A NOVEL GOLD-BASED POROUS MATERIAL FOR A LITHIUM BATTERY

Abstract

The present invention relates to a novel gold-based porous material, the use of said gold-based porous material as a precursor of a negative active material, the preparation process of said gold-based porous material, a novel gold-based porous material comprising lithium, the use of said gold-based porous material comprising lithium as a negative electrode material, a lithium-ion battery comprising said gold-based porous material comprising lithium, and a process for the preparation of said gold-based porous material comprising lithium.

Claims

1. A gold-based porous material, wherein said gold-based porous material comprises a gold porous substrate and a coating comprising a tin gold alloy.

2. The gold-based porous material according to claim 1, wherein it has a cellular structure or a honeycomb-like structure.

3. The gold-based porous material according to claim 1, wherein it is in the form of a film, preferably having a thickness ranging from 10 .Math.m to 100 .Math.m.

4. The gold-based porous material according to claim 1, wherein it is in the form of a macroporous film with nanostructured pore walls.

5. The gold-based porous material according to claim 1, wherein the coating is composed of a tin gold alloy responding to the chemical formula SnAu.

6. The gold-based porous material according to claim 1, wherein it is a supported gold-based porous material comprising said gold-based porous material and a support, said support being coated with said gold-based porous material.

7. A precursor of a negative electrode material, comprising: a gold-based porous material as defined in claim 1 .

8. A process for the preparation of a gold-based porous material as defined in any one of the preceding claims, wherein said process comprises at least the following steps: i) providing a gold porous substrate by a dynamic hydrogen bubble template method, and ii) electrodepositing tin from a solution comprising at least one tin precursor.

9. The process according to claim 8, wherein step i) is performed with constant current.

10. The process according to claim 8, wherein step i) is performed by electrodepositing gold .

11. The process according to claim 8, wherein step ii) is performed by electrodepositing tin from an acidic solution comprising at least one tin precursor, with constant potential.

12. A gold-based porous material comprising lithium, wherein said gold-based porous material comprising lithium includes a gold porous substrate and a coating comprising a lithium tin gold alloy.

13. The gold-based porous material comprising lithium according to claim 12, wherein the coating is composed of a lithium tin gold alloy responding to formula Li.sub.2SnAu.

14. The gold-based porous material comprising lithium according to claim 12, wherein it is a supported gold-based porous material comprising lithium including said gold-based porous material comprising lithium and a support, said support being coated with said gold-based porous material comprising lithium.

15. A negative electrode comprising: a gold-based porous material comprising lithium as defined in claim 12 material.

16. A lithium-ion battery comprising: a positive electrode material, a negative electrode material or a negative electrode material precursor, said negative electrode material or negative electrode material precursor, and said positive electrode material being separated from one another by an electrolyte, wherein the negative electrode material precursor is a gold-based porous material as defined in claim 1, and the negative electrode material is a gold-based porous material having lithium, wherein said gold-based porous material comprising lithium includes a gold porous substrate and a coating comprising a lithium tin gold alloy .

17. A process for the preparation of a gold-based porous material having lithium, wherein said gold-based porous material comprising lithium includes a gold porous substrate and a coating comprising a lithium tin gold alloy, wherein said process comprises at least one step of submitting to a charge a cell comprising: lithium metal or lithium alloy metal as a counter electrode, or a positive electrode material, and a gold-based porous material as defined in claim 1, said counter electrode or positive electrode material, and said gold-based porous material being separated from one another by an electrolyte.

18. The process according to claim 10 wherein step i) is performed by electrodepositing gold on a support, from a solution comprising at least one gold precursor and an acid compound.

Description

PROCESS FOR THE PREPARATION OF THE (SUPPORTED) GOLD-BASED POROUS MATERIAL

[0056] A third object of the present invention is a process for the preparation of a gold-based porous material as defined in the first object of the present invention, wherein said process comprises at least the following steps: [0057] i) providing a gold porous substrate by a dynamic hydrogen bubble template (DHBT) method, and [0058] ii) electrodepositing tin from a solution comprising at least one tin precursor.

[0059] The process of the invention is simple, fast, clean, and leads easily to a highly porous gold-based material. This process can also be easily transferable to pilot production line with microelectronic facilities.

The First Step I

[0060] Thanks to the dynamic hydrogen bubble template method, it is possible to obtain a highly porous gold substrate which is already defined in the first object of the present invention.

