Capacity Enhancement and Structural Study of Fluorine-Doped Co-Free Li- and Mn-Rich Li1.2[Mn0.5Ni0.2Fe0.1]O2(1−x)F2x Layered Oxide Cathodes

Both Co-free and lithium- and manganese-rich layered oxide Li(Li0.2MnxNiyFez)O2 (MNF) cathodes have recently attracted attention in lithium-ion battery (LIB) research due to their high capacities of over 250 mAhg−1, as well as being more eco-friendly and inexpensive than commercial NMC and LiCoO2. However, they still suffer from lower experimental capacity as well as capacity decay, voltage fade, poor rate capability, and thermal instability. In this paper, fluorine (F)-doped Li1.2(Mn0.5Ni0.2Fe0.1)O2(1−x)F2x (MNF502010, x = 0, 0.025, 0.05, 0.075, 0.1) cathode materials have been synthesized in the nanoscale via sol–gel and subsequent solid-phase calcination to address some of these problems. The resulting 5% F-doped MNF502010 cathode demonstrates the advantage of fluorine doping, which makes a significant contribution to the formation of a well-ordered layer structure with a minimal LiM2O4 spinel phase as an impurity. This composition achieves an initial discharge capacity of 252 mAhg−1 (1C = 250 mAhg−1) and a 156 mAhg−1 discharge capacity at 0.3 C on the 100th discharge, with an average voltage fade of 0.24 V. The optimization of fluorine composition results in an enhancement in the activation of the Li2MnO3-type monoclinic phase, as well as an increase in the electronic conductivity compared to the fluorine-free cathode. To understand the structural origin of this improved performance, X-ray absorption spectroscopy (XAS) measurements were carried out on pristine and cycled MNF electrodes.