Thermodynamic analysis of Li-Ni-Co-Mn-H₂O system and synthesis of LiNi_0.5Co_0.2Mn_0.3O₂ composite oxide via aqueous process

Abstract

The constructed potential-pH diagrams of Li-Ni (Co, Mn)-H₂O system indicate that the LiNiO₂, LiCoO₂ and LiMnO₂ are thermodynamically stable in aqueous solution within the temperature range of 25-200 °C and the activity range of 0.01-1.00. A predominant co-region of LiNiO₂, LiCoO₂ and LiMnO₂ oxides (Li-Ni-Co-Mncomposite oxide) is found in the Li-Ni-Co-Mn-H₂O potential-pH diagrams, in which the co-precipitation region expands towards lower pH with rising temperature, indicating the enhanced possibility of synthesizing Li-Ni-Co-Mn composite oxide in aqueous solution. The experimental results prove that it is feasible to prepare the LiNi_0.5Co_0.2Mn_0.3O₂ cathode materials (NCM523) by an aqueous routine. The as-prepared lithiated precursor and NCM523 both inherit the spherical morphology of the hydroxide precursor and the obtained NCM523 has a hexagonal α-NaFeO₂ structure with good crystallinity. It is reasonable to conclude that the aqueous routine for preparing Ncm cathode materials is a promising method with the guidance of the reliable potential-pH diagrams to some extent.

摘要

本文构建的Li-Ni(Co, Mn)-H2O 系电位−pH 图从热力学角度表明,在25≈250 °C 温度范围和 0.01~1.00 活度范围内,LiNiO₂, LiCoO₂ 和LiMnO₂ 可以稳定存在于水溶液中。在Li-Ni-Co-Mn-H₂O 系 电位−pH 图中,同时出现了LiNiO₂、LiCoO₂ 和LiMnO₂ 氧化物(即Li-Ni-Co-Mn 复合氧化物)的优势 区,并且该共沉淀优势区随温度的升高向低pH 方向扩大,表明在水溶液中合成Li-Ni-Co-Mn 复合氧 化物是可能的。在此基础上,本文通过实验采用湿法过程合成了LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523)正极材 料,证明了上述理论的可行性。合成的锂化前驱体和NCM523 材料均继承了氢氧化物前驱体的球形形 貌,同时所得NCM523 材料具有良好的六方α-NaFeO₂ 型晶体结构。在电位−pH 图指导下,湿法合成 工艺将一定程度上成为三元正极材料极具前景的制备方法之一。