Tin Phosphate Electrode Materials Prepared by the Hydrolysis of Tin

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J. Phys. Chem. C 2009, 113, 5316–5323

Tin Phosphate Electrode Materials Prepared by the Hydrolysis of Tin Halides for Application in Lithium Ion Battery Zineb Edfouf, Marıá Jose´ Aragoń, Bernardo Leoń,* Carlos Pe´rez Vicente, and Jose´ L. Tirado Laboratorio de Quı´mica Inorgańica. Edificio C3, Campus de Rabanales 14071 Co´rdoba, Spain ReceiVed: NoVember 11, 2008; ReVised Manuscript ReceiVed: January 22, 2009

The synthesis, structural characterization, and electrochemical evaluation as potential anode material for Li ion battery of new materials based on tin phosphate are studied. Hydrothermal treatments of tin halides are used for the preparation of phosphate oxysalts. 119Sn Mo¨ssbauer spectroscopy is used to discern the oxidation state of tin in the synthesized materials. Samples prepared with SnCl2 have better electrochemical properties than those prepared with SnF2. Also, low concentration favors the formation of nanostructured materials with an improved capacity retention and high specific capacity. Capacities higher than 500 mAhg-1 are observed after 40 cycles for the ex-SnCl2 nanoparticulate sample. 1. Introduction Among the materials which display larger lithium storage capacity than graphite, Li alloys and intermetallic compounds have been studied since the pioneering 1971 study by Dey.1 The formation of alloys with lithium may take place by a reversible electrochemical reaction at sufficiently low voltages to be useful for the anode of Li ion batteries. Nevertheless, not all the elements form alloys with lithium. Other factors should also to be considered such as physical properties (e.g., melting point), atomic weight, and limiting Li/M ratio (i.e., the value of “x” in LixM), always bearing in mind that the capacity of the cell depends on these two latter factors. Price and toxicity are also important in the material selection. At present, the most interesting elements as active Li ion anodes are Si and Sn. Unfortunately, the bulk elements are generally considered unsuitable because the structural changes and large volume expansion associated with lithium insertion decrease the structural integrity of the electrode. Therefore, the formation of cracks during Li extraction induces pulverization of the particles (loss of electrical and mechanical contact) and lead to a significant decay in cycling efficiency and low cell capacities. Some solutions have been proposed to avoid the problems for Li metal alloys, which involve their structure and morphology (i.e., texture, grain size, and shape), as these parameters strongly affect their dimensional stability. Metal embedding on a matrix that absorbs volume changes, or porous materials where the pores change their size during intercalation and desintercalation of Li, allow a good stability of the particles. The use of nanoparticles of active material dispersed on a lithium oxide or lithium salt matrix could not only prevent aggregation and formation of large particles (thus avoiding the cracks) but also decreases the diffusion-path length for lithium insertion and decreases the charge-transfer resistance of the electrodes.2,3 Interesting active materials that can be used for these purposes include intermetallic and ionic compounds. The reaction mechanism can be interpreted in terms of two simple steps: first an irreversible reduction of the cation into metallic state, according to nLi + MXn f M + nLiX, and then the reversible formation of LiM alloys M + yLi T LiyM. We can therefore deduce that * To whom correspondence [email protected].

should

be

addressed.

E-mail:

we have to find materials with minimum irreversible capacity (minimum value of n) and maximum reversible capacity (maximum value of y). A series of compounds that furnish those conditions are SnII compounds. The richest Li-Sn alloy, Li17Sn4 (i.e., y ) 4.25) formed after the reduction of SnII into metallic Sn provides 68% of the total capacity as reversible capacity. Subsequently, the number of publications concerning tin based compounds has increased significantly during the past decade. Particular examples are SnO (840 mAhg-1 of theoretical reversible capacity),4,5 Sn4P3 (802 mAhg-1),6,7 and Sn2P2O7 (554 mAhg-1).8 The aim of the present work is the synthesis, structural characterization, and electrochemical evaluation of new (nano)materials based on tin phosphate, as anodic material for Li ion battery, having improved capacity retention and high specific capacity. The synthesis of materials based on tin and phosphate has been carried out varying different parameters in order to compare the impact of these changes on the size and the morphology of the particles and thus, on their performance when used as active anode materials for Li ion batteries. 2. Experimental Section 2.1. Synthesis. The synthesis procedure carried out in this work has the aim to produce nanosized particles by using a hydrothermal method. Three solutions were mixed: (A) anhydrous SnF2 or dihydrate SnCl2, (B) sodium dodecyl-sulfate (SDS) surfactant, and (C) concentrated phosphoric acid H3PO4 85%. The solution obtained was stirred during one hour at 40 °C and finally heated in an autoclave (filled up to 80% of its total volume) for 6 days at 90 °C. The final solid product was filtered and washed with distilled water and ethanol until negative reaction to sulfate ions. Then, the material was dried under vacuum overnight (7 mbar pressure) at 100 °C in an alumina crucible. Finally, the solid was heated during 5 h at 400 °C under dynamic Ar atmosphere. Six samples were synthesized from the two different starting materials (anhydrous SnF2 and dihydrate SnCl2) by varying the concentration and the order of addition of the starting solutions (see Table 1). 2.2. X-ray Diffraction, Electronic Microscopy and Thermal Analysis. X-ray diffraction (XRD) patterns were recorded on a Siemens D5000, using Cu KR radiation and a graphite

