Application of TRIZ theory in electrode materials

At present, TRIZ theory is widely used in the research of innovation problems. The innovation of engineering problems is rarely used in the field of basic disciplines. After analysis and summary, we found that TRIZ theory can be found everywhere in the research process of electrode materials. TRIZ theory's systematic and comprehensive analysis perspective and innovation principle can be widely applied to various fields, including basic scientific research. TRIZ theory can provide the general direction and ideas for solving problems for the majority of scientific researchers when developing and improving materials.

We regard the positive electrode of lithium iron phosphate as a microsystem and analyze the system as shown in Figure 1:

When the battery is discharged, lithium ions migrate from the negative electrode to the SEI film on the surface of the positive electrode material through the electrolyte, and then enter the positive electrode particles after passing through the SEI film; on the other hand, electrons flow into the positive electrode material particles through the external path and the conduction of the current collector, and complete the reduction reaction in the positive electrode material particles. It can be seen that the rate performance of the battery with lithium iron phosphate as the positive electrode material depends on the speed of the transportation of lithium ions and electrons in the whole process. View the five components of the system:

(1) Electrolyte: The conductivity of lithium ions in the electrolyte is on the order of 10-3 S/cm;

(2) SEI membrane: The SEI membrane formed by lithium iron phosphate and electrolyte is thin and stable. The pore size of the SEI membrane makes the impact of lithium ions passing through it negligible;

(3) Positive electrode material lithium iron phosphate: electronic conductivity is 10-9 S/cm, and the diffusion rate of lithium ions is 10-14 to 10-11 cm2/s;

(4) The current collector is aluminum foil, which is a good conductor of electrons and its effect on the rate is negligible;

(5) External wires are good conductors of electrons and their effect on the rate is negligible.

After analyzing each component in the system, we know that the speed-controlling step is the slowest part of the whole process, that is, the ion conductivity and electronic conductivity of the lithium iron phosphate material are too low. Therefore, the electronic conductivity and ion conductivity of the material must be improved. We conducted a physical field analysis of the role of the lithium iron phosphate component in the system, as shown in Figure 2:

Lithium iron phosphate materials have insufficient capacity to transport electrons. Using the idea of ​​standard solution, we can introduce a third party to help it strengthen its transport function. Therefore, the first research idea is to add excellent conductive agents to lithium iron phosphate materials to enhance the positive electrode active material. For example, adding conductive agents such as graphene and carbon nanotubes can effectively improve the battery's rate performance.

We then use the causal axis method in the three-axis analysis to analyze the root causes of the ion conductivity and electronic conductivity of lithium iron phosphate materials. First, we must understand the crystal structure of lithium iron phosphate, as shown in Figure 3:

The oxygen atoms in the lithium iron phosphate in Figure 3 are arranged in a hexagonal close packing, and the iron and lithium atoms are divided into the octahedral centers of the oxygen atom stacking. The phosphorus atom occupies the 4c ​​position of the tetrahedron of the oxygen atom. On the other hand, on the bc plane, every two FeO6 octahedra share one O atom. At the same time, each FeO6 octahedron shares edges with two LiO6 octahedra, while each PO4 group has one and two common edges with the FeO6 octahedron and the LiO6 octahedron, respectively.

The tetrahedron PO4 between the FeO6 octahedron and the LiO6 octahedron in lithium iron phosphate limits the insertion and extraction of lithium ions during the charge and discharge process, which is the fundamental reason for the low ion conductivity. At the same time, the conduction of electrons can only be carried out through the covalent bond Fe-O-Fe, which is the fundamental reason for the low electronic conductivity of iron phosphate. It can be seen that the structural characteristics of lithium iron phosphate itself are the essential reason for its poor charge and discharge rate performance during the charge and discharge process. However, it is precisely because of the special structure of lithium iron phosphate that it has a series of excellent properties that other positive electrode materials do not have. For example, the covalent bond structure between lithium and oxygen makes it difficult for the material to release oxygen at high temperatures, making the material thermodynamically stable.

Through the analysis of the components of the lithium iron phosphate material subsystem: Li, O, P, Fe atoms, it can be seen that the defects and advantages of lithium iron phosphate materials come from its NaSICON structure. This is a set of physical contradictions. While solving this contradiction, we need to retain the advantages brought by this structure and improve the disadvantages brought by this contradiction. Seek ideas to solve the contradiction through innovative principles:

(1) The structure of lithium iron phosphate material is retained, that is, all its advantages are retained. At the same time, the original micron-sized particles are nano-sized by adopting the idea of ​​breaking the whole into parts in the innovation principle. The nano-sized particles effectively shorten the path required for lithium ions to diffuse in the material and reduce the time required for diffusion, thereby improving the rate performance.

(2) Improve the lithium iron phosphate system with the help of super system components without changing the structure of lithium iron phosphate. The conductivity of lithium iron phosphate itself is poor, so we coat the surface of lithium iron phosphate particles with some materials with good power-off properties to help it conduct electricity.

(3) Retain the NaSICON structure of lithium iron phosphate material, add some other elements into it, replace the original atomic positions, increase the disorder of the crystal, create defects that are conducive to electron and ion conduction, and thus improve electrical performance.

At present, reports on the optimization of lithium iron phosphate can be summarized into three categories based on the above ideas:

(a) Research on improving grain size and nanomaterials. For example, the nano LiFeO4/C composite material synthesized by Liu et al. can achieve a discharge rate of 95 mAh/g at 80C.

(b) The surface of the material is coated with carbon or metal ions with good conductivity. For example, when graphene with good conductivity is coated on the surface of LiFePO4 particles through hydrothermal synthesis, the specific capacity at a 10 C rate can reach 110 mAh/g. The morphology of the coating is shown in Figure 4:

(c) Metal ion doping. For example, Chung et al. [12] doped high-valent metal ions (Nb5+, Ti4+, W6+, etc.) into the position of lithium ions in the LiFePO4 lattice, so that the Fe atoms in the LiFePO4 and FePO+ lattices exist in a mixed valence state. As shown in Figure 5, the electronic conductivity of the material is greatly improved, which improves the rate performance of the battery, and the specific capacity exceeds 60 mAh/g at a rate of 21.5 C.

It is roughly estimated that the number of research reports on lithium iron phosphate positive electrode materials has reached 104, and the specific content can be attributed to the structural analysis of lithium iron phosphate materials analyzed by TRIZ theory, the calculation of electronic conductivity and ion conductivity, and the experimental and theoretical calculation research on three directions of conductivity modification. Through people's unremitting efforts, the energy density and main technical indicators of lithium iron phosphate battery systems have approached their theoretical values, and it is the main battery material for major lithium battery companies in my country to develop battery materials for power vehicles.

The graphite fluoride materials and lithium iron phosphate materials in the high-energy-density primary batteries developed based on graphite fluoride materials face similar problems. They are both materials of phase change reactions and have low electronic conductivity and ionic conductivity. We can use the concept of TRIZ to obtain ideas and solutions for optimizing graphite fluoride materials.