Role of Lithium Excess and Doping in Li - American Chemical

Dec 5, 2011 - Source (GEM Line), at the Rutherford Appleton Laboratory. (Chilton, UK). .... Anyway, the presence of Li in the M2 cavity comes out by ...
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Role of Lithium Excess and Doping in Li1+xTi2 xMnx(PO4)3 (0.00 e x e 0.10) Doretta Capsoni,*,† Marcella Bini,† Stefania Ferrari,† Vincenzo Massarotti,† and Maria Cristina Mozzati‡ † ‡

Department of Chemistry Physical-Chemistry Section, University of Pavia, viale Taramelli 16, 27100 Pavia, Italy CNISM - Department of Physics “A. Volta”, University of Pavia, via Bassi 6, 27100 Pavia, Italy ABSTRACT: The role of lithium excess and Mn doping on lithium titanium phosphate properties is studied by structural (X-ray and neutron diffraction) and spectroscopic (electron paramagnetic resonance and impedance) techniques. The dopant cation is present with 2+, 3+, and 4+ oxidation states in both the lithium and titanium sublattices: Mn2+ is preferentially located on Li sites and Mn4+ on Ti ones, and Mn3+ can occupy both the sites. The refinement of neutron diffraction patterns allowed us to identify the 18e crystallographic site as the preferred location of the excess Li ions. Cationic distribution and valence state are mainly related to sol gel or solid-state synthesis routes. Consequently, the complex behavior of the bulk conductivity can be explained by the presence of Mn on the Li site and the amount of Li excess, while the effect of the sintering degree is comparable in all the samples as revealed by scanning electron microscopy.

1. INTRODUCTION LiTi2(PO4)3 is an ionic conducting compound which finds application in electrochemical devices, such as lithium batteries.1 5 The conducting behavior of the material is well-known,6 and a growing interest in this topic is evidenced by an increasing number of recently published papers.7 9 In fact, this material is of potential interest as an anode in aqueous lithium-ion batteries because it reacts at about 2.5 V vs pure lithium, near the limit of the electrochemical stability, but still exhibits high capacity and a long cycle life.10 In particular, the present research points to an increase in the ionic conductivity of this material that is significantly influenced by microstructure (sintering degree, porosity, crystallite size),11,12 cation doping, and stoichiometry. In this regard, works have been published on Ti substitution with trivalent ions such as Al, Fe, Ga, Sc, In, and Y,13,14 as well as on the increase of the Li content and on the formation of oxygen vacancies.15 The rhombohedral structure of LiTi2(PO4)3 (R-3c space group) is constituted of 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 8- or 10-fold coordination. These sites are arranged in an alternating way along the conducting channels. Usually, the M1 position is preferred,16 while neutron diffraction studies demonstrated that the M2 one could play a relevant role in Li-rich samples or when the temperature increases.17,18 In most cases, the rhombohedral cell is stable also in solid solutions involving ions such as Y, Cr/Mn, Al, Zr, La, Sn, and Hf.19 22 In the case of the Li-rich, Fe-substituted LiTi2(PO4)3 compound,23 structural transitions to orthorhombic forms are observed, depending on the iron content. r 2011 American Chemical Society

The aim of this work is to characterize the structural features and electrical properties of Li-rich, Mn-doped Li1+xTi2 xMnx(PO4)3 (x e 0.10) samples. Two series of samples were prepared, from solid state and sol gel synthesis, to compare the results of the different synthesis methodologies and to relate them to the conductivity behavior, taking into account the sintering conditions, possibly leading to different morphological aspects and porosity, investigated by SEM analysis. In addition, air and argon atmospheres have been used to force Mn ions preferentially toward a single oxidation state, to better investigate its role in affecting structural and electrical properties of the material. The study is performed by means of structural refinement of X-ray and neutron powder diffraction data and EPR spectroscopy. In particular, the Mn distribution on the cationic sites and the possible presence of lithium ions on the M2 cavity will be discussed. The role of lithium excess on conductivity values, obtained by Impedance Spectroscopy (IS) measurements, will be also examined.