10.1021/jp809932k CCC: $40.75  2009 American Chemical Society Published on Web 03/11/2009

Tin Phosphate Electrode Materials

J. Phys. Chem. C, Vol. 113, No. 13, 2009 5317

TABLE 1: Summary of the Different Syntheses sample F1 F2 F3 C1 C2 C3

Sn source SnF2 SnF2 SnF2 SnCl2 SnCl2 SnCl2

concentration 0.144 mol/L 1.44 mol/L 1.44 mol/L 0.121 mol/L 1.21 mol/L 1.21 mol/L

order of addition A+B+C A+B+C A+C+B A+B+C A+B+C A+C+B

monochromator. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained in a JEOL JSM63000 microscope and a JEOL 200CX microscope, respectively. Thermogravimetric (TGA) and differential thermal analysis (DTA) curves were obtained in a SHIMADZU DTG-60AH instrument, under dynamic Ar amosphere. 2.3. Spectroscopic Techniques. For magic angle spinning (MAS) NMR measurements, the samples were transferred to zirconia rotors and sealed inside a glovebox under inert atmosphere with Kel-F caps. The spectra were recorded on a Bruker Avance-400 wide bore magnet at the following resonant frequencies: 6Li, 58.9 MHz, and 31P, 162.0 MHz. The external references were 1 M LiCl and 85% H3PO4, respectively. A single-pulse excitation sequence was used, with a pulse angle of π/2. The delay times were 2 and 50 s for Li and P, respectively. The spinning speed was ca. 30 kHz. Mo¨ssbauer spectra were recorded using a conventional constant acceleration spectrometer. The source was 119mSn in a BaSnO3 matrix at room temperature. The velocity scale was calibrated using a 57Co(Rh) source and metallic iron foil as an absorber. The spectra were fitted to Lorentzian profiles by a less-square method. All isomer shifts reported are given relative to the center of a BaSnO3 spectrum obtained at room temperature. 2.4. Electrochemical Measurements. Lithium test cells were assembled in two-electrode SWAGELOK-type cells, with metallic lithium as anode and a 1 M solution of LiPF6 in ethylene carbonate-diethyl carbonate (EC-DEC) in a 1/1 w/w proportion as electrolyte, supported by porous glass paper (GF/ A, Whatman). The electrodes were prepared by blending the active material (60%) with carbon black (30%) and of polyvinylidene fluoride (PVDF, 10%) dissolved in N-methyl-2pyrrolidone. The slurry was cast into a Cu foil and vacuum dried at 120 °C. The cells were assembled in a MBraun LabMaster 130 glovebox under Ar atmosphere (H2O, O2 < 2 ppm). The cells were cycled in a multichannel microprocessor-controlled system Mac-pile galvanostat, which allows controlling and monitoring the cell charging and discharging processes. For each synthesis three cells were prepared to be cycled, and the results were averaged. 3. Results and Discussion 3.1. XRD. The XRD patterns of the products obtained after hydrothermal treatment and before heating at 400 °C showed the absence of long-range order (low crystallinity or tiny diffraction domains) when using the starting solutions in a low concentration (samples C1 and F1). Nevertheless, the occurrence of some weak peaks for sample F1 was indicative of the presence of SnHPO4 (see Figure S1 of Supporting Information).9 The use of higher concentration during the synthesis resulted in crystalline phases. In the case of the samples F2 and F3, all the reflections can be ascribed to SnHPO4, while no additional reflections ascribable to impurities were detected (for clarity, the peaks corresponding to SnHPO4 are indexed in the pattern of the sample F3). Changing the order of addition of the raw solutions (see experimental section) has no influence on the final product.

