Phase-separated silicon-tin composite as negative electrode material for lithium-sulfur batteries

Abstract

Electrochemical lithium-sulfur secondary cell or battery comprising a negative electrode material; wherein the negative electrode material comprises a composite consisting of tin and silicon in which the tin and silicon exist in separate phases, the composite being further characterized in that a phase of nanoscale islands of silicon with a size of 30 to 70 nanometers is dispersed in a matrix phase of tin, wherein the tin phase is crystalline or amorphous and the silicon phase is amorphous; wherein the tin and silicon composite is capable of allowing the incorporation of lithium during charging of the cell or battery and of releasing lithium during discharging of the cell or battery.

Description

This application is a continuation-in-part of application number 13 / 234209 filed on September 16, 2011, which is now the US patent application with publication number US 2013 / 0071736A1. TECHNICAL AREA This invention relates to the production of negative electrode materials suitable for lithium-sulfur batteries. More specifically, this invention relates to the production and use of composite particles comprising nanoscale islands of amorphous silicon phases embedded in crystalline or amorphous tin phases, as negative electrode material for the cyclic insertion and extraction of lithium during the use of a lithium-sulfur battery. The combination of suitable atomic ratios of tin and silicon, as well as the sizes of the corresponding phases in such composites, enables the insertion of larger quantities of lithium over repeated electrochemical cycles with less damage to the particulate negative electrode material. BACKGROUND OF THE INVENTION Lithium-ion batteries are used as electrical storage systems to power electric and hybrid-electric vehicles. These batteries comprise a multitude of appropriately interconnected electrochemical cells arranged to provide a predetermined electric current at a defined electrical potential. In each such cell, lithium, as lithium ions, is transported from a negative electrode through a non-aqueous lithium-containing electrolyte solution to a lithium-ion-accepting positive electrode, while an electric current is delivered from the battery to an external consumer, such as an electric propulsion motor. To prevent short-circuiting physical contact between the electrodes, a suitable porous separator material, saturated with the electrolyte solution and permeable to the transport of lithium ions within the electrolyte, is used.Graphite was used as the negative electrode material and bonded in a thin electrode layer on a copper current collector. During cell charging, lithium is intercalated into the graphite (lithiation, forming LiC6, approximately 372 mAh / g) and extracted from the graphitic carbon during discharge (delithiation). A suitable particulate material for capturing and storing intercalated lithium during the discharge of each cell is used as the positive electrode material. Examples of such positive electrode materials include lithium cobalt oxide (LiCoO2), a lithium transition metal oxide spinel, such as lithium manganese oxide spinel (LiMnXOY), a lithium polyanion, such as nickel manganese cobalt oxide [Li(NiXMnYCoZ)O2], lithium iron phosphate (LiFePO4), or lithium fluorophosphate (Li2FePO4F), or a mixture of any of these materials. Suitable positive electrode materials are often bonded as thin layers to an aluminum current collector.The electrochemical potential of such lithium-ion cells is typically in the range of about 2 to 4.5 volts. The use of lithium-ion batteries to power electric motors in motor vehicles has led to a need for batteries with higher gravimetric and / or volumetric capacity. While graphitic carbon is a durable and useful lithium-intercalating negative electrode material for lithium-ion cells, it has a relatively low capacity for such lithium intercalation. Other potential negative electrode materials, such as silicon (theoretical capacity, 3600 mAh / g for Li15Si4) and tin (theoretical capacity, 992 mAh / g for Li22Sn5), have significantly higher theoretical capacities for lithium intercalation than graphite. However, the volume change of up to 300% of silicon by volume during lithiation and delthiation processes leads to the fracturing of the active silicon material and / or the loss of electrical contact with the conductive additives or current collectors.Tin also exhibits the same problem of large volume expansion during lithation, which leads to rapid capacity degradation. Lithium-sulfur batteries, like lithium-ion batteries, are rechargeable. They are also notable