The Influence of Different Carbon Sources on the Processing Performance of Lithium Iron Phosphate
- jiangyaoyao0501
- Jun 3
- 6 min read
1.Overview
Pulp preparation is an important process in the preparation of lithium iron phosphate batteries. The quality of the slurry directly affects the subsequent coating process and the performance of the battery. Therefore, improving the processing performance of LiFePO4 slurry plays an important role in enhancing the electrochemical performance and service life of the battery. The three commonly used modification methods, namely carbon coating, ion doping, and particle nanocrystallization, often also affect the processing performance of LiFePO4.
Different carbon sources have a significant impact on the quality of the carbon layer coated on the surface of lithium iron phosphate particles. Theoretically, it will also have an obvious impact on the processing performance of lithium iron phosphate, but there are few relevant research reports at present. In this paper, a series of lithium iron phosphate materials coated with different carbon sources were synthesized by the high-temperature solid-phase method to explore the influence of different carbon source types on the quality of the carbon coating layer of lithium iron phosphate and the influence of carbon source types on the processing performance of lithium iron phosphate.
2.Raw materials
Lithium carbonate (Li2CO3), battery grade; iron phosphate dihydrate (FePO4·2H2O), industrial grade; polyethylene glycol-6000 (PEG-6000); glucose (C6H12O6); sucrose (C12H22O11).
Carbon source 1: 10% of the mass of the LFP finished product of glucose, together with absolute ethanol;
Carbon source 2: 7% (mass fraction) of PEG-6000;
Carbon source 3: 7% (mass fraction) of sucrose.
The above three carbon sources were used for carbon coating, and the carbon mass fractions in the finished products were 1.02%, 1.05%, and 1.04% respectively.
3.XRD and Raman
The crystal structures of the LFP samples coated with different carbon sources are all olivine-type structures, and the space group is Pnmb. When sucrose is used as the carbon source, the proportion of graphitized carbon in the carbon coating layer is the highest. The highly graphitized carbon layer helps to improve the conductivity of the lithium iron phosphate material, thereby enhancing the electrochemical performance of the material.
4.Morphology structure and physical properties
When coated with different carbon sources, the particle morphology of the LFP/C finished product shows a spherical shape, and there are agglomeration phenomena of different degrees in the particles.
When sucrose is used as the carbon source, it has the smallest primary particles, with an average particle size of (297 nm ± 132 nm). This may be because after sucrose is cracked into carbon, it is more evenly coated on the surface of LFP particles, thus better inhibiting the growth of particles. The smaller primary particles are beneficial to shortening the lithium ion transmission path and enhancing the electrochemical performance of the material. At the same time, the regularity and dispersibility of the particles are significantly better than those of other carbon sources, and the particle sizes are different, showing a grading effect. Such a particle size distribution is beneficial to improving the tap density of the material. The compacted density reaches 2.53 g/cm3, which is significantly better than the other two carbon sources, and its resistivity is only 14.2 Ω·cm. The lower resistivity is beneficial to the exertion of the electrochemical performance, indicating that coating with sucrose as the carbon source is beneficial to optimizing the particle morphology of LFP and enhancing the electrochemical performance.
Carbon mass fraction/% | Compacted density /(g·cm-3) | Resistivity /(Ω·cm) | |
Glucose | 1.02 | 2.51 | 59.1 |
PEG | 1.05 | 2.49 | 47.4 |
Sucrose | 1.04 | 2.53 | 14.2 |
When glucose is used as the carbon source for coating, the thickness of the carbon coating layer reaches 8 - 12 nm, showing the characteristics of uneven coating;
When PEG-6000 is used as the carbon source for coating, the carbon coating layer is only 1 - 2 nm, and the coating is relatively uniform;
When sucrose is used as the carbon source for coating, the carbon coating layer of the sample reaches about 5 nm, and the carbon layer has a high degree of uniformity.
Carbon coating is beneficial to improve the conductivity of LFP materials and is beneficial to the electrochemical performance of materials. When the coating thickness is too low, it may lead to incomplete coating, partial exposure of LFP materials, and affect the electrochemical performance;However, when the thickness of the carbon coating is too high, the overly dense carbon layer will affect the wettability of the material with the electrolyte, and may also hinder ion transport, which is not conducive to the electrochemical performance.
