Detailed design rules of high-quality NCM811 and key points of electrode preparation technology-Part 1
Low-cost and high-performance lithium-ion batteries (LIB), as a key technology to promote technological progress, have become the focus of attention. The layered nickel-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) active material, due to its high energy density, high specific capacity and low material cost, is expected to become the next generation LIB battery cathode material. However, lower thermal stability and higher sensitivity to H2O and CO2 may lead to performance degradation and safety degradation (Figure 1). Therefore, process optimization must be performed. In the past few years, electrode production technology for high-performance LIB has been continuously optimized. Although there are some design strategies for improving the stability of NCM811, information about the entire cathode production process is still scarce. Therefore, it is necessary to review the current status and challenges of the entire cathode production process, including from formulation design to drying treatment.
Figure 1. The characteristics of NCM active materials with different stoichiometric compositions.
Recently, the Arno Kwade team of the Battery Laboratory of the Technical University of Braunschweig, Germany, published a review paper in the internationally renowned journal Journal of The Electrochemical Society with the topic "Review—Knowledge-Based Process Design for High Quality Production of NCM811 Cathodes". In terms of quality, cost and environment, the current status and challenges of the entire cathode production process using NCM811 as an active material are considered, and some improvement suggestions are introduced, as well as innovative methods such as water-based or solvent-free processes.
1. The challenge of nickel-rich NCM811
Establishing an effective cathode production chain inevitably requires understanding the characteristics and challenges of active materials. One of the main problems is the oxidation of lattice oxygen and the release of singlet oxygen in the state of about 80% charge and deep delithiation. In addition, due to the reaction between the singlet oxygen and the electrolyte solvent, the release of CO and CO2 is related to the release of oxygen. The release of oxygen is also closely related to the formation of disordered spinel and rock salt phases on the surface of NCM particles. In addition, as the nickel content in NCM increases and the state of charge increases, the number of Ni4+ ions will also increase, resulting in the generation of covalent Ni-O bonds and undesirable side reactions (Figure 2).
Figure 2. The relationship between the amount of Ni4+ ions in various NCM materials and the state of charge.
Another common problem of NCM materials is cation mixing, that is, other metal ions occupy Li+ positions in the crystal lattice. Due to the similar atomic radius, mixing occurs mainly between Ni2+ and Li+ ions. Therefore, as the nickel content increases, the problem of cation mixing becomes serious. As a result, the diffusion path of Li+ is blocked, and the ion mobility decreases. In addition, the microcracks in the active material are caused by the decrease of the unit cell volume in the deep delithiation state (Figure 3a). The volume of the unit cell is mainly controlled by the c-axis. When the lithium content is ≤0.5, the c-axis of the crystal lattice will suddenly decrease (Figures 3b and 3c). The shrinkage of the layered oxide implies the formation of covalent MO-OM peroxide bonds due to the oxidation of lattice oxygen. Therefore, the electrolyte penetrates into the crack and decomposes. Likewise, it will trigger the dissolution of transition metals in the active material.
Figure 3. (a) Microcracks and electrolyte intrusion in NCM active materials with different stoichiometric compositions (b) Crystal structure of NCM811. (C) The lattice parameters a and c and (d) the change of unit cell volume with Li content.
Studies have shown that H2O and CO2 in the air have harmful effects on NCM active materials through side reactions. Therefore, the most important treatment method is the adjustment and control of air parameters. One problem in the air is that strong alkaline lithium-containing compounds (RCL) are formed on the surface of the material. Studies have shown that the reduction of Ni3+ to Ni2+ will lead to the formation of O2- on the surface of NCM particles. According to equations 1 and 2, it may react with atmospheric H2O and CO2 to form RLCs:
The amount of RLC increases with the increase of nickel content and humidity. Li2CO3 is the cause of battery swelling, especially when it exists in a charged state at high temperature, which will increase the risk of fire. The reaction of LiOH with the polyvinylidene fluoride (PVDF) binder may cause the positive electrode slurry to gel during the coating process. In addition, due to the excessive supply of LiOH during the material synthesis process, RCL may also be generated to compensate for the loss of Li2O. According to formulas 3 and 4, in the presence of H2O and CO2, transition metal hydroxides and carbonates are formed on the surface of the original NCM material:
Therefore, the surface impurities of nickel-rich NCM811 are mainly nickel carbonate, which may also cause gas production. In addition, the residual moisture in the battery will react with LiPF6 to form hydrogen fluoride (HF):
A certain degree of design of NCM811 particles can enhance the electrochemical performance of the cathode. The surface coating can effectively reduce the side reactions between the layered oxide and the electrolyte or the atmosphere. For example, the surface of NCM811 (Figure 4a) is modified with a dual conductive layer composed of Li3PO4 and polypyrrole (PPy) or a Li2SiO3 coating. In addition, some coatings can provide doped ions to reduce cation mixing. Supplying an excess of lithium source or increasing the oxygen partial pressure can reduce RLC. In addition, the gradient nanostructure of the active material can improve the performance of the positive electrode, from the core to the particle surface, the nickel content is reduced.
Another strategy to improve the stability of NMC811 is to use a core-shell structure. Materials with a core-shell structure can increase the charging cut-off voltage to a certain extent. Studies have shown that diacetyl oxime reacts with nickel ions to form a nickel-rich core and a manganese-rich shell, which can protect the nickel-rich core from side reactions with the atmosphere or electrolyte. Another core-shell concept is to coat NCM111 on NCM811 during the co-precipitation process to obtain an electrochemically active surface. The original material shows a higher initial capacity, but a lower capacity at a higher temperature. Passivating metal oxide active materials can reduce the formation of metal carbonates in the atmosphere. The cation mixing can be reduced by doping other ions at the Li site or the Ni site, and the cycle stability of the NCM811 positive electrode can be improved. It is recommended to use cations with electronic inert gas configuration or cations with a higher potential than the working potential of the positive electrode. Regarding the prevention of microcracks, NCM particles with a nickel content of ≥0.8 have a radial crystal texture, which can reduce the spread of microcracks on the surface and avoid the random expansion and contraction of primary particles. All in all, surface coating of NCM811 particles, doping with foreign ions or building a core-shell/gradient structure can improve the stability of the material, thereby improving battery performance.
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