Figure 1. 119Sn Mo¨ssbauer spectra of the samples before heating ((a) C3, (b) F3) and after heating under Ar atmosphere at 400 °C ((c) C1, (d) C3, and (e) F3).

On the contrary, for samples C2 and C3 a mixture of SnHPO4 and Sn2ClPO410 is observed. Additionally, the order of addition of the raw solutions seems to affect the relative amount of each compound in the final product, as evidenced from the variation of the relative intensity of their Bragg reflections. The XRD patterns of the samples after heating at 400 °C under Ar atmosphere showed that for sample F1 the weak peaks observed before heating, assigned to SnHPO4, have disappeared leading to a pattern with no observable peaks (see Figure S2 of Supporting Information). Samples F2 and F3 showed the same X-ray diffraction pattern (except for some weak differences in the peak intensities). The indexation of the Bragg reflections evidenced the presence of tin(II) pyrophosphate, Sn2P2O7,11 as a major product, although traces of tin (IV) pyrophosphate, SnP2O7, and tin oxide SnO2 were also detected. In concern for the samples prepared using SnCl2 as the source of tin, the X-ray diffraction pattern of the C1 sample showed no observable peaks. On the contrary, the patterns of C2 and C3 samples show a set of diffraction peaks, evidencing the

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TABLE 2: Refined Hyperfine Parameters of the Mo¨ssbauer Spectra of the Samples C3 and F3 before and after Heating and C1 after Heating at 400 °C under Argon Atmospherea sample F3

heating

δ

∆

Γ

C

attribution

before

3.605 (5) -0.36 (1) 3.44 (1) -0.40 (2) 3.62 (2) -0.17 (6) 3.47 (3) -0.07 (2) 3.37 (2) -0.29 (1) 3.58 3.62 3.37 -0.15 3.35

1.435 (5) 0.21 (2) 1.51 (1) 0.05 (5) 1.45 (2) 0.48 (1) 1.60 (3) 0.57 (1) 1.72 (2) 0.51 (2) 1.33 1.41 1.47 0.42 1.61

0.92 (1) 0.9 (0.1) 0.99 (2) 1.1 (2) 1.44 (4) 1.44 (3) 1.74 (7) 1.75 (7) 1.45 (3) 1.34 (2) 0.74 0.73 0.92 1.02

89 (1) 11 (3) 77 (3) 23 (7) 87 (4) 13 (5) 54 (3) 46 (3) 28.2 (8) 71.8 (7)

SnII SnIV SnII SnIV SnII SnIV SnII SnIV SnII SnIV SnII SnII SnII SnIV SnII

after C3

before after

C1 Sn2ClPO49 SnHPO49 Sn2P2O711 Sn2P2O712

after

78 22

Isomer shift (δ, mm s-1), quadrupole splitting (∆, mm s-1), linewidth (Γ, mm s-1) and contribution (C, %) for different samples: F3, C3, and C1. For comparison, the hyperfine parameters of Sn2ClPO4, SnHPO4, and Sn2P2O7 are also included. a

presence of crystalline phases. From a comparison with the diffraction patterns before heating it is worth noting that: (i) the peaks coming from Sn2ClPO4 are always present; (ii) the peaks previously assigned to SnHPO4 have disappeared; (iii) a new set of lines appears. This set of new peaks is similar to that observed for samples F2 and F3 and thus can be assigned to a mixture of tin pyrophosphates, Sn2P2O7 and SnP2O7, arising from the thermal decomposition of SnHPO4. 3.2. Thermal Analysis. A thermal analysis study has been carried out to gain information about the transformations occurring during the heating of the samples (the TGA and DTA curves of sample F3 are shown in Figure S3 of Supporting Information). Three peaks in the DTA curve were observable. The first peak, centered at 173 °C, is endothermic and was accompanied by a weak weight loss in the TGA curve. This effect can be ascribed to a loss of water molecules occluded in the solid during the synthesis of the precursor. The second peak and the most important in the DTA curve, occurring at ca. 289 °C, is also endothermic. It is accompanied by a considerable weight loss in the TGA curve of ca. 4.3% of the initial mass, which can be assigned to the formation of Sn2P2O7 according to the reaction

2 SnHPO4 f Sn2P2O7+H2O

(1)

