With the increasing demand for lithium-ion batteries, the traditional lithium cobalt oxide material is being replaced by higher capacity NMC and NCA materials, especially high nickel NCA and NMC materials, the specific capacity can reach more than 200mAh / g , Is the high specific energy lithium-ion battery at the present stage of the best choice, which NCA material with its excellent cycle performance, has been in the high-end lithium-ion battery market gradually won a place.
NCA material in the process of circulation is still the problem of cyclical decline, NCA material in the cycle of the layered structure to the rock salt structure is the result of NCA capacity decline is an important reason, and we usually think that NCA material phase change from the outside The outer layer of the primary particles is first transformed into a rock salt structure, and the core remains a layered structure, which we call a 'core-shell structure' decline, but the study by Hanlei Zhang of the State University of New York, In the LiNi0.8Co0.15Al0.05O2 material, the decay mode of the material is not only the 'core-shell structure' phase transition mode, but also the transformation of the 'antinuclear shell structure' form, that is, the internal transformation of the material particles For the rock salt structure, while the surface layer of the particles still remain lamellar structure, the main reason for this phenomenon is the loss of O surface of the surface of the material particles, resulting in the O space will drive the internal O to the surface diffusion, resulting in the proportion of particles within the internal O Significantly reduced, resulting in changes in the phase, Hanlei Zhang that compared to the traditional 'core-shell structure' decline model, this 'anti-core shell structure' decay mode on the NCA material capacity The impact of the decline is greater.
The following figure shows the high-resolution TEM images of the NCA material after 30 cycles. From the figure, we note that the rock salt structure is formed inside the NCA particles and the surface of the particles remains the lamellar structure. The study of the original NCA material shows that the particles are free of rock salt structure before the start of the test, and all the regions are layered, indicating that these rock salt structures are formed during the process of material circulation.
The following figure is a detailed analysis of the rock salt structure area. It can be seen from the figure that there is a layer of spinel structure between the lamellar structure and the rock salt structure, and the boundary between them is marked by the dashed line. It is found that the structure in NCA particles has similar characteristics, that is, the outer part of the rock salt structure is wrapped with a layer of spinel structure, the spinel structure outside the complete layered structure, Hanlei Zhang called 'anti-core shell Structure ', as shown in Figure e.
The following figure shows the electron diffraction pattern and the high-resolution transmission electron microscopy of the rock salt structure region and the lamellar structure region. As can be seen from the graphs b and c, the lamellar region basically maintains the structural integrity and order, The rock salt structure is very disordered, the crystal arrangement is also very chaotic, mainly because the material lost O, the formation of rock salt structure, destruction of the material face-centered cubic FCC structure, causing the crystal structure of the confusion.
Figure d, g, e is a partial enlarged view of the NCA particles. As can be seen from Fig. D, a certain degree of structural transformation occurs in the lamellar structure of the surface. Some transition metal ions appear in the Li position, Part of the cationic mixture, and in Figure e the phenomenon of cationic mixing is significantly worse than Figure d, there are more Li bit occupied by the transition metal element ions, when the 1/4 of the Li bit is occupied The spinel structure is generated, and when all of the Li bits are occupied, the formation of the rock salt is sufficient. Therefore, in the region shown in Fig. E, the structure of the material is closer to that due to the fact that more Li ions are occupied by the transition metal element ions Rock salt structure.
We know that it is necessary to lose part of O in the formation of rock salt structure in the layered structure, and it is difficult to lose O in the interior of NCA particles, which needs O to diffuse to reach the surface of the particles and then diffuse into the environment. Et al. Have shown that the loss of O on the surface of the NCA material particles will cause the O to diffuse inside the particles to fill the surface of the particles without loss of O, resulting in a significant reduction in the surface O of the particles, whereas the internal O The following figure shows the elemental distribution of NCA particles after 30 cycles. From the distribution of O elements in Fig. B, the content of O in the particles is relatively low, and the surface of the particles and the internal contents are not And it is noted that the distribution of Ni elements is relatively uniform and there is no significant loss as the O element, which means that the area where the partial O loss is more leads to an increase in the Ni / O ratio, and the Ni2 + Increasing the number of materials, resulting in material from the layered structure to the spinel structure changes.
