Main content:
The optimization process of the internal materials of the battery includes the improvement of the positive electrode material and the anode and cathode material, the improvement of the electrolyte and the improvement of the diaphragm. These optimization processes help to improve the safety of the battery and reduce the occurrence of problems such as lithium ion battery overcharge, overdischarge, short circuit and overheating of the battery.
Improvement of positive electrode materials
Nickel-rich layered oxide and Li-rich layered oxide are the most promising high-energy cathode materials at present, which can have a high capacity at a high battery state of charge (SOC), meaning that in a state near full charge, the battery's cathode material can store more energy, thereby providing a higher capacity. However, this is also accompanied by the challenge of poor thermal stability of the material and easy to appear safety problems. The thermal instability failure mechanism of Ni-rich and Li-rich layered oxides can be roughly attributed to their structural characteristics, cation mixing, Jahn-Teller distortion, and chemical evolution at the particle boundary.
Both nickel-rich and Li-rich layered oxides are oxygen-containing layered materials. After deep cycling, the surface of the materials changes from layered to rock salt phase, and the lattice oxygen on the surface of the materials is oxidized to gaseous oxygen, resulting in the loss of positive electrode mass. Whereas cation mixing is due to the similar radii of Ni² + and Li + + ions, after removing Li + +, Ni² + migrates from the TM layer to adjacent lithium vacancies in a highly stable state, Li + /Ni² + mixing occurs, leading to irreversible rearrangement and collapse of the material structure, reducing thermal stability. The octahedral oxygen-anion coordination of Li-ion batteries divides the five d orbitals of a TM into two energy levels, t2g and eg. Ni³ + and Mn³ + have a lone electron in the eg energy level and are susceptible to the J-T effect.
This effect destroys the degeneracy between the two g orbitals, reducing the total energy of the solid. For nickel-rich layered oxides, Ni3+ is unstable and tends to form Ni2+ and Ni4+ disproportionately, and an increase in the content of Ni4+ leads to a decrease in structural stability. For lithium-rich layered oxides, the strong J-T effect of Mn³ + leads to lattice distortion that degrades the material during the cycle, affecting thermal stability.
In addition to the above three internal factors, the chemical evolution of the particle surface of the cathode material is also important for its thermal stability. In the calcination process of high-energy oxide active materials, excess lithium salts are required to form highly ordered layered structures. Therefore, the residual lithium salts on the surface inevitably react with moisture and carbon dioxide in the air to form lithium carbonate or lithium hydroxide in subsequent storage and experiments. For this, there are three ways to optimize: volume doping, surface modification, and shape and structure design.
Bulk doping mainly acts on the material itself, changing ion valence and migration and chemical bonding. Table 2 summarizes the effects of introducing various elements at various sites on the thermal stability of nickel-rich and Li-rich layered oxides at a given charging voltage(charging lithium battery).
However, the doped elements are usually non-electrochemically active and cannot provide capacity, which will affect the electrochemical properties of the positive electrode materials to a certain extent. This requires us to find a balance between electrochemical performance and thermal stability. Bulk doping can generally be divided into Li, TM and O sites. Monovalent (Na+, K+) and bivalent (Mg2+) elements are commonly used to occupy the Li position and form Li-O-N (e.g., Li-o-Na, Li-o-mg) bonds, which are more stable than Li-OLI, thus improving structural stability. High-priced cations are usually doped at the TM site.
Al3+ has been shown to enhance the crystal structure stability and thermal stability of positive electrode materials. Al can reduce Li+/Ni2+ mixing and effectively ease the dissolution of transition metals. Doping anions and/or polyanions at the O site can also enhance the thermal and structural stability of the material.
According to research, doping S2- at the O site can weaken the interaction between oxygen and metal atoms, enhancing the stability of the oxygen layer and the migration of Li +. Yang et al. introduced fluoride ions, which improved the structural stability of the material and inhibited the local lattice distortion of the material.
The doping of chloride ions at the O site not only inhibits the precipitation of oxygen but also enhances the migration energy of TM ions in the oxide lattice. In addition, the shortcomings of thermal instability can be mitigated by appropriately increasing cationic mixing, but the internal mechanism and quantitative influence mechanism remain to be studied.
Improvement of anode material
According to the research and development process, the materials of silicon based negative electrode can be divided into amorphous silicon, silicon oxide, low dimensional silicon materials, silicon carbon composite materials, silicon metal composite materials and silicon conductive polymer composites. The advantages and disadvantages of various silicon-based anode materials are summarized in Table 3. Due to its internal isotropic stresses, amorphous silicon protects against structural fractures that occur during the lithiation/delithiation process.
In addition, the working potential of amorphous silicon is higher than that of crystalline silicon, which not only inhibits the generation of lithium dendrites, but also relatively improves the battery cycle stability. However, amorphous silicon is easy to powder, and the stability of the material is low. The cyclic stability of amorphous silicon oxide (SiOx) is a little better than amorphous silicon and can be improved with the increase of oxygen content in SiOx, but its overall stability is still unsatisfactory.
Low-dimensional silicon materials have obvious advantages in improving the performance of lithium-ion batteries, and according to their morphology, they are divided into zero-dimensional silicon nanoparticles, one-nanometer wires/nanotubes, two-dimensional nanofilms/layered silicon and three-dimensional porous silicon/reticulated silicon. However, the problems of volume expansion, battery capacity decay and cycle stability deterioration occur in the electrochemical process of low-dimensional silicon materials.
To overcome these problems, industry researchers have developed composites that combine silicon with other compatible materials. These composites can be divided into coated Si/C and embedded Si/C composites. The Si/C coating can be divided into three structures: solid core shell, hollow core shell and yolk shell. Embedded Si/C can be divided into amorphous carbon and graphene.
The Si/C composite of the coating has a high silicon content, thus increasing the storage capacity of lithium, and the surface coating effectively alleviates the volume expansion of silicon to form a stable SEI film. In contrast, embedded Si/C composites have a lower silicon content but a higher carbon content, resulting in improved cycle stability.
Overall, the structural stability of Si/C composites under large current, toxic contamination during preparation and coulomb efficiency still need to be improved. The silicon metal alloy negative electrode has a high theoretical specific capacity compared with the traditional graphite negative electrode, which has great potential for development. Silicon alloys (SixMy, where M represents metal and x and y represent elemental proportions) can be broadly divided into two classes: the first class M is a metal that cannot be embedded in lithium. These alloys are mainly used to enhance the conductivity of the silicon negative electrode and buffer volume changes.
Electrolyte improvement
The commonly used lithium salt in the electrolyte is lithium hexafluorophosphate LiPF6, and the commonly used solvent is carbonate, but both substances are flammable. In order to improve the thermal stability of lithium-ion batteries and reduce the risk of thermal runaway, there are usually several methods: improve the solvent and lithium salt, use high concentration electrolyte or local high concentration electrolyte, use electrolyte additives, design non-flammable liquid electrolyte, design ionic liquid electrolyte, design gel electrolyte, design solid electrolyte.
Conclusion
The optimization of the internal materials of lithium batteries has significantly improved their thermal stability and safety, but it is still necessary to find a balance between high energy density and safety performance. By improving the positive and negative electrode materials, using innovative electrolyte solutions, and using advanced process technology, the service life and reliability of lithium batteries have been further improved.
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