WO2020206606A1

WO2020206606A1 - Binder composition for cathode of lithium-ion battery, cathode slurry composition, cathode and battery incorporating it

Info

Publication number: WO2020206606A1
Application number: PCT/CN2019/081917
Authority: WO
Inventors: Martin Hoch; Karola Schneiders; Susanne Lieber; Quinchun LIU
Original assignee: Arlanxeo Deutschland Gmbh
Priority date: 2019-04-09
Filing date: 2019-04-09
Publication date: 2020-10-15

Abstract

It provides a binder composition for a cathode of a cell of a lithium-ion battery comprising Ni-Co-containing cathode active material and hydrogenated nitrile butadiene rubber (HNBR) with an average molecular weight of more than 100,000 g/mol to less than 200,000 g/mol, a cathode slurry composition comprising the binder composition, a cathode, a process for manufacturing this cathode, and a lithium-ion battery having one or more cells incorporating this cathode.

Description

[Title established by the ISA under Rule 37.2] BINDER COMPOSITION FOR CATHODE OF LITHIUM-ION BATTERY, CATHODE SLURRY COMPOSITION, CATHODE AND BATTERY INCORPORATING IT

Binder composition for a cathode of a cell of a lithium-ion battery, a cathode slurry composition, a cathode and the battery incorporating it

Field of Invention

The present invention relate to a binder composition for a cathode of a cell of a lithium-ion battery comprising Ni-Co-containing cathode active material and hydrogenated nitrile butadiene rubber (HNBR) with an average molecular weight of more than 100,000 g/mol to less than 200,000 g/mol, a cathode slurry composition comprising the binder composition, a cathode, a process for manufacturing this cathode, and a lithium-ion battery having one or more cells incorporating this cathode.

Background of Invention

Lithium-ion batteries consist of at least two electrodes of different polarities, an anode and a cathode. A separator is generally located between the at least two electrodes. The separator consists of an electrical insulator imbibed with an electrolyte based on Li ⁺ensuring ionic conductivity. The electrolyte is generally a lithium salt which is dissolved in a mixture of non-aqueous solvents such as acetonitrile, tetrahydrofurane, ethylene carbonate or propylene carbonate. A cathode active material of the cathode of a lithium-ion battery allows reversible insertion/removal of lithium ions into/from this cathode, and the higher the mass fraction of this active material in the cathode, the higher its capacity. The cathode must also contain an conductive material and in order to provide the conductive material with sufficient mechanical cohesion, a polymer binder.

The cathodes of lithium-ion batteries are often manufactured by applying a cathode slurry composition on a cathode material and then evaporating a solvent of the cathode slurry composition. The cathode slurry composition is manufactured by dissolving and/or dispersing the polymer binder, the cathode active material, the conductive material and optionally a dispersant in the solvent.

The polymer binder is dissolved and/or dispersed in the cathode slurry composition to improve an adherence between the cathode active material and adhesion of the cathode active material with the cathode material. The polymer binder also facilitates a dispersion of the conductive material in the cathode slurry composition.

During a charging/discharging process of lithium-ion batteries, lithium ions are loaded and unloaded into the cathode active material. Due to this loading and unloading of lithium ions, an expansion and contraction of the cathode and anode material can occur as well as the cathode active material. It is therefore highly desirable to use elastomeric materials as the polymer binder for lithium-ion batteries to facilitate a flexible movement of the cathode active material during use, without delamination from the current collector and ultimately no crack formation.

The polymer binder facilitates dispersion of the cathode active material and the conductive material in the cathode slurry composition. The polymer binder stabilises the cathode active material and the conductive material in the cathode slurry during cathode manufacture and ensures smooth cathodes of lithium-ion batteries with a uniform pore structure. During use, the cohesion of the cathode and its adhesion with the current collector is of vital importance and influenced strongly by the type of binder used. Adhesion is a key property of the polymer binder which eventually determines the long term performance of the lithium-ion batteries.

EP3358651A discloses a conductive material paste composition for a secondary battery electrode comprising: a fibrous carbon nanomaterial; a binder; and a solvent, wherein the binder includes a first copolymer including an alkylene structural unit and a nitrile group containing monomer unit and having a weight average molecular weight of at least 170,000 and less than 1,500,000 g/mol and a second copolymer including an alkylene structural unit and a nitrile group-containing monomer unit and having a weight average molecular weight of at least 10,000 and less than 170,000 g/mol. In Comparative Example 2 on page 29, a binder is disclosed comprising only a hydrogenated nitrile butadiene rubber (iodine value: 20 mg/100 mg) with 35 mass%nitrile group containing monomer units and a molecular weight of 80,000. In Example 7 and 8, binder compositions with PVDF and two different types of HNBR are disclosed whereas one HNBR has a weight average moleculare weight of 100,000 or 200,000 g/mol. HNBR with low molecular weight was produced using high amounts of t-dodecyl mercaptan /TDM) as chain transfer agent.

EP3324468 discloses hydrogenated nitrile butadiene rubber with mooney viscosity of 40 MU or less as a binder for secondary battery electrodes. The effect of the molecular weight of the binder on the electrode characteristics is not disclosed.

EP3316360A discloses binder compositions for positive electrodes comprising HNBR as well as HNBR copolymers comprising butylacrylate or methacrylic acid. The Mooney viscosity of the binder ranges between 35 and 240 Mooney units. High amounts of t-dodecyl mercaptan /TDM) as chain transfer agent were added to reduce the Mooney viscosity.

KR20170111749A discloses a dispersing agent for a positive electrode of a secondary battery comprising HNBR with a weight average molecular weight of 290,000 g/mol.

EP3309879 discloses a positive electrode material mixture including Ni-Co-active material and a binder composition comprising a crystalline binder having a weight-average molecular weight (Mw) of 500,000 g/mol to 900,000 g/mol (such as PVDF) and an amorphous binder having a weight-average molecular weight (Mw) of 200,000 g/mol to 400,000 g/mol (such as NBR or SBR) .

