RELATED APPLICATION This application is based upon provisional application serial no. 60/092,206, filed July 9,1998, and of provisional application serial no. filed December 19, 1998.

Background of the Invention This invention relates to new anode materials for lithium ion batteries. More specifically this invention relates to new anode materials for lithium-ion batteries consisting of intermetallic materials having active and inactive metals with respect to lithium, such as for instance Sn and Cu metals, respectively. Lithium-ion batteries are under development for the consumer electronics market such as cellular phones, laptop computers, and camcorders. Lithium batteries are also being developed for high energy/power systems such as electric vehicles. A major concern about lithium batteries is safety, primarily when metallic lithium is used as the anode material.

This arises because lithium has a very high oxidation potential and thus reactivity in the cell environment. One approach to the safety issue is to use carbon as a host structure for lithium. The carbon can be either graphite or a less crystalline form of pyrolyzed carbon. Lithium-intercalated graphite/carbon electrodes improve the safety of lithium cells, but do not completely overcome the safety problems, because there is still the possibility of depositing metallic lithium at the top of the charge when the voltage of the lithiated carbon/graphite anode approaches that of metallic lithium.

Carbon itself when present in a finely divided form with a high surface area, can be highly reactive particularly if oxygen is released within the cell from a highly oxidizing cathode material such as Li, xCoO2, Li, XNiO2, and Li1 xMn204.

There is, therefore, a need to find alternative anode materials to carbon.

Many binary lithium alloy systems, such as LiXAI, LiXSi and LixSn have been extensively studied in the past. These alloys undergo several phase transitions during charge and discharge; the structures undergo severe lattice expansion and contraction that limits the cycle-life of the electrode. An improvement in cycle-life has been made by using an amorphous tin oxide negative (anode) electrode, in which the tin ions are initially reduced to a metallic state (during the charge process of lithium-ion cells), and thereafter alloyed with lithium to form a series of phases within the LixSn system (0 < x < Several high-temperature Several high-temperature high-temperature in order of increasing lithium content, they are: Li2Sn5, LiSn, Li7Sn3, Li5Sn2, Li13Sn5, <BR> <BR> Li7Sn2 and Li22Sn5 ; they are generated during the electrochemical reaction of lithium with tin at 400°C. At 25°C, the electrochemical profile has indicated that the compositions of the stable phases are Li2Sn5, Li2Sn3, Li7Sn3, Li5Sn2, Li7Sn2 and Li22Sn5. Tin-oxide electrodes suffer an irreversible capacity loss on the first cycle because a significant amount of the lithium is trapped within the charged electrode as lithium oxide. Furthermore, lithium oxide is an insulator, which reduces the electronic conductivity of the electrode, a serious disadvantage since for optimum performance the anode needs to be an electrical conductor.

Summary of the Invention We have invented a new anode for a lithium battery by replacing the oxygen of a tin oxide electrode with a metal such as copper, which has not only excellent electrical properties, but also has a low oxidation potential, and therefore a low reactivity in a lithium cell environment; copper foil is also used as the current collector at the negative electrode in conventional Li, C,,/Lil-xCO02 lithium-ion cells.

More generally, the invention relates to the use of intermetallic compounds, based on the structure types of copper-tin and lithium-copper-tin materials, as defined by their relationship to NiAs type, Ni21n type and lithiated zinc-blende-type materials as anode materials for rechargeable lithium batteries. The invention also includes non- aqueous electrochemical cells incorporating the new anodes and the methods of making the anodes and cells. In the anode, there is one or more of an active metal which can alloy substantially with lithium (such as tin) and, in most instances, there is one or more of an inactive metal which does not substantially alloy with lithium (such as copper). There are other examples in which the anode material components consist of two (or more) active metals that can alloy with lithium, such as tin and germanium.

The invention also includes cells and batteries incorporating the new anode materials with standard state-of-the-art cathodes such as lithium cobalt oxide, lithium nickel oxide or lithium manganese oxides (and various substituted oxides), cathodes now useful in lithium rechargeable batteries.

Brief Description of the Drawings The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.

