Lithium-ion batteries have been beyond the electrochemical stability voltage window of aqueous electrolyte due to their high operating voltage, so most lithium-ion batteries currently use organic solvent systems.In order to reduce the impact of moisture on lithium-ion batteries, lithium-ion batteries All adopt the seal structure design, which objectively increases the difficulty of researching the internal reaction of the lithium-ion battery, so most of the researches on the reaction mechanism of the lithium-ion battery more are conducted after the battery is dissected. With the continuous improvement of analytical techniques, we have given us powerful tools to study the internal reaction mechanism of lithium-ion batteries. For example, the cryo-EM technique, which has just won the Nobel Prize in Chemistry, helped to crystallize Li dendrites during their formation and growth With a new understanding of structural changes, advances in technology have allowed us to reach areas never seen before, and neutron diffraction is such a powerful tool.
Neutron diffraction technology is a use of different materials on the shielding of neutron radiation rate of different materials on the analysis of the technology, with strong penetration of neutron radiation, we can not damage the lithium-ion battery under the premise of Lithium-ion battery internal distribution of Li for in situ analysis of research.Hermholtz from Germany Institute of Energy Storage (HIU) and Karlsruhe Institute of Technology MJ Mühlbauer such as the use of neutron diffraction technology on battery degradation of Li resources It is found that with the aging of the battery, not only the available Li resource in the battery decreases, but also the obvious distribution unevenness of Li in the diameter direction of the battery.
Po-Han Lee et al., Datong University, Taiwan Province, studied the decay mechanism of 18650 lithium-ion batteries in storage process by means of neutron diffraction technique. The results show that lithium ion batteries with 75% SoC due to active Li, NMC and LMO The loss of active material, leading to the most serious capacity decline, followed by 100% SoC battery, due to LMO and negative active material loss, and more active Li and NMC active material loss, making its capacity decline is higher than 50% SoC storage battery.
In general, we believe that lithium-ion batteries in the process of storage there are three main decay mechanism: 1) loss of active Li, the electrolyte will be stored in the process of positive and negative side by side with the continuous reaction, the constant consumption of active Li , Resulting in capacity decline, and the storage battery charge state is higher, the higher the temperature, then the resulting capacity loss is also more serious, so we in the lithium-ion battery storage process will generally choose a lower SoC state And lower temperature.2) The loss of the positive and negative active materials, due to the change of the positive and negative electrode structures during the storage of the lithium-ion battery, results in the part of the active material particles in the electrode being out of contact with the point-to-point network Resulting in the loss of active material, which is complicated by a number of influencing factors, but generally reduces the incidence of side effects will help reduce the loss of this active material 3) The last reason is the increase in battery internal resistance caused by storage, which is mainly Because lithium-ion batteries in the storage process will continue to occur side effects, resulting in loss of active material, SEI continued to grow, leading to the battery resistance continued to increase Effect battery discharge capacity under a large current.
Neutron diffraction using Po-Han Lee can help us to understand the specific gravity of the three reasons occupied in the battery storage capacity lithium ion decline down to help us to better targeted design. Lee study in Po-Han the positive pole of the battery used iNi0.5Mn0.3Co0.2O2 (NMC532) and Li1.1Mn1.9O4 (LMO) mixed system, a negative electrode using a graphite material, the battery capacity is 2.2Ah. these cells are discharged to different discharge depth DOD, and then stored at 60 ℃ for 1, 2, 4, 6 months respectively.
The figure below shows the battery discharge depth DOD different residual capacity after storage at different times, can be seen after six months of storage, the depth of discharge is 0%, the rate of capacity loss of 25%, 50% and 75% respectively of the cell 9.7%, 17.2%, 7.3% and 0.9%, while 50% DOD of the battery storage for 6 months at 25 ° C, the capacity loss is 1%. from the results can be seen for the depth of discharge DOD of the storage capacity of the battery having a reduced failure significant effect, 75% DOD of the battery during storage capacity decline down the least, the same temperature is an important factor affecting the lithium ion battery storage capacity decline drop, at 25 deg.] C the lithium ion battery capacity decline down to significantly lower Battery stored at 60 ° C.
