Research on the performance of soft-packaged lithium-ion batteries

　　Lithium-ion batteries are undoubtedly a new generation of batteries with a high technological content and a wide range of applications. It has high volume ratio energy and mass ratio energy, rechargeable and pollution-free, and possesses the three major advantages of the current battery industry development. It is called "the most promising chemical power source".

　　Lithium-ion battery is a new type of rechargeable battery developed in the past ten years. It has high specific energy, no memory effect, and is green and environmentally friendly. Its discharge voltage is three times that of nickel-cadmium and nickel-hydrogen batteries. Widely noticed, the development is very rapid. At present, only a few countries have achieved industrialization, of which Japan has done the best in industrialization. China is the earliest developing country for the industrialization and development of lithium-ion batteries. After recent years of development, especially under the active promotion of relevant national policies, domestic lithium-ion batteries have made rapid progress, and the various performances of products have made considerable progress. The production chain of lithium-ion batteries has become more and more perfect. The degree of modernization has increased day by day, forming a trend of competing with developed countries in the world. With abundant natural resources and low labor costs, China's lithium-ion battery industry has obvious advantages in international competition.　　 Lithium-ion batteries are undoubtedly a new generation of batteries with a high technological content and a wide range of applications. It has high volume ratio energy and mass ratio energy, rechargeable and pollution-free, and possesses the three major advantages of the current battery industry development. It is called "the most promising chemical power source". There are currently two common lithium-ion battery technologies: liquid lithium-ion batteries with metal shells and polymer lithium-ion batteries. The shell of liquid lithium-ion battery is generally steel shell or aluminum shell, and its plasticity is not as good as polymer lithium-ion battery. However, the manufacturing process of polymer lithium-ion batteries is complicated, especially the low yield, which greatly restricts the industrialization of this battery. The author first recommends the soft-package lithium-ion battery, which is characterized by the comprehensive basic electrochemical performance of liquid lithium-ion batteries and the unique advantages of polymer lithium-ion battery packaging, and uses liquid electrolyte similar to liquid lithium-ion batteries as lithium ion The medium that is transmitted between the positive and negative electrodes, and the outer packaging adopts a multi-layer plastic composite film. The outstanding advantages of soft-packed lithium-ion batteries are their simple manufacturing process, low cost, and high yield. Its goal is to replace the polymer lithium-ion battery market and compete with stainless steel-case lithium-ion batteries. The comparison of three different technologies of stainless steel shell lithium ion battery, polymer lithium ion battery and soft package lithium ion battery is listed in Table 1. This paper studies the various electrochemical properties of the soft-packaged lithium-ion battery and the differences between it and the stainless steel-cased lithium-ion battery to verify the possibility of industrialization of the soft-packaged lithium-ion battery.

　　1, experiment 　　The positive electrode material used in the experimental battery is lithium cobalt oxide, and the negative electrode material is artificial spherical graphite. LiPF/EC/DEC/DMC is the electrolyte, in which LiPF is a conductive salt, and EC (ethylene carbonate)/DEC (diethyl carbonate)/DMC (dimethyl carbonate) is a composite solvent. The outer packaging of the battery is a composite aluminum plastic film, and its structure is five layers of PET (polyester) / glue / Al / glue / PP (polypropylene). The battery tab sealing glue is a modified polyolefin material.　　The battery manufacturing process includes batching, mixing, coating, rolling, slitting, spot welding, winding, sealing, forming, degassing, and volume division. A lithium-ion battery with a stainless steel case was fabricated at the same time under similar process conditions. The size of the soft-packaged lithium-ion battery and the stainless steel-cased lithium-ion battery are both 48mm&TImes;30mm&TImes;4.1mm. The nominal capacity of the two batteries is 500mA·h.　　 The most critical technology in the manufacturing process of soft-packaged lithium-ion batteries is how to overcome the problem of air swelling caused by the battery. On the one hand, the air swelling problem of the battery is related to whether the battery is well sealed, and on the other hand, it is related to the way the battery is formed. In the experiment, the modified polypropylene material was used as the sealing glue between the positive and negative electrode tabs and the composite packaging film (generally, the bonding ability between the metal material and the inner polypropylene of the aluminum-plastic packaging film is poor), which is a good solution The sealing problem of the battery. In the experiment, a pre-charge method was used to form the protective film of the negative electrode material. During the formation of the protective film, a part of the gas will be generated, and then the generated gas will be removed in a vacuum. In this way, the problem of battery gas swelling caused by the formation method is eliminated. 2. Results and discussion 2.1 Rate discharge performance Figure 1 is the rate discharge curve of two lithium-ion batteries, where C is the battery capacity and the voltage. The battery is charged to 4.2V by 1C (500mA) before the test starts. , And continue constant voltage charging for 2h at this voltage, and then stand still for 2h.