DE102019210857A1 - Lithium-ion battery cell and process for their manufacture - Google Patents

Abstract

The invention relates to a lithium-ion battery cell (10) with a cathode (14) which has a current collector (14a) and LiNi0.5Mn1.5O4 applied to the current collector (14a) as cathode active material (14b), with the cathode -Active material (14b) a first layer (18) made of a lithium niobium oxide is applied, and a second layer (20) made of a lithium phosphate is applied to the first layer (20). The invention also relates to a method for producing such a lithium-ion battery cell (10) and an electrically powered motor vehicle (2).

Description

The invention relates to a lithium-ion battery cell with a cathode, with LiNi _0.5 Mn _1.5 O ₄ as the cathode active material. The invention also relates to a method for producing such a lithium-ion battery cell.

A lithium-ion battery cell (Li-ion battery cell) has a number of anodes and cathodes, a separator in each case being arranged between the anodes and the cathodes. The anodes and the cathodes typically each have a, in particular film-like, current conductor with an active material (electrode material) applied to it, into which lithium ions can intercalate (store) and from which lithium ions can deintercalate (remove). Furthermore, the battery cell has an electrolyte with a conductive charge dissolved therein, for example LiPF ₆ (lithium hexofluorophosphate), with a solvent, for example ethylene carbonate or propylene carbonate, and optionally with additional additives.

Lithium-nickel-manganese-cobalt oxide (NMC) with the formula LiNi _x Mn _y Co _z O _{2 is} used as the active material of the cathode, which is also referred to below as cathode active material, where x + y + z = 1 applies. In particular, have a nickel-rich lithium-nickel-manganese-cobalt oxides such as LiNi _0.60 Mn _0.20 Co _0.20 O ₂ (NMC 622) or LiNi _0.80 Mn _0.10 C _0.10 O ₂ ( NMC 811) as cathode active material has a comparatively high specific capacity. However, lithium-nickel-manganese-cobalt oxide is comparatively expensive due to the cobalt (cobalt) and due to the comparatively high nickel content.

As an alternative to this, a lithium metal oxide with a spinel structure, in particular LiNi _0.5 Mn _1.5 O ₄ (LNMO), is used as the cathode active material. This lithium metal oxide is comparatively easy to synthesize and has a comparatively high energy density. The disadvantage is that the specific capacity of the LiNi _0.5 Mn _1.5 O _{4 is} reduced comparatively quickly. This is based on different mechanisms. For example, Mn ^{3+ of} the lithium metal ^{oxide can} react in the presence of the electrolyte to form Mn ²⁺ , which is soluble. Due to the dissolution of the Mn ²⁺ , a phase transition of the lithium metal ^oxide takes place from a spinel structure to a common salt structure, with no storage or removal of lithium ions taking place in areas with such a common salt structure. In addition, the Mn ²⁺ reaches the anode via the electrolyte, the Mn ^{2+ being} reduced to metallic manganese at the anode, which results in the formation of a boundary layer, the so-called SEI (solid electrolyte interphase). Such a boundary layer can prevent or at least restrict ion transport into or out of the anode active material.

In addition, charging the lithium-ion battery cell with a comparatively high voltage can accelerate the oxidation of the electrolyte on the cathode surface, whereby the reaction products can be deposited on the electrodes, so that an intercalation (storage) and deintercalation (removal) of lithium Ions are difficult.

As in “Research Progress in Improving the Cycling Stability of High-Voltage LiNi _0.5 Mn _1.5 O ₄ Cathode in Lithium-Ion Battery” by XiaoLong Xu et al., Nano-micro letters, 9 (2), 22 In summary, the lithium metal oxide can be doped to reduce the loss of capacity or coated with an inorganic material, in particular ZnO, Bi ₂ O ₃ or Al ₂ O ₃ .

The invention is based on the object of specifying a lithium-ion battery cell in which a loss of capacity, in particular due to a reaction of the cathode active material with the electrolyte, and / or a release of manganese ions from the cathode active material, is avoided or at least reduced. Furthermore, a method for producing such a lithium-ion battery cell and an electrically driven motor vehicle are to be specified, the traction battery of which has such a lithium-ion battery.

