Cells with metallic lithium anodes and methods of manufacture

Abstract

The present invention relates to an electrochemical cell capable of storing electrical energy, the cathode of which is capable of reversibly accommodating lithium ions, the anode of which comprises metallic lithium as an active material, the electrochemical cell including a separator disposed between the cathode and the anode, the anode comprising a porous, electrically conductive matrix having an open pore structure, the metallic lithium of the anode being trapped within the pores of the matrix. The present invention further relates to a method for making such a cell.

Description

[Technical Field]

[0001] The invention described below relates to cells having metallic lithium anodes and methods for making such cells. [Background technology]

[0002] The best-known example of a cell with a metallic lithium anode is the lithium-sulfur cell. Like other electrochemical cells capable of storing electrical energy, this cell comprises a cathode and an anode as electrodes, and a separator disposed between the cathode and anode, the cathode comprising sulfur as an active material, and the anode comprising lithium as an active material.

[0003] During discharge of a lithium-sulfur cell, lithium is oxidized at the anode. At the cathode, lithium combines with sulfur to form lithium sulfide, i.e., dilithium sulfide, LiS, in the case of complete discharge, for example, according to the following equation: JPEG0007730321000001.jpg10166

[0004] During the charging operation, the formed lithium sulfide is redissolved, for example, sulfur is formed on the cathode side and lithium on the anode side according to the following equation: JPEG0007730321000002.jpg10166

[0005] However, metallic lithium anodes can also be combined with cathodes that are able to reversibly incorporate lithium in ionic form, for example cathodes based on lithium cobalt oxide (LCO), nickel manganese cobalt (NMC) or lithium iron phosphate (LFP).

[0006] One of the problems that has hindered the marketability of cells with metallic lithium anodes so far arises from the fact that such anodes completely decompose in the event of full discharge. That is, the volume of the anode can become zero upon discharge. This results in a large volume change within the cell, which repeats in the opposite direction upon charging.

[0007] This problem is particularly acute when the cell has a configuration in which multiple anodes and cathodes are stacked in alternating order in layer form, in which case the respective volume changes are added together.

[0008] DE 102014201836 A1 describes a lithium-sulfur cell in which this problem is countered with volume compensation elements intended to compensate for volume changes during charging and discharging of the cell. The volume compensation elements are elastic and are placed in the cell in addition to the electrodes. Volume changes within the cell can cause them to compress or expand as needed. They exert continuous pressure on the electrodes.

[0009] A drawback of this solution is that the installed volumetric compensation elements constitute useless material from an electrochemical standpoint, adversely affecting the volumetric energy density of the cells that are provided with such elements. The installation of such elements also requires an additional step in the manufacture of lithium-sulfur cells, which creates an additional source of defects in highly automated manufacturing processes. Summary of the Invention

[0010] It is an object of the present invention to provide a cell having a metallic lithium anode that is improved over the prior art.

[0011] To this end, the invention proposes an electrochemical cell capable of storing electrical energy having the features set forth in claim 1 and a method comprising the steps set forth in claim 9. The dependent claims give developments of the invention.

[0012] A cell according to the invention always has the following characteristics: (a) the cell includes a cathode capable of reversibly incorporating lithium ions; (b) the cell includes an anode including metallic lithium as an active material; (c) the cell includes a separator disposed between the cathode and the anode; (d) the anode comprises a porous, electrically conductive matrix having an open pore structure; (e) The metallic lithium of the anode is contained in the pores of the matrix.

[0013] The electrically conductive matrix is ​​a central feature of the present invention, since such matrix ensures that the aforementioned volume changes during charging and discharging of the cell, at least on the anode side, are minimized.

[0014] Starting from a charged state in which lithium is at least largely, optionally completely, located in the pores of the cell, discharge causes lithium to decompose in the anode. However, in contrast to cells known from the prior art, the anode does not substantially lose volume, since this is essentially determined by the matrix. During discharge, lithium can be uniformly redeposited in the anode as a result of the electrical conductivity of the matrix. Therefore, non-uniform lithium deposition and the associated local volume increase or dendrite formation can be avoided.

[0015] The cathode of the cell according to the present invention can be a cathode containing sulfur as an active material. Therefore, the cell according to the present invention can be a lithium-sulfur cell. For example, the cathode can contain a mixture of sulfur and an additive for improving electrical conductivity, such as from the group consisting of graphite, carbon black, CNT, and graphene. However, the cathode can also contain sulfur in a chemically modified form, such as sulfur as a polysulfide.

