Bak Seong-Min, Hu Enyuan, Zhou Yongning, Yu Xiqian, Senanayake Sanjaya D, Cho Sung-Jin, Kim Kwang-Bum, Chung Kyung Yoon, Yang Xiao-Qing, Nam Kyung-Wan
Chemistry Department, Brookhaven National Laboratory , Upton, New York 11973, United States.
ACS Appl Mater Interfaces. 2014 Dec 24;6(24):22594-601. doi: 10.1021/am506712c. Epub 2014 Dec 5.
Thermal stability of charged LiNixMnyCozO2 (NMC, with x + y + z = 1, x:y:z = 4:3:3 (NMC433), 5:3:2 (NMC532), 6:2:2 (NMC622), and 8:1:1 (NMC811)) cathode materials is systematically studied using combined in situ time-resolved X-ray diffraction and mass spectroscopy (TR-XRD/MS) techniques upon heating up to 600 °C. The TR-XRD/MS results indicate that the content of Ni, Co, and Mn significantly affects both the structural changes and the oxygen release features during heating: the more Ni and less Co and Mn, the lower the onset temperature of the phase transition (i.e., thermal decomposition) and the larger amount of oxygen release. Interestingly, the NMC532 seems to be the optimized composition to maintain a reasonably good thermal stability, comparable to the low-nickel-content materials (e.g., NMC333 and NMC433), while having a high capacity close to the high-nickel-content materials (e.g., NMC811 and NMC622). The origin of the thermal decomposition of NMC cathode materials was elucidated by the changes in the oxidation states of each transition metal (TM) cations (i.e., Ni, Co, and Mn) and their site preferences during thermal decomposition. It is revealed that Mn ions mainly occupy the 3a octahedral sites of a layered structure (R3̅m) but Co ions prefer to migrate to the 8a tetrahedral sites of a spinel structure (Fd3̅m) during the thermal decomposition. Such element-dependent cation migration plays a very important role in the thermal stability of NMC cathode materials. The reasonably good thermal stability and high capacity characteristics of the NMC532 composition is originated from the well-balanced ratio of nickel content to manganese and cobalt contents. This systematic study provides insight into the rational design of NMC-based cathode materials with a desired balance between thermal stability and high energy density.
采用原位时间分辨X射线衍射和质谱联用（TR-XRD/MS）技术，在加热至600℃的条件下，系统研究了带电的LiNixMnyCozO2（NMC，x + y + z = 1，x:y:z = 4:3:3（NMC433）、5:3:2（NMC532）、6:2:2（NMC622）和8:1:1（NMC811））正极材料的热稳定性。TR-XRD/MS结果表明，Ni、Co和Mn的含量对加热过程中的结构变化和氧释放特性均有显著影响：Ni含量越高，Co和Mn含量越低，相变（即热分解）的起始温度越低，氧释放量越大。有趣的是，NMC532似乎是保持合理良好热稳定性的优化组成，与低镍含量材料（如NMC333和NMC433）相当，同时具有接近高镍含量材料（如NMC811和NMC622）的高容量。通过热分解过程中各过渡金属（TM）阳离子（即Ni、Co和Mn）氧化态的变化及其占位偏好，阐明了NMC正极材料热分解的起源。结果表明，Mn离子主要占据层状结构（R3̅m）的3a八面体位置，但Co离子在热分解过程中更倾向于迁移到尖晶石结构（Fd3̅m）的8a四面体位置。这种元素依赖性阳离子迁移对NMC正极材料的热稳定性起着非常重要的作用。NMC532组成具有合理良好的热稳定性和高容量特性，源于镍含量与锰和钴含量的良好平衡比例。这项系统研究为合理设计在热稳定性和高能量密度之间具有理想平衡的NMC基正极材料提供了思路。
