Briefly describe the effect of temperature on lithium iron phosphate battery

Lithium-ion battery has high working voltage (three times that of nickel-hydrogen and nickel-cadmium batteries), large specific energy (up to 165Wh/kg, which is 3 times that of nickel-hydrogen battery), small size, light weight, long cycle life and self- Low discharge, no memory effect, no pollution and many other advantages. In the new energy industry, lithium iron phosphate batteries are optimistic, the battery cycle life can reach 3,000 or so, the discharge is stable, and is widely used in the fields of power batteries and energy storage.

However, the speed and depth of its promotion and the depth of its application are not satisfactory. In addition to factors such as price and batch consistency caused by battery materials themselves, temperature performance is also an important factor. This paper investigates the effect of temperature on the performance of lithium iron phosphate battery, and also investigates the charge and discharge of the battery pack under high and low temperature conditions.

First, monomer (module) normal temperature cycle summary

The cycle life of the test battery at room temperature can be seen that the long life advantage of the lithium iron phosphate battery is currently 3,314 cycles, the capacity retention rate is still 90%, and the end of life of 80% may be about 4,000 times.

1, monomer cycle

Currently completed: 3314cyc, capacity retention rate of 90%

Affected by the processing technology of the battery core and the grouping process of the module, the inconsistency of the battery after the completion of the PACK has been formed, and the more refined the process, the smaller the internal resistance of the group, and the smaller the difference between the batteries. The cycle life of the following modules is the basic data that most of the current lithium iron phosphate can be achieved. In this way, the BMS is required to periodically balance the battery packs, reduce the difference between the cells, and prolong the service life.

2, module cycle

Currently completed: 2834cyc, capacity retention rate is 67.26%

Second, the monomer high temperature cycle summary

Accelerate the aging life of the battery under high temperature conditions.

1, monomer charge and discharge curve

2, high temperature cycle

The high temperature cycle was completed at 1100cyc, and the capacity retention rate was 73.8%.

Third, the effect of low temperature on charge and discharge performance

The discharge capacity of the battery at 0～-20°C is equivalent to 88.05%, 65.52% and 38.88% of the discharge capacity at 25°C. The average discharge voltage is 3.134, 2.963V and 2.788V, respectively. The average discharge voltage is 20°C. It is 0.431V lower than at 25 °C. It can be seen from the above analysis that as the temperature is lowered, the discharge average voltage and the discharge capacity of the lithium ion battery are both lowered, especially when the temperature is -20 ° C, the discharge capacity and the average discharge voltage of the battery are decreased rapidly.

From the electrochemical point of view, the solution resistance and SEI film resistance do not change much over the whole temperature range, and have little effect on the low temperature performance of the battery; the charge transfer resistance increases significantly with the decrease of temperature, and the temperature is over the entire temperature range. The change is significantly greater than the solution resistance and SEI film resistance. This is because as the temperature decreases, the ionic conductivity of the electrolyte decreases, and the SEI membrane resistance and electrochemical reaction resistance increase, resulting in an increase in ohmic polarization, concentration polarization, and electrochemical polarization at low temperatures. Large, on the discharge curve of the battery, the average voltage and discharge capacity both decrease with decreasing temperature.

The cycle of the cycle at -20 ° C and then cycle at 25 ° C, the battery capacity and discharge platform are reduced. This is because as the temperature decreases, the ionic conductivity of the electrolyte decreases, and ohmic polarization, concentration polarization, and electrochemical polarization increase during low-temperature charging, resulting in deposition of metallic lithium, which causes the electrolyte to decompose, ultimately resulting in The surface of the electrode SEI film is thickened, and the SEI film resistance is increased, and the discharge platform exhibits a discharge platform and a decrease in discharge capacity.

1. Effect of low temperature on cycle performance

Battery capacity attenuation in - 10 ℃ environment quickly, after a 100 - cycle capacity only 59mah/g, 47.8% capacity attenuation; The charging and discharging test will be carried out on the battery which has been placed under low temperature. Its capacity recovered to 70.8mAh/g with a loss of 68%. Therefore, the low temperature cycle of the battery has a great impact on the recovery of the battery capacity.

2. The effect of low temperature on safety performance

Lithium-ion battery charging is a process in which lithium ions are removed from the positive electrode and migrated into the negative electrode material through the electrolyte. Lithium ions are polymerized to the negative electrode, and one lithium ion is trapped by six carbon atoms. At low temperatures, the chemical reactivity decreases, and the lithium ion migration slows down. The lithium ions on the surface of the negative electrode have not been embedded in the negative electrode and have been reduced to metallic lithium, and precipitated on the surface of the negative electrode to form lithium dendrites, which is easy to pierce. The diaphragm causes a short circuit inside the battery, which in turn damages the battery and causes a safety accident.

It can be concluded from the above data that the lithium iron phosphate battery is greatly affected by the temperature. In the application environment where the power battery application field and the temperature are greatly affected, the battery needs to be thermally managed (air-cooled, liquid-cooled, etc.) to improve the battery. Use efficiency to extend battery system life.

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