for their high energy density. The low atomic weight of lithium and the moderate weight of sulfur allow lithium-sulfur batteries to be relatively lightweight. As with lithium-ion cells, the anode, or negative electrode, of a lithium-sulfur cell contains lithium. During discharge, lithium is dissolved from the anode surface into an electrolyte. This electrolyte (e.g., a molten or liquid alkali metal polysulfide salt) is transported through a porous separator to a cathode (positive electrode during cell discharge), which contains a polysulfide (e.g., S8). Upon reaching the cathode, lithium atoms progressively reduce the polysulfide to a lithium-sulfur compound (e.g., Li₂S₃). These chemical changes are reversed when the lithium-sulfur cell is recharged.The low weight and high energy density of lithium-sulfur cells make lithium-sulfur batteries good candidates for vehicle propulsion systems and other electrically energy-consuming devices. Such and similar lithium-ion batteries are described, for example, in US 2013 / 0071736A1. Furthermore, such and similar lithium-sulfur batteries are described, for example, in CN 102130359A. The mechanism underlying capacity loss due to electrode material fractures in lithium-ion and lithium-sulfur cells consists of the loss of electrical contact with the conductive material and the formation of new surfaces, which irreversibly consume active lithium to form new, solid electrolyte interfaces. Both problems reduce the effective cycle capacity of a battery. A more efficient method or material form for using silicon or tin in the negative electrodes of lithium-ion and lithium-sulfur cells remains a need. SUMMARY OF THE INVENTION According to embodiments of this invention, an improved negative electrode material is formed by combining silicon and tin as mixed but immiscible solid phases. The material is suitable for use in negative electrodes for lithium-sulfur batteries. A particulate composite of silicon and tin is formed in which the silicon and tin are phase-separated. Both phases are receptive to the insertion and extraction of lithium atoms. The composite is formed with a two-phase or multiphase structure, the characteristic length of which is in the nanometer to micrometer range. This structure is achieved by controlling the synthesis process for producing the mixed phases so that they are below the critical size for the formation of microcracks due to repeated insertion of lithium into the mixed phases.In a preferred embodiment, limited particle-shaped architectures are formed, wherein the silicon phases are separated in a matrix of a tin phase.Such a microstructure offers several advantages: (1) The tin phase is electrically conductive, allowing electrons to reach the island-like silicon phase particles; (2) the diffusion of lithium in tin is much faster than in silicon, thus reducing the concentration gradient of lithium ions through the larger silicon and tin composite particles, thereby reducing diffusion-induced stress and mitigating the fracture of larger composite particles; (3) if a crack forms in the composite material, the relatively soft tin matrix tends to absorb the elastic stress energy and prevent the microcracks from propagating; and (4) the immiscible characteristics of silicon and tin, as well as their phase separation, minimize electrochemical sintering and therefore prevent particle coalescence, which would otherwise lead to rapid mechanical degradation of the electrode material. Phase-separated particle composites can be produced, for example, from a homogeneous liquid mixture of elemental tin and silicon using fast-curing techniques (e.g., centrifugal melting). In another example, phase-separated composites can be formed by sputtering separate sources of tin and silicon and co-depositing phase-separated mixtures of tin and silicon at predetermined sections as an electrode film on a suitable surface, such as a copper current collector. Generally, it is preferred to produce a phase-separated composite of approximately forty to sixty atomic percent silicon and the remainder tin as a negative electrode material for lithium-sulfur cells. According to embodiments of the invention, the mechanical degradation that occurs when pure tin or silicon is used for lithium storage is mitigated by adjusting the Si / Sn ratio of the tin-silicon composite. We also show that, at suitable Si / Sn ratios, phase separation occurs, with amorphous silicon phase clusters embedded in a generally continuous phase of crystalline or amorphous tin. The resulting negative electrode materials exhibit significant improvements in charge storage capacity along with excellent cycle stability. Therefore, according