In situ multiscale probing of the synthesis of a Ni-rich layered oxide cathode reveals reaction heterogeneity driven by competing kinetic pathways

Nickel-rich layered oxides are envisaged as key near-future cathode materials for high-energy lithium-ion batteries. However, their practical application has been hindered by their inferior cycle stability, which originates from chemo-mechanical failures. Here we probe the solid-state synthesis of LiNi0.6Co0.2Mn0.2O2 in real time to better understand the structural and/or morphological changes during phase evolution. Multi-length-scale observations—using aberration-corrected transmission electron microscopy, in situ heating transmission electron microscopy and in situ X-ray diffraction—reveal that the overall synthesis is governed by the kinetic competition between the intrinsic thermal decomposition of the precursor at the core and the topotactic lithiation near the interface, which results in spatially heterogeneous intermediates. The thermal decomposition leads to the formation of intergranular voids and intragranular nanopores that are detrimental to cycling stability. Furthermore, we demonstrate that promoting topotactic lithiation during synthesis can mitigate the generation of defective structures and effectively suppress the chemo-mechanical failures. Nickel-rich layered oxides, such as NCM622, are promising cathode materials for lithium batteries, but chemo-mechanical failures hinder their practical application. Now the solid-state synthesis of NCM622 has been studied using multiscale in situ techniques, and kinetic competition between precursor decomposition and lithiation has been observed to lead to spatially heterogeneous intermediates and the formation of defects that are detrimental to cycling.