2. EXPERIMENTAL METHODS Li1+xTi2 xMnx(PO4)3 with x = 0.05 and 0.10 were prepared by the sol gel synthesis previously described.24 The formula is charge balanced on the hypothesis that Mn occupies the Ti sites as Mn3+. The samples will be named SG05 and SG10. The other two samples with x = 0.05 were prepared by solid state synthesis: a stoichiometric mixture of Li2CO3 (Aldrich, 99.6%), TiO2 (Merck, >99%), Mn2O3 (Aldrich, g99.999%), and NH4H2PO4 (Aldrich, g99.99%) was treated for 24 h at 473 K, Received: September 8, 2011 Revised: November 21, 2011 Published: December 05, 2011 1244

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Table 1. Lattice Parameters, Cell Volume, Weighted Pattern Discrepancy Factor, Goodness of Fit, and Cation Distribution Obtained by the Rietveld Refinement a/Å sample SS05-Argon

V/Å3

Rwp

GoF

1307.32(3)

6.92

2.15

[Li0.963(6)Mn0.038(6)]M1[Ti1.988(3)Mn0.012(3)]

1307.14(4)

8.81

2.52

[Li0.988(7)Mn0.012(7)]M1[Ti1.962(3)Mn0.038(3)]

1310.45(10)

5.53

1.59

[Li0.995(6)Mn0.005(6)]M1[Ti1.955(3)Mn0.045(3)]

8.5125(2) 20.8889(9)

1310.87(8)

5.79

1.65

[Li0.950(6)Mn0.050(6)]M1[Ti1.950(3)Mn0.050(3)]

8.5181(6)

1312.52(7)

8.58

0.85

[Li0.147]M2[Li0.953(6)Mn0.047(6)]M1[Ti1.947(6)Mn0.053(6)]

c/Å 8.5092(1)

refined stoichiometry

20.8488(4) SS05-Air

8.5101(1) 20.8413(4)

SG05

8.5128(3) 20.8805(11)

SG10: XRPD NPD

20.8878(19)

24 h at 873 K, and 24 h at 1173 K with intermediate grindings in air or in argon. These two samples will be named SS05-Air and SS05-Argon. The SS and SG undoped samples prepared by solid state and sol gel synthesis, respectively, and previously characterized24 will be used for proper comparison to this series of samples. Room-temperature (rt) X-ray powder diffraction (XRPD) data were collected in air on a Bruker D5005 diffractometer with Cu Kα 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.25 Neutron powder diffraction (NPD) measurements were carried out at rt on the SG10 sample, having the highest Li excess, using the Time-of-Flight at the ISIS Spallation Pulsed Source (GEM Line), at the Rutherford Appleton Laboratory (Chilton, UK). The sample was placed in a vanadium can. The Rietveld refinement on NPD data was performed by means of the FULLPROF Suite Program 1.00.26 The PV-TCH profile function modified by Ikeda-Carpenter was used to describe the peak profile. Thermal and occupancy factors of the dopant ion are varied with suitable constraints. In particular, the Mn occupancy is refined by allowing ions to locate on both Lithium 6b (M1) and Titanium 12c crystallographic sites, keeping fixed the total Mn amount to the sample stoichiometry. To investigate Li ion occupancy on M1 and M2 positions, two models were refined on the NPD data of the SG10 sample. In the first model (model A), the lithium excess is located on the 36f crystallographic site, as suggested by Catti et al.23 for Fe-doped lithium titanium phosphate. In the second case (model B), the lithium excess occupies the 18e site, as previously suggested by Tran Qui et al.18 The isotropic thermal factor values for 6b and 36f or 18e crystallographic sites were constrained during the refinement. EPR measurements were performed at about 9.4 GHz at rt with a Bruker spectrometer. Particular care was paid in determining the sample mass and position in the resonant cavity to compare and estimate the signal intensities of the samples. The IS measurements were performed by means of a Frequency Response Analyzer (FRA) Autolab PGSTAT30 apparatus in the frequency range 1 10 6 Hz. Conductivity measurements were carried out as a function of temperature on disk-shaped pellets sintered at the final synthesis temperatures of the samples, covered with silver paste 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. By fitting the impedance spectrum (at each temperature) with a simple model composed by a resistor in parallel to a capacitor and extrapolating the intercept with the real axis, the total bulk resistivity was estimated, and this value is then used to calculate the bulk conductivity. The SEM micrographs were collected with a Zeiss EVOMA10-HR microscope on Au sputtered samples.

