Raising the energy density is an eternal topic for lithium-ion battery research. According to the requirements of the Ministry of Industry and Information Technology, the target of achieving a specific energy of 300 Wh/kg for lithium-ion battery cells will be achieved by 2020. The improvement in the specific energy of lithium-ion batteries is inseparable from the advancement of materials technology and production technology. The increase in coating volume is an effective way to increase the energy density of lithium-ion batteries, but as the amount of coating increases, we will find that the electrical properties of lithium ions, especially the rate performance and cycle performance, have dropped significantly. This is mainly because the electrodes of lithium-ion batteries are mainly porous structures composed of particles, in which the pores are complex and the tortuosity is high, which will significantly increase the resistance to diffusion of Li+ therein. Therefore, how to increase the coating amount while reducing the electrode porosity The degree of curling, increasing the diffusion rate of Li+ in it is a problem that must be solved in the production of ultra-thick electrodes.
To solve the issue of ultra-thick electrode coating, Yu Shuhong of the University of Science and Technology of China and Professor Cui Wei of Stanford University gave their own answers. The researchers were inspired by the conduit structure of natural trees to develop a vertical catheter. Structure of the ultra-thick LCO positive electrode, this structure can reduce the electrode pore curvature, reduce the diffusion resistance of Li + in the electrode, so as to ensure the cycle performance and rate performance of the battery under high load conditions. With the help of technology, LCO coating can reach a maximum of 22.7mAh/g (equivalent to about 160mg/cm2), which is 4-5 times that of traditional processes, and is of great significance for improving the specific energy of lithium-ion batteries.
In order to obtain a biomimetic LCO electrode, Yu Shuhong's team used Scotch pine as a template. First, it cuts Pinus sylvestris into thin slices of 1.5mm in thickness, and then dissolves the lignin in the wood duct by ammonia dissolution to obtain a uniform porous template. (As shown in Figure b below), the LCO precursor sol prepared from LiNO3, Co(NO3)2·6H2O is then immersed in a template tube under a vacuum and dried to form an LCO precursor gel, and finally in an air environment. Sintering at 700°C for 2h, on the one hand, causes LCO to crystallize and on the other hand removes the wood template. As can be seen from the following figure g, the LCO electrode retains the porous structure in the vertical direction after calcination. In order to further enhance the LCO electrode With the load, Yu Shuhong's team also repeated the above-mentioned LCO precursor sol solute process (LCO-2 electrode), which increased the load per unit area by nearly double.
To analyze the effect of the above preparation process on the porosity and sag of the electrode, Yu Shuhong's team used the CT scanning technique to reconstruct the electrode structure, in which Figure a is an ordinary LCO electrode and Figure b is a LCO-1 electrode in which one LCO is immersed once. Figure C shows the LCO-2 electrode immersed twice in the LCO. It can be seen from the figure that the LCO electrode is randomly deposited by the LCO particles. Therefore, the pore structure of the electrode is complex and the pore curvature is relatively high (around 1.5). Affects the diffusion rate of lithium ions. LCO-1 and LCO-2 prepared by the template method have vertical structure pores. Therefore, the electrode has a high porosity and a low degree of buckling (close to 1), which is favorable for the rapid diffusion of Li+. Super thick electrode rate performance.
In order to verify the electrochemical performance of the LCO electrode with a vertical catheter structure, Yu Shuhong's team used the above electrode to prepare a button cell to test the electrochemical performance. The following figure shows the results of different discharge rates after charge from 0.05C to 4.25V. From the perspective of the conventional process, the LCO electrode rapidly declines in performance as the magnification increases. (The LCO electrode capacity produced by the conventional process here is only about 100 mAh/g, which is much lower than that of the normal LCO material. Since there is no detailed information, the Xiaobian cannot judge the specific The reason is that the LCO-1 and LCO-2 electrodes with vertical conduit design have significantly better rate performance than the control group. Figure b shows the LCO-1 electrode with one LCO slurry immersion and the LCO with two LCO slurry immersion. The capacity per electrode area of the 2 electrodes at different magnifications shows that the coating amount of the LCO-2 battery is significantly increased (the LCO-2 load is 24.5 mAh/cm2 at 0.05C, and the LCO-1 load is 13.3. mAh/cm2), Porosity decreases, but due to the structure of the vertical conduit reduces the buckling of the pores, thus greatly improving the rate performance of the electrode. At almost all magnifications, the LCO-2 load is LCO-1. About times, indicating vertical Pipe structure can well improve the high rate performance of the coating amount of the electrode.
The fast charging performance is also an important goal for the power battery. The following figure shows the different rate charging performances of the control LCO electrode and LCO-1 electrode with vertical conduit structure prepared by the traditional process. Compared with the traditional process, it is prepared from the figure a. LCO electrode, LCO-1 electrode polarization during charging is small, the capacity is higher, the capacity of LCO-1 electrode is significantly higher than LCO electrode prepared by traditional process at different charge rates.
The biomimetic LCO electrode developed by Yu Shuhong team has made important progress in increasing the load per unit area of the electrode. It is of great significance in the development of high specific energy lithium-ion batteries. However, we also noticed some problems as a traditional process of the control group. The LCO electrode not only has very poor rate performance, but also has very low capacity. This is not well explained in the text. Secondly, compared with the control group, the LCO electrode of the biomimetic structure has greatly improved the rate performance. However, at a 1C rate, its capacity is only about 50% of the capacity of 0.05C, which is much lower than current commercial lithium-ion batteries. In summary, the biomimetic LCO electrode developed by Yu Shuhong's team improves the electrochemical performance of ultra-thick electrodes. It is of great significance, but this technology still needs further optimization, including the preparation process and the electrochemical performance of the electrode, to enhance its practicality.
