Local Effects of the Electrochemical Reaction of Lithium with

17436

J. Phys. Chem. C 2008, 112, 17436–17442

Local Effects of the Electrochemical Reaction of Lithium with Sn2ClPO4 and SnHPO4: A Combined 31P, 7Li MAS NMR and 119Sn Mossbauer Spectroscopy Study J. I. Corredor, B. Leo´n,* C. Pe´rez Vicente, and J. L. Tirado Laboratorio de Quı´mica Inorga´nica, UniVersidad de Co´rdoba, Edificio C3, planta 1, Campus de Rabanales, 14071 Co´rdoba, Spain ReceiVed: January 24, 2008; ReVised Manuscript ReceiVed: August 29, 2008

In recent years, metallic tin and tin compounds have proven to be interesting electrode materials for lithium batteries. However, detailed information about the mechanism of reaction in phosphate compounds is needed to improve the performance of some previously examined and future electrode materials containing these species. In the present work, powerful techniques for the study of the local environments of the atoms, such as 119Sn Mo¨ssbauer spectrometry and 7Li and 31P MAS NMR, are used to obtain basic information on the processes involved during the discharge/reduction of the electrode material. The results allow extracting valuable information about the possible interactions between the participating atoms and the surrounding framework. Introduction An increasing interest in the electrochemical reactions of lithium with tin and tin compounds has been experienced in recent years, together with the development of successive prototype and commercial products by battery manufacturers, such as the Nexelion Technology. Direct evidence of this trend is that during 2007 only, there have been nearly 200 patents published on lithium batteries in which tin is used. The use of lithium-metal alloys and compounds was early proposed as a substitute for metallic lithium anodes to avoid the difficult and unsafe handling of this highly reducing metal.1 Although the resulting capacities are significantly higher than that of graphite, the marked differences in volume between the metal and the subsequent alloys and compounds led to undesirable mechanical effects that damage the electrode material. Several authors tried to overcome this drawback by using metal compounds instead of the pure metal.2-4 A great variety of crystalline or amorphous forms of tin alloys and compounds have been tested, in which similar electrochemical processes are involved (i.e., lithium-metal alloying/dealloying processes). Tin compounds, in which tin is in a positive oxidation state and accompanied by different anions, such as oxides,5-8 and oxysalts (SnSO4,9 SnSiO3,10 SnBPO6,11 Sn2P2O7,12 Sn2ClPO4,13 and SnHPO414,15) were reported. As compared with pure tin metal electrodes, the improved electrochemical behavior of these compounds was ascribed to the formation of a nonelectrochemically active phase containing lithium and the corresponding anion originally in the tin compound. This phase does not show further reaction with lithium, and it is believed to counteract the deleterious expansion of the lithium-metal alloy formed during the charging of the electrode by avoiding tin particle aggregation. However, little effort has been made to determine the degree of interaction between lithium salts formed during the discharge process and lithium-tin alloys. The aim of this work is to use some powerful solid state techniques, such as 119Sn Mo¨ssbauer spectroscopy and solidstate 31P and 7Li NMR, to understand the lithium reaction mechanisms in Sn2ClPO4 and SnHPO4 and to elucidate the * Corresponding author. Fax: + (34) 957 218 621. E-mail: q72lemob@ uco.es.

possible interactions between the phases that are being formed during the discharge of the cell. Experimental Sn2ClPO4 was synthesized by a precipitation method. An orthophosphoric acid solution was slowly added to a SnCl2 · 2H2O solution under continuous stirring until the precipitation was completed, as described elsewhere.13 The solid was recovered by centrifugation at 10 000 rpm for 10 min and washed several times with distilled water, absolute ethanol, and acetone. Further drying was carried out at 80 °C in air for 1 day and at 110 °C under vacuum for a second 1-day period. The compound SnHPO4 was obtained by a precipitation procedure. H3PO4 (20 mL, 85%) was added to a tin(II) solution (prepared by dissolving 20 g of SnCl2 · 2H2O in propylene carbonate (PC) up to 100 mL) with continuous stirring until the precipitation was completed. The precipitate was separated by centrifugation at 10 000 rpm for 10 min, washed several times with absolute ethanol, and dried at 60 °C in air for 2 h and then at 110 °C under vacuum for 1 day. The electrochemical behavior was tested using two-electrode Swagelok cells of the type Li|1 M LiPF6 (solvent)|tin compound, where the solvent was a mixture of EC (ethylene carbonate) and DEC (diethylene carbonate) in 1:1 weight ratio. The electrodes were prepared as 10-mm-diameter pellets by pressing a mixture of 86% active material, 6% PVDF binder, and 8% carbon black (4N Strem) to improve the mechanical and electronic conduction properties, respectively. Lithium electrodes consisted of a clean 10-mm-diameter lithium metal disk. The electrolyte solution was supported by Whatman porous glasspaper discs and CELGAR films. The disassembly of the cells and the handling of the electrodes to be placed in an adequate sample-holder for further ex situ measurements was carried out inside a glovebox (M Braun, containing less than 1 ppm of O2 and H2O) to avoid any exposure to air. The electrochemical curves were obtained by using a multichannel MacPile system under galvanostatic conditions at C/4 rate; that is, a molar ratio Li/Sn equal to unity was reached in 4 h. For the preparation of partially discharged samples, the electrode materials were held at the desired cutoff voltage (or lithium content) until the voltage varied less than 1 mV/h.