[0016] In further embodiments, the cathode preferably comprises, as an active material, a compound capable of reversibly incorporating lithium in ionic form. For example, the cathode may comprise, as an active material, a layered oxide such as lithium cobalt oxide (LCO), nickel manganese cobalt oxide (NMC), a polyanionic compound such as lithium iron phosphate (LFP), or a spinel compound such as lithium manganese spinel.

[0017] The anode comprises lithium in metallic form, and may also optionally comprise additional materials, such as at least one metal, with which the lithium is alloyed, optionally also disposed within the pores of the matrix.

[0018] The separator ensures that the cathode and anode are spatially and electrically isolated from each other. The separator is preferably a porous fabric, especially a porous membrane or fleece, or a felt or fiber fabric. The separator is preferably made of plastic, for example, polyolefin, polyimide, or polyester.

[0019] The cell preferably contains a liquid electrolyte consisting of a solvent or solvent mixture and a lithium-ion-containing conductive salt. Suitable conductive salts include, for example, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium hexafluorophosphate (LiPF), or lithium tetrafluoroborate (LiBF). Suitable solvents include, for example, organic carbonates, in particular ethylene carbonate (EC), propylene carbonate (PC), 1,2-dimethoxyethane (DME), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), or diethyl carbonate (DEC), and mixtures thereof.

[0020] When the cell is a lithium-sulfur cell, the solvent used can be, for example, a mixture of dioxolane (DOL) and DME. The electrolyte can further include a passivation additive such as lithium nitrate (LiNO).

[0021] However, as an alternative to the separator / liquid electrolyte combination, the cell may also include a polymer electrolyte, an ionic liquid, or a solid-state electrolyte.

[0022] The polymer electrolyte can be, in particular, a gel electrolyte based on, for example, polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVDF-HFP). Ionic liquids (ILs) are salts of organic nature characterized by the steric asymmetry of their cations and anions, which means that they are in a liquid state even at room temperature. ILs contain, for example, imidazolium, pyridinium, or pyrrolidinium ions as cations and, for example, TFSI as anion.

[0023] The solid electrolyte is preferably a polymer solid-state electrolyte based on a monophasic polymer conductive salt complex without any liquid component. The solid polymer electrolyte can include polyacrylic acid (PAA), polyethylene glycol (PEG), or polymethyl methacrylate (PMMA) as the polymer matrix. These can dissolve lithium conductive salts such as LiTFSI, LiPF6, and LiBF4 in them.

[0024] If the cell is a lithium-sulfur cell, the separator can be a protective layer that protects the anode from the electrolyte and the lithium sulfide dissolved therein. This protective layer can be applied to the separator, for example, on the cathode side.

[0025] The cell according to the invention is preferably surrounded by a housing, which is preferably airtight, and can be a solid, self-supporting housing made, for example, from metal or plastic (hard case) or a housing made from film (pouch packaging).

[0026] The open pore structure of the matrix is ​​crucial to the present invention. An open pore structure is known to describe a structure containing many pores that are interconnected by channels or crevices in the pore walls. Therefore, an open pore structure generally has a large internal area.

[0027] Preferably, the cell has at least one of the additional features (a) and (b) immediately below. (a) the matrix has a porosity in the range of 40% to 95%; (b) The pores in the matrix have an average diameter in the range of 2 to 50 μm.

[0028] It is particularly preferred that the two immediately above features (a) and (b) are realized in combination with each other.

[0029] Determining the porosity (pore volume divided by the total volume of the matrix) and pore size distribution is no longer an obstacle today. There are numerous measuring instruments that carry out the corresponding determinations according to standardized methods. The above values ​​relate to determinations according to standards ISO 15901-1 and DIN 66133.

[0030] In a possible development of feature (a) immediately above, the matrix preferably has a porosity in the range of 50% to 95%, particularly preferably 70% to 95%, in particular 80% to 95%.

[0031] In a possible development of feature (b) immediately above, the pores in the matrix preferably have an average diameter in the range from 7.5 to 150 μm, particularly preferably from 9 to 130 μm, in particular from 10 to 120 μm.

[0032] It is particularly preferred that the pores in the matrix are connected by channels having an average diameter in the range from 0.5 to 50 μm, particularly preferably from 1 to 40 μm, in particular from 1 to 25 μm, particularly preferably from 1 to 10 μm.

[0033] Ideally, the matrix consists of a material that does not undergo chemical changes during charging and discharging.

[0034] In particularly preferred embodiments, the cells have at least one of the additional features (a) and (b) immediately below: (a) the matrix comprises carbon formed by carbonization of an organic compound; (b) The matrix contains carbon in a proportion ranging from 50% by weight to 100% by weight.

[0035] It is particularly preferred that the two immediately above features (a) and (b) are realized in combination with each other.

