Patent Application
20150125762
The present invention relates to a lithium electrochemical storage battery of the lithium-air type comprising, within a cell, an original association between a positive electrode material and a negative electrode material, this association having the consequence of resulting in a more secure storage battery and the reactions of which at the electrodes are easily reversible.
The field of the invention may thus be defined as that of energy storage devices, in particular that of electrochemical storage batteries of the lithium-air type.
Energy storage devices are conventionally electrochemical storage batteries operating on the principle of electrochemical cells able to deliver an electric current by the presence in each of them of a pair of electrodes (a positive electrode and a negative electrode, respectively) separated by an electrolyte, the electrodes comprising specific materials able to react according to an oxidation-reduction reaction, in return for which there is production of electrons at the origin of the electric current and production of ions which will circulate from one electrode to the other through an electrolyte.
Devices of this type may be lithium-air storage batteries, which conventionally at each basic electrochemical cell consists of a negative electrode formed with a material based on lithium, which may either be lithium metal or an alloy based on lithium, as specified in FR 2 941 091, and a positive electrode of the air electrode type separated by an electrolyte conducting lithium ions.
The operation of an electrochemical cell of an air-lithium storage battery, is more specifically based on reduction of the oxygen at the positive electrode by the Li+ ions present in the electrolyte and stemming from the negative electrode and on oxidation of the lithium metal at the negative electrode during the discharge process, the reactions occurring at the electrodes may be symbolized by the following electrochemical equations:
2Li++2e−+O2 (g)→Li2O2 (s) (2.91 V vs. Li+/Li)
2Li++2e−+(½)O2(g)→Li2O(s) (3.10 V vs. Li+/Li)
Li(s)→Li++e−
The main obstacles of lithium-air technology are the following:
Indeed, as regards the safety of the storage battery, the latter essentially results from the use of lithium metal or of a lithium alloy at the negative electrode, which, during the discharge process, migrates and reacts with oxygen in order to form lithium peroxide and, during the charging process, may be at the origin of the creation of lithium dendrites.
These lithium dendrites generate the following drawbacks:
As regards the reversibility of the electrochemical reactions at the electrodes, it should be noted that the discharged products, such as Li2O2 or Li2O, which are insoluble are led to being deposited in the porosity of the air electrode, compromising reversibility of the reactions and consequently, the cycling strength of the storage battery.
Several solutions have been proposed for improving safety and the cyclability of storage batteries, notably:
The inventors of the present invention set the goal of proposing a novel architecture of lithium-air storage batteries comprising an original association between a specific positive electrode and a specific negative electrode, which have a safety aspect and for which the reactions at the electrodes are easily reversible.
Thus, the invention relates to a lithium-air storage battery comprising at least one electrochemical cell comprising:
By means of the different constitutive elements of said cell, the following are obtained:
Before going into more details in the discussion of this invention, we specify the following definitions.
In the foregoing and in the following, it is specified that the voltages or potentials are expressed relatively to the reference pair Li+/Li. This pair has an oxidation-reduction potential of −3.02V relatively to the normal hydrogen electrode (NHE).
By positive electrode, from the foregoing and from the following, is conventionally meant the electrode which acts as a cathode, when the storage battery produces current (i.e. when it is in a discharge process) and which acts as an anode when the accumulator is in a charging process.
By negative electrode in the foregoing and in the following, is conventionally meant that the electrode which acts as an anode, when the storage battery produces current (i.e. when it is in a discharge process) and which acts as a cathode, when the storage battery is in a charging process.
As mentioned earlier, the negative electrode of the storage battery of the invention is an air electrode, which is conventionally used in storage batteries from the prior art as a positive electrode and not as a negative electrode as this is the case of the invention.
At this air electrode, the oxygen is reduced during the charging of the cell according to the following electrochemical equations:
2Li++2e−+O2 (g)→Li2O2 (s)
2Li++2e−+(½)O2(g)→Li2O(s)
The air electrode is intended to be in direct contact with air, in order to allow reduction of oxygen and therefore should conventionally have catalytic sites and allow the exchange of electrons, which is expressed by the following properties:
From a structural point of view, an air electrode capable of entering the structure of a storage battery according to the invention may comprise:
The electron conducting material may preferably be a carbonaceous material, i.e. a material comprising carbon in the elementary state.
As a carbonaceous material mention may be made of:
The electron conducting material may also be an electron conducting ceramic belonging to the families of transition element nitrides, such as TiN, carbides of transition element(s) and/or of metalloid element(s), such as TiC, SiC, carbonitrides of transition element(s), such as TiCN, simple oxides of transition element(s), such as TiO and ZnO.
