lithium 18650 li ion battery

Research progress of binders for silicon-based negative electrode materials for lithium 18650 li ion battery

Silicon (Si)-based negative electrode materials have high theoretical specific capacity (4200mAh/g) and a suitable lithium insertion and extraction platform, making them an ideal high-capacity negative electrode material for lithium 18650 li ion battery [1-2]. During the charge and discharge process, the volume change of Si reaches more than 300%. The internal stress generated by the drastic volume change can easily lead to electrode pulverization and peeling, affecting the cycle stability. In lithium 18650 li ion battery, binders are one of the important factors affecting the stability of electrode structure. According to the properties of the dispersion medium, lithium-ion battery binders can be divided into oily binders with organic solvents as dispersants and water-based binders with water as dispersants. Liu Xin et al. [3] reviewed the research progress of binders for high-capacity negative electrodes and believed that the application of polyvinylidene fluoride (PVDF) modified binders and water-based binders can improve the electrochemical performance of high-capacity negative electrodes, but no discussion or comparison was made on binders for silicon-based negative electrodes. The authors of this article reviewed the research progress of binders for silicon-based negative electrode materials and compared the advantages and disadvantages of different types of binders. 1. Oily binders Among oily binders, PVDF homopolymers and copolymers are the most widely used. 1.1 PVDF homopolymer binder In the large-scale production of lithium 18650 li ion battery, PVDF is generally used as a binder, and organic solvents such as N-methylpyrrolidone (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 connected to silicon-based negative electrode materials by weak van der Waals forces and cannot adapt to the drastic volume changes of Si. Traditional PVDF is not suitable for silicon-based negative electrode materials [3-5]. 1.2 PVDF modified binder In order to improve the electrochemical performance of PVDF applied to silicon-based negative electrode materials, some scholars have proposed modification methods such as copolymerization and heat treatment [4-5]. Z.H.Chen et al. [4] found that the ternary copolymer polyvinylidene fluoride-tetrafluoroethylene-ethylene copolymer [P(VDF-TFE-P)] can enhance the mechanical properties and viscoelasticity of PVDF. J. Li et al. [5] found that heat treatment at 300 ° C under argon protection can improve the dispersibility and viscoelasticity of PVDF. The modified PVDF/Si electrode was cycled 50 times at 150 mA/g at 0.17~0_90 V, and the specific capacity was 600 mAh/g. Although the cycle performance of the PVDF/Si electrode was improved after modification, the cycle stability was still not ideal. 2. Compared with oily binders, water-based binders are environmentally friendly, cheap and safer to use, and are gradually being promoted. At present, the most studied silicon-based negative electrode 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 (CMC) binder SBR/CMC has good viscoelasticity and dispersibility and has been widely used in the large-scale production of graphite negative electrodes. W.R Liu et al. [6] found that (SBR/CMC)/Si electrode can be charged and discharged for 60 times at a constant capacity of 1000mAh/g (0~1.2V), and its electrochemical performance is better than that of PVDF/Si electrode, but 60 cycles cannot fully explain the cycle stability. 2.2 CMC binder Compared with SBR/CMC and polyethylene acrylic acid (PEAA)/CMC with better viscoelasticity, some people believe that the lack of elasticity of CMC binder is more suitable for silicon-based negative electrode materials [7-8]. J. Li et al. [7] found that CMC/Si electrode was cycled for 70 times at 150mA/g at 0?17~0.90V, with a specific capacity of 1100mAh/g, which is better than (SBR/CMC)/Si and PVDF/Si electrodes. B. Lestriez et al. [8] found that the electrochemical performance of CMC/Si electrode is better than that of (PEAA/CMC)/Si electrode, because PEAA easily causes carbon black to agglomerate, affecting the cycle stability of the electrode. The carboxyl group of CMC can be connected to Si through chemical bonds (covalent bonds or α bonds [12-13]), with strong connection force, which can maintain the connection between Si particles; and CMC can form a coating similar to the solid electrolyte interface film (SEI) on the Si surface to inhibit the decomposition of the electrolyte. Although the electrode exhibits good electrochemical properties when CMC is used as a binder, the electrode ratio, pH value and CMC substitution degree (DS) will affect the electrochemical properties of CMC/Si electrodes to varying degrees. J.S.Bridel et al. [12-14] found that when m(Si):m(C):<n(CMC)=1:1:1, when full lithium is embedded, the electrode only expands by 48%, and the electrode cycle performance is the best, but at this time the Si content is low and the energy density of the battery is low. M. Gauthier et al. [9, 11] compared the performance of CMC/Si electrodes prepared at different pH values and found that the electrode prepared in a buffer solution of pH = 3 had the best performance. The CMC/micron Si electrode cycled 600 times at 480 mA/g at [3] 005-1_000 V, with a specific capacity of 1600 mAh/g [91]. In addition, appropriately increasing DS is beneficial to improving the electrochemical properties of CMC/Si electrodes. CMC/Si electrodes with DS?1.2 have good cycle performance [10-12]. CMC binder has good application prospects, but CMC has general viscosity, high brittleness, poor flexibility, and is prone to cracking during charge and discharge [13]. In addition, CMC is greatly affected by conditions such as electrode ratio and pH value. Related research needs to be further studied. 