Introduction to Lithium-ion Batteries
Working Principle of Lithium-ion Batteries
A lithium-ion battery mainly consists of a positive electrode, a negative electrode, an electrolyte that conducts lithium ions, and a separator and casing that separate the positive and negative electrodes. The positive electrode material is generally a compound that allows for the reversible insertion and extraction of lithium ions, such as lithium cobalt oxide (LiCoO₂), lithium nickel oxide (LiNiO₂), lithium manganese oxide (LiMn₂O₄), or ternary materials (LiCoₓNiᵧMn₁₋ₓ₋ᵧO₂). The electrolyte is composed of lithium salts (common lithium salts include LiClO₄, LiPF₆, LiBF₄, LiBOB, etc.) dissolved in a specific solvent (mainly a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), propylene carbonate (PC), etc., in a certain proportion). The separator material is generally a polyolefin resin, commonly using single-layer or multi-layer polypropylene (PP) and polyethylene (PE) microporous membranes, such as the Celgard 2300 separator. The negative electrode typically uses materials capable of lithium intercalation, such as petroleum coke, pure graphite, and layered graphite mixed carbon. The reaction in a lithium-ion battery using carbon (C₆) as the negative electrode and transition metal oxide LiMeO₂ as the positive electrode is as follows.
During charging:
During discharge:
During charging, lithium ions are released from the positive electrode and inserted into the negative electrode; during discharging, lithium ions are released from the negative electrode and inserted into the positive electrode. In other words, during charging and discharging, lithium ions move back and forth between the positive and negative electrodes, much like a rocking chair. Therefore, lithium-ion batteries are also called "rocking chair batteries." Their working principle can be illustrated by Figure 1.1.
Under normal charge and discharge conditions, the insertion and extraction of lithium ions between layered carbon materials and oxide particles or between layers in lithium-ion batteries generally only causes changes in the interlayer spacing and does not damage the crystal structure. During charge and discharge, the chemical structures of the positive and negative electrode materials remain essentially unchanged. Therefore, from the perspective of the reversibility of the charge-discharge reaction, the insertion and extraction of lithium ions in the battery materials is an ideal reaction process. Based on these characteristics, lithium-ion batteries are superior in performance to nickel-cadmium and nickel-metal hydride batteries.
Lithium-ion battery classification
Lithium-ion batteries can be classified according to the cathode material used, appearance and size, cell manufacturing method, packaging type, and application characteristics.
Based on the cathode material used, lithium-ion batteries can be divided into lithium cobalt oxide batteries, lithium manganese oxide batteries, ternary lithium batteries, and lithium iron phosphate batteries.
Lithium cobalt oxide batteries have a nominal voltage of 3.7V and an operating voltage range of 2.4~4.2V. Lithium cobalt oxide batteries have a stable structure, high specific capacity, and outstanding overall performance, but their safety is poor and their cost is high, mainly used in small and medium-sized cells. In recent years, high-voltage lithium cobalt oxide materials have been developed, which can increase the upper limit voltage of the battery to 4.3V or 4.35V, thereby effectively improving the battery capacity and energy density. Currently, lithium cobalt oxide batteries have the highest volumetric energy density, reaching 550Wh/kg, making them the only choice for powering high-end mobile phones and other electronic products.
Lithium manganese oxide (MMANO) batteries have a nominal voltage of 3.8V and an operating voltage range of 2.5V to 4.2V. Overcharge protection voltage is 4.28V ± 0.025V, and over-discharge protection voltage is 2.5V ± 0.1V. MMANO batteries are low-cost and have good safety. However, the lithium manganese oxide material itself is relatively unstable and prone to decomposition, producing gas and swelling. Its cycle life decays relatively quickly, its lifespan is relatively short, and its high-temperature performance is poor. It is mainly used in low-cost, medium-to-large-sized cells for manufacturing power batteries.
Ternary lithium-ion (TLC) batteries have a nominal voltage of 3.6V and an operating voltage range of 2.75V to 4.2V. Overcharge protection voltage is 4.28V ± 0.025V, and over-discharge protection voltage is 2.75V ± 0.1V. TLC batteries have good overall performance, are cheaper than lithium cobalt oxide (LCO), and offer improved safety. They can be used in power batteries, and their market share in the cathode material market is increasing year by year. Small lithium-ion batteries using ternary lithium-ion materials are gradually being accepted by the market. Ternary materials can also be blended with lithium cobalt oxide and lithium manganese oxide for use in steel-cased, aluminum-cased, pouch, polymer, and cylindrical lithium-ion batteries, which can significantly reduce battery costs and improve overall performance. Currently, ternary material batteries can achieve an energy density of 180 Wh/kg (26650 steel-cased batteries can reach a capacity of 4600 mAh/kg with a weight of 90g), offering a clear advantage in cost-effectiveness.
Lithium iron phosphate (LFP) batteries have a nominal voltage of 3.2V and an operating voltage range of 2.5~3.75V. Overcharge protection voltage is 3.75V±0.025V, and over-discharge protection voltage is 2.5V±0.1V. The biggest advantage of LFP batteries is the stability and non-decomposition of the positive electrode material, giving them unparalleled safety compared to other positive electrode material systems. LFP batteries have a long cycle life, are abundant in resources, and are environmentally friendly. However, they have a low discharge platform and poor low-temperature performance.
Based on appearance and size, lithium-ion batteries can be divided into cylindrical batteries and prismatic batteries, etc.
Based on cell manufacturing method, lithium-ion batteries can be divided into wound batteries (cylindrical wound and flat wound), stacked batteries, etc.
Based on packaging material type, lithium-ion batteries can be divided into steel-cased batteries, aluminum-cased batteries, plastic-cased batteries, soft-pack batteries, etc.
Based on application characteristics, lithium-ion batteries can be divided into high-temperature batteries, low-temperature batteries, power batteries, and capacity batteries, etc.
Based on their application areas, lithium-ion batteries can be classified as: backup batteries, power batteries, and energy storage batteries.
Applications of Lithium-ion Batteries
It can be said that since the invention of the battery, no other battery product has been used as rapidly and widely as the lithium-ion battery. From the clock power supply for computer CPUs to the large lithium-ion battery packs used in automobiles and submarines, the capacity difference is more than 10 million times. They have wide applications in daily life, medical equipment, electric vehicles, energy storage power stations, aerospace, and the military.
After more than 10 years of popularization, lithium-ion batteries have become the sole power system widely used in digital products such as mobile phones and laptops. Due to their high specific energy, they are also widely used in power tools, electric bicycles, electric buses, wind and solar energy storage power stations, mobile communication base stations, mining lamp power supplies, mining rescue capsule power supplies, military individual soldier power supplies, radios, satellite batteries, and many other applications. According to statistics, in 2011, the market size of China's lithium battery industry reached 39.7 billion yuan, a year-on-year increase of 43%, and the annual output of lithium batteries reached 2.97 billion units, a year-on-year increase of 28.6%. The lithium battery industry has become an important industrial direction of the national economy²¹.
