The electrode potential of lithium-ion batteries is approximately 3V, and the voltage varies depending on the materials used. For example, a typical lithium-ion battery has a rated voltage of 3.7V and a fully charged voltage of 4.2V; while a lithium iron phosphate battery has a rated voltage of 3.2V and a fully charged voltage of 3.65V. In other words, the potential difference between the positive and negative electrodes of a practical lithium-ion battery cannot exceed 4.2V, a requirement based on materials and safety considerations.
If we take the Li/Li+ electrode as the reference potential, let μA be the relative electrochemical potential of the negative electrode material, μC be the relative electrochemical potential of the positive electrode material, and Eg be the difference between the lowest unoccupied energy level and the highest occupied energy level of the electrolyte. Then, the maximum voltage of a lithium-ion battery is determined by these three factors: μA, μC, and Eg.
The difference between μA and μC is the open-circuit voltage (maximum voltage value) of the lithium-ion battery. When this voltage value is within the Eg range, the electrolyte can operate normally. Normal operation means that the lithium-ion battery moves back and forth between the positive and negative electrodes through the electrolyte, but does not undergo redox reactions with the electrolyte, thus ensuring the stability of the battery structure. There are two forms of abnormal electrolyte operation caused by the electrochemical potential of the positive and negative electrode materials:
1. When the electrochemical potential of the negative electrode is higher than the lowest electron-unoccupied energy level of the electrolyte, the electrons of the negative electrode will be captured by the electrolyte, thus the electrolyte will be oxidized, and the reaction products will form a solid-liquid interface layer on the surface of the negative electrode material particles, which may lead to the damage of the negative electrode.
2. When the electrochemical potential of the positive electrode is lower than the highest electron-occupying energy level of the electrolyte, electrons in the electrolyte will be taken away by the positive electrode and oxidized by the electrolyte. The reaction products form a solid-liquid interface layer on the surface of the positive electrode material particles, which may lead to the destruction of the positive electrode.
However, the possibility of damage to the positive or negative electrode is mitigated by the presence of the solid-liquid interface layer, which prevents further electron movement between the electrolyte and the electrodes, thus protecting the electrode materials. In other words, a slightly thin solid-liquid interface layer is protective. This protection is contingent on the electrochemical potentials of the positive and negative electrodes slightly exceeding the Eg range, but not excessively. For example, the reason why graphite is currently the preferred negative electrode material for lithium-ion batteries is that the electrochemical potential of the graphite-related Li/Li+ electrode is approximately 0.2V, slightly exceeding the Eg range (1V~4.5V). However, due to the protective solid-liquid interface layer, the electrolyte is not further reduced, thus halting the continued development of the polarization reaction. However, 5V high-voltage positive electrode materials exceed the Eg range of currently commercially available organic electrolytes by a significant margin, making them highly susceptible to oxidation during charge and discharge. With each charge-discharge cycle, capacity decreases and lifespan is reduced.
Today I understand why the open-circuit voltage of lithium-ion batteries is chosen to be 4.2V. This is because the Eg range of the electrolyte in existing commercial lithium-ion batteries is 1V~4.5V. If the open-circuit voltage is set to 4.5V, it may increase the output power of lithium-ion batteries, but it also increases the risk of overcharging. The dangers of overcharging have been explained in a great deal of information, so I will not go into details here.