The expected weight loss associated to the condensation reaction of phosphate groups is ca. 4.2% of the initial mass, in good agreement with the experimental weight loss observed. This data also agrees with the results previously showed by the XRD patterns of F2 and F3 samples, for which the presence of tin pyrophosphates (accompanied by the absence of SnHPO4) was detected after heating at 400 °C. A third peak in the DTA curve is detected at ca. 353 °C. This peak, together with the absence of a weight loss in the TGA curve, allow us to assign it to a phase transition of Sn2P2O7, from β-phase (triclinic, stable below 350 °C) to R-phase (monoclinic, stable above 350 °C).11 Finally, it is worth noting that no weight gain was observed in any of the TGA curves. This result is in contrast with the XRD patterns of the samples after heating, where traces of tin dioxide were detected. This partial oxidation can be attributed to the experimental device used for heating the samples, which

probably allowed traces of oxygen in the reactor, while this oxidation was absent in the case of TGA experiments. 3.3. 119Sn Mo¨ssbauer Spectroscopy. To obtain further information about the oxidation state of tin in the samples, 119Sn Mo¨ssbauer spectroscopy was carried out. Selected spectra are shown in Figure 1. The refined hyperfine parameters resulting from the fitting of the spectra are included in Table 2, together with the hyperfine parameters of Sn2ClPO4, SnHPO4, and Sn2P2O7 for comparison. The spectra are characterized by the presence of two doublet signals. The signal at higher isomer shift is a resolved and asymmetric doublet, with an isomer shift in the interval from ca. 3.4 and 3.7 mm s-1. These values are in the typical range of SnII phosphate compounds. The asymmetry can be ascribed to Goldanskii-Karyagin effect, commonly observed for SnII compounds. The second signal consists on an unresolved doublet, centered at values slightly lower than 0 mm s-1, and is assigned to SnIV. From a close inspection of the values of the relative contribution of SnII and SnIV, it is worth noting that the heating of the sample favors a partial oxidation of SnII into SnIV, simultaneously with the decomposition of SnHPO4 giving rise to tin pyrophosphate compounds, as observed by XRD. Thus, the spectrum of the sample F3 before heating shows a large contribution of SnII, which is counterbalanced to SnIV after heating. The isomer shift and quadrupole splitting of the SnII signal (δ ≈ 3.6 mms-1, ∆ ≈ 1.44 mms-1) agree with the presence of SnHPO413 and also with the XRD pattern (see Figure S1 of Supporting Information). The signal with lower intensity appearing at δ ≈ -0.36 mms-1 is assigned to SnIV phosphate. Thus, a mixed valence compound can be assumed. By assumption of a similar recoil-free fraction for SnII and SnIV, the following composition can be proposed: SnII0.8SnIV0.1HPO4. Thus, the thermal decomposition proposed in eq 1 should be modified according to heated to 400°C

2SnII0.8SnIV 0.1HPO4 98 0.8Sn2P2O7 + 0.2SnP2O7 + H2O or heated to 400°C

II IV 2SnII0.8SnIV 0.1HPO4 98 Sn1.6Sn0.2P2O7 + H2O

(2)



Figure 3. TEM images of F1 sample after heating at different magnifications, and the Fourier transform of the picture at higher magnification. In brackets, the assignment (hkl and spacing of Sn2P2O7) of the spot.

Figure 2. TEM images of C1 sample after heating at different magnifications and the Fourier transform of the picture at higher magnification. In brackets, the assignment (hkl and spacing of Sn2P2O7) of the spots.

Nevertheless, the weight loss expected for these decomposition reactions is ca. 4.4%. This value is close of that of the decomposition of pure tin(II) hydrogen phosphate according to eq 1 of ca. 4.2%. Moreover, the line width of the signals, ca. 0.9 mm s-1, is somewhat larger than the line width of SnHPO4 synthesized by a simple precipitation procedure,9 of 0.74 mm s-1. This increase in the line width can arise from a local distortion of the structure induced by the presence of SnIV and vacancies partially substituting SnII. The Mo¨ssbauer spectrum of the sample F3 after heating (Figure 1e) basically shows the same signals than before heating (Figure 1b). However, some variations are observed. Thus, the isomer shift of the asymmetric doublet assigned to SnII decreases from 3.61 to 3.44 mm s-1, while the quadrupole splitting increases from 1.44 to 1.51 mm s-1. These new values are in agreement with the transformation of tin hydrogen phosphate into tin pyrophosphate.11,12 Additionally, an increase of the SnIV contribution is observed. Sample C3 before heating (Figure 1a) also shows a large contribution of SnII and a small contribution of SnIV, similarly to sample F3 (Figure 1b). The hyperfine parameters isomer shift and quadrupole splitting agree with tin phosphates. As seen from XRD, this sample is a mixture of SnHPO4 and Sn2ClPO4. The overlapping of the signals corresponding to both compounds result in an increase of the line width as compared with sample F3. After heating, a partial oxidation is also observed. The shift of the SnII signal to lower values and the increase of quadrupole splitting are in agreement

