In this study, we establish a systematic understanding of LMR calcination from a TM hydroxide precursor and Li 2 CO 3 and establish the role of Li 2 CO 3 particle size. We employ a comprehensive suite of in situ and ex situ characterization techniques, including X-ray diffraction, absorption spectroscopy (XAS) and thermogravimetric analysis, to elu-cidate the role of Li 2 CO 3 particle size on the calcination kinetics and pathway. Our work reveals key trends between temperature, particle size, and reaction heterogeneity. Namely, when Li source particle size and TM source particle size are similar, the calcination reaction is homogeneous across secondary particles, without intermediate phases, and concludes at temperatures lower than the melting point of the Li 2 CO 3 . This finding contradicts some previous work on LMR calcination, which has identified intermediate phases during calcination and shown mass loss at the melting point of Li 2 CO 3 . We identify Li 2 CO 3 particle size as an important variable that explains this discrepancy. We find that Li 2 CO 3 particle size tunes reaction heterogeneity across larger length scales than have been previously reported for LMR materials, with the presence of large (>20 µm) Li 2 CO 3 particles leading to inter-secondary particle heterogeneity in oxidation state, lithium concentration, and extent of Li 2 MnO 3 -like ordering. The impact of Li 2 CO 3 particle size on electrochemical behavior is most strongly seen during initial cycles where superstructure ordering present in as-synthesized LMR materials has not yet been erased by the atomic rearrangements that accompany electrochemical cycling in LMR materials. Probing the impact of Li source morphology on synthesis pathway and ultimately performance sheds light on the persistence of the impact of synthetic conditions on LMR materials.