WO2018077937A1

WO2018077937A1 - New binder systems for silicon-containing composite electrodes for lithium ion batteries

Info

Publication number: WO2018077937A1
Application number: PCT/EP2017/077286
Authority: WO
Inventors: Linda-Susann SCHRAMM
Original assignee: Volkswagen Aktiengesellschaft
Priority date: 2016-10-28
Filing date: 2017-10-25
Publication date: 2018-05-03
Also published as: DE102016221298A1

Abstract

The invention relates to an electrode for an electrochemical cell, in particular for a lithium ion battery, having silicon material, conductive carbon black and at least one binder system, wherein the at least one binder system has carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and poly(acrylic-co-maleic) acid (PAMA), preferably consists thereof, in order to improve the cycle stability and/or to increase the performance of a lithium ion battery. The invention further relates to a method for producing an electrode according to the invention, to a method for producing a lithium ion battery having at least one electrode according to the invention, to a lithium ion battery comprising at least one electrode according to the invention, and to the use of the electrode in lithium ion batteries.

Description

description

New binder systems for silicon-containing composite electrodes for lithium-ion batteries

The present invention relates to an electrode for an electrochemical cell, in particular for a lithium-ion battery, comprising silicon material, conductive carbon black and at least one binder system, wherein the at least one binder system for improving the cycle stability and / or to increase the performance of a lithium-ion battery carboxymethylcellulose (CMC), polyacrylic acid (PAA) and poly (acrylic-co-maleic) acid (PAMA), preferably consists of these. The present invention also relates to a method for producing an electrode according to the invention, a method for producing a lithium-ion battery comprising at least one electrode according to the invention, a lithium-ion battery comprising at least one electrode according to the invention and the use of the electrode in lithium-ion batteries.

The lithium-ion battery is today considered a key technology in electric mobility. Secondary lithium-ion batteries are today widely used electrochemical energy stores with energy densities of up to 180 Wh / kg. Secondary lithium-ion batteries are mainly used in the field of portable electronics, tools and bicycles or automobiles. Despite great success, the lithium-ion battery can be significantly optimized in terms of cost, weight, energy density, service life and safety. Especially for automotive applications, however, it is necessary to further increase the energy density of the batteries and the cycle stability in order to achieve a higher range of the vehicles and a longer life of the batteries.

Electrochemical cells such as lithium-ion batteries usually have composite electrodes. Through the use of innovative electrode materials, it is possible to increase the energy density of lithium-ion batteries and thus significantly increase the range of electric vehicles. In this regard, silicon is a promising material that has ten times the capacity of graphite and a similarly low lithiation potential (0.5V vs. Li / Li ^"1" ). Since silicon is the second most abundant material in the earth's crust, and thus low cost manufacturing, it is attractive from an industrial point of view.

Electrodes for lithium-ion batteries in which the electrode material is based on silicon, however, can undergo an extreme volume change when charged or discharged with lithium. This volume change leads to a strong mechanical stress of the active material in the entire electrode structure, which leads by electrochemical stress to a loss of electrical contact and thus the destruction of the electrode under capacity loss.

A major challenge in the commercial application of silicon-containing electrodes is therefore the enormous volume change of the material in the lithiation and delithiation process (Li ₄ Si ₁₅ : 280% to 300% compared to LiC ₆ : 10 to 1 1%). The breathing of the electrodes, ie their mechanical stress, results in the pulverization of the particles of the electrode and, consequently, effects on the preservation of the electrode architecture, which is particularly noticeable at high surface loadings. This results in contact losses of the particles of the electrode with one another, as well as between the electrode and the current collector and is reflected in the deterioration of the electrical conductivity, the creeping loss of capacity and ultimately the uselessness of the affected electrode. Furthermore, there is a constantly rupturing and thus growing solid-electrolyte interface (SEI), which in turn leads to a continuous Li-ion consumption and an increasing internal resistance in the cell. This results in low Coulomb efficiency (C _E ) and reduced cycle stability.