to embodiments of the invention, phase-separated tin-silicon composites are formed as relatively thin layers on a suitable current collector for use as negative electrode material in the cells of a lithium-sulfur battery. The use of this phase-separated combination of tin and silicon increases the lithium storage capacity of each cell, thus improving the volumetric and / or gravimetric capacity of lithium-sulfur batteries. When the phase-separated tin-silicon composite is to be used as negative electrode material in the cells of a lithium-sulfur battery, it may be preferable to introduce the required amount of lithium into the negative electrode material before the negative electrode is installed in a battery. Other features and advantages of the invention will become apparent from a detailed description of embodiments of the invention, which follows in the description. BRIEF DESCRIPTION OF THE FIGURES Fig. 1 is an enlarged schematic representation of some electrochemical cells of an exemplary lithium-ion battery arrangement. Each cell comprises a thin rectangular negative electrode, which may contain the phase-separated tin and silicon composite material of this invention, a similarly shaped positive electrode, and a separator inserted between the electrodes. Fig. 2 is a further enlarged exploded cross-sectional view of one of the electrochemical cells shown in Fig. 1. Fig. 2 also shows the metallic current collectors that carry the respective electrode materials. The cell elements are shown separately for illustrative purposes but are actually in compressed side-to-side contact, and the electrode materials are formed on or bonded to their respective current collectors.Figures 3A-3C are scanning electron microscope (SEM) images showing the surface morphologies of phase-separated SinSn1-n compositions formed by sputtering tin and silicon from separated targets onto a copper strip substrate. The composition in atomic percent ratios is Si40Sn60 in Figure 3A, Si48Sn52 in Figure 3B, and Si68Sn32 in Figure 3C. Figures 4A and 4B are wide-angle circular dark-field (HAADF) scanning transmission electron microscope (STEM) images showing the phase separation of silicon and tin in sputtered films. Silicon is the darker phase, and tin is the lighter phase. Figure 4A shows a higher silicon content (approximately 60 atomic percent silicon and the remainder tin), and Figure 4B shows a lower silicon content (approximately 30 atomic percent silicon). Fig. 5A and Fig.Figure 5B shows galvanostatic charging curves (potential, volts, Li / Li+ versus charge capacity mAh / cm²) at a rate of C / 10 for five different tin / silicon compositions. Figure 5A is at the second cycle and Figure 5B is at the twentieth cycle. Figure 6 is a graph of charge capacity, mAh / g, versus the number of cycles for five different tin / silicon compositions. DESCRIPTION OF PREFERRED EXECUTION FORMS An exemplary and generalized representation of a lithium-ion battery 10 is shown in Fig. 1. The lithium-ion battery 10 shown here has several thin, rectangular electrochemical battery cells 12, each of which is clamped by metallic current collectors. The electrochemical battery cells 12 are stacked side by side in a modular configuration and, in this example, connected in parallel. A lithium-ion battery 10 can be formed from many identical electrochemical cells connected in series or in parallel to create a lithium-ion battery that has the voltage and current required for a particular application. It is understood that the lithium-ion battery 10 shown here is only a schematic representation.Figure 1 is shown to illustrate the relative position and physical interactions of the various components that constitute the electrochemical battery cells 12 (i.e., the electrodes and the separator). It is not intended to provide information about the relative sizes of the electrochemical battery cell components, to define the number of electrochemical battery cells 12 in the lithium-ion battery 10, or to limit the large number of structural configurations that the lithium-ion battery 10 can adopt. The electrochemical cell 12 (one shown), contained in the lithium-ion battery 10, comprises a negative electrode 14, a positive electrode 16, and the separator 18, which is located between the two opposing electrodes 14 and 16. Each of the negative electrode 14, the positive electrode 16, and the separator 18 is wetted with a liquid electrolyte solution, which enables the transport of lithium ions between the electrodes 14 and 16. A metallic negative electrode current collector 20 (typically copper), comprising a negatively polarized strip 22, is arranged between the back-to-back negative electrodes 