SPECIFIC POSITIVE ELECTRODES COMPRISING A SPECIFIC SALT FOR ACCUMULATOR OF THE ALKALI METAL ION TYPE

Abstract

The invention relates to a positive electrode for alkali metal-ion accumulator comprising at least one organic binder and at least one alkali metal salt meeting the following formula (I):

##STR00001##

wherein the X represent an alkali element.

Claims

1. A positive electrode for alkali metal-ion accumulator comprising at least one organic binder and at least one alkali metal salt meeting the following formula (I): ##STR00008## wherein the X represent an alkali element.

2. A positive electrode according to claim 1, wherein, for the formula (I), the X represent the lithium element, the sodium element or the potassium element.

3. A positive electrode according to claim 1, wherein, for the formula (I), the X represent the lithium element.

4. A positive electrode according to claim 1, wherein the organic binder is a polymeric binder.

5. A positive electrode according to claim 1, further comprising at least one electrode active material.

6. A positive electrode according to claim 5, wherein, when the positive electrode is intended for a lithium accumulator, the electrode active material is selected from: metal chalcogenides of formula LiMQ.sub.2, wherein M is at least one metal element selected from the metal elements, such as Co, Ni, Fe, Mn, Cr, V, Al and Q is a chalcogen; chalcogenides of spinel structure, such as LiMn.sub.2O.sub.4; lithiated or partially lithiated materials of formula M.sub.1M.sub.2(JO.sub.4).sub.fE.sub.1−f, wherein M.sub.1 is lithium, optionally partially substituted with another alkali element up to a substitution level of less than 20%, M.sub.2 is a transition metal element of oxidation level +2 selected from Fe, Mn, Ni and combinations thereof, optionally partially substituted with one or more other additional metal elements of oxidation level(s) between +1 and +5 up to a substitution level of less than 35%, JO.sub.4 is an oxyanion wherein J is selected from P, S, V, Si, Nb, Mo and combinations thereof, E is a fluoride, hydroxide or chloride anion, f is the mole fraction of the oxyanion JO.sub.4 and is, generally, between 0.75 and 1 (including 0.75 and 1).

7. A positive electrode according to claim 1, further comprising at least one electronically conductive additive.

8. A positive electrode according to claim 1, said electrode being in the form of a part comprising a composite material comprising a polymer matrix consisting of one or more polymeric binders and comprising, as charges, at least one alkali metal salt of formula (I) and, optionally, at least one electrode active material and, optionally, one or more electronically conductive additives.

9. A positive electrode according to claim 1, said electrode being in the form of a first portion comprising a composite material comprising a polymer matrix consisting of one or more polymeric binders and comprising, as charges, at least one active material and, optionally, one or more electronically conductive additives and comprising a second portion, in the form of a layer deposited on the surface of the first portion, said layer comprising a polymer matrix consisting of one or more polymeric binders and comprising, as charges, at least one alkali metal salt of formula (I).

10. An alkali metal-ion accumulator cell comprising a positive electrode such as defined according to claim 1, a negative electrode and an electrolyte disposed between the positive electrode and the negative electrode.

11. An alkali metal-ion accumulator cell according to claim 10, wherein the negative electrode comprises, as electrode active material, an active material selected from: carbon materials, such as graphitic carbon capable of intercalating lithium; silicon-based compounds, such as silicon carbide SiC or silicon oxide SiO.sub.x; metallic lithium; lithium alloys; lithiated titanium oxides, such as an oxide of formula Li.sub.(4−x)M.sub.xTi.sub.5O.sub.12 or Li.sub.4M.sub.yTi.sub.(5−y)O.sub.12 wherein x and y range from 0 to 0.2, M represents an element selected from Na, K, Mg, Nb, Al, Ni, Co, Zr, Cr, Mn, Fe, Cu, Zn, Si and Mo; non-lithiated titanium oxides, such as TiO.sub.2; oxides of formula M.sub.yTi.sub.(5−y)O.sub.12 wherein y ranges from 0 to 0.2 and M is an element selected from Na, K, Mg, Nb, Al, Ni, Co, Zr, Cr, Mn, Fe, Cu, Zn, Si and Mo; lithium-germanium alloys; or a mixture thereof.

