPD SG10

Li0.980(5)Mn0.020(5)Ti1.98Mn0.020(3)(PO4)3 Li0.959(5)Mn0.041(5)Ti1.975Mn0.009(2)(PO4)3 Li0.929(4)Mn0.071(4)Ti1.95Mn0.030(2)(PO4)3 Li0.921Mn0.079(5)Ti1.95Mn0.021(5)(PO4)3 Li0.965(5)Mn0.035(5)Ti1.95Mn0.065(2)(PO4)3

sample, small peaks pertinent to the LiTiO(PO4) impurity phase are observed. The structural refinement has been carried out by applying the Rietveld method on XRPD and NPD patterns. For the SS04 sample, the refinement has been performed taking into account the crystal structure of the LiTiO(PO4) impurity phase, and its amount has been determined (3.71(3) wt %). Atomic positions and isotropic/anisotropic thermal factors B were also refined and not reported for the sake of simplicity. In particular, the B thermal factors are always positive and lower than 2.0 Å2, except for the 6b lithium site, for which values up to 5.5 Å2 are obtained. Table 1 reports the lattice parameters and the discrepancy factors (Rwp and GoF) obtained by the Rietveld procedure. The SG10 cell parameters increase with respect to those of the SG sample. Table 2 shows the Mn distribution on the Li and Ti octahedral sites obtained by the refinement of the occupancy factors. In all cases, the Mn ions occupy both of the cationic sites, and their amount on the 6b Li crystallographic site increases by increasing the Mn total content. For the x ) 0.10 samples, the results show that the sol-gel synthesis leads to a lower Mn content on the lithium site. The values of the lattice parameters obtained by XRPD and NPD data for the SS10 sample are very close. Li ions occupy the 6b site, as reported also in the literature,4,5 and the Mn ions preferentially locate on the M1 lithium site. In Figure 1, the observed and calculated NPD patterns for the SS10 sample are compared. 3.2. EPR. The r.t. EPR signal of all of the Mn-doped samples is composed by the superposition of a broad line, centered at g = 2, and a hyperfine structure for which four sextets are detectable in the magnetic field range 2700-4000 G. The r.t. spectrum of the SSXX samples is shown in Figure 2. The broad line intensity increases together with the Mn content in the sample, while its line width and line shape remain unchanged. A broader component is instead observed for the SG10 sample, together with a decrease of the signal intensity with respect to the SS10 sample. In Figure 3, the r.t. EPR signals of the two XX ) 10 samples, normalized for sample mass and experimental conditions, are reported. No signals are instead observed for the undoped samples, as expected.

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Figure 1. Observed (black line) and calculated (red line) NPD pattern for the SS10 sample. The peak positions (blue bars) and the difference curve (gray line) are also shown.

Figure 4. Room temperature EPR spectrum of the SS05 sample in the region 2700-4000 G. The four sextets of the hyperfine structure are also indicated. Figure 2. Room temperature EPR spectra of the samples SSXX [XX ) 02, 04, 05, and 10 (a, b, c, and d, respectively)] normalized for sample mass and experimental conditions.

Figure 3. Comparison between the EPR signals of SS10 (black line) and SG10 (red line).

The hyperfine sextets show the same features in all of the samples, concerning their resonant fields, hyperfine splitting, and line width of each single line. The four sextets of hyperfine structure, named in the following S1, S2, S3, and S4, with reference to the ascending order of their resonant fields, are evidenced in Figure 4, where an enlarged view of the spectrum of the SS05 sample is reported as an example. The S2 sextet is centered at g ) 2.04 (B2 ) 3311 G, at our frequency of measurement) with a hyperfine splitting higher than 84 G and

