1.Overview of the lithium-ion battery

As a new generation of green high energy rechargeable battery, lithium-ion Battery has made rapid development in the past 10 years due to its high voltage, high energy density, good cycle performance, low self discharge and no memory effect, and with its excellent high cost-performance advantage in all countries around the world notebook computers, mobile phones, camcorders, weapons and other mobile electronic terminal equipment field has occupied a leading position, it is considered to be a high-tech industry of great significance to the national economy and people's life in the 21st century.

Figure 1 Global lithium-ion Battery Market Growth 2001-2006. In 2002, Global lithium-ion battery sales reached 8.6 billion yuan, up 51% year on year, and sales reached 2.818 billion US dollars, up 18% year on year In 2003, global sales reached 1.255 billion yuan, up 45% year-on-year, but overall prices fell by 16% to us $3.634 billion, up 29% year-on-year; in 2005, sales reached 1.71 billion yuan; in 2006, a conservative estimate reached 2.5 billion yuan; More than 3 billion units are expected to be sold worldwide in 2010, with power and polymer batteries emerging as new growth areas.

The development of lithium-ion battery led to the development of related industries. Copper foil is one of the key materials for the production of lithium-ion battery, and its quality has a direct impact on the production process, performance and cost of the lithium-ion battery. The development of high performance and high added value copper foil for lithium-ion battery is of great significance to the development of copper foil industry, electronics, communications, energy, transportation, aerospace, military and other industries.

2.Performance requirements for copper foil for lithium-ion battery

The lithium-ion battery consists of a positive electrode, a negative electrode, a diaphragm, and an electrolyte. The potential applied to the two electrodes of the battery forces the lithium compounds in the positive electrode to release lithium ions, which are embedded in the graphite anode with the hexagonal sheet structure through the back of the diaphragm, while the lithium ions are separated from the graphite with the sheet structure during discharge, recombined with a lithium-intercalating compound at the positive electrode, the lithium ions move, creating an electric current. The structure of the lithium-ion battery and the chemistry of the charging and discharging process, while simple, require consideration in practical commercial applications. For example, the conductivity, charge-discharge potential, activity, structure stability, magnification and safety of the cathode and cathode materials, as well as the stability, conductivity and environmental adaptability of the electrolyte.

In addition to the above factors, the internal resistance of the lithium-ion battery must be small enough to ensure reliability and long cycle life. This not only depends on the positive and negative electrode activity, but also has a considerable relationship with the collector.

The main material of the lithium-ion battery is metal foil (such as copper foil, aluminum foil) . Its function is to collect the current produced by the active material of the battery so as to form a larger current output. Therefore, the collector should be in full contact with the active material, and the internal resistance should be as small as possible, which is why lithium-ion battery chose to use the higher price of copper foil and aluminum foil. Copper foil with good conductivity, flexibility and moderate potential, resistance to winding and rolling, production technology is more mature, and therefore become the lithium-ion battery of choice materials.

Copper foil is not only the carrier of the lithium-ion battery, but also the collector and conductor of the negative electrons. Therefore, it has special technical requirements, that is, it must have good conductivity, the surface can be evenly coated with the negative material without falling off, and has good corrosion resistance. In order to ensure that the anode material coated on the electrolytic copper foil will not fall off, a suitable binder must be added during the preparation. According to the online understanding of the coating, the current commonly used adhesives are PVDF, PTFE, SBR, LA133 and so on, the bond strength not only depends on the adhesive itself, but also has a great relationship with the surface characteristics of copper foil physical chemistry. When the adhesive strength of the coating is high enough, it can prevent the cathode from breaking off during charging and discharging, or peel off the substrate due to excessive expansion and contraction, and reduce the circulation capacity retention ratio. On the other hand, if the bonding strength is not up to the requirement, the internal impedance of the battery will increase and the cycle capacity will decrease with the increase of cycle times. This requires lithium-ion battery copper foil to have good hydrophilicity. The performance requirements of lithium-ion Battery for Electrolytic Copper Foil (recommended by the enterprise) are shown in Table 1.

3.Development of copper foil for lithium-ion battery

Lithium-ion battery is a high energy battery based on Lithium Ion batteries. The precursor of lithium-ion battery is lithium battery, which uses lithium metal as negative electrode. Because the electrolyte reacts with Lithium in the course of discharging, the lithium dendrite forms on its surface and pierces the battery separator, which seriously affects the use safety and circulation performance of lithium-ion battery, it can't be reused. Because the intercalation potential of lithium in carbon materials is close to that of lithium, and it is not easy to react with organic solvents, and has good intercalation and deintercalation performance, carbon materials are widely used in commercial lithium-ion battery. In 1990, Japan's Nagoura and others developed a lithium-ion battery with an oil coke negative electrode; in the same year, two battery companies, Mo Li and Sony, launched a lithium-ion battery with a carbon negative electrode In 1991, Sony developed a new phase in the application of lithium-ion battery, in which the pyrolytic carbon of polyfurfuryl alcohol resin was used as the Cathode lithium-ion battery.