It is not excluded that the electron conducting material may both contain a carbonaceous material as mentioned above and an electron conducting ceramic, as mentioned above.
The aforementioned catalyst from a functional point of view is a catalyst able to accelerate the electrochemical reactions occurring at the air electrode (whether during a discharging or charging process) and also able to increase the operational voltage, at which these electrochemical reactions occur.
A catalyst fitting these specificities may be:
Preferably, the catalyst used according to the invention is a mixed or simple manganese oxide, a mixed or simple iron oxide, a mixed or simple iron oxide or mixtures thereof.
In order to ensure cohesion between the electron conducting material and the catalyst, the negative electrode may comprise one or several binders, in particular one or several polymeric binders.
Among the polymeric binders which may be used, mention may be made of:
Preferably, the binder used is a binder based on a fluorinated polymer, such as a polytetrafluoroethylene, a polyvinylidene fluoride and mixtures thereof, this type of binder giving the possibility of obtaining a good percolating lattice.
In the negative electrode, the electron conducting material as defined above may be present in a proportion ranging from 40 to 97% by mass based on the total mass of the mixture comprising said material and said binder(s) (which as a counterpart means that the binder(s) may be present in a proportion ranging from 3 to 60% by mass based on the total mass of the aforementioned mixture).
In addition to the presence of at least one electron conducting material, of at least one catalyst and optionally at least one binder, the negative electrode may also comprise a support, intended, as indicated by its name, for supporting the aforementioned ingredients, this support may further contribute to ensuring good mechanical strength of the electrode and good electron conduction and allow diffusion of gases, in particular oxygen. This electrode may thus be described as a supported electrode.
This support may appear as a foam, a grid or further a fibrous support and may be in a material comprising a metal or a metal alloy or further in a carbonaceous material.
Most particularly, this may be a carbon support, a titanium support, a palladium support, a copper support, a gold support, an aluminum support, a nickel support or a stainless steel support.
According to a particular embodiment of the invention, the negative electrode comprises a grid or foam in nickel used as a support for a composition comprising carbon black (acting as an electron conducting material), PVDF (acting as a binder) and manganese oxide (acting as a catalyst), the manganese oxide may appear in the form of nanowires.
As mentioned above, the positive electrode comprises a material for lithium insertion, the discharge voltage of which is greater than 4.5V expressed relatively to the Li+/Li pair.
A material meeting this specificity may be a material of the lithiated material type with a spinel structure, this material being known under the name of <<5V spinel>>.
More specifically, this may be a material fitting the following characteristics:
In addition to the presence of a material for lithium insertion, the positive electrode may comprise:
The electron conducting material may preferably be a carbonaceous material, i.e. a material comprising carbon in the elementary state.
As a carbonaceous material mention may be made of carbon black.
The binder may preferably be a polymeric binder.
Among the polymeric binders which may be used, mention may be made of:
Electron conducting fibers when they are present, may further participate in the good mechanical strength of the positive electrode and are selected for this purpose so as to have a very large Young modulus. Fibers adapted to this specificity may be carbon fibers, such as carbon fibers of the Tenax® or VGCF-H® type. Tenax® carbon fibers contribute to improving the mechanical properties and have good electric conductivity. VGCF-H® carbon fibers are steam synthesized fibers and contribute to improving the thermal and electric properties, the dispersion and the homogeneity.
In addition to the presence of at least one material for lithium insertion, of at least one electron conducting material, of at least one binder and optionally of electron conducting fibers, the positive electrode may also comprise a support, intended, as indicated by its name, for supporting the aforementioned ingredients, this support may further ensure good mechanical strength of the electrode and good electron conduction. The electrode may thus be described as a supported electrode.
This support may appear as a foam, a grid or further a fibrous support and may be in a material comprising a metal or a metal alloy or further in a carbonaceous material.
Most particularly, this may be a carbon support, a titanium support, a palladium support, a copper support, a gold support, an aluminum support, a nickel support or a stainless steel support.
According to a particular embodiment of the invention, the positive electrode comprises an aluminum grid acting as a support, on which is deposited a composition comprising a material of formula LiMn1.5Ni0.5O4 (acting as a material for lithium insertion), carbon black (acting as an electron conducting material), PVDF (acting as a binder) and carbon fibers (acting as electron conducting fibers).
