WO2018227366A1

WO2018227366A1 - Composite binder for lithium ion batteries and its preparation

Info

Publication number: WO2018227366A1
Application number: PCT/CN2017/088035
Authority: WO
Inventors: Jun Yang; Yitian BIE; Jingjing Zhang; Lei Wang
Original assignee: Robert Bosch Gmbh
Priority date: 2017-06-13
Filing date: 2017-06-13
Publication date: 2018-12-20
Also published as: KR20200018449A; JP6952806B2; JP2020524725A; CN110770950A; KR102392241B1; CN110770950B

Abstract

The present invention relates to a composite binder; an electrode material and a silicon-based lithium-ion battery comprising said composite binder, and a process for preparing said composite binder.

Description

Composite Binder for Lithium Ion Batteries and its Preparation

Technical field

The present invention relates to lithium ion batteries and specifically relates to the development and improvement of binder network in silicon-based anode for lithium ion batteries.

Background Art

With the rapid development and popularization of portable electronic devices and electronic vehicles, the demand for lithium ion batteries with increased energy and powder density becomes more and more urgent. Silicon is a promising candidate electrode material for lithium ion batteries owning to its high theoretical specific capacity of 4200 mAh/g for Li_4.4Si.

However, the cycling performance of Si-based electrode material is still not satisfied for industrial application. One of the biggest challenges is binder failure due to repeated volume change of silicon, for example, during lithiation/delithiation process, silicon undergoes dramatic expansion and contraction, which would cause many cracks in both Si-based active materials and electrode. The binder network is known as playing key role in maintaining the electrode integrity during volume change in the electrode and achieving good cycling performance.

Among all kinds of binders, binders comprising carboxyl groups, such as polyacrylic acid (PAA) and carboxymethyl cellulose (CMC) based polymers show better electrochemical performance than non-functional polymers such as PVDF and styrene-butadiene rubber and are thus frequently used. These functional binders show enhanced binding with Si particles via hydrogen bonding and/or covalent chemical bonds between the polar functional groups of the binder and the partially hydrolyzed surface layer of Si particles.

Nevertheless, the bonding formed by carboxyl groups is still not strong enough to endure the extent volume change of silicon. To combat this, three dimensional polymer networks including cross-linked CMC-PAA binder were sequentially developed for Si electrode material, in which the polymer chain was anchored by a cross-linking technique. These designs effectively enhanced the electrochemical performance of Si electrode material by suppressing the adverse effect from their large volume expansion.

Summary of Invention

It is therefore an object of the present invention to provide further modification and improvements to the binder used in silicon-based anode for lithium ion batteries. According to the present invention, a composite binder which is deformable has been proposed for Si-based electrode material.

Specifically, a composite binder comprising crosslinking product of:

a) a binder comprising carboxyl groups； and

b) α-, β-, or γ-cyclodextrin and/or modified α-, β-, or γ-cyclodextrin with a better water solubility than the non-modified corresponding cyclodextrin, is proposed.

Accordingly, the present invention provides a deformable composite binder with three dimensional binding network and enhanced interaction between the binder and silicon-based material for lithium ion batteries.

The present invention further provides an electrode material, which comprises the composite binder according to the present invention.

The present invention further provides a silicon-based lithium ion battery, which comprises the composite binder according to the present invention.

The present invention also relates to a process for preparing the above composite binder, comprising the steps of:

(1) respectively, preparing an aqueous solution of a binder comprising carboxyl groups and an aqueous solution of α-, β-, or γ-cyclodextrin and/or modified α-, β-, or γ-cyclodextrin；

(2) mixing the aqueous solution prepared above under stirring；

(3) drying and dewatering the mixed solution under vacuum；

(4) carrying out in-situ crosslinking reaction.

Brief Description of Drawings

Figure 1 is a schematic illustration of the three dimensional binding network and the corresponding structural formula when cyclodextrin or β-cyclodextrin is added to the PAA.

