Lithium-ion batteries have different intercalation energies when the intercalation reaction occurs between the two electrodes, and in order to obtain the best performance of the battery, the capacity ratio of the two host electrodes should maintain a balanced value. In lithium-ion batteries, the capacity balance represents the mass ratio of the positive electrode to the negative electrode, namely:
In the formula, C refers to the theoretical Coulomb capacity of the electrode, and Δx and Δy refer to the stoichiometric numbers of lithium ions intercalated in the negative and positive electrodes, respectively. It can be seen from the above formula that the required mass ratio of the two poles depends on the corresponding Coulomb capacity of the two poles and the number of their respective reversible lithium ions. Generally speaking, a smaller mass ratio leads to incomplete utilization of the anode material; a larger mass ratio may cause safety hazards due to overcharging of the anode. In short, at the optimal mass ratio, the battery performance is the best.
For an ideal Li-ion battery system, the capacity balance does not change during its cycle, and the initial capacity in each cycle is a certain value, but the actual situation is much more complicated. Any side reaction that can generate or consume lithium ions or electrons may lead to changes in the battery capacity balance. Once the battery capacity balance state changes, this change is irreversible and can be accumulated through multiple cycles, which will have a negative impact on battery performance. Serious impact.
In lithium-ion batteries, in addition to the redox reactions that occur when lithium ions are deintercalated, there are also a large number of side reactions, such as electrolyte decomposition, active material dissolution, metal lithium deposition, etc.
Overcharge reaction of graphite negative electrode:
① The amount of recyclable lithium is reduced;
② Deposited lithium metal reacts with solvent or supporting electrolyte to form Li2CO3, LiF or other products;
③ Metal lithium is usually formed between the negative electrode and the separator, which may block the pores of the separator and increase the internal resistance of the battery.
④ Due to the active nature of lithium, it is easy to react with the electrolyte and consume the electrolyte. This will lead to a decrease in discharge efficiency and loss of capacity. Fast charging, excessive current density, severe polarization of the negative electrode, lithium deposition will be more obvious. This situation tends to occur when the positive electrode active material is in excess relative to the negative electrode active material. However, in the case of high charging rates, metal lithium deposition may occur even if the ratio of positive and negative electrode active materials is normal.
2. Positive electrode overcharge reaction
When the ratio of positive electrode active material to negative electrode active material is too low, positive electrode overcharge is easy to occur.
3. Oxidation reaction of electrolyte when overcharged
Oxidation of any solvent will increase the concentration of the electrolyte, decrease the stability of the electrolyte, and ultimately affect the capacity of the battery.
1. The electrolyte decomposes on the positive electrode
2. The electrolyte decomposes on the negative electrode
Self-discharge refers to the natural loss of capacity when the battery is not in use. There are two cases of capacity loss caused by self-discharge of lithium-ion batteries: one is the loss of reversible capacity; the other is the loss of irreversible capacity. Reversible capacity loss means that the lost capacity can be recovered during charging, while irreversible capacity loss is the opposite. The positive and negative electrodes may have a micro-battery effect with the electrolyte in the charged state, lithium ion intercalation and deintercalation occur, and positive and negative electrodes intercalate and deintercalate. The intercalated lithium ions are only related to the lithium ions in the electrolyte, so the capacity of the positive and negative electrodes is unbalanced, and this part of the capacity loss cannot be recovered during charging.
The positive electrode active material will oxidize and decompose the electrolyte in the charged state, resulting in capacity loss
Copper and aluminum are the most commonly used materials for negative and positive current collectors, respectively.
The current collector corrosion is related to the electrolyte. In the LiPF6-EC/DMC electrolyte, the aluminum foil can be corroded at a voltage of 4.2V (vs. Li/Li+); while in LiBF4-EC/DMC and LiClF4-EC/DMC, the low The voltage of 4.9V cannot corrode aluminum foil, because LiPF6 is easy to generate HF