Patent Application
20170012293
This invention is related to electrochemical current sources, namely the metal-air ones, and in particular to lithium-air current sources and their electrodes, and can be used for the development of various storage capacitors, for example, batteries.
Metal-air current sources usually include an anode made of an active metal, and air permeable, or rather molecular oxygen permeable cathode, separated by electrolyte, containing ions of metal from which the anode is made. The cathode is made in the form of a porous electrically conducting structure with a highly developed surface, made, as a rule, of carbon materials, on whose surface occur reactions of reduction and evolution of molecular oxygen from metal oxide (peroxide) in the process of charge-discharge of a current source.
In particular, during application of lithium as the anode metal in the so-called lithium-air current sources, the occurring electrochemical processes are described as follows.
During the discharge of the lithium-air current source, lithium oxidation takes place on the anode from which the lithium ions transit to an electrolyte, while electrolytic reduction of molecular oxygen, coming from the atmospheric environment through the porous cathode to the cathode-electrolyte boundary, occurs on the cathode. Electrochemical reactions, occurring in such system during the discharge, are described as follows:
Li−e=Li+, on anode:
O2+4Li++4e=2Li2O or 2Li++O2+2e=Li2O2. on cathode:
During the charging of this current source, oxygen contained in lithium oxide oxidizes on the cathode to molecular oxygen and returns back into the atmosphere. Lithium ions are reduced to metallic lithium on the anode. Electrochemical reactions, occurring according to this system during the charging, are described as follows:
2Li2O−4e=4Li+ or Li2O2−2e=2Li++O2. on cathode:
Li++e=Li. on anode—
Lithium-air current sources are unique by their characteristics, because cathode active material is not stored in the source, but comes from the atmospheric environment. The lithium-air current source has the open circuit voltage (Voc) of about 2.91 V, and its theoretical calculated specific energy is 11140 W·hr/kg [K. M. Abraham. A Brief History of Nonaqueous Metal-Air Batteries//ECS Transactions, 3 (42) 67-71 (2008)]. Such current sources can be used, for example, in the manufacturing of batteries for vehicles where rechargeable current sources are required, having the service life of at least 1,000 cycles of charge-discharge, and the value of specific power of at least 400 W/kg.
Various metal-air current sources are known. Thus, in U.S. Pat. No. 5,510,209 a metal-air current source (battery) is described which includes a metallic anode, a composite carbon cathode, and an electrolyte with high ion conductivity, located in the form of polymer film between anode and cathode, on which the processes of reduction and evolution of molecular oxygen during charging and discharging take place, respectively. As the anode metal, the application of such metals as lithium, magnesium, sodium, calcium, aluminum, and zinc were suggested. This current source has a sufficiently high value of specific energy of about 3,500 W·hr/kg (in relation to cathode weight), yet it has a low value of discharge current density, approximately from 0.1 to 0.25 mA/cm2, i.e. it has very low specific power.
The above features are stipulated by the low rate of electrochemical reactions occurring on the anode due to the high energy of activation of these processes. Accordingly, a considerable number of inventions, known from the prior art, are related to various improvements of cathode which are supposed to have the required effect on electrochemical properties of these current sources.
In particular, in order to increase the rate of said reactions, thus increasing the specific power of metal-air current sources on the cathode surface where the reduction of molecular oxygen and the evolution of molecular oxygen from metal oxide (peroxide) in the process of the current source charge-discharge takes place, a catalyst is applied by some manner or another.
Thus, a cathode for a lithium-air current source is known from the application for invention KR 20140056544, said cathode consisting of manganese dioxide with an addition of nanoparticles of noble metals (platinum, palladium, ruthenium, iridium, and gold), applied to a nickel net. However, the use of noble metals in the composition of cathode material results in significantly higher cost of the electrode and the current source in which it is used.
