BACKGROUND OF THE INVENTION Lithium batteries are prepared from one or more lithium electrochemical cells containing electrochemically active materials. Such cells typically include a pair of electrodes and an electrolyte that electronically separates the electrodes but allows ions to pass through it. The electrolyte typically comprises a lithium salt dissolved in one or more solvents, typically nonaqueous (aprotic) organic solvents. Other electrolytes are solid electrolytes called polymeric matrixes that contain an ionic conductive medium, in combination with a polymer that itself may be ionically conductive which is electrically insulating. By convention, during discharge of the cell, the relatively negative electrode of the cell is defined as the anode. Lithium batteries typically have, for example, cells having a metallic lithium anode and vanadium oxide cathode and are charged in their initial condition.

During discharge, lithium-ions from the metallic anode pass through the electrolyte to the positive electrode material and release electrical energy to an external circuit.

Lithium-ion cells typically contain an intercalation anode as the negative electrode in place of the metallic lithium; and a lithium metal oxide as the positive electrode. Carbon anodes, such as coke and graphite are the intercalation materials typically used. The negative electrode materials are used with lithium-containing intercalation positive electrodes in order to form an electroactive couple in a cell. Such cells are not charged in their initial condition.

In order to deliver electrochemical energy, such cells must be charged in order to transfer lithium ions to the anode from the lithium-containing cathode. During discharge the lithium is transferred from the anode back to the cathode. During a subsequent recharge, the lithium is transferred back to the anode where it re-intercalates. Upon subsequent charge and discharge, the lithium-ions are transported between the electrodes. Such rechargeable batteries, having no free lithium, are also called rocking-chair batteries.

Preferred positive electrode materials include LiCoO2, Li [nln2] 02 and Li [nln2n3] 02 (where nl, n2 and n3 are chosen from Ni, Mn, and Co). Preferred negative electrode materials are carbonaceous materials in various forms, including coke, mesocarbons, glassy carbons and graphitic carbons. These materials suffer from low gravimetric capacity (about 370 mAh/g) and low volumetric capacity (about 1000 mAh/cc).

Alternatives to commercial carbonaceous materials are eagerly sought to overcome the limitations of low gravimetric and volumetric capacity and safety concerns that these materials exhibit, and to match the higher capacities of new emerging negative electrode materials. At present, however, there is no viable commercial alternative to carbon (or carbonaceaous materials). Materials to date that have been examined can be grouped into four main classes which include: (1) active/inactive alloys including Sn, Sb, Al, and Si (J. Dahn et al; M. Thackeray et al); (2) main group oxides (Si, Sn oxides-FujiFilm and others); (3) transition metal oxides (L. Nazar et al. , J-M Tarascon et al); and (4) transition metal nitrides (Takeda et al.;<BR> Shodai et al. , Nazar et al) amongst others. The first class of materials exhibit capacity fading, poor gravimetric capacity and difficult preparation techniques. The second and third class exhibit high polarization, large irreversible capacity and capacity fading. The last class exhibits capacity fading, difficulty of preparation and extreme sensitivity to moisture, in addition to being able to be prepared only in a lithiated form.

Accordingly, there exists a need for alternative materials which overcome the limitations of low gravimetric and volumetric capacity and safety concerns that carbonaceous materials exhibit and also to obviate and mitigate at least some of the above presented disadvantages of materials that have been examined.

SUMMARY OF THE INVENTION An electrode comprising metal phosphides for reversible uptake of lithium at low potential in a rechargeable electrochemical cell.

BRIEF DESCRIPTION OF THE DRAWINGS These and other features of the preferred embodiments of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein:

Figure 1. is a sectional view of an electrochemical cell.

Figure 2. shows the results of an X-ray diffraction analysis or a first electrode material showing results obtained, a) before the uptake of lithium, in its pristine state ; b) after uptake of lithium to full discharge; and c) after the extraction of lithium in the voltage window specified.