a line width of each single line of the sextet of about 20-24 G. This suggests that the signal arises from Mn2+ ions negligibly perturbed from neighboring ions with non-null nuclear spin or by close Mn clusters. The resonant fields of the S1 and S4 sextets are B1,4 ) B2 ( 317 G, respectively, while the sextet S3 is centered at a resonant field B3 ) B2 + ∼250 G. For all of these sextets, the hyperfine splitting value is found to be between 86 and 110 G, so that Mn2+ ions can be considered responsible for these signals, too. The line width of each of the S1, S3, and S4 hyperfine lines, where appreciable, is larger than that for the S2 sextet (about 30-36 G). 3.3. NMR. Parts a and b of Figure 5 show the 7Li static and MAS spectra of LiTi2(PO4)3 and of samples SS04 and SS10, respectively. The static spectrum of the undoped sample is typical of a spin I ) 3/2 nucleus, showing the central transition (-1/2; 1/2), and the satellite ones.18 The full width at half-height (fwhh) of the central line is 1090 Hz. From the best fit of the spectrum, we obtained the quadrupolar coupling constant, CQQ ) 45 ( 1 kHz, and the asymmetry parameter, ηQ ) 0.0 in good agreement with the results of Arbi et al.26 The substitution of Li and Ti with Mn determines a progressive enlargement of both the central and the satellite transitions, caused by hyperfine dipolar magnetic interaction, Hhyp, among the Li nuclei and the Mn unpaired electrons.27 The rotation at the magic angle (Figure 5b) removes the first order quadrupolar interaction and the anisotropic part of the dipolar coupling, putting into evidence the isotropic peak and the spinning sidebands manifold (see caption). In the case of LiTi2(PO4)3, the central line has a fwhh of 520 Hz, which is due to residual isotropic contributions. We observe that Li occupies only one site, as expected from the literature.28 In the case of the samples SS04 and SS10, we

Undoped and Mn-Doped LiTi2(PO4)3

Figure 5. 7Li static (a) and MAS (b) spectra of SS, SS04, and SS10 samples, respectively. The stars indicate the spinning sidebands.

observe a progressive enlargement of both the isotropic line and the spinning sidebands manifold, which can be due to incomplete removal of the hyperfine dipolar interaction, and/or to the presence of scalar contributions of this Hamiltonian term (see the Discussion). Parts a and b of Figure 6 show the 31P (I ) 1/2) static and MAS spectra of LiTi2(PO4)3 and of samples SS04 and SS10, respectively. The static spectrum of LiTi2(PO4)3 is characterized by a single peak showing a chemical shift anisotropy (CSA) pattern. The best fit gives the following values for the diagonal elements of the components of the CSA second rank tensor: σ11 ) -13.5 ppm, σ22 ) -27.5 ppm, and σ33 ) -41.0 ppm, in excellent agreement with the results reported in the literature.18 The isotropic chemical shift value is σiso ) -27.3 ppm. The static peaks of samples SS04 and SS10 are Gaussian in nature, and their enlargement with respect to LiTi2(PO4)3 is likely due to the hyperfine dipolar coupling with the Mn unpaired electrons. A rough estimate gives a coupling constant of about 4.8 kHz for SS04 and 5.5 kHz for SS10. The CSA parameters remain nearly unchanged. The MAS rotation (Figure 6b) almost entirely removes both the CSA and Hhyp. The spectrum of LiTi2(PO4)3 is characterized by a main peak at -27.3 ppm, and by a small

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Figure 6. 31P static (a) and MAS (b) spectra of the SS, SS04, and SS10 samples, respectively. The stars indicate the spinning sidebands.

TABLE 3: Binding Energy (BE) and Peak Area Percentage Obtained by Fitting the XPS Mn(2p) Spectra Mn oxidation state

BE/eV

area/%

+2 +3 +4

640.94 641.98 643.04

41 30 29

+2 +3 +4

SG10 640.82 641.67 642.79

24 50 26

SS10

(0.5%) peak at -9.6 ppm, which is likely due to pyrophosphate units.29 The substitution of Ti and Li with Mn makes a shoulder appear at -19.0 ppm (indicated by the arrows in Figure 6b). This shoulder is likely due to P(OTi)3(OMn)1 units, and accounts for 18.2 and 21.8% of the overall spectrum in the case of SS04 and SS10, respectively. 3.4. XPS. High resolution XPS spectra of SS10 and SG10 samples were collected, with the peculiar aim to investigate the Ti and Mn oxidation states and their abundance in the samples.

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Figure 7. Arrhenius plot of the conductivity data of undoped and x ) 0.10 Mn-doped samples.