Various optimization approaches for improving the energy density and lifetime of silicon-containing electrodes are known which minimize volume expansion of the electrodes. These include, for example, the particle diameter and the morphology of the material, for example the use of core-shell or highly porous silicon material, or the change in the electrochemical conditions, for example the change in the voltage or the C-rate, or the addition of SEI-forming additives. Another approach is the use of binders or binder systems. Binders based on fluorinated polymers and copolymers, in particular based on the standard binder polyvinylidene fluoride (PVDF) which is soluble in N-methyl-2-pyrrolidone (NMP), are described in the prior art, as described in EP 1 261 048. However, due to its non-functionalized linear binder chains, PVDF can not cope with the respiration, ie the mechanical stress, of the silicon-containing electrodes. The long chains slide past each other in the volume expansion of the electrode, so that irreversible deformation of the electrodes and thus associated with pulverization, loss of contact within the electrode, as well as between the electrode and the current collector and the rupture of the SEI.

The requirements for such binder or binder systems are manifold. They should be functionalized in the case of silicon-containing electrodes and have a high degree of substitution, a chemical inertness to the electrolyte and other cell components ensure good adhesion to the current collector and the cohesion between the components within the electrode, as well as form a stable SEI.

EP 1 730 800 discloses an electrode material for lithium-ion batteries which comprises 5 to 85% by weight of silicon particles, 0 to 10% by weight conductive carbon black, 5 to 80% by weight graphite and 5 to 25% by weight. % of a binder.

Choi et al. (Journal of Electrochemical Science and Technology, 2015, 6 (2), 35-49) discloses improved binders for electrodes wherein, by cross-linking CMC / PAA (CMC: carboxymethylcellulose, PAA: polyacrylic acid) or PAA / PVA (PVA: Polyvinyl alcohol) obtained binder show a higher stability of electrodes and a lower deformation.

Song et al. (Adv. Funct. Mater., 2014, DOI: 10,1002 / ADFM.201401269) discloses cross-linked PAA-PVA polymer binders which result in higher stability and lower ductility of silicon electrodes.

Jeschull et al. (Journal of Power Sources, 2016, 325, 513-524) discloses silicon electrodes with CMC or PAA binders for increasing the stability of electrodes.

The listed binders CMC and PAA are functionalized and carry carboxylate groups which can form a covalent bond with the silanol groups on the silicon surface.

However, all known from the prior art electrodes with binders or binder systems have too low cycle stability.

It is therefore the object of the present invention to overcome the aforementioned disadvantages, and in particular to provide a suitable electrode, which reduces the volume change of the material in the lithiation and Delithiierungsprozess and the pulverization of the electrode, the contact loss within the electrode, and between the electrode and current collector and a Rupture of the Solid Electrolyte Interface (SEI) decreases and thus leads to a higher Coulomb efficiency and a higher cycle stability. In particular, an improved service life and / or cycle stability, ie cycle stability, of a lithium-ion battery should be provided.

Accordingly, in particular, it should be prevented that the surface layer of an electrode after a plurality of cycles, that is, after a plurality of charge and discharge cycles, mechanically damaged and must be formed again. The object of the present invention is achieved in each case by the teaching of the independent claims.

According to the invention, an electrode for an electrochemical cell, in particular a lithium-ion battery comprising silicon material, conductive carbon black and at least one binder system is provided, the at least one binder system comprising CMC (carboxymethylcellulose), PAA (polyacrylic acid) and PAMA (poly (acrylic)). In this case, CMC, PAA and PAMA serve as binders in the electrode according to the invention.

In a preferred embodiment, the electrode according to the invention for an electrochemical cell, in particular a lithium-ion battery, silicon material, Leitruß, at least one binder system and additionally graphite, wherein the at least one binder system comprises CMC, PAA and PAMA, preferably consists of these.

Surprisingly, electrodes according to the invention have improved surface properties, in particular the pulverization of the electrode material is reduced, which leads to an increase in the Coulomb efficiency and the improvement of the C-rate stability. Advantageously, the pulverization associated with a change in volume of the electrode material, in particular anode material, is reduced, in particular in that a binder system comprising CMC, PAA and PAMA, preferably consisting thereof, is used in the electrode, in particular because a binder system, the CMC, PAA and PAMA has, preferably consists of these, is used in the electrode and the pH is adjusted in the manufacture of the electrode to a certain pH.

Surprisingly, the use of PAMA, in particular as a binder, in combination with the binders CMC and PAA increases the stability by the addition of PAMA, in particular in comparison to a binder system of CMC and PAA which has no PAMA.

Although PAMA is known in the art as a dispersing agent, it is not known as a binder component in a binder system. In the context of the present invention, the components of the binder system comprising CMC, PAA and PAMA, preferably consisting of these, are preferably mixed in a ratio of 1: 1: 1. The binder components are preferably functionalized and carry carboxylate or imide groups, which can form a covalent bond with the silanol groups on the silicon surface. By An increase in the degree of substitution of the binder components can preferably be achieved by further improving the cycle stability and the efficiency.