14 of adjacent electrochemical cells 12. Similarly, a metallic positive-side current collector 24 (typically aluminum), comprising a positively polarized strip 26, is arranged between adjacent positive electrodes 16.The negatively polarized strip 22 is electrically coupled to a negative terminal 28, and the positively polarized strip 26 is electrically coupled to a positive terminal 30. Each electrode material 14, 16 is typically formed on or bonded to the corresponding metallic current collector 20, 24. An applied compression force usually presses the metallic current collectors 20, 24 and their electrodes 14, 16 against the separator 18 to establish close contact between adjacent contacting components. The negative terminal 28 and the positive terminal 30 are connected to a load 50 that consumes electrical energy. For example, a suitable battery pack comprising many identical individual cells can be provided to supply energy to an electric propulsion motor for driving the wheels in a motor vehicle.In such a battery pack, many cells are connected in groups in an electrically parallel arrangement to provide a suitable energy capacity, and many groups are connected in series to provide a suitable electrical voltage potential. An exploded cross-sectional view of the electrochemical battery cell 12 and its associated metallic current collector 20, 24 is shown in general terms in Fig. 2. The negative electrode 14 comprises an inner surface 32 and an outer surface 34 relative to the position of the separator 18. The positive electrode 14 similarly comprises an inner surface 36 and an outer surface 38. The inner surface 32 of the negative electrode 14 may, but need not, have a larger two-dimensional surface area than the inner surface 36 of the positive electrode 16, as shown. The inner surfaces 32, 36 of the negative and positive electrodes 14, 16 face each other when installed in an electrochemical battery cell 12 and are pressed against a negative-side main surface 40 and a positive-side main surface 42 of the separator 18, respectively.Such a pressing arrangement generally occurs uniformly along the entire interface of the main surfaces 40, 42 of the separator 18 and the corresponding sections of the inner surfaces 32, 36 of the electrodes 14, 16. The metallic negative-side current collector 20 is formed on or connected to the outer surface 34 of the negative electrode 14, and the metallic positive-side current collector 24 is formed on or electrically connected to the outer surface 38 of the positive electrode 16. In many embodiments of this invention, a tin-silicon composite negative electrode material is formed directly on the surfaces of a copper negative-electrode current collector 20. Both metallic current collectors 20, 24 engage with the corresponding outer surfaces 34, 38 of the electrodes over a significant section of the interface surface to enable the efficient collection and conduction of free electrons. In many lithium-ion batteries, the components of the electrochemical cells 12 are made of materials that allow them to be generally thin and flexible. In an illustrative example, a typical thickness (T in Fig. 2) of an electrochemical cell 12, extending from the outer surface 34 of the negative electrode 12 to the outer surface 38 of the positive electrode 16, is approximately 80 µm to approximately 350 µm. Each electrode 14, 16 can be approximately 30 µm to 150 µm thick, and the separator 18 can be approximately 20 µm to 50 µm thick. The metallic current collectors 20, 24 can be approximately 5 µm to 20 µm thick. The relatively thin and flexible properties of the elements of the electrochemical cell 12 and their associated metallic current collector 20, 24 enable them to be rolled, folded, bent or otherwise maneuvered into a variety of lithium-ion battery configurations depending on design specifications and spatial constraints.The lithium-ion battery 10 can, for example, include a number of independent electrochemical cells 12 that are manufactured, cut, aligned and stacked on top of each other, or in an alternative embodiment, the cells 12 can be derived from a continuous layer that has been folded back and forth over itself several times. While lithium-ion batteries have been continuously developed and used, for example, to power propulsion motors and the like in motor vehicles, lithium-sulfur batteries are now being considered for such applications. As described, lithium-sulfur cells also use a lithium-containing negative electrode (anode) and electrode material that functions electrochemically like the negative electrode of a lithium-ion cell or battery. Accordingly, the silicon-tin compositions and microstructures