LiNi_1/3Co_1/3Mn_1/3O₂ was synthesized as a cathode material for lithium‐ion batteries by coprecipitation and solid‐state synthesis. The precursor prepared by coprecipitation was sintered at 950–1000 °C for 10–15 h under air. A cathode material annealed at 950 °C for 10 h (NMC950‐10) has 81% capacity retention, whereas another cathode material at 1000 °C for 15 h (NMC1000‐15) has 85% capacity retention at the 80th cycle at a 60 °C cycle test temperature. Cross‐sectional images of pressed electrodes reveal that this difference results from different degrees of particle rupture. Image analysis shows that the percentages of particle rupture in NMC950‐10 and NMC1000‐15 were 44% and 20%, respectively. The measured particle hardness of the cathode material is quantitatively related to the number of ruptured particles in highly pressed electrodes. Therefore, the cathode material with higher particle …​