The life and cycle stability, ie the cycle stability of the electrode and also the lithium-ion battery is significantly increased due to the inventive composition of the electrode, in particular by the at least one binder system, the CMC, PAA and PAMA preferably consists of these.

Surprisingly, the electrode according to the invention has a minimized mechanical and consequent electrical instability, in particular through the use of a binder system comprising CMC / PAA / PAMA, preferably consisting of these.

Advantageously, the method according to the invention enables an in-situ production of the electrode according to the invention, wherein the optimization takes place during the production of the electrode paste and no further method step is required.

The electrode according to the invention, in particular an electrode comprising silicon material, conductive carbon black and at least one binder system, wherein the at least one binder system comprises CMC, PAA and PAMA, preferably consists of these, is characterized by a smaller volume change of the electrode and less pulverization of the particles of the electrode a variety of charging and discharging cycles. The SEI accordingly remains mechanically and / or chemically largely undamaged by the inventively achieved lower volume change, in particular a silicon electrode, during the lithiation and Delithiierungprozesses after a variety of charging and discharging cycles and does not need to be re-formed.

In a preferred embodiment, an electrode according to the invention for an electrochemical cell comprising silicon material, conductive carbon black and at least one binder system, and optionally graphite, is provided, wherein the electrode has 1 to 80 wt .-% silicon material, preferably 5 to 80 wt .-%, preferably 10 to 80% by weight, preferably 10 to 60% by weight, preferably 10 to 50% by weight, preferably 10 to 40% by weight, preferably 10 to 30% by weight, preferably 10 to 20% by weight. %, based on the total weight of the electrode.

In a preferred embodiment, an electrode according to the invention is provided for an electrochemical cell comprising silicon material, conductive carbon black, at least one binder system and optionally graphite, the electrode having from 10 to 90% by weight of graphite, preferably from 10 to 80% by weight, preferably from 10 to 70 Wt .-%, preferably 10 to 50 wt .-%, preferably 10 to 30 wt .-%, preferably 20 to 80 wt .-%, preferably 20 to 70 wt .-%, preferably 30 to 70 wt .-%, preferably 40 to 70 wt .-%, preferably 50 to 70 wt .-%, based on the total weight of the electrode.

In a preferred embodiment, an electrode according to the invention for an electrochemical cell comprising silicon material Leitruß and at least one binder system, and optionally graphite, is provided, wherein the at least one binder system CMC, PAA and PAMA has, and wherein the electrode additionally a further binder, preferably another Binder system comprising at least one binder comprises.

In a preferred embodiment, an electrode according to the invention for an electrochemical cell comprising silicon material, conductive carbon black and at least one binder system, and optionally graphite, is provided, wherein the electrode has 1 to 40 wt .-% Leitruß, preferably 1 to 30 wt .-%, preferably 1 to 20 wt .-%, preferably 1 to 10 wt .-%, preferably 3 to 40 wt .-%, preferably 6 to 40 wt .-%, preferably 3 to 30 wt .-%, preferably 3 to 20 wt. %, preferably 6 to 30 wt .-%, preferably 6 to 20 wt .-%, preferably 10 to 20 wt .-%, based on the total weight of the electrode.

In a preferred embodiment, an electrode according to the invention for an electrochemical cell comprising silicon material Leitruß and at least one binder system, and optionally graphite, is provided, wherein the electrode 1 to 40 wt .-% of the binder system, preferably 1 to 30 wt .-%, preferably 1 to 20 wt .-%, preferably 1 to 10 wt .-%, preferably 1 to 5 wt .-%, preferably 4 to 40 wt .-%, preferably 4 to 30 wt .-%, preferably 4 to 20 wt. -%, preferably 4 to 10 wt .-%, preferably 8 to 40 wt .-%, preferably 8 to 30 wt .-%, preferably 8 to 20 wt .-%, preferably 8 to 10 wt .-%, preferably 10 to 40 wt.%, preferably 10 to 30 wt.%, preferably 10 to 20 wt.%, preferably 4 to 12 wt.%, preferably 8 to 12 wt.%, preferably 10 to 12 wt. %, particularly preferably 8 wt .-%, based on the total weight of the electrode.

