Energy and environment are the two fundamental issues facing sustainable development in the world today. Hybrid electric vehicles, as a green and efficient means of transportation, have been increasingly promoted worldwide. With the increasingly mature battery preparation technology, the Lithium iron phosphate power battery, as an important part of the hybrid electric vehicle drive system, has a significant impact on the vehicle power, economy and safety. Because of its high energy density, long life, high charging and discharging efficiency, wide applicable temperature range, low self discharge, low internal resistance, no memory effect, fast charging, high safety, high reliability, low cost and reusability, it is recognized as the most promising vehicle power battery.
However, in the actual use of power battery, Lithium iron phosphate material has a high impedance, which directly affects the promotion and application of Lithium iron phosphate power battery. In order to solve the problem of high battery impedance and further improve the performance of Lithium iron phosphate battery, Lithium iron phosphate power battery was prepared by replacing part of conductive carbon with carbon nanotubes with excellent conductivity. The influence of temperature on nano Lithium iron phosphate power battery was analyzed and investigated from the battery capacity, voltage platform and discharge curve under different environmental temperatures.
1 Test
1.1 Test Equipment and Objects
DGBELL high and low temperature test chamber, Lithium iron phosphate power battery (single cell 3.2V, 10Ah)
1.2 Experimental Steps
(1) Set the internal ambient temperature of the high and low temperature chamber to -40, -20, -100, 25, 40, 55, and 60 ℃, with a relative humidity of 40%.
(2) Develop charging and discharging methods: The charging method is to charge the battery at a constant current of 0.2C (2A) to 3.65V at (20 ± 5) ℃, change to constant voltage charging until the current drops to 200mA, and stop charging. The discharge method is to let it sit for IH at different ambient temperatures, and then discharge at a constant current of 1C until the voltage drops to the cut-off voltage of 2V, and calculate the discharged capacity.
(3) Formulate the experimental scheme: this experiment takes 25 ℃ as the reference point for temperature test. First, the low-temperature performance test is carried out. From 25 ℃ to -40 ℃, 0, -10, 20 and -40 ℃ are taken as the observation points, respectively. The temperature change rate is 1 ℃/min. At each temperature Test point, the battery for test is placed for 24 hours, and then the temperature performance test at this temperature point is carried out; Then, conduct high-temperature performance testing of the battery. In order to eliminate the impact of low-temperature testing, first restore the temperature of the high and low temperature test chamber to 25 ℃, and use the data measured at this temperature as the reference point for high-temperature testing. Then, conduct high-temperature performance testing of the battery, starting from 25 ℃ to 60 ℃, and investigate the 1C discharge capacity of different lithium-ion batteries.
(4) When the high and low temperature chamber are stabilized to the set temperature conditions, put the single lithium ion battery with standard voltage of 3.2V after standing for 1d into the test chamber for 1 h, so that it can reach Thermal equilibrium.
(5) When the battery discharges to the cut-off voltage of 2.0V, stop discharging, analyze and process relevant data.
2 Results and Discussion
It can be seen from the data that the discharge capacity of the nano Lithium iron phosphate battery at the low temperature stage gradually decreases with the decrease of the ambient temperature, because under the low temperature condition, the concentration of the battery electrolyte becomes larger, and the speed of the lithium ion disengaging from the negative electrode material becomes slower.
In addition, because the internal resistance of the battery becomes larger, the discharge capacity curve decreases, and the discharge cut-off voltage of the lithium ion power battery is reached in advance, the discharge capacity decreases, The discharge efficiency is reduced. At temperatures above 0 ℃, the discharge capacity can basically maintain over 93% of the normal capacity, while at temperatures below 0 ℃, the rate of decrease in discharge capacity of lithium-ion power batteries increases with the decrease in temperature.
For nano Lithium iron phosphate battery, the capacity is 88% at -10 ℃, 75.3% at 20 ℃, and only 47.1% at -40 ℃; At 25-10 ℃, the capacity decay rate is about 9.5%; At -10~-20 ℃, the capacity decay rate is about -12.8%, but the capacity decay rate sharply increases from -20 ℃ to -40 ℃, reaching about 28.2%. Therefore, -20 ℃ can be considered as a low-temperature node of Lithium iron phosphate battery.
When the temperature is slightly higher than room temperature (25 ℃), due to the enhanced material activity inside the lithium-ion battery, the diffusion rate of lithium ions increases, and its discharge capacity increases. In the high-temperature stage, the capacity change of the battery is not very significant, and the maximum capacity change only increases by about 3% compared to the benchmark.
After 55 ℃, the capacity curve is basically unchanged, and at 60 ℃, the capacity is at the same level as the reference point. However, under the condition of high temperature, the physical characteristics of the electrode material of the lithium battery will undergo irreversible attenuation, and the reaction intensity of the electrode material will weaken, so its discharge capacity and discharge efficiency will decline. From this point, it can be seen that long-term use of batteries in environments above 50 ℃ should be avoided as much as possible.
The ideal operating temperature of the lithium-ion battery should be between 18~50 ℃ to ensure that the discharge efficiency is above 80% and meet the power requirements of the whole vehicle. From some references and technical manuals, it can be seen that in order to ensure the service life of the battery itself, the working temperature should be controlled between 20 and 50 ℃.
At – 20 ℃, the discharge capacity of the two different lithium batteries is 75.01% of the Nameplate capacity, and the performance of the nano Lithium iron phosphate battery is better than that of the Lithium iron phosphate battery; At -40 ℃, the discharge performance of nano Lithium iron phosphate battery is more excellent, and the discharge capacity is 47.1% of the Nameplate capacity, while the Lithium iron phosphate battery is only 37.5%. Therefore, the use of carbon nanotubes with excellent conductivity to replace part of the conductive carbon to make Lithium iron phosphate positive plates has greatly improved the charging and discharging performance of Lithium iron phosphate battery.
3 Conclusion
The temperature performance of nano Lithium iron phosphate battery and ordinary conductive carbon Lithium iron phosphate battery was investigated. The experimental results show that the environmental temperature has a great impact on the capacity of the Lithium iron phosphate battery.
The capacity decays rapidly at low temperatures, and the green capacity increases rapidly at high temperatures, but the change rate is lower than that at low temperatures. In addition, carbon nanotubes with excellent conductivity are used to replace some conductive carbon to make Lithium iron phosphate positive plates. The charge and discharge performance of the Lithium iron phosphate battery is greatly improved.
The discharge capacity of the nano Lithium iron phosphate battery at -40 ℃ is 47.1% of the capacity at 25 ℃, The discharge capacity of ordinary Lithium iron phosphate battery is only 37.5% of the capacity at 25 ℃. The excellent electrochemical performance of the battery is mainly attributed to the improvement of the conductivity of the whole battery.
In order to ensure the service life of the battery itself, the operating temperature should be controlled between 20~50 ℃. The temperature characteristics of Lithium iron phosphate battery are clarified, which is of great significance for the design of the battery thermal management system.