As the core component of new energy, the charging and discharging process of power lithium battery
In 2018, the field of new energy vehicles is full of gunpowder, and long battery life has become a heavy duty for various car companies to compete for the domestic market. Major car companies are attracting more and more high-end consumers with new models with ultra-long battery life. At the end of February, the Denza 500 was officially unveiled; at the end of March, Geely officially launched the new Emgrand EV450 model; at the beginning of April, BYD launched three new models, Qin EV450, e5450 and Song EV400, with a battery life of more than 400 kilometers.
However, from a technical point of view, the power battery is the core and the key to determining the ultra-long battery life of electric vehicles. Taking the two charging methods of AC slow charging and DC fast charging as an example, the correct and appropriate use method can not only maximize the power of the power battery, but also prolong the service life of the battery. From the perspective of knowledge popularization, on the basis of the current energy density technology level of power batteries, it is necessary to let consumers understand the charging and discharging process of power batteries and the influence of various battery materials on the charging and discharging capacity, so as to cultivate correct usage habits and prolong power The service life of the battery ensures the long-lasting battery life of the electric vehicle.
Charge and discharge electrons escape each other
At present, there are two popular types of power batteries used by major electric vehicle companies, one is lithium iron phosphate battery, and the other is ternary lithium battery. However, no matter what kind of battery it is, the charging process can be roughly divided into the following four stages, namely the constant current charging stage, the constant voltage charging stage, the full charging stage, and the floating charging stage.
In the constant current charging stage, the charging current is kept constant, the charging capacity increases rapidly, and the battery voltage also increases. In the constant voltage charging stage, as the name implies, the charging voltage will remain constant. Although the charged capacity will continue to increase, the battery voltage will rise slowly and the charging current will also decrease. When the battery is fully charged, the charging current drops below the float switching current, and the charger charging voltage drops to the float voltage. During the float charging phase, the charging voltage will remain at the float voltage.
The charging and discharging process of lithium ion batteries is the process of intercalation and deintercalation of lithium ions. In the process of intercalation and deintercalation of lithium ions, it is accompanied by the intercalation and deintercalation of electrons equivalent to lithium ions (usually the positive electrode is represented by intercalation or deintercalation, and the negative electrode is represented by intercalation or deintercalation). During the entire charging process, the electrons on the positive electrode will run to the negative electrode through the external circuit, and the positive lithium ions Li+ will pass from the positive electrode through the electrolyte, through the diaphragm material, and finally reach the negative electrode, where they stay and combine with the "resident" electrons Together, it is reduced to Li embedded in the carbon material of the negative electrode. The data shows that the carbon as the negative electrode has a layered structure, and it has many micropores. The lithium ions reaching the negative electrode are embedded in the micropores of the carbon layer. The more lithium ions are embedded, the higher the charging capacity.
On the contrary, when the battery is discharged (that is, the process of using the battery), the Li embedded in the negative electrode carbon material loses electrons, the electrons on the negative electrode "moves" to the positive electrode through the external circuit, and the positive lithium ion Li+ crosses the electrolyte from the negative electrode, It crosses the separator material, reaches the positive electrode, and combines with the "resident" electron electrons. Likewise, the more lithium ions returned to the positive electrode, the higher the capacity of the discharge.
Four materials to ensure efficiency
What role do various key materials (such as positive electrode materials, negative electrode materials, diaphragms, electrolytes, etc.) play in the process of charging and discharging power batteries?
The first is the positive electrode material. As far as the positive electrode material is concerned, the active material is generally lithium manganate or lithium cobaltate, lithium nickel cobalt manganate and other materials. The mainstream products mostly use lithium iron phosphate.
The second is the negative electrode material. The negative electrode material is roughly divided into carbon negative electrode, tin-based negative electrode, lithium transition metal nitride negative electrode, alloy negative electrode, nano-scale negative electrode, and nano-materials. Among them, the negative electrode materials actually used in lithium-ion batteries are basically carbon materials, such as artificial graphite, natural graphite, mesophase carbon microspheres, petroleum coke, carbon fiber, pyrolysis resin carbon, etc. As far as nano-oxide materials are concerned, it is reported that according to the latest market development trend of lithium battery new energy industry in 2009, some companies have begun to use nano-titanium oxide and nano-silicon oxide to add traditional graphite, tin oxide and carbon nanotubes. , greatly improving the charge-discharge capacity and the number of charge-discharge times of lithium batteries.
The third is an electrolyte solution, usually a lithium salt, such as lithium perchlorate (LiClO4), lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), and the like. Since the working voltage of the battery is much higher than the decomposition voltage of water, organic solvents are often used in lithium-ion batteries, but organic solvents often destroy the structure of graphite during charging, causing it to peel off, and form a solid electrolyte film on its surface, resulting in electrode passivation. . It may also bring safety problems such as flammability and explosion.
The fourth is the separator. As one of the key components of the battery, the advantages of the separator performance determine the interface structure and internal resistance of the battery, which in turn affects the battery capacity, cycle performance, charge and discharge current density and other key characteristics. Generally speaking, there are several types of commonly used separators, such as single-layer and multi-layer separators. It is understood that some domestic companies will choose slightly thicker diaphragms, and some companies use diaphragms with a thickness of 31 layers. Due to the high technical threshold of diaphragm production, there is still some gap between domestic lithium-ion battery diaphragm technology and foreign countries.
According to the data, the diaphragm is a specially formed polymer film with a microporous structure. After absorbing the electrolyte, it can isolate the positive and negative electrodes to prevent short circuits. At the same time, it provides a microporous channel for the lithium-ion battery to realize the charging and discharging function and rate performance, and realize the conduction of lithium ions. When the battery is overcharged or the temperature changes greatly, the separator blocks current conduction through closed pores to prevent explosion.