further described in this specification are suitable for lithium intercalation when the negative electrode material is used in either a lithium-ion battery or a lithium-sulfur battery.When a negative electrode is prepared with a layer of the silicon-tin composites of the present invention as the negative electrode material, it is necessary to ensure the introduction of a specific amount of lithium into the electrode materials of the cell. In a preferred embodiment, a defined amount of lithium can be incorporated into the silicon-tin composite material, for example, by electrochemical incorporation, electroplating, vacuum deposition, or physical contact with lithium metal in the presence of a suitable electrolyte (lithiation). In another embodiment, a suitable amount of lithium can be introduced into the sulfur-containing positive electrode material before the elements of the lithium-sulfur cell are assembled.During the initial charging of the assembled cell or battery, lithium would be transported through the electrolyte and incorporated into the silicon-tin negative electrode material composite. Implementations of this invention are aimed at producing higher current capacities and more durable materials for negative electrodes of electrochemical lithium-ion and lithium-sulfur cells. Phase-separated tin-silicon composites are produced for this purpose. Several combinations of silicon and tin atomic ratios were co-deposited as thin composite films in a Gamma 1000 sputtering system (Surrey Nanosystems, UK). Twelve thin-film combinations of silicon and tin were produced, each comprising, in atomic percent, 22.7%, 27.2%, 34.4%, 39.9%, 42.7%, 48.1%, 57.6%, 64.4%, 66.3%, 71.5%, 77.3%, and 81.8% silicon, with the remainder being tin. A roughened copper foil was used in each fabrication to ensure good adhesion between the sputtered thin Si-Sn films and the copper current collector. The deposition plasma for each material component (Si and Sn) was generated using radio frequency or direct current, applied to two magnetron guns under an argon flux of 14 sccm. The deposition rates of tin and silicon were controlled separately to obtain the different atomic ratios of these elements specified above.The dynamic pressure during film growth was 3 mTorr and the substrate was kept at room temperature. Ex-situ X-ray diffraction (XRD) was used to investigate the structures of the films deposited on the copper current collectors. All samples were examined using Cu Kα radiation in a Bruker AXS General Area Detector Diffractometer System (GADDS). Diffraction patterns were acquired over a period of 5 minutes at a sample-to-detector distance of 150 mm using a 0.5 mm collimator. The composition of each sample block was determined by electron beam microanalysis (EPMA), while selected samples were characterized using a JEOL 2100F AC transmission electron microscope operated at 200 kV. Scanning transmission electron microscopy (STEM) images were acquired using a wide-angle circular dark-field (HAADF) detector. All electrochemical experiments were performed in a Swagelok cell within an argon-filled glovebox. Pure lithium metal was used as the counter electrode for the half-cell tests. The electrolyte solution was 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 v / v). A Celgard 3501 (1 µm thick microporous polypropylene film with 40% porosity) was used as a separator. Galvanostatic tests for all samples were performed using an Arbin BT-2000 battery test station at a cyclic voltage between 10 mV and 1.5 V (relative to a Li / Li+ electrode). The XRD results exhibit the strong Cu diffraction peaks observed from the substrate and showed multiple Sn signals, indicating that the Sn phase was crystalline. No Si diffraction signals were detected, suggesting the amorphous nature of the Si. This was expected, as the deposition temperature was too low for Si to develop a crystalline phase. No signals representing any other Si-Sn phase were observed in any of the sputtered samples, clearly demonstrating that the Si and Sn were phase-separated, consistent with their known immiscibility. This nanostructure, introduced by phase separation, is crucial for significantly improving their electrochemical performance. The overall mechanism for this is further described in this specification. Based on the well-known Scherrer equation, the average Sn crystallite size, estimated from several X-ray diffraction patterns, was 20 to 50 nm. Those sputtered films containing higher Si contents resulted in smaller Sn grains, as confirmed by the SEM images in Figures 3A–3C, which show that the surface morphology associated with Sn grain size becomes rougher with increasing Si content. Detailed