Li'Li1 / 9Ni1 / 3Mn5 / 9'O2 materials at 30 ℃ and 50 ℃, O diffusion rates were 3'10-13 and 2'10-12cm2 / s, the grain surface of the O loss will occur in the particles The concentration of O is affected by factors such as SoC and temperature. When the O loss rate is fast enough, the surface area is seriously damaged due to O, so the surface of the particle will be affected by the concentration of O. From the lamellar structure directly to the rock salt structure.When the O loss rate is slow, it will promote the O from the inside to the external diffusion of particles, resulting in the loss of particles within the grain, the formation of rock salt structure within the particles, while the particle surface is still to maintain the layer Which means that NCA particles form 'core-shell structure' or 'anti-core shell structure' is mainly affected by the loss rate of O.
Hanlei Zhang studied 120 NCA particles, of which only about 10% of the particles showed a 'anti-core-shell structure' phase transition pattern, and the rest of the particles showed a 'core-shell structure' phase change pattern, that is, two of the NCA materials Species decay mode will exist at the same time, the electrode will exist in the 'core-shell structure' decay, there will be 'anti-core shell structure' decay.
The following figure summarizes the mechanism of the formation of this 'anti-core-shell structure'. First, the surface of the NCA particles is depleted O, resulting in O vacancies, forming a concentration gradient of O in the particles, thereby promoting the migration of O-particles into the particles The O content in the granules is significantly reduced. Because of the large number of O deletions, these regions will change the crystal structure from the lamellar structure to the rock salt structure, thus forming the rock salt structure area - the spinel layer - the layered structure area However, when the O loss is fast enough, a layer of rock salt structure is directly formed on the surface of the NCA particles. Since the diffusion rate of the layer of the salt rock structure layer is relatively slow, it can protect the interior of the particles To the rock salt structure.Therefore, in general, the 'anti-core shell structure' has a greater effect on the capacity decay of the NCA material, because this structure not only destroys the active lamellar structure but also causes the layered structure The area is isolated, causing more loss of capacity, and vice versa if it is the traditional 'core-shell structure', although the formation of the rock surface layer of the rock salt structure will reduce the electrochemical Activity, but also to protect the internal lamellar structure does not change to the rock salt structure, thereby reducing the capacity of NCA material decline.
The transition from lamellar structure to rock salt structure in the process of NCA material is the main reason for the loss of capacity. In general, we believe that this structural transformation will start from the surface of the particles first, and finally form a shell rock structure, The core is the 'core-shell structure' of the layered structure, but Hanlei Zhang has found that the NCA material has a 'anti-core-shell structure' decay pattern in addition to the above-mentioned decay pattern, That is, because the surface O loss rate of NCA particles is relatively slow, so that a certain O concentration gradient is formed between the inside of the particles and the shell, resulting in the internal O-surface diffusion, and the internal particles are relatively low in the O concentration to form the rock salt structure, The surface of the particles is still covered with a layered structure due to the addition of O in the inner layer.
Due to the faster diffusion rate of O in the lamellar structure and the slower diffusion rate of O in the rock salt structure, the rock salt structure of the surface in the traditional 'core-shell structure' can protect the lamellar structure of the inner layer from changing to the rock salt structure And the rock salt structure in the 'anti-core shell structure' can not only play a role in protecting the interior of the particles, but also accelerate the decline in the capacity of the NCA. Therefore, during the NCA use, we can create conditions to improve the rate of O loss on the surface of the NCA material particles Such as raising the temperature, etc.), so as to form a layer of rock salt structure on the surface of its particles, play a protective role, reduce the late cycle of the decline.