Ti(2p) ions are exclusively present in the +4 oxidation state. In both samples, the Mn(2p) spectra show a significant peak broadening, suggesting that Mn ions are present in +2, +3, and +4 oxidation states, with different relative amounts in the two samples. Table 3 summarizes the values of the binding energy and area percentage of each Mn ion signal, obtained by a fitting procedure on the Mn(2p) spectra. 3.5. IS. The Arrhenius plots of undoped and x ) 0.10 samples are shown in Figure 7. For the undoped samples, the SS sample possesses the highest conductivity due to the high sinterization temperature. The Mn doping decreases the conductivity in both the SS and SG series of samples. 3.6. SEM. The SEM micrographs of SG and SS samples are reported in Figure 8a and b, respectively. As expected, the morphologies are different. The SG sample particles show an average dimension of about 10 µm and are covered by fine and very small grains. The SS sample shows agglomerates of rounded particles, with dimensions ranging between about 1 and 7 µm, highly interconnected and sintered. 4. Discussion The structural refinement for all of the samples leads to satisfactory results, as witnessed by the favorable values of the discrepancy factors (Table 1) and by the graphical comparison of the experimental and calculated patterns (Figure 1). The high

Figure 8. SEM micrographs of SG (a) and SS (b) samples.

Capsoni et al. value of the B thermal factors for the 6b lithium site (about 5.5 Å2) is in agreement with the values reported in the literature,4 and justified by the large M1 site dimension. The refined stoichiometry reported in Table 2 clearly demonstrates that the Mn ions are located on both 6b and 12c crystallographic sites. As suggested by the XRPD and NPD results, two remarks can be made: (i) the Mn content on the lithium site increases by increasing the total dopant amount; (ii) for the same stoichiometry (x ) 0.10), the sol-gel synthesis leads to a lower amount of Mn ions substituted on the Li site. The octahedral dimensions give insights on the possible oxidation state of Mn ions. The Li-O bond length in the LiO6 octahedra is about 2.21 Å, while the Ti-O one is 1.90 Å. In agreement with the ionic radii for Mn2+, Mn3+, and Mn4+ ions,30 it is reasonable to expect that Mn2+ preferentially occupies the LiO6 octahedra, while Mn3+ and Mn4+ should be located on the TiO6 ones. These considerations are also supported by the literature for Mn/Cr-doped samples7,9 and for the Mn0.5-xCaxTi2(PO4)3 (0 e x e 0.5) compound.8 A validation of this hypothesis comes out by comparing the refinement results (Table 2) with those obtained by XPS analysis (Table 3) for the x ) 0.10 samples. The Mn2+ percentage revealed by XPS is higher in the SS10 than in the SG10 sample, confirming at least the same trend of Mn content on Li site obtained by the Rietveld refinement. Interesting considerations can be drawn from the EPR data. As previously mentioned, the line width and line shape of the broad line do not change for the solid state samples,10 while the signal intensity increases by increasing the Mn amount, as shown in Figure 9, where the trend of the signal intensity vs the Mn content, obtained after double integration of the derivative signal, is reported. This figure also evidences the EPR lower intensity detected for the SG10 sample with respect to the SS10 one. We recall here that the SG10 EPR line width is broader than the ones of the other samples. The ratio between the EPR intensity values obtained for SS10 and SG10 well reproduces the ratio between the amount of Mn2+ + Mn4+ ions (EPR active ions) calculated from XPS measurements, thus suggesting that the increase of Mn3+ ions (not EPR active) in the SG10 sample is responsible for the EPR intensity decrease observed for this sample. The different amount of each Mn oxidation state, related to the different Mn distribution on Li and Ti cationic sites, can also be considered responsible for the

Undoped and Mn-Doped LiTi2(PO4)3

Figure 9. EPR signal intensity, obtained after double integration of the derivative signals, vs Mn amount in the samples. The open square represents the value of the SG10 sample.

Figure 10. I2/I1 ratio vs Mn content in all of the considered samples. The open square represents the value of the SG10 sample. The inset shows the I1/I2 ratio (full stars) and the Mn/Li values from Table 2 (open diamonds) vs the Mn content.