10.1021/jp800722x CCC: $40.75  2008 American Chemical Society Published on Web 10/15/2008

Reaction of Lithium with Sn2ClPO4 and SnHPO4

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Figure 1. Discharge curve when using Sn2ClPO4 as cathode material in a Li cell. 119Sn

Mo¨ssbauer spectra were recorded using a constant acceleration spectrometer in transmission mode. The γ-ray source of nominal activity 10 mCi was Ba119mSnO3, and the velocity scale was calibrated by using the magnetic sextet spectrum of a high-purity iron foil absorber using 57Co(Rh) as source. Ex situ measurements were performed on partially discharged electrodes at different depths of discharge. For this purpose, the Swagelok cells were opened inside the glovebox, and the electrodes containing the active material were placed on an airtight, specific sample holder transparent to the γ-rays. Experimental data were fitted to Lorentzian profiles by a leastsquares procedure, and goodness of fit was controlled by the classical χ2 test. Isomer shifts were referenced to the center of the BaSnO3 spectrum recorded at room temperature. NMR spectra were recorded using a Bruker ACP-400 spectrometer at a magnetic field of 9.4 T. The samples were ground and mixed with KBr and then placed in a zirconia rotor and sealed, all inside the glovebox. KBr was used as an inert matrix to fill the sample holder and thus to avoid the sample’s moving inside during the NMR recording. The 31P spectra were recorded at a resonance frequency of 161.97 MHz under MAS conditions at a maximum spinning speed of 14.5 KHz. The excitation pulse and recycled delay were fixed at 2 µs and 0.5 s, respectively. H3PO4 (85%) was used as external reference for the phosphorus chemical shifts. 7Li spectra were recorded by setting a solid spin-echo sequence (π/2 - τ - π/2 - τ acquisition). These spectra were recorded at 155.5 MHz, also under MAS. In this case, 8 µs and 2 s were set as excitation pulse and recycle delay, respectively. The chemical shifts were referred to an aqueous LiCl solution. Results and Discussion A. Sn2ClPO4. Figure 1 shows the electrochemical discharge curve of a lithium cell using Sn2ClPO4 as active electrode material. As previously reported,13 the cell potential vs composition curve is characterized by a plateau at ∼1.6 V, extending to 2 Li per Sn atoms. This step can be assigned to the reduction of SnII to Sn0, accompanied by the formation of lithium phosphate and lithium chloride by a conversion reaction. The second step, from 2 to 6.25 Li/Sn, is assigned to the formation of Li-Sn alloys and compounds. The first published X-ray diffraction results confirmed this mechanism.13 Nevertheless, the progressive amorphization of the products during reduction did not allow a clear description of the detailed nature of the reduced phases involved in the later stages of the discharge process. To follow in detail the modifications of the local order in this material during the electrochemical discharge, 119Sn Mo¨ssbauer, 31P, and 7Li NMR spectra were recorded and are discussed below.

Figure 2. Experimental and proposed fitting of 119Sn Mo¨ssbauer spectra of Sn2ClPO4 recorded at different depths of discharge in Li cells: Sn2ClPO4 (solid line), SnIV phosphate (dotted), metallic tin (dotteddashed), Li-Sn alloys (dashed).