Lithium-ion batteries based on binder compositions comprising LiCoO ₂ or LiNiO ₂ as cathode active material have been used in commercialised items, which operate in the high voltage range. However, a major drawback of these binder compositions are electrolyte oxidation during electrochemical processes.

A recent approach to replace LiCoO ₂ to improve cycling stability has seen LiNiO ₂ as an alternative. LiNiO ₂ is less oxidizing versus the electrolyte and is cheaper than the LiCoO ₂ compounds. However LiNiO ₂ is difficult to manufacture in a reproducible way because of its tendency to non-stoichiometry due to the presence of an excess of nickel. In addition LiNiO ₂ suffers from poor thermal stability in its highly oxidized state (Ni ³⁺/Ni ⁴⁺) . Therefore LiNiO ₂ is of no practical importance.

A promising candidate is Lithium-Nickel-Cobalt Oxide such as LiCo _yNi _1-yO ₂, as these materials alleviate the disadvantages for LiCoO ₂ and LiNiO ₂. Although the capacity of LiCoO ₂ is about 130 mAh/g, the capacity of LiCo _yNi _1-yO ₂ with part of the cobalt substituted by nickel, increases to about 150 mAh/g.

However, there is a need to provide a polymer binder for Ni-Co-containing cathode active materials for cathodes of lithium-ion batteries with electrochemical stability, desirable bonding strength, cycling stability over time and which do not break during processing and use.

Therefore an aim of the present invention is to provide a binder composition for cathodes for lithium-ion batteries that overcomes the aforementioned problems.

It was surprisingly discovered by the applicant, that a binder composition comprising a Ni-Co-containing cathode active material and a hydrogenated nitrile butadiene rubber (HNBR) with an average molecular weight of more than 100,000 g/mol to less than 200,000 g/mol as polymer binder can be used to provide cathodes that have optimal balance of resistivity and peel strength. At the same time, hydrogenated nitrile butadiene rubber with lower Mw results also in lower viscosity which is beneficial for the coating of the electrode.

Summary of Invention

In a first aspect the present invention relates to a binder composition for a cathode of a cell of a lithium-ion battery. The binder composition comprising:

- a Ni-Co-containing cathode active material; and

- hydrogenated nitrile butadiene rubber with an average molecular weight of more than 100,000 g/mol to less than 200,000 g/mol.

In a further aspect the present invention relates to a cathode slurry composition. The cathode slurry composition comprising:

- the binder composition,

- at least one conductive material; and

- at least one solvent.

In a further aspect the present invention relates to a cathode comprising:

- a current collector,

- the binder composition; and

- a conductive material.

The binder composition can be used to provide cathodes that have optimal balance of resistivity and peel strength. At the same time, hydrogenated nitrile butadiene rubber with Mw in the aforementioned ranges results also in lower viscosity which is beneficial for the coating of the electrode.

Detailed Description

For a complete understanding of the present invention and the advantages thereof, reference is made to the following detailed description.

It should be appreciated that the various aspects and embodiments of the detailed description as disclosed herein are illustrative of the specific ways to make and use the invention and do not limit the scope of invention when taken into consideration with the claims and the detailed description. It will also be appreciated that features from different aspects and embodiments of the invention may be combined with features from different aspects and embodiments of the invention.

- a Ni-Co-containing cathode active material; and

- hydrogenated nitrile butadiene rubber with an average molecular weight of morethan 100,000 g/mol to less than 200,000 g/mol.

The Ni-Co-containing cathode active material can be a lithium containing complex metal oxide comprising nickel and cobalt that accepts and donates electrons in the cathode of the lithium-ion battery. It further can occlude and release lithium.

the Ni-Co-containing cathode active material is a compound according to Formula 1:

Li _1+aNi _xCo _yMn _zM _wO ₂ [Formula 1]

wherein, M may be at least one of selected from the group consisting of aluminium (Al) , copper (Cu) , iron (Fe) , vanadium (V) , chromium (Cr) , titanium (Ti) , zirconium (Zr) , zinc (Zn) , tantalum (Ta) , niobium (Nb) , magnesium (Mg) , boron (B) , tungsten (W) , and molybdenum (Mo) , and

a, x, y, z, and w represent an atomic fraction of each independent element, wherein -0.5≤a≤0.5, 0<x≤1, 0<y≤1, 0≤z≤1, 0≤w≤1, and 0<x+y+z≤1,

The Ni-Co-containing cathode active material is Lithium-Nickel-Cobalt-Manganese-Oxide (NMC) or Lithium-Nickel-Cobalt-Aluminium-Oxide (NCA) and particularly preferably Lithium-Nickel-Cobalt-Manganese-Oxide (NMC) .

The Ni-Co-containing cathode active material LiNi _0.6Mn _0.2Co _0.2O ₂, LiNi _0.5Mn _0.3CO _0.2O ₂, LiNi _0.7Mn _0.15CO _0.15O ₂, or LiNi _0.8Mn _0.1Co _0.1O ₂, and any one thereof or a mixture of two or more thereof may be used. This increases capacity characteristics and stability of the resulting battery.

A particle size (D50) of the Ni-Co-containing cathode active material is preferably 1 to 50 μm, more preferably 5 to 15 μm.

The polymer binder of the present invention is a hydrogenated nitrile butadiene rubber (NBR) with an average molecular weight of more than 100,000 g/mol to less than 200,000 g/mol as described in detail below.

The hydrogenated nitrile butadiene rubber in the binder composition is a copolymer rubber, comprising an α, β-ethylenically unsaturated nitrile unit and a conjugated diene unit and optionally one or more further monomer units.

The term copolymer encompasses polymer having more than one monomer unit. In one embodiment of the invention, the copolymer is derived exclusively, for example, from the two monomer types (a) and (b) as described below. The term "copolymer" likewise encompasses, for example, additionally terpolymers and quaterpolymers, derived from the two monomer types (a) and (b) and one or more further monomer units.

