A Comparison in Structural Transformation of Li[NixCoyMnz]O2 (x = 0.6, 0.85) Cathode Materials in Lithium-Ion Batteries

本文主要对高镍三元材料（Li(Ni0.85Co0.1Mn0.05)O2,Ni85）和常规低镍三元材料（Li(Ni0.6Co0.2Mn0.2)O2,Ni60）两种三元材料的相变电压范围进行了划分和测定,以研究两种材料相变规律的区别,并进一步分析得出高镍材料在充电过程中的结构稳定性相对较弱的原因。本文主要采用了XRD、dQ·dV-1以及SEM的表征方式对两种材料的相变、结构变化及颗粒表面的形貌进行分析。并得出以下结论,高镍正极在3.0 V ~ 4.2 V范围内充电时经历了H1→M→H2→H3的三次相变过程,最终产物为H3相。而传统Ni60材料在相同电压范围内只经历了H1→M的相变过程,当过充至4.550 V时,Ni60材料可达到H2相,继续过充至5.000 V后,可完成H3相的转变。因此,高镍正极材料在正常充电电压范围内即完成了H3相的相转变过程,其较低的相变电压阈值是其结构稳定性较差的原因。In this paper, the phase transformation voltage ranges of two layered oxide ternary cathode materials, namely, Li(Ni0.85Co0.10Mn0.05)O2 (referred to Ni85, presenting high Ni content) and Li(Ni0.6Co0.2Mn0.2)O2 (referred to Ni60, presenting common low Ni content), were classified and determined. The structural differences between high Ni and common low Ni ternary materials were studied in order to understand the structure instability of high nickel material during the charging process. At the same time, the differential capacity (dQ·dV-1) curves of Ni85 and Ni60 positive electrodes during the charging process were obtained to characterize phase regions, and the corresponding relationship between the cathode and anode phase transfermations was studied. In addition, the phase transformation and surface morphology of Ni85 and Ni60 materials were analyzed by X-Ray diffraction (XRD) and field emission scanning electron microscopy (SEM). It is concluded that the high Ni positive electrode underwent three phase transformations of H1→M→H2→H3 within the normal charging range of 3.0 V ~ 4.2 V, through which the final product was H3 phase, which is relatively unstable. In the same charging voltage range, the traditional Ni60 material only experienced the phase transition from H1 phase to M phase. When overcharged to 4.550 V, Ni60 material could reach H2 phase, and after overcharging to 5.000 V, H3 phase transformation could be completed. The dQ·dV-1 curve reflects the above phase transformation processes, and the variations of characteristic diffraction peaks can be observed on XRD. The cross section SEM images of fresh and fully charged cathodes showed that, the particle crushing degree of Ni85 material was obviously greater than that of Ni60 material under the full charge state. According to the above experimental results, it can be concluded that the H3 phase transformation could be completed within the normal charging voltage range for Ni85 material. Therefore, the lower phase transformation voltage threshold of high Ni material accounts mainly for the poor structure stability.国家重点研发计划项目(2016YFB0100304)通讯作者:李丽娟E-mail:lilijuan@gotion.com.cnContact:Li-JuanLiE-mail:lilijuan@gotion.com.cn合肥国轩高科动力能源有限公司,安徽 合肥 230012Hefei Gotion High-Tech Power Energy Co., Ltd. Hefei 230012, Anhui, Chin