Power Battery Charging and Discharge Process as the Core Component of New Energy

In 2018, the new energy vehicle industry began to smoke, and Changsheng became a heavy weapon for various car companies to compete for the domestic market. Big car companies are using their new, ultra-long-range models to attract a growing number of high-end consumers. At the end of February, Teng 500 officially debuted; At the end of March, Geely officially launched the new EV 450 model; In early April, BYD launched three new models of the Qin EV450, e5450, and Song EV400 in one breath, all of which lasted more than 400 kilometers.

But from a technical point of view, the power battery is the core, is the key to the decision of electric vehicles with ultra-long endurance. Taking AC slow charging and DC fast charging as examples, the correct and suitable use method can not only maximize the power of the power cell, but also extend 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 impact of various battery materials on the charging and discharging ability, so as to cultivate correct usage habits and extend the service life of power batteries. Ensure that electric vehicles continue for a long time.

Charging and discharging electrons escape each other

At present, there are two main types of power batteries used by major electric vehicle companies. One is the lithium iron phosphate battery and the other is the ternary lithium battery. However, regardless of the type of battery, the charging process can be roughly the following four stages, namely the constant current charging phase, the constant pressure charging phase, the full phase, and the floating charging phase.

In the constant current charging phase, the charging current remains constant, the charging power increases rapidly, and the battery voltage also rises. At the constant voltage charging stage, as the name implies, the charging voltage will remain constant. Although the charge will continue to increase, the battery voltage will rise slowly and the charging current will decrease. At the battery filling stage, the charging current drops below the floating charge conversion current, and the charging voltage of the charger is reduced to floating charge voltage. During the charging phase, the charging voltage will remain floating.

The charging and discharging process of a lithium ion battery is a process of intercalating and deintercalating lithium ions. In the process of intercalation and deintercalation of lithium ions, concomitant insertion and deintercalation of equivalent electrons with lithium ions (commonly referred to as embedding or deintercalation of the positive electrode and insertion or deintercalation of the negative electrode). During the entire charging process, the electrons on the positive electrode will run through the external circuit to the negative electrode. The positive lithium ion Li+ passes through the electrolyte from the positive electrode, passes through the separator material, and finally reaches the negative electrode, where it stays in conjunction with the "resident" electrons. Together, it is reduced to Li inlaid in the carbon material of the negative electrode. The data show that the carbon as the negative electrode has a layered structure, and it has many micropores, and lithium ions reaching the negative electrode are embedded in the micropores of the carbon layer, and the more lithium ions are embedded, the higher the charging capacity.

On the contrary, when the battery is discharged (ie, the process of using the battery), Li embedded in the negative carbon material loses electrons, the electrons on the negative pole "move" through the external circuit to the positive pole, and the positive lithium ion Li + from the negative pole. Over the electrolyte, Cross the diaphragm material, reach the positive pole, and combine with the electronic electrons of the "resident". Similarly, the more lithium ions returned to the positive pole, the higher the discharge capacity.

Four major materials for efficiency

What is the role of various key materials(such as positive materials, negative materials, membranes, electrolytes, etc.) in the process of charging and discharging power cells?

The first is a positive polar material. For positive polar materials, its active substances are generally lithium manganese acid or lithium cobalt acid, lithium nickel cobalt manganese acid and other materials, and most mainstream products use lithium iron phosphate.

The second is the negative electrode material. The negative electrode material is roughly divided into carbon negative electrode, tin negative electrode, lithium transition metal nitride negative electrode, alloy negative electrode, nanometer negative electrode, and nanometer material. Among them, the negative electrode materials actually used in lithium ion batteries are basically carbon materials, such as artificial graphite, natural graphite, intermediate phase carbon microspheres, petroleum Coke, carbon fiber, and pyrolytic resin carbon. In terms of Nano oxide materials, it is reported that according to the latest developments in the new energy industry of lithium batteries in 2009, some companies have begun to use Nano titanium oxide and nano Silicon oxide to be added to traditional graphite, tin oxide, and nano carbon tubes. The charge and discharge times of lithium batteries are greatly improved.

The third is an electrolyte solution, usually using lithium salts such as lithium perchlorate(LiClO4), lithium hexafluorophosphate(LiPF6), and lithium tetrafluoroborate(LiBF4). Since the operating voltage of the battery is much higher than the decomposition voltage of water, lithium-ion batteries often use organic solvents, but organic solvents often destroy the graphite structure when charging, causing it to peel off, and forming a solid electrolyte film on its surface leads to electrode passivation. It may also cause safety problems such as flammable and explosive.

The fourth is the diaphragm, as one of the key components of the battery, the advantages of the diaphragm performance determine the interface structure and internal resistance of the battery, which in turn affects the battery's capacity, recycling performance, charging and discharging current density and other key characteristics. In general, there are several types of commonly used diaphragm, such as single-layer and multi-layer diaphragm. It is understood that some domestic companies will choose a slightly thicker diaphragm, some enterprises use diaphragm thickness up to 31 layers. Due to the high technical threshold of diaphragm production, there are still some gaps between domestic lithium-ion battery diaphragm technology and foreign countries.

The data show that the diaphragm is a specially shaped polymer thin film with a microporous structure. After absorbing the electrolyte, it can isolate the positive and negative poles to prevent short circuits. At the same time, the lithium-ion battery is provided with a microporous channel that realizes the function of charge and discharge and the performance of the ratio, so as to achieve the conduction of lithium ions. When the battery is overcharged or the temperature changes greatly, the diaphragm blocks the current conduction by closing the hole to prevent the explosion.

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