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How to store lithium iron phosphate batteries? Tests on factors affecting storage performance

by:dcfpower     2021-03-12
u003cpu003eu003cpu003e   This article takes the cylindrical 9Ah aluminum shell lifepo4 lithium iron phosphate graphite battery as the research object, and studies the battery capacity, voltage and AC during and after storage at 55/45/23 and -10℃ Changes in resistance, etc., and focused on testing the changes in characteristic parameters such as DC internal resistance, power capacity, constant current charging ratio, coulomb/energy efficiency, etc. before and after the battery is stored at 45/23℃, and analyzed the impact of these parameter changes on the entire vehicle The influence of battery pack performance gives the best storage method of lifepo4 lithium iron phosphate graphite system power battery. u003c/pu003eu003cpu003eu003c/pu003eu003cpu003eThe    test takes the cylindrical 32131 aluminum shell lifepo4 lithium iron phosphate graphite battery as the research object, the rated capacity is 9Ah, and the positive and negative electrode active materials are lifepo4 lithium iron phosphate and artificial graphite respectively. For storage at 45/55℃, use DHP200 electric heating constant temperature incubator; use low temperature refrigerator for low temperature; electrical performance test equipment is CT-3008W-5V100A-TF test cabinet; AC internal resistance test equipment is HIOKI3554 battery internal resistance tester, AC1kHz. u003c/pu003eu003cpu003eu003c/pu003eBattery storage experiment 1u003cpu003e  (1) Choose ≥60 single batteries, and charge and discharge at room temperature with 4500mA (0.5C) current in the 3.65~2V interval 3 Weekly, get the battery capacity value before the test, and finally end with 100%, 50%, 0% SOC state (20 batteries of each SOC) to make the capacity stable. After leaving it for 15 hours, measure its voltage, AC internal resistance, etc. After the data is to be tested;u003c/pu003eu003cpu003e  (2) Select 4 batteries with 100%, 50%, 0% SOC respectively, and put them in a 55℃ oven for 28 days; choose 4 batteries with 100%, A total of 12 batteries in 50%, 0% SOC state were placed in an oven at 45°C for 28 days; 4 batteries in 100%, 50%, 0% SOC state were selected and a total of 12 batteries were placed in an air-conditioned room at 23°C for 28 days; Choose 4 batteries in 100%, 50%, 0% SOC state and put them in a refrigerator at -10℃ for 28 days; during the shelving process, test the internal resistance and voltage of these batteries every 7 days;u003c/pu003e u003cpu003e  (3) After the storage is over, put the battery in the test cabinet and charge it for 3 weeks with 4500mA (0.5C) current in the range of 3.65~2V at room temperature to obtain the battery capacity after storage. u003c/pu003eu003cpu003eu003c/pu003eData and discussion: The voltage, internal resistance, and capacity changes of the battery during the 28-day storage of experiment 1u003cpu003e   The voltage during the 28-day storage of the experimental battery ( Open circuit voltage) changes. It can be seen from Figure 1 that the open circuit voltage changes during storage at different temperatures and different SOC states. The best consistency is in the 50% SOC state, and the largest change is in the 0% SOC state. This has a lot to do with the polarization of lifepo4 lithium iron phosphate graphite system batteries in different SOC states. Generally, this series of batteries have the largest polarization when the battery is empty, that is, 0% SOC, and the 50% SOC polarization is the smallest. It can also be seen from the voltage change relationship of 0% SOC at different temperatures in Fig. 1 that the increase in temperature is conducive to the battery quickly reaching a stable state after polarization. Using this principle, when battery modules are assembled in the whole vehicle, batteries with similar polarization states can be quickly selected by heating up. u003c/pu003eu003cpu003eu003c/pu003eu003cpu003eFigure 1 The voltage (V) change graph of the battery during the 28-day storage processu003c/pu003eu003cpu003eu003c/pu003eu003cpu003e  The experimental battery during the 28-day storage process The change of AC internal resistance is shown in Figure 2. It can be seen from Figure 2 that the AC internal resistance test value decreases with the increase of temperature. This is because the higher the temperature, the stronger the conductivity of each component inside the battery. But after storage, return to normal temperature and test again. The internal resistances of all batteries are not much different, but the AC internal resistance of the batteries changes significantly after storage at different SOCs and different temperatures. The internal resistance of the battery after storage at 45/55°C under high temperature and 100% SOC increases significantly. This is because after high temperature and high SOC storage, the SEI on the graphite anode surface of the lifepo4 lithium iron phosphate/graphite system battery thickens and the electrolyte LiPF6 Micro-decomposition makes the SEI components form inorganic salts with greater resistance such as LiF. u003c/pu003eu003cpu003eu003c/pu003eu003cpu003eFigure 2 The AC internal resistance (mΩ) change of the battery during 28 days of storageu003c/pu003eu003cpu003eu003c/pu003eu003cpu003e   listed in Table 1 The battery capacity change data after 28 days of storage, it can be seen from the data that compared to the high SOC state, the low SOC is more conducive to the storage of the battery capacity. It can be seen from the data that, except for the low temperature 0% SOC and the capacity at -10℃ In addition to the loss, the battery capacity under the 0% SOC state has increased to a certain extent. This phenomenon may be due to the cracking of the secondary particles of the cathode material after storage, forming a fresh interface, and re-embedding. Lithium ion activity. In fact, this phenomenon also occurs in batteries that are directly cycled without storage. The capacity of these batteries is gradually increasing during the initial cycle of tens of weeks. u003c/pu003eu003cpu003eu003c/pu003eu003cpu003eTable 1 Storage battery capacity data table before and after 28 daysu003c/pu003eu003cpu003eu003c/pu003eu003c/pu003eu003cpu003eu003c/pu003eu003cpu003e 1 2 Nextu003c/pu003e
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