CELL WITH METALLIC LITHIUM ANODE AND MANUFACTURING PROCESS

Description

[0001] The invention described below relates to a cell with a metallic lithium anode and a method for producing such a cell.

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

[0003] During the discharge of a lithium-sulfur cell, lithium is oxidized at the anode. At the cathode, the lithium combines with sulfur to form lithium sulfides. Upon complete discharge, dilithium sulfide Li 2 S is formed, for example, according to the following formula: S 8 + 16 Li → 8 Li 2 S

[0004] During the charging process, the lithium sulfides formed are dissolved again. Sulfur is formed on the cathode side and lithium on the anode side, for example, according to the following formula: 8 Li 2 S → S 8 + 16 Li

[0005] Metallic lithium anodes can also be combined with cathodes capable of reversibly storing lithium in ionic form, for example, with cathodes based on lithium cobalt oxide (LCO), nickel manganese cobalt (NMC), or lithium iron phosphate (LFP).

[0006] One of the problems that has so far hindered the marketability of cells with metallic lithium anodes is that such anodes are completely degraded during a full discharge. The volume of the anodes can therefore drop to near zero during discharge. This results in massive volume changes within the cell, which are repeated in reverse during charging.

[0007] The problem is particularly critical when the cells have a structure in which several layered anodes and cathodes are stacked in alternating sequence. In this case, the respective volume changes add up.

[0008] DE 102014 201836 A1 describes a lithium-sulfur cell that addresses this problem with volume compensation elements designed to compensate for volume changes during charging and discharging. The volume compensation elements are elastic and are installed in the cell in addition to the electrodes. When volume changes occur within the cell, they can be compressed or expanded as needed. In doing so, they exert continuous pressure on the electrodes.

[0009] The disadvantage of this solution is that the installed volume compensation elements represent dead material from an electrochemical perspective and negatively impact the volumetric energy density of a cell equipped with them. Furthermore, their assembly represents an additional step in the manufacture of lithium-sulfur cells, which represents an additional source of error in highly automated production.

[0010] WO 2017 / 086609 A1 and US Pat. No. 5,510,212 A each disclose the production of a polymer and its subsequent carbonization. This results in porous carbon structures that can be used to produce electrodes, for example, for a lithium-ion battery or a lithium-sulfur battery. The present invention was based on the object of providing improved cells with a metallic lithium anode compared to the prior art.

[0011] To achieve this object, the invention proposes an electrochemical cell capable of storing electrical energy, having the features recited in claim 1, and a method comprising the steps recited in claim 8. Further developments of the invention are the subject of subclaims.

[0012] The cell according to the invention is always characterized by the following features: a. It comprises a cathode which is capable of reversibly absorbing lithium ions, and b. it comprises an anode which contains metallic lithium as the active material, and c. it comprises a separator which is arranged between the cathode and the anode, wherein d. the anode comprises a porous, electrically conductive matrix with an open-pore structure and e. the metallic lithium of the anode is embedded in pores of the matrix.

[0013] Furthermore, the cell according to the invention is always characterized by the following features: f. The cell comprises a metal foil as an electrical conductor for electrical contact with the anode and g. the matrix was formed by carbonization of a porous organic compound directly on the metal foil.

[0014] The electrically conductive matrix is a central feature of the present invention. It ensures that, at least on the anode side, the described volume changes during charging and discharging of the cell are minimized.

[0015] Starting from a charged state in which the lithium is at least predominantly, or possibly even completely, located in the pores of the cell, the lithium is decomposed in the anode during discharge. Unlike in prior art cells, however, the anode loses almost no volume, as this is largely determined by the matrix. During charging, the lithium can then be evenly deposited in the anode due to the electrical conductivity of the matrix. This prevents uneven lithium deposition and the associated local volume increase or even dendrite formation.

