Electrode, electrochemical energy accumulator with an electrode, and method for producing an electrode

Abstract

An electrode for an electrochemical energy accumulator includes a catalyst layer, where the catalyst layer includes an electrically conductive matrix and a chemically active material which is intercalated into the electrically conductive matrix. A protective coating is disposed on the catalyst layer, where the protective coating includes at least one metal oxide and methionine.

Claims

1. An electrode for an electrochemical energy accumulator, comprising: an active material layer, wherein the active material layer comprises: an electrically conductive matrix; and a chemically active material which is intercalated into the electrically conductive matrix; and a protective coating disposed on the active material layer, wherein the protective coating comprises at least one metal oxide and methionine; wherein the protective coating is formed in two layers including a methionine layer containing the methionine and a metal oxide layer containing the at least one metal oxide.

2. The electrode according to claim 1, wherein the protective coating completely encloses the catalyst active material layer.

3. The electrode according to claim 1, wherein the methionine layer is deposited on the catalyst active material layer.

4. The electrode according to claim 1, wherein the metal oxide layer is deposited on the methionine layer.

5. The electrode according to claim 1, wherein the metal oxide layer is an aluminum oxide layer.

6. An electrochemical energy accumulator, comprising: an electrode according to claim 1; a counter electrode; and an electrolyte disposed between the electrode and the counter electrode.

7. The electrochemical energy accumulator according to claim 6, wherein the electrode is a cathode and wherein the counter electrode is an anode.

8. A method for producing the electrode according to claim 1, comprising the steps of: intercalating the chemically active material into the electrically conductive matrix to form the active material layer; disposing the protective coating on the active material layer; applying the active material layer to a substrate; providing of the methionine in powder form; providing a metal-containing precursor compound and a counter compound; forming a methionine layer by depositing the powder methionine on the active material layer; and forming a metal oxide layer by depositing at least one layer of the metal-containing precursor compound on the methionine layer and thereafter applying the counter compound to the deposited metal-containing precursor compound.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically shows a sectional view of a design example of an electrochemical energy accumulator with an electrode and a counter electrode; and

(2) FIG. 2 schematically shows an exemplary operation of a method for the production of an electrode for an electrochemical energy accumulator.

DETAILED DESCRIPTION OF THE DRAWINGS

(3) Parts corresponding to each other are provided in all Figures with the same reference signs.

(4) FIG. 1 shows schematically a longitudinal section of an electrochemical energy accumulator 1, which is formed, for example, as a single cell for a rechargeable battery. e.g., a lithium-sulfur battery.

(5) The electrochemical energy accumulator 1, comprising an electrode 2 and a counter electrode 3, wherein the electrode 2 is formed as a cathode and the counter electrode 3 is formed as an anode. An ion-conducting electrolyte 4 is arranged between electrode 2 and counter electrode 3. Furthermore, an ion-conducting separator 5 is arranged between the electrode 2 and the counter electrode 3.

(6) The electrode 2 comprising a substrate 2.1, is provided with a coated active material layer 2.2. The substrate 2.1 is formed by an electrically insulating material, comprising, for example, polycarbonate. The active material layer 2.2 is formed by a composite material, comprising an electrically conductive matrix and a chemically active material.

(7) The electrically conductive matrix is formed by an electrically conductive, porous and mechanically flexible carbon structure, e.g., graphite or carbon black. The chemically active material, comprising sulfur compounds, especially sulfur, is intercalated into the electrically conductive matrix.

(8) The counter electrode 3 also comprising a substrate 3.1, for example, polycarbonate, which is provided with an active material layer 3.2. The active material layer 3.2 is also formed as a composite material, comprising an electrically conductive matrix and a chemically active material. The electrically conductive matrix for the counter electrode 3, for example, consists of an electrically conductive carbon structure and a silicon structure. The chemically active material herein is lithium or a lithium alloy.

(9) The electrolyte 4 comprising a liquid, non-aqueous electrolyte solution which contains, for example, a non-aqueous solvent and a lithium salt dissolved therein. The separator 5, for example, is formed as a semi-permeable membrane, permeable in particular to ions, which is formed, for example, by a microporous ceramic, a microporous polymeric film or a microporous glass fiber fleece.

