Crystal structure, microstructure and electrochemical properties of hydrothermally synthesised LiMn2O4

Abstract

The hydrothermal synthesis method offers an environmentally benign way of synthesizing Li-ion battery materials with strong control of particle size and morphology, and thereby also the electrochemical performance. Here we present an in depth investigation of the crystal structure, microstructure and electrochemical properties of hydrothermally synthesized LiMn₂O₄, which is a widely used cathode material. A range of samples were synthesized by a simple, single-step hydrothermal route, and the products were characterized by elaborate Rietveld refinement of powder X-ray diffraction data, electron microscopy and electrochemical analysis. A distinct bimodal crystallite size distribution of LiMn₂O₄ was formed together with a Mn₃O₄ impurity. At high LiOH concentration the layered Li_xMn_yO₂ phase was formed. The crystallite sizes and impurity weight fractions were found to be highly synthesis dependent, and the amount of spinel impurity phase was found to correlate with deterioration of the electrochemical performance. The Mn₃O₄ phase can be very difficult to quantify in standard powder X-ray diffraction and due to peak overlap and X-ray fluorescence impurity levels of more than 10% are easily hidden. Furthermore, the spinel LiMn₂O₄ phase can easily be mistaken for the layered Li_xMn_yO₂ phase. The present study therefore highlights the importance of thorough structural characterization in studies of battery materials.