[0036] Particularly preferred variants of carbonizable organic compounds and methods for carbonization are described in EP 2 669 260 A1, the contents of which are fully incorporated into the content of this specification by reference.

[0037] The production of porous, electrically conductive matrices with an open pore structure is carried out from porous organic compounds, in particular from polymers with a porous structure.

[0038] The formation of this porous organic compound, particularly a polymer with a porous structure, is preferably carried out by polymerizing the monomer phase of a monomer-in-water emulsion, for example by ring-opening metathesis polymerization (ROMP) of a suitable diene compound. Water droplets are trapped during polymerization. After subsequent removal of the water, holes remain in their place. The polymer matrix with these holes can be carbonized in a subsequent process, possibly requiring an intermediate step such as an oxidation treatment (see below).

[0039] In the present case, carbonization should be understood to mean the conversion of organic compounds into substantially pure carbon, which conversion is generally carried out at very high temperatures and in the absence of oxygen.

[0040] EP 2669260 A1 describes the formation of unsaturated polymers with porous structures. The starting material is at least one carbon-containing monomer, preferably at least one mono- or polycyclic diene compound, particularly preferably at least one diene compound selected from the group consisting of dicyclopentadiene, norbornene, norbornadiene, cyclooctene, cyclooctadiene, and derivatives thereof. This at least one carbon-containing monomer is converted to the desired unsaturated polymer with porous structure by ring-opening metathesis polymerization (ROMP) in the monomer phase of a monomer-in-water emulsion. The resulting unsaturated polymer preferably has a C=C double bond with at least one oxidizable hydrogen atom in the α-position.

[0041] The resulting unsaturated polymer with a porous structure is subsequently subjected to chemical and / or physical treatment and carbonized by heat treatment, which then produces the desired electrically conductive matrix with an open pore structure.

[0042] The chemical and / or physical treatment preferably comprises an oxidation treatment, in particular the reaction of the unsaturated polymer in an oxidizing atmosphere, preferably at a temperature in the range of 0° C. to 250° C. The purpose of this treatment is to increase the oxygen content in the polymer, particularly preferably to a mass fraction in the range of 25% to 40%, before subsequent carbonization.

[0043] For subsequent carbonization, the polymer can be heated to a temperature in the range of 550° C. to 2500° C., preferably in an oxygen-free atmosphere.

[0044] The properties of the matrix, especially its pore size, can also be specifically tailored in this preparation. This can be achieved by adding different amounts of surfactant to the monomer-in-water emulsion. Preferably, the volume fraction of surfactant is varied in the range of 0.1% to 8% (based on the amount of polymerizable monomer in the emulsion).

[0045] In a possible development, it is preferred if the cell has at least one of the additional features (a) and (b) immediately below: (a) in addition to carbon, the matrix comprises at least one filler having an electrical conductivity higher or lower than that of carbon; (b) The filler is at least one material selected from the group consisting of carbon black, CNT, graphene, and metal particles.

[0046] It is particularly preferred that the two immediately above features (a) and (b) are realized in combination with each other.

[0047] The electrical conductivity of the matrix can be specifically increased or decreased by fillers, which can be introduced, for example, by adding them to the monomer-in-water emulsion described above.

[0048] The matrix preferably contains at least one filler in a proportion ranging from 0.1% to 30% by weight.

[0049] It is further preferred that the cell have at least one of the additional features (a) and (b) immediately below: (a) the cell includes an electrical conductor for making electrical contact with the anode; (b) The electrical conductor is a metal foil.

[0050] It is particularly preferred that the two immediately above features (a) and (b) are realized in combination with each other.

[0051] It is particularly preferred that the electrical conductor is made from nickel or a nickel alloy or copper or a copper alloy.

[0052] In a possible development, the cell has at least one of the additional features (a) and (b) immediately below: (a) The anode matrix forms a layer on the electrical conductor; (b) The layer has an average thickness in the range of 5 to 100.

[0053] It is particularly preferred that the two immediately above features (a) and (b) are realized in combination with each other.

[0054] The main function of the electrical conductor is to carry electrical current to and from the anode. Preferably, one end of the electrical conductor is connected directly to the anode and the other end leads to a terminal of the cell according to the invention which can be coupled to an electrical consumer.

[0055] However, the electrical conductor also acts as a carrier, in particular for the electrically conductive matrix. It is preferred that the matrix covers the electrical conductor so that direct deposition of lithium on the electrical conductor is not possible. In the case of cells containing a liquid electrolyte, this is ensured, for example, by the matrix covering all surfaces of the electrical conductor that can come into contact with the electrolyte.

