ANODE FOR A BATTERY CELL, METHOD FOR MANUFACTURING AN ANODE, AND BATTERY CELL

An anode for a battery cell, including an active material containing silicon, and a current collector to which the active material is applied, and an anode coating which is applied to the active material. The anode coating contains graphite and a binder. A method for manufacturing an anode, and a battery cell which includes at least one anode, are also described.

CROSS REFERENCE

The present application claims the benefit under 35 U.S.C. §119 of German Patent Application No. DE 102015212182.8 filed on Jun. 30, 2015, which is expressly incorporated herein by reference in its entirety.

FIELD

The present invention relates to an anode for a battery cell which includes a silicon-containing active material. Moreover, the present invention relates to a method for manufacturing an anode, and a battery cell which includes an anode according to the present invention.

BACKGROUND INFORMATION

Electrical energy may be stored with the aid of batteries. Batteries convert chemical reaction energy into electrical energy. A distinction is made between primary batteries and secondary batteries. Primary batteries are non-rechargeable, while secondary batteries, also referred to as accumulators, are rechargeable. A battery includes one or multiple battery cells.

In particular, so-called lithium-ion battery cells are used in an accumulator. They are characterized, among other features, by high energy densities, thermal stability, and extremely low self-discharge. Lithium-ion battery cells are used, for example, in motor vehicles, in particular in electric vehicles (EVs), hybrid vehicles (HEVs), and plug-in hybrid vehicles (PHEVs).

Lithium-ion battery cells include a positive electrode, also referred to as a cathode, and a negative electrode, also referred to as an anode. The cathode and the anode each include a current collector, to which an active material is applied. The active material for the cathode is a lithium-metal oxide compound such as LiCoO2in particular. The active material for the anode is silicon, for example. However, graphite is also widely used as active material for anodes.

Lithium atoms are intercalated into the active material of the anode. During operation of the battery cell, i.e., during a discharging operation, electrons flow in an external circuit from the anode to the cathode. During a discharging operation, lithium ions migrate from the anode to the cathode within the battery cell. In the process, the lithium ions are reversibly deintercalated from the active material of the anode, also referred to as delithiation. During a charging operation of the battery cell, the lithium ions migrate from the cathode to the anode. In the process, the lithium ions are reversibly reintercalated into the active material of the anode, also referred to as lithiation.

The electrodes of the battery cell have a foil-like design and are wound to form an electrode winding, with a separator situated in between which electrically and mechanically separates the anode from the cathode. Such an electrode winding is also referred to as a “jelly roll.” The electrodes may also be layered one above the other to form an electrode stack. The electrodes and the separator are surrounded by an electrolyte which is generally liquid. The electrolyte is conductive for the lithium ions, and allows transport of the lithium ions between the electrodes.

Silicon, as the active material of the anode, has a higher storage capacity for lithium ions compared to graphite. However, the liquid electrolyte, together with the contained lithium, deposits on the surface of the active material and is thereby decomposed. In the process, a layer, referred to as a solid electrolyte interphase (SEI), forms. Lithium deposited at that location is no longer available for transporting lithium ions between the electrodes.

During operation of the battery cell, an anode with silicon as the active material experiences volume changes. Such a volume change may be as high as 300%. When lithium ions are intercalated the active material expands, and when lithium ions are deintercalated the active material contracts. Such volume changes may result in deformations of the active material and cracks, even chipping, in the SEI. As the result of further decomposition of the electrolyte, accompanied by further deposition of additional lithium, a new SEI is formed.

A generic battery cell which includes an anode and a cathode, in which the active material of the anode includes silicon, is described in German Patent Application No. DE 10 2012 212 299 A1, for example.

U.S. Patent Application Publication No. 2012/0231326 describes an anode for a battery cell which contains porous silicon and is provided with a coating. The coating is made of carbon, for example.