Figure 4. TEM images of the samples F3 (top, left), C3 (top, right), and C2 (bottom), after heating and the Fourier transform of the last one. In brackets, the assignment (hkl and spacing of Sn2P2O7) of the spot.

with the transformation of tin hydrogen phosphate into tin pyrophosphate. Additionally, the comparison of the concentration effect on the syntheses has been carried out by analyzing two similar syntheses with different concentrations. Thus, the spectrum of the sample C1 after heating (Figure 1c) shows the same peaks observed in all the previous spectra, corresponding to the two oxidation states of tin. However, in this case the contribution of the SnIV signal after heating is considerably higher. 3.4. TEM. Figure 2 shows the TEM images of sample C1 after heating at different magnifications. The particles have a


Edfouf et al.

Figure 5. First discharge/charge cycle of the different samples in Li cells.

Figure 7. Ex situ 6Li MAS NMR of the F1 sample: (c) discharged at 0.8 V, (d) discharged at 0 V, and (e) recharged to 3 V.

Figure 6. Ex situ 31P MAS NMR of F1 sample: (a) the raw material, (b) discharged at 1.5 V, (c) discharged at 0.8 V, (d) discharged at 0 V, and (e) recharged to 3 V.

spherical shape with 10-30 nm size, forming agglomerates of ca. 200-300 nm. The small size of the particles can result in a high reactivity. This fact could justify the strong oxidation observed for this sample after heating (see Mo¨ssbauer section). At high magnification, crystallographic domains smaller than 10 nm are visible. The Fourier transform shows a set of spots corresponding to those domains. The d spacing values determined from the Fourier transforms of the observed images (3.65, 2.22, and 3.25 Å) agree with the structure of the high temperature form of Sn2P2O7 (3.643, 2.232, and 3.233 Å for 101, 310, 210 reflections, respectively).11 For the F1 sample after heating, the results are somewhat different. As we can observe from Figure 3, there is a mixture of sub-micro- and nanoparticles. The Fourier transform of the domain images in which crystallographic fringes are visible shows well-defined spots with spacing assignable to Sn2P2O7.

The particles also show a porous system of ca. 7 nm diameter. A porous texture was previously described for SnHPO4 (obtained from SnF2 and H3PO4), with a porous size of ca. 2-3 nm.14 A carbon-coated tin(IV) phosphate (obtained from SnCl4 and Na2HPO4) was also described, with similar porous system (ca. 10 nm).15 In fact, pore size strongly depends on the organic molecule used as template in the hydrothermal synthesis of tin phosphates.16 Finally, the texture of samples C2, C3, F2, and F3 after heating is somewhat similar to sample F1, i.e., markedly heterogeneous, with the presence of small particles (40-60 nm) forming agglomerates and large particles (up to 1 µm) with different elongated shapes, as shown in Figure 4. On the contrary, the porous system was not observed. The Fourier transform of the image of sample C2 also shows spots ascribable to Sn2P2O7. 3.5. Electrochemical Characterization. The samples heated at 400 °C have been tested in lithium cells. The first discharge curve of all samples is characterized by two steps, as shown in Figure 5. The first step is a plateau centered at ca. 1.5-1.6 V, with extensions that vary between 200 and 300 mAhg-1, depending on the sample. The second step is a smooth voltage drop down to 0 V. A comparison with other tin phosphate systems allows assigning the first step to tin reduction into



Figure 8. Discharge and charge profiles of selected cycles for electrodes using F1, F2, F3, C1, C2, and C3 samples.

