The test method of cyclic aging of lithium ion battery is introduced

First, capacity test method

1. Constant current charge and discharge test:

This is the most basic test method. Under the specified temperature environment (such as normal temperature 25℃), the lithium-ion battery is charged with a constant current until the set charging cutoff voltage is reached, such as for common ternary lithium-ion batteries, the charging cutoff voltage is generally about 4.2V. Then discharge at a constant current until the discharge cutoff voltage is reached, which is usually about 3.0V for ternary lithium-ion batteries. By recording the charging capacity and discharge capacity of each cycle, the attenuation of the battery capacity with the number of cycles can be observed.

For example, when testing a lithium-ion battery for a mobile phone, the charging current is set to 0.5C (C is the nominal capacity of the battery, such as the nominal capacity of the battery is 3000mAh, 0.5C is 1500mA), and the discharge current is also 0.5C, after 100 cycles, It was found that its discharge capacity dropped from the initial 3000mAh to 2700mAh, which intuitively reflects the attenuation of battery capacity.

2. Capacity test under different magnification:

In addition to testing at a single magnification, it is also tested at several different charge and discharge magnifications. For example, charge and discharge cycles are carried out at different magnifications such as 0.2C, 0.5C, 1C and 2C. The capacity performance of the battery is different at different magnification, and the polarization phenomenon of the battery is more serious at high magnification, and the capacity attenuation may be faster.

Taking lithium-ion power batteries for electric vehicles as an example, when cycling at 0.2C rate, the battery may be able to maintain a high capacity utilization rate, but when cycling at 2C rate, due to factors such as polarization and heating inside the battery, its capacity decay speed will be significantly accelerated. By comparing the capacity decay curve at different rates, Battery performance and aging can be evaluated more comprehensively.

Second, internal resistance test method

1. Ac Impedance Test (EIS) :

This method is to apply a small amplitude of AC voltage signal to the battery (general frequency range from a few mHz to a few hundred kHz), and then measure the AC current response of the battery, so as to obtain the impedance spectrum of the battery. In the process of battery cycle aging, its internal resistance will change, and the change of internal resistance at different frequencies can be obtained by EIS.

For example, the new lithium-ion battery is mainly reflected in the ohmic internal resistance of the electrolyte in the high frequency region, and the charge transfer internal resistance and diffusion internal resistance of the electrode reaction in the low frequency region. With the increase of the number of cycles, the internal resistance of the low frequency region may increase due to the change of the electrode surface active substance and the blockage of the lithium ion diffusion channel. Through the analysis of EIS spectrum, we can deeply understand the electrochemical reaction process and aging mechanism inside the battery.

2. Dc internal resistance test:

The method is to apply a short DC pulse current at both ends of the battery (for example, the current pulse duration is 10-30 seconds), and then measure the voltage change at both ends of the battery, and calculate the internal resistance of the battery according to Ohm‘s law. The variation of DC internal resistance with the number of cycles can be observed by testing the internal resistance at different cycle stages.

For example, when testing lithium-ion batteries for power tools, a DC internal resistance test is performed every 10 cycles. At the beginning, the DC internal resistance of the battery may be only tens of milliohm, and with the increase of the number of cycles, the internal resistance may rise to several hundred milliohm, which indicates that the performance of the battery is gradually declining.

Third, cycle life test method

1. Standard cycle life test:

Cyclic testing is carried out according to the charge and discharge system specified by the relevant standards (such as the International Electrotechnical Commission IEC standard or the national standard GB/T, etc.). It generally includes cycling the battery at a specific temperature (such as 25 ° C or 45 ° C at a high temperature) at a certain rate of charge and discharge (such as 0.5C charging and 0.5C discharging) until the battery capacity decays to a certain percentage of the initial capacity (such as 80%), and the number of cycles is recorded as the cycle life of the battery.

Taking lithium-ion batteries for energy storage as an example, according to the GB/T standard test, in a normal temperature environment, after hundreds of cycles, if the battery capacity decays to 80% of the initial capacity, which indicates that the cycle life of the battery under the conditions of use has reached a certain limit, this method can directly assess the service life of the battery under standard conditions of use.

2. Accelerated cycle life test:

In order to shorten the test time, an accelerated aging method is used. For example, the aging process of the battery is accelerated by increasing the charging and discharging ratio and increasing the test temperature. However, this method requires the establishment of a suitable acceleration model to estimate the cycle life under actual use conditions.

For example, in the study of new lithium-ion battery materials, in order to quickly evaluate its performance, the test temperature is increased to 60 ° C, the charge and discharge ratio is increased to 2C, so that the rapid attenuation of battery capacity and other performance changes can be observed in a short time, and then the cycle life under normal temperature and normal ratio is calculated according to the accelerated aging model.

fourth. Voltage characteristic test method

1. Charge and discharge curve test:

During the battery cycle, record the voltage-time or voltage-capacity curve during charging and discharging. The charge and discharge curve of the new battery is generally smooth, and the curve will change with the increase of the number of cycles. For example, the charging curve may show a decrease in platform voltage and an increase in voltage at the end of charging. The voltage drop may be accelerated on the discharge curve.

For lithium-ion batteries for laptop computers, in the initial state, the voltage drop in the discharge curve from full charge to no power is relatively stable, after several cycles, the voltage may decrease significantly in the initial discharge, which reflects the change in the performance of the electrode material inside the battery and the degree of aging.

2. Open circuit voltage test:

After the battery has been standing for a certain period of time (generally a few hours to more than ten hours to ensure that the internal electrochemical balance of the battery is reached), its open-circuit voltage is measured. During battery aging, the open-circuit voltage changes because the chemical balance inside the battery changes.

For example, when monitoring the lithium-ion battery pack, periodically (such as once a week) measure the open circuit voltage of each battery in the battery pack, with the aging of the battery, the open circuit voltage of some batteries may be significantly different from other batteries, which can help judge the consistency and aging of the battery, and find the battery that may have problems in time.