The α, β-ethylenically unsaturated nitrile (a) used, which forms the α, β-ethylenically unsaturated nitrile units, may be any known α, β-ethylenically unsaturated nitrile. Preference is given to (C ₃-C ₅) -α, β-ethylenically unsaturated nitriles such as acrylonitrile, α-haloacrylonitrile, for example α-chloroacrylonitrile and α-bromoacrylonitrile, α-alkylacrylonitrile, for example methacrylonitrile, ethacrylonitrile or mixtures of two or more α, β-ethylenically unsaturated nitriles. Particular preference is given to acrylonitrile, methacrylonitrile, ethacrylonitrile or mixtures. Very particular preference is given to acrylonitrile. The amount of α, β-ethylenically unsaturated nitrile units is typically in the range from 10%to 60%by weight, preferably 20%to 50%by weight, more preferably from 25%to 40%by weight, based on the total amount of 100%by weight of all the monomer units.

If the hydrogenated nitrile butadiene rubber comprises the α, β-ethylenically unsaturated nitrile monomer in the ranges according to this invention, the cathode slurry composition can be stored in a stable condition for a long period of time. Furthermore, a uniform Ni-Co-containing cathode active material layer can be produced easily with an excellent stability against the electrolytic solution. In addition, the conductivity of the lithium ions becomes good and the internal resistance in the battery becomes small. Thus, the output characteristic of the lithium-ion battery can be improved as the cycle characteristics, particularly the high temperature cycle characteristics become excellent.

The conjugated diene (b) , which forms the conjugated diene unit (b) , may be of any type, especially conjugated C ₄-C ₁₂ dienes. Particular preference is given to 1, 3-butadiene, isoprene, 2, 3-dimethylbutadiene, 1, 3-pentadiene (piperylene) or mixtures thereof. Especially preferred are 1, 3-butadiene and isoprene or mixtures thereof. Very particular preference is given to 1, 3-butadiene. The amount of conjugated diene is typically in the range from 40%to 90%by weight, preferably 50%to 80%by weight, more preferably 60%to 75%by weight, based on the total amount of 100%by weight of all the monomer units.

In addition, the hydrogenated nitrile butadiene rubber may optionally contain one or more further copolymerizable monomers in an amount of 0%to 20%by weight, preferably 0.1%to 15%by weight, more preferably 3%to 10%by weight based on the total amount of 100%by weight of all monomer units in the hydrogenated nitrile butadiene rubber. In that case, the amounts of the other monomer units are reduced in a suitable manner, such that the sum total is always 100%by weight.

Preferred further copolymerizable monomers which may be used are, for example, α, β-ethylenically unsaturated carboxylic acids.

The α, β-ethylenically unsaturated carboxylic acid, which forms the α, β-ethylenically unsaturated carboxylic acid unit, may be of any known α, β-ethylenically unsaturated monocarboxylic acid and the derivative thereof or dicarboxylic acid and the derivative thereof. Particular preference is given to monocarboxylic acids such as acrylic acid, methacrylic acid, crotonic acid or cinnamic acid. Especially preferred are monocarboxylic acids such as acrylic acid and methacrylic acid. Very particular preference is given to methacrylic acid. Preferred derivatives of monocarboxylic acids are 2-ethylacrylic acid, isocrotonic acid, α-acetoxy acrylic acid, β-transaryloxy acrylic acid, α-chloro-β-E-methoxy acrylic acid or β-diamino acrylic acid. Preferred dicarboxylic acids are, maleic acid, fumaric acid or itaconic acid. Preferred dicarboxylic acid derivatives are methyl allyl maleate such as methylmaleic acid, dimethyl maleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, fluoromaleic acid; maleic acid esters such as diphenyl maleate, nonyl maleate, decyl maleate, dodecyl maleate, octadecyl maleate, or fuluoroalkyl maleate. Also, acid anhydride which generates the carboxylic acid group by hydrolysis can be used as well. Preferred acid anhydrides are maleic acid anhydride, acrylic acid anhydride, methyl acrylic acid anhydride or dimethyl acrylic acid anhydride. The α, β-ethylenically unsaturated carboxylic acid comprise also α, β-ethylenically unsaturated dicarboxylic acid monoesters such as alkyl, especially C ₄-C ₁₈-alkyl, preferably n-butyl, tert-butyl, n-pentyl or n-hexyl, more preferably mono-n-butyl maleate, mono-n-butyl fumarate, mono-n-butyl citraconate, mono-n-butyl itaconate; alkoxyalkyl, especially C ₄-C ₁₈-alkoxyalkyl, preferably C ₄-C ₁₂-alkoxyalkyl; hydroxyalkyl, especially C ₄-C ₁₈-hydroxyalkyl, preferably C ₄-C ₁₂-hydroxyalkyl; cycloalkyl, especially C ₅-C ₁₈-cycloalkyl, preferably C ₆-C ₁₂-cycloalkyl, more preferably monocyclopentyl maleate, monocyclohexyl maleate, monocycloheptyl maleate, monocyclopentyl fumarate, monocyclohexyl fumarate, monocycloheptyl fumarate, monocyclopentyl citraconate, monocyclohexyl citraconate, monocycloheptyl citraconate, monocyclopentyl itaconate, monocyclohexyl itaconate and monocycloheptyl itaconate; alkylcycloalkyl, especially C ₆-C ₁₂-alkylcycloalkyl, preferably C ₇-C ₁₀-alkylcycloalkyl, more preferably monomethylcyclopentyl maleate and monoethylcyclohexyl maleate, monomethylcyclopentyl fumarate and monoethylcyclohexyl fumarate, monomethylcyclopentyl citraconate and monoethylcyclohexyl citraconate; monomethylcyclopentyl itaconate and monoethylcyclohexyl itaconate; aryl, especially C ₆-C ₁₄-aryl, monoester, preferably monoaryl maleate, monoaryl fumarate, monoaryl citraconate or monoaryl itaconate, more preferably monophenyl maleate or monobenzyl maleate, monophenyl fumarate or monobenzyl fumarate, monophenyl citraconate or monobenzyl citraconate, monophenyl itaconate or monobenzyl itaconate or mixtures thereof; unsaturated polyalkyl polycarboxylates, for example dimethyl maleate, dimethyl fumarate, dimethyl itaconate or diethyl itaconate; or α, β-ethylenically unsaturated carboxylic esters containing amino groups, for example dimethylaminomethyl acrylate or diethylaminoethyl acrylate.