3. RESULTS 3.1. XRPD and NPD. The XRPD and NPD patterns of the samples show the peaks expected for the LiTi2(PO4)3 trigonal structure (JCPDS card n.35-0754), suggesting that the Mn insertion in the cationic framework does not cause any structural transition, as already verified for samples previously studied.24 No additional peaks are observed, and the samples can be considered impurity free, within the detection limit of the XRPD technique. The structural refinement has been carried out by applying the Rietveld method to both XRPD and NPD data. The values of the refined lattice parameters, cell volume, and discrepancy factors (Rwp and GoF) are reported in Table 1. The comparison between the experimental and calculated NPD patterns of the SG10 sample is shown in Figure 1. The satisfactory discrepancy factors and the difference curve of Figure 1 suggest a good quality of the refinement. Lattice parameters, in particular the c axis value, and cell volumes are higher for sol gel samples than for solid state ones. Table 1 also shows the stoichiometry obtained by the refinement of the occupancy factors, varied under the constraints previously reported. Mn ions distribute on both Ti (12c) and Li (6b) crystallographic sites. In general, the Mn amount on the 6b site increases by increasing x in the formula unit for SG samples, while for x = 0.05, the solid state synthesis leads to a higher content of Mn on the lithium site, in particular in the case of the SS05-Argon sample. The lithium occupation of the M2 cavity has been investigated only on the SG10 sample NPD data due to the low X-ray scattering factor of Li ions. In the literature, in the case of Fe-doped Li1.5Fe0.5Ti1.5(PO4)3 phosphate,23 the Li excess has been located by searching the negative peaks (the neutron scattering factor for Li+ is negative) in the Fourier difference map. In our case, the same procedure allows us to determine only Li ions 1245

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Figure 1. Observed (red points) and calculated (black line) NPD pattern for the SG10 sample. In the bottom, the peak position (green bars) and the difference curve (blue line) are also shown.

Figure 2. Room-temperature EPR spectrum of (a) SS05-Argon, (b) SS05-Air, (c) SG05, and (d) SG10. Spectra have been normalized for sample mass and experimental conditions.

present on the regular 6b site and not the lithium excess position, probably due to its small amount. Anyway, the presence of Li in the M2 cavity comes out by comparing the results obtained by refining the model A (Li on 36f site) and B (Li on 18e site). The model A, notwithstanding the quite good discrepancy factors, leads to high isotropic thermal factors, so it will be no more considered herein. The model B appears more reliable, and the relative results are reported in Table 1. The lithium excess x (0.10) and the lithium amount lacking on the 6b site (due to the Mn quantity determined on the same site (0.047)) can be found on the 18e site. The Mn distribution on lithium and

titanium sites obtained by NPD Rietveld refinement well matches the XRPD results. 3.2. EPR. Information on Mn ion oxidation states and their distribution on the cationic sites can be inferred from EPR data. The EPR spectrum of each investigated sample is composed by a set of structured signals, detectable in the magnetic field range 2700 4000 G, superimposed to a broad unstructured line, centered at about g = 2. In Figure 2, the rt EPR spectra of all the samples are reported. The structured signals are attributable to at least four sextets of hyperfine structure coming from Mn ions. The resonant fields of each sextet, whose values, at our frequency of measurements, are 3311 G (corresponding to g = 2.04), 3311 G ( 317 G, and 3311 G + ∼250 G, and the hyperfine splitting values (always higher than 84 G) are the same for all the samples. Only intensity variations of the sextets can be observed, where appreciable, depending on the Mn concentration and on the kind of synthesis and treatment. We had observed the same set of sextets in a previously examined series of Li1 x/2MnxTi2 x/2(PO4)3 samples.24 The analysis of this hyperfine structure, performed as a function of the Mn concentration,24 lead us to consider three different Mn centers as responsible for the four sextets. In detail, Mn2+/Li not perturbed ions should be responsible for the sextet centered at g = 2.04; Mn2+/Li ions perturbed via hyperfine interactions by ions with non-null nuclear magnetic moment (P5+ or Li+ ions) or interacting with neighboring Mn ions should be responsible for the signals centered at 3311 G ( 317 G; and rather insulated Mn/Ti ions could be responsible for the sextet centered at 3311 G + ∼250 G. Opposite to the structured signals, line-shape and line-width of the unstructured broad line are peculiar to each sample. For the SS05-Air sample, the broad line is about 270 G wide and is centered at g e 2 (1.997), and for the other samples a broader line centered at g = 2 is observed. A numerical analysis of the 1246