Previous works demonstrated the relevance of 119Sn Mo¨ssbauer spectrometry16 to characterize the wide variety of lithium-tin phases. In this sense, Dunlap et al.17 have recorded the spectra of several crystalline Li-Sn compounds obtained by direct synthesis. Here, a shift of the weighted averages of isomer shift values of the components as a function of Li/Sn toward low velocities was taken as a proof of the increase of lithium content in the resulting phases. In a further reference,18 the authors claimed good agreement between the profiles of the crystalline alloys and those of the electrodes of SnO and TCO (tin composite oxides) electrochemically reacted with lithium, obtained by in situ measurements. However, the authors found a new signal, at ∼1.5 mm/s, which was assigned to tin atoms at the edge of the tin clusters that are near oxygen. From these results, a mechanism based on two successive and separate reactions of tin reduction and Li-Sn alloying was concluded.18 Other authors19 have interpreted the 119Sn Mo¨ssbauer spectra of electrochemically reduced SnO electrodes in terms of two simultaneous processes that involve the reduction of the oxide to β-Sn and the occurrence of a reduced intermetallic tin in strong interaction with the structural SnO. Previously published results on SnO20,21 concluded that the mechanism cannot be interpreted only in terms of a reduction of SnII to Sn0, followed by a Li-Sn alloying process. In these works, a more complex mechanism was suggested (i) involving the formation of mixed valence compounds LixSnO during the SnII reduction and (ii)

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TABLE 1: Refined Hyperfine Parameters of 119Sn Mo¨ssbauer Spectra of Raw and Lithiated Sn2ClPO4 Shown in Figure 2: Isomer Shift (δ), Quadruple Splitting (∆), Line Width at Half Maximum (Γ), and Contribution hyperfine parameters in mm s-1 and relative contribution sample Sn2ClPO4

1 Li/Sn

1 V (2.16 Li/Sn)

1 V (2.16 Li/Sn, at 77 K)

0.6 V (2.64 Li/Sn, at 77 K)

Sn2ClPO4

SnIV phosphate

δ ) 3.564(2) ∆ ) 1.314(2) Γ ) 0.753(2) 98.2% δ ) 3.554(8) ∆ ) 1.317(8) Γ ) 0.753(2) 83.1% δ ) 3.42(2) ∆ ) 1.35(3) Γ ) 0.72(5) 40.7% δ ) 3.32(2) ∆ ) 0.91(2) Γ ) 1.13(6) 20.8% δ ) 3.33(8) ∆ ) 1.0(1) Γ ) 1.0(3) 11.6%

δ ) -0.25(3) ∆)0 Γ ) 0.9(1) 1.8% δ ) -0.1(1) ∆ ) 0.4(3) Γ ) 1.2(3) 7.3%

0.3 V (4.29 Li/Sn)

0 V (6.13 Li/Sn)

revealing strong interactions between Li-Sn compounds and the inert matrix of Li2O during the alloying process. The discrepancies found in the literature show that the electrochemical reduction and further alloying of tin related compounds is a rather complex subject. It could be elucidated by a more extended study of the local environment of the atoms in the structure for distinct tin related compounds. The different properties of the neighboring atoms could alter the possible interactions between tin and the other surrounding atoms which in turn could be reflected in the Mo¨ssbauer spectra. Mo¨ssbauer Effect. The ex situ 119Sn Mo¨ssbauer measurements were carried out on partially discharged electrodes. The charge passed through the cell was previously selected to achieve a set of spectra that allow us to evaluate the whole range of the discharge process. The 119Sn Mo¨ssbauer spectra recorded at different depths of discharge are shown in Figure 2. The refined hyperfine parameters are included in Table 1. The spectrum of the Sn2ClPO4 consists of an asymmetric doublet centered at ∼3.56 mm/s, which is in the typical range for SnII compounds. The value of the quadrupole splitting, ∼1.31 mm/s, agrees well with the highly distorted environment of tin atoms in Sn2ClPO4. The asymmetry in the intensities of the two components of the doublet is attributed to the Goldanskii-Karyagin effect,22,23 commonly found in SnII compounds. Additionally, another subspectrum of very low intensity was detected at ∼-0.2 mm/ s, which is attributed to traces of SnIV phosphate impurities. The spectrum of the sample discharged at 1 Li/Sn shows the presence of a shoulder, centered at ∼2.40 mm/s, which corresponds to metallic tin, thus confirming the proposed reduction of SnII to Sn0 during the first plateau of the discharge curve. For the samples at 1.0 and 0.6 V, Table 1 shows that the main contribution to the spectra comes from metallic tin, which allows a good refinement of the tin subspectrum. The presence of a quadrupole splitting can arise from the low local order of