insights were obtained using HAADF STEM imaging, which has a high sensitivity to atomic number contrast. As shown in Figures 4A and 4B, the phase separation between the Sn (light features) and Si (dark features) results in discrete amorphous Si nanoparticles dispersed in a continuous crystalline Sn matrix. The Si particle size ranges from 30 to 70 nm for high Si contents (approximately sixty atomic percent silicon in Fig.4A) with an average value of 50 nm and decreases with decreasing Si content in the alloy (approximately thirty atomic percent silicon in Fig. 4B). The Sn thickness between the Si particles was approximately 20 nm for the alloy with high Si content and increased with decreasing Si. The effects of the Si / Sn composition ratios on the galvanostatic charge / discharge behavior were evaluated. A comparison of Si-Sn films with five different Si / Sn composition ratios at a charge rate of C / 10 is shown in Fig. 5A (2nd cycle) and 5B (20th cycle). During the initial cycles (as illustrated by the second cycle, Fig. 5A), the cell capacities are higher with increasing Si content, since the theoretical capacity of Li15Si4 (3580 mAh / g) is much greater than that of Li4,4Sn (992 mAh / g). However, at a very high Si content (Si92 / Sn8), the capacity was lower (approximately 2400 mAh / g, Fig. 6), suggesting that, due to the drastic volume change that occurs and also due to the high intrinsic resistance of Si, not all of the Si is available for the uptake of lithium ions. Still referring to Fig. 5A and Fig.5B The stress profiles of Si-Sn alloys also reflected the charge characteristics of each individual Si and Sn phase. For example, at high Sn content, several plateaus of constant stress for crystalline Sn can be clearly observed. These plateaus correspond to different regions of solid solutions, with each plateau representing regions of coexistence of two phases: Li₂₂Sn₅-Li₇Sn₃, Li₇Sn₃-LiSn, LiSn-Li₂Sn₅, and Li₂Sn₅-Sn during the Li⁺ extraction process from 0 to 1 V. With decreasing Sn content, the plateaus of constant stress for Sn become less visible, while a broad constant stress plateau appears near 0.4 V. This broad, featureless stress characteristic is a typical representation for amorphous Si during charging (delithiation process of the Si alloy). These distinguishing characteristic features of both Si and Sn further show that the Si and Sn were phase-separated from each other. We also investigated the effects of Si / Sn composition ratios on cycle stability. Fig. 5B shows the charge characteristics of Si-Sn films after 20 cycles. The constant voltage plateaus of crystalline Sn disappeared during cycling, suggesting that the tin tends to become amorphous. The cycle performance of all five Si-Sn samples is also summarized in Fig. 6. While capacity retention was better at ratios of 65:35 and 46:54 at.% (Sn:Si) than at higher Si contents, the 46 Sn:54 Si ratio was the most promising composition due to its high specific charge capacity. At 1400 mAh / g, the electrode achieved 90% of its theoretical capacity (1550 mAh / g). This composition offers a good balance between electrical conductivity and the ability to accommodate large volume expansion. In general, it is preferred to use silicon / tin compositions containing a mixture of approximately 50-50 atomic percent of the two elements. Suitable mixtures comprise 40 to 60 atomic percent of silicon and the remainder tin. These compositions have been found to provide improved charge capacity for a greater number of charge and discharge cycles. The compositions are prepared by a process that forms thin films of the silicon / tin composite with the elements in separate phases, in which the silicon phase is amorphous and the tin phase is initially crystalline. Preferably, the amorphous silicon phase is dispersed in a crystalline tin matrix. The phase-separated tin-silicon compositions can be produced from a homogeneous melt of suitable tin-silicon ratios by a rapid-setting process. For example, the process can be carried out as described below: 1. Elemental silicon and tin are melted together in a predetermined atomic ratio. In many embodiments of the invention, the ratios produced are in the range of forty to sixty atomic percent silicon and the remainder tin. 2. The melt is increasingly rapidly set to form particles, flakes, or ribbons. 3. The particles are crushed, as by cryogenic pellet milling at a suitably low temperature between 0°C and down to -30°C under an inert atmosphere to avoid oxidation of the particles, to form substantially uniformly shaped particles of one to five micrometers or similar (or smaller) in their largest dimension. 