Since the silicon-based negative electrode material has a high weight-specific capacity and volumetric capacity, the development of a silicon-based negative electrode is one of the most effective methods for increasing the energy density of a lithium-ion battery. However, as an active material, silicon is in charge/discharge cycles. When inserting and extracting lithium, the volume change reaches 270%, and the cycle life is poor. This volume expansion leads to: (1) Pulverization of silicon particles, and separation of the coating from the copper current collector; (2) Solid electrolyte (SEI) film at Instability during the cycle, the volume expansion causes the SEI to rupture and repetitively form, resulting in failure of the lithium-ion battery.
The compaction process will make the solid phase contact more closely and improve the electron transport performance of the pole piece. However, the porosity is too low and will increase the lithium ion transmission resistance, and the electrode/electrolyte interface charge transfer resistance, and the rate performance will be deteriorated. The graphite electrode porosity is optimally controlled between 20% and 40%, while the silicon-based electrode, after compaction, has poor performance. These pole pieces usually have a porosity of 60% to 70%. The high porosity can coordinate the volume expansion of the silicon-based material. The particles are violently deformed, slowing the pulverization and shedding. However, the high-porosity silicon-based negative pole piece limits the volumetric energy density. So, how does a lithium-based battery-based negative electrode pole piece be fabricated? KarkarZ et al. studied the preparation process of a silicon electrode. .
First, they used two stirring methods to prepare 80wt% silicon, 12wt% graphene, and 8wt% CMC electrode pastes: (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 pH 3 buffer solution (0.17 M citric acid + 0.07 MKOH), and the second step was to add the graphene sheet and water to continue the ultrasonic dispersion.
As shown in Figures 1a and d, for the graphite sheet, the ultrasonic disperse RAM maintains the original morphology of the graphene sheet. The length of the sheet is larger than 10 μm, parallel to the current collector, and the porosity of the coating is higher, and the SM stirring causes the graphene sheet to break. Graphene sheets are only a few microns in length. The porosity of uncompacted RAM plates is about 72%, which is greater than 60% of SM electrodes. For silicon, there is no difference between the two stirring methods. The nano-sheet graphene has good electronic conduction. Capability, RAM dispersion maintains the integrity of graphene sheets, and good battery cycling performance (Figures 3a and b).
Fig.1 Morphology of silicon-based electrode under different stirring modes and compaction pressure
Then, they studied the effects of compaction on the porosity, density, and electrochemical performance of the electrode. As shown in Figure 1, after compaction, the morphology of the graphene sheet and silicon did not change significantly, only the coating was more dense. The pole piece was fabricated into a half cell to test the electrochemical performance. It can be seen from FIG. 2 that:
(1) As the compaction pressure increases, the porosity of the electrode decreases, the density increases, and the volumetric specific capacity increases.
(2) Unconstrained pole piece, RAM porosity is about 72%, which is greater than 60% of SM electrode. And RAM electrode compaction is more difficult, reaching 35% porosity, RAM electrode needs 15T/cm2 pressure, and SM pole piece only 5T/cm2. This is because the graphene sheet is difficult to deform, and the RAM pole sheet keeps the graphene sheet structure and is harder to compact.
(3) Calculate the volumetric capacity based on the volumetric expansion of fully lithiated silicon of 193%. Under the compaction of 20 T/cm2, the volumetric capacity is the largest. The porosity of RAM and SM electrodes is divided into 34%, 27%, and the corresponding volumetric capacity is 1300mAh/ Cm3, 1400mAh/cm3.
Figure 2 Effect of compaction pressure on (a) SM electrode and (b) RAM electrode porosity, density and volumetric capacity
Figure 3 Cyclic performance of uncompacted electrodes
In addition, they also found that the compaction of the compacted poles can improve the cycle performance. When the pole pieces are pressed in real time, the binder and the particles of the living material may break under the frictional force between the particles, and even the bond of the binder itself breaks. The mechanical stability of the sheet is deteriorated, and the cycle performance is cracked (Fig. 4a). In the maturing process, the pole piece is placed in a humidity of 80% for 2 to 3 days. During this process, the binder will migrate, and better. Spreading on the surface of the particles of the living material, more and more reliable connections are established. In addition, the copper foil will corrode when it matures, and the copper foil forms a Cu(OC(=O)-R)2 chemical bond with the binder, and the binding force increases. It also inhibits coating shedding. Therefore, aging treatment can improve the stability and cycle performance of the pole piece. The schematic diagram of the microstructure change of the pole piece of the dispersion-compaction-curing process is shown in Figure 4c. The compaction results in the cracking of the adhesive and circulation. The stability is degraded, and when the binder migrates during curing, the connection is re-established, the microstructure of the pole piece changes, the mechanical stability is improved, and the corresponding cycle performance is improved.
If the pole piece is cured and then compacted, the cycle performance of the pole piece is improved, but the effect is not obvious (Fig. 4b). This is because the aging enhances the mechanical stability of the pole piece, but the subsequent compaction destroys the stickiness. The connection of the knot.
Figure 4 (a) (b) Effects of compaction and curing on the cycling performance of the electrode and (c) Schematic representation of the microstructure evolution during compaction and maturation
Therefore, for the silicon-based electrode, in order to improve the cycle performance, the volume of the buffered silicon expands, the porosity of the pole piece is higher, but in order to increase the volumetric energy density, when the pole piece is reduced in thickness by pressing the pole piece, the pole piece aging process needs to be improved. Electrode microstructure.
