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Due to the high weight specific capacity and volume specific capacity of silicon-based negative electrode materials, developing silicon-based negative electrodes is one of the most effective methods to improve the energy density of lithium-ion batteries. However, as an active substance, silicon undergoes a volume change of 270% during the charging/discharging cycle when lithium is inserted and removed, resulting in poor cycle life. This volume expansion can lead to: (1) pulverization of silicon particles and separation of coatings from copper current collectors; (2) The solid electrolyte (SEI) film is unstable during cycling, and its volume expansion causes SEI to rupture and repeatedly form, leading to the failure of lithium-ion batteries.

The compaction process will make the solid phase contact tighter and improve the electron transfer performance of the electrode. However, a low porosity will increase the resistance to lithium ion transport and the impedance of charge transfer at the electrode/electrolyte interface, resulting in poor rate performance. Generally, the porosity of graphite electrodes is optimized and controlled at 20% -40%, while the performance of silicon-based electrodes deteriorates after compaction. These electrode plates usually have a porosity of 60% -70%. High porosity can coordinate the volume expansion of silicon-based materials, buffer the severe deformation of particles, and slow down pulverization and detachment. However, high porosity silicon-based negative electrode plates limit the volumetric energy density. So, how to prepare silicon-based negative electrode plates for lithium batteries? KarkarZ et al. studied the preparation process of silicon electrodes.

Firstly, they used two stirring methods to prepare 80wt% silicon, 12wt% graphene, and 8wt% CMC electrode slurry: (1) SM: conventional ball milling dispersion process; (2) RAM: Two step ultrasonic dispersion process, the first step is to ultrasonically disperse silicon and CMC in a PH3 buffer solution (0.17M citric acid+0.07MKOH), and the second step is to add graphene sheets and water to continue ultrasonic dispersion.

As shown in Figures 1a and d, for graphite sheets, ultrasonic dispersion RAM maintains the original morphology of the graphene sheet, with a length greater than 10 μ m, distributed parallel to the current collector, and a higher coating porosity. However, SM stirring causes the graphene sheet to fracture, with a length of only a few micrometers. The porosity of the uncompacted RAM electrode is about 72%, which is greater than 60% of the SM electrode. For silicon, there is no difference between the two stirring methods. Nanosheet graphene has good electronic conductivity, and RAM dispersion maintains the integrity of graphene sheets, resulting in good battery cycling performance (Figure 3a and b).

Then, they studied the effect of compaction on the porosity, density, and electrochemical performance of the electrode. As shown in Figure 1, after compaction, the morphology of graphene sheets and silicon did not change significantly, only the coating became denser. Make the polarizer into a half cell to test its electrochemical performance. As shown in Figure 2:

(1) As the compaction pressure increases, the porosity of the electrode decreases, the density increases, and the specific volume capacity increases.

(2) The unconsolidated electrode has a RAM porosity of approximately 72%, which is greater than 60% of the SM electrode. Moreover, it is more difficult to compact the RAM electrode, achieving a porosity of 35%. The RAM electrode requires a pressure of 15T/cm2, while the SM electrode only requires 5T/cm2. This is because graphene sheets are difficult to deform, while RAM electrodes maintain their graphene sheet like structure, making it more difficult to compact.

(3) Calculate the volume specific capacity based on the volume expansion of 193% for fully lithiated silicon. Under 20T/cm2 compaction, the volumetric specific capacity is the highest, with porosity rates of 34% and 27% for RAM and SM electrodes, corresponding to volumetric specific capacities of 1300mAh/cm3 and 1400mAh/cm3, respectively.

In addition, they also found that the solidification treatment of compacted electrode sheets can improve cycling performance. When the polarizer is compacted, the adhesive and active material particles may break under the frictional force between the particles, and even the adhesive itself may break, resulting in a decrease in the mechanical stability of the polarizer and cracking of its cyclic performance (Figure 4a). The curing process involves placing the polarizer in an environment with 80% humidity for 2-3 days. During this process, the adhesive will migrate and better spread on the surface of the active material particles, re establishing more and stronger connections. In addition, the copper foil will corrode during curing, forming a Cu (OC (=O) - R) 2 chemical bond with the adhesive, increasing the bonding force and inhibiting coating detachment. Therefore, maturation treatment can improve the stability and cycling performance of the polarizer. The schematic diagram of the microstructural changes of the polarizer during the dispersion compaction maturation process is shown in Figure 4c. Compaction causes the binder to fracture, resulting in a decrease in cycling stability. However, during maturation, the binder migrates and re establishes connections, causing changes in the microstructure of the polarizer, improving mechanical stability and corresponding cycling performance.

If the polarizer is first matured and then compacted, the cycling performance of the polarizer can be improved, but the effect is not significant (Figure 4b). This is because maturation enhances the mechanical stability of the polarizer, but subsequent compaction breaks the bond of the adhesive.

Therefore, for silicon-based electrodes, in order to improve cycling performance and buffer the volume expansion of silicon, the porosity of the electrode plate needs to be high. However, in order to increase the volume energy density and reduce the thickness of the electrode plate by compacting it, it is necessary to improve the microstructure of the electrode plate through aging treatment.

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