[0061] More preferably, the gold porous substrate is in the form of a macroporous film with nanostructured pore walls

[0062] The DHBT method is a known electrodeposition at high overpotentials, where metal deposition is accompanied by the evolution of hydrogen (H.sub.2) bubbles.

[0063] In one preferred embodiment, step i) is performed with constant current (i.e. galvanostatic deposition). Constant current is preferred by comparison with constant potential for facilitating upscaling.

[0064] Step i) can involve the use of a solution comprising at least one gold precursor, which can be selected from the group consisting of HAuCl.sub.4.3H.sub.2O, AuCl.sub.3, K(AuCl.sub.4), AuBr.sub.3, and mixture thereof.

[0065] The gold precursor preferably comprises gold ions at the oxidation state +III (i.e. Au.sup.3+).

[0066] The concentration of the gold ions or Au.sup.3+ in said solution preferably ranges from 1 × 10.sup.-3 mol/l to 10 × 10.sup.-3 mol/l approximately.

[0067] The solution can further comprise an acid compound, which can be selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof.

[0068] Alternatively, the solution can comprise a salt such as NH.sub.4Cl, which is able to generate H.sub.2 as follows: 2NH.sub.4 + + 2e.sup.- .fwdarw.H.sub.2 + 2NH.sub.3.

[0069] The concentration of the acid compound or the salt in said solution preferably ranges from 1 mol/l to 5 mol/l approximately.

[0070] Step i) can be performed at room temperature (i.e. 18-25° C.).

[0071] More particularly, step i) is performed by electrodepositing gold, preferably on a support, from a solution as defined above.

[0072] The deposition time during step i) preferably ranges from 1 min to 30 min approximately, and more preferably from 10 min to 20 min approximately.

[0073] The support can be as defined in the first object of the present invention.

[0074] When a support is used in step i), a supported gold porous substrate is obtained.

[0075] Advantageously, step i) is carried out by immersing the support in said solution, and applying a given potential vs a reference electrode, or a given current with respect to the geometrical surface area of the support.

[0076] Step i) can be performed under a constant current per surface ranging from about 1 A/cm.sup.2 to 5 A/cm.sup.2, where the surface is the geometrical surface area to which current is applied for deposition.

[0077] Electrodeposition is generally carried out with a 3-electrode configuration, namely a platinium counter electrode, a standard calomel electrode as the reference, and a support as defined in the invention such as Si/SiO.sub.2/Ti/Au as the working electrode.

The Second Step II

[0078] During step ii), electrodeposition of tin is achieved without affecting the morphology of the gold porous substrate. Surprisingly, formation of a SnAu alloy occurs during the electrodeposition step ii).

[0079] In one preferred embodiment, step ii) is performed with constant potential. This is preferred for controlled reduction of Sn.sup.2+ ions on Au surface.

[0080] Step ii) can be performed in an acidic medium, preferably at a pH of less than 2, and preferably ranging from 1) to 1.2.

[0081] The solution comprising at least one tin precursor is preferably an acidic solution.

[0082] The tin precursor preferably comprises tin ions at the oxidation state +II (i.e. Snz.sup.2+).

[0083] The tin precursor can be selected from the group consisting of SnCl.sub.2, SnSO.sub.4, Tin (II) oxalate, Tin (II) citrate, Tin (II) ethylhexanoate, and mixture thereof.

[0084] The concentration of the tin ions or Sn.sup.2+ in said solution preferably ranges from 1 × 10.sup.-3 mol/l to 10 × 10.sup.-3 mol/l approximately.

[0085] To provide an acidic solution, the solution can further comprise an acid compound, which can be selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid, and mixture thereof.

[0086] The concentration of the acid compound in said solution preferably ranges from 0.01 mol/l to 0.1 mol/l approximately.

[0087] The deposition time during step ii) preferably ranges from 1 min to 10 min approximately, and more preferably from 3 min to 6 min approximately.

[0088] Step ii) can be performed at room temperature (i.e. 18-25° C.).

[0089] Step ii) can be carried out by immersing the gold porous substrate in said solution, and applying a given potential vs a reference electrode, or a given current with respect to the surface of the gold porous substrate.

[0090] Step ii) is advantageously performed under a constant potential ranging from about -1.5 to -2.5 vs SCE (saturated calomel electrode as the reference electrode).

