The reason why the maximum voltage of lithium ion cells cannot exceed 4.2V?

The parameter describing the energy storage size of lithium-ion batteries (hereinafter referred to as lithium batteries) is energy density, which is approximately equivalent to the voltage of lithium ion cells and lithium battery capacity. method to achieve the goal. However, limited by the nature of the raw materials used, the capacity improvement is always limited, so increasing the voltage value has become another way to improve the power storage capacity of lithium batteries. As we all know, the nominal voltage of lithium batteries is 3.6V or 3.7V, and the highest voltage is 4.2V. So, why can’t the voltage of lithium batteries get a bigger breakthrough? In the final analysis, this is also determined by the material and structural properties of the lithium battery.

Lithium battery application

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 measures the energy difference between 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 voltage of 4.2V; while a lithium iron phosphate battery has a rated voltage of 3.2V and a full voltage of 3.65V. In other words, the potential difference between the positive electrode and the negative electrode of a practical lithium battery cannot exceed 4.2V, which is a requirement based on materials and safety of 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 range Eg is the lowest electron unoccupied energy level and the highest electron occupied energy level of the electrolyte. level difference. Then, the three factors that determine the highest voltage value of lithium batteries are μA, μC, and Eg.

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 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 a redox reaction 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:

  • When the electrochemical potential of the negative electrode is higher than the lowest electron of the electrolyte and does not occupy the energy level, the electrons of the negative electrode will be captured by the electrolyte, so the electrolyte is oxidized, and the reaction product forms a “solid-liquid interface layer” on the surface of the negative electrode material particles. As a result, the negative electrode may be damaged.
  • 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, and the reaction product will form a “solid-liquid interface layer” on the surface of the positive electrode material particles. As a result, the positive electrode may be destroyed.

However, this possibility of damage to the positive electrode or negative electrode prevents the further movement of electrons between the electrolyte and the positive and negative electrode due to the existence of the “solid-liquid interface layer”, and instead protects the electrode material, that is to say, to a lesser extent The “solid-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 range, 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 relative to the Li/Li+ electrode is about 0.2V, which is slightly beyond the Eg range (1V~4.5V), but because of its “protective properties” “Solid-liquid interface layer”, so that the electrolyte is not further reduced, thereby stopping the continued 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 the charging and discharging process.

Now I understand that the open circuit voltage of the lithium battery is selected as 4.2V because the Eg range of the existing commercial lithium battery electrolyte is 1V~4.5V. If the open circuit voltage is set to 4.5V, the output power of the lithium battery may be improved, but It also increases the risk of overcharging the battery, and the hazards of overcharging have been explained by a lot of information, so I won’t say more here.

According to the above principles, there are only two ways to improve the energy density of lithium batteries by increasing the voltage value. modified.

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