Structural, Electrochemical, and Thermal Properties of Nickel-Rich LiNi_xMn_yCo_zO₂ Materials

Nickel-rich LiNi_xMn_yCo_zO₂ materials (x + y + z = 1, x ≥ 0.6) (NMC) are one of the most promising positive electrode candidates for lithium-ion cells due to their high specific capacity, ease of production, and moderate cost. Conventional NMC materials such as LiNi_0.4Mn_0.4Co_0.2O₂ (NMC442), LiNi_0.5Mn_0.3Co_0.2O₂ (NMC532), LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622), etc. have 20% of costly Co among the transition metal atoms. To lower the Co content while maintaining good electrochemical performance, three series of materials with different transition metal ratios, LiNi_0.6Mn_0.4–xCo_xO₂ (x = 0, 0.1, 0.2), LiNi_0.9–xMn_xCo_0.1O₂ (x = 0.1, 0.2, 0.25), and LiNi_0.8Mn_0.2–xCo_xO₂ (x = 0, 0.1, 0.2), were studied. The materials were synthesized via a coprecipitation/solid state sintering method. Powder X-ray diffraction and electrochemical measurements using coin-type cells were made to characterize the materials. Accelerating rate calorimetry was used to study the reactivity of charged NMC positive electrode materials in the presence of electrolyte at elevated temperatures. NMC721, NMC631, and NMC6.5:2.5:1, which have 50% less Co content than current commercialized NMC622, exhibited excellent specific capacity and thermal stability and therefore deserve careful consideration as next generation materials.