F-doped Li1.15Ni0.275Ru0.575O2 cathode materials with long cycle life and improved rate performance

Abstract

In this study, Li1.15Ni0.275Ru0.575O2 cathode material for lithium-ion batteries is synthesized using a facile solid-state reaction. In particular, the Li1.15Ni0.275Ru0.575O2 cathode material with a layered structure, despite its high initial capacity, deteriorates in both stability and rate performance. In order to overcome the drawbacks, F-doped Li1.15Ni0.275Ru0.575O2 cathode structures (LNROF-x, 0 < x < 0.1) are prepared with varying contents of F as a dopant and characterized. For the F-doped Li1.15Ni0.275Ru0.575O2 samples, if the O2− sites in the structure are replaced by F−, the transition metal ions of Ni2+ and Ru4+ can be partially reduced to Ni+ and Ru3+ with larger ionic radii for charge compensation. Thus, the increased interspace between the transition metal ions caused by their reduction increases the lattice parameter in the F-doped Li1.15Ni0.275Ru0.575O2 structure. Compared to the undoped Li1.15Ni0.275Ru0.575O2, the improved electrochemical properties, i.e., long life cycle and rate performance, of the F-doped Li1.15Ni0.275Ru0.575O2 samples can result from the improved structural stability caused by a stronger bond of metal-F than that of metal-O and an increased Li+-ion diffusion motion caused by an increased Li slab distance. Furthermore, the Li+-ion diffusion coefficients for the samples are measured by cyclic voltammetry and galvanostatic intermittent titration. However, with increasing F-doping amount, the diffusion coefficients for LNROF-0.02, LNROF-0.04, and LNROF-0.06 increase, whereas the diffusion coefficient for LNROF-0.08 with the excessive F-doping decreases because of the increased resistance to Li+ ion motion caused by the Li/Ni anti-site defect. Thus, the amount of F as a dopant in the F-doped Li1.15Ni0.275Ru0.575O2 samples for the LIBs needs to be optimized.