Research on the performance of the hottest flexibl

Research on the performance of flexible packaging lithium ion battery (I)

preface

Lithium ion battery is a new type of rechargeable battery developed in recent ten years. It has high specific energy, no memory effect, and is green and environmental friendly. However, if it is used too frequently, the discharge voltage is three times that of nickel cadmium and nickel hydrogen batteries. Therefore, it has received widespread attention and developed very rapidly once it came out At present, only a few countries have achieved industrialization, among which Japan has done the best China is the developing country with the earliest industrialization development of lithium ion batteries After the development in recent years, especially under the active promotion of relevant national policies, domestic lithium-ion batteries have made rapid progress, various performances of products have made considerable progress, the production chain of lithium-ion batteries has been improved day by day, and the degree of industrialization has been increased day by day, forming a situation of competing with the 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 high technological content and wide applications It has high volume specific energy and mass specific energy, rechargeable and pollution-free. It has three advantages of the current development of battery industry, and is called "the most promising chemical power supply" At present, there are two kinds of lithium-ion battery technology: liquid lithium-ion battery with metal shell and polymer lithium-ion battery The shell of liquid lithium-ion battery is generally steel shell or aluminum shell, which is less plastic than polymer lithium-ion battery The manufacturing process of polymer lithium-ion battery is complex, especially the low yield, which greatly limits the industrialization of this battery The author first introduced the flexible packaging lithium-ion battery, which is characterized by the combination of the basic electrochemical performance of liquid lithium-ion battery and the unique advantages of polymer lithium-ion battery in the shape packaging. The liquid electrolyte similar to liquid lithium-ion battery is used as the medium for the transmission of lithium ions between the positive and negative electrodes, while the shape packaging adopts multi-layer plastic composite film The outstanding advantages of flexible packaging lithium-ion battery are its simple manufacturing process, low cost and high yield. Its goal is to replace the market of polymer lithium-ion battery and form a competitive state with stainless steel shell lithium-ion battery The comparison of three different technologies of stainless steel shell lithium-ion battery, polymer lithium-ion battery and flexible packaging lithium-ion battery is listed in Table 1 In this paper, various electrochemical properties of flexible packaging lithium-ion batteries and the differences between flexible packaging lithium-ion batteries and stainless steel shell lithium-ion batteries are studied to verify the possibility of industrialization of flexible packaging lithium-ion batteries

1 experiment

the positive material of the experimental battery is lithium cobalt oxide, and the negative material is artificial spherical graphite Lipf6/ec/dec/dmc is the electrolyte, in which LiPF6 is the conductive salt and EC (vinyl carbonate)/dec (diethyl carbonate)/dmc (dimethyl carbonate) is the composite solvent The outer package of the battery is composite aluminum-plastic film, and its structure is five layers of pet (polyester)/glue/al/glue/pp (polypropylene) The sealing glue of battery lug is a modified polyolefin material

the battery manufacturing process includes batching, mixing, coating, rolling, slitting, spot welding, winding, sealing, formation, degassing and volume division Lithium ion batteries with stainless steel shells were fabricated at the same time according to similar process conditions The size of lithium-ion battery with flexible packaging and stainless steel shell is 48mm × 30ｍｍ × 4.1ｍｍ. The nominal capacity of the two batteries is 500mah

the most critical technology in the manufacturing process of flexible packaging lithium-ion batteries is how to overcome the problem of air inflation caused by batteries On the one hand, the problem of battery inflation is related to the good sealing of the battery, on the other hand, it is related to the formation mode of the battery In the experiment, the modified polypropylene material was used as the sealing adhesive between the positive and negative electrode ears and the composite packaging film (the bonding ability between general metal materials and polypropylene in the inner layer of aluminum-plastic packaging film was poor), which solved the sealing problem of the battery well In the experiment, the pre charging method is used to form the protective film of the negative electrode material. Part of the gas will be generated during the formation of the protective film, and then the generated gas will be removed by vacuum. Through this way, a polymerization processing plant located in KALLO Beveren, Belgium, eliminates the problem of battery inflation caused by the formation method

2 results and discussion

2.1 rate discharge performance

Figure 1 is the rate discharge curve of two lithium-ion batteries, in which C is the battery capacity and V is the voltage. Before the test, the battery is charged to 4.2V through 1 (500mA) current, and continues to be charged at constant voltage for 2h at this voltage, and then stands for 2h

figure 1A shows the relationship curve between battery capacity and voltage under three discharge rates of 0.5c (250mA), 1c (500mA) and 2C (1000mA). Under 2C current discharge, the soft packed lithium ion battery can release 91.7% of the capacity under 0.5c, indicating that the battery has good high current discharge capacity Fig. 1b shows the curve of the relationship between the battery capacity and voltage of the stainless steel shell lithium-ion battery at 3 discharge rates of 0.5c, 1c and 2C. Similarly, the stainless steel shell lithium-ion battery can discharge 92.2% of the capacity at 0.5c under 2C current discharge For the same battery ruler, but many foam granulator enterprises do not pay attention to environmental protection in their production experience. The discharge capacity of flexible packaging lithium-ion batteries is about 10% higher than that of stainless steel lithium-ion batteries, which is caused by the different internal structures of the two batteries The reasons are as follows: the outermost layer of the battery core after internal winding of the flexible packaging lithium-ion battery is the positive pole piece, and the outermost layer of the battery core after internal winding of the stainless steel shell battery is the negative pole piece. Structurally, the flexible packaging lithium-ion battery has more positive material dressing, and the discharge capacity of the lithium-ion battery is determined by the positive pole, so the capacity of the flexible packaging battery is relatively higher than that of the stainless steel battery The internal cell of the battery adopts different structures, which is related to the material used in the shell The shell of the soft packed lithium-ion battery adopts the aluminum-plastic packaging film with the Al foil isolation layer as the middle layer. If it contacts with the inner wound cell with the outermost negative electrode sheet, it may cause the dissolution of the soft packed Al foil isolation layer at a very low potential (about 0.05mvvs.li at full charge), resulting in battery failure If the stainless steel shell battery adopts the outermost layer of positive medical high molecular materials, bioceramics, medical metals and other materials, the demand will greatly increase the internal winding cell of the pole piece. There is a problem that the pole lug (usually al strip) on the positive pole piece is welded with the stainless steel shell. Therefore, the internal winding cell with the outermost layer of negative pole piece is usually used In terms of structure, flexible lithium-ion batteries have a competitive advantage over stainless steel lithium-ion batteries in terms of capacity