The carboxylic acid group in the α, β-ethylenically unsaturated carboxylic acid increases the binding properties of the cathode active material layer and the current collector.

Further copolymerizable monomers which may be used are, for example,

· aromatic vinyl monomers, preferably styrene, α, β-styrene, α-methylstyrene and vinylpyridine,

· fluorinated vinyl monomers, preferably fluoroethyl vinyl ether, fluoropropyl vinyl ether, o-fluoromethylstyrene, vinyl pentafluorobenzoate, difluoroethylene and tetrafluoroethylene, or else

· α-olefins, preferably C ₂-C ₁₂ olefins, for example ethylene, 1-butene, 4-butene, 4-methyl-1-pentene, 1-5 hexene or 1-octene,

· non-conjugated dienes, preferably C ₄-C ₁₂ dienes such as 1, 4-pentadiene, 1, 4-hexadiene, 4-cyanocyclohexene, 4-vinylcyclohexene, vinylnorbornene, dicyclopentadiene or else

· alkynes such as 1-or 2-butyne,

· copolymerizable antioxidants, for example N- (4-anilinophenyl) acrylamide, N- (4-anilinophenyl) methacrylamide, N- (4-anilinophenyl) cinnamide, N- (4-anilinophenyl) crotonamide, N-phenyl-4- (3-vinylbenzyloxy) aniline, N-phenyl-4- (4-vinylbenzyloxy) aniline or

· crosslinkable monomers, for example divinyl components, for example divinylbenzene; di (meth) acrylic esters, for example ethylene glycol di (meth) acrylate, diethylene glycol di (meth) acrylate, butanediol di (meth) acrylate orpolyethylene glycol di (meth) acrylate, or tri (meth) acrylic esters, for example trimethylolpropane tri (meth) acrylate; self-crosslinkable monomers, for example N-methylol (meth) acrylamide or N, N'-dimethylol (meth) acrylamide.

The hydrogenated nitrile butadiene rubber has an average molecular weight of more than 100,000 g/mol to less than 200,000 g/mol, preferably in the range from 110,000 g/mol to 195,000 g/mol, more preferably in the range from 110,000 g/mol to 180,000 g/mol and most preferably in the range from 110,000 g/mol to 150,000 g/mol.

If the hydrogenated nitrile butadiene rubber comprises the average molecular weight in the ranges according to this invention, the Ni-Co-containing cathode active material can be dispersed stably in the cathode slurry composition. This improves the stability of the cathode slurry composition. Also, the binding properties between the Ni-Co-containing cathode active material particles between each other is improved. Furthermore, the binding properties between the Ni-Co-containing cathode active material layer and the current collector is improved. Thus, the cathode slurry composition and the Ni-Co-containing cathode active material layer are suppressed from falling off during the production step of the cathode. Due to the improved binding properties, the cathode shows excellent cycle characteristics.

The use of high molecular weight hydrogenated nitrile butadiene rubbers as binder in electrodes results in excellent peel strength but also high resistivity, which is unfavored.

The use of low molecular weight hydrogenated nitrile butadiene rubber improves the resistivity but in the same time, peel strength is strongly decreased, which is unfavored.

The use of hydrogenated nitrile butadiene rubber of this invention with a weight-average molecular weight of more than 100,000 g/mol to less than 200,0000 g/mol leads to an excellent balance between the the peel strength and the resistivity.

At the same time, hydrogenated nitrile butadiene rubber with lower Mw results also in lower viscosity which is beneficial for the coating of the electrode.

The hydrogenated nitrile butadiene rubber typically has a polydispersity index (PDI=Mw/Mn where Mw is the weight-average molecular weight and Mn is the number-average molecular weight) of 1.5 to 6, preferably 2 to 5 and more preferably 2.5 to 4.

The hydrogenated nitrile butadiene rubber containing nitrile groups typically has a Mooney viscosity (ML 1+4@100℃) of 10 to 150, preferably of 20 to 120 and more preferably of 25 to 100.

A preferred hydrogenated nitrile butadiene rubber comprises as α, β-ethylenically unsaturated nitrile unit (a) preferably acrylonitrile or methacrylonitrile, more preferably acrylonitrile and as conjugated diene unit (b) preferably isoprene or 1, 3-butadiene, more preferably 1, 3-butadiene.

A process for preparing the aforementioned hydrogenated nitrile butadiene rubber by polymerization of the aforementioned monomers has been described extensively in the literature (e.g. Houben-Weyl, Methoden der Organischen Chemie [Methods of Organic Chemistry] , vol. 14/1, 30 Georg Thieme Verlag Stuttgart 1961) and is not particularly restricted. In general, the process is one in which α, β-ethylenically unsaturated nitrile units and conjugated diene units are copolymerized as desired. The polymerization process used may be any known emulsion polymerization process, suspension polymerization process, bulk polymerization process and solution polymerization process. Preference is given to the emulsion polymerization process. Emulsion polymerization is especially understood to mean a process known per se in which the reaction medium used is typically water (see, inter alia,

Lexikon der Chemie [

Chemistry Lexicon] , volume 2, 10 ^th edition 1997; P. A. Lovell, M. S. El-Aasser, Emulsion Polymerization and Emulsion Polymers, John Wiley &Sons, ISBN: 0471 96746 7; H. Gerrens, Fortschr. Hochpolym. Forsch. 1, 234 (1959) ) . The incorporation rate of the termonomer can be adjusted directly by the person skilled in the art, such that an inventive terpolymer is obtained.