FIG. 1 depicts a schematic illustration of an electrochemical cell; FIG. 2 is a representation of the ideal structure of a high-temperature Cu6Sn5 intermetallic compound, labeled n-Cu6Sn5 ; FIG. 3 is a representation of the ideal structure of NiAs; FIG. 4 is a representation of the ideal structure of Nizin ; FIG. 5a is a [001] crystallographic projection of the high-temperature structure of rl-Cu6Sn5 shown in FIG. 2 showing the displacement of Sn atoms that is required for the transformation to a cubic LixCu6Sn5-type structure (Xmax ~ 13) ; FIG. 5b is a [001] crystallographic projection of the ideal structure of the room-temperature Cu6Sn5 intermetallic compound, labeled n'-Cu6Sn5, showing the displacement of Sn (or Cu) atoms that is required for the transformation to a cubic LixCu6Sn5-type structure; FIG. 6 is a [001] crystallographic projection of the ideal structure of the Ni3Sn2 (and Co3Sn2) intermetallic compound; FIG. 7 shows the X-ray diffraction patterns (CuK, radiation) of a) a standard Cu6Sn5 sample at room-temperature, b) LixCu6Sn5 obtained by electrochemical insertion of lithium at room temperature, c) a standard Li2CuSn sample at room temperature, and d) a chemically delithiated sample of Li2CuSn at room temperature; FIG. 8a is a representation of the ordered room-temperature structure of Li2CuSn ; FIG. 8b is a representation of the disordered high-temperature structure of Li2CuSn ; FIG. 8c is a representation of the zinc-blende type CuSn framework of the Li2CuSn structure shown in FIG. 8a; FIG. 9 is a [001] crystallographic projection of the ordered structure of Li2CuSn shown in FIG. 8a; FIG. 10 shows the in-situ X-ray diffraction data (CuK, radiation) of a Li/LixCU6Snr) cell; FIG. 11 is an electrochemical profile of the first four discharge and three charge cycles of a Li/Sn cell between 1.2 volts and 0.0 volts; FIG. 12 is a graphical representation similar to FIG. 11 for a Li/Cu6Sn6 cell ; FIG. 13 is a graphical representation similar to FIG. 11 for a Li/Cu6Sn5 cell; <BR> <BR> <BR> <BR> FIG. 14 is a graphical representation similar to FIG. 11 for a Li/Cu6Sn4 cell ;<BR> <BR> <BR> <BR> <BR> <BR> <BR> FIG. 15 is a graphical representation similar to FIG. 11 for a Li/Li2CuSn cell ; FIG. 16 is a graphical representation showing the relationship between cell capacity and the first twenty cycles for Li/Sn, Li/Cu6Sn5+j (where b = 0, +1, -1, -3) and Li/L'2CUSn cells cycled between 1.2 volts and 0.0 volts; FIG. 17 is a graphical representation similar to Fig. 16 for Li/Sn and Li/Cu6Sn5+a (where b = 0, +1, -1) cells cycled between 1.2 volts and 0.2 volts ; FIG. 18 shows the X-ray diffraction pattern (CuKa radiation) of a Ni3Sn2 sample at room-temperature; FIG. 19 shows the X-ray diffraction pattern (CuKa radiation) of a Li2AgSn sample at room-temperature; Detailed Description of the Invention As previously stated, the invention relates to a new anode for use in an electrochemical cell 10 having an anode 12 separated from a cathode 16 by an electrolyte 14, all contained in an insulating housing 18 with suitable terminals (not shown) being provided in electronic contact with the anode 12 and the cathode 16, as shown schematically in Fig. 1. Binders and other materials normally associated with both the electrolyte and the anode and cathode are well known and are not described herein, but are included as is understood by those of ordinary skill in this art.

The anodes of the present invention include intermetallics comprising a substantially active element (or elements) which alloys with lithium with a substantially inactive element (or elements) which does not alloy with lithium, or alternatively, comprising a combination of substantially active alloying elements, the anodes having structure types based on the copper-tin compound Cu6Sn5 and lithium-copper-tin derivatives, such as the compound Li2CuSn. Metals which are considered active, that is metals which can form a binary alloy with lithium useful in this invention are preferably one or more of Sn, Si, Al, Ga, Ge, In, Bi, Pb, Zn, Cd, Hg and Sb. The substantially inactive elements which do not form substantial binary alloys with Li are selected preferably from the first row of transition metals and more preferably one or more of Cu, Ni, Co, Fe, Mn, Cr, Ti, V and Sc, and in addition, Mg and Ag along with the noble metals, Pd, Rh, and Ir. More specifically, the preferred combinations of inactive and active metals are Cu and Ni, and one or more of Sn, Ge, Sb, Si and Al. It is understood that the active and inactive metals may be present in a wide variety of ratios as will hereinafter be described with the Cu6Sn5 and Li2CuSn intermetallic compounds, by way of example only. It should be understood that various combinations, relative amounts and relative"activities"of both the inactive metals and the active metals may be used in the context of intermetallic compounds without departing from the true spirit and scope of the present invention.

By way of example only, we selected the eta-phase Cu6Sn5 and the intermetallic phase Li2CuSn to demonstrate the principles of this invention; in this invention, we describe in detail the structural properties and electrochemical <BR> <BR> properties of Cu. Sn5l, (8= -3, -1,0, +1) and Li2CuSn electrode materials in cells with metallic lithium electrodes.

The concept of using an intermetallic compound, MM', where M is an"active" alloying element and M'is an"inactive"element (or elements) can be considered, broadly speaking, analogous on a macroscopic scale to a metal oxide insertion electrode, such as MnO2 in which Mn is the electrochemically active ion and O is the inactive ion. In an MnO2 host electrode, lithium ions are inserted into the structure to a composition LiMnO2 with a concomitant reduction of the manganese ions from a tetravalent to a trivalent state. In Mn02, the ratio of the inactive element to active element is 2: 1; for the uptake of one lithium ion per manganese ion, the capacity of the MnO2 electrode is 308 mAh/g. Manganese oxides, such as the spinel-related phase, l-MnOz, that expand and contract isotopically during lithium insertion and extraction with minimal volume expansion of the unit cell provide greatly enhanced cycling stability compared to manganese oxide structures that expand and contract anisotopically with a large volume increase, as in the ramsdellite-LixMnO2 structure.

