Multiscale Probing and Redesign of the Solid-State Synthesis Toward Defect-Free Ni-Rich Layered Cathodes

Abstract

Nickel-rich lithium layered oxides (LiNixCoyMnzO2, NCM, x+y+z=1, x≥0.6)) have received tremendous attention as the most promising cathode materials for the practical development of high-energy lithium-ion batteries. However, they commonly suffer from rapid capacity decay along with structural and morphological degradation. Previous studies have unraveled that the general failure mechanism involves the mechanical deterioration of electrode particles, originating from micro-cracking in particles due to the build-up of significant anisotropic strains during phase transition and the chemical parasitic reactions with electrolytes through the crevice created. Such chemo-mechanical failures are triggered and aggravated by various structural defects, including internal void spaces, surface reconstruction layers, and intragranular nanopores, typical features of the conventionally synthesized nickel-rich layered oxide materials. It remains elusive how these defects are formed regardless of the use of single-crystalline or polycrystalline particles. A fundamental understanding of the general defect formations is thus indispensable to clarify how the synthesis process induces the formation of intrinsic defects from the precursor to the final product stage and how these defects affect the electrochemical degradation at the primary- and secondary-particle levels. Critical questions that require in-depth study include how the spatially inhomogeneous solid-state reaction begins at the interface of reacting precursors, propagates during the synthesis, and triggers the generation of defects. In this presentation, I will show the hidden synthetic mechanism of nickel-rich layered oxide materials using a combination of high-end and multi-length-scale analysis methods, including aberration-corrected TEM, in situ heating TEM with gas control, and in situ XRD. The kinetic interplay in synthesis of nickel-rich lithium layered oxide and its implications will be discussed in detail. Furthermore, the microstructural pore defects of NCN9235 during the heating process were quantitatively characterized. Based on our mechanistic understanding, we propose a redesign of solid-state synthesis pathway to achieve structurally integrated nickel-rich layered oxide cathodes.