Cationic distribution and electrochemical performance of LiCo 1/3Ni 1/3Mn 1/3O 2 electrodes for lithium-ion batteries

Abstract

LiCo 1/3Ni 1/3Mn 1/3O 2 oxides with layered crystal structure were prepared from co-precipitated Co,Ni,Mn hydroxide precursors. The synthesis temperature was varied between 850 to 950 °C. The extent of Li +,Ni 2+ disorder between the layers was determined by Rietveld refinement of the structural model, based on the XRD powder diffraction patterns. The cationic distribution in the layers was determined using both conventional (9.23 GHz) and high-frequency electron paramagnetic resonance (EPR) spectroscopy (95 GHz and 285 GHz). The high-frequency EPR spectra of LiCo 1/3Ni 1/3Mn 1/3O 2 consisted of two types of signals, which merged into one Lorentzian line at 9.23 GHz. These signals were assigned to Mn 4+ ions having different metal surroundings. It was shown that the value of the g-factor of Mn 4+ was sensitive to the number of Ni 2+ ions included in the first metal coordination sphere of Mn 4+. For oxides prepared at 850 °C, two types of Mn 4+ ions having two distinct environments in respect of the number of Ni 2+ ions were established: Mn 4+ in a Ni-free and Mn 4+ in a Ni-rich environment. On increasing the preparation temperature from 850 to 950 °C, a cationic redistribution process takes place in a way to avoid the contacts between the Ni 2+ and Co 3+ ions. Even at this preparation temperature, the cationic distribution is still non-homogeneous. While the cationic distribution in the layers was changed significantly when the temperature increased from 850 to 950 °CC, the morphology of LiCo 1/3Ni 1/3Mn 1/3O 2 powders underwent a strong change at temperatures above 900 °C. The electrochemical performance of LiCo 1/3Ni 1/3Mn 1/3O 2 was tested in lithium cells using both potentiostatic and galvanostatic modes. Electrochemical impedance spectroscopy (EIS) was used to study the cycling stability of LiCo 1/3Ni 1/3Mn 1/3O 2. The best electrochemical performance was established for the oxides prepared at 900 °C. The process of lithium extraction from and insertion into LiCo 1/3Ni 1/3Mn 1/3O 2 was discussed on the basis of ex-situ EPR experiments.