How many cycles can the lifepo4 batteries last up to?
The cycle performance of a single cell is mainly affected by four aspects:
(1) Material system or cell design
(2) Reliability of cell processing
(3) Working conditions and conditions of battery charge and discharge cycles
(4) Test accuracy.
If you go to the module and system level, you should also consider the consistency of cell processing, the reliability of BMS, the reliability of welding when connected in series and parallel, whether you have an equalization function, and so on.
Analysis of the cycle performance of a single cell:
First, the material system of the cell, or the design of the cell.
1. The theoretical cycle life of lithium iron phosphate material is very long
mainly because the lattice stability of lithium iron phosphate is good, and the insertion and extraction of lithium ions have little effect on the lattice, so it has good reversibility. But this is theoretical. In fact, the performance of each material manufacturer’s raw materials, processing technology, coating or modification process is very different, and even relatively poor materials can be cycled for hundreds of weeks. No. In addition, there will also be a problem of consistency in the material processing process; therefore, in order to have a good cycle life, it is necessary to select lithium iron phosphate materials with excellent cycle performance and no problems in processing;
2. Capacity of different lithium iron phosphate materials
Appropriate compaction density, required conductive agent and binder content are also very different, and some must be used with carbon-coated aluminum foil; therefore, the most suitable positive electrode ratio, compaction density and other parameters must be selected;
3. Other materials in the cell
The life of the negative electrode, electrolyte, separator, and even the current collector, as well as the packaging material, will affect the cycle life of the battery cell. Other materials other than the positive electrode must use materials that match the life of the positive electrode lithium iron phosphate, or even exceed the life of the positive electrode. Design Parameters;
4. Different cell rate performance designs
Different cell rate performance designs also have a great impact on the cell cycle life. For cells with high energy density, the life of high rate cycles will generally be poor; therefore, it is necessary to formulate a cycle condition that is compatible with the cell’s required cycle life. matching design.
Second, the reliability of cell processing
1. The dispersion effect of the positive and negative electrode slurry, and the processing defects of the pole piece; poor dispersion of the slurry will affect the cycle of the cell, which is well known; the processing of the pole piece is a problem that cannot be completely avoided, and each process has Such surface defects may occur, and various abnormal areas of point, wire, and surface have a great impact on the cycle life of the cell, and with the current mass production technology, it is impossible to completely eliminate it, only to ensure that there is no major impact. to security deficiencies;
2. If it is a wound cell, or even some laminated cells, the flatness of the entire cell and the deformation of the winding core will have a great impact on the cycle life. Among them, the winding tension and hot-pressing flatness will all be affected. great influence;
3. The residual amount of electrolyte and the film formation of the negative electrode will also affect the cycle life; this includes the temperature of high temperature formation, formation current, whether the bubbles are completely removed, or whether too much electrolyte is negatively pressured during formation Extraction will affect the lifespan.
Therefore, the cycle life of the cell is good, and every detail of the entire processing process must be controlled in place to ensure that there is no problem, and this control should not only look at some superficial parameters, but should be every detail of the equipment and products.
Third, cycle conditions and conditions
1. Charge and discharge current and charge and discharge depth
It can generally be considered that within the normal current in place, the larger the charge and discharge current, the greater the SOC at the end of charge, the lower the SOC at the end of discharge, and the worse the cycle life;
Regardless of whether the temperature is too high or too low, it will have a negative impact on the cycle performance, and this temperature should be the actual temperature inside the cell, not just the temperature of a certain part detected;
2. External force conditions
Applying a certain force to the cell can effectively improve the adhesion between the pole piece and the diaphragm during the cycle, and prolong the life. However, excessive force will affect the diffusion of the electrolyte and reduce the life.
Fourth, test accuracy
This is mainly related to the test machine or connection. For example, the actual output current is too large or small during the test, the voltage is high or low, and the connection point of the wire is heated, which will affect the life of the test, and may even have a great impact. The effect, the difference of thousands of weeks.
After the above analysis, the lithium iron phosphate battery with 12,000 cycles should be a lithium iron phosphate battery with a long-life design. Select a sample whose processing process is fine and reliable, and has no problems such as winding core deformation. Under the discharge regime (personal estimation is at room temperature, 1C current 80% DOD cycle), under the appropriate preload, it is possible to achieve 12,000 cycles of life through accurate cycle testing (it is also possible that the cut-off condition is 70% of the initial capacity. or 60%).
But this situation, at present, can only exist in the laboratory. The actual engineering application products are difficult to achieve, especially the energy storage system, a container of batteries, it is impossible to measure at present. Even if the manufacturer promises to achieve such a lifespan, it also considers the preconditions for continuous maintenance and replacement in the future.