[0016] The cathode of the cell according to the invention can be a cathode containing sulfur as the active material. The cell according to the invention can therefore be a lithium-sulfur cell. For example, the cathode can comprise a mixture of sulfur with an additive to improve electrical conductivity, for example, from the group consisting of graphite, carbon black, CNTs, and graphene. Alternatively, the cathode can also comprise the sulfur in a chemically modified form, for example, as polysulfide.

[0017] In further embodiments, it is preferred if the cathode comprises, as active material, a compound capable of reversibly incorporating lithium in ionic form. For example, the cathode can comprise, as active material, lithium cobalt oxide (LCO), layered oxides such as nickel manganese cobalt oxide (NMC), polyanionic compounds such as lithium iron phosphate (LFP), or spinel compounds such as lithium manganese spinel.

[0018] The anode comprises the lithium in metallic form. Optionally, it may also comprise at least one other material, for example, at least one metal with which the lithium is alloyed. Optionally, the at least one other material is also embedded in the pores of the matrix.

[0019] The separator spatially and electrically separates the cathode and the anode. The separator is preferably a porous sheet, in particular a porous film, a nonwoven, a felt, or a textile fabric. The separator is preferably made of a plastic, for example, a polyolefin, a polyimide, or a polyester.

[0020] The cell preferably comprises a liquid electrolyte consisting of a solvent or solvent mixture and a conducting salt containing lithium ions. Examples of suitable conducting salts include lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium hexafluorophosphate (LiPF 6 ), or lithium tetrafluoroborate (LiBF 4 ). Suitable solvents include 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), as well as mixtures thereof.

[0021] If the cell is a lithium-sulfur cell, a mixture of dioxolane (DOL) and DME can be used as a solvent. The electrolyte can also contain a passivation additive such as lithium nitrate (LiNO 3 ).

[0022] As an alternative to a separator / liquid electrolyte combination, the cell can also contain a polymer electrolyte, an ionic liquid or a solid electrolyte.

[0023] The polymeric electrolyte can in particular be a gel electrolyte, for example based on polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVDF-HFP).

[0024] Ionic liquids (ILs) are organic salts characterized by the steric asymmetry of their cations and anions, which results in ILs existing in the liquid phase even at room temperature. ILs contain, for example, imidazolium, pyridinium, or pyrrolidinium ions as cations and TFSI as anion.

[0025] The solid-state electrolyte is preferably a polymer solid-state electrolyte based on a polymer-conducting salt complex, which exists as a single-phase solution without any liquid component. A polymer solid-state electrolyte can comprise polyacrylic acid (PAA), polyethylene glycol (PEG), or polymethyl methacrylate (PMMA) as the polymer matrix. Lithium conducting salts such as LiTFSI, LiPF 6 , and LiBF 4 can be dissolved in these.

[0026] If the cell is a lithium-sulfur cell, the separator may have a protective layer that protects the anode from the electrolyte and any lithium sulfides dissolved therein. This protective layer can, for example, be applied to the separator on the cathode side.

[0027] The cell according to the invention is preferably enclosed in a housing. The housing is preferably designed to be gas-tight. The housing can, for example, be a solid, inherently stable housing made of metal or plastic (hard case) or a housing made of a film (pouch packaging).

[0028] Of great importance for the invention is the open-pore structure of the matrix. An open-pore structure is understood to be a structure that has a large number of pores that are interconnected by channels or perforations in the pore walls. As a result, open-pore structures generally have a large internal surface area.

[0029] It is preferred that the cell is characterized by at least one of the immediately following additional features a. and b.: a. The matrix has a porosity in the range of 40 to 95%. b. The pores in the matrix are characterized by an average diameter in the range of 2 to 50 µm.

[0030] Particularly preferably, the two immediately above features a. and b. are implemented in combination with one another.

[0031] Determining porosity (the ratio of pore volume to total matrix volume) and pore size distributions is no longer a challenge. Numerous measuring instruments are available that perform the relevant measurements using standardized procedures. The above values refer to measurements according to ISO 15901-1 and DIN 66133.

