DE102017202442A1 - Flexible pantograph foils - Google Patents

Abstract

The present invention relates to an electrode for a lithium-ion cell comprising a current collector of metal foil and an active layer applied thereto, wherein the lithium-added active layer has a maximum volume expansion ΔV of 33% or more or a maximum change in length ΔL parallel to the metal foil of 10% or has more. The metal foil has an extensibility which is greater than the change in length ΔL of the active layer.

Description

Technical area

The present invention relates to an electrode for a lithium ion cell with a flexible current collector foil.

Technical background

The negative electrode (anode) of a lithium-ion cell typically employs carbon-based intercalation materials, such as graphite, deposited on a copper foil as a current collector.

To increase the energy density there is an effort to make the current collector foil as thin as possible in order to reduce the weight. In addition, an attempt is also being made to replace the carbon-based active material with a higher specific-capacity active material, such as silicon / carbon composites, silicon alloys, or silicon.

However, such silicon-based anode active materials undergo relatively large volume changes of up to about 300% in the uptake of lithium. This corresponds to a change in length of about 60% in each spatial direction. Due to this volume change, the active material layer is subjected to considerable mechanical stresses during cyclization. These can lead to breaks and detachments in the active material coating. In addition, silicon-based active materials are used in very small layer thicknesses, so that the current conductor should also be chosen very thin in order to save weight and to obtain a favorable energy density. Especially with very thin Stromableiterfolien, however, there is a risk that due to the change in volume of the active material layer of the current collector itself can break.

Conversion-active materials such as FeF ₃ , which is currently being investigated as a cathode active material as an alternative to the commonly used intercalation materials, have a similar problem. In the conversion reaction with FeF ₃ , three Li ⁺ / e ^{- are converted} per formula unit, resulting in volume changes up to about 100% in the cyclization, corresponding to a change in length of about 25% in each spatial direction.

Due to the more complex and slower compared to conventional active materials kinetics thin film thicknesses are again sought to increase the reaction rate, with the result that the current conductor should also be as thin as possible in order to achieve a favorable energy density despite the thin active material layer. The change in volume of the active material layer in conjunction with the small layer thickness of the current collector foil can in turn lead to fractures during the cyclization.

Disclosure of the invention

In view of the above problem, it is an object of the present invention to provide an electrode for a lithium ion cell employing a high volumetric expansion active material in cyclization and having a reduced risk of breakage in the active layer or in the current collector.

The metal foils which are conventionally used as a current collector, e.g. Roll-pressed copper foil usually has a relatively high tensile stiffness but only a low ductility, typically in the range of 0.1%.

It has been found that, in conjunction with high volume expansion active materials, the tensile elongation properties of the metal foil used as the current collector are of significant importance, and that the problems described above can be overcome or mitigated by using a flexible, stretchable metal foil as a current collector.

Thus, the invention provides an electrode comprising a current collector of metal foil and an active layer deposited thereon, said lithium receiving active layer having a maximum volume expansion ΔV of 33% or more or a maximum length change ΔL parallel to the metal foil of 10% or more , and the extensibility of the metal foil is greater than the change in length of the active layer.

In this case, the active layer denotes the layer applied to the current collector and containing the active material. It may consist of an active material throughout, such as silicon or an active material that forms lithium alloys, or it may be a composite material of active material and other components, such as an electrically conductive additive and / or an additive having lithium ion conductivity. Binders may also be included.