In a preferred embodiment, the electrode according to the invention for improving the conductivity on at least one conductive additive, preferably, the conductive additive is selected from the group consisting of Al, Cu or Li.

Preferably, the electrode comprises 0.1 to 20 wt .-%, preferably 1 to 18 wt .-%, in particular 5 to 15 wt .-% of the at least one additive, based on the total weight of the electrode. In a particularly preferred embodiment, the abovementioned quantitative constituents of an electrode comprising silicon material, conductive carbon black and at least one binder system add up to 100% by weight, based on the total weight of the electrode.

In a particularly preferred embodiment, the abovementioned quantitative constituents of an electrode comprising silicon material, conductive carbon black, graphite and at least one binder system add up to 100% by weight, based on the total weight of the electrode.

In a particularly preferred embodiment, the abovementioned quantitative constituents of an electrode comprising silicon material, conductive carbon black, at least one binder system and at least one additive add up to 100% by weight, based on the total weight of the electrode.

In a particularly preferred embodiment, the abovementioned quantitative constituents of an electrode comprising silicon material, conductive carbon black, at least one binder system, graphite and at least one additive add up to 100% by weight, based on the total weight of the electrode.

In a preferred embodiment, an electrode according to the invention for an electrochemical cell comprising silicon material Leitruß and at least one binder system, and optionally graphite, is provided, wherein the at least one binder system 5 to 80 wt .-% carboxymethylcellulose (CMC), preferably 5 to 60 wt. -%, preferably 5 to 40% by weight, preferably 10 to 80 wt .-%, preferably 10 to 60 wt .-%, preferably 10 to 40 wt .-%, preferably 20 to 80 wt .-%, preferably 20 to 60 wt .-%, preferably 20 to 40 wt .-%, preferably 30 to 40 wt .-%, based on the total weight of the binder system. In a preferred embodiment, the binder carboxymethylcellulose (CMC) is used in the form of sodium carboxymethylcellulose (NaCMC).

In a preferred embodiment, an electrode according to the invention for an electrochemical cell comprising silicon material Leitruß and at least one binder system, and optionally graphite, is provided, wherein the at least one binder system 5 to 80 wt .-% polyacrylic acid (PAA), preferably 5 to 60 wt. -%, preferably 5 to 40 wt .-%, preferably 10 to 80 wt .-%, preferably 10 to 60 wt .-%, preferably 10 to 40 wt .-%, preferably 20 to 80% by weight, preferably 20 to 60 wt .-%, preferably 20 to 40 wt .-%, preferably 30 to 40 wt .-%, based on the total weight of the binder system. In a preferred embodiment, the polyacrylic acid (PAA) in the at least one binder system of the electrode has a molecular weight of 300k to 550k g / mol, preferably 300k to 500k g / mol, preferably 350k to 500k g / mol, preferably 450k g / mol.

In a preferred embodiment, an electrode according to the invention for an electrochemical cell comprising silicon material, conductive carbon black and at least one binder system, and optionally graphite, is provided, wherein the at least one binder system 5 to 80 wt .-% poly (acrylic-co-maleic) acid (PAMA ), preferably 5 to 60 wt .-%, preferably 5 to 40 wt .-%, preferably 10 to 80 wt .-%, preferably 10 to 60 wt .-%, preferably 10 to 40 wt .-%, preferably 20 to 80 wt .-%, preferably 20 to 60 wt .-%, preferably 20 to 40 wt .-%, preferably 30 to 40 wt .-%, based on the total weight of the binder system.

In a preferred embodiment, an electrode according to the invention for an electrochemical cell comprising silicon material, conductive carbon black and at least one binder system, and optionally graphite, is provided, wherein for the weight ratio CMC: PAA: PAMA (a: b: c) in the binder system: 0.5 < a <2; 0.5 <b <2 and 0.5 <c <2, preferably 0.75 <a <1.75; 0.75 <b <1, 75 and 0.75 <c <1.75, preferably 0.75 <a <1.5; 0.75 <b <1, 5 and 0.75 <c <1, 5, preferably a = 2, b = 1, c = 1, preferably a = 1, b = 2, c = 1, preferably a = 1 , b = 1, c = 2, preferably a = 2, b = 2, c = 1, preferably a = 2, b = 1, c = 2, preferably a = 1, b = 2, c = 2, preferably a = 1, b = 1, c = 1.