12. An alkali metal-ion accumulator cell according to claim 10, wherein the electrolyte is a conductive liquid electrolyte of alkali ions comprising at least one organic solvent and at least one metallic salt.

13. A method for treating the accumulator cell such as defined according to claim 10, comprising a step of forming a passivation layer at the surface of the negative electrode with the X ions from the decomposition of the alkali metal salt of formula (I) by applying a first charge to the abovementioned cell.

14. A use of a salt of following formula (I): ##STR00009## wherein the X represent an alkali element as sacrificial salt for the formation of a passivation layer at the surface of a negative electrode of an alkali metal-ion accumulator, which further comprises a positive electrode comprising said salt and an electrolyte disposed between the positive electrode and the negative electrode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0085] FIG. 1 is a graph illustrating the evolution of the voltage U (in V vs Li.sup.+/Li) depending on the specific capacity C (in mAh/g), the curve a) being representative of the battery obtained with the composite electrode comprising the salt of Example 1, the curve b) being representative of the battery obtained with the composite electrode comprising the salt of Comparative Example 1 and the curve c) being representative of the battery obtained with the composite electrode comprising the salt of Comparative Example 2.

[0086] FIG. 2 is a graph illustrating, for a cell obtained with the electrode comprising the salt of Example 1, the evolution of the voltage U (in V vs Li.sup.+/Li) depending on the specific capacity C (in mAh/g) with the curve a) being representative of the electrode comprising the salt of Example 1 during the first charge, the curve b) being representative of the counter-electrode based on Li.sub.4Ti.sub.5O.sub.12 during the first charge and the curve c) being representative of the counter-electrode based on Li.sub.4Ti.sub.5O.sub.12 during the second discharge.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

EXAMPLE 1

[0087] This example illustrates the preparation of a lithium salt that can be used for preparing a positive electrode in accordance with the invention, this lithium salt meeting the following formula:

##STR00005##

[0088] Nitrilotriacetic acid (1 g) is dispersed in 50 mL of ultra-pure water before adding lithium hydroxide (3 equivalents, 0.65 g). After 24 hours of stirring at ambient temperature, the water is evaporated with the aid of a rotary evaporator. The compound quantitatively obtained is subsequently characterized by x-ray diffraction (DRX) and infrared spectroscopy. The purity and the water content of the hydrated structures are determined by elementary and thermogravimetric analysis. The compound is subsequently vacuum desolvated at 250° C. This compound has a theoretical capacity of 384 mAh/g and a measured capacity of 550 mAh/g and a theoretical released gas volume of 2.5 eq. mol.

Comparative Example 1

[0089] This example illustrates the preparation of a lithium salt that can be used for preparing a positive electrode not in accordance with the invention, this lithium salt meeting the following formula:

##STR00006##

[0090] Ethylenediaminetetraacetic acid (1 g) is dispersed in 50 mL of ultra-pure water before adding lithium hydroxide (4 equivalents, 0.57 g). After 24 hours of stirring at ambient temperature, the water is evaporated with the aid of a rotary evaporator. The compound quantitatively obtained is subsequently characterized by x-ray diffraction (DRX) and infrared spectroscopy. The purity and the water content of the hydrated structures are determined by elementary and thermogravimetric analysis. The compound is subsequently vacuum desolvated at 250° C. This compound has a theoretical capacity of 339 mAh/g and a measured capacity of 424 mAh/g and a theoretical released gas volume of 3 eq. mol.

Comparative Example 2

[0091] This example illustrates the preparation of a lithium salt that can be used for preparing a positive electrode not in accordance with the invention, this lithium salt meeting the following formula:

##STR00007##

[0092] Diethylenetriaminepentaacetic acid (1 g) is dispersed in 50 mL of ultra-pure water before adding lithium hydroxide (5 equivalents, 0.53 g). After 24 hours of stirring at ambient temperature, the water is evaporated with the aid of a rotary evaporator. The compound quantitatively obtained is subsequently characterized by x-ray diffraction (DRX) and infrared spectroscopy. The purity and the water content of the hydrated structures are determined by elementary and thermogravimetric analysis. The compound is subsequently vacuum desolvated at 250° C. This compound has a theoretical capacity of 316 mAh/g and a measured capacity of 407 mAh/g and a theoretical released gas volume of 3.3 eq. mol.