The electrolyte intended for entering the structure of the storage batteries of the invention is a lithium ion conducting organic electrolyte positioned between said negative electrode and said positive electrode, said electrolyte not degrading, when it is subject to a voltage ranging from 3V to 5.5V expressed relatively to the Li+/Li pair, which means that it retains its properties intact after having been subject to such a voltage.
An electrolyte fitting this specificity may be an electrolyte comprising:
As examples, the lithium salt may be selected from the group formed by LiPF6, LiClO4, LiBF4, LiAsF6, LiCF3SO3, LiN (CF3SO2)3, LiN(C2F5SO2), lithium bis(trifluoromethylsulfonyl)imide (known under the acronym of LiTFSI) LiN[SO2CF3]2, lithium bis(oxalato)borate (known under the acronym of LIBOB), lithium bis(fluorosulfonyl)imide (known under the acronym of LiFSI), LiPF3(CF2CF3)3 (known under the acronym of LiFAP), lithium trifluoromethanesulfonate (known under the acronym of LiTf), lithium bis-trifluoromethanesulfonylimide (known under the acronym of Lilm) and mixtures thereof.
The lithium salt may be comprised in the electrolyte, in an amount from 0.3M to 2M.
As an organic solvent belonging to the family of carbonate solvents, mention may be made of ethylene carbonate (known under the acronym of EC), propylene carbonate (known under the acronym of PC), dimethyl carbonate (known under the acronym of DMC), diethyl carbonate (known under the acronym of DEC) and mixtures thereof.
As an organic solvent belonging to the family of lactone solvents, mention may be made of γ-butyrolactone, γ-valerolactone, δ-valerolactone, ε-caprolactone, γ-caprolactone.
As an organic solvent belonging to the family of sulfone solvents, mention may be made of ethylmethylsulfone (known under the acronym of EMS), trimethylenesulfone (known under the acronym of TriMS), 1-methyltrimethylenesulfone (known under the acronym of MTS), ethyl-sec-butylsulfone (known under the acronym of EiBS), ethyl-iso-propylsulfone (known under the acronym of EiPS) and also 3,3,3-trifluoropropylmethylsulfone (known under the acronym of FPMS).
The solvent may be used as a single solvent or as a mixture of distinct solvents which may thereby form a binary solvent or a ternary solvent.
For example, this may be simply EC, a binary solvent EC/EMC (1:1) or a ternary solvent which may comprise three solvents in proportions of (1:1:1) to (1:8:1) or (8:1:1) or further (1:1:8), a specific example being the ternary solvent EC/PC/DMC (1:1:3).
As a stabilization additive, mention may be made, when the latter is a phosphate compound, of tris(hexafluoroisopropyl)phosphate (known under the acronym of HFiP).
As a stabilization additive, mention may be made, when the latter is an anhydride compound, of ethanoic anhydride, propanoic anhydride, benzoic anhydride, butanoic anhydride, cis-butenedioic anhydride, butane-1,4-dicarboxylic anhydride, pentane-1,5-dicarboxylic anhydride, hexane-1,6-dicarboxylic anhydride, 2,2-dimethylbutane-1,4-dicarboxylic anhydride, 2,2-dimethylpentane-1,5-dicarboxylic anhydride, 4-bromophthalic anhydride, 4-chloroformylphthalic anhydride, phthalic anhydride, benzoglutaric anhydride and mixtures thereof.
This additive may be present in the electrolyte in an amount from 0.01% to less than 30% by mass based on the total mass of the electrolyte.
The aforementioned liquid electrolyte may be led, in the electrochemical cells of the storage batteries of the invention, to impregnate a separator, which is positioned between the positive electrode and the negative electrode of the electrochemical cell.
This separator may be in a porous material able to receive the liquid electrolyte in its porosity.
This separator may consist in a membrane in a material selected from glass fiber, a polymeric material (such as polyethylene, polypropylene or a mixture of both of them).
The electrolyte may also be an ionic liquid.
The electrolyte may also consist in a solid electrolyte, for example a ceramic membrane conducting lithium ions, conventionally called LISICON (corresponding to Lithium Super Ionic Conductor), this ceramic membrane may be of the perovskite type, such as (La,Li)TiO3 (known under the acronym of LLTO), of the garnet type, such as Li5La3Ta2O12, Li6La3Zr2O11.5, of the phosphate type, such as Li1+xAlxGe2−x(PO4)3 with 0<x<0.8 (known under the acronym of LAGP) and Li1+xTi2−xAlx(PO4)3 with 0.25<x<0.3/Li1+x−yTi2−xAlxSiy (PO4)3−y with 0.2<x<0.25 and 0<y<0.05 (known under the acronym of LTAP), this membrane being particularly stable in the presence of a lithium insertion material of the positive electrode, while it is not stable in the presence of lithium metal.
