Study on Low Temperature Long Cycle Properties of LiFePO4 Materials
Olivine structured LiFePO4 (LFP) has been widely used in LIBs due to its outstanding advantages such as large theoretical capacity, low cost, environmental protection, safety and thermal stability. However, the low conductivity of the original LiFePO4 itself and the slow lithium ion diffusion rate lead to poor magnification performance and low capacity retention, especially at low temperatures, which limit its wide application in electric vehicles and hybrid vehicle LIBs.
The new multi carbon source technology is used to improve the graphitization degree of carbon coating. Specifically, the formation of metal doped carbon coating with metal elements containing carbon source can effectively improve the conductivity and electrochemical performance of LiFePO4. The author has prepared the kilogram scale test sample, and made the 2Ah soft pack battery, and then cycled it for more than 3000 times to study the long-term cycle performance. When calcium lignosulfonate is used as one of the carbon sources, a thin layer of carbon doped with calcium is formed on the surface of LiFePO4 particles, which can reduce the side reaction between LiFePO4 and the electrolyte and improve the stability of the electrode. In addition, there may be a large number of active functional groups such as hydroxyl, carbonyl, carboxyl and methyl groups in the carbon residue layer after pyrolysis of calcium lignosulfonate, which is conducive to electrolyte penetration into the electrode. Compared with LiFePO4/C composites (LFP-S1) prepared by solid state method, LiFePO4/C composites (LFP-H1) prepared by hydrothermal method have excellent rate performance, cycle life and low temperature performance, and have broad application prospects in the field of lithium ion power batteries.
The material preparation process is as follows. Using a mild hydrothermal process, LiOH · H2O (>99.5 wt%), FeSO4 · 7H2O (>99.5 wt%) and H3PO4 (85 wt%) as raw materials, deionized water as solvent. First, add LiOH and H3PO4 solutions into the autoclave, get a white suspension under stirring, and then add ferrous sulfate solution. The molar ratio of Li: Fe: P is controlled at 2.7:0.97:1. A certain amount of ascorbic acid is added as an antioxidant. After the autoclave is sealed, heated to 180 ° C and maintained for 6 hours, the precipitate obtained by deionization washing is mixed with fructose and calcium lignosulfonate in deionized water to form a uniformly distributed suspension. After the spray drying process, the mixture was annealed at 700 ° C in nitrogen atmosphere for 6 hours, and the layered LiFePO4/C composite was obtained by air flow pulverization. The sample is named LFP-H1.
The reference samples were prepared by solid state method. Add FePO4 · 2H2O, Li2CO3 and C6H12O6 into the ball mill in turn, with the molar ratio of Li, Fe and P of 1.02:0.97:1, and add deionized water to maintain the solid content of 35 - 40 wt%. After sufficient grinding, the fluid mixture with uniform distribution is obtained. The mixture was dried by spray, annealed at 720 ℃ in N2 atmosphere for 10 hours, and the LiFePO4/C reference sample, named LFP-S1, was obtained through air flow pulverization process.
XRD characterization showed that the two samples both showed olivine phase and no impurity phase was formed. For LFP-H1 material, since the content of calcium compound is negligible, no characteristic peak of calcium compound is found. The particle size characterization results show that the D50 values of LFP-S1 and LFP-H1 are 0.86 and 1.81 m respectively. The larger particle size of LFP-H1 may be related to the higher adhesion between LiFePO4 particles caused by the carbon residue generated by the mixed source of fructose and calcium lignosulfonate. Although the secondary particle size of LFP-H1 is higher than that of LFP-S1, the primary particle size of LFP-H1 is smaller, which is conducive to lithium ion diffusion of LFP-H1. The carbon content of LFP-S1 and LFPH1 is 2.07 and 1.91% respectively. The BET of LFP-S1 and LFP-H1 are 14.4 m2/g and 15.6 m2/g respectively, and the appropriate surface area can ensure the good processing performance in the battery manufacturing process. It can be seen from the infrared spectrum analysis that the sample strength of LFP-H1 is low, indicating that the number of residual organic functional groups in LFP-H1 is more than that in LFP-S1, which is mainly because LFP-H1 uses calcium lignosulfonate as the carbon source. LFP-H1 samples with more organic functional groups are more likely to absorb electrolytes with similar polarity and appropriate surface energy, improve the penetration between the positive electrode and the electrolyte, accelerate the insertion/removal reaction of lithium ions, and improve the electrochemical magnification performance.
It can be seen from the SEM characterization results that the LFP-S1 sample is irregular granular, and the particle size is about 100nm-1 m. In addition, it can be seen that the loose residual amorphous carbon derived from glucose is not closely bound to LFP particles. SEM images of LFP-H1 samples show that LFP particles are more evenly distributed, ranging from 150 to 400 nm. There is no excess carbon residue in LFP-H1, indicating that the adhesion between LFP particles and carbon layer is high, which is conducive to improving the conductivity of the sample.
