Characteristics and modification of various lithium battery negative electrode materials

1. Carbon materials
Carbon materials are the most widely used and most common anode materials for commercial use today, mainly including natural graphite, artificial graphite, hard carbon, soft carbon, MCNB (mesocarbon microspheres). Before the next generation of anode materials mature, carbon materials especially However, graphite materials will still be the first choice and mainstream of negative electrode materials.

1.1 Graphite
Graphite is divided into natural graphite and artificial graphite according to the difference between its raw material and processing technology. Because of its low potential for lithium, high initial efficiency, good cycle stability, and low cost, graphite has become an ideal choice for lithium-ion battery applications. Negative material.

Natural graphite: Generally, natural flake graphite is used as raw material, which is modified to make spherical natural graphite for use. Although natural graphite is widely used, there are several disadvantages: ① natural graphite has many surface defects, large specific surface area, and low initial efficiency; ② using PC-based electrolyte, there is a serious co-intercalation phenomenon of solvated lithium ions, which leads to graphite layer expansion Peel off, and the battery performance will fail; ③Natural graphite has strong anisotropy, lithium ions can only be inserted from the end face, and the rate performance is poor and lithium is easily separated.

Modification of natural graphite:
①Aiming at the problems of many surface defects and poor electrolyte tolerance of natural graphite, different surfactants were used for modification. CHENG et al. improved the rate performance of natural graphite by changing the surface of the pore structure and increasing the micropores and lithium intercalation paths on the graphite surface by etching with a strong alkali (KOH) aqueous solution and sintering in a high-temperature oxygen-free atmosphere.

②Aiming at the problem of strong anisotropy of natural graphite, mechanical treatment is often used in industrial production to shape the shape of the particles spherically. The airflow shaper uses wind impact to make the particles rub against each other and cut the edges and corners of the particles. This method does not Doping impurities will be introduced, and the spheroidization efficiency is high, but it will cause a large number of particles to be pulverized, and the yield is low. The mechanical fusion machine uses the high-speed rotation of the material in the rotor, clings to the wall of the device under the action of centrifugal force, and passes between the rotor and the stator extrusion head at high speed. At this moment, the material is simultaneously subjected to extrusion force and shear force, and under the action of friction between particles and particles and between particles and equipment, the surface presents a mechanical melting state to achieve the purpose of spherification. Natural graphite has been spheroidized, and the particle size D50 ranges from 15 to 20 μm. The initial efficiency and cycle performance are significantly improved, and the rate performance is greatly improved.

Artificial graphite: It is generally made of dense petroleum coke or needle coke as a precursor, which avoids the surface defects of natural graphite, but still has problems such as poor rate performance due to crystal anisotropy, poor low temperature performance, and easy lithium decomposition when charging. The modification method of artificial graphite is different from that of natural graphite. Generally, the purpose of reducing the degree of graphite grain orientation (OI value) is achieved through the reorganization of the particle structure. Usually, needle coke precursors with a diameter of 8-10 μm are selected, and easily graphitized materials such as pitch are used as the carbon source of the binder, and processed through a roller furnace to bond several needle coke particles to make a particle size D50 range of 14 ~18μm secondary particles complete graphitization, which effectively reduces the OI value of the material.

1.2 Soft Carbon
Soft carbon, also known as graphitizable carbon material, refers to an amorphous carbon material that can be graphitized at a high temperature above 2500°C. Generally speaking, according to the difference in the sintering temperature of the precursor, soft carbon will produce three different crystal structures, namely amorphous structure, turbulent layer disordered structure and graphite structure. The graphite structure is also the common artificial graphite. Among them, the amorphous structure has attracted extensive attention because of its low crystallinity, large interlayer spacing, and good compatibility with the electrolyte, so it has excellent low-temperature performance and good rate performance.

Soft carbon has high irreversible capacity, low output voltage, and no obvious charge-discharge platform when it is charged and discharged for the first time. Therefore, it is generally not used independently as a negative electrode material, but is usually used as a negative electrode material coating or component. Liu Ping et al. doped a certain proportion of soft carbon in the graphite negative electrode, and found that the low-temperature charging performance of the battery can be improved, and the higher the doping content, the better the low-temperature charging performance, but the cycle performance will decline in the later stage. After experimental demonstration, Doping with 20% soft carbon can achieve a performance balance between low-temperature charging and cycle life.