[0056] The electrical conductor can in principle be any desired sheet metal support, i.e., not only the above-mentioned metal foils, but also metal foams in tape form or metal fleeces in tape form, however, the above-mentioned foils are preferred, especially when they are rectangular supports or in tape form.

[0057] The fact that the matrix is ​​preferably in the form of a layer means that the anode of the lithium-sulfur cell according to the invention is also preferably in the form of a layer together with the cathode and separator, in a preferred embodiment, are therefore also in the form of layers.

[0058] Preferably, the anode, cathode, and separator are combined together to form a composite having the order positive electrode / separator / negative electrode. If the electrodes and separator are in tape form, the composite will likely be in the form of a roll. However, it is also common for multiple composites to be stacked on top of each other.

[0059] The cathode is preferably in the form of a layer having a thickness in the range of 10 μm to 200 μm.

[0060] In particular, when the cell according to the present invention is a lithium-sulfur cell, the cell has at least one of the additional features (a) and (b) immediately below: (a) the cathode comprises a porous, electrically conductive matrix having an open pore structure; (b) Sulfur is placed in this matrix.

[0061] It is particularly preferred that the two immediately above features (a) and (b) are realized in combination with each other.

[0062] The matrix used on the cathode side, which has an open pore structure, preferably has the same composition and structure as the matrix used on the anode side.

[0063] Regardless of the active material selected, it is preferred that the structure of the cathode is similar to that of the anode. Thus, in preferred developments, the cell can have at least one of the additional features (a) to (c) immediately below. (a) the cathode includes an electrical conductor for making electrical contact with the cathode active material; (b) the electrical conductor is a metal foil; (c) A cathode matrix is ​​layered over the metallic conductor.

[0064] It is particularly preferred that the two immediately above features (a) and (b) are realized in combination with each other. If the cell according to the invention is a lithium-sulfur cell, it is preferred that all immediately above features (a) to (c) are realized in combination with each other.

[0065] The described cells can be manufactured with the aid of the method described below, which also forms part of the subject matter of the present invention, and which always comprises the immediately following steps (a) to (c): (a) providing a porous, electrically conductive matrix; (b) incorporating lithium into the pores of the matrix to form an anode containing lithium as the active material; (c) combining the formed anode with a separator and a cathode containing sulfur as the active material;

[0066] The above three steps do not necessarily have to be performed in the specified order, thus step (b) can be performed immediately after step (c).

[0067] Preferably, the method comprises the additional step (a) immediately below, and particularly preferably a combination of the two immediately below steps (a) and (b). (a) carbonizing a porous organic compound to provide an electrically conductive matrix; (b) Carbonization is carried out in the absence of oxygen.

[0068] The properties of the matrix, including carbonization, and in particular the preferred mode of its preparation have already been mentioned. Detailed examples can be found in EP 2 669 260 A1.

[0069] In particular, examples of organic compounds that can be processed into porous organic compounds and that provide a porous matrix by a subsequent carbonization step have already been listed above.

[0070] In a particularly preferred development, the method comprises the additional step (a) immediately below, particularly preferably a combination of the two immediately below steps (a) and (b): (a) for carbonization, a layer of a porous organic compound to be carbonized is formed on a carrier; (b) The carrier used is a metal foil.

[0071] Therefore, it is preferable to first form a layer of the porous organic compound on a carrier. For this purpose, the above-mentioned monomer-in-water emulsion of the diene compound can be applied to the carrier. The metathesis of the diene compound gives a porous organic compound that undergoes the oxidation treatment similarly described above. The porous organic compound, together with the carrier, is exposed to a temperature at which carbonization occurs, thereby obtaining a porous matrix.

[0072] Carriers that can be used include the metal substrates listed above, so-called metal foils.

[0073] In a particularly preferred embodiment, the method comprises the additional step (a) immediately below, particularly preferably the two immediately below steps (a) and (b): (a) introducing metallic lithium into the pores of the matrix by electrochemical deposition; (b) introducing metallic lithium into the pores of the matrix before the anode is combined with the separator and cathode;

[0074] The introduction of lithium into the pores of the matrix can be carried out, for example, by immersing the matrix in a lithium salt solution and connecting it to the negative terminal of a DC voltage source.

[0075] In a further particularly preferred embodiment, the method comprises the additional step (a) immediately below, particularly preferably a combination of the two immediately below steps (a) and (b): (a) introducing metallic lithium into the pores of the matrix by electrochemical deposition; (b) introducing metallic lithium into the pores of the matrix after the anode has been combined with the separator and cathode;

[0076] In this variant, a lithium-ion-containing NMC material can be used, for example, on the cathode side. Electrochemical deposition of metallic lithium into the pores of the matrix is ​​then carried out during the first charge.