With regard to the lithium-ion battery cell, the object is achieved according to the invention by the features of claim 1. With regard to the method, the object is achieved by the features of claim 5 and with regard to the electrically driven motor vehicle with the features of claim 7. Advantageous further developments and refinements are the subject of the dependent claims. The statements in connection with the lithium-ion battery cell also apply accordingly to the method and to the motor vehicle and vice versa.

The lithium-ion battery cell has a cathode with a current collector and with LiNi _0.5 Mn _1.5 O ₄ applied to the current collector as the cathode active material. The cathode active material serves to intercalate into this lithium ions and deintercalate from this lithium ions. Furthermore, a first layer, which is also referred to as the substrate-side layer, made of a lithium niobium oxide is applied to the cathode active material, that is to say on the LiNi _0.5 Mn _1.5 O ₄ .

A second layer, also referred to as the electrolyte-side layer, is applied to the first layer made of the lithium niobium oxide, the second layer is formed by means of a lithium phosphate. The lithium phosphate has a comparatively high electrochemical stability against a reaction with the electrolyte, so that the second layer provides protection for the first layer and for the cathode active material.

Here and in the following, a crystal axis is represented by “[]” and axes that are equivalent to this axis are represented by “<>”.

The substrate-side, ie the first, layer made of the lithium niobium oxide has a comparatively high crystallographic compatibility with LiNi _0.5 Mn _1.5 O ₄ . In other words, a crystal symmetry and / or a lattice parameter, such as the side length of the unit cell and / or an angle between the edges of the unit cell, of the lithium niobium oxide with regard to at least one (lattice plane) crystal facet match those of the LiNi _0.5 Mn _1.5 O ₄ or have only a comparatively small deviation, in particular a deviation of less than 5%.

In the case of the combination of materials shown above, the difference between the modulus of elasticity, the modulus of compression and the modulus of shear of the first layer and the cathode active material as well as the difference between the modulus of elasticity, the modulus of compression and the modulus of shear of the first layer and the second layer are comparatively small . In this way, when the coated cathode active material is subjected to mechanical stress, a layer failure of the first and / or the second layer, such as, for example, flaking, tearing or breakage, is avoided or the risk thereof is at least reduced.

Because of this and because of the crystallographic compatibility of the first layer with the second layer and the first layer with the cathode active material, a comparatively high mechanical stability of the coated cathode active material is advantageously achieved.

For example, the lithium-ion battery cell is provided and set up for a traction battery of an electrically driven motor vehicle, which provides electrical energy for an electric motor for driving the motor vehicle.

The lithium niobium oxide of the first layer is, for example, Li ₃ NbO ₄ or LiNbO ₂ . According to a preferred embodiment, however, the lithium niobium oxide of the first layer is LiNbO ₃ (lithium niobate). Advantageously, lithium niobate has a crystallographic correspondence or only a slight deviation of its [100] crystal axis with the [110] crystal axis of the cathode active material. In addition, the lithium niobate advantageously has a comparatively high ionic conductivity, which is greater than 10 ^-6 S / cm (Siemens per centimeter).

A phosphate here is to be understood as meaning salts and esters of orthophosphoric acid (H ₃ PO ₄ ) and the condensates (polymers) of orthophosphoric acid and its esters. For example, the lithium phosphate has the formula Li ₃ PO ₄ . According to a preferred embodiment, however, lithium diphosphate, that is to say Li ₄ P ₂ O ₇ , is used as the lithium phosphate. Lithium diphosphate advantageously has a comparatively high electrochemical stability with an electrochemical window (voltage window) greater than 5V. The second layer thus provides protection for the cathode active material that is advantageously stable against oxidation or against reduction and for the first layer against a reaction with the electrolyte. In addition, the lithium diphosphate advantageously has a comparatively high ionic conductivity, which is greater than 10 ^-3 S / cm (Siemens per centimeter).

According to an advantageous embodiment, a layer thickness of the first layer and / or the second layer is in each case between 0.5 nm and 2 nm. The layer thickness is preferably 1 nm in each case. In particular, the extent of a layer is below the layer thickness in a direction perpendicular to that To understand the surface on which the layer is applied. Advantageously, because of such a comparatively small layer thickness, intercalation or deintercalation of the lithium ions in or out of the cathode active material is not or only slightly influenced.

