Among many alternative chemical energy storage devices, lithium-ion batteries have become a global research hotspot due to a series of outstanding advantages such as high operating voltage, long cycle life, high energy density, and environmental friendliness. Since the successful commercialization of lithium-ion batteries by Sony in 1991, its applications in computers, communications, and consumer electronics have been extremely successful.
At present, the main development direction of lithium-ion batteries is high energy and high power. At the same time, this is also an inevitable requirement for the continuous upgrading of electronic product terminals and the development of the new energy electric vehicle industry.
Therefore, only by continuing to improve the existing lithium-ion battery related technologies and developing new lithium-ion battery materials, can we adapt to the development trend of economic growth in harmony with environmental protection.
1 Characteristics of ternary cathode materials
The ideal cathode material needs to have the following characteristics.
(1) There are transition metal ions with high redox potential and prone to redox reactions to ensure high charge and discharge capacity and output voltage of lithium ion batteries;
(2) Higher electronic and lithium ion conductivity to ensure good rate performance;
(3) Good structural stability;
(4) High chemical and thermal stability in a wide voltage range;
(5) Easy to prepare, environmentally friendly and affordable.
The ternary cathode material LiNi1-x-yCoxMnyO2 (NCM) has the characteristics of high specific capacity, good structural stability, good thermal stability and low cost. The LiNi1-x-yCoxMnyO2 material can be considered to be partially substituted from the Co layer by Ni and Mn. Increasing the Ni content can increase the material capacity but will reduce the cycle performance and stability. Increasing the Co content can suppress the phase transition and improve the rate performance. Increasing the Mn content is beneficial to improve the structural stability, but it will reduce the capacity. The content of the three transition metals determines the performance of the material. The first discharge capacity, capacity retention rate, specific capacity, and thermal stability cannot be optimized at the same time. Trade-offs are needed. Enterprises can reduce the Ni content appropriately during the production , Increase the proportion of Co, Mn to enhance cycle performance and extend product life.
In 1999, Liu and others for the first time synthesized a ternary transition metal oxide material LiNi1-x-yCoxMnyO2 with different components, and conducted a detailed study of its crystal structure and electrochemical performance. And binary layered materials. In 2001, Ohzuku and others synthesized LiNi1 / 3Co1 / 3Mn1 / 3O2 materials of the same amount as Ni, Co, and Mn. The discharge specific capacity was as high as 200mAh g-1 under the voltage range of 2.5V ~ 4.6V. Since then, ternary materials with different nickel-cobalt-manganese ratios have been developed and synthesized. So far, the research focus of ternary cathode materials mainly includes two types: ① Ni, Mn and other isotypes, such as LiNi1 / 3Co1 / 3Mn1 / 3O2 (NCM111) and LiNi0.4Co0.2Mn0.4O2 (NCM424); Types, including LiNi0.5Co0.2Mn0.3O2 (NCM523), LiNi0.6Co0.2Mn0.2O2 (NCM622) and LiNi0.8Co0.1Mn0.1O2 (NCM811).
2 Problems and challenges
The ternary cathode material NCM has an α-NaFeO2 layered structure, the space point group is Rm, and the lithium and transition metal atoms alternately occupy the center position of the octahedron formed by the oxygen atom. -The transition metal layer is continuously stacked in the direction of the orthorhombic hexahedron [001]. According to the crystal field theory, Ni3 + tends to exist in the form of Ni2 + due to the instability of Ni3 + due to the solitary electron spin in the e-orbit. During the preparation of ternary materials, because the radius of Ni2 + (0.069 nm) and the radius of Li + (0.076 nm) are close to each other, it is very easy for them to occupy each other in the lattice structure. At this time, cations exist between the lithium layer and the transition metal layer. Misalignment.
Compared with the ideal layered structure, the cation mixing leads to a decrease in the lithium layer spacing in the crystal structure, an increase in the activation energy of lithium ion migration, and a transition metal ion occupying the position of the lithium layer also hinders lithium ion diffusion. Therefore, as the cation mixing increases, the rate performance of the material also deteriorates.
In fact, the cation mixed discharge phenomenon of the ternary cathode material not only occurs during its synthesis, but also occurs during the charge and discharge cycle of the battery. Analysis by transmission electron microscopy indicated that there was a case where transition metal elements occupied the position of the lithium layer during the O3 phase layered material LixNi0.5Mn0.5O2 cycle. In addition, under the condition of long-cycle high-voltage charge-discharge cycles, a phase transition occurs in LiNi0.5Co0.2Mn0.3O2, and the Rm phase gradually changes into a spinel-like phase and a rock salt phase. Under the high charge cut-off voltage, the material is deeply delithiated. The existence of a large number of lithium vacancies causes the material structure to be extremely unstable. The transition metal ions migrate from the transition metal layer to occupy the position of the lithium layer. The cations tend to occupy secondary lithium vacancies.
Nickel NCM cathode material has the advantages of high capacity, low cost, and abundant raw material sources. It is a promising lithium ion battery material and is expected to replace LiCoO2 and be commercialized on a large scale. However, during storage of high nickel materials, it is easy to react with CO2 and H2O in the external environment to generate lithium-containing compounds on the surface of the material. In addition, the transition metal ion with high oxidation activity catalyzes the decomposition of the electrolyte in the charged state, and accelerates the consumption of the electrolyte while generating a thicker solid electrolyte interface layer on the surface of the material. In order to obtain an ordered layered structure material, an excessive lithium source is often added during the lithium-mixed sintering preparation process, resulting in lithium residues on the surface of the NCM material. The lithium residue on the surface of the material reacts with H2O and CO2 in the air to form Li2CO3 and LiOH. After immersing the high nickel material in pure water for a period of time, the pH value of the solution is greater than 12. In addition, the material easily forms a gel-like slurry in NMP solvents, which seriously affects the production of electrode pads.
Fresh LiNi0.7Co0.15Mn0.15O2 is composed of primary particles with a particle size of about 150 nm. Its surface is smooth, and residual lithium compounds can be seen on the surface after being left in the air for 3 months. The inspection showed that the surface Li2CO3 content increased from 0.89 wt% to 1.82 wt% after storage in the air, and the LiOH content increased from 0.25 wt% to 0.44 wt%. Therefore, the ternary material storage environment must be protected from air and moisture. During the ternary material charge and discharge cycle, there are spontaneous interface side reactions between the active material and the electrolyte. A large amount of electrolyte decomposition substances are attached to the surface of the electrode material, and the specific composition of these substances is related to the electrolyte used in the battery system. For example, in an electrolyte composed of LiClO4 as a lithium salt and PC as a solvent, the main decomposition product is lithium carbonate. In an electrolyte composed of LiPF6 as a lithium salt and EC and DMC as solvents, the decomposition products are mainly compounds containing P, O, and F. At different test temperatures, test times, and discharge states, the electrolyte decomposition products on the surface of the cathode material are composed of polycarbonate, LiF, LixPFy, and LixPFyOz. The decomposition products of the electrolyte adhered on the surface of the material hinder the migration of lithium ions on the surface of the active material, the interface resistance surges, and the electrochemical performance of the battery deteriorates. Removal of surface lithium residues and suppression of interfacial side reactions are the keys to improving the electrochemical performance of ternary materials.