The high nickel positive electrode with silicon carbon negative electrode has been recognized as a technical route of high specific energy lithium ion power battery in the industry. The high-nickel positive electrode faces many problems, and the preservation of raw materials and high battery production environment are huge challenges.
Earlier, Academician Ouyang Mingga pointed out that under the support of the key projects of the national key R&D plan “New Energy Vehiclesâ€, several teams including Ningde Times, Tianjin Lishen and Guoxuan Hi-Tech have basically realized 300 watt-hour/kg power batteries. R & D, the specific assessment indicators are: battery cell energy density ≥ 300Wh / kg, cycle life ≥ 1500 times, cost ≤ 0.8 yuan / Wh, safety and other national standards.
The technical routes basically adopted by these three teams are similar, all of which are high-nickel ternary and the negative electrode is silicon carbon. The high nickel positive electrode with silicon carbon negative electrode has been recognized as a technical route of high specific energy lithium ion power battery in the industry. The high-nickel positive electrode faces many problems, and the preservation of raw materials and high battery production environment are huge challenges. This article briefly summarizes the environmental factors, especially the effect of humidity on the characteristics of high-nickel cathode materials. Please understand the mistakes.
For nickel-based materials, spontaneous reaction occurs on the surface of the particles, Ni3+ is converted to Ni2+, and O2- is released. When materials with high nickel content (NMC622, NMC811, NCA, etc.) are exposed to the air, it is easier to absorb carbon dioxide and water in the air. .
In this way, Li2CO3 and LiOH layers are formed on the surface of the particles. The ratio of Ni in the material is high and the pH value is higher. Li2CO3 and LiOH consume Li in the material and have no electrochemical activity, so the capacity is attenuated and the surface of the particles is dense. The Li2CO3 layer hinders the diffusion of Li and affects battery performance. LiOH also reacts with PVDF and LiPF6, adversely affecting cell process and performance.
The reaction of materials with air takes place throughout the process of raw material storage, electrode preparation, and pole piece storage. Therefore, for high nickel materials, strict environmental control, especially moisture control, is required from the raw material to the entire battery production process. If the moisture and the material have already reacted, the influence of moisture cannot be removed again by the conventional drying process. The preparation of the electrode slurry and the production of the pole piece need to be carried out in a dry environment. Generally, the production of the high nickel positive electrode battery is carried out. The process requires a dew point of -30 ° C environment.
If the surface of the high-nickel cathode material particles absorbs moisture from the air, the reaction produces LiOH, which has a serious effect on the pole piece manufacturing process. In the preparation process of high nickel positive electrode slurry, PVDF is dissolved in NMP, and the basic group on the surface of the material will attack adjacent CF and CH bonds. PVDF is easy to undergo bimolecular elimination reaction and will form part of the molecular chain. Carbon-carbon double bond.
When the double bond in the PVDF is added, the binding force is also increased, which causes the viscosity of the slurry to increase, and even the slurry forms a gel state. Therefore, in the preparation and coating process, the high-nickel positive electrode slurry has a great influence on the environmental humidity. If the water absorption reaction occurs during the re-process, the properties of the slurry are particularly likely to change, resulting in unstable quality in the manufacturing process of the pole piece. Problems such as poorness, even when the gel slurry is formed, even the coating process cannot be performed.
Moreover, when the double bond in PVDF causes the adhesion force to increase, the brittleness of the pole piece is particularly prone to breakage, and during the process of winding and unwinding of the pole piece rolling and slitting, the process of the pole piece is broken and the process cannot be performed. If the battery is a square winding process, the pole piece may be broken or dropped at the corner of the core.
LiOH reacts with Al foil. After Al is corroded, the mechanical strength is reduced, the electrochemical performance and safety of the battery are affected, and the surface properties of the foil are corroded, the peel strength of the coating is reduced, and the mechanical properties of the pole piece are improved. Electrical performance will be affected.
In addition, LiOH also reacts with LiPF6, consuming Li ions in the electrolyte, and generating HF gas, which can corrode metal parts inside the battery, thereby causing the battery to eventually leak. Moreover, HF destroys the SEI film and continues to react with the main components of the SEI film.
Finally, LiF precipitation occurs inside the battery, causing an irreversible chemical reaction of lithium ions in the negative electrode of the battery, consuming active lithium ions, and the energy of the battery is reduced.
The high-nickel material absorbs the moisture reaction product Li2CO3, which is easily decomposed to generate CO2 gas at a high potential in the state of charge, causing leakage of the battery drum. When the material absorbs enough water, more gas is generated, and the pressure inside the battery becomes larger, which causes the battery to be deformed by force, and there is a danger of battery swelling and liquid leakage.
Therefore, for high-nickel cathode materials, environmental humidity needs to be strictly controlled during raw material storage and battery preparation to produce high-performance lithium-ion batteries.
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