Dry Battery.Oak Ridge National Laboratory Research: Assessment of factors limiting the fast charging capabilities of lithium-ion batteries

　　Cost, energy density, and fast charging characteristics have always been the three important indicators for evaluating power batteries. A careful observation of the press conferences of major brands of electric vehicles in recent years shows that fast charging has always been one of the selling points that businesses have been promoting. Recently, researchers from the Oak Ridge National Laboratory and the University of Tennessee conducted a detailed evaluation of the fast charging limiting factors of the NMC811/graphite system. The results are detailed in Identifying the limiting electrode in lithium ion batteries for extreme fast ging. Electrochemistry Communications, 2018, 97: 37-41. Electrochemistry Communications is one of the few communication publications in the field of electrochemistry. Its current editor-in-chief is Professor R.G. Compton of Oxford University. Although the impact factor of the publication is not high (IF=4.6), most of the papers published are concise, bright and innovative, so they are loved by many people.

　　Highlights:

　　(1) It is fully proved that graphite is the main limiting factor limiting the fast charging capability of batteries;

　　(2) Due to the rapid fading of graphite negative electrode capacity during high-rate charging, N/P may be less than 1, which may easily cause lithium precipitation;

　　(3) When designing battery fast charging, we must also consider diffusion issues under high-rate charging, lithium salt consumption, material selection and load capacity, etc.

　　In order to further accurately evaluate the capacity characteristics of the positive and negative electrodes under different charging rates and eliminate the impact on the electrodes, the author took the positive and negative electrodes of the 50% SOC full battery and made them into symmetrical batteries. Figure 2A and Figure 2B are the charge and discharge curves of NMC811 and graphite symmetric batteries respectively. Figure 2C shows the capacity density attenuation and N/P ratio changes of NMC811 and graphite at different rates. Similar to the power-off results, when the charging rate is higher than 1 C, the capacity of graphite decreases sharply, while NMC811 still maintains good capacity from 1/10 C to 4C. In order to avoid lithium precipitation, the N/P ratio will be greater than 1 during battery design. However, as shown in Figure 2C, the initial N/P = 1.15. As the charging rate increases, the graphite capacity decays too quickly, and N/P < 1 will appear (3 C charging N/P = 1, 4 C charging N/P =0.5), thus lithium evolution is easy to occur (Figure 2D).

　　In addition, the author also used a symmetrical cell to study the EIS spectra of NMC811 and graphite at different temperatures. Comparing Figure 3A and Figure 3B, it can be found that although graphite is an important factor limiting the fast charging capability of the battery, it has a small charge transfer resistance at each test temperature, indicating that the charge transfer resistance is not a factor limiting the fast charging performance of graphite. Figure 3C shows the Arrhenius relationship of NMC811 and graphite symmetric cells at different temperatures, where the slope represents the desolvation energy of each electrode. Although the desolvation energy of Li+ on graphite is small, considering that the thickness of the graphite negative electrode is greater than the thickness of the NMC811 positive electrode, diffusion and lithium salt consumption at high charging rates will become important factors limiting fast charging.

　　Increasing the positive electrode load is one of the effective ways to increase battery energy density. However, as shown in Figure 3D, for NMC532, as the load increases, the capacity attenuation at high rates becomes more and more obvious; and because NMC811 has a higher volumetric energy density, its capacity attenuation is weaker than that of NMC532 at the same load and high rates. a lot of. Therefore, the loading amount and type of cathode material will also affect the fast charging characteristics of the battery and should also be considered during battery design.

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