On January 20-21, 2018, the forum of China EV 100 Forum was held at Diaoyutai State Guesthouse in Beijing. The theme of the forum was "Grasp the global trend of change to achieve high-quality development." NPC Vice Executive Ouyang Minggao, Executive Vice President, Car development of the three core technologies, one of the high-energy lithium-ion battery technology is high.Euyang Ming Gao Academician pointed out that the state's key R & D program 'new energy vehicles' key special support, Ningde era, Tianjin Lishen, Guo Xuan Tech and other teams have basically realized the research and development of 300 watt-hour / kilogram power battery. The specific assessment indexes are as follows: the energy density of the cell is ≥300Wh / kg, the cycle life is ≥1500 times, the cost is ≤0.8 yuan / Wh, the security Etc. These three teams basically use the same technical route at present, are positive high nickel ternary, the negative is silicon carbon. High nickel cathode with silicon carbon anode has been recognized as the industry high-energy lithium-ion power Battery technology route.However, the high nickel cathode is facing many problems, including the preservation of raw materials, high battery production environment is a huge challenge.This paper briefly summarizes the environment Su, especially the impact of humidity on high nickel positive electrode material properties, understanding the wrong place please criticism.
For nickel-based materials, spontaneous reactions occur on the surface of the particles, Ni 3+Change to Ni 2+, Release O 2-, When the material with high nickel content (NMC622, NMC811, NCA, etc.) is exposed to the air, it is easier to absorb the carbon dioxide and water in the air and the following reaction occurs:
This forms Li on the surface of the particles 2CO 3And LiOH layer, the material in the high proportion of Ni, PH value is higher, and Li 2CO 3And LiOH consume Li in the material, and do not have the electrochemical activity, thus causing the capacity to decay, and the dense particle surface Li 2CO 3Layers impede the diffusion of Li, affecting cell performance LiOH also reacts with PVDF and LiPF6, adversely affecting battery technology and performance.
The reaction of the material with the air takes place throughout the process of raw material storage, electrode preparation, pole piece storage, etc. Therefore, strict control of the environment, especially from the raw material to the entire battery production process, is required for high nickel materials, The material has already reacted and the influence of moisture can not be removed again by the conventional drying process. The preparation of the electrode paste and the manufacturing of the pole piece need to be carried out in a dry environment. Generally, the production process of the high-nickel cathode battery needs Dew point -30 ℃ environment.
If the surface of high-nickel cathode material absorbs the moisture in the air, the reaction produces LiOH, which will have a serious impact on the manufacturing process of the pole piece.In the preparation process of high-nickel cathode slurry, PVDF dissolves in NMP and the surface of the material Of the basic groups will attack the adjacent CF, CH bonds, PVDF is very easy to occur bimolecular elimination reaction will form part of the carbon-carbon double bond in the molecular chain, the reaction is as follows:
As the double bond in PVDF increases, its cohesion will also increase, which leads to an increase in viscosity of the slurry and even to a gel state of the slurry.Therefore, the impact of high nickel positive paste on the preparation and coating process and the ambient humidity Huge, if the process of absorbing water, especially easy to cause changes in the nature of the slurry, resulting in the quality of the manufacturing process pole plate instability, poor process consistency and other issues, the formation of gel slurry, even the coating process can not be carried out.
In addition, when the double bond in PVDF increases, the bonding force increases, the fragility of the pole piece is increased and the fracture is particularly easy to occur. During the process of the pole piece rolling and slitting, the process of the pole piece breaks can not be performed. Is a square winding process, the core in the corner will cause the pole piece fracture or fall off the situation.
LiOH will react with Al foil as follows:
6OH -+ 2Al + 6H 2O → 6OH -+ 2Al (OH)3+3H2
After the Al is corroded, the mechanical strength decreases, the electrochemical performance and safety of the battery will be affected, and the surface properties of the foil are corroded, the peel strength of the coating will be reduced, and the mechanical and electrical properties of the electrode plate will be affected.
In addition, LiOH also reacts with LiPF6 to consume Li ions in the electrolyte to generate HF gas, which can corrode the metal parts inside the battery and finally leak the battery, and HF destroys the SEI film and interacts with the main components of the SEI film Ongoing reaction:
ROCO 2Li + HF → ROCO 2H + LiF
Li 2CO 3 + 2HF → H 2CO 3 + 2LiF
Finally, LiF precipitates inside the cell, causing lithium ions to undergo irreversible chemical reactions in the negative plate of the cell, consuming active lithium ions and reducing the energy of the cell.
High nickel material absorbs moisture reaction product 2CO 3The high potential in the state of charge easily decomposed produce CO2 gas, causing the battery drum bag leakage problem.When the material absorbs enough water, the gas generated, the battery internal pressure will become larger, causing the battery stress deformation, there Battery drum up, leakage and other dangerous.
Therefore, for high-nickel cathode materials, environmental humidity must be strictly controlled in the process of material storage and battery preparation to produce high-performance lithium-ion batteries.