According to a method for producing a lithium-ion battery cell, which is designed according to one of the variants presented above, LiNi _0.5 Mn _1.5 O ₄ , that is, the lithium metal oxide with a spinel structure, is initially provided as the cathode active material. The LiNi _0.5 Mn _1.5 O ₄ is expediently provided in powder form. The LiNi _0.5 Mn _1.5 O ₄ is thus formed by means of a large number of (powder) particles.

The LiNi _0.5 Mn _1.5 O ₄ , that is to say the cathode active material, is then coated with the first layer made of a lithium niobium oxide, preferably made of lithium niobate. Accordingly, a layer made of a lithium niobium oxide or lithium niobate is arranged as the first layer on the LiNi _0.5 Mn _1.5 O ₄ . The particles of the LiNi _0.5 Mn _1.5 O ₄ powder are thus provided with the first layer.

Subsequently, the LiNi _0.5 Mn _1.5 O ₄ coated with the first layer is coated with the second layer made of a lithium phosphate, preferably made of lithium diphosphate. In other words, the second layer is applied to the first layer. In this case, the coating of the particles of the LiNi _0.5 Mn _1.5 O ₄ powder advantageously enables the respective particle to be completely enveloped by the two layers.

The LiNi _0.5 Mn _1.5 O ₄ coated with the first layer and the second layer is applied to the current conductor. For example, the coated cathode active material is mixed with a binder, with a solvent and, if appropriate, with an electrically conductive conductive additive, such as in particular conductive carbon black, aluminum or nickel powder. The mixture is then applied to the current collector and dried to form the cathode. Polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, or hydroxypropyl cellulose, for example, is used as the binder, and N-methyl-2-pyrrolidone, for example, is used as the solvent.

In summary, the particles of the LiNi _0.5 Mn _1.5 O ₄ powder are provided with two layers arranged one above the other, which are also referred to as layers. The second layer, that is to say the outer layer with respect to the respective particle, is surrounded by the electrolyte when the lithium-ion battery cell is installed. The first layer, that is to say the inner layer, is arranged between the second layer and the particle of the LiNi _0.5 Mn _1.5 O ₄ powder. The second layer has a comparatively high electrochemical stability and, as a result, a high protective effect for the cathode active material and for the first layer against a reaction with the electrolyte. In this case, the application of the second layer is advantageously simplified due to the crystallographic correspondence of the second layer with the first layer shown above. In addition, because of this, the second layer can be made comparatively thin, so that the electrical conductivity of the cathode is only reduced to a correspondingly small extent.

As an alternative to this, the cathode active material is first applied to the current conductor. For this purpose, the cathode active material is expediently mixed with the binder, with the solvent and optionally with the conductive additive, as shown above, applied to the current conductor and then dried. According to this alternative embodiment, the cathode active material applied to the current conductor and the binder and optionally the conductive additive are then coated with the first layer and then with the second layer.

The so-called sol-gel method or a wet or dry chemical method is used, for example, to apply the first layer and / or the second layer. Alternatively, the respective layer is, for example, in the course of the synthesis of LiNi _0.5 Mn _1.5 O ₄ by adding appropriate precursors, for example by means of surface segregation of the respective layer during calcination of the mixture of LiNi _0.5 Mn _1.5 O ₄ and the precursors. According to an advantageous embodiment, however, a gas phase deposition method is used to apply the first layer and / or the second layer. The atomic layer deposition (ALD) method is preferably used. In this way, it is possible to produce comparatively thin layers, in particular with layer thicknesses less than 2 nm, the chemical composition of the layer being comparatively easy to control and / or set in the atomic layer deposition process.

According to an advantageous embodiment, an electrically driven motor vehicle has a traction battery. This in turn comprises a lithium-ion battery cell, preferably a multiplicity of lithium-ion battery cells, each of which provides electrical energy for driving the motor vehicle, suitably for an electric motor. The respective lithium-ion battery cell is designed according to one of the variants of the lithium-ion battery cell shown above and / or produced according to one of the variants of the method for producing a lithium-ion battery cell shown above.