It is also possible that the preparation of the nitrile butadiene rubber is followed by a metathesis reaction to reduce the molecular weight of the nitrile butadiene rubber or a metathesis reaction and a subsequent hydrogenation, or a hydrogenation only. These metathesis and hydrogenation reactions are sufficiently well known to those skilled in the art and are described in the literature. Metathesis is known, for example, from WO-A-02/100941 and WO-A-02/100905 and can be used to reduce the molecular weight.

The nitrile butadiene rubber is at least partly hydrogenated (hydrogen addition reaction) after the copolymerization . In such at least partly hydrogenated nitrile butadiene rubber, at least some of the C=C double bonds of the repeat unit derived from the conjugated diene have been specifically hydrogenated.

The degree of hydrogenation of the conjugated diene units (b) is preferably 50%or more, preferably 75%or more, more preferably 85%or more and most preferably 95%or more.

The hydrogenation of nitrile butadiene rubber is known, for example from US A 3 700 637, DE A 2 539 132, DE A 3 046 008, DE A 3 046 251, DE A 3 227 650, DE A 3 329 974, EP A-111 412, FR-B 2 540 503. Hydrogenated nitrile butadiene rubber are notable for high breaking strength, low abrasion, consistently low deformation after pressure and tensile stress, and good oil resistance, but in particular for remarkable stability against thermal and oxidative influences.

In another preferred embodiment, said hydrogenated nitrile butadiene rubber is a hydrogenated carboxylated nitrile butadiene rubber (HXNBR) .

In a preferred embodiment, the binder composition comprises

(i) at least one Ni-Co-containing cathode active material and

(ii) at least one polymer binder comprising 10 to 60%by weight α, β-ethylenically unsaturated nitrile-group containing monomers, 40 to 90%by weight conjugated diene monomers with a weight-average molecular weight of more than 100,000 g/mol to less than 200,000 g/mol,

wherein the degree of hydrogenation of the conjugated diene is 50%or more, preferably 75%or more, more preferably 85%or more and most preferably 95%or more.

In a preferred embodiment, the hydrogenated nitrile butadiene rubber is present in the binder composition in an amount of 1 to 30%by weight, preferably 1 to 20%by weight, more preferably 2 to 10%by weight based on the total weight of the binder composition.

The hydrogenated nitrile butadiene rubber may be preferably present in a binder solution in a concentration of 1 to 30%by weight, more preferably in a concentration of 2 to 20%by weight and most preferably in a concentration of 3 to 10%by weight based on the total weight of the binder solution.

In a preferred embodiment of the invention, the polymer binder consists of hydrogenated nitrile butadiene rubber and no further polymer is present.

The polymer binder used in the present invention comprises the above mentioned hydrogenated nitrile butadiene rubber. In a preferred embodiment, the polymer binder consists of the above mentioned hydrogenated nitrile butadiene rubber. In the present invention, by using the binder composition including the hydrogenated nitrile butadiene rubber, the positive electrode for the secondary battery has excellent flexibility and binding property.

The cathode slurry composition for the secondary battery of the present invention comprises the above mentioned Ni-Co-containing cathode active material, the polymer binder, conductive material, solvent and optionally a dispersant. Hereinafter, the embodiment of using the cathode slurry composition as the cathode slurry composition for the Lithium-ion battery will be explained.

In a preferred embodiment, the polymer binder is the hydrogenated nitrile butadiene rubber according to this invention, whereas the hydrogenated nitrile butadiene rubber is present in the cathode slurry composition in an amount of 1 to 10%by weight, preferably 1.5 to 7%by weight, more preferably 2 to 5%by weight based on the total weight of the binder composition.

The cathode slurry composition comprises at least one conductive material. Preferred conductive materials are acetylene black, Ketjen black, carbon black, graphite, expanded graphite, vapor-grown carbon fiber and carbon nanotubes, graphene, graphene oxide and their mixtures. By comprising the conductive material, the electrical connection of the Ni-Co-containing cathode active materials can be improved, and the discharge rate characteristic when using the lithium-ion battery can be improved.

In a preferred embodiment, the conductive material is present in the cathode slurry composition in an amount of 0, 5 to 50%by weight based on the total solid weight of the cathode slurry composition.

The cathode slurry composition comprises at least one solvent. The solvent is not particularly limited as long as the polymer binder can be dispersed or dissolved uniformly, and water or organic solvent can be used. The organic solvent may comprise cyclic aliphatic hydrocarbons such as cyclopentane and cyclohexane; aromatic hydrocarbons such as toluene, xylene and cyclobenzene; ketones such as acetone, methyl ethyl ketone, diisopropyl ketone, cyclohexanone, methylcyclohexane, ethylcyclohexane; chlorine based aliphatic hydrocarbons such as methylene chloride, chloroform, and carbon tetrachloride; esters such as ethyl acetate, butyl acetate, γ-butyrolactone, ε-caprolactone; acylonitriles such as acetonitrile or propionitrile; ethers such as tetrahydrofurane or ethyleneglycoldiethylether; alcohols such as methanol, ethanol, isopropanol, ethyleneglycol or ethyleneglycolmonomethylether; amides such as N-methylpyrrolidone (NMP) , and N, N-dimethylformamide may be mentioned. Preferred solvents are methyl ethyl ketone and N-methylpyrrolidone. Particularly preferred is N-methylpyrrolidone.

The production method of the cathode slurry composition used in the present invention is not particularly limited, and it is produced by mixing the above mentioned binder solution with the cathode active material and the conductive material. The mixing device is not particularly limited as long as the binder solution, the cathode active material and the conductive material can be mixed uniformly; and for example the method of using the mixing device such as the stirring type, the shaking type, and the rotating type may be mentioned. Also, the method using the dispersing kneader such as homogenizer, ball mill, sand mill, roll mill, a planetary kneader such as planetary mixer may be mentioned.