Figure 2 is a schematic illustration showing the carbonyl modification of hydrogen peroxide to β-cyclodextrin.

Figure 3 is an infrared spectrum of the crosslinking product prepared in Example 1.

Figure 4 is a plot showing the cycling performance of the Si electrodes prepared in Examples 1 to 4.

Figure 5 is a plot showing the cycling performance comparison of the Si electrodes prepared in Example 3 and in Comparative Example 1.

Figure 6 is a plot showing the different C-rate of the Si electrodes prepared in Example 3 and in Comparative Example 1.

Figure 7 is a plot showing the cycling performance of the Si electrode prepared in Example 3 at 0.5C.

Figure 8 is a plot showing the cycling performance of the Si electrode prepared in Example 3 at 3C.

Detailed Description of Preferred Embodiments

All publications, patent applications, patents and other references mentioned herein, if not otherwise indicated, are explicitly incorporated by reference herein in their entirety for all purposes as if fully set forth.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification, including definitions, will control.

When an amount, concentration, or other value or parameter is given as either a range, preferred range or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range.

According to the present invention, the present inventor surprisingly found that when α-, β-, or γ-cyclodextrin and/or modified α-, β-, or γ-cyclodextrin such as carbonyl modified β-cyclodextrin is introduced into an binder comprising carboxyl groups, an excellent cycling stability and high coulombic efficiency can be achieved.

Cyclodextrins are a family of compounds made up of sugar molecules bound together in a ring. Typical cyclodextrins are constituted by 6-8 glucopyranoside units, can be topologically represented as toroids with the larger and smaller openings of the toroid exposing to the solvent secondary and primary hydroxyl groups respectively.

In the present context, typical α-, β-, or γ-cyclodextrins contain a number of glucose monomers ranging from six to eight units in a ring, creating a cone shape:

α-cyclodextrin: 6-member sugar ring molecule

β-cyclodextrin: 7-member sugar ring molecule

γ-cyclodextrin: 8-member sugar ring molecule

The formula of α-, β-and γ-cyclodextrin can be referred to as below:

The cone shape can be clearly shown from the below formula:

In the present context, abbreviation CD refers to both α-, β-, or γ-cycllodextrin and/or modified α-, β-, or γ-cyclodextrin. It is advantageous for α-, β-, or γ-cyclodextrin to be modified so as to have a better water solubility than the non-modified α-, β-, or γ-cyclodextrin. In this aspect, for example, CD can be modified by carbonyl groups, amine groups, and the combination thereof. In a preferable embodiment, a carbonyl modified cyclodextrin is used. In a more preferable embodiment, a carbonyl modified β-cyclodextrin is used, which will induce both carboxyl and carbonyl. A schematic illustration can be shown in Figure 2. After modification the ratio of carboxylation is around 15％ to 60％, more preferably from 25％to 45％.

Not bound by theory and as can be shown in Figure 1, it is believed that the improvement on cycling stability and high coulombic efficiency is due to the crosslinking of an binder comprising carboxyl groups and CD. The binder comprising carboxyl groups such as polyacrylic acid (briefed as PAA) are linked together by CD ring, and thus the binder has much improved toughness and mechanical strength due to the uniform 3D network. Besides, the binder can strongly bond with Si by carboxylic groups and hydroxyl functional groups, exhibiting high mechanic strength of adhesion on Si, as well as a particularly recoverable deformation through the reversible morphology change with Si particles. As a result, volume expansion of Si particle is buffered by CD ring, and isolation of Si particle is relieved after volume expansion.

In the present context, the binder which contains carboxyl groups can be any suitable binder as long as it carries carboxyl groups. The preferred binder is selected from the group consisting of polyacrylic acid, carboxymethyl cellulose (hereinafter briefed as “CMC” ) , sodium alginate (hereinafter briefed as “SA” ) , copolymers thereof and combinations thereof.

