In general we often say three yuan first refers to the NMC data, also includes the NCA information, the capacity of the layered materials by their structural stability, the influence of because Ni3 + chemical stability is better than Co elements, and therefore in the process of charging the NMC data also can emerge more Li, increases the capacity of data from a larger progress.
On the contrary, the structural stability of the layered oxide positive electrode data is also affected by the amount of deli. Excessive deli may lead to the collapse of the layered structure of the data. Therefore, in order to ensure the structural stability of NMC data, the charging cut-off voltage of the data should be constrained to ensure the long-term cyclic stability of the data.
Effect of charge cutoff voltage
The amount of lithium removal in NMC data is proportional to the charging cutoff voltage, that is to say, the higher the charging cutoff voltage, the greater the amount of lithium removal in NMC data will be, and the corresponding structure of the data will be more unstable. The following figure is NCM811 data in different charging cutoff voltage, cycle function curve, can see the progress of cutoff voltage, the data capacity to play a significant improvement, but followed by the acceleration of data decline speed.
After comparing the cycle data at different cutoff voltages, it is found that although the specific capacity of 4.6V cutoff voltage is the highest at the fifth discharge, after 53 cycles, its capacity decreases rapidly and is lower than that of NMC111 at 4.5V and 4.4V cutoff voltages. This statement blindly progress in charging cut-off voltage, although the capacity of the data will make great progress, but it will make the cycle stability of the data attack significantly decreased, so according to the planned life of the battery reasonable choice of charging cut-off voltage.
The following figure shows the data of NMC111, NMC532, NMC622, NMC811 and NCA. After 53 cycles at different cut-off voltages, the discharge energy and discharge energy persistence curves can be seen from the figure. It can be seen that after 53 cycles, the battery with the highest discharge energy density is not the battery with the highest cut-off voltage. The highest discharge energy density was obtained at 4.3V cutoff voltage. The NMC622, NMC532 and NCA obtained the highest discharge energy density at 4.4V charging cutoff voltage. The NMC111 obtained the highest discharge energy density at 4.5V charging cutoff voltage.
This is only the data after 53 cycles. With the increase of the number of cycles, the data at a higher cutoff voltage will decline faster. According to the trend of the cycle curve in the figure above, the discharge energy density will be the highest when the cutoff voltage is the lowest. In addition, as can be seen from the figure below, no matter what kind of data with the increase of charging cutoff voltage will lead to the acceleration of capacity decline, especially the NMC111, NMC532 and NMC622 data with low Ni content are more affected by the cutoff voltage, which indicates that the structural stability of several data with low Ni content is worse.
Influence of ambient temperature
In the actual use of lithium ion battery, the stability of the data at high temperature is also to be considered. JohannesKasnatscheew has studied the circulation function of NMC622, NMC811 and NCA data at 60℃, and the results are shown in the following figure. Generally speaking, improving temperature can improve the dynamic conditions in the battery, so as to improve the function of the battery, which can be seen from the capacity of the battery at 60℃, but high temperature will have a certain impact on the cycle stability of the data.
For example, at 20℃ room temperature, three kinds of data in the first 50 cycles, with a relatively close cycle function, but the temperature progress to 60℃, NMC811 and NCA data after 50 cycles of capacity persistence is significantly lower than NMC622 data, this statement NMC622 data has higher thermal stability.
JohannesKasnatscheew has analyzed the factors that affect the function of ternary data cycle, such as charging cut-off voltage and forming voltage and current, and the influence of environmental temperature on the function of NMC and NCA data cycle. Essentially, with the increase of the amount of delithium in NMC data, the structural stability of data will decrease. Affects the circulation function. In addition, high temperature can negatively affect the stability of the data, resulting in a decline in the function of the data cycle. According to the characteristics of NMC data, JohannesKasnatscheew also planned a new charging system, which is not only limited by the cutoff capacity, but also adjusted the charging voltage to ensure that the battery's charging capacity is the same each time, so as to overcome the decline of the charging capacity and discharge capacity caused by the overpotential of the battery and improve the cycle function of the battery.