Thus, a cathode is known for a lithium-air current source, described in the application for invention WO 2013174770, consisting of two layers: a layer, contacting the ambient air and containing catalysts of oxygen electroreduction (manganese, silver, platinum), and a layer contacting the electrolyte which contains catalysts (cobalt or nickel oxides) of electroreduction of oxygen compounds of lithium. Such two-layer structures are usually characterized by high electric resistance on the layer interface which results in an increase in the internal resistance of the current source, and deteriorates its electric characteristics.
Various metal-air current sources are known where cathodes containing catalysts are also described.
Thus, U.S. Pat. No. 7,087,341 describes a metal-air current source containing an anode and a cathode. In addition, the cathode includes a gas diffusion layer, a current collector, and a layer with a catalyst containing carbon particles whose average size does not exceed 10 μm, and catalyst particles. As a catalyst, manganese oxide, cobalt oxide, and nickel oxide were suggested. During the testing of a laboratory electrochemical cell, imitating such a current source, in particular, with a mixture of nickel oxide Ni(II) and cobalt oxide Co(M), the following values were obtained: specific power—35 W/kg, specific energy—80 W·hr/kg. The number of charge-discharge cycles did not exceed 30. It is obvious that this catalyst does not provide the required high operating characteristics of a current source.
A lithium-air current source is known as described in the patent CN 102240574, which consists of a lithium anode, carbon cathode containing catalysts of oxygen reaction, separator and organic electrolyte. As a catalyst, on the cathode complexes of cobalt and manganese with pyridine, 4,4″-bipyridine, pyrazine, and pyrrole are used. Monomer complexes, used as catalysts, are mixed with carbon materials in the process of cathode manufacturing, and are adsorbed on the latter. However, in the process of storage and operating the current source, catalyst molecules, loosely bound with carbon material, may dissolve in the electrolyte, and therefore the efficiency of the electrolyte will decrease from a cycle to a cycle of charge-discharge.
A lithium-air current source is known, which is described in the application US 20120141889, and which consists of a lithium anode, carbon cathode containing traditional catalysts of an oxygen reaction, for example, noble metals, a separator, and an organic electrolyte, in which is dissolved a metal complex, for example, ferrocene. The role of said complex, according to the inventors of this current source, is in the fact that it can be subjected to oxidizing on the cathode, and then oxidizes the product of oxygen electric reduction of oxygen, for example, lithium oxide or peroxide, which may have positive impact on reversibility of the system. At the same time, the oxidized metal complex, in the event of its penetration through a porous separator to anode, may oxidize lithium, which will result in premature battery failure.
A lithium-air current source is known, which is described in the application US 20130157150, and which consists of a lithium anode, carbon cathode, separator, and an organic electrolyte. To provide for the selective delivery of molecular oxygen from the air to the cathode, a membrane made of a porous permeable base is used, onto which membrane a layer of a complex cobalt and polyalkylene amine compound is applied. Selectivity of the system is based on the ability of the said complex to reversibly add molecular oxygen. However, this complex does not have catalytic activity in relation to oxygen reaction on the cathode, and therefore the latter also includes an oxygen reaction catalyst (noble metal compounds, transition metal compounds) applied to a porous carbon carrier.
A lithium-air current source is known, which is described in the application JP 201303721, and which consists of a lithium anode, carbon cathode containing traditional catalysts of oxygen reaction, for example, noble metals, separator, and organic electrolyte. In addition, for selective delivery of molecular oxygen from air to cathode, a membrane is used made of a porous permeable base onto which is applied a layer of a complex compound of cobalt and one of the following ligands: porphyrins, phthalocyanines, and Schiff bases. Selectivity of the membrane is based on the ability of these complexes to add molecular oxygen. Yet the rate of oxygen delivery through such membranes is sufficiently low, and it provides for current density of about 10 μmA/cm2 upon battery discharge. It should be noted that in this case the complex compound, particularly with Schiff base, does not directly participate in charge-discharge processes of the current source, but is rather used for providing the membrane with fractional selectivity to oxygen.