In a preferred embodiment, an electrode according to the invention for an electrochemical cell comprising silicon material Leitruß and at least one binder system, and optionally graphite, is provided, wherein the at least one binder system CMC, PAA and PAMA has, and wherein the electrode comprises a further binder, preferably in one Amount of 1 to 20 wt .-%, preferably 1 to 10 wt .-%, preferably 2 to 10 wt .-%, preferably 5 to 10 wt .-%, preferably 10 to 20 wt .-%, based on the total weight the electrode.

The present invention also relates to a method for producing a lithium-ion battery, comprising the steps of a) providing at least one electrode according to the invention,

b) providing at least one counter electrode, and

c) construction of a lithium-ion battery, comprising the at least one electrode according to the invention and the at least one counterelectrode.

The present invention also relates to a lithium-ion battery comprising at least one electrode according to the invention. The lithium-ion battery according to the invention preferably has a) a housing, b) a battery core with at least one cathode and at least one anode, and c) an electrolyte composition.

In a preferred embodiment, a lithium ion battery having an electrode according to the invention comprises an electrolyte composition, wherein the nonaqueous solvent is an aprotic, nonaqueous solvent, preferably the nonaqueous solvent is selected from the group consisting of propylene carbonate (PC), Ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dioxolane, ethylmethyl carbonate (EMC), fluoroethylene carbonate (FEC), difluoroethylene carbonate, tetrahydrofuran, 1, 2-dimethoxyethane, 2-methyltetrahydrofuran, n-methylpyrrolidone (NMP), acetonitrile, Ethyl acetate and any combination thereof.

The maximum water content in the at least one non-aqueous solvent, preferably in the electrolyte composition, is at most 1000 ppm, preferably less than 500 ppm, preferably less than 50 ppm. In a preferred embodiment, the water content in the at least one non-aqueous solvent, preferably in the electrolyte composition, is at most 0.1% by weight, preferably at most 0.05% by weight, preferably at most 0.005% by weight, based on the Total weight of the solvent, preferably the electrolyte composition.

In a preferred embodiment, the electrolyte composition comprises the at least one non-aqueous solvent in an amount of at least 50% by weight, preferably at least 60% by weight, preferably at least 70% by weight, preferably at least 80% by weight, preferably at least 90% by weight, preferably at least 95% by weight, preferably at least 99% by weight, based on the solvent present in the electrolyte composition.

The present invention also relates to a method for producing an electrode for an electrochemical cell comprising silicon material, conductive carbon black and at least one binder system, the method comprising the steps of: a) providing silicon material, conductive carbon black and at least one binder system, wherein the at least one binder system CMC, PAA and PAMA, preferably consisting of these, b) mixing the components provided in step a) to an electrode paste, c) adjusting the pH of the electrode paste obtained in step b), d) annealing the electrode paste, and e) obtaining the Electrode.

In a preferred embodiment, graphite is additionally provided in step a) and mixed in step b) to form an electrode paste. In a preferred embodiment, in the process for producing an electrode for an electrochemical cell, it is provided that the pH of the electrode paste in step c) is adjusted to 2 to 4, preferably 2 to 3, preferably 3 to 4, preferably 2.5 to 3 , 5, preferably 2.8 to 3.3, preferably 3. In a particularly preferred embodiment, it is provided that the pH value according to step c) is adjusted simultaneously with the mixing of the components in step b).

In a preferred embodiment, in the process for producing an electrode for an electrochemical cell, the temperature during the annealing of the electrode paste in step d) is 100 to 160 ° C., preferably 100 to 140 ° C., preferably 1 10 to 140 ° C., preferably 120 to 140 ° C, preferably 100 to 120 ° C.

In a preferred embodiment, in the process for producing an electrode for an electrochemical cell prior to step d), the viscosity of the electrode paste is adjusted in a further step c2).

The present invention also relates to the use of an electrode according to the invention in lithium-ion batteries.

In the context of the invention, the term "lithium-ion battery" is understood to mean both a primary and a secondary lithium-ion battery, preferably a secondary lithium-ion battery A primary lithium-ion battery is not one Lithium ion rechargeable battery and secondary lithium ion battery is a rechargeable lithium ion battery In a particularly preferred embodiment, an electrode for a secondary lithium ion battery is provided.