The cathode of the secondary battery of the present comprises a current collector, and a Ni-Co-containing cathode active material layer. The Ni-Co-containing cathode active material layer comprises the binder composition (i.e. the Ni-Co-containing cathode active material and the polymer binder) and the conductive material and other components which are added depending on the needs may be comprised as well. The Ni-Co-containing cathode active material layer is formed on said current collector.

The current collector is not particularly limited if this is a material having electric conductivity and electrochemical durability. Preferably, in view of the heat resistance, the current collector is selected from a group consisting of iron, copper, aluminium, nickel, stainless steel, titanium, tantalum, gold and platinum. Among these, aluminium is particularly preferable for current collector of cathode. The shape of the current collector is not particularly limited, and the sheet form current collector having a thickness of about 0.001 to 0.5 mm is preferable, more preferable 1 to 100 μm. It is preferable that the current collector is subject to a roughening treatment in advance before the use, in order to increase the adhering strength with the Ni-Co-containing cathode active material layer. Method of the roughening treatment may include mechanical polishing method, electropolishing method, chemical polishing method, etc. In the mechanical polishing cathode, a coated abrasive cathode in which abrasive particles are fixed, a grinding stone, an emery buff or a wire-brush provided with steel wire can be used. Also, an intermediate layer may be formed on the surface of the current collector to increase the adhering strength and conductivity between the Ni-Co-containing cathode active material layer and the current collector.

The invention further relates to a process for manufacturing a cathode such as defined above, characterized in that it comprises the following steps of:

(1) dissolving the hydrogenated nitrile butadiene rubber in a solvent to form a binder solution; and

(2) mixing the binder solution from step (1) with the Ni-Co-containing cathode active material and a conductive material to form a cathode slurry composition,

(3) applying the cathode slurry composition from step (2) onto a current collector to form a cathode sheet; and

(4) drying the cathode sheet of step (3) .

In a preferred embodiment of this invention, step (1) may be carried out by dissolving the hydrogenated nitrile butadiene rubber in a shaker over night at room temperature. In a preferred embodiment of this invention, the binder solution formed in step (1) has a concentration of 1 to 30%by weight, preferably 1.5 to 20%by weight and more preferably 2 to 15%by weight based on the total weight of the binder solution.

In a preferred embodiment of this invention, step (2) may be carried out in a planetary ball mill.

In a preferred embodiment of this invention, step (3) may be carried out with a bar coater, more preferably with a bar coater with a slit gap of 100 to 200 μm.

In a preferred embodiment of this invention, step (4) may be carried out in an oven, more preferably at a temperature of 70℃ or higher.

In a preferred embodiment, the cathode sheet is calendered to adjust the areal density after the drying step (4) .

The cathodes are punched from the calandered cathode sheet.

The invention further relates to a lithium-ion battery comprising one or more cell comprising the cathode of this invention. A lithium-ion battery according to the invention comprises at least one cell comprising an anode, a cathode such as defined above, a separator and an electrolytic solution based on a lithium salt and on an organic solvent.

As the electrolytic solution for the lithium-ion battery, an organic electrolytic solution can be used wherein the supporting electrolyte is dissolved in an organic solvent.

As the supporting electrolyte, a lithium salt may be used. The lithium salt is not particularly limited, and for example, LiPF ₆, LiAsF ₆, LiBF ₄, LiSbF ₆, LiAlCl ₄, LiClO ₄, CF ₃SO ₃Li, C ₄F ₉SO ₃Li, CF ₃COOLi, (CF ₃CO) ₂NLi, (CF ₃SO ₂) ₂NLi, (C ₂F ₅SO ₂) NLi and the like may be used. Among these, LiPF ₆, LiClO ₄, CF ₃SO ₃Li are preferable since these are easily dissolved in the organic solvent and show a high degree of dissociation. Two or more thereof can be used together. If the supporting electrolytes have a higher dissociation degree, the lithium ion conductivity becomes higher, thus the lithium ion conductivity can be regulated by the type of the supporting electrolyte.

The organic solvent used for the electrolytic solution for the lithium-ion battery is not particularly limited as long as the supporting electrolytes can be dissolved. Carbonates such as dimethyl carbonate (DMC) , ethylene carbonate (EC) , diethyl carbonate (DEC) , propylene carbonate (PC) , butylene carbonate (BC) or methyl ethyl carbonate (MEC) ; esters such as γ-butyrolactone or methyl formate; ethers such as 1, 2-dimethoxy ethane and tetrahydrofuran; sulfur-containing compounds such as sulfolane and dimethyl sulfoxide may or used. The mixed solvent thereof may be used as well. Among these, the carbonates are preferable as it has high dielectric constant, and the stable electric potential is wide. The lower the viscosity of the used solvent is, the higher the lithium ion conductivity is; thus the lithium ion conductivity can be regulated by the type of the solvent.

Further additives can be added to the electrolytic solution. Carbonates such as vinylene carbonate (VC) are preferable as additive.

The concentration of the supporting electrolyte in the electrolytic solution for the lithium-ion battery is typically 1 to 30%by weight, preferably 5 to 20%by weight, based on the total weight of the electrolytic solution. Also, depending on the type of the supporting electrolyte, typically it is used in a concentration of 0.5 to 2.5 mol/L. The ion conductivity tends to decline in case the concentration of the supporting electrolyte is too low or too high.

As separator for the lithium-ion battery known separators such as fine porous films or nonwoven fabrics comprising aromatic polyamide resins or the polyolefin resins such as polyethylene or polypropylene; may be used. For example, the fine porous film formed by the resin such as polyolefin type polymer (polyethylene, polypropylene, polybutene, polyvinyl chloride) and the mixture or the copolymer thereof; the fine porous film formed by the resin such as polyethylene terephthalate, polycycloolefin, polyether sulfone, polyamide, polyimide, polyimideamide, polyaramid, polycycloolefin, nylon, polytetrafuluoro ethylene, the woven fabric wherein the polyolefin type fiber are woven or the non-woven fabric thereof, and the aggregate of the insulating material particles may be mentioned. Among these, the fine porous film formed by the polyolefin type resin is preferable since the thickness of the separator as a whole can be made thinner and the capacity per volume can be increased by increasing the active material ratio in the battery.