Figure 9. Reversible discharge capacity vs cycle number for the potential range from 0.001 to 3V for F1, F2, F3, C1, C2, and C3 samples.

metallic tin, and the second to the formation of lithium-tin intermetallic compounds,9,10,13,17 according to 1st step

2Sn2P2O7 + 8Li

98

4Sn + 17Li

98

4Sn + 2Li4P2O7

(3)

2nd step

Li17Sn4

The irreversible capacity of the first cycle can be then assigned to the charge consumed to reduce Sn cations to metallic Sn and also to the formation of solid electrolyte interphase (SEI).18,19 The impendance spectra of the electrodes were recorded (see Figure S4 of Supporting Information), and it was found that a significant surface layer resistance is developed during the first

plateau. However, on subsequent discharge the resistance still increases reaching ca. 32 ohm at the end of the first discharge. This value is retained on further discharges. Ex situ XRD patterns were recorded for two samples: C1 and F1, after reaction with Li at different points of the first cycle (see Figure S5 of Supporting Information). In contrast to other tin-based systems, peaks of neither metallic tin nor lithium-tin alloys were detected. This can be indicative of: (i) the good dispersion of tin atoms obtained during the preparation and (ii) the impeded aggregation of tin atoms during the reaction with lithium. However, the patterns d and e of of sample F1 show several peaks, arising from the protective material that covered the sample in order to avoid oxidation with air during the analysis. 3.6. 31P and 6Li MAS NMR. Sample F1 was also studied at different depth of discharge by nuclear magnetic resonance. Figure 6 shows the 31P NMR spectra of different lithiated samples. The septet centered at - 144.0 ppm, with coupling constant JP-F ) 708 Hz, can be assigned to P atoms of the residual electrolyte (i.e., PF6- anions). The fact that P atoms are surrounded by six F atoms, with I ) 1/2, results in the observed septet (this signal has been used as internal reference for the chemical shift). For samples a and b (pristine and just at the beginning of the plateau at 1.5-1.6 V), the wide peak centered at ca. -15 ppm can be assigned to Sn2P2O7.20 For sample c, when the complete tin reduction is expected, the signal assigned to Sn2P2O7 has disappeared. Instead, two new signals appear at ca. -2 ppm and +10 ppm, with contributions of 15 and 85%, respectively. The first one is assigned to Li4P2O7,21 while the second one can be assigned to Li3PO4.22 Further discharge down to 0 V (sample d) makes the signal at -2 ppm disappear, remaining only the signal at +10 ppm. Finally, at the end of the charge, neither Sn2P2O7 nor Li4P2O7 (i.e., pyrophosphate groups) are regenerated, being only present the signal at +10 ppm. Such behavior has been previously described for SnO-P2O5 glassy


Edfouf et al.

TABLE 3: Comparison of First and Second Discharge Capacities and the Capacity Retentions of the Electrodes Using F1, F2, F3, C1, C2, and C3 Samples

sample F1 F2 F3 C1 C2 C3

1st discharge capacity (mAhg-1)

discharge capacity at 2nd cycle (mAhg-1)

rate of capacity retention after 20 cycles R20/R2 (%)

rate of capacity retention after 40 cycles R40/R2 (%)

1119 985 1066 1243 1112 1114

733 654 721 762 709 701

71 73 47 76 75 66

66 62

electrodes in rechargeable lithium cells, specifically for the glassy material 67SnO-33P2O5, in which the global composition is equivalent to Sn2P2O7.21 To account for the transformation of lithium pyrophosphate into lithium phosphate, the authors proposed that the P-O-P bond in P2O74- groups might be broken to form P2O64- and PO43- by electrochemical lithium insertion, according to the following reaction

2Li4P2O7 + 2LifLi4P2O6 + 2Li3PO4

(4)

