Lithium-ion batteries, as a new generation of green high-energy rechargeable batteries, have outstanding advantages such as high voltage, high energy density, good cycle performance, low self-discharge, and no memory effect. They have achieved rapid development in the past 10 years and have dominated the field of mobile electronic terminal devices such as laptops, mobile phones, and camcorders in various countries around the world with their excellent cost performance. They are considered to be a high-tech industry of great significance to the national economy and people's lives in the 21st century.
Figure 1 shows the growth of the global lithium-ion battery market from 2001 to 2006. In 2002, global lithium-ion battery sales reached 8.6 billion units, a year-on-year increase of 51%, with sales revenue reaching US$2.818 billion, a year-on-year increase of 18%. The main driver of growth came from the mobile phone market, which accounted for 60% of total sales. In 2003, global sales reached 1.255 billion units, a year-on-year increase of 45%, but the overall price decreased by 16%, with sales revenue reaching US$3.634 billion, a year-on-year increase of 29%. In 2005, sales reached 1.71 billion units. In 2006, it was conservatively estimated to reach 2.5 billion units. It is projected that global sales will exceed 3 billion units by 2010, with power lithium batteries and polymer batteries becoming new growth drivers.
The rapid development of lithium-ion batteries has spurred the growth of related industries. Copper foil is a key material in the manufacture of negative electrode current collectors for lithium-ion batteries, and its quality directly affects the manufacturing process, performance, and production cost of lithium-ion batteries. Research on high-performance, high-value-added copper foil for lithium-ion batteries is of great significance to the development of the copper foil industry, electronics, communications, energy, transportation, aerospace, and other related industries.
Performance requirements for copper foil for lithium-ion batteries
Lithium-ion batteries are primarily composed of a positive electrode, a negative electrode, a separator, and an electrolyte. During charging, the potential applied to the battery electrodes forces the lithium intercalation compound in the positive electrode to release lithium ions, which then pass through the separator and embed into the hexagonal layered graphite negative electrode. During discharging, lithium ions are released from the layered graphite and recombine with the lithium intercalation compound in the positive electrode; this movement of lithium ions generates an electric current. Although the structure and chemical reaction principle of lithium-ion batteries are relatively simple, many issues must be considered in practical commercial applications. These include the conductivity, charge/discharge potential, activity, structural stability of the lithium intercalation/deintercalation materials, rate performance, and safety performance of the positive and negative electrode materials, as well as the stability, conductivity, and environmental adaptability of the electrolyte.
In addition to the factors mentioned above, the internal resistance of a lithium-ion battery must be sufficiently low to ensure reliability and a long cycle life. This depends not only on the activity of the positive and negative electrodes but also significantly on the current collector.
The key material for current collectors in lithium-ion batteries is metal foil (such as copper foil and aluminum foil). Its function is to collect the current generated by the battery's active materials to form a larger current output. Therefore, the current collector should have sufficient contact with the active materials, and its internal resistance should be as low as possible. This is a major reason why lithium-ion batteries use more expensive copper and aluminum foil. Copper foil has good conductivity, flexibility, and a moderate potential; it is resistant to winding and rolling, and its production technology is relatively mature, making it the preferred material for the negative electrode current collector in lithium-ion batteries.
In lithium-ion batteries, copper foil serves as both the carrier of the negative electrode active material and the collector and conductor of negative electrode electrons. Therefore, it has specific technical requirements: it must possess good conductivity, allow for uniform coating of the negative electrode material without peeling, and have good corrosion resistance. To ensure that the negative electrode material coated on the electrolytic copper foil does not peel off, a suitable binder must be added during preparation. According to Coating Online, commonly used binders include PVDF, PTFE, SBR, and LA133. Their bonding strength depends not only on the physicochemical properties of the binder itself but also significantly on the surface characteristics of the copper foil. Sufficiently high bonding strength prevents the negative electrode from pulverizing and peeling off during charge-discharge cycles, or from detaching from the substrate due to excessive expansion and contraction, thus reducing cycle capacity retention. Conversely, if the bonding strength is insufficient, with each additional cycle, the increased degree of coating peeling leads to a continuous increase in the battery's internal impedance and a more severe decrease in cycle capacity. This necessitates that the copper foil used in lithium-ion batteries possess good hydrophilicity. The performance requirements of electrolytic copper foil for lithium-ion batteries (company recommended standards) are shown in Table 1.
Development of copper foil for lithium-ion batteries
Lithium-ion batteries are high-energy batteries developed based on lithium-ion batteries. The prototype of the lithium-ion battery used metallic lithium as the negative electrode. During discharge, the electrolyte reacts with lithium, forming lithium dendrites on its surface, which pierce the battery separator, severely affecting the safety and cycle performance of the lithium-ion battery, making it unusable repeatedly. Because the lithium intercalation potential in carbon materials is close to the lithium's potential and it does not readily react with organic solvents, exhibiting excellent lithium intercalation/deintercalation performance, carbon materials are widely used in commercial lithium-ion batteries. In 1990, Nagoura et al. in Japan developed a lithium-ion battery using petroleum coke as the negative electrode; in the same year, Moli and Sony, two major battery companies, launched lithium-ion batteries using carbon as the negative electrode; in 1991, Sony successfully developed a lithium-ion battery using polyfurfuryl alcohol resin pyrolyzed carbon as the negative electrode, thus ushering in a new era for the application of lithium-ion batteries.
