Nickel-cobalt-manganese ternary material is one of the main materials of the current power battery. The three elements have different meanings for the cathode material, among which the nickel element is to improve the capacity of the battery. The higher the nickel content, the higher the material specific capacity. NCM811 has a specific capacity of 200mAh/g and a discharge platform of about 3.8V, which can be made into a high energy density battery. However, the problem of NCM811 battery is poor safety and fast cycle life decay. What are the reasons affecting its cycle life and safety? How to solve this problem? The following is an in-depth analysis:

NCM811 was made into button battery (NCM811/Li) and flexible pack battery (NCM811/ graphite), and its gram capacity and full battery capacity were tested respectively. The soft-pack battery was divided into four groups for single factor experiment. The parameter variable was cut-off voltage, which was 4.1V, 4.2V, 4.3V and 4.4V, respectively. First, the battery was cycled twice at 0.05c and then at 0.2C at 30℃. After 200 cycles, the soft pack battery cycle curve is shown in the figure below:

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

In this experiment, the parasitic reaction was studied by isothermal calorimetry and the structure and morphology degradation of cathode materials during the cycling process were studied by XRD and SEM. The conclusions are as follows:

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First, structural change is not the main cause of battery cycle life decline

The results of XRD and SEM showed that there was no obvious difference in particle morphology and atomic structure of the battery with electrode and cut-off voltage of 4.1V, 4.2V, 4.3V and 4.4V after 200 cycles at 0.2c. Therefore, the rapid structural change of living matter during charging and discharging is not the main reason for the decline of battery cycle life. Instead, parasitic reactions at the interface between the electrolyte and the highly reactive particles of live matter in the delithium state are the main cause of reduced battery life at the 4.2V high voltage cycle.

(1) the SEM

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A1 and A2 are the SEM images of the battery without circulation. B ~ E are SEM images of positive electrode living material after 200cycle under 0.5C condition and charging cut-off voltage of 4.1V/4.2V/4.3V/4.4V, respectively. The left side is electron microscope image under low magnification and the right side is electron microscope image under high magnification. As can be seen from the figure above, there is no significant difference in particle morphology and breakage degree between the circulating battery and the non-circulating battery.

(2) XRD images

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

(3) Change of lattice parameters

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

1. The lattice constants of uncycled polar plates are consistent with those of NCM811 live powder. When the cycle cutoff voltage is 4.1V, the lattice constant is not significantly different from the previous two, and the C axis increases a little. The lattice constants of the C-axis with 4.2V, 4.3V and 4.4V are not significantly different from those of 4.1V (0.004 angms), while the data on the A-axis are quite different.

2. There was no significant change in Ni content in the five groups.

3. Polar plates with a circulating voltage of 4.1V at 44.5° exhibit large FWHM, while the other control groups exhibit similar FWHM.

In the charging and discharging process of the battery, the C axis has a large shrinkage and expansion. The reduction in battery cycle life at high voltages is not due to changes in living matter structure. Therefore, the above three points verify that structural change is not the main reason for the decline of battery cycle life.

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Second, the cycle life of NCM811 battery is related to the parasitic reaction in the battery

NCM811 and graphite are made into flexible pack cells using different electrolytes. In contrast, 2%VC and PES211 were added to the electrolyte of the two groups, respectively, and the capacity maintenance rate of the two groups showed a great difference after the battery cycle.

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According to the figure above, when the cut-off voltage of the battery with 2%VC is 4.1V, 4.2V, 4.3V and 4.4V, the capacity maintenance rate of the battery after 70 cycles is 98%, 98%, 91% and 88%, respectively. After only 40 cycles, the capacity maintenance rate of the battery with added PES211 decreased to 91%, 82%, 82%, 74%. Importantly, in previous experiments, the battery cycle life of NCM424/ graphite and NCM111/ graphite systems with PES211 was better than that with 2%VC. This leads to the assumption that electrolyte additives have a significant impact on battery life in high-nickel systems.

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

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

Isothermal microcalorimetry was used to analyze the parasitic reaction probability of the battery. Parameters such as polarization, entropy and parasitic heat flow were extracted to make a functional relationship with rSOC, as shown in the figure below:

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Above 4.2V, the parasitic heat flow is shown to increase suddenly, because the highly delithium anode surface reacts easily with the electrolyte at high voltage. This also explains why the higher the charge and discharge voltage, the faster the battery maintenance rate decreases.

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Iii. NCM811 has poor security

Under the condition of increasing the ambient temperature, the reaction activity of NCM811 in charging state with electrolyte is much greater than that of NCM111. Therefore, the use of NCM811 production of battery is difficult to pass the national compulsory certification.

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The figure is a graph of the self-heating rates of NCM811 and NCM111 between 70℃ and 350℃. The figure shows that NCM811 begins to heat up at about 105℃, while NCM111 does not until 200℃. The NCM811 has a heating rate of 1℃/min from 200℃, while the NCM111 has a heating rate of 0.05℃/min, which means that the NCM811/ graphite system is difficult to obtain mandatory safety certification.

High nickel living matter is bound to be the main material of high energy density battery in the future. How to solve the problem of rapid decay of NCM811 battery life? First, the particle surface of NCM811 was modified to improve its performance. The second is to use the electrolyte which can reduce the parasitic reaction of the two, so as to improve its cycle life and safety. The picture

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