Research On Preparation And Electrochemical Performance Of Modified High-Nickel Nickel Cobalt Lithium Aluminate

High nickel nickel cobalt aluminate(LiNi_0.8Co_0.15Al_0.05O₂,NCA)is considered to be a promising candidate for high-nickel cathode materials due to its high reversible stability,excellent processability and storage properties.However,due to the excessive addition of lithium source and the precipitation of surface lattice oxygen during the synthesis of NCA,the excessive accumulation of residual alkaline lithium salts on the surface of the material will result.Excessive alkaline lithium salt will accelerate the hydrolysis of the electrolyte,aggravate the corrosion of HF to the material and generate gas,which will not only reduce the electrochemical performance of the material,but also cut down the safety of the battery.To solve these problems,this thesis adopts the modification strategy of optimizing the residual alkaline lithium salt on the surface and forming an in-situ fast lithium-ion conductor coating layer to modify the high-nickel NCA cathode material.The main contents include the following aspects:1.The Lewis acid WO₃ was introduced to modify the surface residual alkaline lithium salt of the NCA cathode material and reconstruct a fast ion conductor coating layer,Li₂WO₄,on the surface of the NCA,aiming to enhanced the cycle stability and rate performance of the material.The research results exhibit that the NCA@0.02Li₂WO₄（W2-NCA）sample exhibits excellent cycling performance,with a capacity retention rate as high as 86.5%after 200 cycles at 25°C/2 C,and 88.8%after 100 cycles at 55°C/1C,which are much higher than those of the original sample.And at a high rate of 5 C,the W2-NCA sample exhibits the highest discharge specific capacity of 142.6 m Ah·g^-1.2.For the first time,Li H₂PO₄ was introduced to optimize the surface residual alkaline lithium salt of the NCA cathode material and formed an in-situ fast ion conductor coating layer of Li₃PO₄ on the surface of NCA,exhibiting excellent electrochemical performance.The research results show that the p H value of the NCA material modified by the phosphate coating is effectively reduced.The NCA@0.02Li₃PO₄（P2-NCA）coated sample provides a discharge specific capacity of 147.8 m Ah·g^-1 at a high rate of 5 C,which is much higher than that of the pristine sample(126.5 m Ah·g^-1).The capacity retention rate of P2-NCA for 200 cycles at 25°C/2 C is as high as 96.1%,and the capacity retention rate for 100 cycles at 55°C/1 C is as high as 86.2%,which are much higher than the pristine sample.3.The NCA samples modified by the triple composite lithium ion conductor coating（Y（PO₃）₃-Li₃PO₄-YPO₄）were prepared by introducing Y（PO₃）₃ to react with the surface residual alkaline lithium salts of NCA.The research results show that the 2 mol%Y（PO₃）₃ modified NCA electrode（Y2-NCA）exhibits the most excellent rate capability(5 C/156.3 m Ah·g^-1)and cycling stability（2 C/200 cycles/88.3%）,while the pristine sample was only 5 C/123.1 m Ah·g^-1 and 2 C/200 cycles/71.2%.Even at 55°C/1 C,the capacity retention rate of Y2-NCA is still as high as 89.4%after 100 cycles.The above research results all indicate that this surface modification method provides a feasible strategy for enhancing the electrochemical performance and applicability of nickel-rich cathode materials.