In a preferable embodiment, the inventive composite binder comprises crosslinking product of:

a) polyacrylic acid； and

b) carbonyl modified β-cyclodextrin；

wherein polyacrylic acid and carbonyl modified β-cyclodextrin are in a weight ratio of 1: 1 to 10: 1, more preferably from 1: 1 to 8: 1, still further preferably from 1: 1 to 6: 1.

In the present context, the process for preparing the inventive composite binder comprises:

(2) mixing the aqueous solution prepared above under stirring；

(3) drying and dewatering the mixed solution under vacuum；

(4) carrying out in-situ crosslinking reaction.

For silicon-graphite composite anode, capacity retention of 80％over 500 cycles using the inventive composite binder can be achieved. In addition, C-rate performance of silicon-graphite composite anode using different binders in lithium batteries is tested. It was proved that the C-rate performance of batteries using the inventive composite binder was better than those using PAA binder alone.

Another advantage of this invention is that the synthesis process is facile and easy to upscale.

Examples

The following non-limiting examples describe preparation of the electrode comprising the composite binder according to the present invention and compare the performance of the obtained electrodes with those prepared not according to the present invention. The following Examples illustrate various features and characteristics of the present invention, whose scope however is not to be construed as limited thereto:

Example 1

Preparation of the inventive composite binder A1

Synthesis of carbonyl modified β-cyclodextrin

Carbonyl modified β-cyclodextrin was obtained via a simple procedure. Specifically, 2 g β-cyclodextrin (Sinopharm chemical) was added to 2 g H₂O₂ aqueous solution with concentration of 30％, and kept at 80℃ for 24 h in a sealed bottle, ensuring that the β-cyclodextrin fully reacts with H₂O₂. The solution was then dried under vacuum to completely remove all the water and residual H₂O₂.

Synthesis of the crosslinking product of PAA and carbonyl modified β-cyclodextrin

An aqueous solution of PAA (Alfa Aesar, average Mw＝240, 000) and an aqueous solution of the carbonyl modified β-cyclodextrin obtained above were mixed with a weight ratio of 1: 1 under stirring. The above mixed solution was dried and dewatered under vacuum. An in-situ crosslinking reaction was carried out.

As shown in Figure 3, a -COO-ester group is proven to have been prepared. The crosslinking product of PAA and carbonyl modified β-cyclodextrin has a relatively higher intensity than CD but a relatively weaker intensity than PAA at 1650 cm^-1, which is the adsorption peak for -COO-group.

Example 2

Preparation of the inventive composite binder A2

Except that the weight ratio of PAA and carbonyl modified β-cyclodextrin was changed to 2: 1, the inventive composite binder A2 was prepared in the same manner as in Example 1.

Example 3

Preparation of the inventive composite binder A3

Except that the weight ratio of PAA and carbonyl modified β-cyclodextrin was changed to 4: 1, the inventive composite binder A3 was prepared in the same manner as in Example 1.

Example 4

Preparation of the inventive composite binder A4

Except that the weight ratio of PAA and carbonyl modified β-cyclodextrin was changed to 6: 1, the inventive composite binder A4 was prepared in the same manner as in Example 1.

Comparative Example 1

For comparison, PAA (Alfa Aesar, average Mw＝240, 000) in water was used as comparative binder C1.

Preparation of the electrode comprising the composite binder according to the present invention

The working electrodes were prepared by pasting a mixture of active material Si powder, graphite, Super P conductive (40 nm, Timical) , and the above prepared binder at a weight ratio of 35: 45: 7: 13. After coating the mixture onto Cu foil, the electrodes were dried, cut to Φ12 mm disks, pressed at 3 MPa and finally the silicon electrode was thermally treated at 70℃ for 5 h and then increased to 150℃ for another 4 h under vacuum.

Cell assembling and electrochemical test:

The electrochemical performance of the as-prepared composites was evaluated using two electrode coin-type cells.

