The new process increases the electrode/collector adhesion and greatly improves the cycle performance of the Si anode

by:CTECHi     2021-08-05

Lithium Grid News: As the energy density of lithium-ion batteries continues to increase, traditional graphite materials can no longer meet the needs of high-specific energy lithium batteries. The theoretical capacity of Si materials can reach 4200mAh/g, which can effectively increase the ratio of lithium batteries. Energy, but Si material will produce up to 300% volume expansion when lithium is completely inserted, causing the reversible capacity of Si material to decline sharply during the cycle, which seriously hinders the application of Si material. How to enhance the adhesion between the active material and the copper foil and reduce the loss of the active material becomes particularly important for improving the cycle performance of the Si anode material.

The traditional method of improving adhesion is mainly to replace the adhesive with better adhesion, but there are also other ways. For example, Inseong Cho of Hanba University in South Korea, etc. The method of adding a layer of polydopamine coating on the surface increases the adhesion between the active material and the current collector, reduces the delamination of the electrode and the current collector during the cycle, thereby greatly improving the cycle and rate performance of the Si anode material .

In the above picture a, the left side is untreated bare copper foil, and the right side is the copper foil treated with polydopamine. You can see the polydopamine from the color. The color of the copper foil has become significantly darker after treatment. From the XPS test results, it can also be seen that the ordinary Cu foil only has an O1s peak, which corresponds to the oxide on the Cu surface, while the surface of the treated Cu foil is observed The N 1s peak indicates that a polydopamine coating (containing a large amount of amino groups) is formed on the surface of the Cu foil, and the amino groups in the polydopamine can condense with the hydroxyl groups in the PAA binder, thereby greatly improving the negative electrode activity The adhesion between the material layer and the Cu foil current collector.

In the test, Inseong Cho not only prepared polydopamine-coated Cu foil, but also heat-treated part of the treated Cu foil in a vacuum environment at 80°C. Makes the condensation reaction between polydopamine and the PAA binder in the electrode, thereby further enhancing the adhesion between the electrode and the current collector, the test shows that the peel strength of ordinary copper foil is 245.5N/m, while the polydopamine coating The peel strength of the Cu foil after the layer treatment was increased to 297.5N/m, and the peel strength of the polydopamine coating treated Cu foil after heat treatment reached 353.2N/m. Better adhesion also allows Si materials to perform better in terms of cycle performance. From the figure below, we can see that the heat-treated polydopamine coated Cu foil with the best adhesion after 500 cycles (0.5C), The reversible capacity is still as high as 1590mAh/g, and the capacity retention rate is as high as 69%. The remaining reversible capacity of the polydopamine-coated Cu foil is 1361.1mAh/g, and the capacity retention rate is 61%, while the remaining reversibility of ordinary Cu foil without treatment The capacity is only 963.9mAh/g, and the capacity retention rate is only 44%.

The influence of adhesion on the electrochemical performance of Si anode is not only in the cycle performance. From the figure below, we can see that the electrode with better adhesion is in the rate performance. The same performance is excellent. Under the high current density of 9A/g, the heat-treated polydopamine coated copper foil with the strongest adhesion can still exert 1083.3mh/g, and the reversible polydopamine coated copper foil with weaker adhesion The capacity is 800mAh/g, and the weakest common copper foil has the lowest capacity, only 6.7mAh/g. We have already published the previous article on the mechanism of enhanced adhesion to improve the rate performance of silicon carbon anode. A detailed introduction is given in the article, so I won’t repeat it here.

When it comes to the mechanism of improving the performance of the silicon-carbon anode, it has to be said that the volume expansion of the Si material during the lithium insertion process destroys the structure of the Si/C anode. The picture shows the cross-sectional SEM picture of the silicon-carbon anode of three kinds of copper foils. From the picture, it can be seen that the electrodes of the three kinds of copper foils before the test have similar morphologies, but after the pre-cycle, we can see from the figure d below A large number of cracks appeared in the silicon-carbon negative electrode of ordinary copper foil, while the silicon-carbon electrode of polydopamine-coated copper foil (the following figure e) and the heat-treated polydopamine-coated copper foil silicon-carbon electrode (the following figure f) did not appear Obvious cracks. After 50 cycles of the rate test, this trend becomes more obvious. From the figure g below, it can be seen that after 50 cycles of the ordinary copper foil silicon carbon anode, there has been an obvious difference between the electrode layer and the copper foil current collector. Delamination phenomenon, while the more adhesive polydopamine-coated copper foil only has a slight delamination phenomenon, while the most adhesive heat-treated polydopamine-coated copper foil silicon carbon anode has more than 50 cycles There was no obvious delamination phenomenon afterwards, which indicates that stronger adhesion can effectively reduce the delamination and shedding between the electrode/current collector caused by the volume expansion of the Si material during the cycle, and reduce the loss of active materials, thereby Significantly improve the cycle performance of silicon carbon batteries.

In any case, the application of silicon-carbon anode cannot bypass the problem of large volume expansion, a binder with better adhesion and an electrolyte that can form a more stable SEI film They are all commonly used methods to inhibit the destruction of the electrode structure by the volume expansion of Si materials, and the work of Inseong Cho et al. showed us that we can also enhance the gap between the active material and the copper foil by coating the surface of the copper foil. Adhesiveness, reduce the delamination between the electrode layer and the current collector during the cycle, reduce the loss of active materials, and maximize the cycle performance of the Si anode. This idea has an important reference for the design of long-life silicon carbon batteries. value.

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