August 5, 2020
The working principle of three kinds of anode materials for lithium ion batteries is explained in detail
With the acceleration of social development, people's demand and reliance on batteries increase day by day.
The battery is closely related to people's lives and widely used in a variety of small portable electronic devices. With the prominent problem of energy shortage and environmental pollution, the battery has become the core link in large equipment such as electric vehicles and clean energy storage equipment.
Among the many battery systems, as shown in Figure 1, the lithium-ion battery is the most attractive.
At present, the actual capacity of anode materials in lithium-ion batteries is generally low, which has become the focus and difficulty of research.
Understanding the structure and working principle of the common cathode materials of a lithium-ion battery can help us deeply understand the core problem of a lithium-ion battery.
The lithium-ion battery is A device that realizes the mutual transformation of chemical energy and electric energy through the insertion and disinsertion of lithium ions between the positive and negative electrode materials. It is also vividly described as the rocking chair battery, which was first proposed by A. Armand in 1980. Its structure and charging and discharging principle is shown in Figure 2.
Lithium-ion battery cathode candidate materials can divided into the following three categories according to their structure :(1) LiMO2 (M=Co, Ni, Mn) anode materials with layered structure;
(2) LiMn2O4 positive electrode material with spinel structure;
(3) LiFePO4 positive electrode material with olivine structure.
1. Layered LiMO2 (M=Co, Ni, Mn) positive electrode material
LiMO2 (M=Co, Ni, Mn) positive electrode material with a layered structure is developed based on layered LiCoO2 content. It realized by replacing part of Co with Ni and Mn metal, and its composition is similar to layered LiCoO2.
As shown in FIG. 3, Li+ located between regular octahedral plates in a layered arrangement.
Therefore, in charging and discharging, lithium ions can move two-dimensional from the plane in which they located, and the embedding and de-embedding of lithium ions are faster.
The electrochemical process is as follows:
It's all about LiMO2. It's all at xMO2 + xLi+ + xe-
In the layered structure LiMO2 (M=Co, Ni, Mn), different transition metal materials have slightly different synthetic and electrochemical properties. It is summarized as follows :(1) in the layered LiCoO2 structure, the reversible embedding and delamination amount of lithium ions are only 0.5 units. When more than 0.5, the material will undergo an irreversible phase transition, resulting in capacity attenuation.
Therefore, the overcharge resistance of LiCoO2 is weak. The range of X in Li1-XCOO2 is 0≤x≤0.5, and the theoretical capacity is only 156 mAh/g.
Also, lithium cobaltate in a charging state.
0) Oxygen evolution reaction is natural to occur at high temperatures, and oxygen is released.
It's Li0.5CoO2 wetland 0.5 LiCoO2 + 1/6 Co3O4 + 1/6 O2
The theoretical capacity of layered LiNiO2 is 275mAh/g, and the actual size is 190-200 mAh/g.
However, as the ionic radius of nickel ion is smaller than that of lithium-ion, nickel ion tends to occupy the position of lithium-ion during charging and discharging, resulting in cation dislocation the collapse of the local interlayer structure of LiNiO2, resulting in the reduction of material capacity.
LiNiO2 also has many problems, such as poor thermal stability, high heat release, and weak overcharging resistance.
The layered LiMnO2 structure is slightly different from the layered LiCoO2 structure. The oxygen atoms are arranged in a twisted quadrisquare dense heap, presenting a layered rock-salt structure.
The theoretical capacity is 285 mAh/g, but its cycle performance is reduced.
After the removal of lithium, the structure of the material is unstable and will gradually change to a spinel LiMn2O4 structure. At this time, lithium ions will enter the manganese layer, causing capacity attenuation.
In addition, manganese ions are also prone to adverse reactions with the electrolyte and dissolved in the electrolyte.
At high temperatures, the materials are prone to produce heterophase reactions.
3LiMnO2 + 1/2o2 wetland 2 + Li2Mn2O3
Among the ternary positive electrode materials, the most typical lini1-X-Ycoxmnyo2 belongs to the lini1/3Co1/3mN1/3O2 compound nickel-cobalt-manganese ratio of 1:1:1, and its theoretical capacity is 277 mAh/g.
