The guarantee of circulating charge and discharge, what is the difference between the two binders in the silicon anode
The role of the binder in the silicon negative electrode is extremely critical. The most important function of the binder is to provide a strong bonding force to maintain the integrity of the electrode structure and ensure the normal charging and discharging of the lithium ion battery .
The binders involved in most of the studies are mainly two types: PVDF (polyvinylidene fluoride) and CMC (carboxymethyl cellulose).
Let's talk about PVDF, which has a simple molecular chain and good flexibility. The interaction with silicon particles is mainly due to the weak van der Waals force formed by F atoms and H atoms. In view of this weak force, when the silicon particles are inflated to three times the volume of lithium insertion (corresponding to the battery charging process), the force is weakened until it is destroyed, and the result of multiple cycles of charging and discharging is silicon. The particles are pulverized and cracked. At this time, PVDF has no ability to aggregate the particles together. At the microscopic aspect, the electrical contact between the particles is weakened or even disappeared, which directly leads to rapid decay of the macroscopic battery capacity.
Furthermore, CMC, which is a derivative of cellulose, contains a rigid six-membered heterocyclic ring in the molecular chain, and has poor flexibility, as shown in the above figure. Most studies have found that CMC using a rigid molecular chain structure can achieve better capacity retention. This result does not seem to be well understood. Normally, a flexible adhesive has a greater degree of deformation, so it should be slightly better in terms of supporting silicon particle expansion and maintaining the structural integrity of the silicon negative electrode.
The upper right panel shows the corresponding stress-strain curves for the three bulk films, namely the CMC film, the PVDf film, and the SBR-CMC film. As can be seen from the figure, the bulk CMC film is relatively brittle, the elongation at break is only 5-8%, and the volume change of 300% with respect to the maximum of Si particles is simply a drop in the bucket. Even with the super-elastic SBR (styrene-butadiene rubber) as its help, the elongation at break is still below 13%, and PVDF can reach more than 20% under the same conditions. Although the elongation at break of PVDf is much higher than that of CMC, there is no advantage in the cyclic charge-discharge curve. Instead, the cycle stability using CMC is optimal (above left).
Let us first look at the role of CMC in the system:
On the one hand as a dispersant to help a more uniform distribution of carbon black and Si particles;
On the other hand, the bridging model tells us that the molecular chain extended by CMC in solution can be used as a network structure matrix to form an effective bridging structure between carbon black and Si particles to ensure the electrical conductivity of the composite electrode.
Explain again the bridging model, which says that different segments of a polymer chain are adsorbed on different particles, or that the segments that are adsorbed on different particles are intertwined, just like between different islands. By connecting the bridges together, the particles are equivalent to islands, the polymer segments are equivalent to bridges, and the three-dimensional overpasses are cross-connected. After the evaporation of the solvent of the suspension (i.e., the drying process after coating), the bridging morphology of the electrode structure is retained and the distance between the particles is more tight. The bridging action depends on the molecular morphology and molecular weight. Molecular morphological structure refers to the shape of a polymer macromolecule dissolved in a solvent. Theoretically, the higher the molecular weight, the greater the morphological structure extension and the greater the possibility of forming a bridging structure.
The molecular chain of CMC consists of a rigid backbone and other groups (mainly carboxyl, hydroxyl). The rigid backbone can help the CMC molecules extend further, while the electrostatic repulsion between the carboxyl groups makes the molecular form expand more widely (meaning more bridging) The possibility of the maximum expansion scale can reach 260nm (DS = 0.7, Mw = 90,000). For PVDF, the flexible skeleton (C-C skeleton) makes the molecular morphology appear curled, and the structural units are randomly arranged.
And the molecular chain will expand under the action of solvent molecules, so the maximum scale is around 30 nm (Mw = 180000). From this perspective, rigid chains are more dominant in forming bridging structures.
The above figure shows the interaction between CMC and Si material during the pulping and coating process. The dehydration condensation reaction between the carboxyl group (-COOH) in the CMC segment and the hydroxyl group (-OH) of the SiO2 layer of the Si material is the key to ensure the stability of the Si electrode cycle, and the reaction is affected by the CMC self-degree of substitution (DS). Appropriately increasing the degree of substitution is beneficial to increasing the number of covalent bonds generated.
In addition, by lowering the pH of the pulping system, the interaction between CMC and Si can be further improved, and the cycle life of the battery can be improved.
In summary, although CMC is a rigid molecular chain and has a small elongation at break compared with PVDF, it is precisely that its rigid chain expands its molecular morphology and adds more carboxyl groups to the segment. The hydroxyl group increases its range of action and force with the Si particles, thus making it perform better in terms of cycle stability. However, at present, even the expansion problem of Si-based materials using CMC is still present and very serious, and there are many methods for solving the expansion problem, including alloying, porosification, and the like. This paper attempts to provide a solution to the problem by analyzing the mechanism of action of CMC. Perhaps an ideal binder is waiting for us to discover.
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