In the context of the present invention, the term "electrode" is understood to mean a negative or positive electrode, Preferably, the negative electrode is anode during charging, cathode during charging, and generally comprises silicon and / or graphite compounds In the context of the present invention, the term "electrode" is preferably understood to mean an anode (discharge process), in particular an anode of a lithium-ion battery. In the context of the present invention, the term "electrode" is preferably also understood to mean a cathode (discharge process), in particular a cathode of a lithium-ion battery, In a particularly preferred embodiment, the electrode according to the invention is a composite electrode which is at least a binder system, preferably an elastic binder system, preferably a polymeric binder system, preferably a conductive binder system or a combination thereof. Depending on the design of the electrochemical cell, in particular the lithium-ion battery, in which the electrode according to the invention, in particular the composite electrode, is used, the electrode, in particular the composite electrode, may be formed as an anode or as a cathode.

In the context of the present invention, the cycle stability indicates the number of discharge / charge cycles that can be performed until the capacity of a lithium-ion battery is at a certain value, preferably 80% of the initial capacity, that is the capacity after the first full load, has fallen. A lithium-ion battery with a high cycle stability is therefore characterized in that over a large number of charge / discharge cycles, the capacity remains almost constant, preferably constant, and preferably falls by no more than 20% relative to the output capacity , The cycle stability of a lithium-ion battery is independent of the Coulomb efficiency of a lithium-ion battery. Coulomb efficiency, also referred to as Coulomb efficiency, is understood to mean the ratio of the amount of charge taken to the amount taken up. Accordingly, a lithium ion battery can have a high Coulomb efficiency despite poor cycle stability, and vice versa.

In the context of the present invention, the term "C rate" is understood to mean the relative charge or discharge current related to the theoretical specific capacity of the at least one electrochemical cell, in particular the lithium-ion battery (A / Ah).

In the context of the present invention, the term "silicon material" is elemental silicon, preferably highly pure polysilicon, silicon doped with small amounts of impurities, metallurgical silicon containing elemental impurities, such as, for example, in Fe, Al, Ca, Cu, Zr or C, In the context of the present invention, the silicon material may be in the form of silicon oxide or silicon salt If the silicon material comprises or consists of silicon oxide, then the stoichiometry of the silicon oxide SiO _{x is} preferably in the range 0 <x <1.3. The thickness of the silicon oxide layer on the surface of the electrode is preferably 1 to 10 nm, preferably 2 to 8 nm, preferably 3 to 7 nm, preferably 4 to 6 nm, preferably 5 nm.

In a preferred embodiment, the silicon material is present in the form of silicon particles, preferably silicon nanoparticles. The diameter of the silicon particles, preferred Silicon nanoparticles, is preferably less than 50 nm, preferably less than 80 nm, preferably less than 100 nm, preferably less than 150 nm, preferably less than 200 nm, preferably less than 300 nm, preferably less than 400 nm Particle size distribution of the silicon particles, preferably silicon nanoparticles, of d ₉₀ = 80 nm, preferably d ₉₀ = 100 nm, preferably d ₉₀ = 150 nm, preferably d ₉₀ = 200 nm, preferably d ₉₀ = 300 nm, preferably d ₅₀ = 50 nm , preferably d ₅₀ = 80 nm, preferably d ₅₀ = 100 nm, preferably d ₅₀ = 150 nm, preferably d ₅₀ = 180 nm, preferably d ₅₀ = 200 nm, preferably d ₅₀ = 300 nm.

In the context of the invention, the term "conductive carbon black" is understood to mean electrically conductive carbon black. The conductive carbon black preferably has a surface area of more than 200 m ² / g, preferably more than 400 m ² / g, preferably more than 600 m ² / g. preferably more than 800 m ² / g, preferably more than 1000 m ² / g.

In the context of the present application, the term "binder system" is understood as meaning a mixture of different binders as binder components, preferably the binder system comprises as binder components various binders and further additives .The binders preferably form a binder network, preferably by cross-linking, wherein various binders be networked with each other.

In a preferred embodiment, the binder system is conductive. In another preferred embodiment, the binder system is free of conductive particles.

Preferred embodiments of the present invention will become apparent from the subclaims.

