Over the past few decades, lithium-ion batteries with graphite electrodes have been the dominant energy storage technology, with higher efficiency, power density and longevity compared to other rechargeable batteries, such as nickel-cadmium or lead-acid. However, due to the synergistic effect of multiple degradation mechanisms, the battery resistance increases, cycle life and Colombian efficiency decrease. It has always been a challenge to meet the needs of electric vehicles (EV) for fast charging and large-capacity batteries. A mechanochemical model for anodic degradation of nickel-manganese-cobalt (NMC)/ graphite (C) batteries due to SEI growth, lithium plating/stripping, dead lithium storage, recombination OF SEI and rupture of lithium plating film during rapid charging has been reported by the Department of Mechanical Engineering at Iowa State University. "Performance degradation due to anodic failure mechanisms in lithium-ion batteries" was published in the Journal of Power Sources.
Figure 1: a) Represents the schematic diagram of negative electrode degradation of SEI during lithium plating and film cracking; b) Film stress and recombination model; c) Prediction of mechanical properties of composite SEI and lithium plating.
The degradation of the negative electrode generally occurs with the formation and growth of the SEI layer, which consumes the active lithium in the electrolyte. However, under rapid charging conditions where the negative electrode potential becomes negative, metallic lithium starts to be electroplated on graphite together with the SEI film. At the end of charging, the negative electrode potential becomes positive, and during the relaxation period, part of the plated lithium is peeled off. The stripped lithium is reversibly intercalated with the negative electrode. This phenomenon appears as a platform on the voltage-time curve. The remaining electroplated lithium either reacts with the electrolyte to form a new SEI or is trapped between the SEI layers and becomes unreactive. In both cases, irreversible loss of lithium will lead to a decrease in battery capacity. In addition, the fracture caused by the tensile stress of the SEI film due to particle expansion may also cause the consumption of lithium because a new SEI layer is formed on the newly exposed surface of the negative electrode.
Figure 2: Verification of the electrochemical model, prediction of C/20 voltage and time, and comparison with Ge2017.
Use the experimental data of Ge 2017 to establish the initial conditions of the model. The model predictions based on the above experimental parameters are compared with the voltage and time profile measurements reported in Figure 2 to prove the validity of the modeling assumptions and numerical realization. The predicted results of the model are in good agreement with the experimental results, indicating that the numerical implementation can approximate the response of the battery.
Figure 3: The relationship between the negative electrode potential and the charging half-cycle time at different charging rates.
When the charging rate increases from 1 C to 6 C, the anode potential of the graphite particles begins to decrease (Figure 3) and approaches 0 V. At a charging rate of 3 C, the anode potential becomes negative at the end of the charging cycle. As the charging rate increases, the steeper concentration gradient in the anode forces the potential to become increasingly negative in a significant proportion of the charging time. Compared with 3c-6c, the ratio of plating time is 29% ~ 92%.
Figure 4: a) voltage-time curve, b) voltage-time gradient curve of different charging rates during the relaxation period, c) charge-relaxation period current-time curve under 1C charging conditions, d) charge-relaxation period 6C Current-time curve under charging conditions.
The stripped lithium is either reversibly inserted into the electrode, or irreversibly reacts with the anode surface to form SEI. The irreversible part of the coating is unable to carry out further cycles of the battery, thereby reducing the coating efficiency and battery capacity.
Figure 5: Film growth prediction under different charging conditions:
a) Lithium plating film thickness and time; b) SEI film thickness and time; c) Film resistance and time; d) The area mass and C rate of dead lithium stored in the film.
At 1C and 2C charging rates, the SEI film is not plated with lithium, so the resistance of the SEI film increases monotonically with the increase in thickness. Since the lithium plate is charged at a faster rate, it creates a conductive channel through the thin film, thereby reducing the resistance of the thin film. As the volume fraction of lithium plating deposited during charging increases with the increase of C-rate, the film resistance decreases (Figure 5c). During the relaxation process, when the deposited lithium falls off the film, the resistance will be restored. For 3C, since a new SEI is formed during the peeling process, the sheet resistance after relaxation is slightly greater than the sheet resistance after charging. For 4C-6C, the thin film resistance has not fully recovered because some dead lithium is trapped between the SEI layers during the deposition process and has not been peeled off.
Figure 6: a) Lithium electroplating phase volume and time, b) Film hoop stress and time, c) SEI + coating strain energy changes with time at different C rates, d) Normalized film strain energy at different C rates Changes with time; e) The cracking current changes with time under different charging conditions; f) The current component changes with time at 6C.
Figure 7: Crack length contour plots under different C rates and initial SEI thickness
As the hoop stress increases with the C rate, the crack length (lcr) increases, the exposed electrode surface is more, and the crack current extracted is also larger. Crack length is a measure of the tendency of battery degradation and wear.
Figure 8: a) Electroplating efficiency and initial SEI thickness (considering/not considering cracks), b) Relative capacity and C rate (considering SEI-only, SEI + electroplating and SEI + electroplating + film crack models).
Summary and outlook
An anode degradation model that couples SEI growth with lithium electroplating/stripping, dead lithium storage, and SEI film rupture is reported to predict battery capacity. The model analyzes the synergistic coupling of different degradation mechanisms at high carbon rates and their effects on battery capacity. The results show that the negative electrode potential is negative during the charging process. During the relaxation process after charging, the electroplated lithium may be stripped through a reversible or irreversible reaction. The irreversible loss model of dead lithium storage and SEI formation was established. Through the fracture mechanics analysis of SEI film to predict the fracture of SEI. A thicker SEI film and a lower C rate reduce the tendency of the film to break, while a thinner SEI film and a higher C rate increase the possibility of breakage. Quantify the irreversible loss during the electroplating and stripping process by calculating the plating Coulomb efficiency. Under low charge conditions, SEI growth degradation model alone is sufficient to predict battery capacity degradation. However, at higher charging rates, SEI growth and electroplating and SEI cracking mechanisms need to be integrated into the degradation model to accurately predict relative capacity. The coupled mechanical-chemical model established in this study can help identify parameters and alleviate the conditions that lead to anode degradation during the rapid charging of lithium-ion batteries.
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