WO2017008284A1

WO2017008284A1 - Electrode for rechargeable lithium ion battery

Info

Publication number: WO2017008284A1
Application number: PCT/CN2015/084140
Authority: WO
Inventors: Xiaogang HAO; Rongrong JIANG; Jingjun Zhang
Original assignee: Robert Bosch Gmbh
Priority date: 2015-07-15
Filing date: 2015-07-15
Publication date: 2017-01-19

Abstract

The present invention provides an electrode for rechargeable Lithium ion battery, especially an electrode comprising a PEG-PAA (Li) hybrid binder, a rechargeable Lithium ion battery comprising the electrode, and use of the PEG-PAA (Li) hybrid binder in an electrode for rechargeable Lithium ion battery.

Description

Electrode for Rechargeable Lithium Ion Battery

Technical field

The present invention relates to an electrode for rechargeable Lithium ion battery, especially an electrode comprising a PEG-PAA (Li) hybrid binder, and to a rechargeable lithium ion battery including the electrode as well as use of the PEG-PAA (Li) hybrid binder in an electrode for rechargeable Lithium ion battery.

Background arts

Generally, a positive electrode and a negative electrode are included in a rechargeable lithium ion battery and both electrodes comprise material (s) that can intercalate/deintercalate lithium ions.

As for positive active materials for rechargeable lithium ion battery, lithium-transition metal oxides such as LiCoO₂, LiMnO₂, LiNi_1-xCo_xO₂ (0<x<1) and the like are conventionally used. As for negative active materials for rechargeable lithium ion battery, it is well known that silicon can be used since Si can provide a significantly higher capacity than graphite, for example, compound Li₂₁Si₅ has a maximum theoretical capacity of 4, 200 mAh/g, which is considerably higher than the maximum capacity of graphite (372 mAh/g) . However, silicon-based negative active material undergoes a huge volume change associated with the insertion and removal of lithium ion into the silicon material during charge and discharge cycle. The volume of Si-based negative active materials may expand from about 300％to 400％during charge and discharge cycle. As a result of such expansion and contraction, mechanical degradation of silicon materials and electrical isolation of sections may be caused and the electrodes may have a short cycle life.

Binder plays an important role in overcoming the volume expansion issue and ensuring properties of electrodes in a lithium ion battery including mechanical and electrochemical performances.

US2014/0356710 discloses an electrode for rechargeable Lithium ion battery including a binder comprising an acryl-based compound including a repeating unit of – [CH₂-CH (COOR) ] –wherein R may be an alkali metal in an amount of 90-99.5 mol％and a repeating unit of – [CH₂-CH (COOH) ] –in an amount of 0.5-10 mol％. The binder may further include at least one selected from polyvinyl alcohol, carboxyl methylcellulose, polyethylene, polyurethane, polyvinyl alcohol and the like.

EP2816642 A1 discloses an electrode for rechargeable Lithium ion battery including the organic compound (B) as a binder that is hardly soluble in a nonaqueous electrolytic solution, has a π–conjugated structure, and has an electric conductivity at 25 ℃ of 0.1S/cm or less. More preferred examples of the compound (B) include polystyrenesulfonic acid, lithium polystyrenesulfonate, sodium polystyrenesulfonate, styrene-lithium styrenesulfonate copolymer and styrene-sodium styrenesulfonate copolymer.

EP2830141 A1 discloses an electrode for rechargeable Lithium ion battery including a binder selected from polyethylene (PE) , polypropylene (PP) , polyethylene terephthalate (PET) , polyethernitrile (PEN) , polyimide (PI) , polyamide (PA) , polytetrafluoroethylene (PTFE) , styrene-butadiene rubber (SBR) , polyacrylonitrile (PAN) , polymethyl acrylate (PMA) , polymethyl methacrylate (PMMA) , polyvinyl chloride (PVC) , polyvinylidene fluoride (PVDF) , polyvinyl alcohol (PVA) , polyacrylic acid (PAA) , lithium polyacrylate (PAALi) , polyalkylene oxide such as a ring-opened polymer of ethylene oxide and a mono-substituted epoxide, and mixtures thereof, as well as alkali metal salts thereof.