The thickness of the separator is typically 0.5 to 50 μm, preferably 1 to 45 μm, and more preferably 1 to 40 μm. When it is within this range, the resistance of the separator in the battery becomes smaller, and the processing while forming the battery is superior.

The anode active material layer selectively comprise a binder and a conductive agent in addition to the anode active material. The anode active material layer may be prepared by coating a composition for forming an anode, which selectively comprises the binder and the conductive agent as well as the anode active material, on the negative electrode collector and drying the coated anode collector, or may be prepared by casting the composition for forming an anode on a separate support and then laminating a film separated from the support on the anode collector. As the current collector, those mentioned in the cathode of the lithium-ion battery can be mentioned, and it is not particularly limited as long as it is a material having the electric conductivity and the electrochemical durability; however copper is preferable as the anode of the lithium-ion battery.

As the anode active material for the lithium-ion battery anode, for example carbon materials such as amorphous carbon, graphite, natural black lead, mesocarbon microbead and pitch-based carbon fiber, conductive polymer such as polyacene may be mentioned. Also, as the anode active material, a metal such as silicon, tin, zinc, manganese, iron and nickel, the alloy thereof, oxide and sulfate salt of the above metal or alloy can be used. In addition, metal lithium, lithium alloy such as Li-Al, Li-Bi-Cd and Li-Sn-Cd, nitride of lithium-transition metal or silicone can be used as well. As the anode active material, those adhered with the conductivity supplying material on the surface by the surface mechanical modified method can be used as well. The particle diameter of the anode active material is selected depending on balance between the other requirements of the battery, however the 50%volume cumulative diameter of the anode active material is normally 1 to 50 mm, preferably 15 to 30 mm, in view of improvement in battery characteristics such as initial efficiency, load characteristic and cycle characteristic.

The content ratio of the anode active material of the anode active material layer is preferably 90 to 99.9%by weight, and more preferably 95 to 99%by weight based on the total weight of the anode active material layer. By having the content ratio of the anode active material within said range, it can exhibit flexibility and the binding property while showing high capacity.

Also, in the anode for the lithium-ion battery, besides the above mentioned component, the solvent used in the cathode of aforementioned, or the electrolytic solution additives which has function to suppress the electrolytic solution decomposition may be included. These may not be particularly limited, as long as it does not influence the battery reaction.

As the binder composition for the lithium-ion battery anode, known material can be used without any particular limitation. For example, those used in the aforementioned cathode for the lithium-ion battery such as resins such as polyethylene, polytetrafluoroethylene (PTFE) , polyvinylidene fluoride (PVDF) , tetrafluoroethylene-hexafluoropropylene copolymer (FEP) , polyacrylic acid derivative or polyacrylonitrile derivative; soft polymers such as acrylic based soft polymer, diene based soft polymer, olefin based soft polymer or vinyl based soft polymer can be used. These may be used alone or by combining two or more thereof.

The negative electrode for the lithium-ion battery can be produced as same as the aforementioned positive electrode.

As the specific production method of the lithium-ion battery, the cathode and the anode mentioned in above may be layered via the separator, which is then winded or bended depending on the battery shape to fit in the battery case, followed by filling the electrolyte in the battery case and sealing the case. Also, as needed, it is possible to prevent pressure increase inside the battery and overcharge-overdischarge by setting in expanded metal such as a nickel sponge, overcurrent protection element such as fuse and PTC element, and lead plate, etc. The shape of the battery may include coin shape, button shape, sheet shape, cylinder shape, square shape and flattened shape.

The present invention is demonstrated by the following non-limiting examples.

Examples

Test methods

Method of evaluating peel strength

The evaluation of the peel strength was performed according to ASTM D903. The cathode sheet was cut into test strips with a width of 25 mm and 175 mm length. 3M vinyl electrical tape was bonded onto the coated Ni-Co-containing cathode active material surface of the cathode sheet. The peel test was carried out on the test strip through an Instron tensile machine peeling the tape in a 180° direction with 100 mm/min. The peel force was recorded during the test. The peel strength was calculated according to the following formula:

The average peel strength was calculated basing on the data among 50-200 mm displacement and the average of three measurements was taken as peel strength value.

Method of evaluating capacity retention

The coin cell secondary battery was charged and discharged in constant current mode (CC mode 0.2 C rate) for 100 cycles. Capacity retention was determined as the ratio of the discharge specific capacity after 100 cycles over the discharge specific capacity after the second cycle in percent.

Method of evaluating surface resistivity

the surface resistivity of the coated cathode sheet was tested with a four point probe with a spacing of 10mm. the test was repeated three times and an average value recorded.

General method of coin cell fabrication

Step (1) –Dissolution: A certain amount of the polymer binder (HNBR) is dissolved in the solvent (NMP) in a shaker overnight at room temperature to form a binder solution (5 wt. -%) .

Step (2) –Cathode slurry composition preparation: The binder solution from step 1 is mixed with the active material NMC and the conductive material (conductive carbon black Super P) in a planetary ball mill (milling conditions: 28 Hz, 6 minutes, room temperature) to obtain the cathode slurry composition.

Weight ratio: NMC/polymer/NMP/Super P = 80/10/190/10 (Polymer concentration in NMP = 5 wt. -%)

Step (3) –Production of the cathode disc: The cathode slurry composition was applied with a bar coater onto a current collector (aluminum foil) using 2.8 mm/scoating speed to form a cathode sheet. The coater slit gap of the coating machine was adjusted to 150 μm to obtain a pre-determined coating thickness.

Step (4) –Drying: The cathode sheet was dried in an oven at 80℃ for 120 minutes to remove NMP and moisture. After drying the cathode sheet was calendered to adjust the areal density (weight: 12-19 mg/disc; disc area: 201 mm ²; density: 60-95 g/m ²) . From the calandered cathode sheet a cathode disc

was punched using a machine from ShenZhen PengXiang YunDa Machinery Technology Co., Model: PX-CP-S2. The punch edge was sharp without burr.

