Silicon based anode materials have a high theoretical specific capacity and a suitable platform for lithium intercalation and desorption, which is an ideal high-capacity anode material for lithium-ion batteries. In lithium-ion batteries, the battery binders are one of the important factors affecting the stability of the electrode structure.
According to the nature of the dispersion medium, lithium ion battery binders can be divided into oily binders using organic solvents as dispersants and water-based binders using water as dispersants. This paper reviews the research progress of battery binders for silicon-based anode materials and compares the advantages and disadvantages of different types of battery binders.
1. Oily binders
Among the oily battery binders, PVDF homopolymers and copolymers are most widely used.
① PVDF homopolymer binders
In the large-scale production of lithium-ion batteries, PVDF is generally used as battery binders, and organic solvents such as N-methylpyrrolidone (NMP) are used as dispersants. PVDF has good viscosity and electrochemical stability, but poor electronic and ion conductivity, and organic solvents are volatile, flammable, explosive and highly toxic. Moreover, PVDF is only connected to silicon-based anode materials by weak van der Waals force, which cannot adapt to the drastic volume change of Si. Traditional PVDF battery binders are not suitable for silicon-based anode materials.
② PVDF modified binders
In order to improve the electrochemical performance of PVDF applied to silicon-based anode materials, some scholars have proposed modification methods such as copolymerization and heat treatment. The terpolymer polyvinylidene fluoride-tetrafluoroethylene-ethylene copolymer [P(VDF-TFE-P)] can enhance the mechanical properties and viscoelasticity of PVDF. Heat treatment at 300°C under argon protection can improve the dispersion and viscoelasticity of PVDF. The PVDF/Si electrode has been modified, although the cycle performance has been improved, but the cycle stability is still not ideal.
2. Water-based binders
Compared with oil-based battery binders, water-based battery binders are environmentally friendly, cheap and safer to use, and are gradually being promoted. At present, the battery binders of silicon-based anode materials that have been studied more are water-based binders such as sodium carboxymethylcellulose (CMC) and polyacrylic acid (PAA).
① SBR/CMC binders
SBR/CMC has good viscoelasticity and dispersion, and has been widely used in the large-scale production of graphite-based anodes. The (SBR/CMC)/Si electrode can be charged and discharged 60 times at a constant capacity of 1000 mAh/g, and its electrochemical performance is better than that of PVDF/Si electrode, but 60 cycles does not fully explain the cycle stability.
② CMC binders
Compared with SBR/CMC and PEAA/CMC, which have better viscoelasticity, some people think that the CMC battery binder that lacks elasticity is more suitable for silicon-based anode materials. The CMC/Si electrode was cycled 70 times at 0.17-0.90 V at 150 mA/g, and the specific capacity was 1 100 mAh/g, which was superior to (SBR/CMC)/Si and PVDF/Si electrodes. Although the electrode exhibits good electrochemical performance when CMC is used as the battery binders, the electrode ratio, pH value and degree of substitution (DS) of CMC will affect the electrochemical performance of the CMC/Si electrode to varying degrees.
Some scholars compared the performance of CMC/Si electrodes prepared at different pH values, and found that the electrodes prepared in a buffer solution with a pH of 3 had the best performance. In addition, an appropriate increase in DS is beneficial to improve the electrochemical performance of CMC/Si electrodes. CMC battery binders have good application prospects, but CMC is generally viscous, brittle, and poor in flexibility, and the pole pieces are easy to crack during charging and discharging. Moreover, CMC is greatly affected by conditions such as electrode ratio, pH value, etc., and relevant research needs to be done in depth.
③ PAA battery binders
The molecular structure of PAA is simple, easy to synthesize, soluble in water and some organic solvents. Studies have shown that PAA with higher carboxyl content is more suitable for silicon-based anode materials than CMC. PAA can not only form a strong hydrogen bond with Si, but also form a more uniform coating on the Si surface than CMC. The distribution of PAA in the pole piece is relatively uniform, which can form a coating similar to SEI film on the Si surface, inhibit the decomposition of electrolyte, and have better performance than CMC, PVA and PVDF.
Although PAA containing a large amount of carboxyl groups has good viscosity, the carboxyl groups are highly hydrophilic and easily react with residual moisture in the battery, affecting performance. If there are still hydroxyl groups or moisture after the electrode is dried, it will react with LiPF6 in the electrolyte to decompose PF5, decompose the organic solvent, and affect the charge and discharge performance of the electrode.
④ Sodium alginate battery binders
The structure of sodium alginate is similar to CMC, and the arrangement of carboxyl groups is more regular. Sodium alginate is used as a battery binder for silicon-based anode materials, and the prepared sodium alginate/Si electrode is superior to CMC/Si and PVDF/Si electrodes. At present, there are not many reports on sodium alginate, and similar to PAA, sodium alginate has a high carboxyl content and has the problem of strong hydrophilicity.
⑤ Conductive polymer binders
Conductive polymer battery binders are both viscous and conductive, improving electrical conductivity while maintaining structural stability of the electrode sheet. PFFOMB is used as a silicon-based anode material. The prepared PFFOMB/Si electrode was cycled 650 times at 0.01-1.00 V at C/10, and the specific capacity was 2100 mAh/g.
⑥ Other battery binders
In addition to the above-mentioned battery binders, battery binders such as carboxymethyl chitosan, PAN, and PVA can also be used for silicon-based anode materials. Although the above-mentioned battery binders can form strong hydrogen bonds with Si and have good cycle stability, compared with binders such as CMC, PAA, and sodium alginate, the cycle stability is slightly worse.
The development and application of battery binders is one of the effective ways to improve the cycle stability of silicon-based anode materials for lithium-ion batteries in lithium ion battery anode material companies. Application of PVDF modified binder or water-based binder can improve the cycle stability and electrochemical performance of silicon-based anode to a certain extent. Different types of battery binders have their own advantages and disadvantages.
Relatively speaking, PAA, sodium alginate and conductive polymer binders show better cycle stability and electrochemical performance when applied to silicon-based anode materials. The development of water-based battery binders that can form stronger chemical bonds with Si and more uniform coating is an important development direction of binders for silicon-based anode materials. In addition, conductive polymer binders with both viscosity and conductivity also have broad application prospects.