It can be seen from the TEM image of LFP-H1 that there is an ultra-thin and uniform carbon layer of 2-3 nm on the surface of LiFePO4 particles. Compared with LFP-S1, the LFP-H1 samples prepared by hydrothermal method have more uniform particle size, better carbon coating and higher crystallinity, which are crucial to improve the electrochemical lithium intercalation and cycling performance of active materials.
It can be seen from the rate performance and cycle performance tests of the two materials at 2.0-3.8V that not only the capacity of LFP-H1 is higher than that of LFP-S1, but also the difference between the charging and discharging voltage platforms of LFP-H1 is smaller than that of LFP-S1, indicating that the LFP-H1 material has smaller polarization, which is attributed to the high conductive carbon coating layer and LFP particles with good crystallinity. CV and EIS tests further proved that LFP-H1 sample has smaller polarization and higher reversibility. The following figures e and f are EIS at 25 ℃ and 0 ℃ respectively. Because there are a large number of active functional groups on the surface of LFP-H1, some of the active functional groups may react with the electrolyte to form a positive electrode/electrolyte interface (CEI) film, resulting in the cathode/electrolyte interface resistance (Rcei) during the reaction at 0 ℃. The Rcei of LFP-S1 cannot be detected at 0 ° C and room temperature, which may be due to low polarization and negligible reaction interface (CEI) film amount. At 25 ℃, the charge transfer resistance of LFP-S1 and LFP-H1 is about 160 and 40 Ω respectively; At 0 ℃, the charge transfer resistances of LFP-S1 and LFP-H1 are about 210 and 100 Ω, respectively, indicating that the hydrothermal composites have significantly improved charge transfer dynamics. EIS at 0 ℃ shows that LFP-H1 may have better lithium storage performance than LFP-S1 at low temperature. According to the EIS results at 25 ℃, the lithium ion transmission coefficients of LFP-S1 and LFP-H1 are calculated to be 2.3e-12 and 6.4e-12 cm2/s respectively. The conductivity of LFP-S1 and LFP-H1 measured by four probe method is 20.42e-3 and 76.33e-3 S/cm respectively. From the cyclic voltammetric results of 2.0-4.2V, compared with LFP-S1, LFP-H1 has a sharper redox peak and smaller voltage gap, which further confirms that LFP-H1 has smaller polarization and better electrode dynamics, which is related to the better reversibility of lithium insertion and stripping processes.
In order to evaluate the electrochemical performance of practical application, LFP positive electrode and graphite negative electrode are assembled into a soft package battery with a rated capacity of 2Ah, which can light the bulb. The performance of LFP-S1 and LFP-H1 soft packed battery at 0.5-5C magnification shows that the capacity retention rate of LFP-H1 at 5C is 95.9%, which is much higher than that of LFP-S1 (87.4%), indicating that the soft packed battery based on LFP-H1 has good magnification capacity.
The charge and discharge tests at 0.5C at different temperatures are shown in the figure below. At -10 ℃ and -20 ℃, the capacity retention rates of LFP-H1 are 87.3 and 75.1%, respectively, 14.9 and 22.2% higher than that of LFP-S1, indicating that the advantages of LFP-H1 based soft package full battery at lower temperatures are more obvious. These results are consistent with EIS and CV results.
The long cycle performance of LFP-S1 and LFP-H1 was studied by cycling the two samples for more than 3000 times at the charge discharge rate of 1C. Compared with LFP-S1, LFP-H1 sample has better long-term cycle performance at different temperatures of 25, 45 and 60 ° C. After 3368 cycles at 25 ° C, the capacity retention rate of LFP-H1 is still as high as 89.2%, which is far higher than the end point of 80% of the battery life. In contrast, the capacity retention rate of LFP-S1 battery after 3021 times is about 83%, close to the end of its life. At 45 ° C and 60 ° C, the LFP-S1 battery reached its end of life after 1500 and 750 cycles respectively, while the LFP-H1 battery had a longer cycle time.
Then the battery after 750 cycles at 60 ℃ was disassembled and analyzed to observe the structure of the material. There is residual Super P in LFP particles of both samples. For LFP-S1, the side reaction products between cathode and electrolyte were observed on the particle surface. For LFP-H1 sample, after a long cycle, the active substance almost retains its original form, the primary particles are still intact, and there is no obvious carbon residue shedding. The results showed that the modified LFP particles had stable surface and high adhesion with the carbon coating after a long cycle. Therefore, the long cycle life of the hydrothermally synthesized LFP-H1 may be attributed to the high-quality and uniform carbon coating, which can effectively prevent the side reaction between LFP particles and electrolyte, as well as the corrosion of hydrofluoric acid.
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