Figure 3. is a plot obtained from an EVS (Electrochemical Voltage Spectroscopy) voltage/capacity profile for a cell of a first embodiment.

Figure 4. shows the dependence of the discharge capacity, or uptake of lithium in the active material as a function of rate dependence.

Figure 5. is a voltage/capacity profile for a cell embodying a second electrode material metal phosphide.

Figure 6. shows the results of an X-ray diffraction analysis on the second electrode material, a) before the uptake of lithium, in its pristine state ; b) after uptake of lithium to full discharge; and c) after the extraction of lithium in the voltage window specified.

Referring to Figure 1, an electrochemical cell or battery has a negative electrode side 12, a positive electrode side 14, an electrolyte/separator 16 there in between ; all contained in a sealed holder, 18. The negative electrode is the anode during discharge, and the positive electrode is the cathode during discharge. The negative electrode is defined as the material of lower redox potential, and the positive electrode is defined as the material with the higher redox potential. The negative electrode side 12 includes current collector 20, typically of nickel, iron, stainless steel and/or copper foil, and negative electrode material 22. The positive electrode side includes current collector 24, typically of aluminum, nickel, and/or stainless steel and such foils may have a protective conducting coating foil, and a positive electrode active material 26. The electrolyte/separator 16 is typically a solid electrolyte, or a separator and liquid electrolyte. Solid electrolytes typically referred to as polymer electrolytes gel polymeric matrixes which contain an ionic conductive medium. Liquid electrolytes typically comprise a solvent and an alkali metal salt which form an ionically

conducting liquid. In the latter case, the separation between the anode and cathode is maintained, for example, by a relatively inert layer of material such as glass fiber. Any lithium ion containing conducting electrolyte may be used that is stable up to 4.5 volts or more. Any method may be used to maintain the positive and negative electrodes spaced apart and electrically insulated from one another in the cell and essentially any counter electrode 26 can be used that contains lithium such as as lithium metal oxide, lithium metal phosphate or lithium metal silicate. In a typical lithium-ion cell configuration, the negative electrode material 22 has a lower redox potential than the positive electrode material 26.

The negative electrode side 12 is made by mixing a binder, an active material, and carbon powder (particles of carbon) to form the material 22 as a paste. The binder is preferably a polymer. The paste containing the binder, active material and carbon is coated onto a current collector 20.

The electrode 22 comprises a metal phosphide distributed within a carrier, typically and binder and a carbonaceous material such as coke, graphite or acetylene black to act as a conductive additive.

The metal phosphide of electrode 22 may comprise main group phosphides or transition metal phosphides or both main group and transition metal elements. Preferably, the metal phosphides are phosphides of transition metal elements selected from the group comprising Iron, Cobalt, Nickel, Copper, Manganese, Tungsten, Gold, Silver, Molybdinium and Chromium.

More preferably the transition metal elements are selected from the group iron, cobalt and manganese.

The electrode 22 may also include main group elements, preferably selected from the group comprising Sn, In, Bi, Sb and Pb Ge and Si. These may be present as a second phase with amounts preferably less than 1%, more preferably less than or equal to 0.5%.

Alternatively, it can be a substitutional element of the metal phosphide which forms a single phase crystallographic phase.

It is also preferred that the metal phosphide is greater than 50% by weight of the electrode 22, more preferably greater than or equal to 60% by weight and still more preferably greater than or equal to 75% by weight. Examples of the manufacture of electrodes 22 and their performance is such further is further detailed below.

Example 1 A preferred procedure for forming an electrode from an FeP2 compound active material comprised conducting a reaction between iron and red phosphorus, within a sealed quartz tube using metallic Sn as a flux in the reaction. The constituents Fe, P, and Sn were mixed in the stoichiometric ratio 1: 10: 40 and loaded in a quartz tube and sealed under vacuum. The tube and its contents were heated in a tube furnace at 700°C for one week. The tube was opened and washed with 3 portions of 6 molar HC1 of 100 ml for a total starting Fe mass of lg. The HC1 was used to remove the Sn, and the exact amount is not critical. The final product consisted of black air-stable crystals, and their X-ray diffraction pattern contained all of the peaks expected for this material as shown in Figure 2a.