[0091] Electrodeposition is generally carried out with a 3-electrode configuration, namely a platinium counter electrode, a standard calomel electrode as the reference, and a working electrode made of the gold-porous substrate obtained in step i) or the supported gold-porous substrate obtained in step i) such as a Si/SiO.sub.2/Ti/Au/Au DHBT electrode.

[0092] When a support is used in step i), a supported gold-based porous material as defined in the first object of the present invention is obtained in step ii).

[0093] During step ii), deposition of tin on gold porous substrate is homogeneous.

Other Steps

[0094] The process can further comprise between steps i) and ii), a washing step i'), so as to wash the obtained gold porous substrate.

[0095] Step i') can be performed by using water or dionized water, preferably several times.

[0096] The process can further comprise after step i'), a drying step i"), so as to dry the obtained gold porous substrate.

[0097] Step i") can be performed by vacuum drying.

[0098] The process can further comprise after step ii), a drying step ii'), so as to dry the obtained gold-based porous material.

[0099] Step ii) can be performed by vacuum drying.

[0100] The support optionally used in step i) can be previously prepared by conventional microfabrication methods.

[0101] For example, a Ti/Au thin film can be deposited by evaporation on an oxidized silicon substrate so as to obtain a support comprising a silicon substrate sequentially coated with a layer of SiOz (oxidized silicon substrate), a layer of Ti, and a layer of Au.

[0102] The (supported) gold-based porous material comprising lithium

[0103] A fourth object of the present invention is a gold-based porous material comprising lithium, preferably in the form of a film, wherein said gold-based porous material comprising lithium includes a gold porous substrate and a coating comprising a lithium tin gold alloy.

[0104] The gold porous substrate is as defined in the present invention.

[0105] More particularly, the porous structure of the gold-based porous material or the gold porous substrate as defined in the present invention is maintained after incorporating lithium.

[0106] The gold-based porous material comprising lithium provides rate capabilitity with flat discharge profile even at high rates, and robust cycling stability. Furthermore, the gold-based porous material comprising lithium can be used as an electrode material in miniaturized devices, where size and compactness are critical, and cost is mainly determined by the microfabrication process and not by the minute amount of active material involved.

[0107] Said coating comprising a lithium tin gold alloy is a layer deposited on said gold porous substrate. In other words, the thickness of the substrate is greater than the thickness of the coating.

[0108] The coating comprising a lithium tin gold alloy preferably represents an outermost layer of the gold-based porous material comprising lithium. In other words, there are no other layer(s) deposited on said coating comprising a lithium tin gold alloy.

[0109] The coating comprising a lithium tin gold alloy preferably covers at least partly, and more preferably entirely, the gold porous substrate. In other terms, the gold porous substrate is preferably at least partly, and more preferably entirely, covered with the coating comprising a lithium tin gold alloy. In other terms, the porous surface (i.e. the surface of the pores) of the gold porous substrate is preferably at least partly, and more preferably entirely, covered by the coating comprising a lithium tin gold alloy.

[0110] The coating is preferably composed of a lithium tin gold alloy.

[0111] The gold-based porous material comprising lithium has preferably a cellular structure or a honeycomb-like structure.

[0112] Said cellular or honeycomb-like structure is advantageously a hierarchical structure, that-is-to-say a structure exhibiting a multiple porosity. More particularly, the gold-based porous material comprising lithium of the invention comprises a hierarchical porous network composed of interconnected pores having several sizes, such as macropores, mesopores, and micropores.

[0113] The gold-based porous material comprising lithium of the invention can have at least macropores with a diameter ranging from about 0.1 .Math.m to 50 .Math.m, and preferably from about 20 to 40 .Math.m.

[0114] In one preferred embodiment, the gold-based porous material comprising lithium has an apparent porosity p of at least 80% approximately, and more preferably ranging from 85 to 90% approximately.

[0115] The gold-based porous material comprising lithium of the invention can have pores with an internal wall structure comprising multi-branched dendrites and nodules.

[0116] In one preferred embodiment, the gold-based porous material is in the form of a film.

[0117] More preferably, the gold-based porous material comprising lithium is in the form of a macroporous film with nanostructured pore walls.

[0118] The gold-based porous material comprising lithium, and preferably the film, can have a thickness ranging from about 10 .Math.m to 100 .Math.m.

[0119] The lithium tin gold alloy can respond to the following formula : Li.sub.2+.sub.xSnAu, in which x is such that 0 ≤ x < 2.

