Lithium-based batteries are divided into lithium batteries and lithium-ion batteries. Mobile phones and laptops use lithium-ion batteries, which are commonly referred to as lithium batteries. True lithium batteries, due to their higher risk, are rarely used in everyday electronic products.
A lithium-ion battery is a rechargeable battery that primarily functions by the movement of lithium ions between the positive and negative electrodes. During charging and discharging, Li+ ions repeatedly insert and extract between the two electrodes: during charging, Li+ ions extract from the positive electrode, pass through the electrolyte, and insert into the negative electrode, leaving the negative electrode in a lithium-rich state; the reverse occurs during discharging. Batteries that typically use lithium-containing materials as electrodes are representative of modern high-performance batteries.
Working principle of lithium-ion batteries
Lithium-ion batteries use carbon materials as the negative electrode and lithium-containing compounds as the positive electrode. They contain no metallic lithium, only lithium ions. The term "lithium-ion battery" refers to any battery that uses lithium-ion intercalation compounds as the positive electrode material. The charging and discharging process of a lithium-ion battery is essentially the process of lithium-ion insertion and extraction. During this process, an equivalent number of electrons are simultaneously inserted and extracted (conventionally, the positive electrode uses insertion or extraction, while the negative electrode uses insertion or extraction). During charging and discharging, lithium ions shuttle back and forth between the positive and negative electrodes, a process figuratively called a "rocking chair battery."
When a battery is charged, lithium ions are generated at the positive electrode. These lithium ions then move through the electrolyte to the negative electrode. The carbon layer at the negative electrode has a layered structure with many micropores. The lithium ions that reach the negative electrode embed themselves into these micropores. The more lithium ions embedded, the higher the charging capacity. Similarly, when the battery is discharged (i.e., when we use the battery), the lithium ions embedded in the carbon layer at the negative electrode are released and move back to the positive electrode. The more lithium ions that return to the positive electrode, the higher the discharge capacity.
Generally, the charging current for lithium batteries is set between 0.2C and 1C . The higher the current, the faster the charging, but the greater the heat generated by the battery. Moreover, charging with too high a current will not fully fill the battery, because the electrochemical reactions inside the battery require time. It's like pouring beer; pouring too quickly will create foam and prevent the battery from filling completely.
For batteries, normal use is the process of discharging.
Several points to note when discharging lithium batteries:
First, the discharge current should not be too high. Excessive current will cause the battery to overheat, potentially leading to permanent damage. This is not a problem for mobile phones and can be disregarded.
Secondly, never over-discharge! Lithium batteries are most vulnerable to over-discharge. Once the discharge voltage drops below 2.7V , the battery may be rendered unusable. Fortunately, mobile phone batteries have built-in protection circuits. Before the voltage drops to a level that would damage the battery, the protection circuit will activate and stop the discharge. As can be seen from the diagram, the higher the battery discharge current, the smaller the discharge capacity, and the faster the voltage drops.
Lithium-ion battery structure
Positive electrode: active material, conductive agent, solvent, binder, matrix.
The electrode that gains electrons from the external circuit during battery discharge undergoes a reduction reaction. This is typically the electrode with the highest potential. Examples include lithium cobalt oxide and lithium manganese oxide electrodes in lithium-ion batteries.
Negative electrode: active material (graphite, MCMB, CMS), binder, solvent, matrix.
The electrodes that supply electrons to the external circuit during battery discharge undergo an oxidation reaction.
A separator is a device placed between two electrodes to isolate them, thereby preventing direct contact between the active materials on the electrodes and causing a short circuit inside the battery. However, the separator still needs to allow charged ions to pass through in order to form a pathway.
Diaphragm requirements:
1. High ion permeability
2. Appropriate mechanical strength
3. It is an insulator itself.
4. Does not react with electrolyte or electrodes.
Material: Single-layer PE (polyethylene) or three-layer composite PP (polypropylene) + PE + PP
Thickness: Single layer is generally 0.016–0.020 mm , three layers are generally 0.020–0.025 mm .
electrolyte
Housing hardware (steel housing, aluminum housing, cover plate, electrode tabs, insulating sheet, insulating tape)
Lithium-ion battery cell raw materials
cathode materials
Cathode materials have the largest market capacity and the highest added value in lithium batteries, accounting for about 30% of the cost of lithium batteries, with a gross profit margin ranging from 15% to over 70%.
Currently, the cathode materials that are used in large quantities in lithium batteries mainly include lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium cobalt nickel manganese oxide, and lithium iron phosphate.
