Synthesis, characterization, and electrochemical cycling behavior of the Ru-doped spinel, Li[Mn_2-xRu_x]O₄(x=0, 0.1, and 0.25)

Abstract

Pure and Ru-doped Li[Mn_2-xRu_x]O₄ (x=0.1 and 0.25) spinel compounds are prepared by the high temperature solid-state reaction and characterized by X-ray diffraction (XRD), scanning electron microscopy, Brunauer, Emmett, and Teller surface area, density, IR, Raman spectroscopy, and X-ray absorption spectroscopy. XRD studies show single-phase compounds with the cubic spinel structure and a decrease in (Mn-O) and an increase in (Li-O) bond lengths on Ru doping. The electrochemical cycling behavior of Li[Mn_2-xRu_x]O₄ is examined by galvanostatic cycling and cyclic voltammetry (CV). The CV shows that the Mn³⁺ ↔ Mn⁴⁺ couple lies in the range from ~4.0 to ~4.2 V and the spinel-to-double-hexagonal (S-DH) transition at ~4.5 V in agreement with results in the literature. The Ru⁴⁺ ↔ Ru⁵⁺ redox couple operates at ~4.3 V vs Li in the spinel structure. The S-DH phase transition is suppressed on Ru doping. Charge-discharge cycling, up to 25 or 40 cycles, in the voltage ranges 3.5-4.3 and 3.5-4.6 V vs Li at 0.14C rate gave initial charge capacities ranging from 118 to 139 mAh g⁻¹ for various x. Compositions with x=0.1 and 0.25 showed smaller initial capacity loss and smaller capacity fading during cycling as compared to x=0. For x=0.25 in the voltage range 3.5-4.6 V and 2-25 cycles, the capacity fade is only 9% thereby showing the beneficial effect of Ru doping, by way of suppression of S-DH transition, participation of Ru-redox couple, and better electronic conductivity of the compound during cycling, in comparison to pure LiMn₂O₄.