Li-ion battery rate performance is closely related to battery resistance.Battery resistance includes ionic resistance and electronic resistance. The ionic resistance mainly refers to the resistance of lithium ions in the electrolyte in the electrode pores, the resistance of lithium ions through the SEI film, and the charge transfer resistance of lithium ions and electrons at the active material/SEI film interface. And the solid phase diffusion resistance of lithium ions inside the active material,electronic resistance mainly refers to the resistance of positive and negative active materials, current collector resistance, contact resistance between active materials, contact resistance between active material and current collector, and tab welding resistance.In the actual battery development and production process, the ionic resistance part needs to be evaluated at the finished battery end, and the electronic resistance part can be quickly evaluated at the material and pole piece end.Therefore, testing the resistivity of lithium battery positive and negative materials is of great significance to the stability of the material process and the resistance prediction of the finished cell.
The composition structure of the lithium-ion battery is shown in Figure 1.Commonly used positive and negative electrode materials usually include graphite, lithium iron phosphate, nickel cobalt manganese ternary, lithium cobalt oxide, etc. Due to the different chemical composition and crystal structure of different materials, the compaction density and resistivity of the material under pressure also different.This paper tests the compaction density and resistivity of four lithium battery materials, and compares and analyzes the differences in compaction density and resistivity.
Figure 1 Schematic diagram of battery structure
1.Test equipment: PRCD2100 (IEST-Yuanneng Technology) is used to test the resistivity of 4 positive and negative materials, the equipment is shown in Figure 2;
Figure 2. (a) The appearance of PRCD2100; (b) The structure of PRCD2100
2. Test parameters: the applied pressure range is 10-200MPa, and the pressure is maintained for 10s;
3. Sampling quality: lithium cobaltate/ternary powder 2.000g, LFP/graphite powder 0.5000g;
4. Test results:
Figure 3. Comparison of compaction density and resistivity of four different system materials
Compare the compaction density of the above 4 materials: LCO>NCM>LFP>Graphite,Comparison of powder resistivity under 100MPa conditions: LCO> NCM> LFP> Graphite.Among them, when the ratio of nickel, cobalt and manganese in the ternary material system is different, the resistivity of the powder also has a big difference, usually as the nickel content increases, the resistivity of the ternary material decreases.In order to improve the conductivity of the LFP material, the surface is usually coated with carbon or nano-treatment, etc.Therefore, LFP is a material with better conductivity among the commonly used cathode materials, and the rate performance of the corresponding LFP system cell is usually better.
The monitoring of powder resistivity can monitor the stability of the powder manufacturing process, and at the same time help R&D personnel predict the rate performance of the finished cell in advance, and accelerate the development progress.
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