The high-nickel positive electrode paired with a silicon-carbon negative electrode has gained widespread recognition as a key technical route for achieving high specific energy in lithium-ion power batteries. However, the use of high-nickel cathode materials comes with significant challenges, particularly in terms of raw material storage and maintaining a controlled environment during battery production.
Earlier, Academician Ouyang Minggang highlighted that under the support of the national key R&D program "New Energy Vehicles," several leading teams, including CATL, Tianjin Lishen, and Guoxuan Hi-Tech, have made substantial 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 of no more than 0.8 yuan per Wh, and compliance with national safety standards.
These teams have largely adopted similar technical approaches, using high-nickel ternary cathodes combined with silicon-carbon anodes. This combination is now considered a standard path for high-energy-density lithium-ion batteries. Despite this, the high-nickel cathode still faces numerous issues, especially regarding material stability and environmental control during the entire battery manufacturing process.
This article provides a brief summary of environmental factors, with a particular focus on the impact of humidity on the properties of high-nickel cathode materials. Any mistakes or oversights are unintentional and welcome to be corrected.
For nickel-rich materials, such as NMC622, NMC811, and NCA, surface reactions can occur spontaneously, where Ni³⺠is reduced to Ni²âº, and oxygen ions (O²â») are released. When these materials are exposed to air, they tend to absorb carbon dioxide and moisture, leading to the formation of Liâ‚‚CO₃ and LiOH layers on the particle surfaces.
High nickel content increases the pH level, and the presence of these compounds consumes lithium from the material, which is not electrochemically active. This leads to capacity fading and a denser surface layer. 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 the atmosphere occurs throughout the entire process, from raw material storage to electrode preparation and pole piece storage. Therefore, strict environmental control—especially moisture management—is essential from the beginning of the production process to the final battery assembly. Once moisture reacts with the material, conventional drying methods cannot reverse the damage.
Electrode slurry preparation and pole piece production must take place in a dry environment. Typically, the production of high-nickel positive electrode batteries requires a dew point of -30°C to maintain optimal conditions.
If the surface of high-nickel cathode particles absorbs moisture, it can lead to the formation of LiOH, which significantly impacts the pole piece manufacturing process. During the preparation of the high-nickel positive electrode slurry, PVDF is dissolved in NMP, and the basic groups on the material's surface can attack adjacent CF and CH bonds. This can trigger bimolecular elimination reactions in PVDF, forming carbon-carbon double bonds.
The formation of these double bonds increases the binding force, leading to higher slurry viscosity and even gelation. As a result, the slurry becomes difficult to coat, causing instability in the pole piece manufacturing process. Issues like poor coating quality or gel formation can prevent the coating step from being completed.
Additionally, increased adhesion caused by PVDF’s double bonds makes the pole piece more brittle, increasing the risk of breakage during winding, unwinding, and slitting processes. In square winding configurations, the pole piece may crack or fall off at the corners of the core.
LiOH also reacts with aluminum foil, corroding it and reducing its mechanical strength and electrochemical performance. This corrosion weakens the adhesion between the coating and the foil, affecting the overall performance of the pole piece.
Moreover, LiOH reacts with LiPF₆, consuming lithium ions in the electrolyte and generating HF gas. This gas can corrode internal metal components, leading to leaks and eventually damaging the SEI film, which is crucial for battery stability.
Finally, LiF precipitation inside the battery causes irreversible chemical reactions, consuming active lithium ions and reducing the battery’s energy output. Additionally, when Li₂CO₃ forms due to moisture absorption, it can decompose at high voltages, releasing CO₂ gas and causing internal pressure buildup. This can lead to swelling, deformation, and even leakage of the battery.
In conclusion, for high-nickel cathode materials, maintaining strict humidity control during both raw material storage and battery production is essential to ensure the development of high-performance lithium-ion batteries.
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