Characteristics And Modification Of Various Lithium Battery Anode Materials
- Carbon material
- Lithium titanate material
- Silicon-based materials
According to the structural classification of anode materials, this article will briefly introduce the research and development progress of various lithium ion battery anode materials in terms of structural characteristics, performance characteristics, and improvement directions, focusing on the development status and future trends of next-generation high-energy-density battery anode 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.
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.
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.
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.
Lithium titanate material
Advantages and disadvantages of lithium titanate materials
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. The advantages of lithium titanate cell are in the following.
- Zero 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.
- 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;
- Three-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.
Shortcomings are also listed:
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.
Application and precautions of materials
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.
Research and improvement directions of silicon-based materials
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 and 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.
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.
Nano silicon carbon materials
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.
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,and 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, li-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.
- 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.
Scientists 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 composite material and the SiOx material in the prepared SiO2 reacts to generate MgSiO3, which greatly reduces the irreversible loss of the first lithium intercalation, 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
Scientists reported that inert metal lithium powder (SLMP) was directly and uniformly dispersed on the surface of silicon-oxygen electrode, and after activation by rolling and infiltration of electrolyte, SLMP released lithium ions and pre-intercalated silicon-oxygen electrode, which greatly improved The Coulombic efficiency and specific discharge capacity were obtained for the first time.
- Electrochemical pre-lithiation
Scientists have developed an accurate electrochemical pre-lithiation method, which adopts the method of short-circuiting the external circuit, and its pre-lithiation degree and voltage can be monitored in real time, so the amount of lithium intercalation can be effectively controlled to avoid lithium deposition.
The existence of the separator helps to intercalate lithium uniformly and form a stable SEI film. 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.
After continuous replacement and modification by researchers, silicon-based materials have become the most promising next-generation anode materials, but the inherent characteristics of large volume expansion and poor cycle performance limit large-scale applications. In recent years, the improved most of the traditional methods have the problems of complicated process and high cost.