Abstract:

In this paper, the phase transformation voltage ranges of two layered oxide ternary cathode materials, namely, Li(Ni_0.85Co_0.10Mn_0.05)O₂ (referred to Ni85, presenting high Ni content) and Li(Ni_0.6Co_0.2Mn_0.2)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.

Key words: lithium-ion batteries, high-Ni layered oxide cathodes, phase transitions