Lini1-x-ycoxmnyo2 material can adjust the content's performance by adjusting the ratio of Ni, Co, and Mn. However, the stability and safety of the stuff still exist. The mixing of various elements also brings difficulties in the synthesis process.
2. Spinel structure LiMn2O4 positive material
As early as 1983, M. Thackeray and J. Goodenough et al. discovered that manganese spinel (LiMn2O4) could be used as a positive electrode material for lithium-ion batteries, with a theoretical capacity of 148mAh/g.
In spinel LiMn2O4 structure, oxygen is arranged in a cubic dense heap to form its crystal cell skeleton, in which Li+ occupies the position of 8A of oxygen tetrahedron of 1/8 and Mn atom occupies the area of 16d of oxygen octahedron of 1/2.
There are two valence states of manganese in the structure, namely Mn3+ and Mn4+, accounting for 50%. The material structure is shown in FIG. 4.
In the LiMn2O4 structure, the empty oxygen tetrahedron and the oxygen octahedron are connected in coplanar and coplanar ways. These vacancies form three-dimensional lithium-ion diffusion channels. The material has good lithium conductivity, and the lithium-ion diffusion coefficient is 10-10 ~10-8 cm2/s.
The electrode reaction is shown as follows:
It's LiMn2O4 ↔ li1-XMn2o4 + xLi+ + xe-
When lithium-ion embedding and de-embedding occurs, manganese atomic energy stabilizes the oxygen in the cubic dense reactor in the structure, supporting the whole structure, so spinel LiMn2O4 material structure is relatively stable.
The main problem of spinel LiMn2O4 material is that its capacity decay is too fast. The main reasons for its capacity decay are as follows :(1) LiMn2O4 will be converted into tetravalent Li2Mn2O4 during deep discharge or high-power charge and discharge, and Mn in the material will be reduced to trivalent.
This change of valence state will lead to the Jahn-Teller effect resulting in the material deformation, resulting in the increase of cell volume by 6.5%, destruction of material crystal structure, and capacity attenuation.
(2) In the reaction process, Mn3+ will disproportionate to produce Mn4+ and Mn2+, and divalent manganese ions will dissolve into the electrolyte, causing the loss of active substances.
3. Olivine LiFePO4 positive electrode material
In 1997, John B. Goodenough reported that peridot-structured lithium iron phosphate could also be used as a positive electrode material for lithium-ion batteries. The theoretical capacity of LiFePO4 is 170 mAh/g.
LiFePO4 with olivine structure belongs to the orthorhombic system, and its composition is shown in Figure 5.
Oxygen atoms from the basic skeleton of the cell in a slightly twisted hexagonal dense heap. Oxygen atoms Shared by vertices connect the FeO6 octahedron. In contrast, the LiO6 octahedron is connected by typical edges to form chains. Each PO4 tetrahedron shares edges with one FeO6 octahedron and two LiO6 octahedra, respectively.
All oxygen ions are covalently bonded with the pentavalent phosphorus atom. Due to the strong p-O bond, P plays a role in stabilizing the whole framework. The material has excellent thermal stability and strong overcharging resistance.
The electrode reaction is shown as follows:
All LiFePO4 ↔ li1-xfepo4 + xLi+ + xe --
However, in practical application, the capacity and times rate of LiFePO4 material is much lower than the theoretical value, mainly because of the poor conductivity and lithium conductivity of the material.
The calculation results show that in the olivary LiFePO4 structure, the diffusion barrier of lithium-ion is too high in the Direction of Axis A and B, so it can only diffuse along the path of axis C the diffusion barrier is low.
Therefore, in LiFePO4 material, the lithium-ion diffusion channel is one-dimensional, and lithium-ion can only diffuse along the Direction of the C axis (corresponding to the crystal direction ).
Also, because FeO6 octahedrons are only connected by covertices without edges, no continuous network structure is formed, leading to low electronic conductivity of the material.