In an intermetallic system such as Cu6Sn5, the tin atoms can be regarded, broadly speaking, as the electrochemically active component (because it forms a binary LixSn alloy system with Li) and the copper atoms as the inactive component, (because it does not substantially alloy with Li). Lithium insertion into the Cu6Sn5 structure takes place initially in a topotactic reaction to yield a structure, believed to <BR> <BR> have the nominal composition Li., 3Cu6Sn5, or L'2. 17CUSnO. 83, i. e., with a maximum of 13 Li per Cu6Sn5formula unit, which is approximately the same as that of Li2CuSn.

Thus during the reaction, the Cu6Sn5 structure provides a host framework for the uptake of lithium. It is believed that further reaction with lithium necessitates a displacement reaction and a break-up of the LixCu6Sn5 structure ; the lithium combines with the active tin to form a series of LixSn compounds (0< x < 4.4) within a residual copper matrix. In this latter-type reaction, the divided copper atoms/particles that are produced on electrochemical cycling would provide an electronically conducting matrix to contain the lithiated tin particles and to accommodate at least some of the damaging expansion/contraction of the LixSn particles during discharge and charge (note that in a Li/Cu6Sn5 cell, extensive discharge means the formation of a"LixCugSng"composite electrode (with x > -13) consisting of domains of lithiated tin and copper metal). Optimum cycling conditions would be expected from such a composite Cu/LixSn material if it had an ideal microstructure in which the Cu and LixSn particles were evenly distributed with sufficient porosity to accommodate the reacted lithium, thus allowing a minimal isotropic expansion of the overall electrode. Although it is believed that the initial topotactic reaction provides a LixCu6sn5 electrode with superior cycling behavior compared to the subsequent displacement reaction, the invention also includes the composite electrodes that are derived over longer-term cycling by the displacement reaction described above and which can yield higher capacities than can be obtained from the topotactic reaction alone.

The Cu6Sn5 structure exists in a high temperature form and a low temperature form. The transition from the one form to the other takes place at 460K as previously reported. The high-temperature structure of Cu6Sn5, labeled n-Cu6Sn5 and shown in Fig. 2, was first reported in 1928 to have the NiAs-type structure. Simplistically, n- Cu6Sn5 has a defect NiAs structure in which the Cu atoms adopt the octahedral sites occupied by Ni in NiAs, and the Sn atoms occupy the trigonal prismatic sites adopted by As; there is one vacant trigonal prismatic Sn site per Cu6Sn5 unit. A recent analysis in 1994, of the room-temperature Cu6Sn5 structure, labeled n'-Cu6Sn5, has shown that this compound has features of both a NiAs-type and a Ni21n-type structure.

In the well known NiAs structure, which has a hexagonal-close-packed ABACABAC arrangement of atoms (Fig. 3a), the Ni atoms are octahedrally coordinated to six As atoms, whereas the As atoms are coordinated in trigonal prismatic manner to six Ni atoms. The [001] crystallographic projection of the NiAs structure in Fig. 3 shows that the Ni and As atoms are arranged in strings. There are twice as many As strings as Ni strings. The Ni strings are densely packed, whereas the As strings contain half as many As atoms as there are Ni atoms in the Ni strings. In Fig. 3, the NiAs structure is seen to consist of an array of hexagonal columns, each column consisting of alternating Ni strings and As strings, and each column containing an As string at its center.

The [001] crystallographic projection of the Ni21n structure is shown in Fig. 4. In this structure, there is a similar hexagonal arrangement of atom strings as described for NiAs (Fig. 3). In Ni21n, however, strings of Ni atoms (octahedrally coordinated to surrounding In atoms) alternate with strings containing both In and Ni atoms to make up the hexagonal columns. In the In-Ni strings, the In atoms have trigonal prismatic coordination to the Ni atoms in neighboring strings; the Ni atoms in the In-Ni strings reside midway between the In atoms in trigonal bipyramidal interstices (with respect to neighboring In atoms) to provide a very dense structure (p= 9.902 g/ml). Each hexagonal column contains, at its center, a similar string of In and Ni atoms.

The close relationship between the high- and low-temperature forms of the Cu6Sn5 structure and the NiAs and Ni21n structures can be seen in Figs. 3,4, 5a and 5b.