line shape variation observed in the SG10 sample with respect to the SS ones. The broadness of the line, moreover superimposed to structured signals, does not allow us to carefully determine the g value (g = 2) nor to distinguish the two different contributions to the broad line itself, so that we cannot attribute with confidence the origin of the signal to the EPR active Mn2+ or Mn4+ ions. We only can state that the broad signal comes from regions with high Mn concentration, possibly a mixing of Mn ions with 2+ and 4+ oxidation states on the lithium and titanium sites, respectively, consistently with the information we got from the other techniques. Some considerations can be argued also from the EPR structured signals. As previously observed, from the g value of the S2 sextet and from the hyperfine splitting values found for all of the sextets, the Mn2+ ions can be considered responsible for the structured component of the EPR spectra. To try to get information about the number of Mn centers giving rise to the four sextets, we analyzed the dependence of the line intensity of the sextets on the Mn content of the samples. Figure 10 shows the ratio between the amplitudes of the first lines of S2 and S1 sextets (ratio named I2/I1) vs the Mn content. In the SSXX samples, I2/I1 decreases by increasing the Mn content, while for SG10 a clearly higher I2/I1 value is evaluated. Besides, S1 and S4 line intensities can be reasonably considered the same in each sample while a further different behavior vs the Mn

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13877 content can be roughly inferred for S3, for which, however, a careful evaluation of the intensity is hardly feasible: the S3 intensity seems to increase with respect to the S2 one by increasing the Mn content but with a negligible difference between the SS and SG synthesis. The above observations suggest that three different Mn2+ centers seem to give rise to the four sextets of hyperfine structure of the EPR signal. The S2 central sextet can be confidently attributed to Mn2+/Li+ not perturbed ions, which prevail for low Mn concentration or in the SG10 sample. Besides, we can tentatively state that the Mn increase promotes hyperfine interactions with other ions with non-null nuclear magnetic moment (P5+ and/or Li+ ions, which also could give rise to hyperfine interactions causing line broadening) and/or interactions with neighboring Mn ions, with a shift of resonant field and a broadening of the single line of the sextets. Finally, we cannot exclude that the S3 sextet comes from rather insulated Mn/Ti ions. In summary, in the case of the SS synthesis, the increasing Mn substitution favors the intensity increase of the “satellite” signals with respect to the central one, while the SG synthesis strongly favors the Mn/Ti occupation (see Table 2) and the higher Mn concentration on the Ti site corresponds to the predominance of the not perturbed Mn/Li ions upon the more concentrated perturbed centers, so that the intensity of the satellite sextets results to be very low. This evidence fairly agrees with the XRPD results, as shown in the inset of Figure 10 where I1/I2 is reported together with the Mn/Li substitution deduced from Table 2, and with XPS results, showing a lower amount of Mn2+ in the SG10 sample. Concerning the NMR results, we start to discuss the 7Li NMR spectra. As already stated, the addition of Mn causes the onset of hyperfine interactions among the Li nuclei and the unpaired electrons of Mn2+ and Mn4+. The theoretical aspects of this interaction have been largely treated in the literature.27,31 Recently, we discussed in detail the various terms of the Hamiltonian hyperfine interactionsthrough-space dipolar (DP), Fermi contact (FC) shift, and pseudocontact (PS) shiftsas well as their effects on the NMR spectrum, in the case of Mnsubstituted Li4Ti5O12.32 In particular, we showed that FC and PS terms may modify the isotropic chemical shift of Li nuclei perturbed by next-neighbor Mn atoms. In the present study, we observe that the substitution of Li and Ti with Mn does not seem to cause the formation of a peak (or a shoulder) due to Li perturbed by Mn. While we cannot fully rule out that a shifted peak is masked by the residual DP term, this finding is in agreement with the neutron diffraction results showing that Mn chiefly goes on the lithium M1 site. As a matter of fact, M1 sites are separated by at least four bonds, through PO4 tetrahedra. This implies that FC and PS are likely small. 31 P MAS NMR spectra of samples SS04 and SS10 do reveal the existence of a fraction of P nuclei chemically shifted with respect to the 31P isotropic value of LiTi2(PO4)3. The spectra best fit allows one to individuate a second peak, centered at about -19.0 ppm, which can be attributed to P nuclei seeing Mn in their second coordination sphere. Similar results were reported by Arbi et al. in their Li1+xTi2-xAlx(PO4)3 series.26 These authors were able to perform a complex spectral deconvolution, and to assign the resulting contributions to P(OTi)4, P(OTi)3(OAl)1, P(OTi)2(OAl)2, P(OTi)1(OAl)3, and P(OAl)4 structural units. In our case, we observe only one peak beside the main one, likely because of the low quantity of substituting Mn, which makes it statistically unlikely that P atoms can see more than one Mn as a next neighbor. By considering the LiTi2(PO4)3 crystal structure, more quantitative considerations can be made. From the XRPD results, we know that Mn

