However, due to the nature of the raw materials used, capacity increases are always limited. Therefore, increasing the voltage becomes another way to improve the energy storage capacity of lithium batteries. As we know, the nominal voltage of lithium batteries is 3.6V or 3.7V, with a maximum voltage of 4.2V. So why can't the voltage of lithium batteries achieve a greater breakthrough? Ultimately, this is determined by the material and structural properties of lithium batteries.
The voltage of a lithium battery is determined by its electrode potential. Voltage, also known as potential difference, is a physical quantity that measures the energy difference caused by the difference in potential between charges in an electrostatic field. The electrode potential of lithium ions is approximately 3V, and the voltage of a lithium battery 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 level and the highest occupied level of the electrolyte. Then, the maximum voltage of the lithium 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 prevented by the presence of the "solid-liquid interface layer," which inhibits further electron movement between the electrolyte and the positive and negative electrodes, thus protecting the electrode materials. In other words, a mild "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 by too much. For example, the reason why graphite is mostly used as the negative electrode material in current lithium-ion batteries is that its electrochemical potential relative to the 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 stopping the continued development of the polarization reaction. However, 5V high-voltage positive electrode materials exceed the Eg range of current commercial organic electrolytes by a significant margin, making them highly susceptible to oxidation during charge and discharge. With increasing charge and discharge cycles, capacity decreases and lifespan is reduced.
Now it's clear that the open-circuit voltage of lithium-ion batteries is chosen to be 4.2V because the Eg range of existing commercial lithium battery electrolytes is 1V~4.5V. Setting the open-circuit voltage to 4.5V might increase the output power of the lithium battery, but it also increases the risk of overcharging. The dangers of overcharging have been explained in a great deal of information, so I won't go into details here.
Based on the above principles, there are only two ways to increase the energy density of lithium batteries by increasing the voltage value: one is to find an electrolyte that can be matched with high-voltage cathode materials, and the other is to carry out protective surface modification of the battery.
