A high-energy, full concentration-gradient cathode material with excellent cycle and thermal stability for lithium ion batteries

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Abstract
Ni-rich Li[Ni1−xMx]O2 (M = Co, Mn and Al) cathodes have shortcomings of poor thermal stability at the delithiated state and insufficient cycle performance, which are unsatisfied for commercial application in lithium ion batteries. Herein, a nickel-rich lithium transition-metal oxide with the full concentration-gradient structure is designed to overcome those problems. In the full concentration-gradient oxide, the nickel concentration decreases linearly, and the manganese concentration increases gradually, whereas the cobalt concentration remains constant from the center to the surface of each particle based on the energy disperse spectrum (EDS) analysis on the cross-section of a single particle. Firstly, the full concentration-gradient precursor is successfully prepared via a newly developed co-precipitation route. After lithiation at 800 °C, the as-prepared full concentration-gradient and normal oxides could be indexed to a typical layered structure with an Rm space group as detected by X-ray diffraction (XRD). Correspondingly, the full concentration-gradient layered oxide delivers more excellent cycle stability (especially at 55 °C), and thermal stability as compared with the normal layered oxide. It is also found that the Ni dissolution in the electrolyte is more serious, resulting in inferior cycle life for the normal layered oxide. Whereas, the outer layer of the full concentration-gradient oxide is much more stable, contributing to such excellent cycle and thermal stability.

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