EP1244157A discloses a binder composition for rechargeable Lithium ion battery, and the binder is a polymer of an ester of acrylic acid or methacrylic acid with ethylene glycol or polyethylene glycol.

Conventionally used binders, including the above disclosed ones are basically in a chain-like form, which is roughly shown in Figure 1, rather than a network form. Bonding strength of these binders is not high enough to against the breakdown of binder chains associated with the volume expansion of silicon particles during charge and discharge cycle. Meanwhile, currently existed binders have poor wetability to electrolyte.

Summary of the invention

In view of the fore going, after intensive studies on electrodes, especially binders, the inventors found that by using a polyethylene glycol-polyacrylic acid (Li) hybrid (hereinafter referred to as PEG-PAA (Li) hybrid) as the binder of an electrode, the electrode has a high electrode capacity, a good capacity retention with high cycle numbers and a high coulombic efficiency.

Therefore, an object of the present invention is to provide an electrode for rechargeable Lithium ion battery comprising a PEG-PAA (Li) hybrid binder.

Another object of the invention is to provide a rechargeable Lithium ion battery including the above electrode.

Still another object of the invention is use of the PEG-PAA (Li) hybrid as a binder in an electrode for a rechargeable Lithium ion battery.

Brief introduction of the drawings

Figure 1 roughly shows a chain-like structure of the conventionally used binder.

Figure 2 roughly shows a network structure of the binder according to the present invention.

Figure 3 roughly shows a bonding status in an electrode according to the present invention.

Detailed description of the invention

The present invention will be described in details as followings. The materials, methods, and examples herein are illustrative only and, except as specifically stated, are not intended to be limiting. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described herein.

All publications and other references mentioned herein are explicitly incorporated by reference in their entirety.

Unless otherwise defined, all technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the art. In case of conflict, the present specification, including definitions, will control.

Unless stated otherwise, all percentages, parts, ratios, etc., are by weight.

Where a range of numerical values are recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range.

Use of “a” or “an” is employed to describe elements and components of the present invention. This is done merely for convenience and to give a general sense of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise.

Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, or defining ingredient parameters used herein are to be understood as modified in all instances by the term “about” .

The term “normal pressure” used herein means about 0.1 MPa. The term “room temperature” used herein means about 25 ℃.

As mentioned above, one aspect of the invention is to provide an electrode for rechargeable Lithium ion battery comprising a PEG-PAA (Li) hybrid binder.

In the term “PEG-PAA (Li) hybrid” , PEG means polyethylene glycol, and the PEG used in the invention may be prepared from monomer ethylene glycol by using any known polymerization technique.

In some embodiments of the present invention, the PEG used in the PEG-PAA (Li) hybrid binder of the invention has a weight average molecular weight in a range of from about 50 to about 400 kDa, preferably from about 100 to about 300 kDa.

In the term “PEG-PAA (Li) hybrid” , PAA means polyacrylic acid, and the PAA used in the invention may be prepared from monomer acrylic acid by using any known polymerization technique. PAA (Li) means that part of or all of hydrogen atoms in the carboxyl group of PAA may be replaced by Li and a Li salt of PAA may be formed.

In some embodiments of the present invention, the PAA used in the PEG-PAA (Li) hybrid binder of the invention has a weight average molecular weight in a range of from about 200 to about 500 kDa, preferably from about 250 to about 450 kDa.

Particularly, the PAA (Li) has a repeating unit represented by the following Formula 1:

wherein R independently is hydrogen or Li.

In some embodiments of the present invention, the PAA (Li) may include about 70 mol％to 100 mol％, preferably about 80 mol％to about 95 mol％of repeating unit of Formula 1 with R being Li and about 0 mol％to about 30 mol％, preferably about 5 mol％to about 20 mol％of repeating unit of Formula 1 with R being hydrogen.

