Why can't the maximum voltage of a lithium battery break through 4.2V?

The voltage of a lithium battery is determined by the electrode potential. Voltage, also known as potential difference or potential difference, is a physical quantity that weighs the energy difference of electric charges in an electrostatic field due to different potentials. The electrode potential of lithium ions is about 3V, and the voltage of lithium batteries varies with different materials. For example, a general lithium battery has a rated voltage of 3.7V and a full-charge voltage of 4.2V; while a lithium iron phosphate battery has a rated voltage of 3.2V and a full-charge voltage of 3.65V. In other words, the potential difference between the positive electrode and the negative electrode of a lithium battery in practical use cannot exceed 4.2V, which is a requirement based on the safety of materials and use. If the Li/Li+ electrode is used as the reference potential, μA is the relative electrochemical potential of the negative electrode material, μC is the relative electrochemical potential of the positive electrode material, and the electrolyte potential interval Eg is the lowest electron unoccupied energy level and the highest electron occupied energy of the electrolyte. The difference between the levels. Then, it is the three factors of μA, μC, and Eg that determine the highest voltage value of the lithium battery. The difference between μA and μC is the open circuit voltage (the highest voltage value) of the lithium battery. When this voltage value is within the Eg range, the electrolyte can be guaranteed to work normally. Normal operation means that the lithium battery moves back and forth between the positive and negative electrodes through the electrolyte, but does not undergo oxidation-reduction reactions with the electrolyte, thereby ensuring the stability of the battery structure. The electrochemical potential of the positive and negative materials causes the electrolyte to work abnormally in two forms: 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 is oxidized, and the reaction product forms a solid-liquid interface layer on the surface of the negative electrode material particles, which may cause damage to the negative electrode. 2. When the electrochemical potential of the positive electrode is lower than the highest electron-occupied energy level of the electrolyte, the electrons in the electrolyte will be captured by the positive electrode and then oxidized by the electrolyte. The reaction product forms a solid-liquid interface layer on the surface of the positive electrode material particles, resulting in The positive electrode may be damaged. However, the possibility of damage to the positive or negative electrode is due to the existence of the solid-liquid interface layer, which prevents the further movement of electrons between the electrolyte and the positive and negative electrodes, and instead protects the electrode material. The liquid interface layer is protective. The premise of this protection is that the electrochemical potential of the positive and negative electrodes can slightly exceed the Eg interval, but not too much. For example, the reason why most of the current lithium battery anode materials use graphite is because the electrochemical potential of graphite related to Li/Li+ electrodes is about 0.2V, which is slightly beyond the Eg range (1V~4.5V), but because of its protective effect The solid-liquid interface layer prevents the electrolyte from being further reduced, thereby stopping the continuous development of the polarization reaction. However, the 5V high-voltage cathode material exceeds the Eg range of the current commercial organic electrolyte by too much, so it is easily oxidized during charging and discharging. As the number of charging and discharging increases, the capacity decreases and the lifespan decreases. I now understand that the open circuit voltage of the lithium battery is chosen to be 4.2V because the range of the electrolyte Eg of the existing commercial lithium battery is 1V~4.5V. If the open circuit voltage is set to 4.5V, the power output of the lithium battery may be increased, but It also increases the risk of overcharging the battery, and the hazards of overcharging have been explained by a lot of data, so I won't say more here. Disclaimer: Some pictures and content of the articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous: What are the processes for assembling lithium iron phosphate batteries