◆Conversion-type anode materials
◆Lithium metal anode materials
In the charging process of lithium-ion batteries, the negative electrode material plays a crucial role in carrying lithium ions and electrons, and is essential for energy storage and release. From a cost perspective, these materials account for 5% to 15% of the total battery manufacturing cost and are considered one of the indispensable key raw materials for lithium-ion battery production. Like the positive electrode material, the negative electrode material plays an extremely important role in promoting the advancement of lithium-ion battery technology. In recent years, with the increasing demand for improved battery performance-specifically, the pursuit of higher energy density, power density, and better cycle stability and safety-researchers have paid great attention to the negative electrode material, one of the core components of lithium-ion
batteries. An ideal negative electrode material should possess the following characteristics:
(1) Lightweight, accommodating as much Li as possible to optimize specific capacity.
(2) Low redox potential for lithium ion insertion and extraction reactions, which helps to achieve a higher battery output voltage.
(3) Good electronic and ionic conductivity.
(4) Insoluble in electrolyte solvents and does not react with lithium salts. (5) Excellent chemical stability after charging and discharging, high safety performance and cycle life, and low self-discharge rate.
(6) Inexpensive, abundant resources, and environmentally friendly.
Anode materials can be divided into two main categories based on their chemical composition: carbon-based materials and non-carbon-based materials. Carbon-based materials can be further subdivided into graphitic carbon materials and amorphous carbon materials. Non-carbon-based materials include silicon-based, titanium-based, and various metal oxides. Currently, the widely used anode materials on the market mainly include three types: carbon-based materials, lithium titanate (LiTisOi2), and carbon composite materials incorporating silicon. Carbon-based materials can be further divided into graphite (natural and artificial graphite), soft carbon, and hard carbon. Among these categories, artificial graphite holds the largest market share.
Intercalated anode materials
carbon materials
In the development of lithium-ion batteries, the innovation of using carbon-based materials to replace metallic lithium as the anode marks a major breakthrough in this technology. To date, no other type of anode material can match its cost-effectiveness and performance; therefore, it is expected that carbon-based materials will remain the primary choice for large-scale commercial applications for a considerable period. Based on the degree of graphitization, carbon-based materials used as anodes can be classified into three categories: graphite, soft carbon, and hard carbon. Non-graphite carbon materials all exhibit a tendency to transform into graphite during high-temperature processing; however, some substances are more prone to this transformation and are defined as soft carbon; while those that are difficult to complete the process are called hard carbon. Typically, soft carbon can be obtained from raw materials such as coal tar or petroleum pitch; in contrast, hard carbon is mostly synthesized from components such as phenolic resin or sucrose. Currently, one of the most studied subjects in the field of soft carbon is mesophase carbon microspheres. Both graphitic and non-graphitic carbon materials have their own advantages and disadvantages when used as negative electrodes in lithium-ion batteries. Based on this, researchers often use various sub-segments to modify and alter the surface of these carbon materials in order to improve their performance.
Graphite, as a layered material (Figure 5-8), has an internal structure consisting of a hexagonal framework of atoms arranged in an sp2 hybrid state, extending in two dimensions. Within each layer, carbon atoms form a robust hexagonal grid structure with a carbon-carbon atom distance of 0.142 nm and a bond energy of 345 kJ/mol, exhibiting extremely strong stability. In contrast, carbon atoms between different layers are connected by weaker van der Waals forces, with an interaction energy of only 16.7 kJ/mol, corresponding to a measured interplanar spacing of 0.3354 nm. Lithium ions can undergo reversible insertion and extraction between the six carbon layers of graphite, forming LiC6 compounds to store lithium. During this process, the interlayer spacing changes significantly; for LiC6, this value becomes 0.37 nm, thus achieving a theoretical maximum specific capacity of 372 mA·h/g. Furthermore, graphite's excellent conductivity facilitates rapid electron migration within the material. However, when used as a negative electrode material, graphite also presents some drawbacks: its relatively low lithium insertion/extraction voltage plateau can lead to the growth of lithium dendrites during charging or discharging. Once these dendrites penetrate the battery separator, they can cause internal short circuits, potentially leading to fires or even explosions, threatening battery safety.
