In 2018, my country’s new energy automotive production and sales volume increased by 60% for the first time to exceed 1.2 million units, and the main amount of more than 2.2 million, the power lithium battery has also reached 56GWH.
. While we rapidly develop, we will have a rapid development of new energy auto industries, we must also face the recovery problem of huge number of power lithium batteries after gradually entering the scrap period..
The retired power lithium battery continues to be used in the field of energy storage, which is the value of the power lithium battery, which is what we often say, but some lithium-ion batteries will have a capacity diving after life declines to 80% or less. The phenomenon, therefore to meet the needs of lithium-ion battery, first solve the phenomenon of power lithium battery capacity diving at the end of life..
A large number of studies on lithium-ion battery capacity diving, indicating that the reversible capacity of lithium-ion batteries during the circulation process is often caused by the negative surface analysis LI.. The Tobiasc.
Bach (1) of the German Fraunhofsilicate Institute  studied the capacity diving of the cylindrical lithium ion battery in the circulating process, and the results indicate the interior of the battery. The uneven distribution of pressure is a key factor that causes lithium-ion battery in the end of life..
The lithium ion battery can be divided into a soft bag and hard shell, and the battery cells in the hard shell structure will have a significant effect on the impedance of lithium ion batteries.  The pressure is too large or too small to increase the performance of the lithium ion battery, and the pressure inside the actual use in the electrical core also has unevenness, which further leads to the no operation of the current distribution, thereby accelerating the failure of the lithium-ion battery. drop.
The battery used in the experiment is an IHR18650A battery from e-OneMolienergy. The positive electrode material is NCM material, and the negative is very graphite material. The authors have studied three batteries.
The battery A did not circulate, the battery B was in the event of a capacity diving stage, the battery C is in capacity After diving. The figure below shows the XRD diffraction spectrum of three batteries. From the figure, it can be seen that although C battery has severe capacity decline, the crystal structure of NCM still does not have a large change.
. Although the crystal structure of the NCM material is not large from the XRD diagram, the lattice parameters A and C we calculate the material have been discovered that there is a significant difference in the content of Li in the batteries A, B and C (as shown below) From the figure below, we can notice that the loss of battery A activity Li is the smallest, and the lumability of battery B is significantly increased, and the LI loss of battery C is the largest..
Compared to the positive electrode, the negative electrode varies more in life decline, and the negative electrode topography after the three batteries of Figures A, B, and C, can be found that there is no circulating battery A negative electrode completely present black (graphite material The color), and the intermediate position of the negative electrode of the battery B of the capacity diving occurs, and the battery C negative surface after the capacity diving is even more precisely.. The author analyzed the amount of Li quantity in the three battery negative electrodes of A, B and C, which only contains 0.
25 mAh / cm2 of Li, but the amount of LI in the negative electrode in the battery B is 0.55mAh / CM2, and the amount of LI containing 0.85 mAh / cm2 in the negative electrode of the battery C, the amount of Li of the negative electrode in the Li portion reached 1.
87 mAh / cm2, which indicates that a large amount of loss of active Li deposition is in the negative electrode. It is an important factor in causing battery capacity diving. The non-active Li of the negative electrode deposition is important in two forms, one is a non-active metal Li deposited by the surface of the electrode; the other is the compound Li present in the negative electrode SEI film, which we can battery B in the following B And the thickness of the negative particles of C can be seen in the thick SEI film.
The following figure shows the relationship between the reversible capacity of the 18650 battery and the loss of li loss, which can see the loss of the activity LI and the loss of the battery reversible capacity have very strong correlation, indicating the activity of the battery in the circulation process. Loss is an important cause of retroffacious capacity decline and diving of batteries. In order to analyze the reaction mechanism of the negative electrode active Li loss, the author sampling and fabricate a three-electrode battery in different positions shown below, respectively, for analyzing the charge and discharge characteristics of the positive and negative electrode.
. The picture below shows the 5th position of the A battery and the positive and negative DQ / DV curve of the 2 and 3 position of the B battery. From the figure, it is possible to see the 1/3 of the 1/3 of the negative electrode of the 2nd position in the figure.
At 0.1 V peaks, almost completely disappear, NCM’s DQ / DV curve can see that the operating voltage range is significantly narrow, which indicates that the electrode of the position 2 has a significant li loss, compared to the position of B battery 3 The loss of Li is obviously light in position 2. It can be seen that even if there is still a phenomenon that there is still a decline between different locations within the same battery.
This battery is not uniform in the battery in the end of the cycle life. From the figure below, we can see the DQ / DV curve shape about the battery A, all positions is almost very consistent..
However, the loss of the active Li of the battery B, its No. 1 and 2 is obvious than being severe than other locations, while the DQ / DV curve in different positions of the battery C has significant differences, indicating that there is different positions within the C battery. Differential differences in the decline.
Factors that cause uneven decline in the lithium ion battery may include factors such as temperature gradient, uneven pressure and voltage difference, the authors also analyze these factors respectively.. The figure below shows the temperature of the battery surface during the fast charge and discharge process of lithium ion batteries.
We take three points to measure the temperature of the battery at the axial direction of the battery. We observed that the negative electrode LI started from the intermediate position in battery B. Therefore, if it is the effect of temperature, there should be a significant temperature gradient in the axial direction.
. However, the measurement found that the temperature of the three measuring points was 48.4 ¡ã C, 48.
9 ¡ã C and 48.3 ¡ã C, the temperature difference between the intermediate portion and the battery was only 0.6 ¡ã C, such a small temperature difference is insufficient to cause the phenomenon of neutralization Li, so in this experiment Temperature is not caused by inconsistencies of internal decline in battery.
The structure of the battery below, and the shape of the negative electrode LI, it can notice that the shape and position of the negative electrode LI is related to the shape and position of the positive electrode ears, and we can It is seen that the positive pole ear position is in the middle of the cell, which causes the electrical cell deformed and the new part of the local pressure, which is that the new increase of the pressure received by the electrode has caused the occurrence of negative electrode LI phenomena.. In order to verify local pressure, the purpose of negative electrode analysis, the authors also added the expansion of the card to limit the battery outside the 18650 battery.
From the picture below, we can see the position in the clip and battery extreme (red box in the figure). There is a significant analysis of LI, which indicates that local pressure is an important reason for negative electrode analysis..
Tobiasc.bach’s research work shows that the important cause of capacity diving in the end of the lithium-ion battery life is the loss of the active Li, the negative electrode SEI film, and the precipitation of metal Li is an important factor that leads to the loss of LI..
A focus on analyzing Li phenomena shows that the pressure distribution inside the battery is not important causes of negative electrode analyzes Li, especially the local pressure caused by the electrical cell deformation caused by the positive electrode, which will be significant, a significant new bureau The tendency, so the battery is designed and manufactured, installed and used, and the battery is taken as a uniform pressure, preventing the sub-analytical LI, affecting the cycle performance of the lithium-ion batt.