EXAMPLE 2

[0093] This example illustrates the preparation of test electrodes comprising salts prepared in the previous examples, these electrodes being intended just to analyze the electrochemical decomposition of said salts in a context of button cell facing a metal lithium electrode or in a “Pouch Cell” configuration facing an electrode comprising Li.sub.4Ti.sub.5O.sub.12. In this regard, they do not comprise electrode active materials in addition to salts, as opposed to what should be for electrodes intended to operate in a real battery context.

[0094] Three types of these electrodes are prepared:

[0095] an electrode comprising the lithium salt of Example 1;

[0096] an electrode comprising the lithium salt of Comparative Example 1; and

[0097] an electrode comprising the lithium salt of Comparative Example 2.

[0098] The protocol for preparing these electrodes is the following.

[0099] The appropriate salt is mixed with the SuperP® carbon black in a mortar then all of this is dispersed within a solution comprising PVDF in NMP (N-methyl-2-pyrrolidone), in order to obtain an ink, the dry extract of which consists of 60% by weight of salt, 30% by weight of carbon black and 10% by weight of PVDF. This ink is spread on an aluminium sheet having a thickness of 100 μm wet and, after drying at 55° C., the deposit obtained is cut into a disk of 14 mm of diameter, calendered at a pressure of 10 tonnes and vacuum dried.

EXAMPLE 3

[0100] In this example, the electrodes prepared in Example 2 are each assembled, facing a metal lithium counter-electrode with a polyolefin separator soaked with an organic electrolyte (solution of LiPF.sub.6 1M in a mixture of ethylene carbonate/propylene carbonate/dimethyl carbonate in 1/1/3 proportions in volume) within a battery of button cell format. The 3 batteries thus formed are subjected to a charge/discharge cycle at a rate of C/20.

[0101] The results of this cycle are reported in FIG. 1, which illustrates the evolution of the voltage U (in V vs Li.sup.+/Li) depending on the specific capacity C (in mAh/g), the curve a) being representative of the battery obtained with the composite electrode comprising the salt of Example 1, the curve b) being representative of the battery obtained with the composite electrode comprising the salt of Comparative Example 1 and the curve c) being representative of the battery obtained with the composite electrode comprising the salt of Comparative Example 2.

[0102] It becomes apparent from this figure, that with the battery obtained with the composite electrode comprising the salt of Example 1, the decomposition took place at approximately 4.1 V vs Li.sup.+/Li, that it is totally irreversible (no reduction phenomenon) and that it corresponds to 550 mAh/g of equivalent of released lithium ions, which is a substantial improvement in relation to the batteries obtained with the salts of Comparative Examples 1 and 2.

EXAMPLE 4

[0103] In this example, an electrode in accordance with Example 2 (the electrode comprising the salt of Example 1) is assembled facing a counter-electrode based on Li.sub.4Ti.sub.5O.sub.12 (LTO) with a polyolefin separator soaked with an organic electrolyte (solution of LiPF.sub.6 1M in a mixture of ethylene carbonate/propylene carbonate/dimethyl carbonate in 1/1/3 proportions in volume) within a single-side cell of the “Pouch Cell” type including a third metal lithium electrode being used as a reference electrode.

[0104] The cell obtained is subjected to a charge/discharge cycle performed at a rate of C/20 by controlling the potential of the electrodes in relation to the reference electrode. The results are reported, in FIG. 2, which illustrates the evolution of the voltage U (in V vs Li.sup.+/Li) depending on the specific capacity C (in mAh/g) with the curve a) being representative of the electrode comprising the salt of Example 1 during the first charge, the curve b) being representative of the counter-electrode based on Li.sub.4Ti.sub.5O.sub.12 during the first charge and the curve c) being representative of the counter-electrode based on Li.sub.4Ti.sub.5O.sub.12 during the first discharge.

[0105] This experiment makes it possible to demonstrate that the lithium released by the decomposition of the salt is indeed available and inserted into the LTO counter-electrode and that it is possible to deinsert it during the discharge.