In the [001] crystallographic projection of the high-temperature modification, rl-Cu6Sn5, shown in Fig. 5a, the Cu atoms and the Sn atoms are arranged in discrete strings, as in the NiAs structure (Fig. 3). In the [001] crystallographic projection of the room- temperature modification, n'-Cu6Sn5, shown in Fig. 5b, the Cu atoms are arranged in discrete rows as they are in rl-Cu6Sn5 (Fig. 5a), but in this structure the Sn strings also contain some Cu atoms; these Cu atoms are located between the Sn atoms in trigonal bipyramidal sites, one-fifth of which are occupied in the structure. In rl'-Cu6Sn5, there exist hexagonal columns that consist of Cu strings that alternate with Sn-Cu strings; the center of the hexagonal columns contain identical Sn-Cu strings. Thus, r'-Cu6Sn5 may be regarded as having a defect Ni21n-type structure in which 80% of the trigonal bipyramidal sites are vacant. Intermetallic compounds that are assigned either to the NiAs- or the Ni21n-type structure commonly have a composition between that of NiAs and Ni21n. A superstructure is formed if the structure is ordered. Examples of such superstructures are found in room-temperature modifications ofn'-CUgSns, Ni3Sn2, and Co3Sn2 intermetallic compounds. The [001] crystallographic projection of the Ni3Sn2 structure (which is isostructural with Co3Sn2) is shown in Fig. 6; it demonstrates that Ni3Sn2 (and Co3Sn2) is of the same structure type as rl'-Cu6Sn5 (Fig. 5b). In practice, however, it is believed that these structures are not ideally ordered and that some disorder can exist, for example between the copper and tin atom sites in the Cu6Sn5 structures shown in Figs. 5a and 5b.

An important aspect of this invention is the discovery by combined electrochemical and in-situ X-ray diffraction studies of Cu6Sn5 electrodes in lithium cells, that lithium can be inserted topotactically into the Cu6Sn5 structure and that this process is reversible. The X-ray diffraction data of various lithiated and delithiated samples (Fig. 7a-d) showed that the lithiated phase LixCu6Sn5 (Fig. 7b) has an X-ray pattern similar to that of Li2CuSn (Fig. 7c). The structure of Li2CuSn has cubic symmetry. The room-temperature form of Li2CuSn (space group symmetry F-43m), shown in Fig. 8a, is ordered. The Sn atoms and one-half of the Li atoms form two interlinked face-centered-cubic arrays. The Cu atoms and remaining Li atoms occupy interstitial sites located at the center of cubes defined by 4 Sn and 4 Li atoms of the face-centered-cubic arrays. The high-temperature phase of Li2CuSn has higher symmetry, Fm-3m. In this case, the Li and Cu atoms within the cubes are randomly disordered, as shown in Fig. 8b to provide a Heusler-type phase, such as Ni2MnGa, in which the Ni atoms occupy the same sites as the disordered Li and Cu atoms in Fig. 8b, and the Mn and Ga atoms occupy the sites of the Sn and remaining Li atoms, respectively.

Of particular note is that the CuSn framework of the ordered Li2CuSn structure (Fig. 8a) has a zinc-blende-type lattice, as shown in Fig. 8c. Therefore, L'2CUSn may be regarded as having a lithiated zinc-blende-type structure in which the Li atoms occupy the interstitial sites of the CuSn host framework. The invention thus includes zinc-blende-type framework structures as insertion electrodes for lithium batteries. Other examples of such a zinc-blende-type framework structure is provided by the AgSn component in Li2AgSn and by the MgSi component in Li2MgSi.

Additional examples of zinc-blende-type structures are AISb, GaSb, and InSb.

The ordered Li2CuSn structure, as depicted in a second representation of the structure in Fig. 9, bears a close relationship to the rl-Cu6Sn5 structure (Fig. 5a); this relationship, which is seen in the arrangement of the hexagonal columns, makes it easy to understand the topotactic reaction when lithium is inserted electrochemically into n- Cu6Sn5. It is apparent from the X-ray diffraction data in Fig. 10 obtained in-situ in an electrochemical cell that the lithiation process involves a two-phase reaction. The phase transformation from 9-CU6Sn. to the Li2CuSn-type structure can be understood if lithium insertion into the hexagonal columns of rl-Cu6Sn5 displaces the Sn atoms from the center of the columns into the trigonal bipyramidal interstitial sites in neighboring Sn strings, as shown by the arrows in Fig. 5a. [Note that full lithiation of n-Cu6Sn5 with an ideal defect NiAs-type structure would result in the composition Li, 3CU6Sn5 (alternatively, Li2, 7CuSn0. 83 or U2CU {Sno83Li 017} ) s in which 17% of the Sn sites are occupied by Li.] Such a reaction makes the hexagonal channels (5.6 - 6.2 A in diameter) accessible for lithium occupation and creates two interstitial sites per hexagon for the incoming Li atoms (Fig. 9). However, to accommodate the inserted Li atoms, the layers of Cu and Sn atoms shear from their hexagonal-close-packed sequence ABACABAC in the rl-Cu6Sn5 structure to the cubic-close-packed sequence ABCABC of like atoms in the Li2CuSn-type structure.