The composition of a conventional lithium-ion battery negative electrode is graphite + conductive agent + binder + current collector. Graphite and other negative electrode materials need to be coated onto the conductive current collector, and then processed through drying, rolling, and slitting to form the negative electrode. This electrode is then wound or stacked together with the separator material and positive electrode to form a lithium-ion battery. Copper foil, due to its aforementioned advantages, has become the preferred material for the negative electrode current collector in lithium-ion batteries. Industrial copper foil is divided into two main categories: rolled copper foil (RA copper foil) and electrolytic copper foil (ED copper foil). Rolled copper foil has better performance, while electrolytic copper foil has the advantage of lower cost. In the early stages of lithium-ion battery development, due to the lower performance of electrolytic copper foil at the time, battery manufacturers all used rolled copper foil. However, the production process of rolled copper foil is complex and costly, and global production is extremely concentrated in a few companies (such as Nippon Mining, Fukuda Metals, Hitachi Cable, and Microhard in Japan, and Olinbrass in the United States). In recent years, with the improvement of the physical, chemical, mechanical, and metallurgical properties of electrolytic copper foil, as well as its advantages such as simple production process, high efficiency, and low cost, most lithium-ion battery manufacturers at home and abroad have switched to using electrolytic copper foil to make battery negative electrode current collectors. However, some types of high-performance batteries still use rolled copper foil. There are various types of electrolytic copper foil, such as high elongation type, single-sided rough type, double-sided rough type, double-sided smooth type, and double-sided roughened type. A comparison of the important technical indicators of rolled copper foil and electrolytic copper foil is shown in Table 2.
The impact of copper foil quality on lithium-ion battery performance
The basic performance characteristics of lithium-ion batteries include capacity, voltage characteristics, internal resistance, cycle life, storage performance, and temperature characteristics. These characteristics are closely related to the materials used in the entire battery system. Regarding the negative electrode, in addition to the negative electrode active material, the characteristics and quality of the copper foil have a significant impact on battery performance and the negative electrode manufacturing process (coating, rolling, slitting, etc.).
1. Physical properties of copper foil
hydrophilic
The hydrophilicity of copper foil is related to its microstructure and surface roughness, directly affecting its contact and adhesion to negative electrode active materials, electrode fabrication process, and electrode quality. Electrolytic copper foil must possess good adhesion strength to negative electrode active materials to ensure uniform coating without detachment; otherwise, it will affect battery internal resistance and cycle life. This necessitates a certain degree of surface roughness for the copper foil. However, according to Coating Online, higher surface roughness is not always better. Increased surface roughness makes easily wettable surfaces even easier to wet and more hydrophilic, while difficult-to-wet surfaces become even more difficult to wet and less hydrophilic. Negative electrode active materials such as graphite exhibit poor contact, low adhesion, and are prone to detachment from electrolytic copper foil with high surface roughness, directly impacting battery cycle life.
areal density
The areal density of copper foil refers to the mass per unit area, reflecting the uniformity of the copper foil thickness and directly affecting the amount of active material coated on the negative electrode. Excessive fluctuations in the uniformity of copper foil thickness will ultimately affect battery capacity and consistency.
Flexural strength
Different types of lithium-ion batteries have different requirements for the folding resistance of the negative electrode copper foil. Compared with stacked batteries, wound batteries require copper foil with better folding resistance.
Tensile strength and elongation
The copper foil must have sufficient tensile strength and elongation; otherwise, during the flattening process of the negative electrode sheet coated with graphite and other active materials, the contact performance between the copper foil and the active material will deteriorate, resulting in poor dimensional stability and flatness of the negative electrode, and making it prone to problems such as electrode breakage. All of these will affect the yield of negative electrode production, battery capacity, internal resistance, and cycle life.
2. Chemical properties of copper foil
In the production of electrolytic copper foil, raw foil is highly reactive and easily oxidizes with oxygen in the air. Therefore, passivation treatment is necessary to form a protective oxide film (passivation film). According to Coating Online, if the oxide film is semiconductor-grade and too thick, electrons cannot conduct effectively, resulting in high impedance and increased internal resistance in the battery, leading to capacity decay. Conversely, if the oxide layer is too porous, it will reduce the adhesion of the negative electrode active material. Furthermore, the organic electrolytes used in lithium-ion batteries are highly corrosive, thus requiring the copper foil to have excellent corrosion resistance.
3. Copper foil surface quality
The surface quality of copper foil has a significant impact on the negative electrode manufacturing process, manufacturing quality, and battery performance (Table 3). Surface defects will lead to reduced copper foil adhesion, resulting in exposed foil spots, uneven coating on both sides, and severely affect battery capacity, internal resistance, cycle life, and may even directly cause electrode failure. The surface of copper foil used in lithium-ion batteries must be clean and flat, and no defects such as streaks, dents, pinholes, spots, or mechanical damage are allowed.
Development Trends of Copper Foil for Lithium-ion Batteries
With the widespread use of electronic products, higher requirements have been placed on the specifications and quality of lithium-ion batteries, bringing new opportunities for the development of the lithium-ion battery industry and the copper foil industry. At the same time, higher requirements have been placed on the performance and quality of copper foil.
1. Higher requirements are placed on the performance, precision, and consistency of copper foil;
2. Thinner thickness to meet the high volumetric capacity requirements of lithium-ion batteries. Current research focuses on developing copper foil with a thickness of less than 9 μm.
3. Perform micro-treatment on the surface to enhance its antioxidant, corrosion-resistant, and conductive properties, as well as its adhesion strength to the negative electrode active material;
4. To meet the needs of polymer lithium-ion batteries and high-capacity alloy anode materials, develop two-dimensional and even three-dimensional copper foils.