• 1 in schematischer Darstellung ein Kraftfahrzeug mit einer Traktionsbatterie, deren Lithium-lonen-Batteriezellen jeweils eine Anzahl an Kathoden mit LiNi_0,5Mn_1,5O₄ als Kathoden-Aktivmaterial aufweisen,
• 2 schematisch einen Partikel des Kathoden-Aktivmaterials, wobei auf den Partikel eine erste Schicht aus Lithiumniobat und auf diese eine zweite Schicht aus Lithiumdiphosphat aufgebracht ist, und
• 3 in einem Flussdiagramm einen Verfahrensablauf zur Herstellung einer der Lithium-Ionen-Batteriezellen.

Exemplary embodiments of the invention are explained in more detail below with reference to a drawing. Show in it:

• 1 a schematic representation of a motor vehicle with a traction battery, the lithium-ion battery cells of which each have a number of cathodes with LiNi _0.5 Mn _1.5 O ₄ as cathode active material,
• 2 schematically, a particle of the cathode active material, a first layer of lithium niobate being applied to the particle and a second layer of lithium diphosphate being applied to this, and
• 3 in a flow chart a process sequence for producing one of the lithium-ion battery cells.

Corresponding parts and sizes are always provided with the same reference symbols in all figures.

This in 1 shown motor vehicle 2 is electrically powered. This points to the Motor vehicle 2 an electric motor 4th on, which has an inverter 6 with a traction battery 8th connected is. The traction battery 8th has a number of battery cells 10 on which interconnected and to the inverter 8th are connected. Here are only two of the lithium-ion battery cells for the purpose of an improved overview 10 shown.

In each of the lithium-ion battery cells 10 are anodes 12th and cathodes 14th alternately stacked on top of each other, with between the anodes 12th and the cathodes 14th one separator each 16 is arranged so that the anodes 12th and the cathodes 14th are spatially separated from each other. The separators 16 are in the 1 shown hatched and comprise, for example, polyethylene and / or polypropylene.

Each of the anodes 12th each has an anode foil, designed for example as a copper foil, as a current conductor 12a on which both sides, i.e. on their flat sides, with an anode active material 12b (Active material of the anode), for example graphite, is coated.

Each of the cathodes 14th has a cathode foil, for example an aluminum foil, as a current collector 14a on. A cathode active material is on both sides of these 14b (Active material of the cathode) applied. The cathode active material 14b , i.e. the active material of the cathode 14th is formed by means of a lithium metal oxide with a spinel structure, namely by means of LiNi _0.5 Mn _1.5 O ₄ .

The cathode active material 14b is formed from a large number of particles, on each of which a first layer 18th made of a lithium niobium oxide (LiNbO ₃ ), namely lithium niobate, and a second layer 20th from a lithium phosphate, namely lithium diphosphate (Li ₄ P ₂ O ₇ ), is applied. In other words, the particles are each with the first layer 18th made of lithium niobate and with the second layer 20th coated from lithium diphosphate. About the first layer 18th and with the second layer 20th coated cathode active material 14b is in the 1 shown flat dotted. One of the ones with the first layer 18th and with the second layer 20th coated particle is in the 2 shown schematically.

In each of the lithium-ion battery cells 10 a liquid electrolyte, not shown, is also included. This has a conductive salt and a solvent for the conductive salt, lithium hexafluorophosphate being used as the conductive salt and a mixture of ethylene carbonate and diethyl carbonate being used as the solvent.

In the 2 is one of those with the first layer 18th and with the second layer 20th coated particles of the cathode active material 14b shown schematically. The (substrate-side) first layer is here 18th between the particle and the second layer 20th arranged. The second layer, i.e. the outer layer, is in the assembled state of the lithium-ion battery cell 10 surrounded by the electrolyte. The second layer 20th a high electrochemical stability with an electrochemical window (voltage window) greater than 5V and, as a result, a high protective effect for the cathode active material 14b and for the first layer 18th against a reaction with the electrolyte. Applying the second layer 20th is due to a crystallographic agreement or only a slight deviation of the [001] crystal axis of Li ₄ P ₂ O ₇ with the [100] crystal axis of LiNbO ₃ , the [11-1] crystal axis of Li ₄ P ₂ O ₇ with the [101] crystal axis of LiNbO ₃ and the [010] crystal axis of Li ₄ P ₂ O ₇ with the [001] crystal axis of LiNbO ₃ .