1.3 Hard carbon
Hard carbon, also known as non-graphitizable carbon material, is difficult to graphitize at high temperatures above 2500°C, and is generally obtained by heat treatment of the precursor in the range of 500-1200°C. Common hard carbons include resin carbon, organic polymer pyrolytic carbon, carbon black, and biomass carbon. Among them, phenolic resin can be pyrolyzed at 800°C to obtain hard carbon materials, and its initial charging capacity can reach 800mAh/g , interlayer spacing d002>0.37nm (graphite is 0.3354nm), large interlayer spacing is conducive to the intercalation and deintercalation of lithium ions, so hard carbon has excellent charge and discharge performance, and is becoming a new research hotspot for negative electrode materials. However, hard carbon has high irreversible capacity for the first time, lagging voltage platform, low compaction density, and easy gas production are also its shortcomings that cannot be ignored.

2. Lithium titanate material
2.1 Advantages and disadvantages of materials
Lithium titanate material: Lithium titanate (LTO) is a composite oxide composed of metallic lithium and low-potential transition metal titanium, which belongs to the spinel solid solution of the AB2X4 series. The theoretical gram capacity of lithium titanate is 175mAh/g, and the actual gram capacity is greater than 160mAh/g. It is one of the negative electrode materials that have been industrialized at present.

Unique advantages:
①Zero strain: the lithium titanate unit cell parameter a=0.836nm, the intercalation and deintercalation of lithium ions during charging and discharging have almost no effect on its crystal structure, avoiding the structural changes caused by material stretching during charging and discharging, thus having extremely High electrochemical stability and cycle life;

② No risk of lithium analysis: Lithium titanate has a potential of up to 1.55V for lithium, no SEI film is formed on the first charge, high initial efficiency, good thermal stability, low interface impedance, excellent low-temperature charging performance, and can be charged at -40°C;

③Three-dimensional fast ion conductor: Lithium titanate is a three-dimensional spinel structure, the intercalation space of lithium is much larger than that of graphite layer, and the ion conductance is an order of magnitude higher than that of graphite material, which is especially suitable for high-rate charge and discharge.

Lithium titanate also has low battery specific energy due to its low gram capacity and low voltage platform; nano-materials have strong hygroscopicity, resulting in serious high-temperature gas production and poor high-temperature cycles; the material process is complicated and the cost is extremely high, and the cost of the battery cell is the same The energy is more than 3 times that of the lithium iron phosphate battery.

2.2 Application and Precautions of Materials
Application fields: The advantages and disadvantages of lithium titanate are very obvious, and the performances are relatively extreme. Therefore, it is the correct application method to apply to specific subdivision fields and give full play to its strengths. At present, lithium titanate batteries are mainly used in urban pure electric BRT buses, electric hybrid buses, power frequency modulation peak shaving auxiliary services and other fields.

Note: In view of the serious problem of high-temperature gas production of lithium titanate, the current industrial production needs to strictly control the environmental humidity and the introduction of water during operation; add new additives to the electrolyte to inhibit side reactions at the interface between lithium titanate and the electrolyte; improve raw materials Purity, to avoid the introduction of impurities in the manufacturing process.

3. Silicon-based materials
3.1 Research hotspots and improvement directions
Research hotspots: Silicon is considered to be one of the most promising anode materials. Its theoretical gram capacity can reach 4200mAh/g, which is more than 10 times higher than that of graphite materials. At the same time, the lithium intercalation potential of Si is higher than that of carbon materials, and the risk of lithium deposition during charging is small. safer. At present, the research hotspots of silicon-based materials are divided into two directions, namely nano-silicon carbon materials and silicon-oxygen (SiOx) negative electrode materials.
Application problems: ①The huge volume expansion and shrinkage caused by the deintercalation of lithium lead to particle breakage and pulverization and the destruction of the electrode structure, resulting in the failure of electrochemical performance; Liquid and reversible lithium sources lead to accelerated electrode capacity decay and a sharp drop in charge-discharge efficiency.

The direction of improvement: In response to the above problems, scholars have continuously explored new methods to improve the performance of silicon anode materials in recent years. The current mainstream direction is to use graphite as a matrix and mix 5% to 10% of nano-silicon or SiOx to form a composite material and Carry out carbon coating to suppress particle volume change and improve cycle stability.