The CR2016 coin cells were assembled in an argon-filled glove box (MB-10 compact, MBraun) using 1 M LiPF₆ in dimethyl carbonate (DMC) and ethylene carbonate (EC) mixed solvent of 1: 1 by volume, including 10 wt. ％fluoroethylene carbonate (FEC) as electrolyte, PE membrane (Celgard 2400) as separator, and lithium metal as counter electrode. The cycling performance was evaluated on a LAND battery test system (CT 2007 A, Wuhan Land Electronics, Ltd. ) at 25℃ with constant current densities. The cut-off voltage was 0.01 V versus Li⁺/Li for discharge (Li insertion) and 1.2 V versus Li⁺/Li for charge (Li extraction) . The specific capacity was calculated on the basis of the weight of Si-graphite composites. The mass loading of active materials (Si-graphite) in every electrode is about 2 mg/cm².

Figure 4 shows the cycling performance of the cells each including binders prepared in Examples 1 to 4. It can be seen that when the weight ratio of PAA to the carbonyl modified β-cyclodextrin is 4: 1 (Example 3) , the best cycling performance can be achieved. Cycling performance of cells each including binder with a weight ratio of 2: 1 (Example 2) , 6: 1 (Example 4) or 1: 1 (Example 1) were decreasing in order.

Figure 5 shows the cycling performance of the cells each including binders prepared in Example 3 and Comparative Example 1. It can be seen that the capacity of the cell comprising binder of Comparative Example 1 has unfavorably decreased 50％only after 20 circles. In contrast, the cell including binder of Example 3 can maintain its capacity to a high level even after 300 circles.

In addition, Figure 6 shows the C-rate performance of cells each including binders prepared in Example 3 and Comparative Example 1. It is apparent that the cell including binder of Example 3 has dramatically improved the C-rate and cycling performance compared to the cell including binder of Comparative Example 1.

Figures 7 and 8 respectively show the cycling performance of cells including binder of Example 3 at 0.5C and 3C. From the figures, it is clear that their cycling performance is satisfactory.

The present invention has greatly improved electrochemical performances, especially cycling performance via creating a unique binder that flexibly wraps the silicon particles.

Claims

A composite binder, comprising crosslinking product of:

a) a binder comprising carboxyl groups； and

b) α-, β-, or γ-cyclodextrin and/or modified α-, β-, or γ-cyclodextrin with a better water solubility than the non-modified corresponding cyclodextrin.
The composite binder according to claim 1, wherein the binder comprising carboxyl groups are selected from the group consisting of polyacrylic acid, carboxymethyl cellulose, sodium alginate, polysaccharide and combinations thereof.
The binder according to claim 1 or 2, wherein the modified cyclodextrin comprises carbonyl group modification, amine group modification and combinations thereof.
The silicon-based composite according to claim 1 or 2, comprising crosslinking product of:

a) a binder comprising carboxyl groups being polyacrylic acid； and

b) modified α-, β-, or γ-cyclodextrin with a better water solubility than the non-modified corresponding cyclodextrin being carbonyl modified β-cyclodextrin, wherein the polyacrylic acid and the carbonyl modified β-cyclodextrin are in a weight ratio of 1: 1 to 10: 1, more preferably from 1: 1 to 8: 1, still further preferably from 1: 1 to 6: 1.
An electrode material, comprising the composite binder of any one of claims 1 to 4.
A silicon-based lithium-ion battery, comprising the composite binder of any one of claims 1 to 4.
A process for preparing the composite binder of any one of claims 1 to 4, comprising the steps of:

(1) respectively, preparing an aqueous solution of a binder comprising carboxyl groups and an aqueous solution of α-, β-, or γ-cyclodextrin and/or modified α-, β-, or γ-cyclodextrin；

(2) mixing the aqueous solutions prepared above under stirring；

(3) drying and dewatering the mixed solution under vacuum；

(4) carrying out in-situ crosslinking reaction.