During the transformation from n-CugSngto Lii3CUgSn5, all the Cu atoms and one- half of the Sn atoms remain in their original strings. Even though 50% of the Sn atoms are displaced during lithium insertion from one string to another during lithium insertion, <BR> <BR> <BR> the phase transition is topotactic, i. e., a transition in which a definite orientation relationship exists between the two phases and in which some structural reorganization is necessary. In this transformation, one-half of the tin atoms are displaced by approximately 2.7 A from their trigonal prismatic sites in a two-phase reaction to form columns of tin with the other one-half of the tin atoms. Thus, during this reaction, the "inactive"copper subarray and one-half of the tin subarray of the structure remain spatially intact. In the lithiated product, each Sn atom and each Cu atom is coordinated tetrahedrally to neighboring Cu and Sn atoms, respectively, in a zinc-blende-type structure. This topotactic reaction is reversible. Lithium insertion into the room- temperature structure,'-Cu6Sn5 (Fig. 5b), in which Cu atoms partially occupy 20% of the interstitial trigonal bipyramidal sites can occur by a similar reaction mechanism described for rl-Cu6Sn5 above. However, in this case, in order to form a Li2CuSn-type structure, the ideal phase transformation would necessitate the diffusion of Sn atoms in the Sn-Cu strings to interstitial sites in neighboring strings, as shown by the arrows in Fig. 5b. The transformation also necessitates that the Cu atoms diffuse from the Sn- Cu strings to partially occupy the Li sites of an ordered Li2CuSn-type structure. It is expected, therefore, that the fully-lithiated n'-Cu6Sn5 structure would have the composition LigCu6Sn5, and that the atom arrangement, in Li2CuSn notation, would be (Li, 8Cu02) CuSn. Because there are less interstitial sites available in n, -CuSn5 compared with n-Cu6Sn5, the theoretical gravimetric capacity of n'-CU6Sn5 (248 mAh/g) is significantly less than that of #-CU6Sn5 (358 mAh/g) (Table 1).

Thus, the discovery of the lithium insertion reaction into Cu6Sn5 has immediate important implications for the development of a new family of intermetallic electrodes with the Cu6Sn5 and Li2CuSn structure types. The invention therefore extends to include the family of compounds having structures related to the Cu6Sn5 and Li2CuSn structure types shown in Figs. 5a, 5b, 8a, 8b and 8c.

For example, the Ni3Sn2 (and Co3Sn2) structure shown in Fig. 6 is remarkably similar to that of rl'-Cu6Sn5 (Fig. 5b), having strings of Ni, and strings of Sn-Ni arranged in identical fashion to the strings of Cu and Sn-Cu in n'-Cu6Sn5. The difference between Ni3Sn2(Co3Sn2) and rl'-Cu6Sn5 structures is only in the number of interstitial trigonal bipyramidal sites occupied by the Ni (Co) and Cu atoms, respectively, in the Sn strings. For example, in Ni3Sn2, 50% of the interstitial trigonal bipyramidal sites are occupied by Ni, in contrast to the 20% occupation by Cu in n'-CU6Sn5. Therefore, using the analogy to n'-Cu6Sn5, Ni3Sn2 may, in principle, accommodate lithium to the <BR> <BR> <BR> <BR> composition Li3Ni3Sn2, or in Li2CuSn notation, to the composition (Li, 5Ni 05) NiSn; it offers a lower gravimetric capacity (195 mAh/g) and volumetric capacity (1755 mAh/ml) compared to rl-Cu6Sn5 and #'-CU6Sn5 (Table 1). Because of the high number of Ni atoms that must be accommodated in the Li sites of Li3Ni3Sn2, it is anticipated that the phase transition should be more difficultto accomplish in LixNi3Sn2 electrodes compared to Cu6Sn5 eiectrodes. This hypothesis is supported by electrochemical data from nickel- tin electrodes that are inferior to the data derived from the analogous copper-tin materials. With this structural argument, electrode materials of this invention that are close to the ideal NiAs composition and structure are expected to perform better than those electrodes close to the Ni21n composition and structure.

In a further embodiment of the invention, Table 2 illustrates the wide variety of compounds that can exist with the Li2CuSn structure. The hexagonally shaped channels in Li2CuSn-type structures accommodate the inserted lithium. A particular embodiment of the invention therefore includes those isostructural materials in which non-lithium metals, M, occupy the Cu and Sn atom sites of Li2CuSn shown in Fig. 8a.

One such example is Li2AgSn, the X-ray diffraction pattern of which is shown in Fig. 19.

Another particularly attractive example is Li2MgSi because Mg and Si are relatively light and inexpensive materials. The invention also includes those structures in which at least some M sites are occupied by Li. This is believed to occur in LixCu6Sn5 electrodes. For example, Cu6Sn5 can be represented CuSno. 830." where 0 refers to a vacancy. On lithiation, therefore, the ideal Li ;, CuSn-type structure is reached at the composition Li2Cu (Sno. 83Lio."), oralternatively Li2."CuSno, 83, oralternatively Li, 3Cu6Sn5.

