The role of natural graphite in lithium-ion batteries

Lithium ion battery, also known as rocking chair battery, its main components are positive electrode, negative electrode, diaphragm and electrolyte. Currently, spinel LiMn2O4 or nickel base oxide is generally adopted as the positive electrode of lithium ion power battery, while graphite is the main negative electrode, and LiPF6 carbonate (EC, EMC) organic solution is used as the electrolyte. LiMn2O4 is considered the safest material and the cheapest anode material, and has been used in a variety of power batteries. Li(NiCo)O2 has a high capacity but poor safety performance, which needs to be improved by doping modification and limiting its voltage. Considering the safety and battery cost of the vehicle, the LiFePO4 LiFePO4 has good safety and long service life, and is the most suitable anode material for the lithium ion battery used in the automobile power battery.

The energy density of lithium ion battery largely depends on the anode materials. From the commercialization of lithium ion battery to the present, the most mature anode materials and the most widely used are carbon materials, among which graphite is still the most important. Graphite has a six-membered ring carbon net layered structure, carbon carbon is SP2 hybridized between the layer is the molecular force connection. There are two different crystal structures in graphite: hexahedral graphite (2H) and rhombohedral graphite (3R). The accumulation structure of 3R phase is ABCABC. Two phase can change each other, 2 h is phase thermodynamic stability, in the graphite is more, about four 5 of the total in the lithium ion battery anode materials, natural graphite and artificial graphite has been the largest anode materials used, but due to artificial graphite in the process of production need high temperature treatment, the production cost greatly increased and the environmental impact, relative to the artificial graphite, natural graphite has many advantages, its low cost, high degree of crystallization, purification, crushing, grading, mature technology, low charge and discharge voltage platform, theory of high specific capacity, these for its application in lithium ion battery industry laid a good foundation.

Natural graphite is divided into amorphous graphite (soil-like graphite or microcrystalline graphite) and flake graphite. Theoretical capacity is 372mAh/g. The purity of amorphous graphite is low, and the crystal plane spacing (d002) of graphite is 0.336nm. It is mainly a 2H crystal plane sequencing structure, that is, the graphite layer according to ABAB... In order, the orientation of individual microcrystals presents anisotropy, but after processing, the microcrystalline particles interact with each other to a certain extent, forming massive or granular particles with isotropic properties. And the formed block particles are easy to be crushed into well-shaped particles.

In the process of lithium ion implantation and de-implantation, the volume changes little and the structure is relatively stable, but the reversible specific capacity is only 260mAh/g and the irreversible specific capacity is more than 100mAh/g. Flake graphite has high crystallinity, large unit structure and obvious anisotropy. This structure determines that the volume of graphite changes greatly in the process of lithium implantation and de-implantation, leading to the destruction of graphite layer structure, which leads to large irreversible capacity loss and dramatic deterioration of cycle performance.

As a lithium-ion battery anode graphite, microlite ink and flake graphite has for the first time the irreversible capacity big shortcoming, and flake graphite cycle performance and large current charge and discharge performance is poor, so, when use, the researchers tend to focus on the modification of natural graphite is studied, to improve its own structural faults, improve the performance of the battery. Among them, the modification of graphite anode mainly includes surface treatment, surface coating and element doping, etc. The modification research will be described in detail below.

Modification of graphite anode materials

1. Surface oxidation

Surface oxidation mainly in irregular electrode interface (sawtooth and rocking chair) production of acidic groups (such as - OH, - COOH, etc.) before the intercalated-li these groups can prevent solvent molecules were embedded and improve the wettability between the electrode/electrolyte, reduce impedance interface, for the first time when the intercalated-li into carboxylic acid lithium salt and surface - Oli groups, form a stable SEI film. In addition, oxidation can remove some of the defective structure of graphite, and the nano-level micropores generated can be used as additional lithium storage space to improve lithium storage capacity.

Surface oxidation usually includes gas phase oxidation and liquid phase oxidation. Mainly air gas phase oxidation, O2, O3, CO2, C2H2 gases as oxidant, such as gas-solid interface reaction with graphite, reduce active points on the surface of the graphite, reduce the irreversible capacity loss for the first time, at the same time, generate more pores and nanopores, adding lithium ion storage space, to improve the reversible capacity, improve the cathode performance. Wu Yu equal to the ordinary natural graphite under 500 ℃ in air oxidation modification to treat. The stability of modified graphite structure was improved, and the number of nanometer pores and channels were increased. In addition, the oxide layer formed during oxidation is closely bound to graphite to form a dense passivation film, which prevents the solvation reaction of electrolyte to graphite and improves the reversible capacity of graphite. Liquid phase oxidation method is to use cerium sulfate, sulfuric acid, nitric acid, hydrogen peroxide and other strong oxidant solution, through the liquid-solid phase reaction to achieve. The surface oxidation of natural graphite with saturated solution of sulfuric acid and ammonium persulfate was carried out, and the reversible capacity of graphite was increased to 349mahg-1, and the coulomb efficiency was improved for the first time.

2. Surface coating

The surface modification of graphite anode materials mainly includes carbon cladding, metal or nonmetal and its oxide cladding and polymer cladding. The reversible specific capacity, initial coulomb efficiency, cycling performance and charge-discharge performance of the electrode can be improved by surface coating. The starting point of surface coating modification of graphite materials is as follows:

By means of surface cladding, the specific surface area of graphite is reduced, and the lithium consumed by SEI film is reduced, so as to improve the material's first coulomb efficiency.

By surface coating, the active points on the graphite surface are reduced, the surface properties are uniform, the co-embedding of solvent is avoided, and the irreversible loss is reduced.

3. Amorphous carbon cladding

In cover a layer of amorphous carbon graphite "core - shell structure of C/C composites, the amorphous carbon contact with solvent, solvent to avoid direct contact with the graphite, stop by solvent molecules were embedded in the graphite layer stripping phenomenon, expand the scope of the choice of the electrolyte, the kingdom of equality will natural flake graphite made into spherical graphite, in its surface coating a layer of nanometer graphitized carbon materials with core - shell structure modification of spherical graphite, modification of spherical graphite tap density improved significantly, and the reversible capacity of up to 365 mah g - 1, at the same time, For the first time, the coulomb efficiency and cycle stability were also significantly improved.

Lithium ion batteries with high capacity, high voltage, high cycle stability, high energy density, no environmental pollution and other excellent properties of popularity, known as green energy and the dominant power in the 21st century, has a broad prospect of civilian and defense applications, expanding its application field, not only has been widely and successfully applied to all kinds of portable electronic products, has begun to develop in the direction of power battery. At present, lithium ion battery and its key materials have become a focus of science and technology and industry in many countries. Until now, lithium ion batteries have been commercialized with the most mature anode materials and the most widely used carbon materials, among which graphite is still the most important. Natural graphite has the advantages of low cost, high crystallization degree, mature purification, crushing and grading technology, low charging and discharging voltage platform and high theoretical capacity. However, the structural defects of natural graphite lead to low efficiency and poor circulation for the first time. Therefore, it is imperative to develop methods to modify natural graphite.

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