A further electrochemical investigation on solutions to high energetical power sources: isomerous compound 0.75Li1.2Ni0.2Mn0.6O2·0.25LiNi0.5Mn1.5O4

Abstract

An isomerous layered/spinel 0.75Li_1.2Ni_0.2Mn_0.6O₂·0.25LiNi_0.5Mn_1.5O₄ cathode material with outstanding electrochemical properties has been synthesized by a reasonable design of introducing high-power spinel LiNi_0.5Mn_1.5O₄ material to fill up the surface gaps of pristine lithium-rich layered Li_1.2Ni_0.2Mn_0.6O₂ material with a molar ratio of 25 : 75. Morphological characterization reveals that the octahedral spinel LiNi_0.5Mn_1.5O₄ particles are successfully coated into the surface gaps of the Li_1.2Ni_0.2Mn_0.6O₂ secondary particle, forming a special alternant structure with spherical and octahedral particles on the surface. Interestingly, some hollow sections are also observed in 0.75Li_1.2Ni_0.2Mn_0.6O₂·0.25LiNi_0.5Mn_1.5O₄ material, confirmed from the TEM images. The structural characterization demonstrates that this isomerous compound is more well-defined α-NaFeO₂ configured, more enlarged in Li layer spacing and lower in cation disordered degree. The exquisite morphology and ideal structure endow this nanocrystal-assembled composite significantly enhanced electrochemical performance with high capacity, good rate capability and excellent cycling stability, compared with the pristine Li_1.2Ni_0.2Mn_0.6O₂. It delivers a discharge capacity of 135 mA h g⁻¹ even at an ultrahigh current density of 2000 mA g⁻¹ (10 C). Moreover, the superior cycling stability is also observed with high discharge capacities of 254 mA h g⁻¹ and 222 mA h g⁻¹ at 0.5 C and 1 C after 100 cycles with capacity retention of 98% and 94%, respectively. Moreover, the fast-charging test results are indicative of the fact that this layered/spinel cathode could be used in practical application. Its discharge capacity is 176 mA h g⁻¹ at 1 C after 50 cycles with the charge rate of 10 C. Furthermore, the composite can endure high current charging and discharging even at a high cut-off potential (5.0 V), whereas the pristine Li_1.2Ni_0.2Mn_0.6O₂ material cannot. Therefore, we absolutely believe that this isomerous layered/spinel 0.75Li_1.2Ni_0.2Mn_0.6O₂·0.25LiNi_0.5Mn_1.5O₄ cathode is a promising candidate for the commercial development of advanced LIBs.