Lithium Storage Principle of Graphite Anode Materials

Lithium Storage Principle of Graphite Anode Materials

Graphite has the advantages of low cost, abundant reserves, low voltage platform, and small volume change during intercalation and deintercalation, and is an ideal anode material for lithium-ion batteries. Graphite materials are currently the mainstream of the anode electrode market. The specific capacity of marketed graphite anode electrodes is above 330 mAh/g, and the first Coulombic efficiency is higher than 90%.

Graphite has a layered structure suitable for intercalation and deintercalation of lithium ions, and can form lithium-graphite intercalation compounds.

The graphite crystal is a layered structure, and the carbon atoms are arranged in a hexagonal and two-dimensional direction to form a graphite sheet. The structural parameters of the graphite crystal are mainly La, Lc, d(002) and G. La represents the average width of the graphite crystal along the a-axis direction, Lc represents the average height of the graphite crystal along the c-axis direction, and d(002) is the distance between two adjacent graphite sheets. The degree of graphitization G is the degree of ordering of the carbon material, and the larger the G value, the closer its properties and structure are to ideal graphite.

The carbon atoms in the graphite crystal layer are covalently bonded to form a hexagonal structure, and the layers are bonded by weak Van Der Waals (Van der Waals force). This special structure allows lithium to intercalate between the layers of graphite crystals without destroying the two-dimensional network structure of graphite, thereby expanding the interlayer spacing. This process is reversible, so the intercalation and delithiation of graphite materials are also reversible. The intercalation of lithium ions into graphite forms an interlayer compound, usually denoted LixC6, where the size of x is related to factors such as the type and structure of the material, the composition of the electrolyte, and the rate of Li+ movement. When x=1, LiC6 is formed, which is a first-order lithium-graphite interlayer compound, and the maximum theoretical specific capacity of graphite is reached at this time.

The process of lithium ion migration to the graphite anode can be roughly divided into the following four steps:

(1) Diffusion of solvated lithium ions in the electrolyte;

(2) The solvated lithium ions reaching the surface of the graphite anode electrode begin to desolvate;

(3) Desolvated Li ions pass through the solid electrolyte (SEI) membrane and intercalate between graphite layers with charge transfer;

(4) Lithium ions diffuse inside the graphite particles.

Finally, lithium accumulates in graphite through phase transitions between different intercalation stages.

There are three ways of lithium storage in graphite materials: interlayer lithium storage, end face lithium storage and surface lithium storage. The graphite interlayer provides the main lithium storage capacity, and this part of lithium ions is represented as LiL, that is, interlayer lithium storage. The more developed the graphite layer (Larger La), the more lithium inserted and the greater the lithium storage. When the voltage is lower than 0.25V, the vast majority of lithium ions begin to intercalate into the layers and form different orders of graphitic intercalation compounds. In the initial lithium intercalation stage, the lithium ion concentration is low, and a single lithium ion layer is preferentially formed due to the existence of the repulsive force between lithium ions. As the concentration of lithium ions increases, the valence state of lithium ions changes to +ɛ, the higher the lithium content, the more ɛ tends to 0, and the anode electrode will form graphite layers with different orders of 4th, 3rd, 2nd, and 1st orders. compound until saturation is reached.

It is believed that the intercalation process of lithium ions between graphite layers is an intercalation process from high-order to low-order. During this process, the graphite sheets gradually expanded within the stress range, the graphite interlayer spacing increased from 0.335 nm to 0.370 nm, and the intercalation capacity gradually increased. During the discharge process, lithium ions are extracted from the graphite layers, and the graphite crystals return to their original state. The exposed carbon atoms at the edge of the graphite sheet are in an amorphous state with high energy, which is the active site of lithium ions. The bonding of carbon atoms and lithium ions on the graphite surface is similar to that of the edge, and this part of the lithium ions is represented as LiS, that is, the surface lithium storage.

The above is the introduction of the lithium storage mechanism of graphite materials. The good layered structure of graphite materials is conducive to the insertion and extraction of lithium ions during the charging and discharging process, so it has become the mainstream of the development and utilization of lithium ion battery anode materials. Although graphite materials also have their own defects, it is difficult to see other alternative materials that can be commercialized on a large scale in the short term.