Recently, researchers from 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 lithiumion batteries for extreme fastging*, *Electrochemistry Communications*, 2018, 97:37-41. *Electrochemistry Communications* is one of the few communication journals in the field of electrochemistry, currently edited by Professor R.G. Compton of Oxford University. Although the journal's impact factor is not high (IF=4.6), its published papers are mostly concise, clear, and highly innovative, thus attracting many readers.
Highlights:
(1) This fully demonstrates that graphite is a significant limiting factor for the fast charging capability of batteries;
(2) High-rate charging can lead to lithium plating because the graphite anode capacity decays too quickly and the N/P ratio may be less than 1.
(3) When designing a fast-charging battery, it is also necessary to consider the diffusion problem under high-rate charging, lithium salt consumption, material selection and load capacity, etc.
To further accurately evaluate the capacity characteristics of the positive and negative electrodes under different charging rates and to eliminate the influence on the electrodes, the authors used the positive and negative electrodes of a 50% SOC full cell to fabricate symmetrical cells. Figures 2A and 2B show the charge-discharge curves of NMC811 and graphite symmetrical cells, respectively, while Figure 2C shows the capacity density decay and N/P ratio changes of NMC811 and graphite at different charging rates. Similar to the results of coin cell charging, the capacity of graphite decreases sharply when the charging rate exceeds 1C, while NMC811 maintains good capacity retention from 1/10C to 4C. To prevent lithium plating, the N/P ratio is designed to be greater than 1. However, as shown in Figure 2C, the initial N/P = 1.15. As the charging rate increases, the capacity decay of graphite decreases too quickly, resulting in an N/P < 1 (N/P = 1 at 3C charging and N/P = 0.5 at 4C charging), which easily leads to lithium plating (Figure 2D).
Furthermore, the authors investigated the EIS spectra of NMC811 and graphite at different temperatures using symmetric cells. Comparing Figures 3A and 3B reveals that although graphite is a significant factor limiting the fast-charging capability of the battery, it exhibits relatively low charge transfer resistance at all test temperatures, indicating that charge transfer resistance is not a limiting factor for graphite's fast-charging performance. Figure 3C shows the Arrhenius relationship between 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 relatively low, considering that the graphite anode thickness is greater than the NMC811 cathode thickness, diffusion and lithium salt consumption at high charge rates will become significant limiting factors for fast charging.
Increasing the cathode loading is one of the effective ways to improve battery energy density. However, as shown in Figure 3D, for NMC532, the capacity decay becomes more pronounced at high rates as the loading increases; while NMC811, due to its higher volumetric energy density, exhibits much weaker capacity decay at the same loading and high rate compared to NMC532. Therefore, the cathode material loading and type also affect the battery's fast-charging characteristics and should be considered during battery design.
