In this work, nickel-rich, layered-structure LiNi0.65Co0.08Mn0.27O2 cathode materials were synthesized and compared with materials of the same overall composition, but with a concentration gradient throughout the particles: the Ni concentration is higher at the center of the particles and lower at surface, while the Mn concentration is higher at the surface and lower at the center. The synthesis parameters of the co-precipitation method were optimized, with two different annealing protocols for the final products. Both generations of gradient materials provided superior capacity and rate capability than their respective non-gradient materials, at normal operating potentials and temperatures, e.g. 30 oC up to 4.3 V vs. Li.
TEM measurements were used for morphological and structural characterization of pristine (uncycled) and cycled electrodes. We explored the evolution of the gradient structure during cycling, comparing energy dispersive X-ray Spectroscopy (EDX) line scans in order to determine whether the particles’ concentration gradients are stable during cycling. Prolonged cycling, even at elevated temperatures, did not change the initial concentration profiles determined by the synthesis.
The layered-to-spinel (rhombohedral layered R3m to cubic spinel phase, Fd3m) transition is commonly observed in layered materials containing Mn & Ni during prolonged cycling, with Ni-rich layered compounds often showing slower spinel formation during cycling. TEM, electron diffraction studies showed that spinel phase formed on constant concentration particles upon electrodes cycling, but no spinel phase was detected in full concentration gradient particles from cycled electrodes.
