As the specific energy of lithium-ion batteries continues to increase, the specific capacity requirements for positive and negative materials are also increasing. The traditional LiCoO2 material has a capacity of only about 140mAh/g, which cannot meet the needs of new generation high-specific-energy batteries. Increasingly, the price of cobalt has become the last straw to suffocate LCO. Therefore, people have turned their attention to NCM materials with higher capacity and more competitive prices. Compared with LCO materials, NCM has achieved a significant increase in specific capacity (NCM 622 With a capacity of 170-180mAh/g or so, and because of the greatly reduced use of Co, NCM also has a clear advantage over LCO materials in terms of price, which makes NCM the new darling of lithium-ion power batteries. However, NCM materials still There is a serious problem - poor high-temperature cycle performance, NCM material capacity degradation greatly accelerated at high temperatures, seriously affecting the life of lithium-ion batteries.
Recently, SiyangLiu et al. of Fudan University in Shanghai conducted an in-depth study on the mechanism of high-temperature cycling at 55°C of NCM622 materials. Studies have shown that the metal cations in the surface layer of NCM622 will undergo a serious mixing phenomenon during high temperature and high voltage cycling. This leads to a significant increase in the charge exchange impedance. In addition, high temperature and high voltage cycles will also exacerbate the decomposition of LiPF6 on the electrode surface, increase the content of LiF and NiF2, and increase the interface/electrolyte interface impedance.
Siyang Liu first synthesized the NCM622 material by solid-phase method. The XRD pattern shows that the synthesized NCM622 material has a well-developed lamellar a-NaFeO2 structure. The following figure shows the first charge of the NCM622 synthesized by Siyang Liu under different cutoff voltage conditions. The discharge curve, as can be seen from the figure, as the cut-off voltage is gradually increased to 4.3V, 4.5V and 4.7V, the material capacity reaches 176, 201.3 and 218.1mAh/g, respectively, although higher cutoff voltage can bring more The high capacity, however, also results in a drastic reduction in the cycling performance of the NCM622. From the following figure b it can be seen that when the cut-off voltages are 4.3V, 4.5V and 4.7V respectively, the NCM622 material is cycled at 55°C for 50 cycles. The rates were 96.3%, 90.7% and 78.9%, respectively. It can be seen that the cut-off voltage has an important influence on the cycle performance of NCM622 materials.
The study of the decay mechanism of NCM622 materials at different cutoff voltages shows that a higher cutoff voltage will significantly increase the interface resistance of NCM622 materials. The following figure shows the EIS results of NCM622 materials with different cutoff voltages and different cycle times. All the curves are composed of two arcs and a straight line, which indicates that there are two interfaces on the material surface: The electrolyte decomposes on the surface of the NCM622 to form an interfacial film. Siyang Liu uses the equivalent circuit in figure c below. The EIS results were fitted. Siyang Liu thinks that Rs1 is the impedance of the interfacial film and Rct is the charge exchange impedance. When the cut-off voltage is 4.3V, 4.5V and 4.7V, the Rs1 of the material is 17, 20 and 21.6W, respectively. After 25 cycles, Rs1 increased to 18.7, 23.4 and 28.2 W, respectively, indicating that a higher cut-off voltage will cause the growth and reconstruction of the interface film of NCM622 material, thereby increasing the interface resistance.
The change of the charge exchange resistance Rct during the cycle is even more pronounced. It can be seen from the figure that the Rct of the NCM622 material has only slightly increased after 25 cycles of the cutoff voltage of 4.3 V, but after 25 cycles of the cutoff voltage of 4.5 V and 4.7 V The Rct of the material increased by 2 and 8 times, respectively. This may be because the higher cut-off voltage resulted in more Li from the NCM622 material, which led to the increase of material Li/Ni mixing and irreversible phase change of the material. Causes the material's charge exchange impedance to increase.
EIS analysis shows that the increase of the interfacial impedance of materials is closely related to the capacity decline of materials, but the mechanism of action among them is still not clear. The following figure shows the SEM images of new electrodes and electrodes after cycling under different voltages. We can see that After the cycle, the number of cracks on the surface of the electrode showed a significant increase. In particular, cracks on the surface of the electrode after cycling increased more severely. These cracks on the surface of the electrode cause partial loss of the active material and Al foil. , The connection of the conductive network, causing the loss of active material, resulting in some of the capacity decline.
Usually we think that the side reactions mainly occur at the electrode/electrolyte interface, so the electrode/electrolyte interface is more susceptible to erosion, so Siyang Liu tested the surface of NCM622 after cycling under different voltages by HRTEM. TEM images We noticed that the new NCM622 has a well-developed crystal structure. After cycling 50 times at a cutoff voltage of 4.3 V, the body of the NCM622 material still maintains a well-developed layered structure, but some areas are observed on the surface of the material. The phenomenon of transition metal ion mixing appeared. When the cutoff voltage was increased to 4.5V, the crystal structure of the material decreased more seriously after 4.7V. It can be seen from the figure that excessive Li elution resulted in a high cutoff voltage. Metal cations enter the Li layer, which blocks the diffusion channels of Li, decreases the active sites of Li, leads to an increase in the interfacial charge exchange resistance and a decline in the reversible capacity, which is consistent with previous EIS analysis results.
It is also worth noting that at higher cut-off voltages, some holes can be observed on the surface of the material after cycling. This is mainly due to the release of O in the material and the dissolution of transition metals at higher cutoff voltages.
For the mechanism of the electrode/electrolyte interface resistance Rs1 increase, Siyang Liu used XPS to analyze the surface of the NCM622 material and found that the electrolyte decomposition product showed a significant increase after cycling. In particular, LiF cycles at a cutoff voltage of 4.3V. The LiF content of the rear electrode surface was 8.9%, but the LiF content on the electrode surface increased to 14.9% and 17% after the cutoff voltage was increased to 4.5V and 4.7V. At the same time, we also found that the surface of the electrode was after cycling NiF2 through XPS analysis. The significant increase in the content of the electrolyte indicates that the decomposition of the electrolyte on the surface of the NCM622 material is accompanied by the dissolution of transition metal elements. Siyang Liu believes that this is mainly due to the corrosion of the NCM 622 material caused by the decomposition of LiPF6 to HF, resulting in transition metal elements. Dissolved.
Siyang Liu's work shows that the cycling of NCM622 materials at high temperatures and high cut-off voltages causes an increase in the mixing of transition metal elements and Li in the material on the electrode surface, causing the crystal structure of the NCM 622 to decay, resulting in increased charge-exchange impedance and The reversible capacity is reduced. Cycling at high temperature and high voltage also causes the decomposition of LiPF6 on the electrolytic surface, resulting in an increase in the content of LiF and NiF2 on the surface of NCM622, resulting in an increase in the electrode/electrolyte interface resistance of NCM622.
