New Progress on Silic-based Negative Material Binder for Lithium-ion Battery

The theory of Silicon(Si) base negative electrode material is higher than the capacity(4200mAh/g) and the embedded lithium removal platform is more suitable. It is an ideal high-capacity negative electrode material for lithium ion batteries. During the process of charging and discharging, the volume change of Si reaches more than 300 %. The internal stress caused by the dramatic volume change easily leads to the pulverization and peeling of the electrode, affecting the stability of the cycle.

In lithium-ion batteries, the binder is one of the important factors affecting the stability of the electrode structure. According to the properties of dispersion media, lithium-ion battery binders can be divided into oily binders with organic solvents as dispersants and waterborne binders with water as dispersants. Liu Xin and others summarized the progress of research on the binders for the negative electrode of high capacity and believed that the application of PVDF modified binders and waterborne binders can improve the performance of high-capacity negative electrode electrochemistry. However, there is no discussion or comparison of the binders used for Silicon negative electrode.

In this paper, the research progress of Silicon-based negative electrode binders is reviewed and the advantages and disadvantages of different types of binders are compared.

1, oily binder

Among the oily binders, PVDF homopolymers and copolymers 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 the organic solvent N-methylpyrrolidone(NMP) is used as a dispersant. PVDF has good viscosity and electrochemical stability, but the electron and ion conductivity is poor, and the organic solvent is volatile, flammable, explosive, and toxic; Moreover, PVDF is only connected to the silicon-based negative electrode material by Ruofandehuali and can not adapt to the dramatic volume changes of Si. Traditional PVDF is not suitable for silicon-based negative electrode materials.

1.2 PVDF modified binders

In order to improve the electrochemical properties of PVDF applied to silicon-based negative electrode materials, some scholars have proposed modification methods such as copolymerization and heat treatment. Z.H. Chen et al. found that the ternary copolymer PTFE-tetrafluoroethylene-ethylene copolymer[ P(VDF-TFE-P)] It can enhance the mechanical properties and viscoelasticity of PVDF. J.Li et al. found that heat treatment at 300 °C and argon protection can improve the dispersion and viscoelasticity of PVDF. The modified PVDF/Si electrode has a 150mA/g cycle of 0.17 to 0.90V 50 times, and the specific capacity is 600mAh/g. The PVDF/Si electrode has been modified and the cyclic performance has been improved, but the cyclic stability is still not satisfactory.

2, waterborne binders

Compared with oily binders, waterborne binders are environmentally friendly, cheap, and safer to use, and are gradually promoted. At present, more researches on silicon-based negative materials binders are carboxymethyl cellulose sodium(CMC) and polyacrylic acid(PAA) other waterborne binders.

2.1 Butyl benzene rubber(SBR) / carboxymethyl fiber decarbonate(CMC) binder

SBR/CMC has good viscoelasticity and dispersion, and it has been widely used in large-scale graphite negative electrode production. W.RLiu et al. found that(SBR/CMC) / Si electrodes can be charged and discharged at a constant capacity of 1000mAh/g(0 to 1.2 V) 60 times, and the electrochemical performance is better than that of PVDF/Si electrodes, but the 60 cycles do not fully account for the cycle stability.

2.2 CMC binders

Compared to SBR/CMC and PEAA / CMC, which have better viscoelasticity, some people think that inelastic CMC binders are more suitable for silicon-based negative electrode materials. J.Li et al. found that the CMC/Si electrode has 70 cycles at 0.17 to 0.90 V with a specific capacity of 1100 mAh/g at 150 mA/g, which is superior to(SBR/CMC) / Si and PVDF/Si electrodes. B. Lestiez et al. found that the electrochemical properties of the CMC/Si electrode were superior to those of the(PEAA/CMC) / Si electrode because PEAA easily reunited carbon black and affected the cyclic stability of the electrode.