Sn

δ ) 2.40(2) ∆)0 Γ ) 0.79(6) 9.6% δ ) 2.49(3) ∆ ) 0.43(3) Γ ) 1.2(1) 59.3% δ ) 2.577(8) ∆ ) 0.52(1) Γ ) 1.17(2) 79.2% δ ) 2.56(2) ∆ ) 0.32(2) Γ ) 1.02(2) 88.4%

alloy site 1

alloy site 2

δ ) 2.34(2) ∆ ) 0.49(5) Γ ) 0.83(8) 46.4% δ ) 1.87(2) ∆ ) 0.39(3) Γ ) 0.94(3) 59.9%

δ ) 2.26(2) ∆ ) 1.10(3) Γ ) 0.83(4) 53.6% δ ) 1.98(2) ∆ ) 1.35(3) Γ ) 0.8(1) 40.1%

the in situ formed metallic tin (i.e., low crystallinity). Concerning the sample at 1 Li/Sn, the small contribution of the subspectrum of metallic tin, appearing as a shoulder of the main subspectrum of Sn2ClPO4, makes a good refinement of the former difficult. This is why for this sample, metallic tin has been refined using a singlet.The decrease in the intensity of the doublet assigned to Sn2ClPO4 is not proportional to the increase of the new singlet of metallic tin due to the low value of the recoil-free factor of metallic tin.24,25 This low value has been reported for both crystalline Sn and amorphous Sn, formed in situ during the reduction of SnII compounds in lithium cells. Thus, it results in an apparent increase in the intensity of the SnIV phosphate from 1.8 to 7.3%, but this augmentation cannot be interpreted as an increase in the SnIV phosphate impurity content. At the end of the plateau (sample at 1 V, equivalent to 2.16 Li/Sn) the complete conversion of SnII into Sn0 is expected. The Mo¨ssbauer spectrum of this sample recorded at room temperature shows the subspectrum of metallic tin, with a contribution of ∼60%, and the subspectrum of Sn2ClPO4, with a contribution of 40%. As in the precedent case, this is due to the low recoil-free factor of metallic tin, as compared with Sn2ClPO4. To partially palliate this disadvantage, the spectrum was recorded at 77 K, where an increase in the recoil-free fraction is expected, especially for metallic tin. Thus, the contribution of the subspectrum of metallic tin increases up to ∼80% and is centered at 2.57 mm/s, as expected for β-Sn. For an additional decrease in temperature, an increase in its relative contribution is expected. Thus, the presence of the signal at ∼3.42 mm/s can be attributed to traces of Sn2ClPO4, due to kinetic effects, having its origin in the presence of particles with low electrical contact or electrolyte accessibility or both. The spectrum of the sample at 0.6 V is similar to that of the sample at 1.0 V, but the contribution of the metallic tin increased up to ∼90%.

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Figure 4. Experimental and proposed fitting of 7Li NMR MAS spectra of Sn2ClPO4 recorded at different depths of discharge.

Figure 5. Discharge curve when using SnHPO4 as cathode material in a Li cell. Figure 3. Top: 31P NMR spectrum of Sn2ClPO4 at different depths of discharge in Li-cells. Bottom: refinement of the spectrum of Sn2ClPO4.

TABLE 2: Results of the Refinement of the 31P NMR Spectra of Sn2ClPO4, Shown in Figure 3 δ/ppm Γ/Hz C/%

peak 1

peak 2

peak 3

peak 4

peak 5

-15.9 186 72.0

-12.3 166 18.6

-10.2 200 1.5

-8.6 284 2.4

-6.1 572 5.5

In the spectrum of the sample at 0.3 V, neither Sn2ClPO4 nor metallic tin are detected. Instead, two new subspectra are observed, centered at ∼2.34 and 2.26 mm/s. Finally, at 0 V, these two subspectra are shifted to lower values of velocity, of 1.87 and 1.98 mm/s, which agrees with Li-rich phases, as compared with the sample at 0.3 V. Although the low crystallinity of the products does not allow the assignment to any precise local environment, as could be done for crystalline Li-Sn phases,17 the isomer shift values are consistent with the formation of Li-Sn alloys. MAS NMR. Figure 3 shows the 31P MAS NMR spectra of Sn2ClPO4 at different depths of discharge. The spectrum of Sn2ClPO4 is characterized by two main signals at ∼-12 and -16 ppm (Table 2). The spectrum of the sample discharged at 1 Li/Sn shows a progressive decrease in intensity, while a new peak with a complex profile centered at ∼5 ppm is observed,

