To reveal how the performance of LiNi_xCo_yMn_zO₂ (NCM) changes as a function of transition metal (TM) composition, the effect of TM ratio on the structure, morphology, electrochemical performance and thermal behavior of different NCMs was systematically investigated. Increasing Ni content leads to higher reversible capacity but worse rate capability and cycling stability. Inhibited kinetics of Li⁺ and poor electronic conductivity cause major capacity loss in Ni-rich NCMs. Comparison of thermal behaviors was carried out via in-situ and ex-situ micro-calorimetry. With the decrease of Ni content, the thermal stability is significantly improved. The oxygen release related to phase transitions was evaluated based on Li residuals in delithiated NCMs, which verifies that Ni-rich materials exhibit severer structural deterioration, lower onset temperature and more heat release. Comprehensive characterization identifies that LiNi_0.5Co_0.3Mn_0.2O₂ strikes a well-balanced combination of electrochemical performance and safety features.