The volume expansion ΔV of the active layer is the volume difference between the minimum volume V _min and the maximum volume V _{max of} the material _composing the active layer normalized to V _min as 100%. The same applies to the change in length ΔL parallel to the film. V _min or L _min corresponds to the state with minimal lithium loading, ie the oxidized state, and V _max or L _max corresponds to the state with maximum lithium loading, ie the reduced state of the material making up the active layer.

$$\Delta \text{V}=\text{100\%*}\left({\text{V}}_{\text{max}}-{\text{V}}_{\text{min}}\right)/{\text{V}}_{\text{min}}$$

wobei bei V_max und V_min das Volumen des Materials der aktiven Schicht bei maximaler bzw. minimaler Lithiumbeladung darstellen, und L_max und Lmin die zugehörigen Längen darstellen.Thus, ΔV and ΔL are defined by the following formulas: $$\Delta \text{L}=\text{100\% *}\left({\text{L}}_{\text{Max}}-{\text{L}}_{\text{min}}\right)/{\text{L}}_{\text{min}}$$

$$\Delta \text{V}=\text{100\% *}\left({\text{V}}_{\text{Max}}-{\text{V}}_{\text{min}}\right)/{\text{V}}_{\text{min}}$$

where at V _max and V _{min represent} the volume of the material of the active layer at maximum and minimum lithium loading, respectively, and L _max and Lmin represent the associated lengths.

$$\Delta \text{V}=\text{100\%*}\left[\left(\Delta \text{L/100\%}\right)+{1)}^3-1\right]$$

In general, the volume change of the active layer is approximately isotropic, so that ΔL and ΔV can be converted into each other as follows: $$\Delta \text{L}=\text{100\% *}\left[\left(\Delta \text{V / 100\%}\right)+{1)}^{1/3}-1\right]$$

$$\Delta \text{V}=\text{100\% *}\left[\left(\Delta \text{L / 100\%}\right)+{1)}^3-1\right]$$

The methods for determining ΔL and ΔV are not particularly limited, and may depend on the kind of the active material or the composition of the active layer.

If the active layer consists only of materials which are both compact and nonporous both in the oxidized and the reduced state and have a defined density and composition, ΔL and ΔV can be calculated from the known substance parameters (ie density and molecular weight of the oxidized and reduced steps). the respective pure substances are calculated.

Furthermore, ΔL and ΔV can also be measured directly. For example, the measurement can be made on a test electrode during operation by means of a dilatometer, or it can be performed in a special cell with in-situ electron microscopy in a plan view or cross section. Alternatively, light microscopic or electron micrographs of the charged and uncharged electrodes can be compared. ΔL can be read off, for example, from the change in thickness of the active layer. For the determination of ΔV, for example, the microscopic observation of the cross section comes into question.

Examples of anode active materials with high volume expansion during cyclization include alloying agents such as silicon (ΔV = 300%), aluminum, gallium, germanium, tin, silver, alloys of these, or even phosphorus. Optionally, these materials can also be used as composite material with carbon. Preferred are silicon and silicon / carbon composites. As further examples, low-potential conversion active materials can be cited against Li / Li +, such as metal oxides, phosphides, nitrides, and sulfides, eg, Fe ₂ O ₃ . Examples of cathode active materials with large ΔV or ΔL are in particular conversion materials based on fluoride, such as FeF ₃ (ΔV = 100%), or even sulfides with high potential to Li / Li +, such as CuS.

All of these anode and cathode active materials are capable of stoichiometrically accepting and releasing a high ratio of lithium per formula unit, which explains the large volume change.

If a combination of active materials is used in the active layer, ΔV and ΔL are generally the weighted average of the components.

When using composite materials containing passive components such as electrically conductive additives, lithium ion conductors or binders whose volume does not change during cyclization, ΔV and ΔL decrease accordingly. If the active layer has pores, then ΔV and ΔL also decrease because the volume change is partially absorbed by filling the pores.

In this way, depending on the type of active material and proportion of passive components, ΔV or ΔL values can be set in a range of usually 33 to 330%.

As a metal foil for the current collector are metal foils of copper, nickel, aluminum or alloys of these into consideration. For electrodes with a strongly negative potential of less than 1 V compared to Li / Li + copper foil is preferably used, otherwise aluminum foil is preferred. The film thickness is preferably 20 .mu.m or smaller, in the case of copper foil particularly preferably 10 .mu.m or less, and in the case of aluminum foil particularly preferably 15 .mu.m or less.