[0032] In possible further developments of the immediately preceding feature a., the matrix preferably has a porosity in the range from 50% to 95%, particularly preferably from 70% to 95%, in particular from 80% to 95%.

[0033] In possible further developments of the immediately preceding feature b., 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.

[0034] The pores in the matrix are particularly preferably connected by passages having an average diameter in the range of 0.5 µm and 50 µm, particularly preferably in the range of 1 to 40 µm, in particular in the range of 1 to 25 µm, most preferably from 1 to 10 µm.

[0035] Ideally, the matrix consists of a material that does not change chemically when the cell is charged and discharged.

[0036] In particularly preferred embodiments, the cell is characterized by at least one of the immediately following additional features a. and b.: a. The matrix comprises carbon formed by the carbonization of the organic compound. b. The matrix comprises carbon in a proportion ranging from 50 to 100 wt.%.

[0037] Particularly preferably, the two immediately above features a. and b. are implemented in combination with one another.

[0038] Particularly preferred variants of carbonizable organic compounds and also of carbonization processes are described in EP 2 669 260 A1, the content of which is hereby incorporated in its entirety by reference into the present description.

[0039] Most preferably, the porous, electrically conductive matrix with an open-pore structure is produced from a porous organic compound, in particular from a polymer with a porous structure.

[0040] The formation of this porous organic compound, particularly the polymer with the porous structure, preferably occurs by polymerizing the monomer phase of a monomer-water emulsion, for example, by ring-opening metathesis polymerization (ROMP) of a suitable diene compound. During polymerization, water droplets are trapped. After subsequent removal of the water, voids remain in their place. The resulting polymer matrix with these voids can be carbonized in a subsequent step, although intermediate steps such as oxidative treatment (see below) may be necessary.

[0041] In this context, carbonization refers to the conversion of an organic compound to almost pure carbon. Such a conversion usually occurs at very high temperatures and in the absence of oxygen.

[0042] EP 2 669 260 A1 describes the formation of an unsaturated polymer with a porous structure. The starting material used 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, nobornadiene, cyclooctene, cyclooctadiene, and derivatives thereof. This at least one carbon-containing monomer is converted in a monomer phase of a monomer-water emulsion by ring-opening metathesis polymerization (ROMP) to form the desired unsaturated polymer with the porous structure. The unsaturated polymer thus formed preferably has C=C double bonds with at least one oxidizable hydrogen atom in an α-position.

[0043] The resulting unsaturated polymer with the porous structure is then subjected to chemical and / or physical treatment and then carbonized by thermal treatment. This carbonization then results in the desired electrically conductive matrix with the open-pore structure.

[0044] The chemical and / or physical treatment preferably comprises an oxidative treatment, in particular the reaction of the unsaturated polymer in an oxidative atmosphere, preferably at temperatures in the range of 0 °C to 250 °C. The purpose of this treatment is to increase the oxygen content in the polymer before subsequent carbonization, particularly preferably to a mass fraction in the range of 25% to 40%.

[0045] 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.

[0046] The properties of the matrix, especially its pore size, can be specifically adjusted with this manufacturing variant. For this purpose, varying amounts of a surfactant can be added to the monomer-in-water emulsion. The volume fraction of the surfactant is preferably varied in the range of 0.1% to 8% (based on the amount of polymerizable monomer in the emulsion).

[0047] In a possible further development, it is preferred that the cell is characterized by at least one of the immediately following additional features a. and b.: a. In addition to carbon, the matrix contains at least one filler that has a higher or lower electrical conductivity than the carbon. b. The filler is at least one member from the group consisting of carbon black, CNTs, graphene, and metal particles.

[0048] Particularly preferably, the two immediately above features a. and b. are implemented in combination with one another.

[0049] The filler can be used to specifically increase or decrease the electrical conductivity of the matrix. To incorporate the filler, it can be added, for example, to the monomer-in-water emulsion mentioned above.

[0050] Preferably, the matrix comprises the at least one filler in a proportion in the range of 0.1 to 30 wt.%.

