Comparative analysis of the performance of commercial lithium iron phosphate and ternary power lithium-ion batteries

Introduction: The safety hazard of ternary battery is higher than that of lithium iron phosphate. In domestic and foreign reports on the burning and explosion of electric vehicles, the number of safety accidents caused by ternary battery is significantly higher than that of lithium iron phosphate battery.

At present, the mainstream power batteries in China are divided into two types, lithium iron phosphate batteries and ternary batteries, according to different cathode materials. It is precisely because of the difference in the positive electrode materials that the performance of the two batteries is quite different.

Lithium iron phosphate material has stable structure and good thermal stability, and its cycle performance and safety are better than ternary materials; ternary materials have high energy density, but due to the unstable structure of the material during charging and discharging, phase transitions are prone to occur, so its cycle The performance is worse than lithium iron phosphate; the thermal stability of the ternary material is poor, so the safety hazard of the ternary battery is higher than that of the lithium iron phosphate. In the domestic and foreign reports on the burning and explosion of electric vehicles, the safety accident caused by the ternary battery The amount is significantly higher than that of lithium iron phosphate batteries.

1. Experimental process

This article is based on the relevant test procedures of GB/T 31484-2015 "Cycle Life Requirements and Test Methods for Power Batteries for Electric Vehicles" and GB/T 31486-2015 "Electrical Performance Requirements and Test Methods for Power Batteries for Electric Vehicles", the 75Ah iron phosphate The cell cycle life and room temperature discharge capacity of lithium and ternary NCM622 batteries, as well as the room temperature discharge capacity and room temperature rate performance of the module (5 cells connected in series) were tested. Photographs of battery cells and modules are shown in Figure 1. The weight of lithium iron phosphate battery is 1.81kg, the weight of module is 9.05kg; the weight of ternary battery is 1.34kg, and the weight of module is 6.70kg.

Figure 1 Commercialized lithium iron phosphate battery (a) monomer and (b) module; commercialized ternary battery (a) monomer and (b) module

1.1 Battery cell performance test

The upper limit of the charging voltage of the lithium iron phosphate battery is 3.8V, and the discharge termination voltage is 2V; the upper limit of the charging voltage of the lithium iron phosphate battery module is 19V. The upper limit of the ternary battery cell charging voltage is 4.2V, and the discharge termination voltage is 3.0V; the upper limit of the ternary battery module charging voltage is 21.5V.

1.1.1 Cell cycle life test

a) Discharge with 1I1 (I1=75A) to the final voltage;

b) Set aside for 30 minutes;

c) Charge with constant current of 1I1 current to the upper limit of voltage and switch to constant voltage charging, and stop charging when the charging current drops to 0.05I1;

d) Set aside for 30 minutes;

e) Discharge with 1I1 to the discharge termination condition, and record the discharge capacity;

f) Follow b) to e) for 500 consecutive cycles.

1.1.2 Cell discharge capacity test at room temperature

a) Constant current charging with 1I1 current to the upper limit of voltage, then constant voltage charging, stop charging when the charging current drops to 0.05I1, and put it aside for 1h;

b) At room temperature, the battery is discharged with a current of 1I1 until the discharge reaches the final voltage;

c) Measure the discharge capacity and record the specific discharge energy;

d) Repeat steps a) to c) 5 times. When the range of 3 consecutive test results is less than 3% of the rated capacity, the test can be terminated early and the average of the last 3 test results is taken.

1.2 Battery module performance test

1.2.1 Module room temperature discharge capacity test

a) The battery module (5 cells connected in series) is charged at 1I1 to the upper limit of the voltage and then converted to constant voltage charging. When the charging current drops to 0.05I1, the charging is stopped. If the battery voltage exceeds the termination voltage of 0.1V during the charging process, the charging will stop. , Put it aside for 1h after charging;

b) At room temperature, the battery is discharged with a current of 1I1 until the voltage of any single cell reaches the discharge termination voltage;

c) Measure discharge capacity and discharge specific energy;

d) Repeat steps a) to c) 5 times. When the range of 3 consecutive test results is less than 3% of the rated capacity, the test can be terminated early and the average of the last 3 test results is taken.