Figure 5-8 Schematic diagram of graphite layered structure
Graphite is mainly divided into two categories: natural graphite and artificial graphite. Natural graphite, abbreviated as NG (natural graphite), refers to a high-carbon material extracted from nature and obtained through simple processing. It possesses two different morphologies of layered crystal structure: hexagonal and rhombic. This material is not only abundant in reserves, but also low in cost and environmentally friendly. However, in lithium-ion battery applications, due to the uneven distribution of surface activity and large grain size of natural graphite powder particles, its surface crystal structure is easily damaged during charge-discharge cycles, leading to uneven SEI film coverage and affecting the battery's initial coulombic efficiency and rate performance. To overcome these challenges, researchers have developed various techniques to improve the properties of natural graphite, such as spheroidization, oxidation surface treatment, fluorination, and surface carbon coating, aiming to optimize its surface characteristics and microstructure.
Artificial graphite can be produced by high-temperature graphitization of easily graphitized carbon materials. This type of material is widely used as the anode material in lithium-ion batteries. Compared to natural graphite, artificial graphite exhibits significant advantages in terms of long cycle life, high-temperature storage capacity, and high-rate performance, making it one of the preferred anode materials for lithium-ion batteries used in new energy vehicles in China. Due to its large specific capacity and relatively low cost, artificial graphite is also widely used in power batteries and mid-to-high-end consumer electronics products. Statistics show that in 2021, artificial graphite accounted for 84% of all anode material shipments.
Non-graphite carbon materials are mainly divided into two categories: hard carbon and soft carbon. Hard carbon refers to a type of carbon material that is difficult to transform into a graphite structure even at extremely high temperatures (above 2800℃). These materials are usually obtained by pyrolyzing certain polymers. Specifically, common sources of hard carbon include various resin carbons (such as phenolic resins, polyfurfuryl alcohol resin PFA-C, and epoxy resins), carbon formed by the pyrolysis of specific polymers (such as polyvinyl alcohol (PVA), polyvinylidene fluoride (PVDF), and polyacrylonitrile (PAN), and carbon black products like acetylene black). During the preparation process, a large number of lattice defects are formed inside the hard carbon, which allows lithium ions not only to intercalate between carbon layers but also to fill these defect regions, thus giving anodes made from this material a high specific capacity (between 350 and 500 mA·b/g), which is very beneficial for improving the overall capacity of lithium-ion batteries. However, the aforementioned lattice defects also lead to low initial coulombic efficiency and poor cycle stability when hard carbon is used as an anode material. To date, due to these problems, hard carbon has not been widely used in commercially produced lithium-ion batteries, and there are still some obstacles before it can be used on a large scale.
Soft carbon refers to amorphous carbon materials that readily graphitize under high-temperature conditions (above 2800℃). These materials include pitch, needle coke, petroleum coke, and carbon fibers. Due to the low level of graphitization in soft carbon, its structure contains numerous defects, allowing it to reversibly accommodate more lithium ions; simultaneously, the larger interlayer spacing promotes electrolyte penetration. Therefore, based on these characteristics, soft carbon materials exhibit high capacity during the initial discharge. However, precisely because of its structural instability, its irreversible capacity is also relatively high. Furthermore, the irregularity of the internal structure of soft carbon leads to varying energy distributions of lithium-ion active sites, resulting in a lack of a defined voltage plateau during charge and discharge, which limits its practical applications.
Titanium dioxide
Titanium dioxide (TiO2) shows great potential as a negative electrode material for lithium-ion batteries, not only due to its feasibility for large-scale production and low cost, but also because it exhibits excellent safety and stability at an operating voltage of 1.5V (relative to Li/Li). In addition, TiO2 possesses a series of remarkable properties: high electrochemical activity, strong oxidizing power, good chemical stability, abundant natural resources, and diverse crystal structures.
These advantages collectively make TiO2 one of the ideal negative electrode material choices for lithium-ion batteries (especially in the field of hybrid electric vehicles).
Theoretically, each unit mass of TiO2 can store one lithium ion, corresponding to a capacity of 330 mA·h/g, which is almost twice the theoretical capacity of LiTiO2. However, in practice, it has been found that achieving this theoretical maximum lithium storage capacity is quite difficult. Many factors influence the lithium ion insertion and extraction efficiency in titanium dioxide, including but not limited to the material's crystallinity, particle size, internal structural characteristics, and specific surface area. It is worth noting that TiO2 exists in various crystal phases, the most well-known being the rutile and anatase types in the tetragonal crystal system, and the brookite type in the orthorhombic crystal system.