To analyze the decay mechanism of lithium-ion batteries at high SoC and high temperatures, Po-Han Lee performed an ICP (Capacity Increment) analysis of cells stored in different SoC states as shown in the following figure The peaks represent a reaction. It can be seen from the figure that after 1, 2, 4 and 6 months of storage, the ICP curve has not changed obviously in 50% DOD storage at 25 ° C and 100% DOD storage at 60 ° C, Indicating that lithium-ion battery stored within the side effects of relatively small.
In Figure c, we can see that after a battery of 75% DOD (25% SoC) is stored at 60 ° C for one month, the peak at 3.47V shifts to a higher voltage, and the peak intensity at 3.63V becomes significant , Indicating that the capacity decline of the battery in this case is mainly due to the loss of active Li and the loss of NMC, whereas in Figure d, the peak of 3.47V at 50% DOD (50% SoC) Shifted more and the 3.64V peak decreased gradually with storage time during storage, indicating that 50% DOD (50% SoC) compared to 75% DOD (25% SoC) The loss of active Li and NMC during the storage of the battery under battery was somewhat more.We also observed that the loss of active Li and the loss of NMC at 25% DOD (75% SoC) were also significantly lower than those of 50% DOD ) Cells, the peak at 3.93 V shifts to higher voltages, indicating a significant loss of LMO active species.Comparing Figure e and Figure f below, we find that 0% DOD (100% SoC ) Than the batteries stored at 25% DOD (75% SoC) have less active Li and NMC losses, and so far the factors contributing to this phenomenon are unclear.
With the help of high-resolution neutron diffraction technology, Po-Han Lee performed an in-situ analysis of the cells in a fully charged state and a fully discharged state. The results are shown in the following figure. It can be seen that in the charged state, a Value and the value of a and c of NMC crystal are almost the same as that of the new battery. However, in the fully discharged state, the values of a and b of the LMO and NMC are smaller than those of the new battery, but the value of c of the NMC material is higher than that of the new battery (Loss of active Li), the decrease of the a value of the LMO material is mainly due to the loss of the active LMO material caused by the dissolution of the Mn2 + element due to storage at a higher potential.
Po-Han Lee According to the neutron diffraction results obtained at different depths of discharge of the total cathode material lattice structure changes and changes in Li content, can be seen from the table below 25% DOD (75% SoC) battery in the storage process The loss of active Li and NMC is the most serious, and the loss of LMO is also serious.At the 0% DOD (100% SoC) battery, the a value of LMO decreases most seriously after storage, which means that LMO The material loss is also the most serious, while the loss of active Li and NMC is second only to 25% DOD (75% SoC).
In order to verify the above conclusion, Po-Han Lee used neutron diffraction technique to analyze the phase change of the negative electrode material during the charge-discharge process of Li-ion battery. The result is shown in the following figure. Li will be pulled out from the positive electrode during embedding Therefore, the loss of Li can be inferred by analyzing the number of LiC12 and LiC6 phases in the negative electrode. According to the result of neutron diffraction, it can be seen that the Li loss of the battery stored in 25% DOD (75% SoC) is the most serious. The phase transition at 2q = 87 and 90 degrees can be used to infer the loss of negative active material. It can be seen that the lithium ion battery faces the problem of negative active material loss during storage, especially at 0% DOD (100% SoC) battery negative active material loss is the most serious.
Po-Han Lee's research shows that the most influential factors on lithium-ion battery storage performance are the temperature, the state of charge of the battery, for example, the same 50% SoC state, and the loss of capacity at 25 ° C for 6 months is only 1% Up to 3.9% at 60 ° C. The state of charge of the battery also has a significant effect on the battery's storage performance, with a maximum loss of capacity of 17.2% at 75% SoC, followed by 100% SoC and a capacity loss of 9.7 % (The mechanism responsible for this phenomenon needs further study.) With the help of neutron diffraction technology, let's gain an understanding of the mechanism of capacity decay. Batteries stored in 75% SoC have active Li and NMC Of the losses is the most serious, leading directly to the largest capacity decline, and 100% SoC storage battery, LMO material and negative active material during storage loss is the most serious, so the capacity loss is second only to 75% SoC storage battery , While storage at lower temperatures and at lower SOCs provides better storage performance due to less active material loss.
