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What is the difference between the two adhesives in silicon cathode for the protection of cyclic charge and discharge

- Dec 26, 2018 -

Adhesives play an extremely important role in silicon cathode. The most important role of adhesives is to provide strong adhesion force to maintain the integrity of electrode structure and ensure the normal charging and discharging of lithium ion battery.

There are two main types of binders involved in most studies: PVDF (polyvinylidene fluoride) and CMC (carboxymethyl cellulose).

Let's start with PVDF, which has a simple molecular chain and good flexibility. The interaction between PVDF and silicon particles is mainly the weak van der Waals force formed by F atoms and H atoms. Given this force is weak, so the silicon particles embedded lithium (corresponding to the battery charging process), the volume expansion to three times the force will be weakened until destruction, and many times of charging and discharging cycles result of silicon particle crushing cracking, PVDF particles don't have the ability to put together at this time, in terms of micro electrical contact between the particles weakened or even disappeared, a direct result of the rapid attenuation of macro battery capacity.

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Moreover, CMC, which is a derivative of cellulose, has a rigid heterocyclic ring of six yuan in the molecular chain, with poor flexibility, as shown in the figure above. Most studies have found that CMC with rigid molecular chain structure can achieve better capacity retention rate. This result does not seem to be very well understood. Normally, the binder with good flexibility has a greater degree of deformation, so it should be slightly better at absorbing the expansion of silicon particles and maintaining the structural integrity of silicon cathode.

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YouShangTu shows three ontology membrane namely CMC SBR - CMC and membrane PVDf membranes for each stress-strain curve CMC membranes can be seen from the diagram, ontology is relatively fragile, elongation at break is only 5-8%, relative to the Si particles maximum 300% of the volume change is a drop in the ocean And even elastic super good SBR (styrene butadiene rubber) for the use of the elongation at break are below 13%, while under the condition of same PVDF can reach more than 20%Although the elongation at break of PVDf is much higher than that of CMC, it does not have any advantage in the cycle charging-discharging curve. On the contrary, the cycle stability of CMC is the best (the figure on the left).

On the other hand model tells us that the CMC in solution is even extend the molecular chain can be used as a mesh structure matrix, an effective bridge between carbon black and Si particles even to explain the bridge structure to ensure the electrical conductivity of the composite electrode model, the model that is the difference between the polymer chain segment adsorption on the different particles, or between different particles on the adsorption of segment produces mutual entanglement, like through the bridge together between different islands, particle is equivalent to the island, polymer chain segment is equivalent to the bridge, and is a three-dimensional overpass cross connection each otherAfter the suspension of the solvent evaporation (i.e., the drying process after coating), even the retention of the electrode structure in the form of the bridge, and the distance between the particles closer bridge even effect depends on the morphology structure and molecular weight molecules morphological structure refers to the dissolved in solvent in the shape of the polymer molecules, in theory, the greater the extension of shape and structure of the higher molecular weight, form a bridge even the possibility of structure, the greater the CMC of the molecular chain including rigid frame and other groups (mainly for the carboxylHydroxy) rigid skeleton can help CMC molecules stretch farther, and electrostatic repulsion between the carboxyl, makes the molecular form expand wider (means the possibility of even more bridge), expand the scale biggest can reach 260 nm (DS = 0.7 Mw = 90000) for the PVDF, flexible skeleton skeleton (C - C) makes the molecular form curls, random arrangement and the molecular chain structure unit will give under the effect of solvent molecules, so the biggest scale in about 30 nm (Mw = 180000)From this perspective, rigid chains are more advantageous in forming bridged structures.

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The figure above shows the interaction between CMC and Si materials during pulping and coating. Dehydration and condensation reaction between carboxyl group (-cooh) in CMC chain segment and hydroxyl group (-oh) in SiO2 layer of Si material is the key to ensure the cycling stability of Si electrode. This reaction will be affected by the degree of substitution of CMC itself (DS). Increasing the degree of substitution appropriately is conducive to increasing the number of covalent bonds.

In addition, by reducing the pH value of the pulping system, the interaction between CMC and Si can be further improved to improve the battery cycle life.

In conclusion it can be seen that although compared with PVDF, CMC for molecular chain rigidity and elongation at break are also small, but precisely because of its rigid chain to expand its molecular form, along with chain segment containing carboxyl, hydroxyl, increased its scope with Si particles and forces, so that made it perform better in terms of cycle stability. However, at present, the expansion problem of si-based materials using CMC still exists and is very serious, and there are many methods to solve the expansion problem, including alloying, porosity and so on. This paper attempts to provide you with a solution to the problem through the analysis of the mechanism of CMC action, perhaps there is an ideal binder waiting for us to find.