Why can't the highest voltage of lithium ion battery break through 4.2v?

The parameter to describe the energy storage size of lithium ion battery is energy density, which is approximately equivalent to the product of voltage and lithium battery capacity in numerical value. In order to effectively improve the storage capacity of lithium battery, people will generally increase the battery capacity to achieve the purpose. However, due to the nature of the raw materials used, there is always a limit to the capacity increase, so increasing the voltage value becomes another way to improve the capacity of lithium battery storage. As you know, the nominal voltage of lithium battery is 3.6v or 3.7v, and the maximum voltage is 4.2v. So why isn't the voltage of lithium batteries getting a bigger breakthrough? Ultimately, this is also determined by the material and structural properties of lithium batteries.

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 of charge in electrostatic field due to different potential. The electrode potential of lithium ion is about 3V, and the voltage of lithium battery varies with different materials. For example, the rated voltage of general lithium-ion batteries is 3.7v and the full voltage is 4.2v; The rated voltage of lithium iron phosphate battery is 3.2v and the full voltage is 3.65v. In other words, the potential difference between the positive electrode and the negative electrode of a practical lithium ion battery cannot exceed 4.2v, which is a requirement based on the material and safety of use.

If Li/Li+ electric pole reference potential, set mu A as the negative material relative electrochemical potential, mu C as the positive material relative electrochemical potential, electrolyte potential interval Eg is the difference between the lowest electron occupied level and the highest electron occupied level. So, the maximum voltage of lithium battery is the three factors: mu A, mu C, mu Eg.

The difference between mica and micc is the open circuit voltage (maximum voltage) of the li-ion battery. When the voltage is within Eg range, the electrolyte can work normally. "Normal operation" means that the lithium ion battery moves back and forth between positive and negative electrodes through the electrolyte, but does not REDOX with the electrolyte to ensure the stability of the battery structure. The electrochemical potential of anode and cathode materials results in abnormal electrolyte operation in two forms:

1. When the electrochemical potential of the negative electrode is higher than the lowest electron occupied energy level of the electrolyte, the electrons of the negative electrode will be taken by the electrolyte, so that the electrolyte is oxidized, and the reaction product forms a "solid liquid interface layer" on the particle surface of the negative electrode material, which may lead to the destruction of 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, which will be oxygenated by the electrolyte, and the reaction products will form a "solid-liquid interface layer" on the particle surface of the positive electrode material, which may lead to the destruction of the positive electrode.

However, the possibility that the positive or negative electrode may be damaged prevents the electrons from further moving between the electrolyte and the positive and negative electrodes due to the existence of the solid-liquid interface layer, which protects the electrode material instead. In other words, the light solid-liquid interface layer is "protective". The protective premise is that the electrochemical potential of the positive and negative electrodes can slightly exceed the Eg range, but not too much. Now, for example, the lithium ion battery anode materials are mostly use graphite, is because the graphite relative to the electrochemical potential of the Li/Li + electrode is about 0.2 V, slightly beyond the Eg interval (1 V ~ 4.5 V), but because of the "protective" "solid-liquid interface layer", make the electrolyte will not be further reduction, to stop the development of polarization reaction. However, the 5V high-voltage anode material exceeds the Eg range of commercial organic electrolytes too much, so it is easy to be oxidized in the charging and discharging process. As the number of charging and discharging increases, the capacity decreases and the service life decreases.

Now understand that the open circuit voltage of lithium ion battery was chosen as 4.2 V, because the existing commercial lithium battery electrolyte Eg range from 1 V to 4.5 V, if might be able to set the open circuit voltage of 4.5 V lithium battery output power, but also increase the risk of battery overcharge, and the harm of overcharge considerable data has shown that there is no longer say more.

According to the above principles, there are only two ways for people to improve the energy density of lithium battery by increasing the voltage value. One is to find electrolyte that can match the positive electrode material with high voltage value; the other is to carry out protective surface modification on the battery.