In order to shorten the charging time of electric vehicles, scientists have been actively looking for new solutions. Recently, the team of academician Yu Shuhong of the University of science and technology of China, in cooperation with the teams of Professor Yao Hongbin and Professor Ni Yong, is committed to solving the contradiction between high energy density and fast charging performance of lithium-ion batteries. A new type of double gradient graphite cathode material is proposed and prepared, realizing 60% charging of lithium-ion batteries in 6 minutes. Relevant achievements were recently published in scientific progress.
High energy density and fast charging performance are a pair of contradictions
At present, electric vehicles driven by lithium-ion batteries are favored by people because of their energy conservation and environmental protection. However, the charging time of electric vehicles is much longer than that of traditional fuel vehicles, which greatly reduces the sense of use experience.
"This is mainly because the poor rate performance of graphite negative electrode in lithium-ion battery limits the rapid charging capacity of electric vehicles." Lu Leilei, co-author of the paper and special associate researcher of Hefei National Research Center for micro scale material science, University of science and technology of China, explained to China Science Daily.
Energy density and power density are two important parameters to evaluate battery system. The energy density determines the amount of energy that can be stored per unit mass / volume, while the power density determines the charging and discharging ratio of the battery. Ideally, the higher the two parameters, the better the performance of lithium-ion battery. However, high energy density and fast charging performance are a pair of contradictions, which is a "one after another" process.
"High energy density usually means that the active material load of the battery monomer is relatively high and the electrode is relatively thick, so it has a long lithium ion transmission path and limits the charge discharge ratio," Lu said
Therefore, in order to improve the rate performance of graphite negative electrode, the traditional strategy is to make the graphite electrode porous or thin. "However, these methods often sacrifice the energy density of the prepared battery." Lu Lei said frankly.
Is there a solution that can achieve both high energy density and fast charging performance? Yu Shuhong's team decided to start with designing the electrode structure to improve the fast charging performance of lithium-ion batteries while ensuring the energy density.
"Queuing" graphite particles to speed up charging
The research team first constructed a new particle level theoretical model to optimize the two parameters of particle size distribution and electrode porosity distribution in the electrode structure at the same time, so as to improve the fast charging performance of graphite negative electrode.
Lu Lei introduced that the traditional two-dimensional model usually simplifies the particles to be homogeneous spherical and the pores to be evenly distributed. In fact, graphite particles are mostly of different sizes and shapes, usually arranged in a fairly random order. At the same time, the shape and size of the holes are also unevenly distributed.
The new particle level theoretical model is a three-dimensional model based on real graphite particles, which is very close to the real electrode structure.
In the particle level theoretical model, researchers "queue up" again according to the order of graphite particle size, and adjust the distribution of electrode porosity at the same time. Specifically, the graphite particles closer to the top of the battery are smaller and have higher porosity, and the particles closer to the bottom are larger and have lower porosity.
"We call this structure a double gradient electrode." Lu Lei said that the simulation results show that under the condition of high current density charging, this new structure shows excellent fast charging performance compared with the traditional random homogeneous electrode and single gradient electrode.
The ideal structure model has been found, and the next step is how to realize it in the electrode.
In the traditional electrode preparation method, due to the high viscosity of the slurry, the prepared graphite slurry is stable and not easy to settle. Therefore, the prepared electrode, including graphite particle size and porosity, is usually evenly distributed. "Just like instant milk powder, any part is homogeneous," Lu said
How to construct a "heterogeneous" structure? The research team has developed a self-assembly technology of low viscosity non polymer binder slurry. The copper coated graphite negative electrode particles and copper nanowires are mixed in ethanol solution to make the slurry. Using the difference of sedimentation velocity of graphite particles with different sizes in the slurry, the double gradient structure optimized by simulation calculation is successfully constructed to obtain the electrode.
The researchers found that the lithium-ion battery based on the new double gradient graphite cathode material was charged from zero to 60% and 80% in 5.6 minutes and 11.4 minutes, respectively, while maintaining high energy density.
How far is the 6-minute fast charging technology from industrialization
How long does it take to complete 100% charging?
"In fact, this is a misunderstanding. Usually, the evaluation of battery fast charging performance is based on the time when the battery is charged to 60% or 80% capacity." For example, electric vehicle manufacturers usually recommend charging the vehicle to 80% to maintain battery life, Lu said.
Speaking of the biggest highlight of this study, Lu Lei said, "it is to complete the process from the establishment of a hypothesis and theoretical model to the re verification of experiments, which provides a new idea to overcome the contradiction between the high energy density and fast charging performance of lithium-ion batteries."
"There is still a certain distance from industrialization." Lu Lei admitted that, for example, the current preparation methods in the laboratory are difficult to achieve large-scale production, and the design of double gradient structure is difficult to maintain the consistency of electrodes.
"At present, the team is gradually solving these problems. I hope that one day this more efficient battery can power electric vehicles." Lu Lei said.
"This electrode structure design provides a new idea to solve the problem of fast charging. It is suggested to further evaluate its performance in large batteries and pay more attention to the electrode preparation cost." One reviewer said.