In-depth analysis of the causes of NCM811 battery life degradation

Nickel-cobalt-manganese ternary material is one of the main materials of current power batteries. The three elements have different meanings for cathode materials. Among them, nickel is to increase battery capacity. The higher the nickel content, the greater the specific capacity of the material. The specific capacity of NCM811 can reach 200mAh/g, the discharge platform is about 3.8V, and it can be made into a battery with high energy density. However, the problem with the NCM811 battery is that it has poor safety and rapid cycle life decay. What are the reasons that affect its cycle life and safety? How to solve this problem? Here is an in-depth analysis:

Make NCM811 into button battery (NCM811/Li) and soft pack battery (NCM811/graphite), and test its gram capacity and full battery capacity respectively. Divide the soft pack batteries into 4 groups for single factor experiments, the parameter variable is the cut-off voltage, and its values are 4.1V, 4.2V, 4.3V, 4.4V. First, the battery was cycled twice at a rate of 0.05C, and then cycled at 30°C at a rate of 0.2C. After 200 cycles, the cycle curve of the soft pack battery is shown in the figure below:

It can be seen from the figure that under the condition of a higher cut-off voltage, the gram capacity of active matter and the battery capacity are both high, but the gram capacity of the battery and the material decays faster. On the contrary, at a lower cut-off voltage (below 4.2V), the battery capacity decays slowly and the cycle life is longer.

This experiment uses isothermal calorimetry technology to study parasitic reactions, and uses in-situ and ex-situ XRD and SEM to study the structural and morphological degradation of cathode materials during cycling. conclusion as below:

1. Structural changes are not the main reason for the attenuation of battery cycle life

The results of ex-situ XRD and SEM data show that the uncycled battery pole pieces and the battery with cut-off voltages of 4.1V, 4.2V, 4.3V, and 4.4V respectively, after being cycled at 0.2C for 200 times, the particle morphology and atomic There is no obvious difference in structure. Therefore, the rapid structural change of the active material during charge and discharge is not the main reason for the degradation of the battery cycle life. On the contrary, the parasitic reaction between the electrolyte and the interface of the highly active active material particles in the delithiation state is the main reason for the shortened battery life under the 4.2V high voltage cycle.

(1) SEM

a1 and a2 are the SEM pictures of the battery without cycling. b~e are the SEM images of the positive active material after a 200 cycle cycle under the condition of 0.5C and the charge cut-off voltage is 4.1V/4.2V/4.3V/4.4V. The left side is at low magnification and the right side is high magnification. Download the electron microscope picture. It can be seen from the above figure that there is no significant difference in particle morphology and degree of fragmentation between the recycled battery and the uncycled battery.

(2) XRD

As can be seen from the above figure, there is no obvious difference between the five in terms of peak shape and position.

(3) Changes in lattice parameters

As can be seen from the table, the following points:

1). The lattice constant of the non-circulated pole piece is consistent with that of the NCM811 active material powder. When the cycle cut-off voltage is 4.1V, its lattice constant is also indistinguishable from the former two, and the c-axis has a small increase. Looking at the c-axis lattice constants with cycle cut-off voltages of 4.2V, 4.3V, and 4.4V, there is no significant difference from 4.1V (the difference is 0.004 angstroms), while the data on the a-axis is quite different.

2). There was no significant change in Ni content in the five groups of comparative tests.

3). The pole piece with a cycling voltage of 4.1V at 44.5° exhibits a larger FWHM, while the other comparison groups are closer.

During the charging and discharging process of the battery, the c-axis showed great contraction and expansion. Under high voltage, the decrease in battery cycle life is not due to changes in the structure of the living material. Therefore, the above three points verify that structural changes are not the main reason for the degradation of battery cycle life.

2. The cycle life of the NCM811 battery is related to the parasitic reaction in the battery

NCM811 and graphite are made into soft pack batteries, and they use different electrolytes. The two groups of comparative experimental battery electrolytes were added with 2% VC and PES211, but the battery capacity maintenance rate after cycling showed a big difference.

As can be seen from the above figure, when the cut-off voltage of the battery with 2% VC is 4.1V, 4.2V, 4.3V, 4.4V, the capacity retention rate of the battery after 70 cycles is 98%, 98%, 91%, 88%, respectively . In the battery with PES211 added, the capacity retention rate dropped to 91%, 82%, 82%, 74% after only 40 cycles. The important thing is: In the previous experiment, the cycle life of the NCM424/graphite and NCM111/graphite systems with PES211 was better than that with 2% VC. This leads to the hypothesis that in high nickel material systems, electrolyte additives have a great impact on battery life.

It can also be seen from the above data that the cycle life under high voltage is much worse than that under low voltage. By fitting functions to polarization, △V and the number of cycles, the following figure is obtained:

It can be seen that the ΔV of the battery is small when the battery is cycled at a low cut-off voltage, and when the voltage rises above 4.3V, the ΔV rises sharply and the battery polarization increases, which greatly affects the battery life. It can also be seen from the figure that the ΔV change rates of VC and PES211 are different, which further verifies that the electrolyte additives are different, and the degree of polarization and speed of the battery are also different.

Use the isothermal microcalorimetry method to analyze the parasitic reaction probability of the battery, and make a functional relationship with rSOC by extracting parameters such as polarization, entropy, and parasitic heat flow, as shown in the following figure:

The figure shows that above the 4.2V voltage, the parasitic heat flow suddenly rises. This is because the surface of the positive electrode, which is highly de-lithiated under high voltage, is very easy to react with the electrolyte. This also explains why the higher the charge-discharge voltage, the faster the battery capacity retention rate drops.

3. NCM811 has poor security

Under the condition of continuously increasing the ambient temperature, the activity of NCM811 reacting with the electrolyte in the charged state is far greater than the activity of NCM111 reacting with the electrolyte. Therefore, it is more difficult for batteries made by NCM811 to pass national compulsory certification.

This figure is a graph of the self-heating rate of NCM811 and NCM111 between 70°C and 350°C. The figure shows that at around 105°C, NCM811 began to generate heat, but NCM111 has not yet started to generate heat until 200°C. NCM811 starts at 200°C and has a heating rate of 1°C/min, while NCM111 is still 0.05°C/min. This also means that it is difficult for NCM811/graphite batteries to pass mandatory safety certification.

High nickel active material will inevitably be the main material for high energy density batteries in the future. How to solve the problem of rapid decay of NCM811 battery life? One is to improve the performance of NCM811 by modifying the surface of the particles. The second is to use an electrolyte that can reduce the parasitic reaction between the two, thereby improving its cycle life and safety.