[0077] It is also possible to partially fill the matrix with metallic lithium, combine the partially filled matrix with the cathode and separator, and then completely fill the matrix during the first charge, which allows excess lithium to be introduced into the cell to compensate for losses during the first charge and discharge cycles.

[0078] If the cell according to the invention is a lithium-sulfur cell, in some preferred embodiments, the cathode can be filled with lithium sulfide and connected to a separator and a porous, electrically conductive matrix as described above disposed on an electrical conductor similar to that described above to provide a cell. When a charging voltage is applied to the cell, the lithium sulfide dissolves and the matrix fills with lithium from the cathode. [Example]

[0079] Further features of the present invention and the advantages obtained from the present invention will be apparent from the following examples and the figures used to illustrate the present invention. The examples described below serve only for illustration and for a better understanding of the present invention and should not be considered as limiting in any way.

[0080] (1) 8.00 mL (60 mmol) of dicyclopentadiene (Sigma-Aldrich) and 0.084 mL (1.9 × 10 -2 10 mmol) of surfactant Pluronic® 121 (poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol); Mn = 4400 g / mol; Sigma-Aldrich) was initially charged into the reaction vessel. The mixture of the two components was stirred at 400 rpm. 33 mL of deionized water was added dropwise with constant stirring. After the addition of water, the mixture was stirred for another hour until a homogeneous emulsion was obtained. At the end of the stirring operation, the emulsion was dissolved in 1 mL of toluene and 8 mg (8.4 × 10) of initiator. -3 The mixture was mixed with 10 mmol of N,N-bis(mesityl)-4,5-dihydroimidazol-2-yl (Sigma-Aldrich).

[0081] (2) The emulsion resulting from step (1) was applied in a layer thickness of 100 μm to a 12 μm thick copper foil. The resulting layer with the copper foil was then heated to 80° C. Curing for 4 hours gave a white, mechanically stable layer. This layer was washed repeatedly with dichloromethane and acetone and dried under vacuum.

[0082] (3) The layer obtained from step (2) was exposed to atmospheric oxygen at room temperature for 4 weeks. At the end of the 4-week period, the oxygen content in the layer was 31.54% (determined by elemental analysis).

[0083] (4) In a subsequent step, the copper foil containing the layer was subjected to a temperature of 900°C in an argon atmosphere for 2 hours, which carbonized the layer.

[0084] (5) 1.13 cm from the copper foil coated with the carbonized layer 2 A portion of the anode was cut out and placed in a Swagelok cell as the negative electrode, and charged and discharged at a charge rate of 0.1 C and a discharge rate of 0.1 C. During the first charge, metallic lithium was deposited in the pores of the anode.

[0085] The positive electrode used was an NMC cathode (active material: lithium nickel manganese cobalt oxide (LiNi 0.33 Mn 0.33 Co 0.33 The electrolyte used was a mixture of EC and EMC (volume ratio 3:7) with 2% by volume of vinylene carbonate (VC) and 1M LiPF6 added. The separator used was a commercially available polyolefin separator. [Brief explanation of the drawings]

[0086] [Figure 1] The charge-discharge results are shown in Figure 1. As can be seen, virtually no capacity loss was observed during the charge-discharge test.

[0087] [Figure 2]2 is a schematic diagram of an embodiment of a cell 100 of the present invention. The cell includes an anode 101, a cathode 102, and a separator 103. The cell 100 includes an electrical conductor 101a for making electrical contact with the anode 101 and an electrical conductor 102a for making electrical contact with the cathode 102. Both the electrical conductor 101a and the electrical conductor 102a are metal foils. The anode 101 includes a porous, electrically conductive matrix 101b having an open pore structure, and the pores are filled with lithium.

Claims

1. (a) providing a porous, electrically conductive matrix; (b) incorporating lithium into the pores of the matrix to form an anode containing lithium as the active material; (c) combining the formed anode with a separator and a cathode comprising sulfur as an active material; 1. A method for manufacturing an electrochemical cell, comprising the steps of: (d) the porous organic compound is carbonized to provide an electrically conductive matrix; (e) for carbonization, a layer of the porous organic compound to be carbonized is formed on the carrier; (f) the carrier used is a metal foil; (g) introducing metallic lithium into the pores of the matrix by electrochemical deposition; (h) introducing metallic lithium into the pores of the matrix before the anode is combined with the separator and cathode; method.

2. 10. The method of claim 1, comprising the additional steps of: Carbonization is carried out in the absence of oxygen.

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