3.2 Nano silicon carbon materials
Material design: The initial research on nano-silicon carbon materials mainly focused on the low-capacity direction of 400-500mAh/g, and the material structure mainly includes core-shell type and embedded type. At the beginning of the design, Li Hong’s team considered increasing the graphite content of the matrix as much as possible, alleviating the strain of lithium deintercalation, and reducing the rebound; in addition, the type, content and sintering process of the surface coating agent were optimized to improve the integrity of the coating layer and introduce liquid phase dispersion. process, improve the uniformity of dispersion, and better play the nano-silicon size effect.

Optimizing the battery chemical system: In addition to material design, the battery chemical system is also optimized by studying binders, conductive agents and electrolytes. The 600-cycle capacity retention rate of 400mAh/g silicon carbon materials is over 80%. On this basis, through optimization Granular structure, developing high power type materials. At present, lithium-ion batteries made of low-capacity materials have been mass-produced in the industry, but from the actual results, the improvement of battery specific energy is extremely limited.

Preparation process of doped nano-silicon: due to the low graphite content of high-capacity silicon-carbon anodes, the research focus is on the problems of poor cycle stability and poor charge-discharge efficiency caused by the volume expansion of silicon particles, and at the same time it is necessary to deal with the new problems of difficult dispersion and poor processability . Starting from raw materials, Li Hong’s research group developed a low-cost, high-efficiency doped nano-silicon preparation process, supplemented by gas phase coating, which reduces the specific surface area of the material and improves its surface characteristics and processing performance. It is blended with graphite to make a 500mAh/g negative electrode material, and the compaction density is properly reduced during the application process, and the capacity retention rate of 500 cycles can reach 80%.

The preparation process of composite materials: a preparation process of large-scale silicon-carbon composite materials, using a micro-nano composite structure, so that nano-silicon is uniformly dispersed in a three-dimensional conductive network. In cooperation with Ningbo Institute of Materials, the negative electrode material of 600mAh/g is made by mixing with graphite, and the lithium-rich phase material is selected for the positive electrode. The energy density of the developed pouch battery is as high as 374Wh/kg.

3.3 SiOx material
Lithium supplementation: The reversible capacity of SiOx materials is as high as 1500-2000mAh/g, and the volume expansion during the lithium intercalation process is only 120% (nano-silicon materials can reach more than 300%), which greatly improves the cycle life of Si-based materials. However, during the first intercalation process of SiO material Li, Li4SiO4 with no electrochemical activity will be generated, resulting in the initial efficiency of SiOx materials being far lower than that of graphite and silicon carbon materials, which has also become the main obstacle for the application of SiOx materials. Therefore, for The research on SiOx materials mainly focuses on how to reduce the first irreversible capacity. For this reason, researchers have developed different methods of lithium supplementation in an attempt to compensate for the active lithium consumed by the negative electrode during the first charging process.

Granulation: Grind and mix SiO, MgO and Si materials by ball milling to obtain nanoscale particles, and use spray drying to granulate. The MgO component in the prepared composite material reacts with SiO2 in the SiOx material to form MgSiO3. The irreversible loss of the first lithium intercalation is greatly reduced, and the first efficiency of SiOx materials is increased by more than 8%. The preparation method of the material is simple and efficient, and has the potential for large-scale production.

Lithium ion pre-intercalation: Use inert lithium metal powder (SLMP) to directly and uniformly disperse on the surface of the silicon-oxygen electrode. After activation by rolling and infiltration of the electrolyte, SLMP will release lithium ions and pre-intercalate the silicon-oxygen electrode, greatly improving the first Coulombic efficiency and Discharge specific capacity.

Electrochemical pre-lithium: The degree of pre-lithiation and voltage can be monitored in real time by using the method of short-circuiting the external circuit, so the amount of lithium intercalation can be effectively controlled to avoid lithium deposition. The existence of the separator is helpful for uniform lithium intercalation and the formation of a stable SEI membrane. After pre-lithiation, the first coulombic efficiency of the full battery composed of NCA can reach 85.34%, and the cycle stability is also improved.

Research direction: The pre-lithiation process of SiOx materials is still in the laboratory stage due to high requirements on the environment and cannot be applied on a large scale. Therefore, the follow-up research will focus on the pre-lithiation of positive electrode materials and the pre-lithiation of SiOx materials.

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