ZrO_(2)包覆高镍LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)正极材料提高其循环稳定性的作用机理

Effects of ZrO_(2)Coating on Ni-Rich LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)Cathodes with Enhanced Cycle Stabilities

高镍三元材料作为一种锂离子电池正极材料,因其较高的放电比容量而得到科学界和工业界的广泛关注。研究表明,高镍三元材料的比容量与材料中的Ni含量呈正相关,但Ni含量的增加也会加剧循环过程中的界面副反应,材料表面释氧以及结构转变等问题。本文采用ZrO_(2)包覆LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)材料,利用X射线衍射证明,在高温处理下ZrO_(2)包覆物中的Zr^(4+)会掺杂进LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)材料表面晶格中,使得X射线衍射谱中的(003)衍射峰左移。电化学测试证明在4.3和4.5 V的截止电压下,改性最优的材料在1C循环100周后容量保持率分别从84.89%和75.60%提高到97.61%和81.37%,同时发现循环稳定性的提升主要来自材料表面的Zr^(4+)掺杂。X射线光电子能谱证明Zr^(4+)表层掺杂后材料的Ni化合价由Ni3+向Ni^(2+)转变,透射电子显微镜观察到Zr^(4+)的表层掺杂使得材料表面的层状结构发生重构,从而稳定了材料体相结构,提高了材料整体的循环稳定性。

With the development of electric vehicles(EVs)and hybrid electric vehicles(HEVs),the demand for lithium ion power batteries with high energy density and long cycle life has continuously increased in the recent years.According to the“Made in China 2025”plan,the energy densities of lithium ion batteries need to reach 300 Wh·kg−1 in 2020.Due to their high discharge capacities and work voltages,Ni-rich layered materials have attracted considerable attention from the science and industry fields as one of the most promising cathodes to achieve high energy density.According to previous reports,the discharge capacities of Ni-rich cathodes were positively correlated to their Ni content.However,the increased Ni content can aggravate the side reactions between the cathode and electrolyte,induce O loss,and trigger structural transformation from the surface to bulk.In this study,ZrO_(2)was coated on LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)with a simple wet chemical method to improve its cycle performance.The scanning electron microscopy(SEM)with energy dispersive spectroscopy(EDS)demonstrated that Zr was only detected in the ZrO_(2)-coated samples and was mainly distributed at the surface of the secondary particles of the LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)cathodes.The X-ray diffraction(XRD)indicated that Zr^(4+)in ZrO_(2)migrated into the layered surface structure of LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)based on the shift of the(003)peak to a lower angle,which was considered as a lattice expansion along the c axis.Under the cut-off voltage of 4.3 and 4.5 V,the capacity retentions of the LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)cathodes improved from 84.89 to 97.61%and 75.60 to 81.37%,respectively,after 100 cycles at 1C.This was mainly attributed to the doped Zr^(4+)in surface structure as opposed to the ZrO_(2)coating.The X-ray photoelectron spectroscopy(XPS)indicated that the Ni3+at the surface of LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)was reduced to Ni^(2+)after the Zr^(4+)surface doping due to charge balance.Rietveld refinement also indicated that the Li+/Ni^(2+)cation disordering improved after the Zr^(4+)in ZrO_(2)doped into NCM surface structure.The raised cation disordering may be triggered by the increased content of Ni^(2+)and their migration into Li layers due to the similar ion radius of Li+(0.076 nm)and Ni^(2+)(0.069 nm).A structure-reconstructed layer at the surface of LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)was formed after the Zr^(4+)doping,which had been confirmed by transmission electron microscope(TEM).It was determined that this structure-reconstructed layer can hinder the side reactions at the interface and stabilize the bulk structure during cycles;thus,the cycle stability of LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)material was improved.