Therefore, when sucrose is used as the carbon source for coating, the carbon coating layer has a moderate thickness and good uniformity, and excellent electrochemical performance is expected.
5.Electrochemical performance test
When carbon coating is carried out with sucrose as the carbon source, the electrochemical performance is the best. This may be because the carbon layer coated on the surface of LFP particles after sucrose cracking has a moderate thickness and good uniformity of the carbon layer, which can effectively improve the conductivity of the LFP material and has good wettability with the electrolyte. When carbon coating is carried out with sucrose as the carbon source, the polarization of the LFP material is the smallest.
When carbon coating is carried out with PEG-6000 as the carbon source, the charge-discharge performance of the LFP material at high rates is better. This may be because the thinner carbon coating layer is more conducive to the deintercalation and intercalation of lithium ions when the polarization is greater at high rates.
When using sucrose as the carbon source for coating, the carbon coating layer with a moderate thickness can effectively prevent the direct contact between the electrolyte and LFP particles, and can also prevent the shedding of the carbon layer during the long-term cycling process, ensuring the smooth deintercalation and intercalation of lithium ions and enhancing the cycling performance of the LFP material.
0.1C discharge specific capacity/(mA·h·g-1) | 0.1C first coulombic efficiency/% | 1.0C discharge specific capacity/(mA·h·g-1) | 1.0C first coulombic efficiency/% | |
Glucose | 144.9 | 95.6 | 132.2 | 91.7 |
PEG | 149.3 | 96.9 | 140.0 | 92.0 |
Sucrose | 154.6 | 98.1 | 141.5 | 92.1 |
6.Processing performance test
The rheological and thixotropic properties of the slurry are important parameters for evaluating the processing performance of LFP materials. Under the condition of the same pulp preparation process and the same slurry solid content of 49% (mass fraction), the slurries of the rheological and thixotropic properties of LFP all show typical non-Newtonian fluid characteristics, that is, the viscosity of the slurry gradually decreases with the increase of the shear rate and finally reaches a stable range. According to the shear rate, the coating process of the LFP slurry can be divided into a high-shear rate process in the initial coating stage and a low-shear rate process in the self-leveling stage of the slurry. A high viscosity of the slurry at high shear rates is not conducive to the coating of the slurry on the surface of the current collector and affects the leveling of the electrode sheet.
The slurry viscosity of the LFP material coated with glucose as the carbon source is the lowest, and the slurry dispersibility is the best. Thixotropy refers to a reversible property in which the viscosity of a material changes when subjected to a shear force and then returns after the shear is stopped, representing the dependence of fluid rheology on time. When carbon coating is carried out with glucose as the carbon source, the processing performance is significantly better than the other two carbon sources.
Solid content /% | Discharge viscosity /(mPa·s) | 60s viscosity /(mPa·s) | 185s viscosity /(mPa·s) | Recovery rate/% | |
Glucose | 49 | 10738 | 136010 | 133100 | 97.86 |
PEG | 49 | 17197 | 252170 | 146120 | 57.95 |
Sucrose | 49 | 17526 | 132760 | 111110 | 83.69 |
7.Finally:
Three kinds of lithium iron phosphate materials coated with carbon sources were synthesized by the high-temperature solid-phase method, and the influence of carbon source types on the physical properties, electrochemical performance, and processing performance of lithium iron phosphate was explored.
It was proved that when carbon coating is carried out with sucrose as the carbon source, the degree of graphitization of the carbon coating layer on the surface of lithium iron phosphate particles is the highest, and the thickness of the carbon layer is about 5 nm, which is uniform and moderate. The good carbon coating effect makes the electrochemical performance of lithium iron phosphate with sucrose as the carbon source very excellent. Its charge-discharge specific capacity at a 0.1C rate reaches 154.6 mA·h/g, and the initial coulomb efficiency is 98.1%, which is the best; and after charge-discharge at different rates from 0.2C to 5.0C, the discharge specific capacity recovery rate at a 0.2C rate reaches 99.5%. At the same time, after 110 cycle tests at a 1.0C rate, the discharge specific capacity retention rate reaches 98.2%, which is also the highest. When glucose is used as the carbon source for coating, under the same solid content condition, the discharge viscosity of the slurry is 10738 mPa·s, and the slurry dispersibility is the best; and the results of the three-stage thixotropy test also show that its viscosity recovery rate is 97.86%, which is the highest, showing excellent processing characteristics.
