Snx[BPO4]1−x composites as negative electrodes for lithium ion cells: Comparison with amorphous SnB0.6P0.4O2.9 and effect of composition

doi:10.1016/j.jssc.2009.10.015

Journal of Solid State Chemistry

Volume 183, Issue 1, January 2010, Pages 65-75

https://doi.org/10.1016/j.jssc.2009.10.015 Get rights and content

Abstract

A comparative study of two Sn-based composite materials as negative electrode for Li-ion accumulators is presented. The former SnB_0.6P_0.4O_2.9 obtained by in-situ dispersion of SnO in an oxide matrix is shown to be an amorphous tin composite oxide (ATCO). The latter Sn_0.72[BPO₄]_0.28 obtained by ex-situ dispersion of Sn in a borophosphate matrix consists of Sn particles embedded in a crystalline BPO₄ matrix. The electrochemical responses of ATCO and Sn_0.72[BPO₄]_0.28 composite in galvanostatic mode show reversible capacities of about 450 and 530 mAh g⁻¹, respectively, with different irreversible capacities (60% and 29%). Analysis of these composite materials by ¹¹⁹Sn Mössbauer spectroscopy in transmission (TMS) and emission (CEMS) modes confirms that ATCO is an amorphous Sn^II composite oxide and shows that in the case of Sn_0.72[BPO₄]_0.28, the surface of the tin clusters is mainly formed by Sn^II in an amorphous interface whereas the bulk of the clusters is mainly formed by Sn⁰. The determination of the recoilless free fractions f (Lamb–Mössbauer factors) leads to the effective fraction of both Sn⁰ and Sn^II species in such composites. The influence of chemical composition and especially of the surface-to-bulk tin species ratio on the electrochemical behaviour has been analysed for several Sn_x[BPO₄]₁₋_x composite materials (0.17<x<0.91). The cell using the compound Sn_0.72[BPO₄]_0.28 as active material exhibits interesting electrochemical performances (reversible capacity of 500 mAh g⁻¹ at C/5 rate).

Abstract

Galvanostatic discharge-charge curves of ATCO and Sn_0.72[BPO₄]_0.28 composite materials at C/5 rate, respectively.

Introduction

Lithium ion batteries have become considerable interest in various fields of applications ranging from electronic devices, to electric and hybrid vehicles or space applications.

Since the first Li-ion battery commercialized by Sony, using Li_xC₆ as negative electrode, an intensive research has been undertaken to search for new negative materials with the aim to increase their energy density.

Many metallic anode materials, pure metals or alloys, have been studied showing higher capacities than graphite (372 mAh g⁻¹). Among them, metallic Sn is known to have one of the highest theoretical capacities when used as a lithium storage electrode, with a value of 991 mAh g⁻¹ corresponding to a fully lithiated composition of Li_4.4Sn. However, the use of tin or its alloys in rechargeable cells has been hindered by drastic volume changes occurring during lithium alloying–dealloying reactions, which cause cracking of the electrode material and finally loss of electrical contact between the single particles. As a consequence the capacity fades rapidly. To overcome this disadvantage, several methods have been proposed. One consists in the use of superfine materials, such as Sn/SnSb_x, Sn/SnAg_x (200–400 nm) [1], [2], nano-Sn (∼10 nm) [3], [4] and nano-SnSb (20–100 nm) [5]. Intermetallic compounds have been widely investigated, for example Cu₆Sn₅ [6], [7], CoSn₂ [8] and Mg–Sn [9], [10]. A feature article recently published summarizes the state-of-the-art in the field of pure metal as negative materials [11].

The announcement of the Stalion battery by Fuji Photo Film Co. [12] using a new class of anode material based on an amorphous tin composite oxide (ATCO) of the form SnM_xO_y (where M is a group III or V glass forming element such as B^III, P^V and Al^III) has stimulated intense discussion on the use of Sn-based composites as negative electrode for lithium ion batteries [13], [14], [15]. In such amorphous materials, showing a high reversible capacity (∼600 mAh g⁻¹), the Sn^II atoms are dispersed into an oxide glass, typically a borophosphate amorphous material. For the composition Sn^IIB_0.6P_0.4O_2.9 the lithium insertion mechanism has been studied in detail by means of X-ray diffraction (XRD), X-ray absorption (XAS), in situ and ex situ ¹¹⁹Sn transmission Mössbauer spectroscopy (TMS) [16], [17]. This study revealed a complex reduction mechanism in two steps. During the first step the main reaction corresponds to a Sn^II→Sn⁰ reduction leading to the in-situ formation of a Sn/amorphous oxide composite. The second step corresponds to a Li–Sn alloying process with the formation of Li–Sn bonds. The reversible part of this second step can be explained from the formation of small particles of Li–Sn alloys in interaction with the oxygen atoms in the glass matrix. Although the vitreous support presents an interesting dispersal effect, the main drawback of these ATCO materials is the large irreversible capacity in the first discharge (400 mAh g⁻¹) due to the reduction of Sn^II to Sn⁰.

