How is the energy density of lithium batteries

As an active material, silicon has a volume change of 270% when lithium is inserted and extracted during the charge/discharge cycle, and the cycle life is poor. This volume expansion will cause: (1) the crushing of silicon particles and the separation of the coating from the copper current collector; (2) the instability of the solid electrolyte (SEI) membrane during the cycle, and the volume expansion will cause the SEI to break and continue to repeat Formation, leading to the failure of lithium batteries. The compaction process will make the solid phase contact closer and improve the electron transport performance of the pole piece. However, if the porosity is too low, it will increase the lithium ion transport resistance, and the electrode/electrolyte interface charge transfer resistance, and the rate performance will be worse. Generally, the porosity of graphite electrodes is optimized and controlled at 20%-40%, while the performance of silicon-based electrodes deteriorates after compaction. These pole pieces usually have a porosity of 60%-70%. High porosity can coordinate the volume expansion of silicon-based materials. , The buffer particles are violently deformed to slow down powdering and shedding. However, the high-porosity silicon-based negative pole piece limits the volumetric energy density. So, how to prepare the silicon-based negative electrode of a lithium battery? KarkarZ et al. studied the preparation process of silicon electrodes. First, 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 for continuous ultrasonic dispersion. Regarding graphite flakes, the ultrasonic dispersion RAM maintains the original morphology of the graphene flakes. The flake length is greater than 10μm, distributed parallel to the current collector, and the porosity of the coating is higher. SM stirring causes the graphene flakes to break, and the length of the graphene flakes is only a few microns . The porosity of the uncompacted RAM pole piece is about 72%, which is greater than 60% of the SM electrode. Regarding silicon, there is no difference between the two stirring methods. Nano-sheet graphene has good electronic conductivity, RAM dispersion maintains the integrity of the graphene sheet, and battery cycle performance is good (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 the graphene sheet and silicon did not change significantly, but the coating was more dense. The pole piece is made into a half-cell to detect electrochemical performance. It can be seen from Figure 2: (1) As the compaction pressure increases, the electrode porosity decreases, the density increases, and the volume specific capacity increases. (2) Without compacted 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 requires 15T/cm2 pressure, and SM pole piece only needs 5T/cm2. This is because the graphene sheet is difficult to deform, and the RAM pole piece maintains the graphene sheet structure and is more difficult to compact. (3) Calculate the volume specific capacity based on the volume expansion of fully lithiated silicon by 193%. Under 20T/cm2 compaction, the volume specific capacity is the largest. The porosity of RAM and SM electrode is 34% and 27%, and the corresponding volume specific capacity is 1300mAh/cm3 and 1400mAh/cm3, respectively. In addition, they also found that compacting pole piece curing solution can improve cycle performance. When the pole piece is compressed, the binder and the particles of the living substance may break under the friction between the particles, or even the bond of the binder itself is broken, so that the mechanical stability of the pole piece deteriorates and the cycle performance is cracked (Figure 4a). The maturation process is to place the pole piece in an environment with a humidity of 80% for 2 to 3 days. During this process, the binder will migrate, spread better on the surface of the living material particles, and re-establish more and stronger connections. In addition, the copper foil will corrode during aging, and the copper foil will form a Cu(OC(u003dO)-R)2 chemical bond with the binder, which will increase the bonding force and prevent the coating from falling off. Therefore, the aging solution can improve the stability and cycle performance of the pole piece. The schematic diagram of the microstructure change of the pole piece during the dispersion-compaction-curing process is shown in Figure 4c. The compaction causes the binder to break and the cycle stability deteriorates. During curing, the binder migrates and re-establishes the connection, and the microstructure of the pole piece occurs. Changes, improved mechanical stability, and correspondingly improved cycle performance. If the pole piece is cured first and then compacted, the pole piece cycle performance is improved, but the effect is unclear (Figure 4b). This is because maturation enhances the mechanical stability of the pole piece, but the subsequent compaction breaks the bonding of the adhesive. Therefore, regarding silicon-based electrodes, in order to improve the cycle performance and buffer the volume expansion of silicon, the porosity of the pole piece should be high. However, in order to increase the volume energy density, when the pole piece is compacted to reduce the thickness of the pole piece, it is necessary to carry out the pole piece maturation to solve the improvement. Electrode microstructure. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous post: Detailed explanation of lithium battery electrolyte technology and development trends