The extensibility of the metal foil must be greater than the length expansion of the active layer and is therefore at least 10%, preferably at least 20%, more preferably at least 50%. The greater the extensibility of the metal foil, the greater may be according to the invention, the volume or length expansion of the applied active layer on her. The extensibility can be determined as breaking elongation according to ISO 6892.

In addition, metal foil is preferably as elastic as possible and has a high elongation at break. The elongation to the elastic limit (0.2% proof stress) is preferably at least 1%, more preferably at least 5%.

Metal foils with such extensibility and elasticity can be obtained, for example, by soft annealing or by suitable choice of the alloy. Alternatively, mechanical processing, e.g. into a suitable expanded metal into consideration.

To produce the electrode according to the invention, the active material layer is deposited on the metal foil according to a method known per se. The layer thickness is generally 10 to 100 .mu.m, preferably 20 to 70 microns.

For example, a silicon or tin layer may be deposited by vapor deposition techniques, such as vapor deposition or sputtering. For tin or tin alloys, electrochemical plating may also be considered. For applying composite material layers, a slurry coating in which a suspension of the active material, typically in combination with passive ingredients such as binders, electrically conductive additives, and optionally a lithium ion conductor is applied to the film, followed by drying and pressing or sintering, respectively ,

Depending on the type of active material, the electrode according to the invention is used as the anode or cathode in a lithium-ion cell. The type of cell is not particularly limited, and all cell types are suitable which are suitable for the use of the respective active material. Since most of the candidate active layers with large volume expansion are primarily intended for solid state cells, the electrode is typically used in solid state cells. However, the present invention is not limited thereto, and use in liquid electrolyte cells or gel electrolyte cells may also be considered.

The construction of the cell is not particularly limited. However, the larger the track length of the electrode, the more pronounced is the possible absolute offset due to the change in volume, which may be particularly important for web lengths used in winding-type cells. Therefore, use in stack-type cells, for example, in pouch (pouch) construction, is preferred.

Claims (9)

An electrode for a lithium-ion cell comprising a metal foil current collector and an active layer deposited thereon, said lithium-receiving active layer having a maximum volume expansion ΔV of 33% or more or a maximum length change ΔL parallel to the metal foil of 10% or more; Elongation of the metal foil is greater than the change in length ΔL of the active layer, wherein ΔV and ΔL are defined by the following formulas: $$\Delta \text{V}=\text{100\% *}\left({\text{V}}_{\text{Max}}-{\text{V}}_{\text{min}}\right)/{\text{V}}_{\text{min}}$$

$$\Delta \text{L}=\text{100\% *}\left({\text{L}}_{\text{Max}}-{\text{L}}_{\text{min}}\right)/{\text{L}}_{\text{min}}$$

where at V _max and V _{min represent} the volume of the material of the active layer at maximum and minimum lithium loading, respectively, and L _max and L _{min represent} the associated lengths. Electrode according to Claim 1 wherein the extensibility of the metal foil is 20% or greater. Electrode according to Claim 1 or 2 wherein the metal foil has a thickness of 20 μm or less. Electrode according to one of Claims 1 to 3 wherein the active layer has a thickness of 20 to 70 μm. Electrode according to one of Claims 1 to 4 in which the metal foil is a copper foil, and the active layer contains as active material silicon, tin or their alloys. Electrode according to one of Claims 1 to 4 wherein the metal foil is an aluminum foil and the active layer contains as active material a conversion active material. Electrode according to one of Claims 1 to 6 wherein the active layer is composed of a composite material containing an anode or cathode active material, a lithium ion conductive solid state electrolyte, and optionally a binder and / or an electrically conductive agent. Lithium-ion cell containing at least one electrode according to one of the Claims 1 to 7 includes. Lithium ion cell according to Claim 8 , which is constructed as a stack-type cell in bag form.