Specific negative electrode based on lithium and lithium electrochemical generator comprising such a negative electrode

Abstract

A negative electrode for a lithium electrochemical generator, wherein it comprises, as active material, a lithium and calcium alloy, wherein the calcium is present in the alloy to the extent of 2% to 34% of atomic.

Claims

1. A negative electrode for a lithium electrochemical generator, the negative electrode consisting of an active material, the active material comprising a lithium and calcium alloy, the calcium is present in the alloy in an amount of 2% to 34% atomic and wherein the negative electrode is in the form of a sheet or plate having a thickness ranging from 5 to 200 μm.

2. The negative electrode according to claim 1, wherein the lithium and calcium alloy consists of lithium and calcium.

3. The negative electrode according to claim 1, wherein the calcium is present in the alloy in an amount of 2% to 15% atomic.

4. The negative electrode according to claim 1, wherein the lithium and calcium alloy comprises CaLi.sub.2.

5. The negative electrode according to claim 1, which is self-supporting.

6. An electrochemical generator comprising at least one electrochemical cell comprising: a positive electrode comprising an active material comprising a lithium-insertion material, a negative electrode, and an electrolyte conducting lithium ions disposed between said negative electrode and said positive electrode, characterised in that the negative electrode is as defined according to claim 1.

7. The electrochemical generator according to claim 6, wherein the lithium-insertion material is selected from the group consisting of: a lithiated oxide comprising at least one transition metal element selected from the group consisting of: a lithiated oxide of the formula LiMO.sub.2, where M is an element selected from the group consisting of Ni, Co, Mn, Al, Mg and mixtures thereof; and a lithiated oxide with a spinel structure, a lithiated phosphate comprising at least one transition metal element having a formula LiM.sup.1PO.sub.4, where M.sup.1 is selected from the group consisting of Fe, Mn, Co and mixtures thereof, manganese dioxide, and mixtures thereof.

8. The electrochemical generator according to claim 7, wherein the lithiated oxide with a spinel structure is LiMn.sub.2O.sub.4.

9. The electrochemical generator according to claim 7, wherein the lithium-insertion material comprising the lithiated phosphate comprising at least one transition metal element is LiFePO.sub.4.

10. The electrochemical generator according to claim 7, wherein the lithium-insertion material is manganese dioxide.

11. The electrochemical generator according to claim 6, wherein the positive electrode further comprises at least one electron-conducting additive and/or at least one binder.

12. The electrochemical generator according to claim 11, wherein the electron-conducting additive is selected from the group consisting of carbon blacks, acetylene blacks, graphite, carbon fibres, carbon nanotubes, titanium, nickel, aluminium, stainless steel and mixtures thereof.

13. The electrochemical generator according to claim 11, wherein the binder is a polymeric binder.

14. The electrochemical generator according to claim 6, wherein the electrolyte conducting lithium ions is a liquid electrolyte comprising at least one organic solvent and at least one lithium salt.

15. The electrochemical generator according to claim 14, wherein the at least one organic solvent is selected from the group consisting of carbonate solvents, ether solvents, amide solvents, sulfoxide solvents, lactone solvents, lactam solvents, nitrile solvents, ester solvents, sulfite solvents and mixtures thereof.

16. The electrochemical generator according to claim 14, wherein the at least one lithium salt is selected from the group consisting of LiPF.sub.6, LiClO.sub.4, LiBF.sub.4, LiAsF.sub.6, LiSbF.sub.6, LiAlCl.sub.4, LiGaCl.sub.4, LiCF.sub.3SO.sub.3, LiC(SO.sub.2CF.sub.3).sub.3, lithium bis(trifluoromethylsulfonyl)imide, LiN[SO.sub.2CF.sub.3].sub.2, LiSCN, lithium nitrate LiNO.sub.3, lithium bis(oxalato)borate, lithium bis(fluorosulfonyl)imide, LiPF.sub.3(CF.sub.2CF.sub.3).sub.3, lithium trifluoromethanesulfonate LiCF.sub.3SO.sub.3, lithium fluorosulfonate LiSO.sub.3F, LiC.sub.6F.sub.5SO.sub.3, LiO.sub.3SCF.sub.2CF.sub.3, LiO.sub.2CCF.sub.3, LiB(C.sub.6H.sub.5).sub.4 and mixtures thereof.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a graph illustrating the change in the tensile force F (in N) as a function of the elongation L (in mm) for materials illustrated in example 1 below, curve a) relating to that obtained with the material consisting solely of metallic lithium, curve b) relating to that obtained with the material comprising calcium at 2% atomic and curve c) relating to that obtained with the material comprising calcium at 4% atomic.

(2) FIG. 2 is a graph illustrating the change in the voltage U (in V) as a function of the charge Q (in mAh) with curve a) for the electrochemical generator comprising the material comprising calcium at 2% atomic and curve b) for the electrochemical generator comprising the material comprising calcium at 8% atomic of example 1 below.