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The Journal of Physical Chemistry C spectra allowed us to deepen the nature of this broad line. For SS05-Air a pure Lorentzian line-shape is obtained. For the other samples, the broad line results to be the sum of two components: the narrower one, Lorentzian (L-component), shows the same g-factor and line-width detected for SS05-Air, except for SG05, for which a higher value of line-width is obtained; the broader one, Gaussian (G-component), is centered at g > 2 (about 2.05) and shows a line-width of about 900 G for both the SG samples, while it is broader (about 1100 G) for SS05-Argon. Figure 3 shows, as an example, the experimental and computed (from numerical analysis) derivative EPR signal of SG05. 3.3. IS. The Arrhenius plots of Li-rich doped samples are shown in Figure 4. The undoped SG and SS sample conductivity data24 are also reported for comparison. The undoped SS sample shows the highest conductivity values in the entire investigated temperature range. Among the samples considered in this work,

Figure 3. (a) Experimental (thin) and computed from numerical analysis (thick) derivative EPR signal of SG05. (b) Lorentzian (narrower) and Gaussian (broader) components of the broad line.

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the SG10 one presents the highest conductivity values, and the SG05 and SS05-Air show conductivity data very close to that observed for the undoped SG one. A peculiar behavior is instead observed for the SS05-Argon sample, whose conductivity, markedly low at rt, reaches the values of the other samples for higher temperatures. More details concerning this behavior will be presented in the Discussion section. 3.4. SEM. The grain morphology of the two series of samples, as expected, is very different. As an example, the micrographs of SS05-Air and SG05 samples are shown in Figure 5. The solid state sample presents wide squared particles of about 5 10 μm linked together to form extended agglomerates (Figure 5a). The sol gel samples, independently of stoichiometry, show small rounded particles of about 100 nm that form large agglomerates (Figure 5b). SEM analysis has also been carried out on the pellets of the same samples to verify the sintering degree and porosity. The micrographs are shown in Figure 5c and 5d. The powder morphologies and size are retained also in the pellets, and a high degree of sintering is evident in both the samples.

4. DISCUSSION The comparison of structural and spectroscopic results suggests interesting considerations regarding the valence state of Mn ions and their distribution on the cationic sites. EPR spectra arise from Mn2+ and Mn4+ ions, considering that signals coming from Mn3+ (3d4) are not expected in our experimental configuration.24,27 In Figure 6, the EPR intensities (areas), obtained after double integration of the first-derivative EPR signals, are reported as a function of the total Mn content of the samples. For x = 0.05, the lowest value is detected for SS05-Argon and the highest for SS05Air. A factor of about two is observed between the EPR intensities of the SG-doped samples. The main contribution to the EPR signal intensity values comes from the broad line that can be ascribed to sample regions with high Mn concentration. The Lorentzian line shape of the narrower component (L-component) of the broad line, possibly coming from exchange interactions between similar ions via

Figure 4. Arrhenius plot of the bulk conductivity of Li-rich, Mn-doped samples in comparison to the stoichiometric undoped ones. 1247

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Figure 5. SEM micrographs of SS05-Air and SG05 powder (a and b, respectively) and of broken pellets of the same samples after conductivity measurements (c and d, respectively).

Figure 6. EPR intensity values of all the spectra obtained after double integration of the first-derivative EPR signals.

identical paths, and its g-value, lower than 2, suggest it can arise from Mn4+ ions in ordered regions. The attribution of the origin of this component to Mn4+ ions is further supported by the spectrum (purely Lorentzian line), centered at g < 2, detected for the SS05-Air sample, that was prepared in an oxidizing atmosphere just to stabilize the 4+ oxidation state of the Mn ions.

On the contrary, the Gaussian character of the broader component (G-component) of the broad line and its g-factor, higher than 2, suggest this component coming from Mn2+ ions in disordered sample regions. On the other hand, on the basis of ionic radii,28 Mn2+ and Mn4+ ions are expected to occupy the 6b lithium site and the 12c titanium site, respectively. The Mn3+ radius is instead compatible with both the dimensions of Ti and Li ions. As a support for the above considerations, the estimated values of the intensities of the two (L- and G-) EPR components have been compared to the Mn abundance on Li and Ti crystallographic sites, as achieved from XRPD refinements (Table 1). A good agreement is found between the narrower L-component intensity and the Mn/Ti amount and between the broader G-component intensity and Mn/Li amount, as shown in Figure 7. These observations definitively suggest to attribute the broader G-component mainly to Mn2+ on the Li sublattice and the narrower L-component mainly to Mn4+ on the Ti sublattice. The deviations in the trend of the curves based on EPR signals and the Rietveld results (Figure 7) can be related to the amount of Mn3+ in the samples and to its distribution in the different crystallographic sites. In fact, Mn3+ is not revealable from EPR but is included in the total Mn amount determined in the refinement. Besides, the Mn3+ presence can influence both the Lorentzian and Gaussian component of the broad line depending on its location, namely, Ti or Li sites. For example, the line-shape 1248