In addition, U.S. Patent Application Publication No. 2012/0100438 A1, German Patent Application No. DE 11 2012 001 289 T5, and U.S. Patent Application Publication No. 2013/0189575 A1 describe anodes made of porous silicon for battery cells, which are provided with a carbon coating.

SUMMARY

An anode for a battery cell is provided. The anode includes an active material which contains silicon. The anode also includes a current collector to which the anodic active material is applied, and an anode coating which is applied to the anodic active material. The anodic active material is preferably designed as a monolith, and has a maximum thickness of 75 microns.

According to an example embodiment of the present invention, the anode coating applied to the anodic active material contains graphite and a binder. The anode coating may thus be applied to the anodic active material relatively easily, namely, in the form of a slip layer and preferably with the aid of a doctor knife.

In addition, the graphite contained in the anode coating acts as an active anode material, and may thus pick up lithium ions during charging of the battery cell.

In a first charging operation of the battery cell, a stable protective layer, referred to as a solid electrolyte interphase (SEI), forms, in particular on the anode coating. This protective layer, which is impermeable to electrolyte, prevents contact of electrolyte with the silicon of the anodic active material.

The anodic active material advantageously has porosity. The anodic active material is thus porous, and has pores. The maximum diameter of the pores of the anodic active material is preferably approximately 50 nanometers. Due to the porosity, during a charging operation the anodic active material is able to expand without destroying the protective layer.

The porosity is at least 20%, preferably between 60% and 80%, of the volume of the active material.

The binder in the anode coating preferably contains carboxymethylcellulose (CMC). The binder in the anode coating may also contain other substances, in particular styrene butadiene rubber (SBR), polyacrylic acid (PAA), lithium polyacrylic acid (LiPAA), alginic acid (alginate), and polyvinyl alcohol (PVA). Mixtures of such substances are also possible.

The anode coating contains binder in a proportion of between 2% and 20%. The proportion of binder is preferably 5% to 10%.

The remaining portion of the anode coating may include up to 100% graphite. However, it is also possible for the remaining portion of the anode coating to contain conductive carbon black in addition to graphite. The quantity ratio of the remaining portion of the anode coating is preferably between 100% graphite to 0% conductive carbon black and 75% graphite to 25% conductive carbon black.

According to one advantageous refinement of the present invention, an intermediate layer is situated between the current collector and the anodic active material. The intermediate layer forms a relatively good electrically conductive transition between the silicon of the anodic active material and the current collector.

The intermediate layer advantageously contains carbon black and a binder. An intermediate layer designed in this way increases the adhesion between the silicon of the anodic active material and the current collector.

The binder in the intermediate layer preferably contains carboxymethylcellulose (CMC). The binder in the intermediate layer may also contain other substances, in particular styrene butadiene rubber (SBR), polyacrylic acid (PAA), lithium polyacrylic acid (LiPAA), alginic acid (alginate), and polyvinyl alcohol (PVA). Mixtures of such substances are also possible.

The intermediate layer contains binder in a proportion of between 2% and 20%. The proportion of binder is preferably 5% to 10%.

Moreover, a method for manufacturing an anode according to the present invention is provided. An anode coating containing graphite and a binder, in the form of a slip layer, is doctored over an anodic active material containing silicon.

The anodic active material is preferably produced by initially creating a monolithic wafer. Porosity is subsequently introduced into the monolithic wafer by electrochemical etching, for example.

According to one advantageous refinement of the present invention, the anodic active material is applied to a current collector with the aid of an intermediate layer containing carbon black and a binder. The anodic active material is adhered to the current collector with the aid of the intermediate layer.

Moreover, a battery cell is provided which includes at least one anode according to the present invention.

A battery cell according to the present invention is advantageously used in an electric vehicle (EV), in a hybrid vehicle (HEV), in a plug-in hybrid vehicle (PHEV), or in a consumer electronic product. Consumer electronic products are understood in particular to mean mobile telephones, tablet PCs, or notebooks.

