The ideal anode material must have the following seven conditions: low chemical potential, and the anode material to form a large potential difference, so as to obtain a high power battery; High cycle capacity should be available; In the anode material, Li+ should be easy to be embedded and detached, with higher coulombic efficiency, so that the charging and discharging voltage can be relatively stable in the process of Li+ removal. Good electronic and ionic conductivity; Good stability, electrolyte compatibility; For the source of materials should be rich in resources, low cost, simple process; Safe, green and pollution-free.
Currently, there are few materials that meet the above conditions. Therefore, it is urgent to study new cathode materials with high energy density, good safety, low price and easy availability, which is also a hot topic in the field of lithium battery research. Currently, lithium ion battery anode materials mainly include carbon materials, transition metal oxides, alloy materials, silicon materials and other silicon materials, transition metal nitride materials containing lithium and lithium titanate materials.
1. The carbon materials
Including graphite, graphene, carbon nanotubes.
Carbon nanotube (CNT) is a kind of special carbon material with relatively complete graphitized structure. Due to its special structure, the negative electrode has a small depth, short travel distance and fast speed when removing lithium, and it has a small polarization effect when charging and discharging with large current, which is very helpful to improve the performance of lithium battery with large multiplier and fast charging and discharging. When carbon nanotubes are used as the anode material of lithium ion battery, there are some prominent problems such as high irreversible capacity of lithium battery, low coulomb efficiency of first charge and discharge, not obvious charging and discharging platform and serious voltage lag. The carbon nanotube is directly used as the anode material. Some data show that its first discharge capacity is 1,500 ~ 1,700 mah /g, but the reversible capacity is only 400mAh/g. With the further charge and discharge cycle of lithium battery, the reversible capacity is lower and the decay rate is faster. This leads to further applications in lithium batteries.
However, CNT can be compounded with graphite negative electrode, silicon-based composite negative electrode, lithium titanate, tin base and other kinds of materials, making full use of its unique hollow structure, good electrical conductivity, large specific surface area and other advantages, and using it as a carrier or additive to improve the electrochemical performance of the original system of negative electrode materials. Experimental results show that CNT can not only buffer the volume change of the composite anode material during the removal of lithium, but also improve the multiplier performance and cycle life of the composite anode material.
As the most cutting-edge carbon material, graphene has very excellent electrochemical properties. It is possible to be used as anode material of lithium battery. Some experimental results showed that 3d graphene sheets with jungle structure morphology were prepared by using natural graphite as raw material, stripping through chemical reaction and reducing with hydrazine hydrate. The 3d graphene sheets had some excellent characteristics of both hard carbon and soft carbon negative electrode, and showed excellent electrochemical characteristics of capacitors in the voltage interval higher than 0.52v.
3. The silicon anode
The silicon negative electrode is considered to be the ideal choice for the next generation of lithium ion batteries due to its ultra-high specific capacity of 3590mAh/g. Silicon anode material greatly improves the energy density of lithium ion batteries, which is the urgent need of a series of new technology fields such as portable electronic products, unmanned aerial vehicles, new energy vehicles and energy storage battery systems. Its low cycle life seriously hinders its commercial application. The low cycle life of silicon cathode is due to its large volume expansion during charging and discharging. However, due to the volume expansion effect of silicon negative electrode, the mechanical stability of nano-silicon particles and electrode plates becomes worse, the contact between active particles is not good, and the stability of surface SEI passivation film decreases, which leads to the challenges of the life and safety performance of lithium batteries.
4. Silicon negative electrode
Currently, silicon-carbon composite anode materials are basically core-shell structures, which are composed or coated with Si nanoparticles on the surface of graphite based on spherical or artificial graphite, and then coated with amorphous carbon, carbon nanotubes or graphene on the outside. The principle and essence of carbon coating are as follows: the volume expansion of Si negative electrode is jointly undertaken by graphite and coating layer, so as to avoid or reduce the pulverization of silicon negative electrode material due to huge volume change and stress in the process of delamination of lithium. The effects of carbon coating are as follows: 1. Preventing the agglomeration of nano active particles; Prevents electrolyte penetration into the center and maintains a stable interface and SEI.
Carbon anode materials have good cyclic stability and excellent electrical conductivity, and lithium ions have no obvious influence on the spacing between layers. To a certain extent, they can buffer and adapt to the volume expansion of silicon, so they are often used to compound with silicon.
5. Tin-based anode material
Tin is one of the early anode materials in lithium charge and also a hot field. Tin-based anode materials have high specific capacity and are considered to have great potential to replace traditional graphite anode materials. However, the disadvantages are also obvious: serious volume expansion, electrode pulverization and particle agglomeration during charging and discharging, resulting in rapid attenuation of capacity and low conductivity of lithium-ion batteries. To develop and find effective methods for preparing tin anode and composite materials, and to improve the electrical conductivity of composite anode electrode materials is the key to improve the electrochemical performance of tin anode and the premise of its large-scale application.
6. Tin oxide anode material
SnO2 negative electrode material has attracted much attention from academic and industrial circles due to its high specific capacity (1494 mAh/g). It is also a research hotspot in the field of negative electrode, which has been involved by many companies. Their faces in the process of charge and discharge cycle and tin anode also some problems: the irreversible capacity big, coulomb efficiency is low, at the same time will exist in the process of charging intercalated-li larger volume expansion, the volume expansion ratio can reach 250% ~ 300%, and likely to occur in the process of circular particle agglomeration, restricted its market application.
The experimental results show that the preparation of carbon-based SnO2 composite negative electrode material can effectively inhibit the agglomeration of negative SnO2 particles, alleviate the serious volume expansion effect when lithium is embedded, and improve the stability of negative SnO2 in charge and discharge cycle. Experimental results show that using graphite carbon material as the carrier can not only disperse SnO2 particles evenly, but also effectively inhibit the agglomeration of SnO2 particles and improve the cyclic stability and frequency of the material. Therefore, the composite negative electrode material based on SnO2 will be the future development direction of tin oxide.
How to make use of the advantages of various materials to develop high performance, low cost and safe anode materials is the direction of our joint efforts.