With the continuous improvement of the energy density of lithium-ion batteries, the traditional lithium cobalt oxide material has been facing the fate of being eliminated. Although the high-voltage lithium cobalt oxide developed in recent years has been increased in capacity, compared with the higher capacity Ternary materials, there is not much advantage, and with the gradual maturity of high-nickel NMC and NCA technology, lithium cobalt oxide market share is rapidly losing high-nickel ternary material generally refers to the Ni content of 0.8 The above NMC and NCA materials, ternary material capacity depends mainly on the content of Ni elements, the higher the Ni content will be higher capacity, for example, some manufacturers on the market today introduced high-nickel NMC type of material, the specific capacity has reached 200mAh / g or more.But Ni element in the material to bring higher capacity, but also lead to the thermal stability of the material decreased, especially at high potential, Ni4 + has a strong oxidizing, resulting in the electrolyte in the material Decomposition of the surface, causing the capacity decline and internal resistance increased.
There are mainly two ways to solve this problem: 1) Surface coating, for example, coating a layer of ceramic oxide such as Al2O3, MgO on the surface of NMC can not only improve the interfacial stability of the high-nickel material but also reduce Surface alkalinity to improve the processability of high nickel materials in production; 2) element doping, for example, Al elements in NCA material, although they can not participate in the electrochemical reaction of charge and discharge, the addition of Al element can stabilize the material Lattice, to enhance the cycle performance of materials, but the excessive addition of non-reactive Al elements will affect the material capacity to play in order to solve this problem, Jianguo Duan, Central South University, such as the development of a gradient doping technology in the preparation of NCA From the inside to the outside of the particles, the concentration of Al gradually increases and the Al concentration on the particle surface is the highest. This technology not only solves the problem of cycling stability of high nickel NCA material (1000 cycles, 92.4% capacity retention) NCA is also a good material to improve the surface alkalinity, easy to water problems.
It is easy to see from the above introduction gradient doping technology for the preparation of high nickel ternary material, is a very powerful tool that can not only improve the surface stability of NCA material, but also not to its capacity Have a great impact.In order to solve the high cycle stability of nickel NCA materials, especially in the high temperature and high voltage cycling stability problems, the Chinese Academy of Sciences Tao Chen, etc. using gradient doping technology in LiNi0.8Co0.15Al0.05O2 The incorporation of a small amount of boron element, gradient doping technology makes NCA particle surface boron element is significantly higher than the internal particle, the particle surface enriched boron element NCA particles improve the surface stability, reducing the high temperature cycling process The thickness of the SEI film on the surface of the NCA particles reduces the surface-induced particle crack and improves the cycling performance of the high-nickel NCA material.
In the experiment, Tao Chen used H3BO3 as a boron source to gradient-process NCA material to form Li'Ni0.8Co0.15Al0.05 '(BO3) x (BO4) yO2-3x-4y (Bx + y- NCA, x + y = 0, 0.01, 0.015, 0.02) The SEM image of the NCA material with different doping ratios of the precursor and different B elements can be seen in the figure below.Compared with the non-doping NCA material (figure c) After the doping of NCA material (mainly the higher doping ratio of B0.015-NCA (lower panel e) and B0.02-NCA (lower panel f)), the primary particle packing is more compact and the particle surface is clearer (Probably caused by the reduction of surface decomposition lithium salt, such as LiOH / Li2CO3, etc.).
Gradient doping technology is focused on the word "gradient", that is, doping elements can not be uniformly incorporated into the NCA particles inside, or lose the technical advantages of gradient doping.Using XPS on B0.015- The elemental analysis of NCA reveals that the concentration of element B gradually decreases from the surface of the particle to the core of the particle (c), and the element B in the surface layer is significantly higher than that of the particle core to form a gradient-changing structure XPS analysis also showed that the content of Ni2 + on the surface of NCA particles doped with B element is obviously higher than that of NCA particles without doping, and the higher Ni2 + content on the NCA particles helps to improve the structural stability of NCA material and improve the NCA Material circulation performance.
Good surface stability can significantly improve the electrochemical properties of NCA materials in Li-ion batteries. The graph below shows the electrochemical performance of NCA doped with different B elements. The results are also summarized in the following table. From It is easy to see in the table below that the initial discharge capacity and first-time Coulombic efficiency decrease slightly with the addition of B elements in the NCA material. For example, the NCA material without doping has an initial discharge capacity of 192.6 mAh / g and a first efficiency of 90.7 %, But the initial discharge capacity of B0.02-NCA material is only 185.9mAh / g, the first efficiency is only 83.1% .But the initial capacity gap in the cycle performance has been compensated, as can be clearly seen in Figure b The doping of B element remarkably improved the cycling performance of NCA material. The cycling of 200 times of NC (200-4.3V) was carried out 200 times, while the capacity retention of pure NCA was only 74.5%. However, the doping B0.015-NCA and The capacity retention of B0.02-NCA material was 96.7% and 97.2%, respectively, showing excellent cycling performance.
To investigate the long-term cycle performance of B-element-doped NCA, Tao Chen used a more stringent test regime, as shown in Figure a, with a voltage of 2.8-4.5V in the range of 100 cycles at a 2C rate of 37.2mAh / g, while the boron-doped B0.015-NCA capacity decreased only by 7.4 mAh / g, whereas this difference became more pronounced at the more severe high temperature (55 ° C, lower panel b) cycle test, indicating Gradient doped B elements significantly improve the cycle stability of NCA materials.
NCA material cycle performance improvement can not be separated from the material / electrolyte interface stability improvement, the following picture shows the surface morphology of the electrode after the cycle, you can see pure NCA material (a, b below) after the cycle, The surface of the particles cracks due to the volume change in the circulation, and the surface of the particles is covered with a thick electrolytic decomposition product. However, the surface of B0.015-NCA doped with B elements does not change obviously after the circulation, This is due to the stronger BO bond, which reduces the generation of cracks. The more stable surface structure of the B0.015-NCA material also reduces the decomposition of the electrolyte.
Tao Chen Gradient doping technique is a good solution to the problem of instability and poor interfacial stability of high-Ni materials. It is a good solution to the particle cracks that occur during the cycling process by enriching more B elements on the particle surface And electrolyte decomposition to reduce the NCA material during the cycle of crystal structure changes and reduce battery polarization and voltage decay, significantly improve the NCA material cycle stability, especially at high cut-off voltage and high temperature and other harsh Circumstance stability of the environment.B element gradient doping technology is a very effective method to improve the cycle performance of high nickel NCA material.
