The high-nickel positive electrode paired with a silicon-carbon negative electrode has emerged as a leading technical route for achieving high specific energy in lithium-ion power batteries. Despite its promising potential, the high-nickel cathode material faces numerous challenges, particularly in terms of raw material stability and maintaining a controlled production environment throughout the battery manufacturing process.
Earlier, Academician Ouyang Minggang highlighted that under the national key R&D program "New Energy Vehicles," several teams including CATL, Tianjin Lishen, and Guoxuan Hi-Tech have made significant progress in developing 300 Wh/kg power batteries. The key performance indicators include an energy density of at least 300 Wh/kg, a cycle life of over 1500 times, a cost below 0.8 yuan/Wh, and compliance with national safety standards.
These teams primarily use a similar technical approach: high-nickel ternary materials combined with silicon-carbon anodes. This combination is widely recognized as a viable path toward achieving high-energy-density lithium-ion batteries. However, the high-nickel cathode still presents many difficulties, especially regarding material preservation and the need for a highly controlled production environment.
This article provides a brief overview of environmental factors, with a particular focus on the impact of humidity on the properties of high-nickel cathode materials. We hope this summary helps clarify some of the challenges associated with these materials.
Nickel-based materials are prone to surface reactions, where Ni³⺠is reduced to Ni²âº, and oxygen ions (O²â») are released. When high-nickel materials such as NMC622, NMC811, or NCA are exposed to air, they tend to absorb carbon dioxide and moisture from the environment. This leads to the formation of Liâ‚‚CO₃ and LiOH layers on the particle surfaces.
With a higher nickel content, the pH level increases, and these byproducts consume lithium within the material, which is not electrochemically active. As a result, the material's capacity decreases, and the particle surfaces become denser. The Li₂CO₃ layer hinders lithium ion diffusion, negatively affecting battery performance. Meanwhile, LiOH can react with PVDF and LiPF₆, further degrading cell performance and processability.
The reaction between the material and air occurs throughout the entire production chain—starting from raw material storage, through electrode preparation, and up to pole piece storage. Therefore, strict environmental control, especially humidity management, is essential from the beginning to the end of the battery production process. Once moisture interacts with the material, conventional drying methods cannot reverse the damage.
Electrode slurry preparation and pole piece manufacturing must be carried out in a dry atmosphere. Typically, the production of high-nickel cathode batteries requires a dew point of -30°C to maintain optimal conditions. If moisture is absorbed by the cathode particles, it can lead to the formation of LiOH, which significantly impacts the pole piece manufacturing process.
During the preparation of high-nickel cathode slurry, PVDF is dissolved in NMP. The basic groups on the material’s surface can attack CF and CH bonds in PVDF, leading to bimolecular elimination reactions. This can result in the formation of carbon-carbon double bonds, increasing the slurry's viscosity and even causing it to gel. Such changes can make coating processes difficult or impossible, leading to quality issues like poor adhesion or incomplete coating.
Additionally, increased adhesion due to PVDF double bonds can cause the pole piece to become brittle, making it prone to cracking during winding, slitting, or other mechanical processes. In square winding configurations, the pole piece may break or fall off at the corners of the core, leading to production failures.
LiOH also reacts with aluminum foil, corroding it and reducing its mechanical strength. This corrosion affects both the electrochemical performance and safety of the battery. It also weakens the adhesion between the coating and the foil, further impacting the mechanical integrity of the electrode.
Moreover, LiOH reacts with LiPF₆, consuming lithium ions in the electrolyte and producing HF gas. This gas can corrode internal metal components, leading to leaks and long-term degradation of the battery. Additionally, HF can damage the SEI film, continuing to react with its main components and causing irreversible damage.
Finally, LiF precipitation inside the battery can lead to irreversible chemical reactions in the negative electrode, consuming active lithium ions and reducing the battery’s overall energy capacity.
When high-nickel materials absorb moisture, the resulting Li₂CO₃ can decompose into CO₂ gas at high voltages, causing internal pressure buildup and potentially leading to battery leakage. If enough moisture is absorbed, the internal pressure increases significantly, leading to deformation, swelling, or even liquid leakage.
In conclusion, controlling humidity is critical during the storage of high-nickel cathode materials and throughout the battery manufacturing process to ensure the production of high-performance lithium-ion batteries.
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