As we can see from the prior art, at the present time, metal-air and, in particular, the best of them, i.e. lithium-air current sources have the service life of about several cycles of charge-discharge at the best specific power of no more than several dozens of W/kg. As a result, having the principally enormous potential in terms of service life and specific energy, metal-air current sources, developed up to date, do not have characteristics sufficient for their practical utilization, for example, in automobile industry. At the same time, it should be acknowledged that electric parameters of such current sources, to the great extent, depend on electrochemical properties of cathode.
A group of inventions has been applied: a cathode and a metal-air current source which form a single inventive concept—achieving the possibility of creating metal-air current sources with improved characteristics of specific energy, specific power, and the number of charge-discharge cycles.
One of the objects of the invention is a cathode for metal-air current sources, including a base of porous electrically conductive material, permeable for molecular oxygen, onto whose operating surface is applied a polymer complex compound of transition metal with Schiff base.
The use of such cathode in a metal-air current source, whose composition also includes an anode and an electrolyte, separating both electrodes, comprising ions of active metal from which the anode is made, brings about the following result. Each fragment of the said polymer complex compound of the transition metal with Schiff base acts as a highly efficient reaction center capable of concentrating the molecular oxygen coming through the porous cathode base, and metal ions coming from the electrolyte. The result of coordination of all components of the oxygen reduction process on the reaction center of such catalyst is the decrease in energy consumption for this reaction and an increase in its rate, which provides for an increase in specific energy and specific power of the current source as the energy reserve system.
One of the reasons for limitation of service life (the number of charge-discharge cycles) of metal-air current sources is blocking the surface of a catalyst, applied to cathode, by large nonconductive and insoluble crystals of active metal oxide and peroxide. The complex compound of a transition metal with Schiff base, used in this invention, which, as it was discovered by the authors of the invention, acts in this system as a catalyst, consists of discrete reaction centers in which nanocrystalline products of electric reduction of oxygen are formed. Oxide (peroxide) nanocrystals are oxidized reversibly upon charging the current source, which makes it possible to obtain a larger number of charge-discharge cycles of the system.
It is preferred to use porous carbon material with a developed surface as the material of the cathode. Carbon materials have low densities (specific weight), sufficient mechanical strength, a high degree of the development of the surface, which is easily variated with known methods, and at the same time they are chemically inert, have good adhesion to a polymer complex compound that is suggested to be used according to this invention.
As a polymer complex compound of transition metal with Schiff base, which is suggested to apply to the operating surface of the cathode base, the compound of poly-[M(R, R′-Salen)] type is suggested to be used which has the following structure
wherein M is the transition metal selected from the group of nickel, palladium, platinum, cobalt, copper, and iron;
Salen is the residue of bis(salicylaldehyde)ethylene diamine in the Schiff base; R is a substituent in the Schiff base selected from the group H, CH3O—, C2H5O—, HO—, or —CH3;
R′ is a substituent in the Schiff base selected from the group H or any other of the halogens,
n is the degree of polymerization having the value up to 200000.
Also, as the polymer complex compound of transition metal with Schiff base, the compound of poly-[M(R, R-Saltmen)] type can be used, which has the following structure
wherein M is the transition metal selected from the group of nickel, palladium, platinum, cobalt, copper, and iron;
Salen is the residue of bis(salicylaldehyde)tetramethyl ethylene diamine in the Schiff base;
R is a substituent in the Schiff base selected from the group H, CH3O—, C2H5O—, HO—, or —CH3;
R is a substituent in the Schiff base selected from the group H or any other of the halogens,
n is the degree of polymerization having the value up to 200000.
Also, as the polymer complex compound of transition metal with Schiff base, the compound of poly-[M(R, R-Salphen)] type can be used, which has the following structure
wherein M is the transition metal selected from the group of nickel, palladium, platinum, cobalt, copper, and iron;
Salen is the residue of bis(salicylaldehyde)-o-phenyl diamine in the Schiff base;
R is a substituent in the Schiff base selected from the group H, CH3O—, C2H5O—, HO—, or —CH3;
R′ is a substituent in the Schiff base selected from the group H or any other of the halogens,
n is the degree of polymerization having the value up to 200000.