[0120] In a preferred embodiment, the coating comprising a lithium tin gold alloy is in direct physical contact with the gold porous substrate.

[0121] The gold-based porous material comprising lithium is preferably obtained by submitting the gold-based porous material as defined in the first object of the present invention to at least one charge in a lithium battery.

[0122] Indeed, when submitting the gold-based porous material as defined in the first object of the present invention to at least one charge in a lithium battery, lithium is inserted into at least one part of the tin gold alloy, and preferably the whole tin gold alloy, so as to form a lithium tin gold alloy.

[0123] More particularly, the porous structure of the gold-based porous material as defined in the present invention is maintained after cycling.

[0124] Preferably, the lithium tin gold alloy responds to the following formula : Li.sub.2SnAu (i.e. x = 0). This lithium tin gold alloy is very stable and can be used as a negative electrode material.

[0125] In one preferred embodiment, the gold-based porous material comprising lithium is a supported gold-based porous material comprising lithium.

[0126] The supported gold-based porous material comprising lithium may include the gold-based porous material comprising lithium as defined in the fourth object of the present invention and a support, said support being coated with said gold-based porous material comprising lithium.

[0127] The support can be as defined in the first object of the present invention.

[0128] A fifth object of the present invention is the use of a gold-based porous material comprising lithium, preferably of a supported gold-based porous material comprising lithium, as defined in the fourth object of the present invention, as a negative electrode material.

[0129] The gold-based porous material comprising lithium displays a good cycling stability, whatever the mass loadings of said gold-based porous material comprising lithium.

[0130] For example, long-term cycling involves the lithiation of Li.sub.2SnAu into further Li.sub.xSnAu lithiated phases, with an × value varying between 2 to ∼4.

[0131] Surprisingly, unlike Li.sub.xSn, the variation in volume during lithiationdelithiation process of Li.sub.2AuSn is very low: first lithiation of Li.sub.2AuSn to Li.sub.3AuSn has minor volume change of ∼26% with following steps having lower associated volume expansion while transitioning from individual lithiated phases (total calculated volume expansion of 44.4%). Moreover, the void space of the porous structure can accommodate this limited volume expansion of the anode active material, making porous Li.sub.2SnAu a very promising anode for long-term cycling microbatteries.

[0132] A sixth object of the present invention is a lithium-ion battery comprising: [0133] a positive electrode material, [0134] a negative electrode material or a negative electrode material precursor, [0135] said negative electrode material or negative electrode material precursor, and said positive electrode material being separated from one another by an electrolyte, [0136] wherein the negative electrode material precursor is a gold-based porous material as defined in the first object of the present invention and the negative electrode material is a gold-based porous material comprising lithium as defined in the fourth object of the present invention .

[0137] The positive electrode material can comprise a positive electrode active material, for example selected from the group consisting of lithiated metal oxides (LiCoOz, LiNiOz, LiMn.sub.2O.sub.4 etc); lithium phosphates (LiFePO.sub.4, Li.sub.3V.sub.2(PO.sub.4).sub.3, LiCoPO.sub.4, LiMnPO.sub.4, LiNiPO.sub.4; and active materials of LiMOz lamellar oxide type with M representing a mixture of at least two metals chosen from Al, Ni, Mn and Co, such as LiNi.sub.⅓Mn.sub.⅓Co.sub.⅓O.sub.2 (NMC family), LiNi.sub.0.8CO.sub.0.15Al.sub.0..sub.05O.sub.2 (NCA family) or LiNi.sub.0..sub.5Mn.sub.0..sub.5O.sub.2.

[0138] The positive electrode material can further comprise at least one polymeric binder, and optionally a material conferring electronic conduction.

[0139] The electrolyte can be a solid electrolyte, a polymer gelled electrolyte, or a liquid electrolyte.

[0140] The electrolyte is preferably a non-aqueous electrolyte.

[0141] The electrolyte preferably comprises at least one lithium salt.

[0142] The lithium salt can be selected from LiPF.sub.6, LiClO.sub.4, LiBF.sub.4, LiNO.sub.3, LiN(SO.sub.2CF.sub.3), LiAsF.sub.6, LiCF.sub.3SO.sub.3, Li(CF.sub.3SO.sub.2).sub.3C, LiB(C.sub.2O.sub.4).sub.2, LiBF.sub.2(C.sub.2O.sub.4), LiN(C.sub.4F.sub.9SO.sub.2)(CF.sub.3SO.sub.2), and mixtures thereof.

