Service life is an indicator that we are very concerned about in the use of lithium-ion batteries. Generally speaking, the service life of lithium-ion batteries is mainly affected by two factors:
1) Use time;
2) Number of cycles.
According to the decay speed of lithium-ion batteries, we can divide the decay process of the battery into the linear decay process in the early stage and the non-linear decay process in the later stage. The typical characteristic of the nonlinear decay process is that the capacity of the battery decays sharply in a short period of time, which is what we usually call capacity diving, which is very unfavorable for the use of power batteries and the use of cascades.
1. The influence of operating voltage window
With the expansion of the battery operating voltage window, the node at which the battery has a non-linear decay is significantly earlier. For example, compared to the battery with a voltage window of 1.2V (3.0-4.2V), the battery with a voltage window of 0.94V (3.17-4.11V) has a linear decay The length of the descending segment increases by about 42%. This is mainly due to the increase in the dissolution of the transition metal elements of the positive electrode under the wider electrochemical window. The migration of the dissolved transition metal elements to the surface of the negative electrode leads to the acceleration of the growth of the negative electrode SEI film, which leads to the accelerated decline of the negative electrode kinetic conditions, so the negative electrode The earlier precipitation of metallic lithium leads to the earlier appearance of non-linear decay.
There is a very close correlation between the increase in the ohmic impedance and charge exchange impedance of the battery and the reversible capacity decline of the battery. Therefore, we can use the BMS system to track the change of the battery's internal resistance to predict the nonlinear decline.
2. The influence of charge and discharge rate
Since the non-linear degradation of lithium-ion batteries is mainly caused by the precipitation of metallic lithium on the negative electrode surface, the charge and discharge current is also closely related to the occurrence of non-linear degradation of lithium-ion batteries. The cycle performance of the battery under different charge and discharge currents Curve, the battery's non-linear decay has the greatest impact on the charging current of the battery. A battery charged at a rate of 1C shows a trend of non-linear decay almost from the beginning, but if we reduce the charging current to 0.5C then the battery The time node at which the non-linear decay occurs will be greatly delayed, and the impact of the discharge current on the non-linear decay of the battery is almost negligible. This is mainly because as the charging current increases, the polarization of the negative electrode will also increase significantly, which leads to a significant increase in the risk of lithium precipitation in the negative electrode. The precipitated metallic lithium with a porous structure will promote the decomposition of the electrolyte, thereby accelerating the decomposition of the electrolyte. The decline in the kinetic performance of the negative electrode leads to the early appearance of non-linear decay.
The increase of battery internal resistance is also very closely related to the appearance of battery non-linear decay, so we can also use BMS to monitor the battery internal resistance to predict the appearance of battery non-linear decay.