Another object of the invention is a metal-air current source including a cathode, as it has been described above, and an anode made of material including, at least, one chemically active metal. In addition, anode and cathode are separated by electrolyte containing the ions of the said chemically active metal contained in the composition of anode.
As the material for the manufacturing of anodes of metal-air current source, alkali metal, rare earth metal, or transition metal can be used. Such metals have negative electrode potential and therefore they are preferred to be used as anode material.
In particular, as an alkali metal, lithium, which has the most negative electrode potential, can be used. In addition, as an electrolyte, for example, in this current source with a lithium anode, a solution of lithium trifluoromethanesulfonate in dimethyl ether of tetraethylene glycol at the mole ratio of these components of about from 1:2 to about 1:8, preferably 1:4 can be used. The said range is determined by the solubility of lithium trifluoromethanesulfonate salt in dimethyl ether of tetraethylene glycol as a solvent. Selection of the electrolyte is determined by the fact that it ensures high ion conductivity, is stable within the wide range of voltages (the area of electrochemical stability), and also it does not chemically interact with lithium, which excludes self-discharge of a lithium-air current source with such electrolyte.
Also, an alloy, comprising one or several chemically active metals, can be used as material for anode manufacturing. In particular, lithium-silicon alloy, lithium-aluminum alloy, lithium-tin alloy, or lead-sodium alloy can be used. The said alloys have sufficient negative electrode potential and, at the same time, provide for higher thermodynamic (corrosion) resistance and mechanical strength of anode.
The invention is explained by the following graphic materials.
An implementation of this invention is shown below using as an example of a lithium-air current source (
The polymer coating 4 of cathode 2 can be applied by means of electrochemical polymerization, for example, by electrochemically oxidizing the monomer [Co(Salen)] on the surface of the base 3 of a porous, permeable for molecular oxygen, carbon material at potential of 1.05 V in relation to a standard silver-chloride electrode in deaerated acetone solution comprising 10−3 mol/l of the said monomer, and 0.1 mol/l of tetrafluoroborate tetraethyl ammonium, for approximately 10 to 30 min. As the base 3 material, the material can be used which comprises Carbon Super P® brand carbon manufactured by TIMCAL Company.
Investigations, conducted, among others, by one of the authors of this invention, demonstrated that polymer complex compounds of transition metal with Schiff base have a specific stack structure with polymer fragments connected to each other through donor-acceptor interactions between the metallic center of one polymer fragment and ligand of another polymer fragment [I. E. Popeko, V. V. Vasiliev, A. M. Timonov, G. A. Shagisultanova. Electrochemical Behaviour of Palladium (II) Complexes with Schiffs Bases, Synthesis of Mixed-Valent Pd(II)-Pd(IV) Complexes//Russian J. Inorg. Chem. 1990, V. 35, N 4, P. 933].
In
The possibility of achieving the above-mentioned result, related to the energy parameters of the investigated current source, is related to properties of polymer complex compounds of transition metal with Schiff base, discovered as a result of investigations by the inventors of the present invention. The polymer complex compounds, for example, of poly-[Co(Schiff)] type, have strong chemical affinity to molecular oxygen. In the air environment, such polymers are capable of interaction with molecular oxygen due to formation of bridges of metal-oxygen-metal type between metallic centers [El-Ichiro Ochiai. Electronic structure and oxygenation of bis(salicylaldehyde)ethylenediiminicobalt(II)//J. Inorg. Nucl. Chem. 1973. V. 35. P. 1727]. This interaction is shown In
It was demonstrated that the oxygen concentration in the polymer is approximately 500 higher than in the air. In addition, this polymer-bound oxygen has a longer and therefore looser connection between atoms of oxygen than a molecule of free molecular oxygen. This means that the bound oxygen transited to a more active state due to the action of poly-[Co(Schiff)] polymer which acted in this system as a catalyst.