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TABLE 4: Cation Neighborhood in the Range 1-5 Å for the SS Sample atom 1

atom 2

number of neighbors

d1,2 (Å)

Li

O2 Ti P O1 O2 Li P P O2 O1 Ti Ti O1 Li

6 2 6 3 3 1 3 3 2 2 2 2 2 2

2.2655 2.9618 3.4984 1.8801 1.9759 2.9618 3.2745 3.3410 1.5271 1.5273 3.2745 3.3410 3.3424 3.4984

Ti

P

occupies the M1 lithium site, whereas a non-negligible part (∼50 and ∼30% in the SS04 and SS10 samples, respectively) occupies the Ti site. However, from the crystal structure, we know that both Li and Ti sites see six phosphorus as next neighbors, and nearly at the same distance (see Table 4, reporting the atom-atom distances obtained by the refinement of atoms position for the SS sample). Therefore, from the point of view of the effects on the P chemical shift, it is practically the same if Mn substitutes Li or Ti. Since each Mn sees six P atoms, we would expect to have 24 and 60% of perturbed P for the samples SS04 and SS10, respectively. In contrast, our NMR results give 18.2 and 21.8% which points to a progressive clustering of Mn, in agreement with the EPR results. The conductivity data shown in Figure 7 put into evidence different aspects. The synthesis procedure significantly influences the conductivity behavior of the undoped samples; in particular, the conductivity values of the SS sample are higher than that of the SG one. This is in agreement with the lower sinterization degree of the sol-gel synthesis, witnessed by the SEM micrographs (Figure 8). On the other hand, the influence of porosity on the conductivity is well-known in the literature.18 Independently of the synthesis method, the Mn doping lowers the conductivity of the material. This is expected because the Mn substitution occurs also on the 6b lithium site as Mn2+, a really immobile ion,9 and for x ) 0.10, this conductivity decrease is particularly consistent for the SS sample, where a higher amount of Mn2+ is found on the lithium site, as obtained by XRPD and XPS data. Another aspect to be taken into account concerning the lower conductivity of the Mn-doped samples is the Li amount on the 6b site, too: the lithium content in the synthesis stoichiometry is lower than 1, which is instead the value used in the undoped samples. By taking into account all of the factors affecting our conductivity data, our results put into evidence that the Mn substitution on the lithium site plays a more relevant role than the porosity and Li content of the samples. In fact, the sample showing the lower conductivity in the entire examined temperature range is the SS10, notwithstanding its high degree of sinterization and the same Mn total content of the SG10 sample. 5. Conclusions The combined use of several structural and spectroscopic techniques allowed us to characterize in detail the effect of Mn doping on the LiTi2(PO4)3 cathode material. Mn ions are present in +2, +3, and +4 oxidation states and distribute on both the

