The most comprehensive and classic analysis of the reasons for capacity degradation in lithium-ion batteries

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The most comprehensive and classic analysis of the reasons for capacity degradation in lithium-ion batteries: The fundamental reason is that lithium-ion batteries have different insertion energies when insertion reactions occur between their two electrodes; in order to achieve optimal battery performance, the capacity ratio of these two electrodes must be maintained at an equilibrium level. In lithium-ion batteries, capacity balance is expressed as the mass ratio of the cathode to the anode, that is: γ = m+/m- = ΔxC- / ΔyC+. Here, C refers to the theoretical coulombic capacity of the electrode, while Δx and Δy represent the stoichiometric coefficients of lithium ions inserted into the anode and cathode, respectively. As can be seen from the above equation, the mass ratio required for the two electrodes depends on their respective Coulomb capacities and the number of reversible lithium ions in each. Generally speaking, a smaller mass ratio leads to incomplete utilization of the anode material ； A higher mass ratio may pose safety risks due to overcharging of the negative electrode. In short, the battery performance is best at the optimized quality ratio. For an ideal Li-ion battery system, the capacity remains unchanged throughout its cycle life, with a constant initial capacity in each cycle; however, the actual situation is much more complex. Any side reaction that can generate or consume lithium ions or electrons may lead to changes in the battery’s capacity balance. Once this balance is altered, such changes are irreversible, and they can accumulate over multiple cycles, having a severe impact on the battery’s performance. Reply to view the full text [Note: If you like it, please consider giving a positive rating as well.] In lithium-ion batteries, in addition to the redox reactions that occur during the insertion and extraction of lithium ions, there are numerous side reactions as well, such as electrolyte decomposition, dissolution of active materials, and deposition of metallic lithium, as shown in Figure 1. Arora et al. compared these processes of capacity degradation with the discharge curves of half-cells, allowing us to clearly see the possibility of capacity degradation during battery operation and its underlying causes. 1. Overcharging: 1. Overcharging reaction of the graphite anode: When a battery is overcharged, lithium ions tend to be reduced and deposited on the surface of the anode: Li++e→Li(s). The deposited lithium covers the anode surface, preventing lithium from intercalating. The reasons for reduced discharge efficiency and capacity loss include: ① A decrease in the amount of recyclable lithium ； ②The deposited metallic lithium reacts with the solvent or supporting electrolyte to form Li2CO3, LiF, or other products ； ③Lithium metal typically forms between the negative electrode and the separator, which may block the pores of the separator and increase the internal resistance of the battery. ④Due to the highly reactive nature of lithium, it readily reacts with the electrolyte, consuming it. This leads to a decrease in discharge efficiency and a loss of capacity. With fast charging, the current density is too high, causing severe polarization at the negative electrode, and lithium deposition becomes more pronounced. This situation tends to occur when the cathode active material is in excess relative to the anode active material. However, at high charging rates, metal lithium deposition may still occur even if the ratio of the cathode and anode active materials is normal. 2. Positive electrode overcharging reaction: When the ratio of the positive electrode active material to that of the negative electrode active material is too low, positive electrode overcharging is likely to occur. Capacity loss due to overcharging of the cathode is mainly caused by the formation of electrochemically inert substances (such as Co3O4, Mn2O3, etc.), which disrupts the capacity balance between the electrodes; this capacity loss is irreversible. (1) LiyCoO2 → (1-y)/3 + yLiCoO2y