[0051] Particularly preferably, the electrical conductor for electrically contacting the anode consists of nickel or a nickel alloy or copper or a copper alloy.

[0052] In a possible further development, the cell is characterized by at least one of the immediately following additional features a. and b.: 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 μm.

[0053] Particularly preferably, the two immediately above features a. and b. are implemented in combination with one another.

[0054] The primary function of the electrical conductor is to supply and discharge electrical current to and from the anode. Preferably, one end of the electrical conductor is directly connected to the anode, while the other end leads to a terminal of the cell according to the invention that can be coupled to an electrical load.

[0055] Furthermore, the electrical conductor also serves, in particular, as a carrier for the electrically conductive matrix. The matrix preferably covers the electrical conductor in such a way that direct deposition of lithium on the conductor is excluded. In the case of a cell containing a liquid electrolyte, for example, this is ensured if the matrix covers all surfaces of the conductor that could come into contact with the electrolyte.

[0056] In principle, the conductor can be any flat metal substrate, i.e., in addition to the aforementioned metal foil, it can also be a strip-shaped metal foam or a strip-shaped metallic fleece. However, the aforementioned foil is preferred, especially when it is present as a rectangular substrate or in strip form.

[0057] The statement that the matrix is preferably formed as a layer already implies that the anode of the lithium-sulfur cell according to the invention is preferably also formed as a layer. Accordingly, in preferred embodiments, the cathode and the separator are also formed as layers.

[0058] Preferably, the anode, cathode, and separator are combined to form a composite body with the sequence positive electrode / separator / negative electrode. In the case of strip-shaped electrodes and separators, the composite body is preferably in the form of a coil. However, several composite bodies are often stacked on top of each other.

[0059] The cathode is preferably present as a layer with a thickness in the range of 10 µm to 200 µm.

[0060] In particular, if the cell according to the invention is a lithium-sulfur cell, the cell is preferably characterized by at least one of the immediately following additional features a. and b.: a. The cathode comprises a porous, electrically conductive matrix with an open-pore structure, and b. sulfur is embedded in this matrix.

[0061] Particularly preferably, the two immediately above features a. and b. are implemented in combination with one another.

[0062] The matrix used on the cathode side with the open-pore structure preferably has the same properties 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 resembles the structural design of the anode. Thus, in preferred embodiments, the cell may be characterized by at least one of the following additional features a. to c.: a. The cathode includes an electrical conductor for electrically contacting the cathode's active material. b. The electrical conductor is a metal foil. c. The cathode matrix forms a layer on the metallic conductor.

[0064] Particularly preferably, the two immediately above features a. and b. are implemented in combination with one another. If the cell according to the invention is a lithium-sulfur cell, all immediately above features a. to c. are preferably implemented in combination with one another.

[0065] The described cell can be produced using the method described below, which is also the subject of the present invention. This always comprises the immediately following steps a. to c.: a. Providing a porous, electrically conductive matrix, and b. Incorporating lithium into pores of the matrix to form an anode containing the lithium as the active material, and c. Combining the formed anode with a separator and a cathode containing sulfur as the active material.

[0066] The three steps do not necessarily have to be performed in the specified order. Step b can certainly be performed after step c.

[0067] Furthermore, the method according to the invention always comprises the following steps: d. To provide the porous, electrically conductive matrix, a layer of the porous organic compound to be carbonized is formed on a support and e. a metal foil is used as the support and f. the porous organic compound is carbonized on the support.

[0068] It is preferred that the process is characterized by the immediately following additional step a.: a. Carbonization takes place in the absence of oxygen.

[0069] The properties of the matrix and, in particular, preferred methods of its production, including carbonization, have already been discussed. Detailed examples of this can be found in EP 2 669 260 A1.

[0070] In particular, examples of organic compounds have already been mentioned above which can be processed into the porous organic compound, which leads to the porous matrix via the subsequent carbonization step.

