In lithium-ion batteries, the binder is one of the important factors affecting the stability of the electrode structure. According to the nature of the dispersing medium, lithium-ion battery binder can be divided into oil-based binder with organic solvent as dispersant and water-based binder with water as dispersant. Liu Xin et al [3] reviewed the research progress of binder for high-capacity negative electrode. Thinking about the application of polyvinylidene fluoride (PVDF)-modified binders and water-based binders, Can improve the performance of high-capacity negative electrode electrochemistry. However, there is no discussion or comparison for binders for silicon-based negative electrodes.

In this paper, the authors provide an overview of the research progress on binders for silicon-based anode materials and compare the advantages and disadvantages of different types of binders.

1.     Oil-based binder

Among the oil-based binders, homopolymers and copolymers of PVDF are the most widely used.

1.1  PVDF Homopolymer Binder

In the large-scale production of lithium-ion batteries, PVDF is commonly used as a binder, and organic solvents such as N-methyl pyrrolidone (NMP) are used as dispersants. PVDF has good viscosity and electrochemical stability, but poor electronic and ionic conductivity, Organic solvents are volatile, flammable, explosive and highly toxic; Moreover, PVDF is only bonded to the Si-based anode material by weak van der Waals forces and cannot accommodate the dramatic volume change of Si. Conventional type PVDF is not suitable for silicon-based anode materials [3 -5].

1.2  PVDF modified binder

In order to get the improved electrochemical performance of PVDF applied to silicon-based anode materials, Some scholars have proposed modification methods such as copolymerisation and heat treatment [4-5]. Z. H. Chen and other scholars [4] found that: The terpolymer polyvinylidene fluoride-tetrafluoroethylene-ethylene copolymer [P(VDF- TFE-P)] enhances the mechanical properties and viscoelasticity of PVDF. J. Li and other scholars [5] found that. Heat treatment at 300°C and under argon protection improves the dispersion and viscoelasticity of PVDF. The modified PVDF/Si electrode was cycled 50 times at 150 mA/g at 0.17 ~ 0_ 90 V with a specific capacity of 600 mAh/g. By modifying and treating the PVDF/Si electrode, the cycling performance was improved, but the cycling stability was still unsatisfactory.

2.     Water-based binder

Compared to oil-based binders, water-based binders are environmentally friendly, inexpensive and safer to use, and are gradually gaining popularity. Currently, the more researched silicon-based anode material binders are water-based binders such as sodium carboxymethyl cellulose (CMC) and polyacrylic acid (PAA).

2.1 Styrene-Butadiene Rubber (SBR)/Sodium Carboxymethyl Cellulose Abatement (CMC) Binder

SBR/CMC has good viscoelasticity and dispersibility, and has been widely used in the large-scale production of graphite-based negative electrodes. W. R Liu and other scholars [6] found that：(SBR/CMC)/ Si electrodes can be charged and discharged 60 times at 1000 mAh/g constant capacity (0 ~ 1.2 V)， Electrochemical performance better than PVDF/Si electrode，however, 60 cycles is not an adequate indication of cyclic stability.

2.2 CMC Binder

Compared to the more viscoelastic SBR/CMC and polyethylene acrylic acid (PEAA)/CMC. Some people think: CMC binders that lack elasticity are more suitable for silicon-based anode materials [7-8]. J. Li and other scholars [7] found that: CMC/Si electrodes were cycled 70 times at 150 mA/g at 0.17 ~0.90 V, specific capacity of 1100 mAh/g, superior to (SBR/ CMC)/Si and PVDF/Si electrodes. B. Lestriez and other scholars [8] found that: The electrochemical performance of the CMC/Si electrode is superior to that of the (PEAA/CMC)/Si electrode, the reason is that PEAA tends to agglomerate carbon black, which affects the cycling stability of the electrode. Through chemical bonding (covalent or a-bonding [12-13]) the carboxymethyl group of CMC can be attached to Si, Because of the strong bonding force, the connection between the Si particles can be maintained; And CMC can form a solid electrolyte phase interface film (SEI)-like coating on the surface of Si, which inhibits the decomposition of the electrolyte.

Although the electrode exhibits good electrochemical properties when CMC is used as a binder, however, the degree of substitution (DS) of CMC and the electrode ratio, pH value, etc., will affect the electrochemical performance of the CMC/Si electrode to different degrees. J. S. Bridel and others [12-14] found that: When m(Si):m(C):<n(CMC) = 1:1:1, only 48% expansion of the pole piece when fully lithium embedded, the electrode has the best cycling performance, but at this time the Si content is low and the energy density of the battery is low. M. Gauthier and other scholars [9, 11] compared the performance of CMC/Si electrodes prepared at different pH values, the best performance of the electrodes was found to be prepared in pH = 3 buffer solution, where the CMC/micron Si electrode was cycled 600 times at [3] 005 ~ 1000 V at 480 mA/g, Specific capacity of 1 600 mAh/g [91]. In addition, an appropriate increase in DS is conducive to improving the electrochemical performance of CMC/Si electrodes, CMC/Si electrodes with DS < 1.2 have better cycling performance [10-12].