[0143] When the electrolyte is a liquid electrolyte, it can further comprise an aprotic solvent.

[0144] The aprotic solvent can be selected from ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl propyl carbonate, dipropyl carbonate, ethyl methyl carbonate, vinylene carbonate, 1,3-dimethoxyethane, 1,3-diethoxyethane, 1,3-dioxolane, tetrahydrofurane, and mixtures thereof.

[0145] When the electrolyte is a liquid electrolyte, the battery can further comprise a separator impregnating said liquid electrolyte.

[0146] The separator acts as electrical insulator and allows the transport of ions.

[0147] It is noted that in the battery of the present invention, the negative electrode material or the negative electrode material precursor does not require to comprise a polymeric binder and/or a material conferring electronic conduction such as a carbon-based compound.

[0148] A seventh object of the present invention is a process for the preparation of a gold-based porous material comprising lithium, preferably of a supported gold-based porous material comprising lithium, as defined in the fourth object of the present invention, wherein said process comprises at least a step of submitting to a charge a cell comprising: [0149] lithium metal or lithium alloy metal as a counter electrode or a positive electrode material as defined in the invention, and [0150] a gold-based porous material, preferably a supported gold-based porous material, as defined in the first object of the present invention, [0151] said counter electrode or positive electrode material, and said gold-based porous material being separated from one another by an electrolyte.

[0152] The electrolyte is as defined in the sixth object of the present invention.

[0153] The present invention is illustrated in more detail in the examples below, but it is not limited to said examples.

Example 1: Preparation of a Gold-Based Porous Material

[0154] A support was first prepared so as to receive the gold porous substrate.

[0155] Accordingly, a Ti(100 nm)/Au(300 nm) thin film is deposited by evaporation on an oxidized silicon substrate which is electrochemically pretreated by cycling the potential at a scan rate of 100 mV s.sup.-1 between -0.3 and +1.7 V versus saturated calomel electrode (SCE) in 1 M H.sub.2SO.sub.4 until a stable voltammogram is obtained. Thus, the obtained support comprises a silicon substrate sequentially coated with a thin layer of SiO.sub.2, a thin layer of Ti, and a thin layer of Au.

[0156] A solution was prepared by mixing dionized water, HAuCl.sub.4.3H.sub.2O, and sulfuric acid, such that the concentration of gold ions [Au.sup.3+] in said solution is 2x10.sup.-3 mol/l, and the concentration of sulfuric acid is 3 mol/l.

[0157] Electrodeposition of gold on said support is performed by immersing the support in said solution, and applying at room temperature a constant current of 5 A/cm.sup.2, in a 3-electrode configuration, namely a Pt counter electrode, a standard calomel electrode as the reference, and the support previously prepared as the working electrode.

[0158] The deposition time during step i) is about 20 min.

[0159] A supported porous gold substrate in the form of a film is obtained, washed several times in de-ionized water, and vacuum drying for 30 min.

[0160] Then, a solution was prepared by mixing dionized water, SnCl.sub.2, and hydrochloric acid, such that the concentration of tin ions [Sn.sup.2+] in said solution is 7.5x10.sup.-3 mol/l, and the concentration of hydrochloric acid is 0.02 mol/l.

[0161] Electrodeposition of tin on the gold porous substrate is performed by immersing the supported porous gold substrate in said solution, and applying at room temperature a constant potential of -2 V vs SCE, in a 3-electrode configuration, namely a Pt counter electrode, a standard calomel electrode as the reference, and the supported porous gold substrate previously prepared as the working electrode.

[0162] The deposition time during step i) is about 10 min.

[0163] SnAu alloy is thus formed with mass loadings of ~2.9 mg cm.sup.-2 of SnAu per min.

[0164] The gold-based porous material obtained is further dried to remove any moisture content.

[0165] The supported gold-based porous material is in the form of a film having a thickness of 55-75 .Math.m.

[0166] FIG. 1 represents SEM images at different magnifications of the gold porous substrate (FIG. 1a) and of the gold-based porous material (FIG. 1b) obtained in example 1.

[0167] SEM images were obtained on a Hitachi S-4800 field emission electron microscope.