showing a broad and intense shoulder at a lower chemical shift. The profile can be decomposed into two signals, centered at ∼5 and 0 ppm (inset in Figure 3). The spectrum of the sample at 2.16 Li/Sn (Figure 3) does not show the signal at ∼0 ppm, and the peaks ascribable to Sn2ClPO4 are very weak, as expected at this depths of discharge. At 0.3 V, traces of [PO4] groups ascribable to Sn2ClPO4 are still visible, but with a relative contribution lower than 1%. However, the main resonance for these samples is now the peak located at ∼5 ppm. The signal at 5 ppm appears at values slightly lower than that of crystalline Li3PO4, which has been earlier reported at 10.6 ppm.15 Because no Bragg reflections of the in situ Li3PO4 formed during the electrochemical reaction are detected by X-ray diffraction,14,15 these signals should be due to amorphous phosphates. Thus, for example, the [PO4] groups in xNa2O · (1 - x)P2O5 glasses typically appear in the 12-14 ppm range.26 In lithium phosphates, the signals are upfield shifted by ∼4-7 ppm due to the increase in cationic potential by changing Na for the more electronegative Li,27 and the signal of [PO4] groups appears in the 5-10 ppm range. This interval of parts per million is consistent with the signal at ∼5 ppm observed in our experimental spectrum, which then can be attributed to amorphous lithium phosphate. Concerning the broad signal appearing at ∼0 ppm, it may be assigned to (i) [PO4] surface groups of freshly formed Li3PO4

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Figure 6. Experimental and proposed fitting of 119Sn Mo¨ssbauer spectra of SnHPO4 recorded at different depths of discharge in Li cells: SnHPO4 (solid line), Sn3(PO4)2 (dotted), metallic tin (bold), and Li-Sn alloys (dotted-dashed).

TABLE 3: Results of the refinement of the 31P NMR spectra of raw and lithiated Sn2ClPO4, shown in Figure 3: chemical shift (δ), linewidth (Γ), relative contribution (C) excluding LiPF6. In all cases the central peak of the septet corresponding to LiPF6 appears at - 148 ppm, with JP-Cl ) 710 Hz, and Γ ≈ 190 Hz Sn2ClPO4a

sample 1 Li/Sn 1V 0.3 V (4.29 Li/Sn) 0 V (6.13 Li/Sn)

a

δ/ppm Γ/Hz C/% δ/ppm Γ/Hz C/% δ/ppm Γ/Hz C/% δ/ppm Γ/Hz C/%

14.3 1.6 0.9 1.0

signal 1

signal 2

4.5 828 29.3 5.4 870 98.4 4.9 761 99.1 5.0 820 99.0

0.0 1214 56.4

Only the contribution was refined.

in contact with clusters of metallic tin formed simultaneously, which may modify the shielding of P atoms to some extent. Since Sn is more electronegative than Li, the resonance is shifted to higher field; that is, to lower ppm values, as compared with the signal of bulk lithium phosphate. (ii) Possible intermediate compounds formed during the reaction involving the simultaneous presence of Li+ and SnII cations compensating their charge by coordinating phosphate groups. Unfortunately, actual data do not allow unequivocally discriminating between these possibilities.

Figure 4 shows the 7Li MAS NMR spectra. The spectrum of the sample obtained at 1 V shows only one intense signal centered at ∼0 ppm, which results from two overlapping components ascribed to Li3PO4 + LiCl (because both signals appear at very close values, they have been treated as one signal) and a narrow and weak peak ascribable to the electrolyte. Further discharge of the sample allowed us to detect a progressive broadening of the signal, and the development of a new component shifted to higher frequency, with increasing contribution until the end of the discharge. The chemical shift value is highly coincident with those ascribed to Li-Sn alloys in the literature,15,28 according to an expected increase of the covalence for the bonding of both metallic elements. Moreover, their appearance matches well with abrupt changes in the Mo¨ssbauer spectra profiles which were ascribed above to the formation of alloys. The spectrum recorded at the end of the discharge curve (6.13 Li/Sn) shows the same previously described signal, with strong variation in intensity. The downfield signal, at 8.5 ppm, can be assigned to Li-rich Li-Sn phases of low conductivity, where the Knight shift is not present. This value is far from the 114 ppm shift reported for crystalline LixSn phases, where the observed high value is due to the Knight shift.29 Owing to the sensitivity of NMR to the structural and electronic environment of lithium in metallic hosts and due to the low crystallinity of the product, this component cannot be ascribed to a single particular environment. Moreover, the absence of Knight shift can be related to the small size of alloy particles, which would hinder the free motion of the conduction electrons.30 B. SnHPO4. Figure 5 shows the discharge curve of a lithium cell using SnHPO4 as active material in the positive electrode. The first step, extending up to ∼3 Li/Sn, was tentatively assigned to tin and hydrogen reduction, according to13

