Mn4+ rich surface enabled elevated temperature and full-cell cycling performance of LiMn2O4 cathode material

Abstract

LiMn2O4 (LMO) cathode exhibiting improved electrochemical performance is reported. X-ray diffraction confirms spinel cubic structure in the bulk with localized structural integrity confirmed by high-resolution Transmission Electron Microscopy (TEM) analysis showing lattice fringes with spacing of 0.48nm corresponding to (111) of spinel LMO. X-ray photoelectron spectroscopy (XPS) study quantified the Mn4+/Mn3+∼2 instead of 1 on the surface of pristine LMO nanoparticles. Mn4+ rich surface improved elevated temperature cycling stability inhibiting Mn-dissolution. The surface rich Mn4+ and almost equal concentration of Mn4+ and Mn3+ in the sub-surface/bulk was confirmed by XPS analysis upon ion-etching. At room temperature, high discharge capacity of ∼110 mAh/g at 2C rate and ∼102 mAh/g at 10C rate is reported for long cycles (over 500). Cycling at 55°C, capacity retention of 81.2% and 72% at the end of 200 cycles for 1C and 10C discharge rates respectively are testified for the electrochemical stability. This is superior elevated temperature performance of LMO electrodes especially, without any surface coating or doping. To demonstrate LMO cathode’s potential, a full-cell against Li4Ti5O12 and commercial graphite anodes were tested that exhibit discharge capacity of 95 mAh/g and 82 mAh/g respectively with retention of ∼82% over 100 cycles. Finally, electrodes after first charge and discharge have been investigated by ex situ XPS to correlate the oxidation states of manganese with pristine LMO.