CMC binder has a good application prospect, but CMC is generally sticky, brittle, and not very pliable, the pole piece is prone to cracking during charging and discharging [13].Moreover, CMC is strongly influenced by conditions such as electrode ratio and pH value，further studies are needed.

2.3  PAA Binder

PAA has a simple molecular structure, is easy to synthesise and is soluble in water and some organic solvents. Some studies have shown that PAA with higher carboxyl group content is more suitable than CMC for 15% of silicon-based anode materials. Magasinski and other scholars [15] found that: PAA can not only form strong hydrogen bonding interactions with Si, but also form a more homogeneous cladding on the surface of Si than that of CMC,PAA/Si electrodes were cycled 100 times at 0.01 ~ 1.00 V with C/2, specific capacity of 2400 mAh/g. S. Komaba and other scholars [16] found that: PAA is more evenly distributed in the pole piece, it can form SEI-like coating on Si surface and inhibit electrolyte decomposition, PAA outperforms CMC, polyvinyl alcohol (PVA) and PVDF.

Scholars such as M. Hasegawa [17-18] have argued that.: PAA containing a large number of carboxyl groups has good adhesion, but the hydrophilicity of the carboxyl groups is strong, easily reacts with residual moisture in the battery and affects performance. If hydroxyl groups or moisture are still present after drying the electrode, will react with LiPF6 in the electrolyte to decompose PF5 (>601C),decompose the organic solvent and affect the charging and discharging performance of the electrode. If PAA was vacuum heat-treated at 150-200 t for 4-12 h, the carboxyl group of PAA was partially condensed, it not only reduces the hydrophilicity of the electrode, but also enhances the structural stability of the electrode. B. Koo et al. scholars heat-treated CMC and PAA for 2h at 150 t, the resulting CMC-PAA/Si electrode was cycled 100 times at 0.005-2.000 V at 1.5 A/g, the specific capacity is 1500mAh/g.

2.4  Sodium alginate binder

The structure of sodium alginate is similar to that of CMC and the carboxyl groups are more regularly arranged. Sodium alginate was used as a binder for silicon-based anode materials by I. Kovalenko and other scholars, the prepared sodium alginate/Si electrode was cycled 100 times at 0.01~1.00 V at 4.2 A/g with a specific capacity of 1700 mAh/g, superior to CMC/Si and PVDF/Si electrodes. Currently, there are few reports on sodium alginate, and similar to PAA, sodium alginate has a high content of carboxyl groups and suffers from high hydrophilicity.

2.5  Conductive Polymer Binders

Conductive polymer binder with both adhesive and conductive properties to improve conductivity while maintaining structural stability of the pole piece. G. Liu and other scholars used poly(9.9-dioctylfluorene-co-fluorenone-co-methylbenzoic acid) (PFFOMB ) for silicon-based anode materials,t he prepared PFF0MB/Si electrode was cycled with C/10 at 0.01~1.00 V for 650 times, and the specific capacity was 2100 mAh/g. The polyaniline (PAni)/Si electrode synthesised and prepared in situ by H. Wu and other scholars was cycled at 0.01-1.00 V for 5000 cycles at 6.0 A/g, and still had a specific capacity of 550 mAh/g.

2.6  Other binders

In addition to the above binders, carboxymethyl chitosan, polyacrylonitrile (PAN) and PVA can also be used in silicon-based anode materials. 500 mA/g completed methyl chitosan/Si electrode cycled 50 times at 0.12~1.00 V with a specific capacity of 950 mAh/g[s]. The specific capacity of PAN/Si electrode and PVA/Si electrode were maintained at 600mAh/g124-251 after 50 cycles at 0.005~3.000V with C/2.Although all of the above binders can form strong hydrogen bonds with Si and have good cyclic stability, but the cyclic stability was slightly lower than that of binders such as CMC, PAA and sodium alginate.

3.     Conclusion

Binder development and application is one of the effective ways to improve the cycling stability of silicon-based anode materials for lithium-ion batteries. The application of PVDF-modified binder or water-based binder can improve the cycling stability and electrochemical performance of silicon-based anode to a certain extent. Different types of binders have their own advantages and disadvantages, Comparatively, PAA, sodium alginate and conductive polymer binders showed better cycling stability and electrochemical performance when applied to silicon-based anode materials.

The development of aqueous binders capable of forming a stronger chemical bond connection with Si and a more homogeneous coating is an important development direction for the binders of Si-based anode materials. In addition, conductive polymer binders, which are both adhesive and electrically conductive, also have promising applications.

(Source: Research Institute of Tsinghua University in Shenzhen、Shenzhen Lithium Battery Active Electrode Materials Engineering Laboratory）