SnHPO4 + 3 Li f Sn + 1 ⁄ 2 H2 + Li3PO4

(1)

The second step, from 3 to 7.14 Li/Sn, corresponds to the Li-Sn alloys formation. Its extension, ∼4.14 Li/Sn, is close to the theoretical value of 4.4, which corresponds to the usually described Li22Sn5 phase. The structure of Li-M phases was recently revisited and established the composition of the more Li-rich phase to be Li17Sn4,31,32 for which the Li/Sn ratio is 4.25, closer to the experimental value observed in Figure 5. Mo¨ssbauer Effect. Figure 6 shows the experimental and refined 119Sn Mo¨ssbauer spectra of SnHPO4 discharged in lithium cells at different depths of discharge. The pristine compound is characterized by an asymmetric split signal centered at ∼3.62 mm/s, a value that is in the typical range of SnII compounds. The origin of the asymmetry can be assigned to a Goldanskii-Karyagin effect, commonly found in SnII compounds. Additionally, traces of SnO2 were detected, which contribution was estimated to be ∼0.9%. Taking into account the high value of the recoil-free fraction in SnO2 as compared to SnHPO4, its quantitative contribution is expected to be considerably lower than 0.9%. A visual inspection of the spectra in Figure 6 of the samples obtained at 1.5 Li/Sn and at 1.0 and 0.6 V allows drawing the following remarks: (1) At 1.5 Li/Sn, the spectrum is composed of three doublets with isomer shifts ∼3.62, 2.99, and -0.12 mm/s, which are related to the SnHPO4 (57%), Sn3(PO4)2 (34%), and SnO2 (5%) compounds, respectively, and one singlet with isomer shift ∼2.47 mm/s corresponding to metallic Sn (3%). (2) From 1.5 Li/Sn to 1 V, the intensity of the doublet related to SnHPO4 drops down from 57.3 to 8.7%, Sn3(PO4)2 increases

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TABLE 4: Refined Hyperfine Parameters of 119Sn Mo¨ssbauer Spectra of Raw and Lithiated SnHPO4, Corresponding to Figure 6: Isomer Shift (δ), Quadruple Splitting (∆), Line Width at Half Maximum (Γ), and Contributiona hyperfine parameters in mm s-1 and relative contribution sample pristine

1.5 Li/Sn

1 V (3.21 Li/Sn)

0.6 V (4.28 Li/Sn)

SnO2

SnHPO4

Sn3(PO4)2

Sn

δ ) -0.35(5) ∆ ) 0.60(6) Γ ) 0.7(1) 0.9% δ ) -0.12(6) ∆ ) 0.65(7) Γ ) 0.8(1) 5.1% δ ) -0.06(1) ∆ ) 0.67(4) Γ ) 0.83(7) 6.1% δ ) 0.1(1) ∆ ) 0.5(1) Γ ) 0.7(1) 2.8%

δ ) 3.621(1) ∆ ) 1.410(2) Γ ) 0.729(2) 99.1% δ ) 3.621 ∆ ) 1.410 Γ ) 0.69(2) 57.3% δ ) 3.621 ∆ ) 1.410 Γ ) 0.66(4) 8.7% δ ) 3.621 ∆ ) 1.410 Γ ) 0.83(3) 2.1%

δ ) 2.99 ∆ ) 1.84 Γ ) 0.82(2) 34.1% δ ) 2.99(2) ∆ ) 1.84(2) Γ ) 0.89(1) 77.0% δ ) 2.99 ∆ ) 1.84 Γ ) 0.89(3) 79.8%