The preparation method of the PAA (Li) used in the present invention is not particularly limited as long as the above requirements are satisfied. In an embodiment, the PAA (Li) may be prepared by adding lithium hydroxide to polyacrylic acid aqueous solution under stirring at room temperature. In another embodiment, the PAA (Li) may be prepared by co-polymerizing monomer acrylic acid and monomer lithium salt of acrylic acid under suitable condition known in the art.

In a preferred embodiment of the invention, the molar ratio between PEG and PAA (Li) in the PEG-PAA (Li) hybrid binder of the invention is in a range of from 1: 1.5 to 1.5: 1, preferably about 1: 1.

It is believed that by a external action applied to the mixture of PEG and PAA (Li) , such as stirring or milling, hydrogen bonds may be formed between the PEG and PAA (Li) , and thus, the PEG-PAA (Li) hybrid according to the present invention has a structure represented by the following Formula 2:

A network structure of the binder according to the present invention is roughly shown in Figure 2. A dimensional structure of the binder together with an electrode active material and a conductive material is roughly shown in Figure 3.

In an embodiment of the present invention, the PEG-PAA (Li) hybrid may be formed prior to the preparation of electrode. For example, the PEG-PAA (Li) hybrid binder may be prepared by the following process:

-dissolving PEG in water to prepare a solution A；

-dissolving PAA in water and adding LiOH thereto to obtain a PAA (Li) aqueous solution B；

-dropwise adding the solution A to the solution B slowly under vigorous stirring to form a homogeneous PEG-PAA (Li) hybrid at room temperature.

In the above process, the water used may be any kind water such as de-ionized water. The concentration of PEG in the solution A may be 0.02-0.15 g/ml, preferably 0.08-0.12 g/ml. The concentration of PAA (Li) in the solution B may be 0.03-0.12 g/ml, preferably 0.05-0.10 g/ml.

Dissolving steps of PEG and PAA (Li) may be conducted under stirring in order to ensure a complete dissolve. The stirring may be conducted by any known means in the art, for example, magnetic stirring, manual stirring and the like. Stirring speed is not particularly limited and may be adjusted practically as long as the PEG and PAA (Li) are completely dissolved.

In the step of adding the solution A to the solution B, vigorous stirring means that the stirring may be conducted at a speed of 300-1000 rpm, preferably 500-800 rpm.

The viscosity of the hybrid should be controlled in a range of from 1.0 x 10³ to 2.0 x 10⁴ mPa·S, preferably from 5.0 x 10³ to 1.5 x 10⁴ mPa·S. The viscosity may be measured by, for example, Brookfield viscometer. If the viscosity is too high, a uniform mixture of the electrode can not be formed and the electrode active materials can not be dispersed in the binder homogeneously. If the viscosity is too low, precipitate may be formed during preparation of electrode and a good binding strength for the electrode active material can not be achieved.

The final viscosity of the hybrid may be controlled by the concentration of PEG and/or the concentration of PAA (Li) , and may also controlled by adding or removing water from the obtained hybrid.

In an embodiment of the present invention, an electrode may prepared by mixing the PEG-PAA (Li) hybrid binder, an electrode active material and a conductive material to form a composition, coating the composition on a current collector such as copper foil or aluminum foil, and drying the coated current collector at an elevated temperature, for example, 50-150℃, preferably 80-150 ℃ for about 1-5 h, preferably 1-3h. In another embodiment of the present invention, the PEG-PAA (Li) hybrid may be formed during the preparation of electrode. For example, an electrode may be prepared by the following process:

-dissolving PAA in water and adding LiOH thereto to obtain a PAA (Li) aqueous solution；

-adding an electrode active material and an conductive material to the above aqueous solution to form a slurry；

-adding PEG solution to the slurry and milling to form a homogenously mixture, therein a PEG-PAA (Li) hybrid binder is formed； and

-coating the mixture onto a current collector and drying the coated current collector at an elevated temperature, so as to produce an electrode.

In the above process, stirring may be used if necessary and may be adjusted practically as long as a homogeneous mixture may be formed. The milling may be conducted by, for example, ball-milling or other milling devices known in the art, such as ball-milling at 100-300rpm at room temperature.