4.In some embodiments, the particles can be annealed, if necessary, to form phase separation by creating nanoscale islands of amorphous silicon embedded in a tin matrix. Such phase-separated silicon-tin composite particles are considered the active material for a negative electrode of a lithium-ion cell or a lithium-sulfur cell. 5. The active material particles are bonded to a suitable metallic current collector, preferably a copper current collector, forming a negative electrode. For example, particles of active electrode material can be mixed with a suitable polymeric binder, such as polyvinylidene fluoride (PVDF), and carbon black in a weight ratio of, for example, 80:10:10. An inert liquid medium can be used temporarily to distribute the mixture over one or both opposing surfaces of a thin current collector strip.The liquid medium is removed, and the mixture with the polymeric binder is bonded to the recipient film surfaces. The electrical conductivity of the silicon-tin composite is further increased by the carbon black particles. Accordingly, phase-separated films are formed to a predetermined thickness over a predetermined area on a copper current collector for assembly as negative electrode material in lithium-ion or lithium-sulfur electrochemical cells. Lithium can be introduced into the silicon-tin composite negative electrode material during electrode fabrication, initial charging, or other cell operation. The total amount of negative electrode material and its lithium content are selected to create the desired electrode capacity for the respective cell. Implementations of the invention have been described for the purpose of illustration and not to limit the scope of protection of the invention.

Claims

Electrochemical lithium-sulfur secondary cell or battery comprising a negative electrode material; wherein the negative electrode material comprises a composite consisting of tin and silicon in which the tin and silicon exist in separate phases, the composite being further characterized in that a phase of nanoscale islands of silicon with a size of 30 to 70 nanometers is dispersed in a matrix phase of tin, wherein the tin phase is crystalline or amorphous and the silicon phase is amorphous; wherein the tin and silicon composite is capable of allowing the incorporation of lithium during charging of the cell or battery and of releasing lithium during discharging of the cell or battery. Electrochemical lithium-sulfur secondary cell or battery according to claim 1, in which the tin and silicon composite consists of forty to sixty atomic percent silicon and the remainder tin. Electrochemical lithium-sulfur secondary cell or battery according to claim 1, in which the tin and silicon composite consists of forty-eight to fifty-two atomic percent silicon and the remainder tin. Electrochemical lithium-sulfur secondary cell or battery according to claim 1, in which the tin and silicon composite is characterized by crystals in the tin phase with a size in the range of twenty nanometers to fifty nanometers. Electrochemical lithium-sulfur secondary cell or battery according to claim 1, in which the negative electrode material is the composite of tin and silicon, an adhesive film of tin and silicon which is sputtered as negative electrode material onto a surface of a metal current collector. Electrochemical lithium-sulfur secondary cell or battery according to claim 1, in which the tin and silicon composite is an adhesive film of tin and silicon that is sputtered as a negative electrode material onto a surface of a metal current collector, wherein the sputtered film has a thickness of about five micrometers. Electrochemical lithium-sulfur secondary cell or battery according to claim 1, in which the tin and silicon composite was formed by rapid solidification of a molten liquid of elemental tin and silicon into solid particles of the phase-separated tin and silicon composite. Electrochemical lithium-sulfur secondary cell or battery according to claim 7, in which the solid particles are reduced in size to about one to five micrometers and then mixed with a polymeric binder and carbon black and bound as negative electrode material to a metal current collector. Electrochemical lithium-sulfur secondary cell or battery according to claim 7, in which the solid particles were reduced in size by cryogenic ball milling at a temperature below 0°C, down to -30°C, in a non-oxidizing atmosphere. Electrochemical lithium-sulfur secondary cell or battery according to claim 1, wherein the negative electrode material comprises a composite of tin and silicon containing embedded lithium prior to the first charging of the cell or battery.

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