Lithium nickel oxide batteries have the worst safety (prone to fire when overcharged), the lowest high temperature tolerance (decomposes at high temperatures), and the highest difficulty in synthesis.
Lithium cobalt oxide was the first material to be commercially applied, and its technology has now matured and is widely used in small, low-power portable electronic products such as mobile phones, laptops, and digital electronic products.
Lithium iron phosphate (LFP) is a cathode material for lithium-ion batteries with excellent electrochemical performance, a very stable charge-discharge platform, and structural stability during charge and discharge. Furthermore, this material is non-toxic, non-polluting, safe, can be used in high-temperature environments, and has a wide availability of raw materials, making it a hot research topic in the battery industry.
Anode material
Anode materials account for a relatively small proportion of the cost of lithium batteries, and mainly include carbon anode materials and non-carbon anode materials.
Carbon anode materials: widely used in commercial lithium-ion batteries.
Advantages: safe, long cycle life, inexpensive, and non-toxic.
Disadvantage: Low mass-to-energy ratio.
Non-carbon anode materials: classified by composition into lithium transition metal nitrides, transition metal oxides, and nano-alloy materials.
Advantages: It has a very high volumetric energy density.
Disadvantages: poor cycle stability, large irreversible capacity, high preparation cost, and not yet industrialized.
The future goal of anode materials is to improve capacity and cycle stability by combining carbon materials with various high-capacity non-carbon anode materials to develop high-capacity, non-carbon composite anode materials.
Diaphragm material
The commercially available membrane materials are mainly polyolefin membranes, primarily made of polyethylene (PE) and polypropylene (PP). PE products are mainly produced by wet processes, while PP products are mainly produced by dry processes.
Comparison of characteristics between PE and PP products:
1. PP is more resistant to high temperatures, while PE is more resistant to low temperatures;
2. PP has a lower density than PE;
3. PP has a higher melting point and closed-cell temperature than PE;
4. PP products are more brittle than PE products;
5. PE is more sensitive to environmental stress.
The main membrane materials include single-layer PP, single-layer PE, PP+ceramic coating, PE+ceramic coating, double-layer PP/PE, double-layer PP/PP and triple-layer PP/PE/PP, etc. The first two types of products are mainly used in the field of 3C small batteries, while the latter types of products are mainly used in the field of power lithium batteries.
In the field of lithium-ion battery separator materials, double-layer PP/PP separators are mainly produced by Chinese companies and used in mainland China. This is primarily because, at present, no Chinese company possesses the technology and capability to fabricate a double-layer composite film from PP and PE. Globally, the separators used in automotive lithium-ion batteries are mainly triple-layer PP/PE/PP, double-layer PP/PE, and PP+ceramic coating, PE+ceramic coating, and other separator materials. Meanwhile, other new separator materials are emerging and beginning to be applied; however, due to limited quantity and high price, they are mainly used in the manufacturing of lithium-ion batteries. These products mainly include coated polyester films (PET, Polyethylene Terephthalate), cellulose films, polyimide films (PI), polyamide films (PA), spandex or aramid films, etc. The advantages of these separators are high temperature resistance, low-temperature output, long charge cycle life, and moderate mechanical strength. Overall, lithium-ion battery separator materials are showing a clear trend of diversification.
electrolyte
The key to lithium-ion battery electrolyte materials lies in high safety and high environmental adaptability. The main development will focus on: new solvents (widening the operating temperature range), ionic liquids, new lithium salts (improving environmental adaptability), and additives (flame retardants, redox shuttles, protecting the positive and negative electrode films, etc.). These will be matched with new positive and negative electrode materials to improve safety, power and capacity, and ultimately be safely and conveniently applied to electric vehicles, energy storage, aerospace and a wider range of fields.
Manufacturing process of lithium-ion batteries
Main equipment used in lithium-ion battery production
Vacuum planetary mixer
Application: To uniformly mix various battery materials into a slurry.
Electrode coating machine
Application: To uniformly coat the stirred slurry onto metal foil. The coating thickness is accurate to less than 3 micrometers.
Roller press
Application: The coated electrode sheets are further compacted to improve the energy density of the battery.
Electrode slitting equipment
Ultrasonic welding conductive handle equipment
Winding machine
Application: To wind the manufactured electrode sheets into a battery.
glove box
Application: To ensure the electrolyte and the core are sealed together in low humidity environments.
Liquid injection machine
Application: To ensure high-precision, streamlined injection of electrolyte into battery packaging materials under vacuum.
Chemical testing equipment
Purpose: To charge and activate the prepared battery, generate voltage, and test the battery capacity.