Let us examine processes of discharging and charging the current source according to this invention.
In the process of discharging the lithium-air current source (see
The described reaction of the reduction of oxygen occurs very fast, because the reduced oxygen and lithium ions are concentrated in the same reaction zone of the polymer fragment at a close distance from each other, which alleviates chemical interaction between lithium and oxygen resulting in the formation of lithium oxide. Usually, the used reduction catalysts, as a rule, adsorb and concentrate only one reagent, which usually is oxygen. The polymer complex of transition metal, demonstrating catalytic properties upon its utilization in this invention, “attracts” reagents, which are lithium ions and oxygen. The discharge process is completed after the total cathode surface is covered with a thin layer of discharge products.
In the process of charging the current source, completed according to this invention, as a result of application of the positive electric potential to cathode 2 in relation to anode 1, metallic centers 8 of fragments of the polymer coating 4 become oxidized and transferred to oxidized state with +3 degrees of oxidizing. Metallic centers, which in this case are cobalt atoms, in this oxidized state are the strong oxidizing agents capable of oxidizing the lithium oxide back to molecular oxygen, which leaves the reaction zone and exits into the atmospheric environment through porous carbon material of base 3 of the cathode 2. In this case, the polymer coating 4 operates as an electrochemical catalyst. In addition, it remains in the oxidized state due to positive potential applied to cathode 2 from the external power source. Lithium ions 7 diffuse back to the lithium anode 1, where they are reduced to metallic lithium.
In the described discharge process, polymer coating of cathode remains stable within the entire range of operating potentials; no irreversible changes occur in the polymer structure. As a result of charging the examined lithium-air current source, lithium oxide (peroxide) transforms back into oxygen and lithium ions. In addition, the total cathode surface is freed of the said products formed in the process of discharge of the current source. Together, all this makes it possible to significantly increase the number of charge-discharge cycles of the current source in comparison with the known ones.
Thus, high operating characteristics of the lithium-air current source, pursuant to this invention, are achieved due to the following circumstances.
Higher specific power of discharge is achieved due to higher rate of reduction of oxygen and higher rate of oxygen diffusion through the polymer coating of the cathode.
In accordance with this invention, the longer service life and efficiency of charge-discharge of the lithium-air current source is achieved due to high reversibility of chemical and electrochemical transformations taking place on the surface of polymer coating of its cathode, and due to absence of irreversible processes in it, related to blocking of operating (active) surface of the cathode caused by nonconductive and insoluble crystals of lithium oxide or peroxide.
Upon manufacturing the cathode, carbon material (Super P, 80% by weight, manufactured by TIMCAL Company) and polyvinylidene fluoride (PVDF, 20% by weight) as a binder were directly mixed in N-methyl-2-pyrrolidone solvent. After this, the produced mass was applied to a gas permeable layer made of carbon nonwoven material manufactured by the Hollingworth & Vose Company, brand 8000030, with application density of 1.0±mg/cm2. Then, this blank was dried for 12 hours at 100° C. in vacuum to remove residues of the solvent. Thus the base of a future cathode was obtained. Then the poly-[Co(Saltmen)] coating was applied to the cathode base in the form of a film. The application process was completed in a sealed box, filled with argon, with the total concentration of water and oxygen of less than 10−5%. The process of polymerization was performed in acetone nitrile solution containing 1 mmol/l of [Co(Saltmen)] monomer, and 0.1 mol/l of (C2H5)4NBF4 tetrafluoroborate tetraethyleneammonium, and it included two cycles under the conditions of changing the potential from 0 to 1.4 V in relation to silver-chloride electrode at the rate of 50 mV/sec.
