(Invited) Stabilizing High-Nickel Content Layered Cathode Materials through Introducing Hierarchical Valence Gradient
Authors: Lin Ruoqian , Bak Seongmin , Shin Youngho , Xin Huolin L. , Yang Xiao-Qing 
Published: December 2020

Stabilizing high-nickel content layered cathode materials through introducing hierarchical valence gradient

Ruoqian Lin1, Seongmin Bak1, Huolin L. Xin2, Xiao-Qing. Yang1,*

1 Chemistry Division, Brookhaven National Laboratory

2 Department of Physics and Astronomy, University of California, Irvine
High Ni-content layered oxide as cathode materials for lithium-ion batteries have attracted great research interest recently. Taking advantage of two-electron transfer per Ni by the Ni2+/Ni4+ redox, high-Ni-content cathode materials can potentially provide much higher capacity than traditional layered cathode materials such as LiCoO2 or Li Ni1/3Mn1/3Co1/3O2 (NMC333). However, the structure and surface instability of this type of materials renders poor capacity retention and thermal stability, which need to be addressed for this material to be used as lithium-ion battery cathode material for electric vehicles. To overcome all these challenges, we investigated a new type of secondary particle engineering that has a uniform Ni:Mn:Co 811 chemical composition throughout the particle but with the nickel oxidation state spatially changing from Ni2+ to Ni3+ along the radial direction of the particle from the surface to the center. The goal of this study is to understand how this new gradient structure slows down the performance deterioration and structural degradation of high-Ni-content materials. Also, this study will differentiate the effect of Ni valence gradience from the concentration gradience, which has been widely utilized to improve the surface and structural stability of high-Ni-content cathode materials. By combining in-situ/ex-situ multi-scale multi-probe characterization tool, including electron and synchrotron X-ray imaging and spectroscopy, the LiNi0.8Mn0.1Co0.1O2 with Ni valence gradience is investigated at multiple length scales. We found that nanopores and micro cracks are more favored to be formed in the center of the secondary particles rather than on the surfaces, which indicates that degradation could initiate from the center bulk rather than particle surfaces. It is also found that Ni valence gradience along the radial direction across the secondary particle is well retained during electrochemical cycling, which implies that lower valence state Ni on the surfaces can stabilize the material surfaces and mitigate the internal structure degradation. Time-resolved in-situ thermal stability studies show that, compared with regular LiNi0.8Mn0.1Co0.1O2 materials, introducing Ni valence gradience greatly enhances the thermal stability and prohibits oxygen release related thermal runaway. It is also found that calcination atmosphere is one of the key factors that can tune the chemical hierarchical structure of high-Ni-content cathode materials, which provides valuable guidance for future material design and scaling up synthesis.

Acknowledgement

The work done at Brookhaven National Laboratory was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the U.S. DOE through the Advanced Battery Materials Research (BMR) Program, including Battery500 Consortium under contract No. DE-SC0012704. This research used the National Synchrotron Light Source II and the Center for Functional Nano-materials (CFN), U.S. DOE Office of Science User Facilities operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. Thework done by UCI was supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Award Number: DE-EE0008444. Dr. Y. Shin was supported by the Office of Vehicle Technologies of the U.S. Department of Energy under contract number DE-AC02-06CH11357.