Basic knowledge of lithium battery
A battery that uses a lithium compound as the active material for the positive electrode plate, graphite and the like as the negative electrode plate material, and an anhydrous organic substance as the electrolyte is called a lithium ion battery, or lithium battery for short.
Since its introduction in the 1990s, lithium batteries have become a hot spot in the field of power battery applications due to their high energy density, long cycle life, no memory effect, and good environmental protection. At present, lithium batteries have become the main body of power batteries for electric vehicles.
According to the electrolyte used in lithium batteries, it can be divided into two categories: liquid lithium batteries (LIB) and polymer lithium batteries (LIP). The positive and negative materials of the above two lithium batteries are the same, and the basic principles are also similar.
There are many kinds of cathode materials for lithium batteries, mainly lithium cobalt oxide, lithium manganate, lithium nickel oxide, ternary materials (nickel, cobalt, manganese), lithium iron phosphate, etc. The corresponding names are lithium cobalt battery, lithium manganese battery And so on, the battery with ternary material as the positive electrode is called ternary lithium battery.
1). Structural principle
Take lithium iron phosphate battery (LiFPO4) as an example. Lithium iron phosphate power battery appeared in 2002, but it has been used in the electric vehicle industry for a long time. Therefore, many electric vehicles in the society currently use this type of battery. Lithium iron phosphate batteries have the following characteristics.
①High efficiency output. The discharge rate during standard discharge is (2~5)C, the discharge rate can reach 10C during continuous high current discharge, and the discharge rate can reach 20C during instantaneous pulse discharge (10s).
②Good performance at high temperature. When the external temperature is 65C, the internal temperature is as high as 95℃, and the temperature can reach 160℃ when the battery is discharged.
③The safety of the battery is good. Even if the battery is injured internally or externally, the battery will not burn or explode.
④After 500 cycles, the discharge capacity is still greater than 95%.
The structure and working principle of the lithium iron phosphate battery are shown in Figure 1. As the positive electrode of the battery, LiFePO4 is connected to the positive electrode of the battery by an aluminum box; in the middle is a polymer separator, which separates the positive electrode from the negative electrode, lithium ions (Li+) can pass, but electrons (e–) cannot pass; the negative electrode material is graphite , The copper foil is connected to the negative electrode of the battery. Between the upper and lower ends of the battery is the electrolyte of the battery, and the battery is hermetically sealed by a metal casing. When a lithium iron phosphate battery is charged, Li+ in the positive electrode migrates to the negative electrode through the polymer separator; during the discharge process, Li+ in the negative electrode migrates to the positive electrode through the separator. Lithium batteries are named after lithium ions move back and forth during charge/discharge.
The inside of a lithium battery is mainly composed of a positive electrode, a negative electrode, an electrolyte and a separator. The difference in the materials of the positive and negative electrodes and the electrolyte and the difference in the process make the battery have different performance, especially the positive electrode material has the greatest impact on the performance of the battery.
There are two types of lithium batteries, square and cylindrical, as shown in Figure 2 and Figure 3.
The anode materials of liquid lithium batteries use carbon materials, mainly graphite, microbead carbon, petroleum coke, carbon fiber, cracked polymer and cracked carbon, etc.; cathode materials mainly include LiCoO2, LiNiO2, LiMn₂O4, etc., of which LiCoO2 is widely used and is reversible. Performance, discharge capacity, charge/discharge rate, voltage stability, etc. are all very good. The electrolyte is liquid, and its solvent is anhydrous organic. The diaphragm adopts polyolefin microporous membrane, such as PE, PP or composite membrane. The shell is made of steel or aluminum, and the cover assembly has the function of welding and power failure.
Polymer lithium battery, also known as polymer lithium battery, is a second-generation lithium battery. The polymer lithium battery is composed of multiple films. The first layer is a metal box collector, the second layer is a negative electrode, the third layer is a solid electrolyte, the fourth layer is a positive electrode, and the fifth layer is an insulating layer. The negative electrode adopts polymer conductive materials, polyacetylene, artificial graphite, polyaniline or poly-p-phenol. The positive electrode adopts LiCoO2, LiNiO2, LiMn2O4 and Li(CFSO2)2, etc. The electrolyte is a colloidal electrolyte, such as LiPF6, organic carbonate mixture, etc.
2). Features and applications
a. The working voltage is high. The operating voltage of lithium battery cells is 3.6V, which is 3 times that of nickel-cadmium batteries and nickel-hydrogen batteries, and nearly twice that of lead-acid batteries.
b. Higher than energy. The specific energy is as high as 150W-h/kg, which is twice that of Ni-MH batteries and 3-4 times that of lead-acid batteries, so the weight is 1/4 to 1/3 of that of lead-acid batteries with the same energy; small size, energy density It is as high as 400W.hL, and its volume is 1/3~1/2 of that of lead-acid batteries.
c. Long cycle life. The number of cycles can reach 1,000 times (more than 10 years or 200,000 kilometers). In terms of capacity retention of 60%, the number of 100% charge/discharge cycles of the battery pack can reach more than 600 times, the service life can reach 3 to 5 years, and the life span is 2 to 3 times that of lead-acid batteries.
d. Low self-discharge rate. The monthly self-discharge of lithium batteries is only 6% to 8%, which is much lower than other types of power batteries.
e. No memory effect. Lithium batteries can be charged anytime, anywhere.
f. No pollution. There are no toxic substances in lithium batteries, so they are called “green batteries.”
s. Light weight. The battery pack provides a more reasonable structure and a more beautiful appearance of the design conditions and design space.
a. High cost. Lithium batteries are mainly expensive as cathode materials, but based on the price per unit of energy, they are only higher than lead-acid batteries.
b. Lithium batteries must have special protection circuits to prevent overcharging.
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At present, there are three main types of lithium power batteries used by global automakers, namely, nickel diamond lithium aluminate batteries (lithium cobalt oxide batteries) represented by Tesla, lithium iron phosphate batteries represented by BYD, and Japan Lithium manganese oxide batteries represented by automobiles. For example, Nissan’s Leaf uses a stacked compact lithium battery pack composed of 48 battery cells, providing an output power of more than 90kW. In addition, electric vehicles such as Mitsubishi’s i-MiEV and Chevrolet Volt use lithium batteries. Figure 4 shows the lithium battery used in the MercedesBenz S400 Hybrid hybrid vehicle.