Due to the design according to the present invention of an anode, a stable protective layer, referred to as a solid electrolyte interphase (SEI), forms which prevents subsequent contact of electrolyte with the anodic active material. The overall volume of the monolithically designed anodic active material, including the contained pores, changes only insignificantly during a charging operation and during a discharging operation. Therefore, the anodic active material is able to expand during a charging operation without destroying the protective layer. Thus, no significant mechanical stresses develop between the anodic active material and the anode coating. In this way, cracks in the protective layer as well as chipping of the protective layer, with unavoidable deformation of the anodic active material, are still largely avoided during subsequent charging operations and discharging operations of the battery cell. The formation of the protective layer on the anode coating thus results in passivation of the anodic active material, which increases the cycle stability of the anode.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

A battery cell2is schematically illustrated inFIG. 1. Battery cell2includes a cell housing3having a prismatic design, in the present case a cuboidal design. In the present case, cell housing3has an electrically conductive design and is made of aluminum, for example. However, cell housing3may also be made of an electrically insulating material, for example plastic.

Battery cell2includes a negative terminal11and a positive terminal12. A voltage provided by battery cell2may be tapped via terminals11,12. In addition, battery cell2may also be charged via terminals11,12. Terminals11,12are situated spaced apart from one another on a top surface of prismatic cell housing3.

An electrode stack which includes two electrodes, namely, an anode21and a cathode22, is situated within cell housing3of battery cell2. Anode21and cathode22each have a foil-like design, and are stacked to form an electrode stack with a separator18situated in between. It is also possible to provide multiple electrode stacks in cell housing3. An electrode winding, for example, may also be provided instead of the electrode stack.

Anode21includes a current collector31, which has a foil-like design. Current collector31of anode21has an electrically conductive design and is made of a metal, for example copper. Current collector31of anode21is electrically connected to negative terminal11of battery cell2.

Anode21also includes an anodic active material41which likewise has a foil-like design. Anodic active material41contains silicon as the base material. Anodic active material41is designed as a monolith. Anodic active material41has a maximum thickness of 75 microns. Anodic active material41also has a porous design, and has pores55. The maximum diameter of pores55of anodic active material41is approximately 50 nanometers.

During the production of the anodic active material41, for example a monolithic wafer is initially created. Porosity is subsequently introduced into the monolithic wafer by electrochemical etching, for example.

However, it is also conceivable to produce a monolithic layer of silicon with the aid of chemical vapor deposition (CVD) and to subsequently introduce porosity into this layer by electrochemical etching, for example. This method is particularly suited for producing relatively thin anodic active material41, in particular in a thickness of less than one micron.

An intermediate layer61is situated between current collector31and anodic active material41. In the present case, intermediate layer61of anode21includes carbon black and a binder. The binder in intermediate layer61contains carboxymethylcellulose (CMC). The binder in intermediate layer61may also contain other substances, in particular styrene butadiene rubber (SBR), polyacrylic acid (PAA), lithium polyacrylic acid (LiPAA), alginic acid (alginate), and polyvinyl alcohol (PVA). Mixtures of such substances are also conceivable.

Intermediate layer61of anode21is used for contacting anodic active material41with current collector31. Intermediate layer61of anode21ensures relatively good adhesion of anodic active material41on current collector31. In addition, intermediate layer61of anode21results in a relatively good electrically conductive transition between anodic active material41and current collector31.

An anode coating51is applied to anodic active material41. In the present case, anode coating51includes graphite and a binder. Conductive carbon black may also be contained in anode coating51. The binder in anode coating51likewise contains carboxymethylcellulose (CMC). The binder in anode coating51may also contain other substances, in particular styrene butadiene rubber (SBR), polyacrylic acid (PAA), lithium polyacrylic acid (LiPAA), alginic acid (alginate), and polyvinyl alcohol (PVA). Mixtures of such substances are also possible.