[0168] FIG. 2 represents Grazing XRD pattern of the gold porous substrate before and after Sn electrodeposition with several peaks matching SnAu alloy.

[0169] The crystallographic structures were analyzed by grazing incidence X-ray diffraction (XRD) measurements on a Bruker D8 Advanced X-ray diffractometer with Cu Kα radiation (1.54184 A°), operating at 40 kV and 40 mA.

Example 2: Preparation of a Gold-Based Porous Material Comprising Lithium

[0170] The gold-based porous material obtained in example 1 (0.8 cm.sup.2) was tested using a Li-ion half-cell (EL-Cell) assembled in a glove box with purified argon, with lithium foil as counter and reference electrode, and glass fiber separator soaked with 1 M LiPF.sub.6 in ethylene carbonate (EC) / diethyl carbonate (DEC) (1:1 volume ratio).

[0171] FIG. 3 represents in situ XRD pattern recorded after 1 cycle and 2000 cycles of charge/discharge [cell cycled between cycling potential - 0.02 and 1.50 V vs Li/Li.sup.+, 2 formation cycles at 0.1 C rate, followed by 2000 cycles at 3 C rate, where 1 C = 1 mA/cm.sup.2]. The configuration used is two electrode systems with Li metal foil as counter as well as reference electrode. After a certain number of cycles, only a stable Li.sub.2SnAu phase remains for lithiation.

[0172] FIG. 3 shows that the electrode after one cycle of lithiationdelithiation is constituted of SnAu, its lithiated counterpart i.e. Li.sub.2SnAu, and unalloyed Au underneath. After a large number of 2 000 cycles, the peaks corresponding to Li.sub.2SnAu and unalloyed Au are quite conspicuous, whereas SnAu peak has completely disappeared. This indicates that even during long cycling, the porous conducting 3D scaffold of gold porous substrate stays intact, providing the much-needed electronic conductivity. Also, as the SnAu signal almost completely disappears, it indicates that the following reaction : Li.sub.2SnAu + x Li .Math. Li.sub.2+.sub.xSnAu is providing the reversible capacity.

[0173] Presence of Li.sub.2SnAu after long cycling indicates efficient Li.sup.+ transport properties in the electrode. The electrochemical lithiation of conformally deposited SnAu alloy not only gives high reversible specific areal capacity, but also results in intermediates, which favor Li.sup.+ ion diffusion kinetics, displaying its superiority as a prospective anode material for Li-ion microbatteries.

Example 3: Electrochemical Characterizations of the Gold-Based Porous Material Comprising Lithium

[0174] The gold-based porous material obtained in example 1 (0.8 cm.sup.2) was tested using a Li-ion half-cell (EL-Cell) assembled in a glove box with purified argon, with lithium foil as counter and reference electrode, and glass fiber separator soaked with 1 M LiPF.sub.6 in ethylene carbonate (EC) / diethyl carbonate (DEC) (1:1 volume ratio).

[0175] FIG. 4 represents the potential (in V vs Li.sup.+/Li) as a function of the discharge capacity (in mAh/cm.sup.2) after 10 minutes of Sn deposition. The electrode exhibits high specific capacity at low C-rate (0.1 C), of 7.3 mAh/cm.sup.2, which is much higher than most reported microbattery anodes.

[0176] FIGS. 5a and 5b represent the specific capacity (in .Math.Ah/cm.sup.2) as a function of the number of cycles for different C rates. In case of FIG. 5b, the C-rate performance is performed with small current steps (0.25 C).

[0177] FIG. 5c represents the discharge capacity (in .Math.Ah/cm.sup.2 ) and the coulombic efficiency (in %) as a function of the number of cycles at 3 C rate.

[0178] FIGS. 5a and 5b show the robustness of the electrode towards rate fluctuations. The electrode displays extraordinary stability towards rate change with 156 .Math.Ah/cm.sup.2 at 4 C rate (FIG. 5a) and reversible capacity retention upon reversal to lower rates (816 .Math.Ah/cm.sup.2 at 1 C, FIG. 5b).

[0179] Extra long-term cyclability of the electrode is evaluated at 3 C rate (FIG. 5c). The electrode exhibits excellent stability testifying the reversible transition from Li.sub.2SnAu to Li.sub.4.2SnAu. More impressively, the electrode shows superior cyclability, sustaining 30 000 cycles with a limited capacity decay below 10 nAh/cm.sup.2 per cycle.