Synthesis of Al-doped LiMn2O4spinels by mechanical alloying and rotary heating

Abstract

LiMn2O4 has been widely studied as the cathode material for Li-ion batteries because of its low cost, environmental merit and relatively high voltage [1–3]. However, it shows significant capacity fading and poor cycleability. A very effective way for improving the cycling performance of LiMn2O4 is to synthesize manganese-substituted LiMx Mn2−x O4 spinel phase by doping with divalent or trivalent ions (M = Al, Mg, Co, Ni, Fe, Ti, Zn and Cr, etc.) [4–8]. The conventional synthesis method of LiMn2O4 is direct solid-solid reaction of oxides and carbonate of manganese and either LiOH, or Li2CO3 and LiNO3 at high temperatures to enhance the diffusion process and obtain a well-ordered structure. Due to this high temperature, there are some disadvantages such as broader particle size distribution, longer reaction time or other unwanted phases. However, solid-state reaction technology is easy and simple, and is one of the most suitable techniques for industrialization. The obstacles to the solid-state reaction are mainly the higher synthesis temperature and longer high temperature reaction time, together with the difficulties in the effective controls of chemical composition and microstructures. In recent years great progress has been mode in developing new synthesis methods of LiMn2O4 [9–13]. One of the main approaches is mechanical alloying. The works of Kosova et al. [11–13] on LiMn2O4 showed that mechanical alloying significantly decreased the synthesis temperature and the resulting products exhibited tolerable electrochemical performances. Mechanical alloying can produce particle and grain sizes down to the nanoscale, together with structure defects. Energetic lattice defects, combined with short diffusion distances, are the driving forces for faster solid-state alloying and chemical reactions at low temperatures. The high temperature furnaces for solid-state synthesis, at present, are mainly periodical kilns or furnaces. The powders in these equipments are static. Therefore the temperature field within the powders is often nonuniform during heating or cooling, especially for mass production, which will result in non-uniform chemical composition and non-uniform particle size distribution in a batch of synthesized powders. On the other hand, these mechanically alloyed initial powders easily grow during the following high temperature treatment,