Anode21has a layered design with multiple layers, and includes current collector31, intermediate layer61situated thereon, anodic active material41situated on the intermediate layer, and anode coating51situated on the anodic active material. Anode coating51faces separator18of battery cell2. Current collector31and anode coating51enclose anodic active material41and intermediate layer61.

Cathode22includes a cathodic active material42which has a foil-like design. Cathodic active material42includes a lithium-metal oxide compound, for example lithium-cobalt oxide (LiCoO2), as base material. Cathode22also includes a current collector32, which likewise has a foil-like design. Cathodic active material42and current collector32are placed flatly against one another and joined together.

Current collector32of cathode22has an electrically conductive design and is made of a metal, for example aluminum. Current collector32of cathode22is electrically connected to positive terminal12of battery cell2.

Anode21and cathode22are separated from one another by separator18. Separator18likewise has a foil-like design. Separator18has an electrically insulating design, but is ionically conductive, i.e., is permeable for lithium ions70.

FIG. 2shows a schematic sectional representation of anode21directly after manufacture of battery cell2. This means that neither a charging operation nor a discharging operation of battery cell2has yet taken place.

Anode21, as previously mentioned, has a layered design and includes current collector31, intermediate layer61situated thereon, anodic active material41situated on the intermediate layer, and anode coating51situated on the anodic active material.

Electrolyte15, which in the present case is liquid, surrounds anode21. Electrolyte15contacts primarily anode coating51. In addition, electrolyte15penetrates into pores55of anodic active material41, and in the process contacts boundary surfaces of pores55.

Free lithium ions70are present in electrolyte15. Free lithium ions70are thus present on the surface of anode coating51facing away from anodic active material41, and also in pores55of anodic active material41.

In a subsequent charging operation of battery cell2, lithium ions70, which are still free, migrate to anode21and are intercalated into anodic active material41, also referred to as lithiation. Lithium ions70are able to penetrate anode coating51.

FIG. 3shows a schematic sectional representation of anode21fromFIG. 2during operation in battery cell2. This means that charging operations as well as discharging operations of battery cell2have already taken place.

Decomposition of liquid electrolyte15has taken place at the surface of anode coating51, and liquid electrolyte15together with contained lithium ions70has deposited on the surface of anode coating51. A protective layer75, known as a solid electrolyte interphase (SEI), has thus formed on the surface of anode coating51.

Decomposition of liquid electrolyte15has also taken place at the boundary surfaces of pores55of anodic active material41. Liquid electrolyte15together with contained lithium ions70has hereby deposited on the boundary surfaces of pores55of anodic active material41. A protective layer75, known as a solid electrolyte interphase (SEI), has likewise formed on the boundary surfaces of pores55of anodic active material41.

Resulting protective layer75is permeable to lithium ions70. However, resulting protective layer75is impermeable to electrolyte15. Further contact of electrolyte15with anode coating51and with anodic active material41is thus prevented.

In a subsequent charging operation of battery cell2, free lithium ions70migrate to anode21and are intercalated into anodic active material41. In the process, anodic active material41expands. Due to pores55, sufficient free space is present for the expansion of anodic active material41.

During intercalation of lithium ions70, porous anodic active material41therefore expands predominantly in the direction of its pores55. The diameter of pores55of anodic active material41thereby decreases. The change in the overall volume of monolithically designed anodic active material41, including contained pores55, is insignificant.

Thus, also no significant mechanical stresses develop between anodic active material41and anode coating51applied to anodic active material41.

Cracks in protective layer75as well as chipping of protective layer75at the boundary surfaces of pores55of anodic active material41and at the surface of anode coating51are thus largely avoided. Protective layer75which is present is thus maintained during subsequent charging operations and discharging operations.

The present invention is not limited to the exemplary embodiments described here and the aspects highlighted therein. Rather, numerous modifications within the range set forth in the claims are possible which are within the scope of activities carried out by those skilled in the art.