Step (5) –Assembly of the lithium-ion secondary battery: Assembly and pressing of the Lithium-ion secondary battery is carried out in a glove box. The assembly comprises the coin cell casing top (2032 type; negative side) , the nickel sponge, the lithium disc (as anode) , the porous separator (Celgard 2340) , the cathode disc and the casing bottom (positive side) . All parts were assembled layer-by-layer. The electrolyte solution was dropped in during the assembly step in order to completely fill the free volume of the coin cell. Finally, the coin cell case was pressed by the press machine in the glovebox. An open-circuit voltage test was performed to check, whether short-circuit took place or not.

Table 2: Determination of the peel strength of the cathode sheet, surface resistivity and capacity retention of the lithium-ion battery

*inventive examples; n.d. = not determined

The results in Table 2 show that HNBR binders with a weight-average molecular weight of more than 100,000 g/mol lead to cathode sheets with a higher peel strength (>0.5 N/m) than HNBR binders with less than 100,000 g/mol. A high peel strength is preferred since the cohesion of the active material and the adhesion of active material to the current collector is important for the longevity of the lithium ion battery.

The results in Table 2 further show that HNBR binders with a weight-average molecular weight (Mw) of less than 200,000 g/mol lead to cathodes with a low surface resistivity ( <50 %) than HNBR binders with more than 200,000 g/mol. This means that the conductive material in the slurry are well and stable dispersed. A low resistivity leads to better electron transport in the cell. In addition, materials with an average molecular weight (Mw) of more than 100,000 g/mol lead to good capacity retention (>80%) which is preferred since the longevity of the battery cell is thus increased.

The combination of Ni-Co-containing cathode active material such as NMC and NCA with HNBR with a molecular weight of more than 100,000 g/mol and less than 200,000 g/mol as the polymer binder leads to lithium-ion cells with remarkably higher capacity retention than using either PVDF as the polymer binder or a non-Ni-Co-containing active material such as LiCO as the active material. A high capacity retention is preferred to obtain long lifetime of the battery.

Having thus described the present invention and the advantages thereof, it should be appreciated that the various aspects and embodiments of the present invention as disclosed herein are merely illustrative of specific ways to make and use the invention.

The various aspects and embodiments of the present invention do not limit the scope of the invention when taken into consideration with the appended claims and the foregoing detailed description.

What is desired to be protected by letters patent is set forth in the following claims.

Claims

A binder composition for a cathode of a cell of a lithium-ion battery, the binder composition comprising:

- a Ni-Co-containing cathode active material; and

- hydrogenated nitrile butadiene rubber with an average molecular weight of more than 100,000 g/mol to less than 200,000 g/mol.
The binder composition according to claim 1, wherein the Ni-Co-containing cathode active material is a lithium containing complex metal oxide.
The binder composition according to anyone of the above claims, wherein the Ni-Co-containing cathode active material is a compound according to Formula 1:

Li _1+aNi _xCo _yMn _zM _wO ₂ [Formula 1]

wherein, M may be at least one of selected from the group consisting of aluminium (Al) , copper (Cu) , iron (Fe) , vanadium (V) , chromium (Cr) , titanium (Ti) , zirconium (Zr) , zinc (Zn) , tantalum (Ta) , niobium (Nb) , magnesium (Mg) , boron (B) , tungsten (W) , and molybdenum (Mo) , and

a, x, y, z, and w represent an atomic fraction of each independent element, wherein -0.5≤a≤0.5, 0<x≤1, 0<y≤1, 0≤z≤1, 0≤w≤1, and 0<x+y+z 1, more preferably Lithium-Nickel-Cobalt-Manganese-Oxide (NMC) or Lithium-Nickel-Cobalt-Aluminium-Oxide (NCA) and particularly preferably Lithium-Nickel-Cobalt-Manganese-Oxide (NMC) .
The binder composition according to anyone of the above claims, wherein the hydrogenated nitrile butadiene rubber is selected from at least one of a hydrogenated nitrile butadiene rubber (HNBR) , a hydrogenated carboxylated nitrile butadiene rubber (HXNBR) or any combination thereof.
The binder composition according to anyone of the above claims, wherein the hydrogenated nitrile butadiene rubber has a hydrogenation degree of 50%or more.
The binder composition according to anyone of the above claims, wherein the hydrogenated nitrile butadiene rubber comprises:

- 10 to 60%by weigh α, -ethylenically unsaturated nitrile monomer units, and

-40 to 90%by weight conjugated diene monomer units,

based on a total weight of monomer units in the rubber.
The binder composition according to anyone of the above claims, wherein the hydrogenated nitrile butadiene rubber is present in the binder composition in an amount of 1 to 30%by weight based on the total weight of the binder composition.
A cathode slurry composition comprising:

- the binder composition according to any of claims 1 to 7,

- at least one conductive material; and

- at least one solvent.
The cathode slurry composition according to claim 8, wherein the at least one conductive material is selected from at least one of acetylene black, Ketjen black, carbon black, graphite, expanded graphite, vapor-grown carbon fiber, carbon nanotube, graphene, graphene oxide or any mixture thereof.
The cathode slurry composition according to any one of claims 8 -9, wherein the at least one conductive material is present in an amount of 1 to 10%by weight based on the total solid weight of the cathode slurry composition.
The cathode slurry composition according to any one of claims 8 -10, wherein the at least one solvent is selected from at least one of N-methylpyrrolidone (NMP) , methyl ethyl ketone (MEK) or any combination thereof.
A cathode comprising:

-a current collector,

- the binder composition according to any of claims 1 to 7; and

- a conductive material.
The cathode according to claim 12, wherein the current collector is selected from the group consisting of iron, copper, aluminium, nickel, stainless steel, titanium, tantalum, gold and platinum or aluminium foil.
A lithium-ion battery comprising the according to any one of claims 12-13.