Lithium nickel oxide, the chemical formula is LiNiO2, and lithium cobalt oxide are isomers. Some of their physical properties are shown in Table 1-1. However, there is still no synthetic pure and stable structure of lithium nickel oxide as a positive electrode material. First, LiNiO2 with no strict stoichiometric ratio was synthesized. Many reports are Li1-yNi1+yO2 in the form of nickel-rich compounds. On the microstructure, some nickel atoms are in the position of the lithium ion layer and are connected to the NiO2 layer. Thereby reducing the expansion coefficient of lithium and the energy capacity of the electrode; second, the lithium-deficient compound is unstable due to the high equilibrium oxygen partial pressure, and the combination with organic solvents is prone to danger, so that this type of battery is also unstable. The structure is a non-quantitative compound of Li1.8 Ni1+yO2, which is actually a mixture of LiNiO2 and Li2NiO2.
Table 1-1: Some physical property data of LiMO2 (M=Ni,Co)
We will discuss the use of other elements to replace part of nickel for material modification in a later article. The substitution of other elements will maintain the regularity of its structure and ensure that the nickel can be in the position of the nickel atomic layer; the substitution does not occur redox reactions, so that lithium ions can be stably deintercalated, thereby maintaining the stability of the material structure and preventing the occurrence of lithium content Any crystal phase change at low or zero. Unlike cobalt and nickel, manganese cannot form a stable LiMnO2 with the same structure as LiCoO2, but it can form a stable spinel type. There are many structures with a Mn/O ratio of 1/2. Other structures may have different lithium contents. Is stable.
The non-stoichiometric defects of nickel-based materials are mainly manifested as lithium defects and oxygen defects. Because when the temperature is lower than 600℃ in the air, the divalent nickel ions cannot be completely oxidized to trivalent nickel ions; when the temperature is higher than 600℃, the reduction of trivalent nickel ions to divalent nickel ions is inevitable, so it is difficult in the air The real stoichiometric LiNiO2 is obtained.
Someone used β-Ni1=xCoxOOH and LiOH to prepare samples at 450℃ and showed good electrochemical performance. The study found that the impedance of the electrode increases with the increase of the lithium defects of the Li1-xNi1+xO2 sample. The higher the lithium defect of the sample, the greater the structural change during the cycle. The larger the lithium defect value of the synthesized product, the smaller the specific discharge capacity and the worse the cycle stability. It is difficult to completely oxidize Ni2+ to Ni3+ and the evaporation of Li2O at high temperature is the main reason for the formation of lithium defects.
Different starting materials have a great influence on the electrochemical performance of synthesized LiNiO2. The electrochemical performance of LiNiO2 synthesized with LiOH·H2O and Ni(OH)2 is the best; while using carbonate as the raw material, the electrochemical performance of LiINiO2 synthesized under oxygen atmosphere is significantly worse. The specific reason is not clear. .
The non-quantity ratio product [Li+1-yNiy2+]3b[Ni3+1-y Niy2+ ]3aO2 interlayer divalent nickel ions will be oxidized into trivalent nickel ions with a smaller ion radius in the later stage of delithiation, causing the collapse of the structure near the ion. In the subsequent lithium insertion process, Lithium ions will be difficult to insert in the collapsed position, resulting in a decrease in the amount of lithium inserted, resulting in a loss of capacity in the first cycle.
The divalent nickel ions between the layers and the layers should be oxidized at the same time in the early stage of delithiation, and the lithium ions around the interlayer divalent nickel ions will be preferentially extracted, and the capacity loss mainly occurs in the first cycle of the first cycle of delithiation. In addition, if the delithiation is charged to a high voltage to generate a high delithiation product, then the Ni-O layer structure will be determined by the majority of Ni4+ with a smaller radius, and a small amount of Ni3+ with the Jahn-Teller effect will pass through the tetrahedral gap. Transfer to lithium ion vacancies, resulting in greater capacity loss. The greater the deviation of the unmetered ratio of the synthesized product, the greater the electrode capacity loss in the first cycle and the greater the charge capacity loss under high voltage. Therefore, the synthetic LiNiO2 should be as close as possible to the ideal stoichiometric ratio.
The structure of LiNiO2 can be changed or modified by doping to achieve better results.
Since A13+ and Ni3+ have similar ionic radii, the valence is very stable. The introduction of about 25% of aluminum ions can control the deintercalation capacity in the high-voltage region, thereby improving its overcharge and cycle resistance. For example, using LiOH·H2O, Ni(OH)2 as raw materials and aluminum powder synthesized in the air at 700 ℃, the pure phase material LiAlyNi1-yO2, the introduction of aluminum can change the crystal structure change during Li deintercalation, so that 0<x< 0.75 The whole area is a single-phase insertion and extraction reaction, and its cycle performance is improved. The DSC spectra of LixNiO2 and LiAl1/4Ni3/4O2 in the fully charged state (4.8V) show that the doping of aluminum is also beneficial to improve its thermal stability.
The LiGa0.02 Ni1.98O2 material synthesized by doping with Ga has been significantly improved in both cycle performance and specific capacity. Its reversible specific capacity reaches 200mA·h/g, and it can still maintain 95% of the initial capacity after 100 weeks of cycling. , And resistant to overcharge. XRD spectra under different charging states show that Ga doping stabilizes the layered structure of LNiO2. The introduction of alkaline earth metal elements such as Mg, Ga, and Sr can not only improve the cycle performance of LiNiO2, but also improve the electrical conductivity of LiNiO2, which is conducive to rapid charge and discharge. The doping of alkaline earth metal elements leads to the generation of defects, which is conducive to the rapid transfer of charges, thereby improving its cycle performance and rapid charging and discharging capabilities.
The influence of doping Mg and F on the electrochemical performance of LiNiO2 solid solution. F doping prevents the migration of Ni3+. Part of the Mg occupies the lithium site can reduce the large change of the c-axis at the end of charging, stabilize the structure and improve the performance, but the amount of doping If it is too large, it will cause a larger capacity attenuation.
Since the doping of a certain element alone cannot solve all the disadvantages of LiNiO2, choosing a combination of multiple elements to dope has become the main research direction at present. Table 1-2 lists the thermal decomposition temperature and heat of decomposition of the composite doped product and other positive electrode materials. It can be seen from Table 1-2 that the combined doping is not only beneficial to improve the cycle performance, but also beneficial to the improvement of its thermal stability. Among them, LiNi0.7Co0.2Ti0.05Mg0.05O2 has the best overall performance.
Table 1-2: The initial temperature of various cathode materials when oxygen is lost in the state of charge