Fig. 5: Mechanistic understanding of the roles of Li stoichiometry in tuning crystal growth and inter-particle fusion during the calcination process.

a Initial microstructure of the primary particles with an average size of ~35 nm (measured by SEM; Fig. S8). Scale bars: 100 nm. b Computationally simulated primary particle microstructure after calcination at 600 °C for 12 hours (Li/TM = 1.0), with an average particle size similar to the initial value (consistent with SEM observation as given in Fig. S15 (a)). c, d Computationally simulated primary particle microstructure after calcination at 720 °C for 12 hours with a Li/TM ratio of 0.95 and 1.05, respectively. e Comparison between the experimentally observed (black squares) and computationally predicted primary particle size (red line) as a function of the Li/TM ratio obtained after calcination at 720 °C for 12 hours. The two distinct regions, where lithiation-induced crystallization is dominant, and where the  liquid phase sintering becomes predominant, are clearly demarcated. f, g Schematic representation of the sintering induced particle growth mechanism observed during the calcination of Ni-rich cathode primary particles with different amounts of lithium salt. TM(OH)₂ precursors, LiOH salt particles, and lithiated LiTMO₂ particles are denoted by orange, green and blue, respectively. Scheme (f) indicates that, under lithium poor condition (Li/TM < 1.0) or stoichiometric amount (Li/TM ≈ 1.0), all the lithium is consumed in the reaction with the cathode precursors, and with no excess lithium salt, no fusion among individual grains occurs during calcination. On the contrary, scheme (g) indicates that with the presence of excess lithium salt (Li/TM > 1.0), a significant amount of the non-reacted molten lithium salt exists adjacent to the cathode primary particles. This excess lithium salt in liquid phase acts as a sintering aid and leads to a substantial amount of sintering-induced particle growth through inter-particle fusion.