[0071] According to the invention, the layer of the porous organic compound is first formed on the support. For this purpose, for example, the above-described monomer-in-water emulsion of a diene compound can be applied to the support. Metathesis of the diene compound produces the porous organic compound, which is then subjected to the oxidative treatment also described above. Subsequently, the porous organic compound, together with the support, can be exposed to a temperature at which carbonization occurs and the porous matrix is obtained.

[0072] In particularly preferred embodiments, the process is characterized by the immediately following additional step a., particularly preferably by a combination of the two immediately following steps a. and b.: a. The metallic lithium is introduced into the pores of the matrix by electrochemical deposition. b. The metallic lithium is introduced into the pores of the matrix before the anode is combined with the separator and the cathode.

[0073] The introduction of lithium into the pores of the matrix can be achieved, for example, by immersing the matrix in a lithium salt solution and connecting it to the negative pole of a DC voltage source.

[0074] In further particularly preferred embodiments, the process is characterized by the immediately following additional step a., particularly preferably by a combination of the two immediately following steps a. and b.: a. The metallic lithium is introduced into the pores of the matrix by electrochemical deposition. b. The metallic lithium is introduced into the pores of the matrix after the anode is combined with the separator and the cathode.

[0075] In this variant, a lithium-ion-containing NMC material can be used on the cathode side. The electrochemical deposition of the metallic lithium in the pores of the matrix then occurs during the first charge.

[0076] It is also possible to partially load the matrix with metallic lithium, combine the partially loaded matrix with the cathode and separator, and fully load the matrix during the initial charge. This allows an excess of lithium to be introduced into the cell to compensate for losses during the initial charge and discharge cycles.

[0077] If the cell according to the invention is a lithium-sulfur cell, it is also possible in some preferred embodiments to load the cathode with lithium sulfides and interconnect it to form a cell with a separator and a porous and electrically conductive matrix (also described above) arranged on the described electrical conductor. When a charging voltage is applied to the cell, the lithium sulfide dissolves, and the matrix is filled with lithium from the cathode.

[0078] Further features of the invention and advantages resulting from the invention will become apparent from the following exemplary embodiments and the drawings, with the aid of which the invention is explained. The embodiment described below serves merely to explain and improve the understanding of the invention and is not to be understood as limiting in any way. Example

[0079] (1) 8.00 mL (60 mmol) of dicyclopentadiene (Sigma-Aldrich) and 0.084 mL (1.9*10 -2< mmol) of the surfactant Pluronic ®< 121 (poly(ethylene glycol)-blockpoly(propylene glycol)-blockpoly(ethylene glycol); M n = 4400 g / mol; Sigma-Aldrich) were placed in a reaction vessel. The mixture of the two components was stirred at 400 rpm. 33 ml of deionized water was added dropwise with continuous stirring. After addition of the water, the mixture was stirred for another hour until a uniform emulsion was obtained. At the end of the stirring process, an initiator (8 mg (8.4*10 -3< mmol) N,N-bis(me-sityl)-4,5-dihydroimidazol-2-yl; Sigma-Aldrich) dissolved in 1 ml of toluene was added to the emulsion. (2) The emulsion resulting from step (1) was applied to a 12 µm thick copper foil in a layer thickness of 100 µm. The resulting layer was then heated to 80 °C with the copper foil. After 4 hours of curing, a white and mechanically stable layer was obtained.This was rinsed several times with dichloromethane and acetone and dried in vacuo. (3) The layer resulting from step (2) was exposed to atmospheric oxygen for four weeks at room temperature. After the four weeks, the oxygen content in the layer was 31.54% (determined by elemental analysis). (4) In a subsequent step, the copper foil with the layer was exposed to a temperature of 900 °C for 2 hours in an argon atmosphere. This carbonized the layer. (5) A 1.13 cm2< section was cut out of the copper foil coated with the carbonized layer and installed as a negative electrode in a Swagelok cell and cycled at a C charge rate of 0.1 and a C discharge rate of 0.1. During the first charge, metallic lithium was deposited in pores of the anode. An NMC cathode (active material lithium nickel manganese cobalt oxide (LiNi 0.33 Mn 0.33 Co 0.33 O 2 )) is used, as electrolyte a mixture of EC and EMC (volume ratio 3:7) with an addition of 2 vol.% vinylene carbonate (VC) and 1M LiPF 6 A commercially available polyolefin separator was used as the separator.

