In the last issue, we conducted an in-depth discussion and summary on the development of cathode materials for lithium-ion batteries, and cathode materials also played a mainstay role in the development of lithium-ion batteries. Today we will talk about the past and present of lithium-ion battery anode materials.

In the development of lithium-ion batteries, the emergence of a material has completely changed the fate of the entire lithium-ion battery and even subverted the pattern of the entire energy storage field, that is, lithium intercalation compounds. Intercalation ( ) refers to the reversible movement of particles (molecules, atoms, ions) into network positions in a lattice of appropriate size. The positive and negative electrode materials of lithium-ion batteries are compounds that can intercalate lithium ions and electrons. The charge compensation of the electrode material maintains the overall electrical neutrality when the electrode material intercalates lithium ions. The use of lithium intercalation compounds instead of metal lithium anodes not only avoids the formation of lithium dendrites due to uneven deposition on the electrode surface during the electrochemical cycle of metal lithium electrodes but also prevents the formation of lithium dendrites due to uneven dissolution of metal lithium. Electrochemically active "dead lithium" greatly improves the safety performance, thereby accelerating the commercialization of lithium-ion batteries. Cathode materials with high energy density, excellent cycle stability, and safety are essential to improve the overall performance of lithium-ion batteries. This requires that these materials must have the following characteristics at the same time: (1) must have relatively low density and relatively high capacity to accommodate Li+ per unit mass, excellent cycle stability, mass capacity, and volume capacity. For example, graphite can form LiC6 during the process of deintercalation of lithium ions, thus ensuring the high theoretical capacity of graphite (372 mA hg-1). But at the same time, it also limits the improvement of the theoretical capacity of carbon materials, because in lithium-carbon alloys (LiCx), the case of x>6 is impossible; (2) The ideal negative electrode material is in the process of changing the solubility of lithium ions, the reaction The voltage should be as close as possible to metallic lithium. Because when the negative electrode material and the positive electrode material of 4V are assembled into a battery, its working voltage is lower than 4V. For example, the voltage of graphite to lithium metal is 0.15-0.25V; (3) The ideal negative electrode material cannot be dissolved in the electrolyte, and cannot react with the electrolyte and its salts. Studies have shown that for ethylene carbonate (EC), a protective film (, SEI) of a passivation layer will be formed on the surface of graphite electrodes during charge and discharge. This prevents further reactions between the electrolyte and electrodes while acting as a diffusion channel for the Li+ reaction, thereby improving cycling stability.  (4) The ideal negative electrode material should have good electron conduction and lithium-ion conduction capacity, and the electrode material should have small electron conduction resistance and more lithium-ion reaction sites; (5) The negative electrode material should be selected as rich in resources as possible, Inexpensive, widely sourced, and simple preparation process materials to reduce battery costs; (6) The positive electrode material should be stable in the air, non-toxic, and environmentally friendly. In lithium-ion battery anode materials, materials with the above advantages are mainly divided into three categories: (1) carbon materials; (2) alloy materials such as silicon and germanium; (3) materials such as transition metal oxides/sulfides, below we Introduce them one by one.

1. Carbon material

Carbon materials have become an inevitable choice for anode materials due to their abundant reserves, excellent electrical conductivity, and good cycle stability. According to the degree of graphitization of carbon materials, carbon materials can generally be divided into graphite materials (natural graphite and modified graphite) and amorphous carbon (hard carbon and soft carbon).

Graphite material: Graphite has a layered structure, the carbon atoms in the same layer are arranged in a regular hexagon, and each layer is combined by van der Waals force. Lithium ions can intercalate between graphite layers to form lithium-graphite intercalation compounds. In addition to graphite, the lithium storage mechanism of other carbon materials is similar. Graphite materials have good electrical conductivity, high crystallinity, and a stable charge-discharge platform (Figure 1). They are currently the most commercialized cathode materials for lithium-ion batteries. my country's natural graphite is rich in minerals and low in price, but natural graphite has poor liquid absorption, no cross-linked sp3 structure in the molecule, and ink sheet molecules are prone to translation, resulting in poor cycle performance of graphite negative electrodes. To improve the application range of graphite, scientists modify natural graphite to obtain modified graphite. Modified graphite can be locally disordered or form nanoscale particles in various materials through surface modification and structural adjustment of the original material. Pores, pores, channels, and other structures can increase the intercalation and deintercalation reactions of lithium ions, so they have the advantages of high density, high capacity, and long life (Table 1).