(10) The chemically active materials of the electrode 2 and counter electrode 3 can be integrated homogeneously over the entire electrode 2 or counter electrode 3 into their electrically conductive matrices. The chemically active materials are used for a chemical reaction between the electrode 2 and counter electrode 3, especially during charging and discharging of the electrochemical energy accumulator 1, as described in more detail below using the example of a lithium-sulfur battery.

(11) When the electrochemical energy accumulator 1 is discharged, the energy stored in the counter electrode 3 intercalated lithium is oxidized into lithium ions and electrons. The lithium ions migrate through electrolyte 4 to the electrode 2, while at the same time the electrons are transferred via an external circuit S from the counter electrode 3 to electrode 2. An energy consumer 6 is arranged in the outer circuit S, which is supplied with energy by the electron current. In the electrode 2, the lithium ions are absorbed by a reduction reaction in which the sulfur is reduced to lithium-sulfide. The electrochemical reaction, when discharging the electrochemical energy accumulator 1 can be described as follows:

(12) Counter electrode 3: Li.fwdarw.Li.sup.++e.sup. and

(13) Electrode 2: S.sub.8+2Li.sup.++e.sup..fwdarw.Li.sub.2S.sub.8.fwdarw.Li.sub.2S.sub.6.fwdarw.Li.sub.2S.sub.4.fwdarw.Li.sub.2S.sub.2.fwdarw.Li.sub.2S.

(14) When charging the electrochemical energy accumulator 1, an energy source (not shown) is connected to the electrode 2 and the counter electrode 3. In the electrode 2, lithium from lithium-sulfide is oxidized into lithium cations and electrons, whereby the lithium cations migrate via the electrolyte 4 and the electrons via the outer circuit S back to the counter electrode 3.

(15) The storage of the chemically active material, i.e., lithium ions, for example, during the charging process of the electrochemical energy accumulator 1, as well as the retrieval of the chemically active material during the discharge of the electrochemical energy accumulator 1 leading to very strong volumetric changes of the electrode 2 and the counter electrode 3. This is also called breathing of the electrode 2 or the counter electrode 3.

(16) When discharging the electrochemical energy accumulator 1, short-chain polysulfides, e.g., Li2S2, and/or long-chain polysulfides, e.g., Li2S8, Li2S6, which may not have been fully formed into elemental sulfur in the electrode 2, diffuse out from the active material layer 2.2 of the electrode 2 and migrate via the electrolyte 4 to the counter electrode 3, as this electrode in the electrolyte 4 is not soluble. At the counter electrode 3, a lithium-sulfide layer can be formed by means of the polysulfides, significantly reducing the service life of the electrochemical energy accumulator 1. Additionally, the chemically active material embedded in the electrically conductive matrix of electrode 2 is successively degraded and the risk of a short circuit between electrode 2 and counter electrode 3 increases significantly.

(17) To prevent the chemically active material from dissolving out of the electrode 2 or at least reduce it, the active material layer 2.2 shall be provided with a protective coating 2.3, which is described below in more detail.

(18) The protective coating 2.3 is preferably formed in two parts, comprising a methionine layer 2.3.1 containing methionine and a metal oxide layer 2.3.2 containing at least one metal oxide. The methionine layer 2.3.1 is directly on the active material layer 2.2 and therefore forms a lower or inner layer of the protective coating 2.3.

(19) The methionine contained in the methionine layer 2.3.1 is a sulfurous, proteinogenic amino acid, also known as 2-amino-4-methylmercaptobutyric acid, and due to its excellent chemical stability, it is suitable for use in electrode 2 as part of the protective coating 2.3.

