Particle Size and Microstructure Control of Ni-Rich Layered Oxide Cathodes for Next Generation Li-Ion Batteries

Ni-rich layered oxides are very promising candidates for cathode active materials in Li-ion batteries. Especially LiNiO2 (LNO) with its theoretical capacity of 275 mAh/g could be designated to replace state of the art layered oxide materials containing unethically mined cobalt. Yet, the synthesis of LNO is providing some difficulties due to cation mixing, the occupation of Li sites with Ni2+, and the structural instability during cycling due to occurring phase transitions, which results in irreversible capacity loss and capacity fading during cycling, as well as a decreased capacity retention. Emerging oxygen from the structure during cycling enacts as an additional safety risk.
Optimising the synthesis of the LiNiO2 precursor Ni(OH)2 via a precipitation reaction in a stirred tank reactor as well as the high temperature solid-state reaction to lithiate the precursor to form the final LiNiO2 is suggested to mitigate some of these issues from the start by carefully controlling the particle size and morphology to prevent internal mechanical strain during the phase transitions, improve Li diffusion within the structure and reduce potential cation mixing. Analytical techniques like X-ray diffraction (XRD) and refinements of the resulting diffraction pattern, scanning electron microscopy (SEM), laser diffraction, tap density, Brunauer Emmett Teller (BET) analysis and magnetic measurements were used to examine the crystal structure and the materials properties, as well as electrochemical cycling to study morphological influences on the cycling behaviour of the as-prepared materials. Advanced methods like thermogravimetric analysis (TGA), in situ X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS), operando muon spin relaxation spectroscopy and in situ X-ray diffraction combined with computer tomography (XRD-CT), help understand internal processes and mechanisms during operation.
Ni(OH)2, the precursor for LNO, was successfully synthesised with a quasispherical morphology of secondary particles and a seemingly dense packing of primary particles. Different particle sizes can be produced on demand by tweaking the reaction conditions. LiNiO2 was produced with improved annealing conditions, yielding a reliably repeatable low cation mixing of below 2 % while maintaining spherical-like secondary particles with randomly aligned cuboid primary particles in the core and radially aligned primary particles towards the surface of the secondary particle assembly. These improved materials show enhanced cycling behaviour with initial capacities above 250 mAh/g while sustaining a capacity retention over 100 cycles above 80 %.
Additionally, doping of the optimised LNO gives an overview of the properties enhanced or mitigated through the dopant and will give new insights and opportunities to combine specific targeted dopants to improve battery performance and life even more.