In recent years, with the gradual expansion of the electric car market, electric vehicles began to appear more and more frequently in our daily life. As a product closely related to the safety of our lives and property, the safety of electric vehicle battery is particularly important Power battery in the factory are required to meet the overcharge, over discharge, acupuncture and short circuit and other harsh safety tests to ensure the safety of the battery in use.
In the current lithium-ion battery design, all the Li elements in the battery are provided by the positive electrode. When charging, Li is removed from the positive electrode and embedded in the negative electrode, and the discharge process is exactly the opposite. Under normal circumstances, The amount of delithiation of the positive electrode is controlled within a reasonable range, so that the potential of the positive electrode material can be fully utilized without causing collapse of the positive electrode structure, thereby ensuring the safety and cycle life of the lithium ion battery. However, in special cases, for example, BMS damage, malfunctions, etc. can cause overcharging of lithium-ion batteries, causing safety problems and battery performance damage.
Generally, we think that in the case of over-charging, the lithium-ion battery will react internally: 1) electrolyte decomposition, over-charging leads to the rise of the positive electrode potential. When the positive electrode potential is higher than 4.5 V, conventional organic carbonate electrolysis Liquid begins to decompose, resulting in safety issues; 2) negative analysis Li, Li-ion battery design in the negative capacity will be higher than the positive, we call redundancy or NP ratio, in general, the negative electrode capacity will be higher than the positive 10-30 %. But in the overcharge, the positive will prolapse excess Li, which exceeds the capacity of the negative electrode, resulting in the precipitation of Li in the negative electrode surface, which will not only lead to battery performance damage, under severe conditions can lead to internal short circuit, Leading to safety accidents; 3) the structure of the cathode material collapses. At present, the mainstream of ternary materials and LCO materials of lithium ion batteries belong to a layered structure with a theoretical capacity of 270 mAh / g, but only part of Li can be reversibly removed, For example, for a LCO material, the reversible capacity is about 140 mAh / g. Continuing to remove Li causes the layered structure of the positive electrode material to lose support and structural collapse, resulting in failure of the positive electrode material.
For the layered structure of the cathode material in the over charge will occur structural damage, the United States Argonau National Laboratory, Sandia National Laboratories and Oak Ridge National Laboratory Javier Bareño et al on the NMC532 material in the overcharge in the structure It is found that the NMC532 material does not collapse structurally as we imagine after overcharged but maintains the layered structure but forms a layer containing more C and O on the surface of the positive electrode Electrolyte decomposition products.It was also found that after overcharge, the negative SEI film consumes more Li, and part of the lithium in the form of metal Li precipitated in the negative electrode, resulting in the discharge of electricity to 0% SoC Li content of NMC532 material appears Significant decline.
Javier Bareño and other first ORNL Oak Ridge National Laboratory using NMC532 / graphite prepared 1.5Ah soft battery, and then in the Sandia National Laboratory SNL will be soft pack batteries were charged to 100%, 120%, 140%, respectively, 160%, 180%, and 250% SoC states with 250% SoC charging resulting in cell leakage, and then discharging the above cells to 0% SoC for dissection to investigate the effect of overcharge on the cathode material structure.
The figure above shows the XRD pattern of the positive electrode charged to different SoC states and then discharged to 0% SoC. In comparison with several other diffraction peaks, we found that although these positive electrodes have undergone different overcharges, the positive electrode material The basic structure has not been destroyed, only a small increase in the width of the diffraction peak, which means that the battery material inside a certain degree of stress.
This can also be testified from the SEM image (bottom). As can be seen from the figure below, the particle shape of the NMC532 material is still clearly recognizable despite the overcharge test. The secondary particle morphology is not A significant change occurred.
Although structurally no significant structural changes were observed in the NMC532 material, from the local Li content (as shown in the following figure), when charged to more than 120% SoC, although discharged to 0% SoC, the NMC532 material The Li content still can not be restored to the initial state, and the higher the charging SoC, the less Li contained in the positive electrode material after complete discharge This is partly because the structure of the positive electrode material decays, but the more likely cause is overcharge process The growth of the negative SEI film consumes Li or part of Li precipitates on the negative electrode surface in the form of metallic Li, consuming more Li, resulting in a significant decrease of Li re-embedded in the positive electrode.
In order to further study the influence of overcharge on the structure of NMC532 material, Javier Bareño analyzed the battery materials with different degrees of overcharged by XPS. The main changes of the positive electrode material are reflected in the intensity changes of the two diffraction peaks O1s and P2p, It can be seen that there are two types of O in the material, one is the more electronegativity O at 530 eV, for example, the oxygen in the metal oxide and the oxygen in NMC 532, and the other One is the less electronegative O corresponding to 533 eV, for example, the O element in organic matter.On the basis of the quantity of these two O's, the number of more electronegativity O corresponding to 530 eV is overcharged to 180 % SoC did not change much before, mainly overcharge to 250% after a larger increase, but the content of O in organic matter increased with the degree of overcharge increased, indicating that in the process of overcharge The positive electrode surface produced more electrolyte decomposition products.P2p peak corresponding to the decomposition of the electrolyte P2O5, after overcharging the intensity of this peak was significantly increased, indicating that the anode surface of the electrolyte decomposition products increased,Consistent with the results of the above O1s.
From the above analysis, the Li content in the positive electrode material rapidly decreases with the increase of the degree of overcharge after the overcharge exceeds 140% SoC, and these losses of Li are largely consumed by the growth of the negative SEI film Or formed on the surface of the negative electrode Li precipitation.From XRD study found that excessive de-Li did not lead to the destruction of the structure of the positive electrode NMC532, overcharged positive electrode material still maintained a layered structure.But overcharge has led to the electrolyte in the positive electrode surface Decomposition, with the increase of the degree of overcharge, positive electrolyte decomposition products are also a corresponding increase in the overcharge to 250% SoC, will lead to thermal decomposition of the electrolyte, resulting in more gas, resulting in the battery Leakage.