(3) FIG. 3 is a graph illustrating the change in the potential E (in V) as a function of the test time t (in s) with curve a) for the electrochemical generator comprising the material comprising calcium at 8% atomic and curve b) for the electrochemical generator comprising the material comprising solely metallic lithium of example 1 below.

(4) FIG. 4 is a graph illustrating the change in the charging voltage U (in V) at 0.9 mA (C/10) at 20° C. as a function of the capacity per unit mass C (in mAh/g of LiFePO.sub.4) with curve a) for the first electrochemical generator, curve b) for the second electrochemical generator and curve c) for the third electrical generator of example 2 below.

(5) FIG. 5 is a graph illustrating the change in the charging voltage U (in V) at 100 μA at 20° C. as a function of the capacity per unit mass C (in mAh/g) with curve a) for the first electrochemical generator, curve b) for the second electrochemical generator and curve c) for the third electrochemical generator of example 3 below.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

Example 1

(6) The following example illustrates the synthesis of several materials consisting of a lithium and calcium alloy and the determination of the mechanical properties thereof.

(7) More precisely, a lithium and calcium alloy is prepared comprising calcium at 2% atomic, a lithium and calcium alloy comprising calcium at 4% atomic, a lithium and calcium alloy comprising calcium at 8% atomic and a lithium and calcium alloy comprising calcium at 14.76% atomic.

(8) To do this, metallic lithium and metallic calcium are melted on a heated plate in an argon atmosphere in the required proportions, and, after mixing, quenching of the molten medium obtained is carried out in order to rapidly form a solid material.

(9) The solid materials obtained are analysed by X-ray diffraction, the resulting diagrams illustrating for each of the materials the presence of metallic lithium and the presence of calcium in the form of CaLi.sub.2.

(10) The various solid materials were also able to be rolled in the form of sheets at thicknesses ranging from 20 to 30 μm, such thicknesses not being achievable as easily with solid materials based on metallic lithium alone since they become sticky and easily tear at thicknesses below 100 μm.

(11) Tensile tests at 1 mm/minute were also carried out with test pieces 100 μm thick obtained with the material comprising calcium at 2% atomic and the material comprising calcium at 4% atomic and with, as a reference, a material consisting solely of metallic lithium.

(12) The results are set out in FIG. 1 illustrating the change in the tensile force F (in N) as a function of the elongation L (in mm), curve a) relating to that obtained with the material consisting solely of metallic lithium, curve b) relating to that obtained with the material comprising calcium at 2% atomic and curve c) relating to that obtained with material comprising calcium at 4% atomic.

(13) It is clear that a start of rupture occurs as soon as a force of 2 N is applied for the material consisting solely of metallic lithium while a force of 9 N is necessary for the material comprising calcium at 2% atomic and a force of 12 N for the material comprising 4% atomic in order to obtain this same start of rupture. Thus these results demonstrate that calcium confers a beneficial effect on the mechanical properties for the material when it is included and are consistent with what is observed with the rolling tests.

(14) The functioning of the material comprising calcium at 2% atomic and calcium at 8% atomic, as a negative electrode, was also tested in an electrochemical generator system where it faces a metallic lithium electrode.

(15) The two materials comprise the same quantity of lithium and have respectively a thickness of 100 μm for the material comprising calcium at 2% atomic and a thickness of 160 μm for the material comprising calcium at 8% atomic.

(16) The resulting electrochemical generators are subjected to tests, at 20° C., intended to show the change in the voltage U (in V) as a function of the charge Q (in mAh), the results being set out in FIG. 2 with curve a) for the electrochemical generator comprising the material comprising calcium at 2% atomic and curve b) for the electrochemical generator comprising the material comprising calcium at 8% atomic.

(17) It is clear from these curves that each of these materials is completely discharged on the lithium anode.

(18) To finish, the material comprising 8% calcium was tested in an electrochemical generator facing graphite and cycling it at 200 μA for 3 mAh in comparison with an electrochemical generator comprising solely metallic lithium facing graphite, the curves being set out in FIG. 3 illustrating the change in the potential E (in V) as a function of the test time t (in s) (curve a) for the electrochemical generator comprising the material comprising calcium at 8% atomic and curve b) for the electrochemical generator comprising the material comprising solely metallic lithium).

(19) It is clear from these curves that the lithium and calcium alloy can be used in an electrochemical generator at a reversible active material.