1.2.2 Module room temperature rate discharge performance test

a) The battery module is charged at 1I1 to the upper limit of the voltage to constant voltage charging, and the charging is stopped when the charging current drops to 0.05I1. If the battery voltage exceeds the termination voltage of 0.1V during the charging process, the charging is stopped, and the charging is left for 1h;

b) At room temperature, the battery module discharges with 3I1 current until the voltage of any single cell reaches the discharge termination voltage;

c) Measure the discharge capacity.

2. Results and discussion

The cycle life test results of commercial lithium iron phosphate batteries and ternary battery cells are shown in Figure 2. The first week discharge capacity of lithium iron phosphate battery is 77.80Ah (mass specific capacity 42.98Ah/kg), the discharge capacity after 500 cycles is 72.01Ah (mass specific capacity 39.78Ah/kg), and the capacity retention rate is 92.56%. The first week discharge capacity of the ternary battery is 77.82Ah (mass specific capacity 58.07Ah/kg), the discharge capacity after 500 cycles is 70.69Ah (mass specific capacity 52.75Ah/kg), and the capacity retention rate is 90.84%.

Figure 2 Cycle life test results of commercial lithium iron phosphate batteries and ternary battery cells

It can be seen that the cycle stability of lithium iron phosphate batteries is higher than that of ternary batteries, and ternary batteries have more advantages in terms of specific capacity, and their rated voltage is also higher than that of lithium iron phosphate. Therefore, it is foreseeable that the energy density of the ternary battery is higher than that of the lithium iron phosphate battery, and the subsequent discharge capacity test will specifically verify this.

The discharge capacity curve obtained from the discharge capacity test of the commercial lithium iron phosphate battery and the ternary battery cell is shown in Figure 3. The discharge capacity of the lithium iron phosphate battery for 3 cycles is 78.56, 78.73 and 78.79Ah (specific capacity respectively 43.40, 43.50, 43.53Ah/kg), according to the standard, the average value is 78.69Ah (specific capacity 43.48Ah/kg) ) As the discharge capacity. The discharge capacity of the ternary battery for 3 cycles is 76.98, 77.78 and 77.89 Ah (specific capacity is 57.38, 58.04, 58.13 Ah/kg respectively), and the average value is 77.55 Ah (specific capacity 57.87 Ah/kg) as the discharge capacity.

Figure 3 Discharge curves of commercial lithium iron phosphate batteries and ternary battery cells

Figure 3(a) and Figure 3(b) show the three discharge capacity curves of lithium iron phosphate and ternary batteries, respectively. The lithium iron phosphate battery has a gentle discharge voltage platform between 3.25 and 3.15V, while the ternary battery The discharge platform is 4.05-3.35V, and the voltage platform is wider. The three discharge energies of lithium iron phosphate were recorded as 248.90Wh (specific energy 137.51Wh/kg), 248.15Wh (specific energy 137.10Wh/kg), 247.31Wh (specific energy 136.64Wh/kg), and the average specific energy was 137.08Wh /kg; and the three-time discharge energy of the ternary battery is 278.78Wh (specific energy 208.04Wh/kg), 282.36Wh (specific energy 210.72Wh/kg), 282.88Wh (specific energy 211.10Wh/kg), and the average specific energy is 209.95Wh/kg. It can be seen that because the specific capacity and voltage platform of the ternary battery are higher than those of the lithium iron phosphate battery, the specific energy of the commercial ternary battery is higher than that of the lithium iron phosphate battery.

3. Conclusion

The cycle stability of commercial power lithium iron phosphate batteries is better than that of ternary batteries, and its durability is even better. Commercial ternary batteries have a high discharge voltage platform and high specific capacity. The energy density of ternary battery cells and modules exceeds 200Wh/kg, which is higher than lithium iron phosphate batteries (approximately 136Wh/kg), so it can provide longer time for electric vehicles. The mileage. Both lithium iron phosphate batteries and ternary batteries have excellent discharge capabilities at high rates. Electric car manufacturers basically select the battery system based on the car’s index parameters. For electric car buyers, they should first understand whether the battery system used in electric cars is a lithium iron phosphate battery or a ternary battery. Car model selection based on actual needs.

（来源：译自锂电联盟会长）