| In the face of social development and the fuel crisis,renewable energy vehicles have entered a period of rapid development;however,unsatisfactory lifetime has forced people to vigorously improve the performance of lithium ion batteries.As the core parts of restricting the overall performance of lithium ion batteries,cathode materials have received more and more attention,and most of researches have been focused on high nickel cathode materials.While,the cycle stability and capacity issues of high nickel materials need to be further improved.Herein,surface modification and internal structure optimization were used to improve the electrode/electrolyte interface stability,interface charge transport,and internal structure stability of high nickel materials.(1)The strong alkaline of the high nickel material is undesirable for manipulation.Hence,we report an approach for modifying the surface of LiNi0.8Co0.5Al0.05O2(NCA)with an ultra-thin SiOx layer to alter the surface properties of NCA.The ultra-thin SiOx layer was successfully modified by calcination after the siloxane molecules being hydrolyzed and linked to NCA particles.The static water contact angle and XPS confirmed the success of modification;however,no difference between the pristine and modified samples was found by XRD,SEM,and HRTEM characterization.The ultra-thin SiOx layer could greatly improve the cycle stability of NCA with the capacity retentions of 200 cycles at 2 C current under high temperature(55℃)and at 1 C current under high water environment(200 ppm)were 68.53%and 68.67%,respectively.According to EIS and XPS analysis,it was concluded that the ultra-thin SiOx layer could suppress side reactions at the electrode/electrolyte interface,reduce the impedance during cycling,and hence,obtain excellent cycling performance.Finally,a simple voltage fading experiment was used to verify the effective isolation of the ultra-thin SiOx layer from the electrolyte to electrode materials.(2)The Ti-doped Li4SiO4 dual-functional layer was successfully coated on LiNi0.82Co0.15Al0.03O2(NCA82)cathode material by a simply and efficient wet-chemical method.EDX and TEM tests showed that the coating layer was evenly distributed on the surface of NCA82 particles when the Si:Ti is 4:1(NCA@SiTil).Cyclic voltammetry indicated that the abrupt volume change of the unit cell during the cycling was suppressed for coated sample and hence,improving the cycling stability with the capacity retention after 150 cycles at 2 C reaching 85.59%.EIS data showed that the dual-functional layer can effectively reduce the charge transfer resistance at the interface.It was confirmed by the lithium ion diffusion coefficients before and after cycling that dual-functional layer could reduce the side reaction of the electrode/electrolyte interface without hindering the charge transfer.(3)To optimize the performance of LiNiO2 with minimal modification of the pristine structure,a facile solid-state approach,based on the interdiffusion of elements at the solid/solid interface,is developed to achieve uniformly Al-doped LiNiO2 using alumina coated Ni(OH)2 spheres as the precursor.The resulting LiNi0.95Al0.05O2 material exhibits excellent discharge capacity(209.9 mAh g-1 at 0.1 C)and cycling stability(capacity retention of 85.10%after 200 cycles at 0.5 C).This is ascribed to the improved reversibility of the phase transitions by Al-doping as revealed by in-situ XRD characterization.The Al-doping also endows the material with superior rate capability due to the enlarged interlayer spacing in the structure and alleviation of the side reactions at the electrode/electrolyte interface,favorable for lithium ion diffusion.An optimal amount of doped Al is necessary for ensuring the structural stability and interface ionic conductivity of the LiNiO2 spheres.Thus,the present strategy may provide an opportunity to optimize the performance of LiNiO2,with uniform doping of a small amount of Al,producing a promising cathode material for advanced lithium ion batteries. |
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