In some embodiments of the present invention, the drying may be conducted stepwise from low temperature to high temperature, for example, firstly drying under 50-80 ℃, preferably 50-70 ℃ for about 5-30min, preferably 10-20min； then drying under 80-100 ℃, preferably 50-70 ℃ for about 5-30min, preferably 10-20min； and followed by drying under 100-150 ℃, preferably 100-130 ℃ for about 30min-5h, preferably 1-3h.

In some embodiments of the present invention, the drying may be conducted in one step, for example under 100-150 ℃, preferably 100-130 ℃ for about 1-5h, preferably 1-3h.

Typically, an electrode for rechargeable Lithium ion battery includes a current collector, an electrode active material, a binder and a conductive material, each of these components will be discussed as below.

Current collector

Current collectors conventionally used in the art can be used in the present invention and there is no particular limitation.

As for a negative electrode, examples of the current collector include a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, a polymer substrate coated with the above conductive material, and combinations thereof.

As for a positive electrode, examples of the current collector include a stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel that is surfaced-treated with carbon, nickel, titanium, silver, and the like.

The shape of the collector is generally a sheet-like one, but also may be those produced by roughing the surface of the sheet-like ones, as well as nets or perforated metals, and the like.

Electrode active material

As for negative active material suitable for the present invention, it may be Si, SiO_x (0<x<2) , a Si-C composite, Si alloy or a combination thereof. In some embodiments of the present invention, Si alloy includes an alloy of Si with an alkali metal, an alkaline earth metal, Group 13 to 16 elements, a transition element, a rare earth element, or a combination thereof except for Si. Specifically, Si alloy includes an alloy of Si with Mg, Ca, Sr, Ba, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Tr, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, P, As, Sb, Bi or a combination thereof, preferably with Ti, Fe, Cu, Al or a combination thereof.

In another embodiment of the present invention, the negative active material may include nanosilicon or consist of nanosilicon.

Preferably, the negative active material may be contained in the electrode of the present invention excluding the current collector in an amount of 90wt％to 96wt％more preferably 92wt％to 96wt％.

As for positive active material suitable for the present invention, it may be LiCoO₂, LiMnO₂, LiNi_1-xCo_xO₂ (0<x<1) , Li_1.2Ni_1-x-yCo_xMn_y O₂, LiNi_1-x-yCo_xMn_yO₂, LiFePO₄, LiNi_0.8Co_0.15Al_0.05O₂ and the like.

Preferably, the positive active material may be contained in the electrode of the present invention excluding the current collector in an amount of 90wt％to 98wt％more preferably 94wt％to 96wt％.

Binder

In some embodiments of the present invention, the above defined binder may be used alone in the electrodes.

In some embodiments of the present invention, the above defined binder may be used together with one or more binders conventionally used in the art, for example, polyvinyl alcohol, carboxyl methylcellulose, hydroxypropylcellulose, diacetylcellulose, polyvinylchloride, carboxylated polyvinylchloride, polyvinylfluoride, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, a styrene-butadiene rubber, an acrylated styrene-budadiene rubber, an epoxy resin, and nylon.

Preferably, the binder may be contained in the electrode of the present invention excluding the current collector in an amount of 1wt％to 10wt％more preferably 1wt％to 5wt％.

Conductive material

In some embodiments of the present invention, the electrode may contain a conductive material to improve electrical conductivity of the electrode. The conductive materials usable in the present invention are not particularly limited, and materials known and conventionally used in the art can be used in the electrode of the present invention as long as they do not cause any chemical change.

Examples of the conductive material usable in the present invention include one or more selected from a carbon-based material of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, a carbon fiber, and the like； a metal-based material such as metal powder or a metal fiber including copper, nickel, aluminum, silver, and the like； a conductive polymer such as a polyphenylene derivative； or a mixture thereof.

Preferably, conductive material may be contained in the electrode of the present invention excluding the current collector in an amount of from 0 wt％to about 15 wt％, preferably from 0.5 wt％to 10 wt％.