δ ) 2.47(2) ∆)0 Γ ) 0.99(7) 3.5% δ ) 2.54(2) ∆)0 Γ ) 0.73(4) 6.2% δ ) 2.49(2) ∆)0 Γ ) 0.89(3) 15.3%

0.3 V (5.44 Li/Sn)

0 V (7.14 Li/Sn)

a

alloy site 1

alloy site 2

δ ) 2.294(7) ∆)0 Γ ) 0.87(1) 20.4% δ ) 2.171(8) ∆ ) 0.79(1) Γ ) 0.85(1) 44.3%

δ ) 2.27(7 ∆ ) 0.96(1) Γ ) 0.87(2) 79.6% δ ) 2.084(7) ∆ ) 1.60(1) Γ ) 0.85(3) 55.7%

The parameters without error (in parenthesis) have been fixed during the fitting.

from 34.1 to 79.8%, and the intensity of the singlet corresponding to metallic Sn slightly increases to 6%. (3) At 0.6 V, the relative contribution of the doublet corresponding to SnHPO4 still decreases to 2%, the contribution of Sn3(PO4)2 remains almost unchanged, and metallic Sn increases to 15%. The presence of SnO2 is clearly detected as a signal centered at ∼0 mm/s, where its relatively high contribution is due to its high value of the recoil-free fraction, and not to an actual high amount. The refined hyperfine parameters are included in Table 4. These data evidence that the amount of Sn3(PO4)2 formed cannot be neglected. Thus, although the simple mechanism based on eq 1 represents in a correct way the global reduction step, it has to be decomposed in two substeps. A possible mechanism is as follows: first H+ is reduced to H2 to yield Sn3PO4 and Li3PO4, according to

observed in other tin-based systems. Anyway, the lack of crystallinity of the products, evidenced by the absence of Bragg reflection in the X-ray diffraction patterns13 and probably influenced by the dispersion originated by the H2 evolution, does not allow the assignment to any precise local environment, as previously found for crystalline Li-Sn phases.17 MAS NMR. A detailed study of SnHPO4 by NMR has been previously published.15 The MAS NMR spectra of 7Li and 31P at different depths of discharge led to the following conclusions: (i) the integrity of phosphate groups is preserved during the discharge and charge and(ii) the evolution of H2 is indirectly evidenced by the formation of Li3PO4, according to eqs 2 and 3.

SnHPO4 + Li f 1 ⁄ 2 H2 + 1 ⁄ 3Li3PO4+1 ⁄ 3Sn3(PO4)2

The study of two different tin compounds containing phosphate anions in lithium test cells by using highly specific spectroscopies for 31P, 7Li, and 119Sn nuclei has allowed us to unequivocally identify the different steps during the electrochemical reduction process. In both Sn2ClPO4 and SnHPO4, cell discharge leads to a first conversion reaction that yields nanodispersed metallic tin, lithium orthophosphate, and other side products, depending on the composition of the initial solid. Thus, LiCl is the main side product from the chlorophosphate, whereas protons are reduced to H2 in the case of the hydrogenphosphate, in which conversion takes place by a two-step mechanism involving tin orthophosphate. In a second stage of the discharge, poorly conducting nanoparticles of lithium-tin intermetallics are observed by the isomer shift values in the Mo¨ssbauer spectra and absence of Knight shift in the 7Li MAS NMR. The reluctance of phosphate groups to participate in the electrochemical processes, as revealed by 31P MAS NMR, makes them suitable matrix components to accommodate volume changes in the Li-Sn intermetallics.

(2) then

SnII

is reduced to

Sn0,

according to

1 ⁄ 3 Sn3(PO4)2 + 2 Li f Sn + 2 ⁄ 3 Li3PO4

(3)

The fact that the reduction of SnII to Sn0 is not completely reached at the end of the first plateau can be related to kinetic effects, probably induced by the H2 formation, which can result in a highly dispersed material, avoiding a good electrical contact between the particles of the in situ formed tin phosphate. Moreover, the discharge curve shown in Figure 5 does not allow differentiating between both reactions, since only one plateau is observed. It imply that reactions 2 and 3 take place simultaneously, resulting in the coexistence of metallic tin and Sn3(PO4)2, as observed in the 119Sn Mo¨ssbauer spectra. Finally, the spectra of the samples obtained at 0.3 and 0 V show a complex profile, with a decrease of the weighted average isomer shifts of the components as a function of Li/Sn, as

Conclusions

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