In this instance, 17% of the Sn sites are occupied by Li. The reaction of Cu6Sn5 with lithium to form Li, 3Cu6Sn5 provides an electrode with a theoretical gravimetric capacity and a theoretical volumetric capacities of 358 mAh/g and 2506 mAh/ml, respectively, compared to the theoretical capacities of graphite, viz., 372 mAh/g and 818 mAh/ml (Table 1). A particular advantage of Cu6Sn5-type electrodes, therefore, is that they offer a significantly enhanced volumetric capacity to carbon.

It has been demonstrated that it is possible to extract the lithium from the Li2CuSn structure, both chemically and electrochemically. Complete extraction of lithium would result in a copper-tin electrode with approximately the same CuSn ratio as in rl-Cu6Sn5. Chemical extraction of lithium with NOBF4 from Li2CuSn results in a Cu6Sn5-type structure, as shown by the strong similarity between the room- temperature X-ray diffraction data of a chemically treated Li2CuSn sample (Fig. 7d) and a standard Cu6Sn5 sample (Fig. 7a). These X-ray diffraction data clearly confirm the reversibility of the reaction xLi + Cu6Sn5 < LixCu6Sn5 In the high-temperature structure of Li2CuSn shown in Fig. 8b, the Cu atoms and one half of the Li atoms are disordered over the sites they normally occupy in the ideal, ordered structure at room-temperature (Fig. 8a). In practice, it is believed that at room temperature some disorder exists between these sites. This invention thus extends to include these disordered structures.

In MnOz, the ratio of inactive element to active element is 2: 1 which is approximately twice the ratio in Cu6Sn5 (1.2 : 1). Therefore, it is believed that increasing the inactive copper content as a separate Cu phase in the intermetallic electrode would provide greater cycling stability; conversely, increasing the active tin content as a separate Sn phase probably would reduce the cycling stability of the intermetallic electrode.

Li/CU6Sn5,, cells were evaluated with electrodes having b values of -1 (Cu6Sn4, copper-rich), 0 (Cu6Sn5) and +1 (Cu6Sn6, tin-rich). The electrochemical behavior of a standard Li/Sn cell was determined, for comparison. The electrochemical profiles of the first two discharge - and subsequent charge cycles of a Li/Sn cell and LiCu6Sn5+ cells (b = 0, 1) charged and discharged between 1.2 and 0.0 V are shown in Figs. 11-14. All cells show an initial steep drop in voltage followed by a voltage plateau during discharge at approximately 400 mV; the latter process appears to correspond initially to the insertion of lithium into the Cu6Sn5 component and on subsequent cycling to the formation of at least some Li7Sn3 within a copper matrix. It is believed that reaction beyond the Li7Sn3 composition, i. e., when the cell voltage decreases to 0 V, occurs with the successive formation of phases with increasing lithium content, Li5Sn2, Li7Sn2 and Li22Sn5, as reported for Li/Sn cells at 25°C, but within a copper matrix. It can be seen from the current interrupts that occur every hour during discharge and charge that the impedance of each cell improves dramatically after the first discharge. This improvement in performance can be attributed to better contact between electrolyte and active material and improved particle to particle contact that occurs during the early "conditioning"of the cells.

Electrochemical extraction from the intermetallic phase L'2CUSn occurs with a steadily increasing voltage to the cut-off potential, 1.2 V (see Fig. 15). On the subsequent discharge, the electrode does not show the characteristic plateau at 400 mV, but rather shows a continuous decrease in voltage to 0V.

The relatively poor performance of Li2CuSn electrodes compared to Cu6Sn5 electrodes observed in these initial tests is attributed to the high reactivity of the Li2CuSn phase (particularly in moist air), and to the difficulty of assembling Li2CuSn electrodes and cells without some degradation of the electrode. From this viewpoint, it is clearly advantageous to load cells with unlithiated electrodes, such as Cu6Sn5 rather than with Li2CuSn eiectrodes.

The addition of copper to a tin electrode decreases the theoretical capacity of the tin electrode, whereas increasing the tin content increases the capacity, see Table 3; Table 3 gives the theoretical capacity of Li2CuSn in terms of its ideal fully- delithiated composition CuSn; it has the same composition as Cu6Sn6 (õ = 1) .

Complete removal of lithium from Li2CuSn corresponds to a capacity of 273 mAh/g.

Compared to the theoretical capacity of 372 mAh/g for LiC6, the gravimetric capacity of the copper-tin electrodes, when lithiated to a 1: 1 Li: Sn ratio is too small for it to be of practical interest for lithium-ion cells. However, for compositions reaching higher lithium content (for example, Li7Sn3 to Li44Sn) , the available capacity becomes more attractive, see Table 3. Note that in Li, 3Cu6Sn5, which is believed to represent the maximum uptake of Li by the ideal rl-Cu6Sn5 structure, the Li: Sn ratio is 2.6 : 1 which is close to that in Li7Sn3 (2.3 : 1). Because lithium-metal alloy systems have high crystallographic densities, they provide significantly higher volumetric capacities than lithiated carbon, LiC6 (~750 mAh/ml) . For example, Cu6Sn5 has a density of 8.28 <BR> <BR> <BR> <BR> g/ml ; it will, therefore, provide a volumetric capacity of approximately 2450 mAh/ml when discharged to a Li7Sn3 composition (Li"67Cu6Sn5) . A major advantage of these intermetallic electrodes, therefore, is that they will occupy less volume than lithiated carbon electrodes for a given capacity value.