(20) The metal oxide layer 2.3.2 is deposited on the methionine layer 2.3.1 and forms an upper or outer layer of the protective coating 2.3. For example, the metal oxide layer 2.3.2 formed as aluminum oxide layer. The metal oxide layer 2.3.2 is an oxygen compound of a metal and has electrically insulating properties. Further, the metal oxide layer 2.3.2 is ion-selective and permeable to ions of the counter electrode 3, e.g., lithium ions. This is made possible by specifying a pore size for the metal oxide layer 2.3.2 of less than 2 micrometers and a layer thickness of more than 3.5 micrometers. Also, the metal oxide layer 2.3.2 is chemically stable compared to the electrolyte 4. This is particularly advantageous because, due to the semi-permeability of the metal oxide layer 2.3.2, the electrolyte 4 penetrates at least partially into the metal oxide layer 2.3.2 and into the methionine layer 2.3.1.

(21) Both the methionine layer 2.3.1 and the metal oxide layer 2.3.2 are permeable to the chemically active material of the counter electrode 3, but impermeable to the chemically active material of the electrode 2. In particular, the methionine layer 2.3.1 and the metal oxide layer 2.3.2 are impermeable to polysulfide compounds.

(22) At a boundary surface between the methionine layer 2.3.1 and the metal oxide layer 2.3.2 they are cross-linked with each other, so that the methionine layer 2.3.1 and the metal oxide layer 2.3.2 are connected with each other in the form of a so-called in-situ composite.

(23) FIG. 2 shows an exemplary operation of a method for the production of electrode 2 described in FIG. 1.

(24) In a first step S1, the active material layer 2.2 is applied to the substrate 2.1. The active material layer 2.2 can be applied to the substrate 2.1, e.g., by means of sputter coating or atomic layer deposition.

(25) In a second step S2, the methionine layer 2.3.1 is deposited on the active material layer 2.2. Also, methionine powder is distributed as evenly as possible on the active material layer 2.2, and deposited, for example, by means of electrostatic deposition. Here, the methionine is electrically charged, e.g., by means of high voltage or friction, and then transferred to the active material layer 2.2. A distribution of the methionine on the active material layer 2.2 is preferably applied by means of a uniform distribution of finely divided crystals on the surface of the active material layer 2.2. The active material layer 2.2 can additionally be charged with an electrical potential to improve the electrostatic adhesion of the methionine to the active material layer 2.2. Alternatively, the formation of the methionine layer 2.3.1 can also be controlled by means of powder coating. A homogeneous formation of the methionine layer 2.3.1 is not absolutely necessary, since the methionine layer 2.3.1 is partially detached from the active material layer 2.2 under a later influence of the electrolyte 4. A temperature during the deposition of methionine corresponds to a room temperature of about 25 C.

(26) In a third step S3, the metal oxide layer 2.3.2 is formed on the methionine layer 2.3.1. This is done, for example, by means of atomic layer deposition. First, a metal-containing precursor compound and a counter compound are provided. The metal oxide layer 2.3.2 is then formed by means of depositing at least one layer of the metal-containing precursor compound on the methionine layer 2.3.1 and then applying the counter-component to the deposited metal-containing precursor compound.

(27) Using the particular example of a deposition of aluminum oxide, for example trimethylaluminum can be used as a metal-containing precursor compound which is used in presence of water as a counter compound. For deposition, trimethylaluminum is introduced into a reaction chamber in which electrode 2 is also located. After a specified time, an excess of precursor molecules is removed by pumps to prevent an unwanted chemical reaction at a later point in time. Afterwards, water vapor, for example, is introduced into the reaction chamber as a counter compound, which reacts with the trimethylaluminum to become aluminum oxide. Excess water and any reaction products are then also removed by means of pumps.

(28) The deposition of the metal oxide layer 2.3.2 is performed at a temperature between 80 C. and 100 C. and is therefore below a melting temperature of sulfur, which is above 127 C. The sulfur intercalated in the electrically conductive matrix of the active material layer 2.2 is thereby not affected during deposition.

(29) With the electrode 2 produced in this manner, dissolution of the sulfur intercalated in the electrically conductive matrix is reduced, preferably avoided, even at high temperatures. If sulfur is nevertheless dissolved out of the active material layer 2.2 in the event of an unscheduled reaction or if the temperature is exceeded for a short time, it remains electrically contacted and can be stored again in the electrically conductive matrix in later charging cycles.