More recently ex-situ Sn particles dispersion into an electrochemically inactive matrix has been proposed as new strategy [18]. This procedure leads to composite materials like Sn/BPO₄ or Sn/CaSiO₃ in which Sn particles are embedded in a crystallized BPO₄ or CaSiO₃ matrixes. During the synthesis of such composites via a ceramic route, Sn and oxide particles react to form an interface between them that improves the cohesion of the composite. In the case of Sn:BPO₄ composite, as evidenced by ¹¹⁹Sn conversion electron Mössbauer spectroscopy (CEMS) which is sensitive to tin atoms within layer of few hundreds of nm, this interface has been identified as a Sn^II amorphous borophosphate [19], [20]. Such composites show a high reversible capacity (550 mAh g⁻¹) close to that obtained for the best ATCO materials (600 mAh g⁻¹) and a lower irreversible capacity of 200 mAh g⁻¹. The same concept of in-situ formation of active atoms dispersed in inert phases is used for materials with lower irreversible charge losses, the best results so far having been obtained with a composite of active SnFe₂ and inactive SnFe₃C [21], [22].

In the first part of the present work, we report a comparative study between ATCO and a new Sn/BPO₄ composite material obtained by ex-situ dispersion of Sn into an electrochemically inactive BPO₄ matrix. The recoilless free fractions f (Lamb–Mössbauer factors) of the different tin species have been evaluated from ¹¹⁹Sn TMS spectra recorded at different temperatures, in order to determine the effective fractions of each species.

In the second part, the influence of the Sn/BPO₄ ratio on the electrochemical performances of the electrode material has been studied.

Section snippets

Synthesis

The Sn_0.72[BPO₄]_0.28 composite was prepared by a two-steps solid state reaction method. In a first step, BPO₄ was synthesized from equimolar amounts of NH₄H₂PO₄ (Acros Organics) and H₃BO₃ (Acros Organics) as phosphoric and boric precursors. The starting materials were ground in an agate mortar and placed in an alumina crucible. De-ionized water was added in order to form a homogeneous white paste that was heated in a furnace to 380 °C for about 13 h.

In the second step, BPO₄ was mixed with Sn

Results and discussion

The XRD patterns of both the SnB_0.6P_0.4O_2.9 ATCO and the Sn_0.72[BPO₄]_0.28 composite material are shown in Fig. 1.

As expected, the XRD data of SnB_0.6P_0.4O_2.9 show the absence of long-range order in the pristine glass sample that confirms its amorphous structure. The diffraction lines observed for the Sn_0.72[BPO₄]_0.28 composite are those of pure β-Sn and BPO₄ and have been indexed according to two tetragonal units. The cell parameters have been determined from a least-squares fit using an

Influence of the Sn/BPO₄ ratio on the electrochemical performances

The XRD patterns (Fig. 8) of the different Sn_x[BPO₄]₁₋_x compositions show a progressive inversion of the intensities of the borophosphate and Sn lines from x=0.17 to 0.91. For the last composition (x=0.91), the small quantity of borophosphate matrix and the important amount of dispersed tin do not make it possible to see the amorphous part characteristic of the interface formed between the two species observed for other composition between 20 and 34 2θ(°).

The Mössbauer spectra recorded at room

Conclusion

The comparative Mössbauer study of amorphous SnB_0.6P_0.4O_2.9 and Sn_0.72[BPO₄]_0.28 composite has shown the presence of similar sites in the two materials. The SnB_0.6P_0.4O_2.9, obtained by in-situ dispersion of SnO into an oxide matrix, exhibits the presence of two Sn^II sites with specific Mössbauer parameters of δ=3.02(2) mm s⁻¹ and Δ=2.06(3) mm s⁻¹ for Sn₁^II and δ=3.29(2) mm s⁻¹ and Δ=1.70(3) mm s⁻¹ for Sn₂^II. The Sn_0.72[BPO₄]_0.28, obtained by ex-situ dispersion of commercial β-Sn into a BPO₄ matrix,

Acknowledgments

This authors express their sincere gratitude to Centre National d’Etudes Spatiales CNES-France (Contract no 60255/00), to CNES-Région Languedoc Roussillon (Contract no 07-011380) to SAFT-France (Contract no 016113) and to the Agence Nationale de la Recherche (Contract no ANR-07-Stock-E-03-02) for financial supports.

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The diffraction patterns observed for all the composites are those of pure β-Sn and BPO4. Indexation and cell parameters determination were performed elsewhere [7]. In the enlargements reported in Fig. 1, it is possible to discern the influence of the reaction time on the resulting composite obtained with a reaction time of 7 h.
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¹: Present address: Laboratoire Matériaux et molécules en milieu amazonien (L3MA/UMR ECOFOG/IESG), 2091 Route de Baduel—BP 792, 9337 Cayenne.

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Snx[BPO4]1−x composites as negative electrodes for lithium ion cells: Comparison with amorphous SnB0.6P0.4O2.9 and effect of composition

Abstract

Abstract

Introduction

Section snippets

Synthesis

Results and discussion

Influence of the Sn/BPO4 ratio on the electrochemical performances

Conclusion

Acknowledgments

Solid State Ionics

J. Power Sources

J. Power Sources

J. Power Sources

Solid State Ionics

Electrochem. Acta

Solid State Sci.

J. Power Sources

J. Power Sources

Nucl. Inst. Meth.

J. Solid State Chem.

J. Inorg. Nucl. Chem.

J. Power Sources

Chem. Mat.

J. Mater. Chem.

Electrochem. Solid-State Lett.

Chem. Mat.

Chem. Mater.

J. Mater. Chem.

Sn_x[BPO₄]₁₋_x composites as negative electrodes for lithium ion cells: Comparison with amorphous SnB_0.6P_0.4O_2.9 and effect of composition

Influence of the Sn/BPO₄ ratio on the electrochemical performances