Spinel-type solid solutions involving Mn4+ and Ti4+:Crystal chemistry, magnetic and electrochemical properties

This study reports the structural and magnetic properties of spinel systems Li4Mn5-xTixO2 (4-5-12 series) and LiNi0.5Mn1.5-xTixO4 (LNMTO series), both based on Mn4+ substitution by Ti4+. Intermediate compositions covering the whole range of compositions (0 <= x <= 5 and 0 <= x <= 1.5, respectively) were prepared by solid state reaction. The 4-5-12 system forms a continuous spinet solid solution, whereas the spinel phase range in LNMTO stops before the end member LiNi0.5Ti1.5O4, which is multi-phased with a major hexagonal phase component. Cell parameters and (Mn,Ti)-O distances increase monotonically with titanium content in both series. In the LNMTO series, the end member LiNi0.5Mn1.5O4 is known to form a superstructure with Ni/Mn cation ordering. Neutron diffraction and Raman spectroscopy show that this order is lost when Ti is substituted, even at low level (x=0.15). The LNMTO crystal chemistry is also complicated by the presence of partial cation inversion, and the presence of a secondary rocksalt-type phase that modifies the spinel stoichiometry. Magnetic properties are characterized by a competition between ferromagnetic and antiferromagnetic interactions: no magnetic ordering is achieved, in agreement with B-site cation frustration and disorder. Electrochemical measurements show that the Ti3+/4+ and Mn3+/4+ redox couples behave independently in the 4-5-12 series, and that titanium decreases the high-potential electrochemical redox activity of LNMTO because of its blocking character for electron transfer to and from the nickel sites in the spinet structure. (C) 2010 Elsevier Ltd. All rights reserved.