High-Rate Performances of the Ru-Doped Spinel LiNi_0.5Mn_1.5O₄: Effects of Doping and Particle Size

Abstract

Novel ruthenium-doped Li−Ni−Mn−O spinels with a well-defined stoichiometric composition of LiNi0.5−2xRuxMn1.5O4 were designed and synthesized by traditional solid-state reactions (SS) and a polymer-assisted method (PA), respectively. The characterization of these compounds was carried out by inductively coupled plasma emission spectrometry, X-ray diffraction, Fourier transform infrared spectra, scanning electron microscopy, transmission electron microscopy, conductivity measurements, and electrochemical tests. Both the SS and PA LiNi0.5−2xRuxMn1.5O4 samples have a desirable atomic ratio but exhibit different particle sizes. Intriguingly, the Ru-doped spinels synthesized by the PA method can deliver a capacity of 135 mAh g−1 even at an extremely high discharge rate of 1470 mA g−1 (10 C rate), and excellent cyclic performances at the 10 C charge/discharge rate for 500 cycles are achieved. The improvements can be attributed to the trace Ru doping and reduction of particle size, which give rise to several benefits, including (i) elimination of LixNi1−xO impurity, (ii) increasing the electronic conductivity and lithium diffusivity of electrodes mainly resulting from the wider conduction band formed by overlapping the Ru 4d orbitals with the O 2p orbitals, and (iii) a good structural stability through preventing a pile-up of Li+ ions on surfaces of particles favored by fast Li+ ion transportation. The present results have verified the ability of Ru doping to improve high-rate performances of spinel-structured transition metal oxides.