The capacity vs. cycle no. for Li/Sn and Li/Cu6Sn5+ (6 = 0, +1, -1, -3) and Li/Li2CuSn cells cycled over the range 1.2 to 0 V for the first 20 cycles is shown in Figure 16. Although a pure tin electrode provides a significantly higher capacity on the initial discharge (-670 mAh/g) than copper-tin electrodes, the capacity of the pure tin electrodes decreases rapidly on cycling, dropping to -115 mAh/g after 10 cycles. By contrast, the copper-tin electrodes Cu6Sn5+ (6 = 0, +1, -1, -3) show lower capacity, but significantly better stability to electrochemical cyciing. Cu6Sn6, Cu6Sn5, and Cu6Sn4 electrode powders that were not subjected to high-energy ball milling deliver 350,340 and 440 mAh/g on the initial discharge, and 180,175 and 280 mAh/g after 10 cycles, respectively. The copper-rich electrodes CurSn4 (6 = -1) and CU6Sn2 (6 = -3) show the greatest cycling stability after 20 cycles, in agreement with the hypothesis that higher concentrations of inactive component in composite copper-tin electrodes should enhance electrochemical recharge ability. Reducing the voltage window to 1.2 to 0.2 V improves the cycling stability of the copper-tin electrodes even further; in this case, excellent cycling stability is achieved but at the expense of some capacity. For example, over this voltage range, the Cu6Sn4 electrode delivers an initial capacity of 165 mAh/g, which increases on cycling as the electrode is"conditioned"; it delivers a steady 190 mAh/g after 10 cycles, see Fig.

17. This extra stability is attributed to two major factors: 1) the reversible topotactic reaction of lithium into the Cu6Sn5 component, and 2) to the presence of an inactive, electronically copper component that serves to counter the deleterious volumetric expansion/contraction of any binary LixSn alloys that might be formed during cycling, particularly towards the end of discharge when the lithium concentration at the copper-tin intermetallic surface exceeds the limit allowed by the topotactic lithium insertion reaction, i. e., in excess of the lithium content in Li, 3Cu6Sn5.

This invention thus also embodies electrode intermetallic host structures that transform on lithiation to a lithiated zinc-blende-type structure in the presence of an additional substantially inactive (non-alloying) element (or elements) with respect to lithium, or alternatively, an additional substantially active alloying element (or elements) with respect to lithium, or both.

Experimental Example 1 Cu6Sn5 was synthesized by reacting metallic tin and metallic copper in stoichiometric amounts at 400 °C under argon for 12 hours. The sample was ground by mechanical milling to an average particle size of less than 10 microns. X- ray diffraction data, collected on a Siemens D5000 powder diffractometer with CuKa radiation showed that the Cu6Sn5 product was an essentially single-phase product as shown in Fig. 7a.

Example 2 Copper-rich and tin-rich materials Cu6Sn5+i5 (o=~1, -3) , i. e., Cu6Sn6, Cu6Sn4, and Cu6Sn2 were prepared in identical fashion to Cu6Sn5 in Example 1 with the required excess/deficiency of copper. The samples were ground by mechanical milling to an average particle size of less than 10 microns. The copper-rich and tin-rich materials showed the characteristic peaks of Cu6Sn5 and additional peaks of copper and tin metal, respectively.

Example 3 Li2CuSn was prepared by heating lithium metal, copper metal and tin metal at 900 °C under argon for 3 hours. The sample was ground by mechanical milling to an average particle size of less than 40 microns. X-ray diffraction data, collected on a Siemens D5000 powder diffractometer with CuKa radiation showed that the Li ;, CuSn product was an essentially single-phase product as shown in Fig. 7c.

Example 4 Li2CuSn was chemically delithiated by reaction with NOBF4 in acetonitrile at room temperature. X-ray diffraction data, collected on a Siemens D5000 powder diffractometer with CuKa radiation showed that the product was an essentially single- phase product with a Cu. Sn5-type structure as shown in Fig. 7d.

Example 5 Ni3Sn2 was synthesized by reacting metallic tin and metallic nickel in stoichiometric amounts at 650 °C under argon for 12 hours. The sample was ground by mechanical milling to an average particle size of less than 40 microns. X-ray diffraction data, collected on a Siemens D5000 powder diffractometer with CuKa radiation showed that the Ni3Sn2 product was an essentially single-phase product as shown in Fig. 18.

Example 6 Li2AgSn was prepared by heating lithium metal; silver metal and tin metal at 900°C under argon for 3 hours. The sample was ground by mechanical milling to an average particle size of less than 40 microns. X-ray diffraction data, collected on a Siemens D5000 powder diffractometer with CuKa radiation showed that the L'2AgSn product was an essentially single-phase product as shown in Fig. 19.