The present invention will be explained in more detail with reference to the following embodiments and figures. It shows

Figure 1 is a C-rate test to illustrate the stepwise optimization of

Binder system,

FIG. 2 shows a cyclic voltammogram of an electrode with CMC / PAA / PAMA as binder system

(FIG. 2a) and a dilatometric measurement of an electrode with CMC / PAA / PAMA as the binder system (FIG. 2b),

FIG. 3 shows the influence of the molecular weight of PAA, Figure 4 Scanning electron micrographs of the surfaces of aged

Electrodes (half cells in EC: DEC: DMC with 10% FEC) (Figure 4a) and cross sections of the respective raw and cyclized electrodes in comparison (Figure 4b).

example 1

Electrode manufacturing:

The conductive carbon black (12% by weight) (Super PC65-Timcal) is first predispersed in Ultra-Turaxx (IKA ULTRATURRAX® T 25 digital) in 3/5 of the amount of binder and then the silicon material (20% by weight). (NP 180 WACKER) with the Ultra-Turaxx (5 min at 13000 rpm, 5 min at 16 000 rpm, and 1 min 22 000 rpm). Subsequently, in the dissolver (IKA®) (2h at 1300 rpm), graphite (60% by weight) (SLGF6 -Timcal) and the remaining binder (2/5) are added. The binder content in the entire paste is 8 wt .-% (based on the total weight of the electrode) and is composed of:

Binder system (1) 2.7% by weight (NA-CMC-Mw: 700 k g / mol), 2.7% by weight (PAA-Mw: 250 k g / mol), 2.7% by weight ( PAMA - Mw: 3k g / mol) (in each case based on the total weight of the electrode),

Binder system (2) 2.7% by weight (NA-CMC-700k g / mol), 2.7% by weight (PAA-450k g / mol), 2.7% by weight (PAMA-Mw: 3k g / mol) (in each case based on the total weight of the electrode) together.

The pH is checked and the viscosity is adjusted with an appropriate amount of distilled water and ethanol (4: 6). The electrode sheets are then drawn onto a 10 μπΊ copper foil with a squeegee and dried in air. Subsequently, electrodes are punched, annealed (120 ° C overnight) and installed in half-cells against lithium in coin cell format.

Comparative example

As a comparison to the electrodes prepared in Example 1, other binder components were combined, each of which can function as a base or acid and can covalently bond with the silanol groups of the silicon material on the silicon surface, for example: PAA (Mw: 250k g / mol) and PVA (Mw: 31 kg / mol) and CMC (Mw: 700 k g / mol). By using these binder components, the proportion of alcohol groups is increased and introduced with CMC a sterically demanding and with PVA a less steric binder component in the system. The former should favor the formation of a three-dimensional network, and the latter should hinder the reaction unhindered can participate. In addition, the binder content in the paste can be increased. The active material content was increased to obtain a comparable capacity of the electrodes. In addition, the molecular weights of the different components can be varied.

Results

Comparative measurements have shown that the inventive system of Example 1 has a better cycle stability and better efficiency than the system according to the comparative example. The best result regarding the choice of binder components could be achieved with the binder system CMC / PAA / PAMA. These results were further improved by increasing the molecular weight from 250k to 450k.

Electrochemical investigations

FIG. 1 shows the results of a C-rate test in which the corresponding cells were subjected to different current intensities over 5 cycles. As can be seen in FIG. 1, the electrodes (1, 4) according to the invention show significantly better cycle stability, in particular a better relative capacitance C _R (1) as a function of the number of cycles n than the base CMC (3, 6) or a binder composition CMC and PAA (2, 5). Also, the efficiency, the Coulomb efficiency C _E , is higher in the electrodes (4) according to the invention than in the base CMC electrode (6) or an electrode with a binder composition of CMC and PAA (5).

If the absolute capacity between start - at C / 10 - and at the end of the test - at C / 10 - compared with each other, an increase in capacity from 651 mAh / g (CMC) to 733 mAh / g (CMC / PAA) on 763 mAh / g (CMC / PAA / PAMA), which corresponds to an improvement of 17%.

The efficiency can be increased from 92.6% (CMC) to 94.6% (CMC / PAA / PAMA) at the beginning, after the second cycle. At the end of the test, the efficiency is also improved from 96.1% to 98.0%.

FIG. 1 also shows that the electrodes according to the invention exhibit markedly improved cycle stability even at high currents, indicating a lower internal cell resistance and improved electrical and / or ionic conductivity within the electrode and good preservation of the electrode architecture. FIG. 2 shows a cyclic voltammogram of an electrode with CMC / PAA / PAMA as binder (FIG. 2a) and a dilatometric measurement of an electrode with CMC / PAA / PAMA as binder (FIG. 2b). From the cyclic voltammogram shown (FIG. 2a), it can be seen that the binder, consisting of CMC / PAA / PAMA, does not undergo any electrochemical side reaction in the applied voltage window. The delithiation is voltage-dependent and occurs in stages (7, 8, 9, 10). Likewise, the lithiation of the electrode in stages (1 1, 12, 13). The binder is not only electrochemical but also chemically inert. This is evident from the dilatommogram shown in FIG. 2b. In this case no current was applied for 100 hours and the open cell potential (14) of the half cell was measured with an electrode according to the invention. In this case, no volume increase (15) is detectable after relaxation of the measuring apparatus, whereby the chemical inertness of the electrode with CMC / PAA / PAMA as a binder compared to the electrolyte and other cell components is given.