To our knowledge, the NMR spectrum of Li4P2O6 has not been reported. Since only one peak at ca. +10 ppm is observed in the NMR spectrum, the peak of Li4P2O6 might overlap the peak of Li3PO4.18 The NMR spectrum after charging suggests that the matrix consisting of P2O64- and PO43- groups formed during the first discharge is retained during the charge. The chemical shift range of 7Li and 6Li NMR is very narrow as compared with 31P NMR. However, 6Li has much higher resolution than 7Li and can provide better information. Thus, we have recorded the 6Li MAS NMR spectra of samples c, d, and e, as Figure 7 shows. The signal of Li3PO4 is expected at ca. 0.3-0.4 ppm, while the signal of Li4P2O7 should appear in the range -0.2 to -0.3 ppm, with line width values of ca. 30-40 Hz.23 The spectrum of sample c shows a large peak centered at ca. 0.2 ppm. It could be tentatively assigned to Li3PO4. However, the presence of an overlapping signal coming from Li4P2O7 cannot be excluded. In fact, the large line width of the signal (ca. 80 Hz ≈ 1.4 ppm) makes difficult the identification of the possible lithium phosphates, especially because the signal of Li4P2O7 is expected to have a weak relative contribution (15%) as compared with Li3PO4 (85%), as previously shown from 31P NMR data. The sample obtained at the end of the discharge (sample d in Figure 7) shows a complex profile close to 0 ppm, which can be assigned to a mixture of lithium phosphates. Additionally, two new signals appear: (i) at ca. 4.4 ppm, assigned to lithium-tin intermetallic compounds, and (ii) at ca. 0.7 ppm that corresponds to Li+ of the residual electrolyte. Finally, after the charge of the cell, the signal assigned to the Li-Sn compounds disappears, while the signals of lithium phosphates and the electrolyte remain. These results confirm that the reversible part of the electrochemical curve can be attributed to the formation/deformation of intermetallic lithium-tin compounds. 3.7. Cycling Performance. A comparison of charge-discharge profiles at different cycle number is illustrated in the Figure 8. A significant loss of capacity is observed during the first ten cycles. Then, a less marked decrease is observed, and a good correspondence between the capacity values for charge and discharge is also noted. On the other hand, the shape of the discharge and charge profiles shown in Figure 8 suffers little change from the fifth cycle to the following cycles. The variation

66 68

of the capacity on discharge upon the number of discharge/ charge is represented in Figure 9. The capacities and their retention after 20 and 40 cycles for all the syntheses are given in Table 3. In general, all the samples obtained using tin chloride during the synthesis show higher capacity and better capacity retention than the equivalent samples obtained using tin fluoride. The best capacity retention was achieved by the sample C1, which delivers the higher capacity value, ca. 762 mAhg-1, in the second discharge (the capacity of the first discharge was not considered representative of the reversible capacity, as it contains a large irreversible part due to tin reduction and SEI formation). The capacity after 40 cycles is still higher than 500 mAhg-1. The capacity retention after 40 cycles (defined as R40/ R2) is ca. 66%. As we have noted before, C1 sample shows nanosized particles by TEM, which can be the reason to have such good electrochemical behavior. Electrodes for lithium ion batteries based on Li alloys are known to exhibit poor cycling stability due to very large volume changes, aggregation of particles, and consequent degradation on cycling. A valuable strategy to overcome this problem is the use of a matrix that buffers the volume changes of the Li alloying element.4 Also in micro- and nanostructured matrix alloys, the volume expansion due to the insertion of lithium causes much less cracking and pulverization of the electrodes.2 The electrochemical properties and good cyclability of this sample can be compared with a mesoporous tin phosphate/Sn2P2O7 composite14 with an initial capacity of ca. 721 mAhg-1 and ca. 587 mAhg-1 at the 30th cycle. Compared to C2 and C3 samples (prepared with high concentration), the C1 sample gives higher capacity and better retention. Thus, the capacity of the sample C2 is slightly lower than that of C1, i.e. 709 mAhg-1 for the second cycle and 480 mAhg-1 after 40 cycles (having the best rate of capacity retention after 40 cycles equal to R40/R2 ) 68%), while for C3 the capacity decrease more rapidly, reaching 403 mAhg-1 after 30 cycles. The second most interesting sample giving a high capacity is F1, which gives a value of 733 mAhg-1 in the second cycle and 487 mAhg-1 after 40 cycles. This sample can be compared to C2 which have quite the same range of capacity and capacity retention. The F2 sample has also a good retention rate but gives lower capacities than the F1, C1, and C2 samples described before. It compares well to C3, which has higher starting capacity and lower retention after 20 cycles. Also, the material giving the lowest capacity values is F3, which loses its capacity very quickly to reach a 290 mAhg-1 after only 25 cycles. The bad behavior showed by this material is maybe caused by the large particles and high agglomeration observed by TEM. 4. Conclusions In summary, the following remarks can be done for the behavior of different materials obtained in this study: (1) Samples prepared with SnCl2 have better electrochemical properties than those prepared with SnF2.

