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−xTixO12 (“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 spinel 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 spinel structure.