Dependence of Structural Defects in Li2MnO3 on Synthesis Temperature

Li2MnO3, an electrode material for Li ion batteries, belongs to the C2/m space group and is known to have a cubic-close packed (ABC...) layered structure, in which the transition-metal layer is supposed to have an ordered atomic arrangement with Li atoms at the 2b site and Mn atoms at the 4g site. However, recently, it has been reported that this compound usually does not exhibit such an ideal structure and instead contains a large number of structural defects, not only stacking faults but also mixing of Li and Mn atoms between the 2b and 4g sites. To elucidate the effect of such structural defects on the electrochemical behavior, we examined the crystal structure of Li2MnO3 synthesized at various temperatures by simultaneously analyzing the stacking faults and cation mixing using FAULTS, a Rietveld code. Our examination showed that the crystals consist of both disordered and ordered domains; the disordered domains contain a large number of stacking faults along the c axis and have considerable Li/Mn atomic mixing within the transition-metal layer, whereas the ordered domains have almost no defects. At low synthesis temperatures, the disordered domains are dominant. However, the ordered domains increase at the expense of the disordered domains above 770 degrees C and become dominant at higher temperatures. It is also found that the degree of cation mixing in the disordered domains remains almost constant irrespective of synthesis temperature. The crystalline defects such as stacking faults or Li/Mn cation mixing are expected to promote the formation of smooth Li percolation paths. The decreasing of the disordered domains leads to dramatically decreased capacity. This indicates that the observed capacities of Li2MnO3 can be determined by the relative amounts of the ordered/disordered domains in the structure.