The conventional lithium-ion battery anode is composed of graphite + conductive agent + binder + current collector. The anode materials, such as graphite, are coated on the conductive current collector, and the anode electrodes are made by drying, rolling and cutting, and are then wound or laminated together with the diaphragm material and the positive electrode to form a lithium-ion battery. Copper foil is the preferred material for lithium-ion battery because of the advantages described above. Industrial Copper Foil calendering copper foil (Ra copper foil) and electrolytic copper foil (ED copper foil) two categories. Calendered copper foil has better performance, while electrolytic copper foil has the advantage of lower cost. At the beginning of the development of lithium-ion battery, because of the low performance of electrolytic copper foil at that time, battery manufacturers all adopted calendered copper foil. But the process is complex and costly, and global production capacity is heavily concentrated in a few companies (such as Nippon Mining in Japan, Fukuda, Itach IC, microhard and Olin brass in the US) . In recent years, with the improvement of physical, chemical, mechanical and metallurgical properties of electrolytic copper foil, as well as the advantages of simple production process, high efficiency and low cost, most of the lithium-ion battery manufacturers at home and abroad use electrolytic copper foil to make negative current collector, but some types of high-performance batteries still use calendered copper foil. There are many types of electrolytic copper foil, such as high elongation type, single-sided hair type, double-sided hair type, double-sided light type, double-sided coarsening type, etc. . The main technical specifications of calendered copper foil and electrolytic copper foil are compared in Table 2.

4.Effect of quality of copper foil on lithium-ion battery performance

The basic properties of lithium-ion battery include capacity, voltage characteristics, internal resistance, cycle life, storage performance, temperature characteristics, and so on, which are closely related to the overall battery system materials. For the negative electrode, in addition to the negative active material, the characteristics and quality of copper foil have a great influence on the battery performance and negative electrode manufacturing process (coating, rolling, cutting, etc.) .

1. Physical Properties of copper foil

1.1 hydrophilicity

The hydrophilicity of copper foil is related to its structure and Surface roughness, and directly affects the contact ability, adhesion ability, electrode making process and electrode quality. The electrolytic copper foil must have a good bond strength to the negative active material so that the negative material can be evenly coated without falling off, otherwise it will affect the internal resistance and the cycle life of the battery, etc. , this requires copper foil surface to have a certain degree of roughness. However, according to the Surface roughness online, the bigger the better, as the surface roughness increases, the easier to wet the surface becomes wetter, more hydrophilic, surfaces that are hard to wet become harder to wet and less hydrophilic. Negative active materials such as graphite have poor contact with the electrolytic copper foil with large Surface roughness, low adhesion and easy to fall off, which directly affects the cycle life of the battery.

1.2 surface density

The surface density of copper foil is the mass per unit area, which reflects the thickness uniformity of copper foil and directly affects the coating quantity of active substance on negative electrode. If the thickness uniformity of copper foil fluctuates too much, the battery capacity and consistency will eventually be affected.

1.3 folding resistance

Different types of lithium-ion battery require different folding resistance of the negative copper foil. Compared with the laminated battery, the winding battery requires better folding resistance of copper foil.

1.4 tensile strength and elongation

Copper foil must have enough tensile strength and elongation, otherwise the contact performance between copper foil and active material will be worse during the process of flattening the negative plate coated with active material such as graphite, it makes the size stability and flatness of the negative electrode worse, and at the same time, it is easy to cause the problem of electrode plate breaking. These will affect the negative production yield, battery capacity, internal resistance and cycle life.

2. Chemical Properties of copper foil in electrolytic copper foil production, the raw foil has strong activity and is easy to oxidize with oxygen in the air, so passivation treatment must be carried out to form a layer of oxide protective film (passivation film) . According to the coating on-line understanding, if the oxide film is semiconductor, too thick, the electron is difficult to conduct, the impedance is larger, will increase the battery internal resistance, which will cause the battery capacity attenuation; if the oxide layer is too loose, will reduce the adhesion of the negative active material. In addition, organic electrolytes for lithium-ion battery are highly corrosive and therefore require good corrosion resistance of copper foil. 

3. Surface quality of copper foil surface quality of copper foil has obvious influence on negative electrode fabrication process, fabrication quality and battery performance (table 3) . The surface defects will lead to the reduction of adhesion of copper foil, the appearance of exposed foils and negative and positive surfaces, which will seriously affect the capacity, internal resistance and cycle life of the battery, and even lead to the scrap of the electrode. The surface of the copper foil for lithium-ion Battery shall be clean and smooth without any defects such as streaks, depressions, pinholes, spots and mechanical damage.

5.Development trend of copper foil for lithium-ion battery

With the extensive application of electronic products, higher requirements on the specification and quality of lithium-ion battery have been put forward, which also brings new opportunities for the development of lithium-ion battery industry and copper foil industry, at the same time, the performance and quality of copper foil put forward higher requirements: 1, Copper Foil performance, precision, consistency requirements are higher; 2, thickness is thinner, to meet the high volume requirements of the lithium-ion battery. The current direction is to develop copper foils with thickness less than 9M. 3. micro-treatment of the surface to enhance oxidation resistance, corrosion resistance and conductivity, as well as adhesion strength with negative active materials; 4. Development of 2D or even 3d copper foils to meet the needs of Polymer lithium-ion battery and high capacity alloy negative electrode materials. 

Source: Coating online information