Fig.1 Charge-discharge curves of graphite, hard carbon, and soft carbon

Amorphous carbon material: Hard carbon refers to a carbon material that is difficult to graphitize above 2500°C. Common hard carbons include resin carbon and carbon black; on the contrary, soft carbon refers to highly graphitized carbon materials, and common soft carbon materials include carbon fibers and carbon microspheres. Due to the presence of some micropores or defects in the structure of hard carbon materials for Li+ storage and deintercalation, hard carbon materials have higher discharge capacity than graphite (Table 1). However, the application of hard carbon as an anode material has been limited due to low cycle efficiency, large voltage variation with capacity, and lack of a stable discharge platform (Fig. 1). The negative electrode capacity of the lithium-ion battery assembled with soft carbon as the negative electrode is about 250 mA h/g. Soft carbon is insensitive to the electrolyte and will not cause the decomposition of the electrolyte. The passivation layer formed by lithium and electrolyte on the graphite surface is not easy to decompose. Good overcharge and over-discharge performance. However, the potential of soft carbon to lithium is relatively high, about 1 V, resulting in a low terminal voltage of the battery, which limits the capacity and energy density of the battery.

Table 1 Performance comparison of common anode materials

Although human beings are on the road to exploring the unknown world, it is difficult, but the improvement of any kind of good material will bring about the innovation of energy.  In 2010, Geim of the University of Manchester won the Nobel Prize in Physics for his pioneering research on the two-dimensional material graphene, which brought the research of carbon materials into a new era. new stage.

Graphene is a special nanoscale material. Due to its excellent electrical properties, good thermal conductivity, and excellent mechanical properties, it is regarded as the "miracle material" of the 21st century, which will bring great benefits in the fields of automobiles and energy. Great innovation. The energy density of lithium batteries made of graphene is as high as /kg, which is 5 times that of traditional power lithium batteries. After the successful development of graphene batteries, the battery life of new energy vehicles will be greatly improved, and the promotion of new energy vehicles is expected to be resolved. However, due to the low density of graphene, poor dispersion performance, and high price, the development of this type of composite material is limited.

2. Alloy materials such as silicon and germanium

Si, Ge, Sn, and other metals (semi-metals) and their alloys are considered potential substitutes for future lithium battery anode materials due to their high specific capacity, and have been extensively studied in recent years; Si material is a typical representative of them.  The specific capacity of Si is as high as -1, which is more than ten times that of commercial anode material graphite; its discharge platform is relatively low (~0.2 V); in addition, Si is abundant in nature and friendly to the environment. Material. Based on the above advantages, Si material has received extensive attention. Similar to Ge and Sn, the lithium storage mechanism of Si materials is an alloying reaction, that is, Si and lithium gradually react to form a series of intermediate phases Li1.71Si, Li2.33Si, Li3.25Si, and finally form an alloy phase Li4.4Si().

Figure 3. Volume expansion of Si particles, SEI film rupture, and material detachment from the current collector after lithiation

For silicon materials, the biggest challenge comes from two aspects: ①The volume of silicon changes greatly (~300%) during the charge-discharge cycle, making it difficult to form a stable SEI) film. This causes the electrode material to be easily pulverized and separated from the current collector during charge and discharge (as shown in Figure 3), resulting in poor cycle performance and rapid capacity loss of the battery.  ② The intrinsic electrical conductivity of Si is poor (about 10-3Scm-1, which increases to about -1 after complete lithiation), and the diffusion coefficient of Li+ ions in Si is low. This makes it difficult to fully utilize the capacity of Si materials, and the rate performance of Si materials is therefore limited.

To solve the above two challenges, researchers have been trying to combine carbon materials with silicon; on the one hand, this can improve the conductivity of Si electrodes, and on the other hand, it can alleviate the huge volume expansion of Si during charging and discharging. the problem, improve the structural stability of the electrode material. By constructing Si-C composites with different morphologies (as shown in Figure 4(a) and 2(b)), the researchers obtained better cycle performance and rate performance than pure Si. In addition, hollow silicon electrodes (such as hollow silicon particles, hollow silicon nanotubes, etc., as shown in Figures 4(c) and (d)) are also widely favored. Due to its unique structure, the problem of volume expansion of hollow silicon electrode materials is effectively improved.

