Performance of three soft-package lithium-ion batteries

Lithium-ion battery is a new type of rechargeable battery developed in the past decade. It has high specific energy, no memory effect, is green and environmentally friendly, and its discharge voltage is three times that of nickel-cadmium and nickel-metal hydride batteries. Therefore, it has attracted widespread attention and developed very rapidly since its introduction. At present, only a few countries have achieved industrialization, among which Japan has done the best industrialization work. my country is the earliest developing country to develop lithium-ion battery industrialization. After development in recent years, especially under the active promotion of relevant national policy orientation, domestic lithium-ion batteries have made rapid progress, and various performances of products have made considerable progress. The lithium-ion battery production chain has been increasingly improved, and the degree of industrialization has been increasing, forming a situation 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 high technological content and wide applications. They have high volume energy and mass energy, are rechargeable and pollution-free, and have the three major advantages of the current development of the battery industry. They are called "the most promising chemical power source". There are currently two common types of lithium-ion battery technology: liquid lithium-ion batteries with metal shells and polymer lithium-ion batteries. The shell of liquid lithium-ion batteries is generally steel or aluminum, and its plasticity is not as good as that of polymer lithium-ion batteries. The manufacturing process of polymer lithium-ion batteries is complex, especially the low yield rate, which greatly limits the industrialization of this battery [3]. The author first recommends soft-package lithium-ion batteries, which are characterized by the combination of the basic electrochemical properties of liquid lithium-ion batteries and polymer lithium-ion batteries. The battery has unique advantages in packaging, and uses a liquid electrolyte similar to that of a liquid lithium-ion battery as a medium for lithium ion transmission between the positive and negative electrodes, while a multi-layer plastic composite film is used in the packaging. The outstanding advantages of soft-package lithium-ion batteries are their simple production process, low cost, and high yield rate. Their goal is to replace the market for polymer lithium-ion batteries and to compete with stainless steel shell lithium-ion batteries. A comparison of three different technologies, stainless steel shell lithium-ion batteries, polymer lithium-ion batteries, and soft-package lithium-ion batteries, is listed in Table 1. This paper studies the various electrochemical properties of soft-package lithium-ion batteries and the differences between them and stainless steel shell lithium-ion batteries, in order to verify the feasibility of the industrialization of soft-package lithium-ion batteries.

1 Experiment

The positive electrode material used in the experimental battery is lithium cobalt oxide, and the negative electrode material is artificial spherical graphite. LiPF6/EC/DEC/DMC is the electrolyte, in which LiPF6 is the conductive salt and EC (ethylene carbonate)/DEC (diethyl carbonate)/DMC (dimethyl carbonate) is the 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 ear sealing glue is a modified polyolefin material. The battery manufacturing process includes ingredients, mixing, coating, rolling, slitting, spot welding, winding, sealing, formation, degassing and capacity division. Lithium-ion batteries with stainless steel shells were also manufactured under similar process conditions. The dimensions of soft-package lithium-ion batteries and stainless steel shell lithium-ion batteries are both 48mm×30mm×4.1mm. The nominal capacity of the two batteries is 500mAh.

The most critical technology in the manufacturing process of soft-package lithium-ion batteries is how to overcome the gas expansion problem generated by the battery. The gas expansion problem of the battery is related to whether the battery seal is good on the one hand, and on the other hand it is related to the battery formation method. In the experiment, modified polypropylene material was used as the sealing glue between the positive and negative electrode ears and the composite packaging film (the bonding ability between general metal materials and the inner layer of polypropylene of the aluminum-plastic packaging film is poor), which solved the sealing problem of the battery well. In the experiment, a pre-charging method was used to form a protective film for the negative electrode material. A part of the gas will be generated during the formation of the protective film, and then the generated gas is removed by vacuum. In this way, the battery gas expansion problem caused by the formation method is eliminated.