Example 2

(20) The following example illustrates a first electrochemical generator according to the invention in the form of a button battery comprising: a negative electrode consisting of a 14 mm diameter disc and composed of a lithium and calcium alloy to the extent of calcium at 2% atomic, said electrode having a thickness of 100 μm and a surface area of 1.54 cm.sup.2; a positive electrode consisting of a 16 mm diameter disc, said electrode being composed of a layer comprising LiFePO.sub.4 (90.5% by mass), polyvinylidene fluoride (5% by mass) and Super P® carbon black (4.5% by mass), this layer being deposited on an aluminium strip having a thickness of 20 μm and the LiFePO.sub.4 grammage being approximately 30 mg/cm.sup.2, that is to say a battery capacity of approximately 4.5 mAh/cm.sup.2; between the negative electrode and the positive electrode, a Celgard 2400° separator in the form of a disc with a diameter of 16 mm and a surface area of 2 cm.sup.2, said separator being impregnated with an electrolyte consisting of a mixture of carbonate solvents EC:PC:DMC in respective proportions by mass of 1/1/3 and a lithium salt 1 M LiPF.sub.6.

(21) Another electrochemical generator according to the invention (referred to as the second electrochemical generator) was produced, this electrochemical generator meeting the same specificities as those illustrated above, except that the alloy of the negative electrode comprises 15% by mass calcium.

(22) As a comparison, another electrochemical generator not in accordance with the invention (referred to as the third electrochemical generator) was produced in this example, this electrochemical generator meeting the same specificities as those of the electrochemical generators in accordance with the invention except that the negative electrode is made from pure lithium.

(23) For these three electrochemical generators, the change in the charging voltage U (in V) at 0.9 mA (C/10) at 20° C. is determined as a function of the capacity per unit mass C (in mAh/g of LiFePO.sub.4), the results being set out in the accompanying FIG. 4 (curve a) by the first electrochemical generator, curve b) for the second electrochemical generator and curve c) for the third electrochemical generator).

(24) In these curves, it is clear that, whatever the material used for the negative electrode, the overvoltages are equivalent and characteristics of lithium-LiFePO.sub.4 technology. The capacity per unit mass of LiFePO.sub.4 is almost entirely restored. The use of a lithium and calcium alloy for the negative electrode discharging facing a positive electrode comprising LiFePO.sub.4 is therefore possible without impairing performance compared with non-alloyed metallic lithium.

(25) Furthermore, the presence of calcium in the lithium alloy constituting the negative electrode helps to make the alloy easier to roll than pure lithium, which makes it possible to envisage the use of such an alloy to form thinner negative electrodes and thus larger developed surfaces, which may prove a major asset in electrochemical generators intended for power applications.

Example 3

(26) The following example illustrates a first electrochemical generator according to the invention in the form of a button battery comprising: a negative electrode consisting of a 14 mm diameter disc and composed of a lithium and calcium alloy to the extent of calcium at 2% atomic, said electrode having a thickness of 100 μm and a surface area of 1.54 cm.sup.2; a positive electrode consisting of a 16 mm diameter disc, said electrode being composed of a layer comprising MnO.sub.2 (80% by mass), polyvinylidene fluoride (10% by mass) and acetylene black (10% by mass), this layer being deposited on an aluminium strip having a thickness of 20 μm and the manganese dioxide grammage being between 5.6 and 6.2 mg/cm.sup.2, that is to say a battery capacity of between 3.45 and 3.81 mAh; between the negative electrode and the positive electrode, a Celgard 2400® separator in the form of a disc with a diameter of 16 mm and a surface area of 2 cm.sup.2, said separator being impregnated with an electrolyte consisting of a mixture of carbonate solvents EC:PC:DMC in respective proportions by mass of 1/1/3 and a lithium salt 1 M LiPF.sub.6.

(27) Another electrochemical generator according to the invention (referred to as the second electrochemical generator) was produced, this electrochemical generator meeting the same specificities as those illustrated above, except that the alloy of the negative electrode comprises 15% by mass calcium.

(28) By way of comparison, another electrochemical generator not in accordance with the invention (referred to as the third electrochemical generator) was produced in this example, this electrochemical generator meeting the same specificities as those in accordance with the invention except that the negative electrode is made from pure lithium.

(29) For these three electrochemical generators, the change in the discharge voltage U (in V) at 100 μA at 20° C. is determined as a function of the capacity per unit mass C (in mAh/g of MnO.sub.2), the results being set out in the accompanying FIG. 5 (curve a) for the first electrochemical generator, curve b) for the second electrochemical generator and curve c) for the third electrochemical generator).

(30) From these curves, it is clear that, whatever the material used for the negative electrode, the overvoltages are equivalent and characteristic of lithium-MnO.sub.2 technology (between 2.5 and 3 V). The use of a lithium and calcium alloy for the negative electrode discharging facing a positive electrode comprising MnO.sub.2 is therefore possible without impairing the performance compared with non-alloyed metallic lithium.

(31) Furthermore, the presence of calcium in the lithium alloy constituting the negative electrode helps to make the alloy easier to roll than pure lithium, which makes it possible to envisage the use of such an alloy performing thinner negative electrodes and thus larger developed surfaces, which may prove a major asset in electrochemical generators intended for power applications.