New developments of LiNi_o.₅Mn_0.5O ₂ electrodes comprising micro- and nano-metric particles: Synthesis, electrochemical behavior, and surface chemistry in Li-ion cells

The aim of this work was a comparative study of the electrochemical behavior of LiNi_0.5Mn_0.5O₂ electrodes prepared from micro- or nano-particles of the active material, in Li-ion cells. Nano-particles of the layered LiNi_0.5Mn_0.5O₂ oxide were synthesized by a novel, modified self-combustion reaction, and micron sized particles were prepared via a solid-state reaction method. It was established that capacities up to 180-200 mAh/g could be reached during prolonged cycling of these electrodes even at elevated temperatures (60°C) in LiPF₆/alkyl-carbonates solutions. We have found that nano-LiNi_0.5Mn_0.5O₂ electrodes demonstrate a faster kinetics and more reversible electrochemical behavior, compared to their micro-sized counterparts. It was proposed that a stable electrochemical behavior of nano-LiNi_0.5Mn_0.5O₂ electrodes under conditions of high temperature cycling/aging and high oxidation potentials (4.5-5 V), may be related, in part, to stabilization of the electrode/solution interphase by a unique surface chemistry. Surface films thus developed on the nano- and micro-LiNi_0.5Mn_0.5O₂ particles contain polycarbonates, LiF, Ni and Mn oxides and fluorides, as established by FTIR and XPS techniques. An important finding is nearly constant impedance of nano- and micro-LiNi_0.5Mn_0.5O₂ electrodes during aging and cycling after these electrodes being stabilized at elevated temperatures.