In a preferred embodiment of the present invention, the electrode is a negative electrode for rechargeable Lithium ion battery.

Another aspect of the invention is to provide a rechargeable lithium ion battery including the above electrode.

Still another aspect of the invention is use of the PEG-PAA (Li) hybrid according to the present invention as a binder in an electrode for a rechargeable lithium ion battery.

Examples

The present invention will be further described and illustrated in details with reference to the following examples, which, however, are not intended to restrict the scope of the present invention.

1. Preparation of binder B-1 according to the present invention

6 g of PEG (Mw＝200KDa) was dissolved in 94 ml deionized water under stirring at room temperature to form solution A.

7.2 g of PAA (Mw＝450KDa) was dissolved in 70 ml deionized water at room temperature under stirring, then 4.2 g of LiOH was added to the solution under stirring, after that, stirring was continued for about 15 hours at room temperature to give a Li salt of PAA having a degree of salt formation of about 9.6wt％which was marked as solution B.

The solution A was dropwise added into the solution B under vigorous stirring with a speed of 300 rpm for 60 minutes to form a PEG-PAA (Li) hybrid, which was marked as binder B-1.

2. Preparation of negative electrode E-1 according to the present invention

40 g of Si-Al-Fe alloy (trade name: L-20772 CV7, from 3M) , 40 g of graphite, 10g of binder B-1, 2 g of carbon black and 8g of KS6 were mixed in 100 ml of water to form a negative electrode active slurry. The slurry was coated on a copper foil and then dried at 50 ℃ for 10 min to evaporate water, followed by compression, so as to prepare a negative electrode E-1 having a thickness of 30 μm and a diameter of 12 mm in disk shape.

3. Preparation of negative electrode E-2 according to the present invention

7.2 g of PAA (Mw＝450kDa) were dissolved in 70 ml of deionized water under stirring at room temperature. Then, 4.2 g of LiOH was added thereto to adjust the pH of the solution to about 7 and form a Li salt of PAA having a degree of salt formation of about 9.6wt％. Thereafter, the obtained solution of PAA (Li) , 40 g of Si-Al-Fe alloy (L-20772 CV7, from 3M) , 40 g of graphite, 2 g of carbon black and 8g of KS6 and 100 ml of water were mixed together to form a slurry A.

6 g of PEG (Mw＝200kDa) was dissolved in 94 ml of deionized water under stirring at room temperature to form a PEG solution.

The above obtained PEG solution was added to the slurry A under stirring, and then the obtained mixture was charged into a ball mill to conduct milling at 200 rpm for 60 minutes so as to form a slurry B, therein a PEG-PAA (Li) hybrid was formed.

The slurry B was coated on a copper foil and then dried at 110 ℃ for 30 minto evaporate water, followed by compression, so as to prepare a negative electrode E-2 having a thickness of 30 μm and a diameter of 12 mm in disk shape.

3. Preparation of comparative negative electrode CE-1

The comparative negative electrode CE-1 was prepared in a substantially same way as that of negative electrode E-1 except that 10 g of PAA (Li) was used.

4. Preparation of comparative negative electrode CE-2

The comparative negative electrode CE-2 was prepared in a substantially same way as that of negative electrode E-1 except that 10 g of PAA (Li) and PEG (in a molar ratio of 1: 1, no hybrid was formed) was used.

5. Preparation of rechargeable lithium ion battery cell according to the present invention

A rechargeable lithium ion battery cell was prepared using the above negative electrode, a lithium metal as positive electrode, a polypropylene separator, and an electrolyte solution comprising 1M LiPF₆, ethylene carbonate (EC) : ethyl methy carbonate (EMC) : dimethyl carbonate (DMC) in a weight ratio of 3: 5: 2, 2wt％vinyl carbonate and 5wt％fluoro ethylene carbonate.

