Due to the high specific capacity and volumetric capacity of silicon-based anode materials, developing silicon-based anodes is one of the most effective methods to improve the energy density of lithium-ion batteries. However, as an active material, silicon undergoes a volume change of up to 270% during lithium insertion and extraction in charge/discharge cycles, resulting in poor cycle life. This volume expansion leads to: (1) the crushing of silicon particles and the separation of the coating from the copper current collector; (2) the instability of the solid electrolyte interphase (SEI) membrane during cycling, with the volume expansion causing the SEI to rupture and repeatedly re-form, leading to the failure of the lithium-ion battery.
The compaction process makes the solid-phase contact tighter, improving the electron transport performance of the electrode. However, too low porosity increases lithium-ion transport resistance and charge transfer impedance at the electrode/electrolyte interface, resulting in poor rate performance. Generally, the porosity of graphite electrodes is optimized to be controlled between 20% and 40%, while the performance of silicon-based electrodes deteriorates after compaction. These electrodes typically have a porosity of 60% to 70%. High porosity can coordinate the volume expansion of silicon-based materials, buffer severe particle deformation, and slow down pulverization and shedding. However, high-porosity silicon-based anode electrodes limit the volumetric energy density. So, how should silicon-based anode electrodes for lithium batteries be prepared? Karkar Z et al. studied the fabrication process of silicon electrodes.
First, they used two stirring methods to prepare an electrode slurry of 80 wt% silicon, 12 wt% graphene and 8 wt% CMC: (1) SM: conventional ball milling dispersion process; (2) RAM: two-step ultrasonic dispersion process. The first step was to ultrasonically disperse silicon and CMC in a pH3 buffer solution (0.17M citric acid + 0.07MKOH), and the second step was to add graphene sheets and water to continue ultrasonic dispersion.
As shown in Figures 1a and 1d, for graphite sheets, ultrasonically dispersed RAM maintained the original morphology of the graphene sheets, with sheet lengths greater than 10 μm, distributed parallel to the current collector, and exhibiting higher coating porosity. In contrast, SM stirring caused the graphene sheets to break, resulting in sheet lengths of only a few micrometers. The porosity of the uncompacted RAM electrode was approximately 72%, greater than the 60% of the SM electrode. For silicon, there was no difference between the two stirring methods. Nanosheet graphene exhibits good electronic conductivity, and RAM dispersion maintains the integrity of the graphene sheets, resulting in good battery cycle performance (Figures 3a and 3b).
They then investigated the effects of compaction on the porosity, density, and electrochemical performance of the electrodes. As shown in Figure 1, after compaction, the morphology of the graphene sheet and silicon did not change significantly; only the coating became denser. The electrodes were then used to fabricate half-cells to test their electrochemical performance. Figure 2 shows the results.
(1) As the compaction pressure increases, the electrode porosity decreases, the density increases, and the volumetric capacity increases.
(2) The uncompacted electrode has a RAM porosity of approximately 72%, which is greater than the 60% of the SM electrode. Moreover, RAM electrodes are more difficult to compact. To achieve a porosity of 35%, RAM electrodes require a pressure of 15 T/cm², while SM electrodes only require 5 T/cm². This is because graphene sheets are difficult to deform, and RAM electrodes maintain the sheet-like structure of graphene, making them more difficult to compact.
(3) The volumetric specific capacity was calculated based on the 193% volume expansion of fully lithiated silicon. The volumetric specific capacity was the largest under a compaction of 20T/cm2. The porosities of the RAM and SM electrodes were 34% and 27%, respectively, corresponding to volumetric specific capacities of 1300mAh/cm3 and 1400mAh/cm3.
Furthermore, they found that the curing treatment of compacted electrodes can improve cycling performance. During electrode compaction, the binder and active material particles may break under the frictional force between the particles, or even the bonds in the binder itself may break, resulting in poorer mechanical stability and reduced cycling performance (Figure 4a). The curing process involves placing the electrode in an environment with 80% humidity for 2-3 days. During this process, the binder migrates and spreads better on the surface of the active material particles, re-establishing more and stronger bonds. In addition, the copper foil corrodes during curing, forming Cu(OC(=O)-R)2 chemical bonds with the binder, increasing the bonding force and inhibiting coating peeling. Therefore, curing treatment can improve electrode stability and cycling performance. The schematic diagram of the microstructure changes of the electrode during the dispersion-compaction-curing process is shown in Figure 4c. Compaction leads to binder breakage and poor cycling stability, while curing causes binder migration and re-establishment of bonds, changing the microstructure of the electrode, improving mechanical stability, and correspondingly improving cycling performance.
If the electrode is cured before compaction, the electrode's cycle performance is improved, but the effect is not significant (Figure 4b). This is because curing enhances the mechanical stability of the electrode, but subsequent compaction disrupts the bonding of the binder.
Therefore, for silicon-based electrodes, in order to improve cycle performance and buffer the volume expansion of silicon, the electrode porosity needs to be high. However, in order to improve the volumetric energy density, when compacting the electrode to reduce the electrode thickness, it is necessary to perform electrode curing treatment to improve the electrode microstructure.