The anode was made of 700 μm-thick lithium foil. The current source was assembled in an R2031 steel body (coin-type). In the body lid, contacting with cathode and operating as a current lead, 21 holes were made with a diameter of 1 mm for providing of oxygen access to cathode. The cathode and anode were separated by a paper porous separator. As a lithium-containing electrolyte, LiCF3SO3 solution (manufactured by Aldrich Company) in dimethyl ether of tetraethylene glycol (TEGDME) was used at 1:4 mole ratio of the components.
The current source was subjected to charging and discharging on a CT-3008W testing unit from NEWARE (China). Charging and discharging were conducted at 100 μA direct current, which conformed to current density of 35 μA per 1 g of carbon in the composition of cathode. During the testing, the current source was placed in a container filled with oxygen under pressure approximately 10% higher than atmospheric pressure.
Charge-discharge curves of the current source, described in Example 1, are shown in
During the manufacturing of the cathode, carbon material (Super P, 80% by weight, made by TIMCAL Company), and polyvinylidene fluoride (PVDF, 20% by weight), as a binder, were directly mixed in N-methyl-2-pyrrolidone. Then, the produced mass was applied to the gas permeable layer made of a stainless steel net, at application density of 0.4±0.1 mg/cm2. After this, the blank was dried for 12 hours at 100° C. in vacuum for removing the residues of the solvent. Thus, the base of the future cathode was obtained. Then, poly-[Co(Saltmen)] coating in the form of a film was applied to the base of the cathode. The application process was performed in the argon-filled sealed box with the total concentration of water and oxygen of less than 10−5%. The process of polymerization was conducted in acetyl nitrile solution, containing 1 mmol/l of monomer [Co(Saltmen)], and 0.1 mol/l of tetrafluoroborate tetraethyleneammonium (C2H5)4NBF4, and it included two cycles under the conditions of changing the potential from 0 to 1.4 V in relation to silver-chloride electrode at the rate of 50 mV/sec.
The anode was made of 700 μm thick lithium foil. In the lid of the body, contacting with cathode and acting as a current lead, 21 holes with the diameter of 1 mm were made to provide for oxygen access to cathode. The cathode and anode were separated by a paper porous separator. LiCF3SO3 lithium trifluoromethanesulfonate solution (manufactured by Aldrich Company) was used in dimethyl ether of tetraethylene glycol (TEGDME) at 1:4 mole ratio of the components.
Also, a control current source was made that was different from the described experimental one, in accordance with this invention, only by the fact that its cathode did not have the said poly-[Co(Saltmen)] polymer coating.
Both current sources (experimental and control ones) were tested under the same charge-discharge conditions on a CT-3008W unit from the NEWARE Company (China). Charging was conducted at the direct current of 100 μA, which corresponded to the current density of 35 mA per 1 g of carbon in the composition of cathode. In the process of testing, both current sources were in the air atmosphere at the room temperature.
In
specific energy—1550 W·hr/kg (per electrode weight);
specific power—300 W/kg (per electrode weight).
Despite the fact that the results obtained upon utilization of polymer cobalt compounds with Schiff base in the current source are presented in the examples, similar results can be obtained also upon utilization of other polymer metal complexes with Schiff bases, for example, complexes of nickel, manganese, and other transition metals.
Thus, the results of the experiments prove that the use of cathode in metal-air current source, where the operating surface of cathode has a coating of polymer complex compound of transition metal with Schiff base, results in higher specific electric characteristics of these current source in comparison with the systems of the same application, known from the prior art. This is achieved because the said polymers, as it was discovered by the inventors, act in this system as catalysts of cathode reactions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014137372 | Sep 2014 | RU | national |
This application is a continuation, claiming priority to International Patent Application No. PCT/US2015/058738, filed on Nov. 3, 2015, which claims priority to and the benefit of Russian Patent Application No. RU2014137372, filed on Sep. 15, 2014, the entire disclosures of each of which are incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/US2015/058738 | Nov 2015 | US |
| Child | 15275527 | US |