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Figure 7. (a) Intensity of the EPR narrower Lorentzian component (see text) and Mn/Ti values from Table 1 for all the investigated samples and (b) intensity of the EPR broader Gaussian component (see text) and Mn/Li values from Table 1 for all the investigated samples.

of the SS05-Air EPR spectrum, especially the lack of the G-component, and the high intensity value of the L-component reveal the 3+ oxidation state for Mn ions on Li sites and the prevailing presence of Mn4+ ions on the Ti site, consistently with the oxidizing treatment in air of the sample. Besides, in the SG05 sample, where Mn mainly occupies the Ti sites according to the Rietveld refinement (Table 1), the L-component is broader with respect to the other samples, and this fact can be interpreted in terms of Mn3+ ions on Ti sites that interfere with the exchange narrowing effect of Mn4+ ions. Concerning the SS05-Argon sample, the thermal treatment in an inert atmosphere did not favor the formation of Mn4+, as confirmed by the very low L-component intensity, and the G-component is very broad thus suggesting that a noticeable amount of Mn3+ ions is present on Li sites together with Mn2+ ions. The high amount of Mn3+ in this sample is also supported by the experimental value of the EPR signal area, being the lowest among the x = 0.05 samples (Figure 6). Finally, we observe that for the SG10 sample the Mn ions distribute equally on Li and Ti sites, as shown by NPD and XRPD techniques. The slightly lower value of the EPR G-component intensity with respect to the L-component (Figure 7) suggests the presence of a slightly higher Mn3+ amount on the Li site with respect to the Ti site. Some structural information on lithium excess can be obtained from NPD analysis of the SG10 sample. Two models were compared, differing in the lithium excess location, i.e., into the 18e site18 or in the 36f23 one. The results of the refinements, both with satisfactory discrepancy factors but differing in the isotropic thermal factor values, suggest the location of lithium excess into the 18e crystallographic site as more probable, as proposed by Tran Qui et al.18 for Li-rich indium-doped samples. This site also hosts the lithium amount removed by the manganese occupation of the regular 6b site.

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The inspection of the lattice parameters and cell volumes (Table 1) can further suggest interesting considerations on the cation distribution on the sites. The cell volume is higher in sol gel samples with respect to solid state ones for the same x value, while for sol gel samples it remains constant by varying x. Different reasons can explain the cell expansion: (i) the presence of Li excess on the M2 site, leading to a maximum repulsion along the c axis14 and (ii) the presence of the Mn2+ on the Li site (ionic radii: Mn2+ = 0.83 Å, Li+ = 0.76 Å).28 No significant changes in cell parameters can instead be expected when Mn3+ substitutes Li ions or when Mn3+ and Mn4+ substitute the Ti ones (ionic radii: Mn3+ = 0.645 Å, Mn4+ = 0.53 Å, Ti4+ = 0.605 Å).28 The lattice parameters of the SS undoped sample previously investigated24 (a = 8.5079(1) Å and c = 20.8531(4) Å) are really comparable with those obtained in the Li-rich SS Mn-doped samples (Table 1); this is in agreement with the presence of a very low Li excess in the samples and of the Mn ions, preferentially in the 3+ oxidation state, on the Li site, as suggested by EPR data. The higher cell volume generally observed for the sol gel samples, with respect to the solid state ones, can be explained taking into account that the SG synthesis route involves lower temperature and time of sintering. Besides, by comparing the lattice parameters of the SG samples considered in this work with those obtained for the SG undoped (a = 8.5107(2) Å and c = 20.8688(10) Å) and of the SG x = 0.10 Mn-doped without Li excess (a = 8.5113(3) Å and c = 20.8852(11) Å) samples previously reported,24 we can observe that the lattice parameters of the two (Li-rich and not Li-rich) SG10 samples are comparable,24 so that the cell expansion with respect to the SG undoped sample cannot be attributed mainly to the presence of Li excess on the M2 site but rather to the Mn distribution on the cationic sublattices. The slightly higher c value observed for SG10 with respect to SG05 can indeed be attributed to the presence of a higher Li excess amount. The Arrhenius plot of the conductivity data shown in Figure 4 clearly evidences a complex behavior ascribable to different factors such as the Mn distribution on the cationic sites and its oxidation states and the different amount of Li excess of the samples. The effect of Li excess on the conductivity has already been reported in the literature for Li-rich samples doped with different ions.13,14 By comparing the bulk conductivity values of our samples with literature data, we can underline the good performances of the sol gel materials, in particular, of the SG10 sample, notwithstanding the low sintering temperature.14 In this case, the Li excess occupying M2 sites, though lower than values usually reported (0.3 e x e 0.5),14,18,23 could play a favorable role on conductivity. For the SG05 sample the low Li excess amount is not sufficient to increase the conductivity with respect to the undoped SG one. The conductivity behavior of solid state samples can be explained on the basis of the cationic distribution as determined by the Rietveld refinement and from EPR data. In fact, the presence of Mn on the lithium site justifies the lowering of the conductivity with respect to the SS undoped sample, and as in the case of the SG05, the Li excess on the M2 site is too low to balance the decrease in conductivity. In the case of the SS05-Air sample, the conductivity is really comparable to that of SG and SG05 samples. The SS05-Argon shows a very different behavior in comparison to the other samples: the conductivity, that initially is very low, increases rapidly with the temperature reaching, at about 300 °C, the values obtained for SS and SG10 samples. This fact can be explained on the basis of a variation of Mn distribution on the cationic sites that takes place 1249