The x-ray diffraction was conducted using Cu Ka radiation, X = 1. 54178A. The pattern evident in the lower trace is consistent with a single phosphide compound, FeP2. This is evidenced by the position of the peaks in terms of the scattering angle 20, x axis. The X- ray pattern showed no peaks due to the presence of other phosphides, or metallic iron, indicating the reaction is essentially complete. A small amount of residual Sn was apparent in the diffraction pattern upon very close examination, which was determined to be less than 0.5% by elemental analysis. Chemical analysis for P and Fe by ICP (inductively coupled plasma) showed an elemental ratio of 1.9, close to the expected result of 2.0.

The chemical analysis and X-ray pattern confirmed that the product was indeed the nominal general formula FeP2, corresponding to the more generic nominal formula MxM'zPy.

The term minimal general formula refers to the fact that the relative proportion of atomic species may vary; especially if the material was prepared by deposition techniques as a thin film or as an amorphous material, and that varying amounts of Sn can be incorporated in the material either as a second phase or as a solid solution.

The FeP2, prepared as described above, was tested in an electrochemical cell with a lithium counter electrode 26. The electrolyte 16 was ethylene carbonate (EC) and dimethylcarbonate (DMC) in a weight ratio of 1: 1 and including a 1 molar concentration of LiPF6 salt. In the examples that follow, the material described herein was tested in the configuration using lithium metal as the counter electrode, to fully describe its properties for lithium insertion at low potential. This embodiment of the cell uses the material described in the invention as the positive electrode, with the counter electrode being comprised of lithium and hence acting as the negative electrode.

The cell was cycled at constant current density between about 0.2 and 2. 0V and the performance monitored as shown in Figure 3. The charging conditions are potentiodynamic control using a voltage step of 5mV in a voltage window 0.1-2. 9V. The equivalent current density based on the equilibrium current is 31 mA/g or 0.35 mA/cm2, corresponding to a rate of C/4, where for C/n, n refers to the number of hours for the uptake or extraction of one lithium. As this active material is tested with a counter electrode 16 of lithium, that is in this cell the active material of electrode 22 functions as the cathode in the cell for the purpose of testing, here discharge refers to the process of lithium uptake within the FeP2 electrode 22 and charge refers to the extraction of lithium from FeP2.

The voltage profile of the cell clearly shows and highlights the high degree of uptake of lithium by the active material, FeP2, and the unexpected degree of reversibility of the material. The negative electrode contained about 8.8 milligrams of FeP2 material, The total electrode weight including the binder and conductive carbon diluent was about 11. 8 milligrams. The negative electrode showed a performance of about 1300 mAh/g on the first discharge, corresponding to the uptake of about 6 Li, or a volumetric capacity of 6630 mAh/cc. On charge, full extraction of lithium was observed, indicating the reaction is substantially reversible.

Further x-ray diffraction analysis was conducted on the metal phosphide material FePa after the insertion of 6 lithium to a potential of 0. 1V. As shown in Figure 2b the reflections of FeP2 completely disappear on lithium insertion. The pattern contains, in additions to the reflections of the aluminum holder, only weak, broad reflections attributable to Li3P. On extraction of lithium, no reflections are visible (Figure 2c), indicating the material becomes amorphous.

Figure 4 to show the rate dependence of the discharge capacity under conditions of constant current cycling at various current densities (corresponding to the C/n rates shown on the Figure). The results show that there is little dependence of the capacity on the current density, showing the reaction is not substantially kinetically limited.

Example 2 In a further embodiment, an electrode using Cobalt Phosphide was prepared.

