REVERSIBLE ELECTROCHEMICAL INSERTION OF LITHIUM IN FINE GRAINED POLYCRYSTALLINE POWDERS OF SnO

Fine grained SnO2 powders have been obtained using an unconventional method. It deals with the well known polymerization method starting from the metallic halide SnCl4 with polyethylene oxide (PEO). With this method, SnO2 powders, which are free from water and hydroxyl group contaminations and possess small crystallite size (≈50 A∘), are obtained by appropriate pyrolysis of the polymer. Consequently, these powders show good ability to insert reversibly lithium ions in the Li/Li+/LixSnO2 cell. Indeed, by minimizing the size of the crystallites, the formation of defect-bonds is favored, particularly at the crystallite surface, acting as reversible (de)grafting sites of Li+. Finally, an easy-to-carry out method to determine the chemical diffusion coefficient of lithium has been proposed.


INTRODUCTION
SnO 2 has attracted considerable attention due to its variety of applications.1-3 In powder form, for instance, it can be used as a sensing material for a combustible gas sensor or as an electrocatalyst for organic oxidation. 3These powders have generally been obtained through two different routes: one involves the oxidation of elemental tin with acids (HNO 3, H 2SO4, etc.), and the other utilizes the well-known sol-gel route. 4The sol-gel route has been found to give finer crystallites compared to the former method.However, samples free from water and hydroxyl groups are hardly achievable.
We propose here an unconventional and easy-to-carry out method for the preparation of fine dry particles of SnO 2 powders.It uses the well known polymerization method starting from the respective halides SnCI 4 with polyethy- lene oxide (PEP). 5The method has been shown to give not only very fine particles, obtained after eliminating the polymer by heating, but also particles free from water and OH group impurities.
We also investigated here the ability to insert reversibly lithium ions into the fine-grained SnO 2 powders in Li/Li+/LixSn02 cells.We have indeed reported elsewhere (for transition metal oxides such as LiFe203 or Li2_xNiO2) that by minimizing the crystallite size, we favor the formation of dangling and weak bonds at the surface; therefore, the later becomes electrochemically active for lithium *Present address: Institut de Chemie de la Matib.raCondensbe de Bordeaux, CNRS, Universit6 Bordeaux I, Chateau Brivazac, Ave du Dr. A. Schweitzer, 33600 Pessac, France.
insertion.6 In the case of transition metal oxides, it was the cationic "d" orbitals that were involved in the electrochemical processes; 6 on the other hand for SnO2, it is the cationic 5s orbitals that would be involved.

EXPERIMENTAL
2.1-SAMPLE PREPARATION PEO (Aldrich, M. W. 500,000)was first added to acetonitrile.The solution was stirred at room temperature in an ultrasonic bath until a satisfactory homogeneity was achieved.SnCI 4 5H2 (99.99%,Aldrich) was then added to the solution in the proportion [CH2CH2O]/Sn 8. Once a complete homogeneity of the "sol" was achieved, the "sol" was casted into a telfon mould.Evaporation of the solvent at about 40C under a stream of dry air gave a polymer film of typical thickness 100/zm.Finally, fine-grained SnO 2 powders were obtained by eliminat- ing the polymer with slow heating (lC/mn) in dry oxygen, up to 450C, and maintained for 2 hrs at this temperature.
The experimental procedure involved is schematically illustrated in Fig. 1.
2.2-SAMPLE ANALYSIS X-ray diffraction measurements were obtained using a Phillips PW 1050 spectrom- eter and CuKa radiation.The average crystallite size, D, was calculated from the well known Scherrer's formula D 0.9A/fl/2"cos 0 (1) where A is the X-ray wavelength and 0 the diffraction angle, fl/2 is the corrected width of the main diffraction peak at half height.The IR spectra in the absorbance mode were recorded on a Perkin-Elmer 983G spectrometer between 4000 and 200 cmwith an average resolution of 5 cm-.The experiments were performed on SnO 2 powders dispersed in Nujol and sandwiched between two cesium iodide disks.  .3-ELECTROCHEMICAL MEASUREMENTS Electrochemical (de)insertion of lithium was realized in bottle-type cells having two electrodes.The cathode consisted of 35 mg of SnO 2 powder and 6 mg of carbon black (i.e., --15 wt%).The cathode was prepared by intimately mixing the powders (previously outgassed at 180C for 5 hrs) and pressing in the form of a disk in a 1.3 cm diameter stainless steel die.Lithium metal was used as both reference and anode.The electrolyte was a 1 M LiCF3SO3-propylene carbonate solution impregnated into a glass filter paper.The propylene carbonate (Aldrich 99 + %) was further dried by fractional distillation under 4 , molecular sieves.
The lithium triflate (Aldrich 97%)was kept under vacuum at 150C during 72 hrs.The experiments have been then carried out in an air-free and moisture-free glove box maintained under argon atmosphere.