FIG. 3 shows the influence of the molecular weight of the PAA used in the binding system of the electrode according to the invention on the cycle stability. It can be seen that the use of PAA with a molecular weight of 450 k g / mol (17, 19) shows a further improvement with respect to the gravimetric capacity C _G and the Coulomb efficiency C _E as a function of the number of cycles n compared to the use of PAA having a molecular weight of 250k g / mol (16, 18).

Microscopic examinations

FIG. 4a shows scanning electron micrographs of the surfaces of aged electrodes (half cells in EC: DEC: DMC with 10% FEC), and FIG. 4b shows cross sections of the respective raw (upper row) and cyclized (lower row) electrodes in comparison. The scanning micrographs of the electrode surfaces in FIG. 4a clearly show that, compared to a binder system without additional PAMA (left), the use of PAMA as an additional binder component advantageously results in reduced cracking (right). This indicates that the electrode architecture is better preserved, which is associated with the improvement of the efficiency and leads to a better cycle stability (see Figure 1). The cross sections of the respectively uncyclized (upper row) and aged (lower row) electrodes shown in FIG. 4b clearly show that the electrode according to the invention, which contains CMC / PAA / PAMA as binder, leads to significantly less irreversible deformation of the electrodes (Figure 4b). In this case, an improvement of 89% could be achieved in the present example.

Claims

claims

An electrode for an electrochemical cell comprising silicon material, carbon black and at least one binder system, characterized in that the at least one binder system comprises CMC, PAA and PAMA.

2. An electrode according to claim 1, characterized in that the electrode has 10 to 60 wt .-% silicon material, based on the total weight of the electrode.

3. An electrode according to claim 1 or 2, characterized in that the electrode has 30 to 70 wt .-% graphite, based on the total weight of the electrode.

4. Electrode according to one of the preceding claims, characterized in that the electrode has 6 to 20 wt .-% Leitruß, based on the total weight of the electrode.

5. Electrode according to one of the preceding claims, characterized in that the electrode has 4 to 12 wt .-%, in particular 8 wt .-% of the binder system, based on the total weight of the electrode.

6. Electrode according to one of the preceding claims, characterized in that the at least one binder system 20 to 40 wt .-% CMC, based on the total weight of the binder system.

7. Electrode according to one of the preceding claims, characterized in that the at least one binder system 20 to 40 wt .-% PAA, based on the total weight of the binder system.

8. An electrode according to claim 7, characterized in that the PAA in the at least one binder system has a molecular weight of 300k to 550k g / mol, preferably 450k g / mol.

9. Electrode according to one of the preceding claims, characterized in that the at least one binder system 10 to 40 wt .-% PAMA, based on the total weight of the binder system.

10. Electrode according to one of the preceding claims, characterized in that for the weight ratio CMC: PAA: PAMA (a: b: c) in the binder system is: 0.5 <a <2, 0.5 <b <2 and 0, 5 <c <2.

1 1. A method for producing a lithium-ion battery, comprising the steps a) providing at least one electrode according to one of claims 1 to 10, b) providing at least one counter electrode and c) construction of a lithium-ion battery, comprising the at least An electrode according to any one of claims 1 to 10 and the at least one counter electrode.

12. Lithium-ion battery, comprising at least one electrode according to one of claims 1 to 10.

13. A method for producing an electrode for an electrochemical cell, characterized in that the method comprises the following steps: a) providing silicon material, conductive carbon black and at least one binder system, wherein the at least one binder system comprises CMC, PAA and PAMA, b) mixing the c) adjusting the pH of the electrode paste obtained in step b), d) annealing the electrode paste, and e) obtaining the electrode.

14. The method according to claim 13, characterized in that the pH of the electrode paste in step c) to 2 to 4, in particular to 2.8 to 3.2, preferably to 3, is set.

15. Use of the electrode with at least one binder system after one

Claims 1 to 10 in lithium-ion batteries.