The 2 is not to scale for the purpose of better visibility. So a (particle size) is maximum expansion a of the particles between 0.5 µm and 20 µm. a layer thickness b the first layer 18th and a layer thickness c the second layer 20th is between 0.5 nm and 2 nm.

That in the 3 The flowchart shown represents a method sequence for producing a lithium-ion battery cell 10 , especially one of the lithium-ion battery cells 10 of the electrically powered motor vehicle 2 .

The first step here is I. the cathode active material provided as a powder 14b , so the LiNi _0.5 Mn _1.5 O ₄ , with the first layer 18th made of lithium niobate, coated. In other words, it will be the first layer 18th applied to the particles of the LiNi _0.5 Mn _1.5 O ₄ powder.

In a second step II be the ones with the first layer 18th coated particles of the LiNi _0.5 Mn _1.5 O ₄ powder with the second layer 20th coated from lithium diphosphate.

For the coating process of the first step I. and the second step II a vapor deposition process is used in each case. The atomic layer deposition method (ALD) and the corresponding precursors are preferably used. In this way, the layer thicknesses between 0.5 nm and 2 nm are realized.

The one with the first layer 18th and with the second layer 20th coated particles of the LiNi _0.5 Mn _1.5 O ₄ powder are used in a third step III mixed with a binder, with a solvent and with an electrically conductive conductive additive, in particular conductive carbon black, aluminum or nickel powder. The mixture is applied to the current arrester 14a applied and forming the cathode 14th dried. Polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, or hydroxypropyl cellulose, for example, is used as the binder, and N-methyl-2-pyrrolidone, for example, is used as the solvent.

The invention is not restricted to the exemplary embodiment described above. Rather, other variants of the invention can also be derived from this by the person skilled in the art without departing from the subject matter of the invention. In particular, all of the individual features described in connection with the exemplary embodiment can also be combined with one another in other ways without departing from the subject matter of the invention.

List of reference symbols

2

Motor vehicle

4th

Electric motor

6th

Inverter

8th

Traction battery

10

Lithium-ion battery cell

12

anode

12a

Anode foil / current arrester

12b

Anode active material

14th

cathode

14a

Cathode foil / current arrester

14b

Cathode active material

16

separator

18th

first layer

20th

second layer

a

Expansion of a particle of the cathode active material

b

Layer thickness of the first layer

c

Layer thickness of the second layer

I.

Coating of the cathode active material with the first layer

II

Coating of the cathode active material coated with the first layer with the second layer

III

Application of the cathode active material coated with both layers to the current conductor

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Non-patent literature cited

• XiaoLong Xu et al., Nano-micro letters, 9 (2), 22 [0006]

Claims (7)

Lithium-ion battery cell (10), comprising - a cathode (14) with a current collector (14a) and with LiNi _0.5 Mn _1.5 O ₄ applied to the current collector (14a) as cathode active material (14b), - wherein a first layer (18) made of a lithium niobium oxide is applied to the cathode active material (14b), and - a second layer (20) made of a lithium phosphate is applied to the first layer (18). Lithium-ion battery cell (10) according to Claim 1 , characterized in that lithium niobate is the lithium niobium oxide. Lithium-ion battery cell (10) according to Claim 1 or 2 , characterized in that lithium diphosphate is the lithium phosphate. Lithium-ion battery cell (10) according to one of the Claims 1 to 3 , characterized in that a layer thickness (b or c) of the first layer (18) and / or the second layer (20) is in each case between 0.5 nm and 2 nm, in particular in each case 1 nm. Process for the production of one according to one of the Claims 1 to 4th formed lithium-ion battery cell (10), - in which LiNi _0.5 Mn _1.5 O ₄ as cathode active material (14b) is coated with the first layer (18), - in which the first layer (18 ) coated LiNi _0.5 Mn _1.5 O _{4 is} coated with the second layer (20), and - in which the LiNi _0.5 Mn ₁ coated with the first layer (18) and with the second layer (20) _{, 5} O _{4 is} applied to the current conductor (14a). Procedure according to Claim 5 , characterized in that a gas phase deposition process is used to apply the first layer (18) and / or the second layer (20). Electrically powered motor vehicle (2) with a traction battery (8) with a lithium-ion battery cell (10), which according to one of the Claims 1 to 4th trained and / or after Claim 5 or 6 is made.

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