RESULTS AND DISCUSSION
3.1-CRYSTALLINE STRUCIRE AND TEXTURE Figure 2 shows the X-ray diffractograms for SnO 2 commercial powder (Aldrich) (a) and for the powder issued from the polymeric route (b).Both spectra are characteristic of the rutile structure of SnO2, as expected.In case (b) the X-ray diffraction lines are very broad accounting for the nanocrystalline nature of the powder.The main size of the crystallites, > 800 (Fig. 2a) and 70 (Fig. 2b), was estimated using eq. ( 1 and the main diffraction peaks, corresponding to the (110) plane.
3.2-1NFRA-RED SPECTROSCOPY Most interestingly, adsorbed water or hydroxyl groups do not occur in the fine- grained SnO 2 powder, as shown below.The presence of adsorbed water and 4000 2000 1200 400 /ave number (cm-1 FIGURE 3 IR absorption spectra of the fine grained SnO 2 powders issued from the polymeric route.hydroxyl groups is indeed normally revealed in the IR spectra.Adsorbed water is best characterized by its deformation mode OH 2 occurring near 1620 cm -.This absorption band is not observed here (Fig. 3).Moreover, no broad bands between 3500 and 3100 era-, involving the stretching modes of water or hydroxyl groups, 7 do not occur (Fig. 3).

3.3-ELECTROCHEMISTRY
3.3.1-1nfluence of grain size on the reversible electrochemical insertion of lithium in Sn02 electrodes: Fig. 4 illustrates the third discharge curve of the Li/Li+/LixSnO2 cells.In agreement with our model, 6 the highest rate of lithium inserted (x 0.5) occurs for the fine-grained powder electrode.The latter exhibits a good cycling reversibil- ity (Fig. 5).
3.3.2-Aneasy-to-carry out method to determine the chemical diffusion coefficient of lithium a-The Model An important parameter one has to know is the chemical diffusion coefficient, D, of lithium ions in the electrodes.In general, D is obtained using galvanostatic and potentiostatic methods s or using a.c.impedance spectroscopy. 9However, the results so obtained notably differ due to measurement techniques and conditions. 1 2.5 2.0 1..5 cmmercial powd:r 0.I 0.2 0.3 0.4 0.5 X in Li.SnO FIGURE 4 Third discharge curves of Li/Li+/LixSnO2 cells between voltage limits of 1.5-3.0V vs Li.The current density was 50/A/cm 2 for an electrode weight of 35 rag.
We propose here an original and easy-to-carry out method to determine D; based on the evolution of the X-ray diffractograms (XRD) of the polycrystalline electrodes upon Li + insertion as shown below.
When the lithium ions are electrochemically inserted into the electrodes, the X-ray diffraction peaks sometimes disappear in favor of new peaks; that occurs, for instance, when new phases are formed.However, the diffraction peaks can also be broadened.This later event will happen for the SnO 2 electrodes because the surface packing density of the atoms in the lattice planes is too large and, therefore prevent efficient lithium insertion (that will be shown below).
D is related to the (average) distance, L, covered by the Li + ions between two positions, within the (average) time, t, by the relation of Einstein's diffusion: 11 L 2= 2D-t. (2) When the later event occurs, L can be related to the decrease in the size of the crystallite.This decrease should, in turn, be evaluated from the enlargement A dhk 2L, (Fig. 6), of the X-ray diffraction peaks due to the lithium insertion.

1"1"
Li + FIGURE 6 Decrease of the size of crystallite upon lithium insertion. (1)well crystallized remaining region of composition SnO_; (2) amorphous region of composition LixSnO2, formed during lithium insertion.Adhk --d-d where d and di designate respectively the crystallite size of pure and inserted sample.
Adhkl 2L can be expressed in the equation:

Adhk
(3) COS 0 f12 1 deduced from relation (1)./31 and f12 are the corrected widths of the main diffraction peaks at half height, of pure and lithium-inserted samples, respectively.
Finally, from equations (2) and (3)one gets: Let us recall that our approach is no longer valid for electrodes whose inside- crystallite structure is well adapted for lithium intercalation (such as Li NiO2, Li -xCoO2 )- b-Application to LiSnO2 electrodes As expected only a textural change, namely a reduction of the size of the crystallites, is observed upon Li + insertion, accounting for a significant broadening of the X-ray diffraction peaks (Fig. 7).
The chemical diffusion coefficients of lithium ions in the electrodes have been estimated for different x values from equations (3) and (4).One gets for LiSnO2 electrodes arising from commercial powder: D(x=0.15 7(+0.5)x 10-10 cm2/sec for the (110) lattice plane.
On the other hand, equations (3) and (4) cannot be used to calculate D for LiSnO2 electrodes issued from the fine-grain powder: indeed, much lower (--ten thousand times lower)values are obtained in contradiction with the normally  (a) commercial powder; (b) fine-grained powder.
expected higher diffusion of lithium ions in such electrodes.In fact, this result gives an indirect but strong evidence that the lithium insertion in the latter electrodes occurs mainly within the inter-crystallite (grain boundary) region as we have already reported for other materials. 6 4. CONCLUSIONS In this paper, we have proposed an original method of preparing fine-grained tin oxide powder, free from water and hydroxyl groups.Related to the fine-grained texture of the samples, we have shown that they are able to sustain long-term electrochemical cyelability.From the unexpected low value of the chemical diffu- sion coefficient of lithium ions in the electrode, we have concluded that the lithium insertion occurs mainly within the inter-crystallite region.

FIGURE
FIGURESchematic diagram of the steps involved in obtaining fine-grained SnO 2 powder.

FIGURE 7
FIGURE 7Broadening of the X-ray diffraction peaks upon Li + insertion in the LixSnO2 electrodes.