Lithium battery anode materials, capacity, life, safety, are the main performance requirements of the most concerned.
The theoretical capacity density of lithium batteries depends on the theoretical capacity of positive and negative materials. How does the theoretical capacity of materials come from?
1. theoretical capacity of materials
How many active lithium ions can be brought by each mole of material molecule. By dividing the Coulombic electric quantity of all lithium ions by the molar mass of the material, the Coulombic electric quantity per unit mass can be obtained. After unit transformation, it is equivalent to the corresponding ampere-hour per unit mass.
Take carbon as an example: LiC6 exists in graphite. Six C atoms can store one lithium ion. The molar mass of lithium ion is 6*12=72g. A molar of LiC6 will transfer 1 mole of electron by complete reaction.
Electricity per mole:
(6.02 x 10 ^ 23)* (1.602176 x 10 ^ - 19C)/3.6
= 9.645009/3.6 = 2.6792*10^4mAh
The number of electrons per mole is 6.02 x 10 ^ 23.
The charge per electron is 1.602176 *10^-19C.
Carbon atomic weight 12;
The storage capacity per unit mass of graphite negative electrode is 2.6792*10^4/(12*6)=372.1 mAh/g.
In this way, we can understand how the theoretical capacity of negative material is derived. The theoretical limit of material determines the theoretical limit of lithium battery. In practice, there will be a discount and then a discount.
Then, from the calculation formula, the determinants of the theoretical capacity of the electrode material are the molecular weight of the material and the number of active lithium ions corresponding to each molecule.
2. Anode Materials for Research and Application
Molecular structure determines the theoretical limit of material charge capacity, and the actual physical structure of material also affects the capacity.
Currently, the types of anode materials are generally divided into carbon materials, silicon matrix and its composite materials, nitride anodes, tin-based materials, lithium titanate, alloy materials and so on. Among them, carbon materials are the mainstream, and most commercial lithium battery production is carbon material negative electrode.
2.1 carbon materials
There are many kinds of carbon materials, such as natural graphite anode, artificial graphite anode, mesophase carbon microsphere (MCMB), soft carbon (such as coke), hard carbon anode, carbon nanotubes, graphene, carbon fiber and so on. Among them, natural graphite anode and artificial graphite anode are widely used.
Graphite is widely used as negative pole for many reasons.
Firstly, the low potential and the discharge platforms range from 0.01V to 0.2V make it easy for the battery to obtain a higher output voltage. Secondly, the layered graphite stacking structure makes lithium ions shuttle freely between layers, which has less obstacle. The van der Waals force between graphite layers prevents the graphite from deformation and collapse when it contains lithium ions.
Finally, element C is abundant on the earth, easy to obtain directly, and easy to realize by manual processing and manufacturing.
The defect of graphite negative electrode is also very obvious. It is easy to react with electrolyte to form SEI film. The aging and thermal runaway of the core are largely due to the aging and stabilization of SEI film. This leads to a definite upper limit on the lifetime of graphite negative lithium batteries.
2.1.1 Natural Graphite
Natural graphite is the original form of carbon in nature, which is easy to obtain directly. Its basic layered structure is suitable for the insertion and removal of lithium ions. However, it is easy to react with electrolyte and has poor recyclability. Generally, it can not be directly used for commercial purposes, but is modified for use.
2.1.2 Artificial Graphite
Gemstones are considered natural, but artificial graphite has properties that natural graphite does not possess. Artificial graphite is a kind of carbon material which is easy to be graphitized and fired at high temperature. It forms large voids in the interior, which brings advantages for lithium ion storage. Artificial graphite is the preferred material for power batteries because of its good cycling ability and ability to withstand the test of high current charging and discharging.
2.1.3 Other Graphite Materials
Graphitized mesophase carbon microspheres, soft carbon and hard carbon, are all processed from high carbon content materials. There are common problems in cycle life, and they have not been applied too much for the time being.
2.1.4 Carbon Materials: Carbon Nanotubes and Graphene
Carbon nanotubes (CNTs) are hollow tubes with nanometer diameter and micrometer length, which are usually closed at both ends. It has excellent conductivity and thermal conductivity. In engineering, it is used more and more widely.
Carbon nanotubes (CNTs) are directly used as negative electrodes. They are helpful for lithium batteries in high rate discharge, but their reversible capacity is low and their life is short, so they can not be used directly for the time being. The current research direction is to use carbon nanotubes in combination with other materials, in order to give play to its advantages of conductivity, heat conduction and lithium intercalation.
Micromodel of Carbon Nanotubes
Graphene is known as the king of new materials, and its discoverer won the Nobel Prize in Physics. Graphene is described as a two-dimensional material consisting of a single layer of carbon atoms with high strength, high conductivity and thermal conductivity.
Graphene can theoretically improve the capacity and charge-discharge ratio of the battery when it is applied to the negative electrode of the battery. It's nice to think about the news of charging for 8 minutes and lasting 500 kilometers. It is difficult to prepare graphene in batches, and there is not much experimental data.
2.2 Silicon Anode Material
Silicon anode materials, with theoretical capacity of 3590 mAh/g and similar properties to carbon, have been an important field in the research of anode materials and are considered to be the most likely alternative to carbon materials. The main disadvantage is that when lithium ion is embedded, the interaction between ion and silicon substrate is too significant, which makes the distance between layers of material increase significantly, and the charging and discharging process.