This work establishes the calcination mechanism of LMR using coprecipitated (oxy)hydroxide precursors, and investigates the importance of Li 2 CO 3 particle size. We quantitatively track the calcination reaction, both in situ and via quenched materials, and reveal solid-solid and Mismatch between Li-source and TM-source particle size is likely also a source of intersecondary-particle reaction heterogeneity in other classes of positive electrode materials. In systems such as stoichiometric NMC materials, which lack substantial atomic rearrangements during electrochemical activation, these heterogeneities may have a more persistent impact on electrochemical performance.