Figure 4. Si-C composites: (a) carbon-coated Si particles and graphene composites; (b) Si-C hollow sphere composites and hollow Si electrode materials (c) Si hollow spheres and (d) Si hollow Tube

Silicon materials are currently entering the stage of industrialization. For example, the battery of Tesla 3, an electric vehicle that has been launched recently, adds 10% Si to the graphite negative electrode, and its energy density can reach 1/kg. Nevertheless, silicon materials still have a long way to go from scientific research to practical application. In general, the application prospect of Si material is promising—high energy density and rich content, but the road are tortuous—large volume expansion and unstable structure.

3. Metal oxide/sulfide materials

Transition metal compounds that realize the lithium storage function based on the conversion reaction mechanism, as electrode materials for lithium-ion batteries, have attracted the attention and research of many researchers, and are a new generation of electrode materials for lithium-ion batteries. Ion batteries with great potential. The metal compound itself usually does not have the function of intercalating lithium, and the metal elements contained in it cannot form an alloy with lithium but can be combined with lithium through a conversion reaction (or phase inversion reaction) mechanism (or continuous lithium insertion, conversion reaction two-step mechanism) The ions undergo a multi-electron reversible redox reaction, thereby achieving the same lithium storage capacity as conventional lithium-ion battery electrode materials.

Figure 5 (a) Schematic diagram of the formation of SnO2 hollow spheres: from the inside to the outside by curing (b) and (c) SnO2 into hollow spheres (d) a-@SnO2 bell-shaped (e) cycle diagram (f) the first circle of hollow spheres Charge and discharge curve

In 2000, P. et al first reported that transition metal oxides can be used as negative electrode materials for lithium-ion batteries, and then discovered some simple transition metal compounds (fluoride, sulfide, and phosphide, etc.), such as FeF3, NiF2, FeS2, CoS2, And NiP2, etc. can also undergo transformation reactions with lithium ions. When the particle size of such transition metal compound materials is in the nanometer range, due to nanoscale effects (increased surface free energy and enhanced material reactivity), the redox reaction between them and lithium ions will show a high degree of reversibility, Graphite has a lithium storage capacity increased by 2 to 4 times, and its lithium storage capacity can be further improved through the interface charge lithium storage mechanism at a low electrode potential. For example, the composite material with SnO2 as the shell a- as the core (a-@SnO2, as shown in Figure 5), greatly improves the lithium storage capacity. The first discharge capacity is as high as hg-1, much higher than that of pure SnO2 hollow spheres. Mainly due to the reversible reaction between a- and Li,: a-+6Li++ 6e-↔2Fe+, Fe nanoparticles are embedded into the 2D lattice of Li2O. The core-shell structure not only improves the stability of SnO2 but also improves the stability of a-.

At present, metal compound negative electrode materials mainly include oxides of Sn, Co, Fe, Ni, Ti, Cu, Mo, Mn, and their composite oxide materials. Although this type of material has great market application value, it has great limitations in the actual commercialization process, and it is difficult to practically apply to commercial lithium-ion batteries. The main disadvantages are that the first Coulombic efficiency is not high (generally less than 75%), the actual discharge platform is high (1~2 V), and the intrinsic conductivity is poor, resulting in unsatisfactory cycle performance. In addition, it is difficult to prepare uniform nano-sized metal compound particles on a large scale. Based on the above shortcomings, the current main research trends of metal compound anode materials include composite modification, structure and morphology control (porous structure, core/shell, special morphology, etc.), preparation method improvement, etc.

Figure 6 SEM images of a series of metal sulfide nanomaterials prepared by Suzhou Nano

The development of carbon materials is in full swing and has long occupied the number one position in anode materials. Alloy materials and transition metal compounds have good genes, but also problems. How to make use of the advantages of various materials to develop high-performance, low-cost, and safe cathode materials is the direction of our material people's joint efforts. Perhaps our scientific research results of dancing with chickens and lighting up night battles will be submerged in the long river of history shortly. Perhaps the lithium-ion battery we are pursuing is not perfect, but it is undeniable that it will eventually be in the human body. play an important role. It has left a strong mark in the history of storage.

This article was contributed by Wang Xiaochun, Ma Nan, Ni Ling, and Material Xiaobing from the new Energy academic group of Material People, and edited by Material Niu. This article is the second in a series of articles about the debate on lithium battery technology by the Material People's New Energy Research Group. Later, we will continue to discuss this topic with you from the perspective of electrolyte (electrolyte) and industrial structure!