Comprehensive Study of Li+/Ni2+ Disorder in Ni-Rich NMCs Cathodes for Li-Ion Batteries

Aleksandra A. Savina,Anatolii V. Morozov,Elena D. Orlova,Sergey A. Abakumov,Artem M. Abakumov

doi:10.3390/sym13091628

Abstract

The layered oxides LiNixMnyCozO2 (NMCs, x + y + z = 1) with high nickel content (x ≥ 0.6, Ni-rich NMCs) are promising high-energy density-positive electrode materials for Li-ion batteries. Their electrochemical properties depend on Li+/Ni2+ cation disordering originating from the proximity of the Li+ and Ni2+ ionic radii. We synthesized a series of the LiNi0.8Mn0.1Co0.1O2 NMC811 adopting two different disordering schemes: Ni for Li substitution at the Li site in the samples finally annealed in air, and close to Ni↔Li antisite disorder in the oxygen-annealed samples. The defect formation scenario was revealed with Rietveld refinement from powder X-ray diffraction data, and then the reliability of semi-quantitative parameters, such as I003/I104 integral intensity ratio and c/(2√6a) ratio of pseudocubic subcell parameters, was verified against the refined defect concentrations. The I003/I104 ratio can serve as a quantitative measure of g(NiLi) only after explicit correction of intensities for preferred orientation. Being normalized by the total scattering power of the unit cell, the I003/I104 ratio depends linearly on g(NiLi) for each disordering scheme. The c/(2√6a) ratio appears to be not reliable and cannot be used for a quantitative estimate of g(NiLi). In turn, the volume of the R3¯m unit cell correlates linearly with g(NiLi), at least for defect concentrations not exceeding 5%. The microscopy techniques such as high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and electron diffraction tomography (EDT) allow us to study the materials locally, still, there is no proper quantitative approach for comprehensive analysis of defects. In the present work, the TEM-assisted quantitative Li+/Ni2+ disordering analysis with EDT and HAADF-STEM in six Ni-rich NMC samples with various defects content is demonstrated. Noteworthy, while PXRD and EDT methods demonstrate overall defect amounts, HAADF-STEM allows us to quantitatively distinguish regions with various disordering extents. Therefore, the combination of mentioned PXRD and TEM methods gives the full picture of Li+/Ni2+ mixing defects in Ni-rich NMCs.

Highlights

Owing to higher specific capacity, higher energy density and reduced use of high-cost and relatively scarce cobalt compared to conventional LiCoO2 positive electrode for Li-ion batteries (LIBs), layered mixed lithium and transition metals (TM) oxides
We suggest the quantitative analysis of high-resolution HAADF-STEM images via statistical parameter estimation theory as a tool for discriminating the local regions with different degrees of anti-site disorder
The crystallographic parameters of the NMC811 materials were first obtained through the Rietveld refinement from Powder X-ray diffraction (PXRD) data

Summary

Introduction

Owing to higher specific capacity, higher energy density and reduced use of high-cost and relatively scarce cobalt compared to conventional LiCoO2 positive electrode (cathode) for Li-ion batteries (LIBs), layered mixed lithium and transition metals (TM) oxides. LiNix Mny Coz O2 (NMCs, x + y + z = 1) are considered to be the most promising candidates for positive electrodes (cathodes) for next-generation LIBs [1,2]. NMCs with high nickel content (termed Ni-rich NMCs, where x ≥ 0.6) are of great interest, as such materials can provide high specific capacity The crystal structure of NMCs is of α-NaFeO2 type (space group R3m) consisting of TMO2 layers built up with edge-sharing TMO6 octahedra and separated by layers of octahedrally coordinated Li cations, being an ordered derivative of the rock-salt structure.

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Results

Conclusion