2 Results and Discussion

2.1 Rate discharge performance

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

Figure 1a shows the relationship curve between battery capacity and voltage of soft-package lithium-ion battery at three discharge rates of 0.5C (250mA), 1C (500mA) and 2C (1000mA). Under 2C current discharge, the soft-package lithium-ion battery can release 91.7% of the capacity at 0.5C, indicating that the battery has good large current discharge capability. Figure 1b shows the relationship curve between battery capacity and voltage of stainless steel shell lithium-ion battery at three discharge rates of 0.5C, 1C and 2C. Similarly, under 2C current discharge, the stainless steel shell lithium-ion battery can release 92.2% of the capacity at 0.5C. For the same battery size, the discharge capacity of soft-package lithium-ion battery is about 10% higher than that of stainless steel shell lithium-ion battery. This is due to the different internal structures of the two batteries. The reasons are as follows: the outermost layer of the battery cell after winding inside the soft-package lithium-ion battery is the positive electrode sheet, and the outermost layer of the battery cell after winding inside the stainless steel shell battery is the positive electrode sheet. The outermost layer is the negative electrode sheet. Structurally, the soft-package lithium-ion battery has more positive electrode material coating, and the discharge capacity of the lithium-ion battery is determined by the positive electrode, so the capacity of the soft-package battery is relatively higher than that of the stainless steel battery. The internal cells of the battery adopt different structures, which is related to the material used in the outer shell. The outer shell of the soft-package lithium-ion battery is an aluminum-plastic packaging film with an Al foil isolation layer as the middle layer. If it contacts the internal wound cell with the negative electrode sheet as the outermost layer, the Al foil isolation layer of the soft package may be dissolved at an extremely low potential (about 0.05mVvs.Li when fully charged), causing the battery to fail. If the stainless steel shell battery uses an internal wound cell with the positive electrode sheet as the outermost layer, there is a problem of welding the tabs (usually Al strips) on the positive electrode sheet to the stainless steel shell, so the internal wound cell with the negative electrode sheet as the outermost layer is usually used. Structurally, soft-package lithium-ion batteries have a competitive advantage over stainless steel shell lithium-ion batteries in terms of capacity.

2.2 High and low temperature performance

Figure 2 shows the high and low temperature discharge curves of two lithium-ion batteries. The test process was carried out in a high and low temperature test chamber. Before the test began, the battery was charged to 4.2V with a current of 1C, and continued to charge at a constant voltage for 2 hours at this voltage, and then stood for 2 hours. As can be seen from the figure, the discharge capacity of the soft-package lithium-ion battery at high temperature of 45°C and low temperature of -10°C reached 102.8% and 87.8% of the capacity of the battery at room temperature of 25°C, and the battery has good high and low temperature performance. The high and low temperature performance of the lithium-ion battery with a stainless steel shell, its discharge capacity at high temperature of 45°C and low temperature of -10°C reached 103.2% and 93.4% of the capacity of the battery at room temperature of 25°C. As can be seen from the figure, the discharge capacity retention rate of the soft-package lithium-ion battery at low temperature is close to the capacity retention rate of the stainless steel shell lithium-ion battery at low temperature.

2.3 Cyclic stability

Figure 3 is a comparison of the cycle performance of two lithium-ion batteries, where n is the number of cycles, c is the battery capacity, and the battery charge and discharge cycle system is 1CmA charged to 4.2V, and then charged at this voltage for 2h and then left to stand for 10min, and then 1CmA discharged to 3.0V. After 300 cycles, the discharge capacity of the soft-package lithium-ion battery remains at about 90.6% of the initial capacity, while the discharge capacity of the stainless steel shell lithium-ion battery remains at 91.3% of the initial capacity. It can be seen from the figure that the lithium-ion battery made by replacing the conventional metal shell with the aluminum-plastic composite film packaging shell has good cycle stability performance. The structure, material and performance of the tab sealant of the aluminum-plastic composite film can completely isolate the interaction between the inside of the battery and the external environment. The aluminum-plastic composite film can complete a good sealing function similar to that of the metal shell.

Testing of other properties of soft-packaged lithium-ion batteries, such as storage performance, self-discharge performance and various safety performance, has been completed, and the products have basically met the testing requirements of the national standard "General Specification for Lithium-ion Batteries for Cellular Phones GB/T18287-2000". Soft-packaged lithium-ion products are currently undergoing market tests and are mainly used in MP3, wireless headphones, car DVDs and other applications.

3 Conclusion

Research on the electrochemical properties of soft-packaged lithium-ion batteries, such as rate discharge capability, high and low temperature discharge capability, and battery cycle stability, shows that soft-packaged lithium-ion batteries have very high application value. It is different from ordinary metal-cased lithium-ion batteries and polymer lithium-ion batteries. It is a new type of lithium-ion battery with the advantages of high energy-to-weight ratio, good shape plasticity, and simple production process, and is expected to be mass-produced.