6b (M1) and 12c crystallographic sites. Both NPD structural refinement and 7Li NMR-MAS spectroscopy suggest that Li ions exclusively occupy the M1 site. Mn2+, present on the 6b site, is responsible for the lower conductivity shown in the doped samples with respect to the undoped ones and prevails as a conductivity limiting factor on the porosity-sinterization effects. Among the two proposed synthesis routes of Mn-doped LiTi2(PO4)3, the sol-gel one allows one to obtain impurity free samples at lower temperature with a lower amount of Mn ions on the lithium sites, thus determining a minor impact on the conductivity behavior of the investigated system. Acknowledgment. We are grateful to Dr. R. I. Smith (ISIS) for ToF neutron diffraction data collection and Dr. B. Hinrichsen for helpful discussion on the application of the TOPAS program to the neutron patterns. References and Notes (1) Sebastian, L.; Gopalakrishnan, J. J. Mater. Chem. 2003, 13, 433. (2) Tarascon, J.-M.; Armand, M. Nature 2001, 414, 359. (3) Robertson, A. D.; West, A. R.; Ritchie, A. G. Solid State Ionics 1997, 104, 1. (4) Aatiq, A.; Menetrier, M.; Croguennec, L.; Suardc, E.; Delmas, C. J. Mater. Chem. 2002, 12, 2971. (5) Alami, M.; Brochu, R.; Soubeyroux, J. L.; Gravereau, P.; Leflem, G.; Hagenmuller, P. J. Solid State Chem. 1991, 90, 185. (6) Woodcock, D. A.; Lightfoot, P. J. Mater. Chem. 1999, 9, 2907. (7) Tran Qui, D.; Hamdoune, S.; Soubeyroux, J. L.; Prince, E. J. Solid State Chem. 1988, 72, 309. (8) Sobiestianskas, R.; Dindune, A.; Kanepe, Z.; Ronis, J.; Kezionis, A.; Kazakevicius, E.; Orliukas, A. Mater. Sci. Eng., B 2000, 76, 184. (9) Aatiq, A.; Delmas, C.; El Jazouli, A. J. Solid State Chem. 2001, 158, 169. (10) Aatiq, A.; Menetrier, M.; El Jazouli, A.; Delmas, C. Solid State Ionics 2002, 150, 391. (11) Aatiq, A.; Delmas, C.; El Jazouli, A.; Gravereau, P. Ann. Chim. Sci. Mater. 1998, 23, 121. (12) Cretin, M.; Fabry, P. J. Eur. Ceram. Soc. 1999, 19, 2931. (13) Hoshina, K.; Dokko, K.; Kanamura, K. J. Electrochem. Soc. 2005, 152, A2138. (14) Wong, S.; Newman, P. J.; Best, A. S.; Nairn, K. M.; MacFarlane, D. R.; Forsyth, M. J. Mater. Chem. 1998, 8, 2199. (15) Arbi, K.; Lazarraga, M. G.; Ben Hassen Chehimi, D.; AyadiTrabelsi, M.; Rojo, J. M.; Sanz, J. Chem. Mater. 2004, 16, 255. (16) Arbi, K.; Rojo, J. M.; Sanz, J. J. Eur. Ceram. Soc. 2007, 27, 4215. (17) Kazakevicˇius, E.; Sˇalkus, T.; Dindune, A.; Kanepe, Z.; Ronis, J.; Kezˇionis, A.; Kazlauskiene`, V.; Misˇkinis, J.; Selskiene`, A.; Selskis, A. Solid State Ionics 2008, 179, 51. (18) Parıs, M. A.; Sanz, J. Phys. ReV B 1997, 55, 14270. (19) Catti, M.; Comotti, A.; Di Blas, S.; Ibberson, R. M. J. Mater. Chem. 2004, 14, 835. (20) Aono, H.; Sugimoto, E.; Sadaoka, Y.; Imanaka, N.; Adachi, G.-Y. J. Electrochem. Soc. 1990, 137, 1023. (21) Wang, B.; Greenblatt, M.; Wang, S.; Hwu, S.-J. Chem. Mater. 1993, 5, 23. (22) Luo, J.-Y.; Chen, L.-J.; Zhao, Y.-J.; He, P.; Xia, Y.-Y. J. Power Sources 2009, 194, 1075. (23) Bruker AXS (2005). TOPAS V3.0: General profile and structural analysis software for powder diffraction data. User Manual Bruker AXS, Karlsruhe, Germany. (24) WSOLIDS program: (Eichele, K. - Univ. Tuebingen, Ge) and MestReNova (Mestrelab, Spain). (25) Chiodelli, G.; Lupotto, P. J. Electrochem. Soc. 1991, 9, 2703. (26) Arbi, K.; Mandal, S.; Rojo, J. M.; Sanz, J. Chem. Mater. 2002, 14, 1091. (27) Mustarelli, P.; Massarotti, V.; Bini, M.; Capsoni, D. Phys. ReV. B 1997, 55, 12018. (28) Nairn, K. M.; Forsyth, M.; Greville, M.; MacFarlane, D. R.; Smith, M. E. Solid State Ionics 1996, 86-88, 1397. (29) Mustarelli, P. Phosphorus Res. Bull. 1999, 10, 25. (30) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751. (31) Grey, C. P.; Dupre`, N. Chem. ReV. 2004, 104, 4493. (32) Capsoni, D.; Bini, M.; Massarotti, V.; Mustarelli, P.; Chiodelli, G.; Azzoni, C. B.; Mozzati, M. C.; Linati, L.; Ferrari, S. Chem. Mater. 2008, 20, 4291.

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