The storage battery of the invention may be included in a sealed enclosure supplied with oxygen for its operation.
A storage battery specific to the invention is a storage battery comprising at least one electrochemical cell comprising:
The storage batteries of the invention may be made by conventional techniques within the reach of one skilled in the art, for example by stacks of various constitutive elements of the storage battery (i.e., a negative electrode, a positive electrode and a separator), this stack being maintained in a casing.
The positive electrode and the negative electrode may be prepared beforehand, before their incorporation into the storage battery; this preparation may consist for each of these electrodes in the succession of the following steps:
More specifically, the preparation of an accumulator according to the invention may comprise:
The invention will now be described, with reference to the following examples, given as an indication but not as a limitation.
The following example illustrates the preparation of a lithium-air storage battery including a negative air electrode (anode) and a positive electrode including a material with a high potential (which is expressed relatively to the Li+/Li pair) and a specific electrolyte.
The preparation of this storage battery comprises:
a) Manufacturing the Negative Electrode
0.85 g of Super C65 carbon are mixed with 24.5 g of N-methyl-2-pyrrolidone (known under the acronym of NMP), 0.426 g of polyvinylidene fluoride (known under the acronym of PVDF) and 0.14 g of manganese oxide nanowires in the a phase, in return for which an ink is obtained. This ink is then coated on a nickel grid with a height of 200 μm. The assembly resulting from this coating is dried in an oven at 60° C. for 24 hours. A negative electrode is made by means of a cylindrical punch with a diameter of 14 mm. Once it is obtained, this electrode is then dried in vacuo and at 80° C. for 48 hours.
b) Manufacturing the Positive Electrode 0.476 g of LiMn1.5Ni0.5O4, 0.016 g of Super C65 carbon, 0.016 g of carbon fibers and 0.027 g of polyvinylidene fluoride are mixed into 2.5 g of N-methyl-2-pyrrolidone (known under the acronym of NMP) for 20 minutes, in return for which an ink is obtained. This ink is then coated on an aluminum grid and then the resulting assembly is dried in an oven at 60° C. for 24 hours. An electrode is made by means of a cylindrical punch with a diameter of 14 mm. Once it is obtained, this electrode is then dried in vacuo and at 80° C. for 48 hours. The capacity of the obtained electrode is 0.83 mAh.
c) Manufacturing the Electrolyte
The electrolyte is prepared by mixing in a glove box 100 mL of ethylene carbonate, 100 mL of propylene carbonate and 300 mL of dimethyl carbonate, to which are added 151.9 g of LiPF6. The mixture is homogenized by stirring for 72 hours.
d) Assembling the Cell
The storage battery was assembled in a glove box filled with argon comprising an oxygen and water proportion of less than 0.1 ppm.
The storage battery was made as illustrated in
a positive electrode 7;
The electrolyte impregnates both aforementioned discs in an amount of 150 μL.
The resulting storage battery is subject to a cycling test.
To do this, the storage battery was introduced into a sealed glass enclosure, in which a flow of pure oxygen at a pressure of 1 atmosphere allows good exposure of the storage battery to oxygen.
The positive and negative terminals of the storage battery are then connected to a battery test system (potentiostat, galvanostat). The storage battery was subject to a charging cycle at C/20 (0.041 mA) up to 2.15 V as illustrated in the appended
Example 2 again uses the same conditions as Example 1 to within a few exceptions.
In order to make the air anode, 0.300 g of Super C65 carbon, 9.8 g of N-methyl-2-pyrrolidone, 0.150 g of polyvinylidene fluoride and 0.10 g of manganese oxide nanowires in the alpha phase are mixed for 15 minutes.
The positive electrode, as for it, is manufactured under conditions similar to those of Example 1, except for the respective amounts of the reagents, which are the following: 0.531 g of LiMn1.5Ni0.5O4, 0.018 g of Super C65 carbon, 0.018 g of carbon fibers and 0.030 g of polyvinylidene fluoride.
The capacity of the electrode is 1.1511 mAh.
The electrolyte and the assembly of the cell are similar in all points to the Example 1 mentioned above.
The cell was subject to 4 charging/discharging cycles between 1.6 V and 0.56 V at C/20 (0.057 mA) as illustrated in the appended
| Number | Date | Country | Kind |
|---|---|---|---|
| 12 53791 | Apr 2012 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/058354 | 4/23/2013 | WO | 00 |