A preferred procedure for forming the CoP3 compound active material is described.

The basic procedure comprised conducting a reaction between a metal compound, preferably Co and red phosphorus, within a sealed tube. The synthesis of the cobalt phosphide was performed by heating stoichiometric amounts of metallic cobalt and red phosphorus (1: 3 ratio) at 650°C for 24h, in a stainless steel tube sealed under argon. The tube was opened and the product was removed. The final product consisted of black air-stable crystals, and their X-ray diffraction pattern contained all of the peaks expected for this material as shown in Figure 5a. The x-ray diffraction was conducted using Cu Ka radiation, X-1. 54178Å. The pattern evident in the lower trace is consistent with a single phosphide compound, CoP3.

This is evidenced by the position of the peaks in terms of the scattering angle 20, x axis. The X-ray pattern showed no peaks due to the presence of other cobalt phosphides, or metallic cobalt, indicating the reaction is essentially complete.

The X-ray pattern demonstrate that the product was indeed the nominal general formula CoP3, corresponding to the more generic nominal formula MxM'zPy. The term minimal general formula refers to the fact that the relative proportion of atomic species may vary; especially if the material was prepared by deposition techniques as a thin film or as an amorphous material.

The CoP3, prepared as described above, was tested in an electrochemical cell with a lithium counter electrode. The electrolyte was ethylene carbonate (EC) and dimethylcarbonate (DMC) in a weight ratio of 1: 1 and including a 1 molar concentration of LiPF6 salt. The cell was cycled at constant current density between about 0.2 and 2. 0V with performance as shown in Figure 6. The charging conditions are potentiodynamic control using a voltage step of 5mV in the voltage window 0.1 to 2.9V, corresponding to a current density of 31 mA/g or 0.35 mA/cm2, corresponding to a rate of C/4 where for C/n, n refers to

the time taken for one complete charge or discharge of one lithium. As this active material is tested with a counter electrode 26 of lithium, that is in this cell the electrode 22 functions as the cathode in the cell for the purpose of testing, here discharge refers to the process of lithium uptake within the CoP3, and charge refers to the extraction of lithium from CoP3.

The voltage profile of the cell clearly shows and highlights the high degree of uptake of lithium by the active material, CoP3, and the good reversibility of the material of the invention. The negative electrode contained about 6.0 milligrams of CoP3 material. The total electrode weight including the binder and conductive carbon diluent was about 10.0 milligrams, i. e. 60% by weight of the electrode 22. The negative electrode showed a performance of about 1500 mAh/g on the first discharge, corresponding to the uptake of about 9 Li (after the carbon contribution was removed). On charge, extraction of lithium was observed corresponding to the removal of 6 Li (1000 mAh/g), indicating the reaction is substantially reversible.

Figure 5b shows the diffraction pattern of the active material, CoP3 after the insertion of 9 lithium to a potential of 0. 1 V. The reflections of CoP3 completely disappear on lithium insertion. The pattern contains, in additions to the reflections of the aluminium holder, only weak, broad reflections attributable to Li3P. On extraction of lithium, no reflections are visible (Figure 5c), although TEM images coupled with selected area electron diffraction patterns and XPS measurements reveal that nanoparticles of LiP are formed on charge.

Electrodes have also been prepared using manganese phosphide which have exhibited similar properties to those described above. The active material was present in the order of 75%. The form of manganese phosphate used was MnP4, in particular 6-MnP4 and 4-Mnp4.

Other forms of manganese phosphide may be used such as 2-MnP4.

The above examples have been obtained using a single metal phosphide. However, metal phosphides using a pair of different metals may also be utilized to provide a metal phosphide of the general form MXM'zPy. Typically, one of the pair of metals may for example be tin and the other iron. The ratio of the iron would be 1: 1 although the ratios may usefully vary between 0. 5 : 1 and 2: 1.