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The Journal of Physical Chemistry C during the measurement, performed in air, and that is not reversible by lowering the temperature. Namely, the Mn2+ ions, initially on Li sites, oxidize to Mn3+ by increasing the temperature and migrate on Ti sites to allow a better Li mobility. These hypotheses are confirmed by EPR data that evidence a signal intensity decrease of about 50% for the sample analyzed after the conductivity measurement. In addition, the numerical analysis of this EPR signal allowed us to attribute the whole signal intensity decrease to a lowering of the broad G-component intensity, which is related to Mn2+ on Li sites. The importance of the degree of sintering on the conductivity behavior of the investigated class of phosphates is wellknown.11,14,20 For this reason, the SEM study has been carried out on both the prepared powders and the sintered pellets used for the IS measurements. Notwithstanding the great difference in grain morphology of the as-prepared sol gel and solid state samples (Figure 5), a high density and no residual porosity are observed on the pellets for both series of samples (Figure 5c and d). This evidence suggests that the degree of sintering acts on the conductivity behavior in a comparable way for the two series of samples, independently of the applied synthesis route. Rather, the synthesis affects the cation distribution and, as a consequence, the amount of lithium in the M2 sites. So, the proper choice of synthesis conditions is important to obtain satisfactory values of the conductivity in the Li-rich, Mn-doped LiTi2(PO4)3 samples.

5. CONCLUSIONS A structural and spectroscopic combined study of Li-rich, Mn-doped LiTi2(PO4)3 samples was performed by means of XRPD and NPD with Rietveld refinement and EPR techniques. Thanks to the EPR spectroscopy, we found that the Mn ions are present in the 2+, 3+, and 4+ oxidation states in different amount in the 6b and 12c crystallographic sites, depending on the synthesis (sol gel or solid state) and on sample treatment. The numerical analysis of the EPR spectra, supported by structural data, allowed us to determine that Mn2+ prefers the Li site and Mn4+ the Ti one, and Mn3+ could be present on both sites. In particular, the use of air or argon atmosphere during the synthesis and also the synthesis route can drive the oxidation state of manganese toward a specific value, modulating the cation distribution. It has been demonstrated that these factors are mainly responsible for the conductivity behavior of the samples and prevail on the sintering degree, which seems independent of the synthesis route. The Rietveld refinement performed on the NPD data has been useful to clarify that the lithium excess locates on the 18e crystallographic sites and not on the 36f one.

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’ AUTHOR INFORMATION Corresponding Author

*Phone: 39 382-987213. Fax: 39 382 987575. E-mail: capsoni@ unipv.it.

’ REFERENCES (1) Robertson, A. D.; West, A. R.; Ritchie, A. G. Solid State Ionics 1997, 104, 1–11. (2) Sebastian, L.; Gopalakrishnan, J. J. Mater. Chem. 2003, 13, 433–441. (3) Tarascon, J. M.; Armand, M. Nature 2001, 414, 359–367. (4) Wolfenstine, J.; Allen, J. L.; Sumner, J.; Sakamoto, J. Solid State Ionics 2009, 180, 961–67. (5) Chang, C. M.; Lee, Y. I.; Hong, S. H. J. Am. Ceram. Soc. 2005, 88, 1803–1807. 1250

dx.doi.org/10.1021/jp208671z |J. Phys. Chem. C 2012, 116, 1244–1250