[0080] The cycling results are in Figure 1 As can be clearly seen, almost no capacity reduction was observed during the cycling tests.

[0081] In Figure 2An embodiment of a cell 100 according to the invention is shown schematically. This comprises the anode 101, the cathode 102, and the separator 103. The cell 100 comprises an electrical conductor 101a for electrically contacting the anode 101 and an electrical conductor 102a for electrically contacting the cathode 102. Both the electrical conductor 101a and the electrical conductor 102a are metal foils. The anode 101 comprises a porous, electrically conductive matrix 101b with an open-pore structure, in whose pores lithium is embedded.

Claims

1. Electrochemical cell having the features that a. it comprises a cathode capable of reversibly accommodating lithium ions and b. it comprises an anode containing metallic lithium as active material and c. it comprises a separator arranged between the cathode and the anode, wherein d. the anode comprises a porous, electrically conductive matrix having an open-pored structure and e. the metallic lithium of the anode is incorporated in pores of the matrix and f. the cell comprises a metal foil as an electrical conductor for making electrical contact with the anode and g. the matrix was formed directly on the metal foil by carbonization of a porous organic compound.

2. Cell according to Claim 1 having at least one of the following additional features: a. the matrix has a porosity in the range from 40% to 95%; b. the pores in the matrix are characterized by an average diameter in the range from 2 to 50 µm.

3. Cell according to either of Claims 1 or 2 having at least one of the following additional features: a. the matrix comprises carbon formed by carbonization of the organic compound; b. the matrix comprises the carbon in a proportion in the range from 50% to 100% by weight.

4. Cell according to Claim 3 having at least one of the following additional features: a. in addition to the carbon the matrix contains at least one filler having a higher or a lower electrical conductivity than the carbon; b. the filler is at least one member of the group comprising carbon black, CNT, graphene and metal particles.

5. Cell according to any of the preceding claims having at least one of the following further features: a. the matrix of the anode forms a layer on the electrical conductor; b. the layer has an average thickness in the range from 5 to 100.

6. Cell according to any of the preceding claims having the following additional features: a. the cathode comprises a porous, electrically conductive matrix having an open-pored structure and b. sulfur is incorporated in this matrix.

7. Cell according to Claim 6 having the following additional features: a. the cathode comprises an electrical conductor for making electrical contact with the active material of the cathode; b. the electrical conductor is a metal foil; c. the matrix of the cathode forms a layer on the metallic conductor.

8. Process for producing an electrochemical cell, in particular having the features of any of the preceding claims, comprising the steps of: a. providing a porous, electrically conductive matrix and b. incorporating lithium in pores of the matrix to form an anode containing the lithium as active material and c. combining the anode formed with a separator and a cathode containing sulfur as active material, characterized in that to provide the electrically conductive matrix d. a layer of the porous organic compound to be carbonized is formed on a carrier and e. a metal foil is used as the carrier and f. the porous organic compound is carbonized on the carrier.

9. Process according to Claim 8 comprising the following additional steps: a. the carbonization is carried out in the absence of oxygen.

10. Process according to either of Claims 8 or 9 comprising either of the following additional steps: a. the metallic lithium is introduced into the pores of the matrix by electrochemical deposition; b. the metallic lithium is introduced into the pores of the matrix before the anode is combined with the separator and the cathode.

11. Process according to any of Claims 8 to 10 comprising either of the following additional steps: a. the metallic lithium is introduced into the pores of the matrix by electrochemical deposition; b. the metallic lithium is introduced into the pores of the matrix after the anode is combined with the separator and the cathode.