Lithium-ion battery has a high operating voltage, than can be large (up to 165wh/kg, nickel-metal hydride battery 3 times times), small size, light weight, long cycle life, low self-discharge, no memory effect, no pollution and many other advantages. Lithium iron phosphate batteries in the new energy industry is optimistic, the battery cycle life can reach about 3,000 times, discharge stability, is widely used in power batteries and energy storage and other fields. But its popularization speed and application field breadth, the depth is not satisfactory. In addition to the price, the battery material itself caused by the batch consistency factors, its temperature performance is also an important factor.
This paper investigates the effect of temperature on the performance of lithium iron phosphate batteries, and also investigates the charging and discharging of the battery pack at high and low temperatures.
One, monomer (module) at room temperature Cycle summary
Normal temperature Test battery cycle life can be seen, lithium iron phosphate battery long life advantage, currently do 3,314 cycles, capacity retention rate is still 90%, and reach 80% of the end of life may have to do about 4,000 times.
1, Monomer cycle
Currently completed: 3314cyc with a capacity retention rate of 90%. Affected by the processing technology of the core and the group process of the module, the battery in the pack after the completion of the inconsistency has been formed, the more exquisite the process of the group of internal resistance, the smaller the difference between the core.
The cycle life of the following modules is currently the majority of lithium iron phosphate can do basic data, so that in the process of use of BMS will need to regularly balance the battery pack, reduce the difference between the core, prolong the service life.
2, module cycle
Currently completed: 2834cyc with a capacity retention rate of 67.26%.
Second, the monomer high temperature cycle summary
Accelerating the aging life of the battery under high temperature conditions.
1, monomer Charge and discharge curve
2, high temperature cycle
The high temperature cycle completes 1100cyc with a capacity retention rate of 73.8%.
Effect of low temperature on charge and discharge performance At 0~-20 ℃ temperature, the discharge capacity is equivalent to 88.05%, 65.52% and 38.88% of the capacitance of 25 ℃ of the temperature; The average discharge voltage is 3.134, 2.963V and 2.788V, and a 20 ℃ discharge average voltage is reduced by 0.431V at 25 ℃.
The above analysis shows that with the decrease of temperature, the average discharge voltage and discharge capacity of Li-ion battery are reduced, especially when the temperature is 20 ℃, the discharge capacity and the average discharge voltage of the battery fall faster.
Fig. 1 Electric curve of lithium iron phosphate battery under different temperature From the electrochemical angle analysis, the solution resistance, the SEI film resistance changes little in the whole temperature range, the effect on the battery low temperature performance is small; The charge transfer resistance increases significantly with the decrease of temperature, and the change of temperature in the whole temperature range is significantly greater than the solution resistance and the SEI film resistance.
This is because with the decrease of the temperature, the ionic conductivity of the electrolyte decreases, the SEI film resistance and the electrochemical reaction resistance increases, resulting in ohmic polarization at low temperature, concentration polarization and electrochemistry polarization are increased, the discharge curve on the battery shows that the average voltage and discharge capacity are reduced with the temperature.
Figure 2 Battery charge and discharge 5 times after low temperature As shown in Figure 2, the capacity of the battery and the discharge platform have been reduced by circulating 5 times under the -20℃ and then circulating at 25 ℃.
This is because as the temperature decreases, the ionic conductivity of the electrolyte is reduced, the ohmic polarization in the process of low temperature charging, the concentration polarization and electrochemical polarization increase, resulting in metal lithium deposition, the electrolyte decomposition, resulting in the electrode surface SEI film thickening, the SEI film resistance increased, in the discharge curve performance for the discharge platform and discharge capacity reduction.
1. Effect of low temperature on cyclic performance
Fig. 3 0.5C magnification cycle curve of Li-ion battery at room temperature
Figure 4 Li-Ion battery-10 ℃ temperature 0.5C magnification cycle curve As can be seen from the figure, the battery in the 10 ℃ environment, the capacity attenuation is faster, 100 cycles after the capacity of only 59mah/g, capacity attenuation 47.8%; The battery at low temperature will be charged and discharged at room temperature, and the capacity recovery performance of the inspection period. Its capacity is restored to 70.8mah/g with a capacity loss of up to 68%.
This shows that the low temperature cycle of the battery has a great impact on the recovery of battery capacity.
2, low temperature impact on safety performance Lithium-ion battery charging is the process by which lithium ions migrate from the cathode to the anode material through the electrolyte, and the lithium ions converge to the anode, capturing a lithium ion from six carbon atoms.
At low temperature, the chemical reaction activity is reduced, while the lithium ion migration slows down, the lithium ions on the cathode surface has not been embedded in the negative electrode has been first reduced to metal lithium, and precipitated on the anode surface of the formation of lithium dendrite, which easily pierced the diaphragm caused by short-circuit inside the battery, and thus damage the battery, resulting in safety incidents