2.3 PAA binder PAA has a simple molecular structure, is easy to synthesize, and is soluble in water and some organic solvents. Studies have shown that PAA with a higher carboxyl content is more suitable for silicon-based negative electrode materials than CMC [15%. Magasinski et al. [15] found that PAA can not only form strong hydrogen bonds with Si, but also form a more uniform coating on the Si surface than CMC. The PAA/Si electrode was cycled 100 times at 0.01~1.00V at C/2, and the specific capacity was 2400mAh/g0S. Komaba et al. [16] found that PAA is more evenly distributed in the electrode, can form a coating similar to SEI film on the Si surface, inhibit electrolyte decomposition, and has better performance than CMC, polyvinyl alcohol (PVA) and PVDF. M. Hasegawa et al. [17-18] believe that PAA containing a large number of carboxyl groups has good viscosity, but the carboxyl groups are highly hydrophilic and easily react with residual water in the battery, affecting performance. If hydroxyl groups or water still exist after the electrode is dried, they will react with LiPF6 in the electrolyte to decompose PF5 (>601C), decomposing the organic solvent and affecting the charge and discharge performance of the electrode. If PAA is heat treated in vacuum at 150-200t for 4-12h, the carboxyl groups of PAA will partially condense, which can not only reduce the hydrophilicity of the electrode, but also enhance the structural stability of the electrode [1^7]. B. Koo et al. [19] heat treated CMC and PAA at 150t for 2h, and the resulting c-CMC-PAA/Si electrode cycled 100 times at 0.005-2.000V at 1.5A/g, with a specific capacity of 1500mAh/g. 2.4 Sodium alginate binder The structure of sodium alginate is similar to that of CMC, and the arrangement of carboxyl groups is more regular. I. Kovalenko et al. [20] used sodium alginate as a binder for silicon-based negative electrode materials. The prepared sodium alginate/Si electrode cycled 100 times at 0.01-1.00V at 4.2A/g, with a specific capacity of 1700mAh/g, which is better than CMC/Si and PVDF/Si electrodes. At present, there are not many reports on sodium alginate. Similar to PAA, sodium alginate has a high carboxyl content and is highly hydrophilic. 2.5 Conductive polymer binders Conductive polymer binders have both viscosity and conductivity, which can improve the conductivity while maintaining the stability of the electrode structure. G. Liu et al. [21] used poly (9,9-dioctylfluorene-co-fluorenone-co-methylbenzoic acid) (PFFOMB) for silicon-based negative electrode materials. The prepared PFFOMB/Si electrode was cycled 650 times at 0.01-1.00V at C/10, and the specific capacity was 2100mAh/g. H. Wu et al. [22] synthesized and prepared polyaniline (PAni)/Si electrode in situ, which was cycled 5000 times at 0.01-1.00V at 6.0A/g, and the specific capacity was still 550mAh/g. 2.6 Other binders In addition to the above binders, binders such as carboxymethyl chitosan, polyacrylonitrile (PAN) and PVA can also be used for silicon-based negative electrode materials. Carboxymethyl chitosan/Si electrode was cycled 50 times at 500mA/g at 0.12~1.00V, and the specific capacity was 950mAh/g[s]. PAN/Si electrode and PVA/Si electrode were cycled 50 times at C/2 at 0.005~[3]000V, and the specific capacity remained at 600mAh/g124-251. Although the above binders can form strong hydrogen bonds with Si and have good cycle stability, their cycle stability is slightly worse than that of binders such as CMC, PAA and sodium alginate. 3. Conclusion The research and development and application of binders is one of the effective ways to improve the cycle stability of silicon-based negative electrode materials for lithium 18650 li ion battery. The application of PVDF-modified binders or water-based binders can improve the cycle stability and electrochemical performance of silicon-based negative electrodes to a certain extent. Different types of binders have their own advantages and disadvantages. Relatively speaking, PAA, sodium alginate and conductive polymer binders show good cycle stability and electrochemical performance when applied to silicon-based negative electrode materials. Developing water-based binders that can form strong chemical bonds with Si and more uniformly coat it is an important development direction for binders of silicon-based negative electrode materials. In addition, conductive polymer binders that have both viscosity and conductivity also have broad application prospects.

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