Example 7 The intermetallic materials were evaluated as electrodes in coin cells (size 1225) 12 mm diameter and 2.5 mm high against a counter lithium electrode. The cells had the <BR> <BR> <BR> <BR> configuration: Li/1 M LiPFs in ethylene carbonate, diethyl carbonate (50: 50) /lntermetallic electrode. For the intermetallic electrodes, laminates were made containing approximately 6 mg (81 %) of the intermetallic compound, intimately mixed with 10% (by mass) binder (Kynar 2801) and 9% (by mass) carbon (XC-72) in tetrahydrofuran (THF).

Metallic lithium foil was used as the counter (negative) electrode. Cells were charged and discharged at constant current (generally at 0.1 mA), within the voltage range of either 1.2 to 0.0 V, or 1.2 to 0.2 V. The electrochemical data from the various intermetallic electrodes are shown in various electrochemical plots, as in Fig. 10: a Li/Cu6Sn5 cell; Fig. 11: a Li/Sn cell (reference); Fig. 12: a Li/Cu6Sn6 cell, Fig. 13; a <BR> <BR> <BR> <BR> Li/Cu6Sn5 cell ; Fig. 14: a Li/Cu6Sn4 cell ; Fig. 15: a Li/Li2CuSn cell; Fig. 16: a collective plot of Li/Sn, Li/Cu6Sn6, Li/Cu6Sn5, Li/Cu6Sn4, Li/Cu6Sn2 and Li/Li2CuSn cells; and Fig.

17: a second collective plot of Li/Sn, Li/Cu6Sn6, Li/Cu6Sn5 and Li/Cu6Sn4 cells.

Table 1 A comparison of the theoretical gravimetric and volumetric capacities of various negative electrode (anode) materials based on CU6Sn. and Li2CuSn-type materials with LiXC6.

Volumetric capacities are calculated using the approximate density of the delithiated electrodes LixC6 #-LixCu6Sn5 #'-LixCu6Sn5 LixNi3Sn2 LixCo3Sn2 LixMgSi x"", ; 1 13 9 3 3 2 Th. Gr. Cap. 372 358 248 195 194 1023 (mAh/g) Approx. density of 2.2 7.0 8.3 9.0 9.0 2.2 delithiated electrode (g/ml) Th. Vol. Cap. 818 2506 2058 1755 1746 2251 (mAh/ml) Table 2.

A list of examples of materials with a lithiated zinc-blende-type structure where the sum of the non lithium atoms equals 2 in a Li2AB structure and A and B are non-lithium atoms.

Li2AgxBi2-x Li2AgxPb2-x Li2AgxSb2-x Li2AgSn Li2AuBi Li2AuGax Li2AuGe Li2AuxIn2-x <BR> <BR> <BR> Li2AuPb Li2AUxSb2 x Li2AuSn -- <BR> <BR> <BR> <BR> <BR> <BR> Li2CdPb Li2CdSn Li2CuSb Li2CuSn Li2GeSn Li2IrAI Li2IrGa Li2Irln <BR> <BR> <BR> Li2MgXln <BR> <BR> <BR> <BR> <BR> <BR> <BR> Li2MgSi Li2PdSb Li2PdGa Li2Pdln <BR> <BR> <BR> <BR> <BR> <BR> Li2PdPb Li2PdSb Li2PtAI Li2PtGa <BR> <BR> <BR> <BR> <BR> <BR> Li2Ptln Li2PtSb Li2RhAI Li2RhGa Li2Rhln Li2ZnxCd2-X Li8.96Zn3.20Pb3.84 LizZnSb Crystallographic Parameters for Li2CuSn Space Group: F-43m ; a = 6.27 A Atom Wyckoff x y z Occupancy Site (%) Li1 4 (b) 0.5 0.5 0.5 100 Li2 4 (c) 0.25 0.25 0.25 100 Cu 4 (d) 0.75 0.75 0.75 100 Sn 4 (a) 0 0 0 100 Table 3.

Relative Capacities of Sn, CuSn, and Cu6SnS+ =0, +1) Electrode 6 in Theor. Capacity* Theor. Capacity* Theor. Capacity* CU6Sn5+# (mAh/g) to LiSn (mAh/g) to Li7Sn3 (mAh/g) to Li44Sn composition composition composition Sn - 226 527 994 CuSn (+1) 147 343 647 (from Li2CuSn) Cu6Sn4 -1 125 292 551 CU6Sn5 0 137 320 604 Cu6Sn6 +1 147 343 647 *Based on mass of starting electrode Intermetallic electrodes based on the Cu6Sn5and Li2CuSn structure types have been discovered to be useful negative electrodes for lithium-ion cells. The new anodes that operate substantially by a topotactic reaction mechanism may provide a significant improvement over the cycling capability of state-of-the-art binary lithium alloy electrodes.

Cycling stability of these electrodes is however, gained at the expense of the capacity of the binary lithium alloy systems.

While there has been disclosed what is considered to be the preferred embodiment of the present invention, it is understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.