The parameter that expresses the energy storage capacity of lithium-ion batteries (hereinafter referred to as lithium batteries) is energy density, which is approximately equivalent to the product of voltage and lithium battery capacity. In order to effectively increase the storage capacity of lithium batteries, people generally use increasing battery capacity The method achieves the goal. However, due to the nature of the raw materials used, the capacity increase is always limited, so increasing the voltage value becomes another way to improve the storage capacity of lithium batteries. As everyone knows, 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 greater breakthrough? After all, 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 difference in energy produced by different electric potentials in an electrostatic field. 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 material and use safety.
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 work" 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 ways:
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, and the electrolyte will be oxidized, and the reaction product will form a "solid-liquid interface layer" on the surface of the negative electrode material particles. As a result, the negative electrode may be damaged.
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. As a result, the positive electrode may be damaged.
However, this possibility of damage to the positive electrode 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, that is, to a lesser degree 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 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 relative to the Li/Li+ electrode is about 0.2V, which is slightly beyond the Eg range (1V~4.5V), but because of the "protective nature" The "solid-liquid interface layer" prevents the electrolyte from being 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 charge and discharge process. As the number of charge and discharge increases, the capacity decreases and the lifespan decreases.
Now I understand that the open circuit voltage of the lithium battery was chosen to be 4.2V because the existing commercial lithium battery electrolyte Eg range 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.
According to the above principles, if people want to increase the energy density of lithium batteries by increasing the voltage value, there are only two ways to find, one is to find an electrolyte that can match the high voltage value of the cathode material, and the other is to provide a protective surface for the battery. modified.
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