Abstract:

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, ZrO₂ was coated on LiNi_0.8Co_0.1Mn_0.1O₂ 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 ZrO₂-coated samples and was mainly distributed at the surface of the secondary particles of the LiNi_0.8Co_0.1Mn_0.1O₂ cathodes. The X-ray diffraction (XRD) indicated that Zr⁴⁺ in ZrO₂ migrated into the layered surface structure of LiNi_0.8Co_0.1Mn_0.1O₂ 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_0.8Co_0.1Mn_0.1O₂ 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 Zr⁴⁺ in surface structure as opposed to the ZrO₂ coating. The X-ray photoelectron spectroscopy (XPS) indicated that the Ni³⁺ at the surface of LiNi_0.8Co_0.1Mn_0.1O₂ was reduced to Ni²⁺ after the Zr⁴⁺ surface doping due to charge balance. Rietveld refinement also indicated that the Li⁺/Ni²⁺ cation disordering improved after the Zr⁴⁺ in ZrO₂ doped into NCM surface structure. The raised cation disordering may be triggered by the increased content of Ni²⁺ and their migration into Li layers due to the similar ion radius of Li⁺ (0.076 nm) and Ni²⁺ (0.069 nm). A structure-reconstructed layer at the surface of LiNi_0.8Co_0.1Mn_0.1O₂ was formed after the Zr⁴⁺ 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_0.8Co_0.1Mn_0.1O₂ material was improved.

Key words: Lithium ion battery, LiNi_0.8Co_0.1Mn_0.1O₂, Cathode material, ZrO₂ coating, Zr⁴⁺ doping, Surface structural reconstruction