Examples 1-4

Rechargeable lithium ion battery cell C-1, C-2, C-3 (comparative) and C-4 (comparative) were fabricated respectively using the negative electrode E-1, E-2, CE-1 and CE-2. Discharge capacity, capacity retention and coulombic efficiency were evaluated by the following methods and the results were shown in Table 1.

Discharge capacity

Rechargeable lithium ion battery cells C-1, C-2, C-3 (comparative cell) and C-4 (comparative cell) were charged and discharged at 0.1C within a voltage range of 1.5V to 0.01V, and discharge capacity of these cells are measured. Results are shown in Table 1.

Capacity retention

A capacity ratio of capacity at 50^th cycle relative to capacity at 1^st cycle under a condition of 1C was measured with regard to rechargeable lithium ion battery cells C-1, C-2, C-3 (comparative cell) and C-4 (comparative cell) , and results are shown in Table 1.

Coulombic efficiency

Rechargeable lithium ion battery cells C-1, C-2, C-3 (comparative cell) and C-4 (comparative cell) were charged and discharged at 0.1C, and then, charge capacity and discharge capacity were measured. Coulombic efficiency, i.e., the ratio of the discharge capacity relative to the charge capacity, was calculated, and results are shown in Table 1.

Table 1

It can be seem from Table 1 that the rechargeable lithium ion battery cells C-1 and C-2 (according to the present invention) show excellent discharge capacity, retention capacity and coulombic efficiency.

The present invention is illustrated in details in the embodiments； however, it is apparent for those skilled in the art to modify and change the embodiments without deviating from the spirit of the invention. All the modifications and changes should fall in the scope of the appended claims of the present application.

Claims

An electrode for rechargeable Lithium ion battery comprising a PEG-PAA (Li) hybrid binder.
The electrode according to claim 1, wherein the PEG has a weight average molecular weight in a range of from 50 to 400 kDa, preferably from 100 to 300 kDa.
The electrode according to claim 1 or 2, wherein the PAA has a weight average molecular weight in a range of from 200 to 500 kDa, preferably from 250 to 450 kDa, and the PAA (Li) has a repeating unit represented by Formula 1:

wherein R independently is hydrogen or Li.
The electrode according to claim 3, wherein the PAA (Li) includes 70 mol％to 100 mol％, preferably 80 mol％to 95 mol％of repeating unit of Formula 1 with R being Li and 0 mol％to 30 mol％, preferably 5 mol％to 20 mol％of repeating unit of Formula 1 with R being hydrogen.
The electrode according to any one of claims 1 to 4, wherein the molar ratio between PEG and PAA (Li) is in a range of from 1: 1.5 to 1.5: 1, preferably about 1: 1.
The electrode according to any one of claims 1 to 5, wherein the PEG-PAA (Li) hybrid is prepared by the following process:

-dissolving PEG in water to prepare a solution A；

-dissolving PAA in water and adding LiOH thereto to obtain a PAA (Li) aqueous solution B；

-dropwise adding the solution A to the solution B slowly under vigorous stirring to form a homogeneous PEG-PAA (Li) hybrid at room temperature.
The electrode according to any one of claims 1 to 6, wherein the electrode comprises or consists of a current collector, a Si-based negative active material, the PEG-PAA (Li) hybrid binder and a conductive material.
The electrode according to claim 7, wherein the Si-based negative active material is selected from Si, SiO_x (0<x<2) , a Si-C composite, Si alloy or a combination thereof.
An electrode, prepared by the following process:

-dissolving PAA in water and adding LiOH thereto to obtain a PAA (Li) aqueous solution；

-adding an electrode active material and an conductive material to the above aqueous solution to form a slurry；

-adding PEG solution to the slurry and milling to form a homogenously mixture, therein a PEG-PAA (Li) hybrid binder is formed； and

-coating the mixture onto a current collector and drying the coated current collector at an elevated temperature, so as to produce an electrode.
A rechargeable lithium ion battery including the electrode according to any one of claim 1 to 9.
Use of the PEG-PAA (Li) hybrid as defined in any one of claims 1 to 6 as a binder in an electrode for a rechargeable lithium ion battery.