Tin Phosphate Electrode Materials (2) The use of low concentration of the precursors during the hydrothermal treatment favors a good electrochemical behavior of the final product in the Li cell. The behavior of those materials can be compared with the results given by the nonmesoporous tin phosphate used as anode for Li ion batteries19 in which the retention of capacity was only of 34% after 50 cycles compared with 66% as average of capacity retention in our case. On the other hand, the irreversible capacity is lower than in other materials as the mesoporous/ crystalline composite based on tin phosphate, in which the irreversible capacity reaches 900 mAhg-1 vs an average of 350 mAhg-1 for our materials.24 The high capacity loss in that case is believed to be due to the reaction of the electrolyte with the high surface area presented by the porous system, which should be very weak in our case. This comparison makes our materials interesting to be used as anodes for Li ion batteries. Acknowledgment. The authors are indebted to EC (ALISTORE), MEC (MAT2008-05880), and Junta de Andalucıá (FQM288). Z.E. is grateful to EC for her ERASMUS MUNDUS grant. Supporting Information Available: Figures described in the text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Dey, A. N. J. Electrochem. Soc. 1971, 118, 1547. (2) Yang, J.; Winter, M.; Besenhard, J. O. Solid State Ionics 1996, 90, 281. (3) Whitehead, A. H.; Elliott, J. M.; Owen, J. R. J. Power Sources 1999, 81-82, 33. (4) Courtney, I. A.; Dahn, J. R. J. Electrochem. Soc. 1997, 144, 2045.

J. Phys. Chem. C, Vol. 113, No. 13, 2009 5323 (5) Chouvin, J.; Olivier-Fourcade, J.; Jumas, J. C.; Simon, B.; Biensan, Ph.; Fernańdez Madrigal, F. J.; Tirado, J. L.; Pe´rez Vicente, C. J. Electroanal. Chem. 2000, 494, 136. (6) Kim, Y. U.; Lee, S. I.; Lee, C. K.; Sohn, H. J. J. Power Sources 2005, 141, 163. (7) Leon, B.; Pe´rez Vicente, C.; Tirado, J. L. J. Electrochem. Soc. 2006, 153, A1829. (8) Wan, K.; Li, S. F. Y.; Gao, Z.; Siow, K. S. J. Power Sources 1998, 75, 9. (9) Elidrissi Moubtassim, M. L.; Corredor, J. I.; Lloris, J. M.; Pe´rez Vicente, C.; Tirado, J. L. J. Electrochem. Soc. 2002, 149, A1030. (10) Fernańdez Madrigal, F. J.; Pe´rez-Vicente, C.; Tirado, J. L. J. Electrochem. Soc. 2000, 147, 1663. (11) Chernaya, V. V.; Mitiaev, A. S.; Chizhov, P. S.; Dikarev, E. V.; Shpanchenko, R. V.; Antipov, E. V.; Korolenko, M. V.; Fabritchnyi, P. B. Chem. Mater. 2005, 17, 284. (12) Lees, J. Y.; Flinn, P. A. J. Chem. Phys. 1968, 48, 882. (13) Corredor, J. I.; Leoń, B.; Pe´rez Vicente, C.; Tirado J. L. J. Phys. Chem. C, in press. (14) Kim, E.; Son, D.; Kim, M. G.; Cho, J.; Park, B.; Ryu, K. S.; Chang, S. H. Angew. Chem., Int. Ed. 2004, 43, 5987. (15) Kim, E.; Kim, Y.; Kim, M. G.; Cho, J. Electrochem. Solid State Lett. 2006, 9, A156. (16) Kim, E.; Kim, M. G.; Kim, Y.; Cho, J. Electrochem. Solid State Lett. 2005, 8, A452. (17) Behm, M.; Irvine, J. T. S. Electrochim. Acta 2002, 47, 1727. (18) Huang, L.; Wei, H. B.; Ke, F. S.; Cai, J. S.; Fan, X. Y.; Sun, S. G. Colloids Surf., A 2007, 308, 87. (19) Lee, J. G.; Son, D.; Kim, C.; Park, B. J. Power Sources 2007, 172, 908. (20) Amornsakchai, P.; Apperley, D. C.; Harris, R. K.; Hodgkinson, P.; Waterfield, P. C. Solid State Nucl. Magn. Reson. 2004, 26, 160. (21) Konishi, T.; Hayashi, A.; Tadanaga, K.; Minami, T.; Tatsumisago, M. J. Non-Cryst. Solids 2008, 354, 380. (22) Hayashi, S.; Hayamizu, K. Bull. Chem. Soc. Jpn. 1989, 62, 3061. (23) Alam, T. M.; Conzone, S.; Brow, R. K.; Boyle, T. J. J. Non-Cryst. Solids 1999, 258, 140. (24) Kim, E.; Kim, M. G.; Kim, Y.; Cho, J. Electrochem. Solid-State Lett. 2005, 8, A452.

JP809932K

Tin Phosphate Electrode Materials Prepared by the Hydrolysis of Tin

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