6F22 carbon battery

Evaluation of factors limiting the fast charging capability of 6F22 carbon battery

Cost, energy density, and fast charging characteristics have always been the three important indicators for evaluating power batteries. A careful observation of the launch conferences of major brands of electric vehicles in recent years shows that fast charging has always been one of the selling points that merchants have been working hard to promote. Recently, researchers from Oak Ridge National Laboratory and University of Tennessee have 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 journals in the field of electrochemistry, and the current editor-in-chief is Professor R.G. Compton of Oxford University. Although the journal has a low impact factor (IF=4.6), most of the papers published are concise and innovative, so they are loved by many people. Highlights: (1) It is fully proved that graphite is the main limiting factor of the battery's fast charging capability; (2) Due to the rapid capacity decay of the graphite negative electrode at high-rate charging, N/P may be less than 1, which is prone to lithium plating; (3) The design of battery fast charging must also consider diffusion problems, lithium salt consumption, material selection and load capacity under high-rate charging. In order to further accurately evaluate the capacity characteristics of the positive and negative electrodes at different charging rates and eliminate the influence on the electrodes, the authors took the positive and negative electrodes of the 50% SOC full battery and made them into symmetrical batteries. Figures 2A and 2B are the charge and discharge curves of NMC811 and graphite symmetrical batteries, respectively, and Figure 2C shows the capacity density decay and N/P ratio changes of NMC811 and graphite at different rates. Similar to the results of the power-withdrawal test, when the charging rate is higher than 1C, the capacity of graphite decreases sharply, while NMC811 has a good capacity retention from 1/10C to 4C. In order to avoid lithium plating, the N/P ratio is always greater than 1 when designing the battery. However, as shown in Figure 2C, the initial N/P=1.15, as the charge rate increases, the graphite capacity decays too quickly, and the phenomenon of N/P<1 will occur (3C charging N/P=1, 4C charging N/P=0.5), which makes lithium precipitation very easy to occur (Figure 2D). In addition, the authors also used symmetric batteries 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 ability 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 batteries 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 charge rates will become important factors limiting fast charging. Increasing the positive electrode loading is one of the effective ways to increase the energy density of the battery. However, as shown in Figure 3D, for NMC532, as the load increases, the capacity decay at high rates becomes more obvious; and because NMC811 has a higher volume energy density, its capacity decay at the same load and high rate is much weaker than that of NMC532. Therefore, the load and type of positive electrode materials will also affect the fast charging characteristics of the battery, and should also be considered when designing the battery.

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