Lithium iron phosphate batteries are widely used in the field of new energy vehicles due to their excellent cycling performance, high safety, and relatively low cost. However, its cycle life curve shows a characteristic of "fast decay in the early stage and flattening in the middle and late stages". Compared to ternary batteries, lithium iron phosphate batteries experience more significant capacity degradation in the early stages of cycling, limiting their potential applications in long-life scenarios. It is of great significance to study the early attenuation mechanism and explore improvement methods to enhance its market competitiveness.

Analysis of attenuation reasons

1. Comparison with ternary batteries

Under the same testing conditions, the capacity retention rate of lithium iron phosphate batteries after 200 cycles is 95%, while that of ternary batteries is 97%. Research has found that the difference in initial Coulombic efficiency between the two is the fundamental reason for the difference in decay rate:

a. The positive electrode of a ternary battery has a lower initial Coulombic efficiency (about 88%), while the negative electrode has a higher initial Coulombic efficiency (about 92%). After the first charge and discharge, about 4% of the active lithium remains in the negative electrode, which can compensate for some of the consumption of active lithium in subsequent cycles;

b. The first coulombic efficiency of the positive electrode of lithium iron phosphate battery is relatively high (about 95%), while the first coulombic efficiency of the negative electrode is relatively low (about 92%). After the first charge and discharge, there is no excess active lithium in the negative electrode, which makes it more sensitive to the consumption of active lithium in the early stage of cycling. ICP and XRD analysis further confirmed that the lithium content in the negative electrode of the ternary battery was always higher than that of the lithium iron phosphate battery, and as the cycle progressed, the lithium content in the negative electrode of the ternary battery gradually decreased, indicating that the lithium stored in the negative electrode was gradually consumed during the cycle, delaying capacity decay.

2. The early degradation mechanism of lithium iron phosphate batteries.

By comparing the capacity loss at 1C and 0.05C rates, it was found that the loss rates of the two were basically the same, indicating that the capacity degradation was mainly not caused by polarization, but by irreversible consumption of active lithium. Further characterization of the negative electrode SEI film using ICP, EDS, DSC and other methods showed that:

a. As the cycle progresses, the lithium content in the negative electrode gradually increases, the lithium element content in the SEI film increases, and the heat release of the SEI film increases;

b. In the first 50 cycles, the capacity decay rate was 3.3%, the polar expansion rate was 3.3%, and the pressure growth rate was 33.6%; In 50-100 cycles, the attenuation rate drops to 1.2%, the expansion rate drops to 1.6%, and the pressure growth rate drops to 1.4%.

This indicates that the rapid expansion of the negative electrode volume during the initial cycle leads to frequent rupture and repair of the SEI film, consuming a large amount of active lithium, which is the main reason for the rapid capacity decay. As the cycle progresses, the polar structure tends to stabilize, the degree of SEI film damage decreases, and the decay rate slows down accordingly.

Improvement measures

Based on the above mechanism, the article proposes multiple improvement strategies from the perspectives of positive and negative electrode material design and process optimization, and verifies their effectiveness through experiments:

1. Reduce the specific surface area of the positive electrode: reduce the occurrence of side reactions, lower the consumption of active lithium, and thus slow down capacity decay.

2. Optimize the negative electrode OI value (orientation index): The smaller the OI value, the smaller the volume expansion of graphite during lithium insertion, and the lower the degree of SEI film damage. The experiment showed that after the OI value decreased from 9.33 to 5.55, the capacity decay rate decreased from 3.3% to 2.4% after 100 cycles.

3. Control the negative electrode coating amount: Excessive coating amount will exacerbate electrode expansion and increase the risk of SEI film damage. The coating amount increased by 30%, the rebound rate of the polarizer increased by 9%, and the capacity decay rate increased by 1.0%.

4. Reduce the expansion rate of negative electrode adhesive: the expansion rate of the adhesive film is reduced by 20%, the rebound rate of the electrode is reduced by 2%, and the capacity decay rate is reduced by 0.5%. The improvement effect was verified by ICP, EDS, DSC and other methods, and the results showed that the optimized battery had significant improvements in negative electrode lithium content, lithium element content in SEI film, and SEI film heat release, proving the effectiveness of the above measures.