The carboxymethyl group of CMC can be connected to Si by chemical bonds(Covalent bonds or bonds), and the connection force is strong to maintain the connection between Si particles; And the CMC can form a coating similar to the solid electrolyte phase boundary mask(SEI) on the Si surface to inhibit the decomposition of the electrolyte.

Although the electrode performance of the CMC as a binder is good, the electrode ratio, pH, and CMC substitution(DS) all affect the electrochemical performance of the CMC/Si electrode to varying degrees. J.S. Bridel et al. found that when M(Si): M(C):

CMC binders have good application prospects, but the CMC is generally viscous and brittle, and the softness is poor. When charged and discharged, the electrode is prone to cracking, and the CMC is greatly affected by electrode ratio, pH, and other conditions. The relevant research still needs to be deep. person.

2.3 PAA binder

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 negative electrode materials than CMC. A. Magasinski et al. found that PAA can not only form a strong hydrogen bond with Si, but also form a more uniform envelope than CMC on the Si surface. The PAA/Si electrode circulates at C/2 at 0.01 to 1.00 V 100 times, with a specific capacity of 2400mAh/g. S. Komaba et al. found that the distribution of PAA in the polar film was relatively uniform, and it could form a similar SEI membrane coating on the Si surface, inhibit electrolyte decomposition, and perform better than CMC, polyvinyl alcohol(PVA) and PVDF.

M. Hasegawa et al. believe that although PAA containing a large number of carboxyl groups has good viscosity, the carboxyl groups are relatively hydrophilic and easily react with the residual water in the battery, affecting performance. If there is still hydroxyl or water after the electrode is dried, PF5(& GT; At 60 °C), the organic solvent is decomposed and the charging and discharging performance of the electrode is affected. If the PAA vacuum heat treatment is 4 to 12 H at 150-200 °C, the partial condensation of the PAA carboxyl group can not only reduce the hydrophilicity of the electrode but also enhance the structural stability of the electrode. B. Koo et al. performed 2 H thermal treatment of CMC and PAA at 150 °C: the resulting c-CMC-PAA / Si electrode was recycled at 1.5A/g at 0.005 ~ 2.000 V 100 times, with a specific capacity of 1500 mAh/g.

2.4 Sodium alginate binder

The structure of sodium alginate is similar to that of CMC, and the carboxyl groups are arranged more regularly. I. Kovalenko et al. used sodium alginate as a binder for silicon-based negative electrode materials. The prepared sodium alginate / Si electrode was recycled 100 times at 4.2A/g at 0.01 ~ 1.00 V, with a specific capacity of 1700 mAh/g. 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 a strong hydrophilic problem.

2.5 Conductive polymer binders

The conductive polymer binder has both viscosity and conductivity and can increase the conductivity while maintaining the stability of the electrode structure. G. Liu et al. used polymeric(9,9-dioctyl fluorene-copyleuranone-copylaminone-methylbenzoic acid)(PFFOMB) for silicon-based negative electrode materials. The PFF0MB/Si electrode was prepared with C/10 in 0.01 ~ 1.00 V cycle 650 times. The specific capacity is 2100mAh/g. H. Wu et al. synthesized and prepared polyaniline(PAni) / Si electrodes with 6.0 A/g at 0.01 to 1.00 V cycles 5,000 times, with a specific capacity of 550 mAh/g.

2.6 Other binders

In addition to the above binders, binders such as carboxymethyl chitosan, polyacrylonitrile(PAN) and PVA can also be used in silicon-based negative electrode materials. The finished methylchitosan / Si electrode circulates at 500 mA/g at 0.12 to 1.00 V 50 times, the specific capacity is 950 mAh/g, the PAN/Si electrode and the PVA/Si electrode are C/2 at 0.005 ~ 1.000 V 50 times, The specific capacity is maintained at 600 mAh/g. Although the above binders can form strong hydrogen bonds with Si and have good cyclic stability, the cyclic stability is slightly worse than that of CMC, PAA, and sodium alginate binders.

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