Electrochemical and structural comparison of zinc-cobalt doped spinels with other doped lithium manganese spinels

Abstract

Abstract. Doping of lithium manganese spinels by zinc and cobalt of the type (LilxZnx)(Mn2-2x Co2x)O4 may stabilize the delithiated spinels and may offer some indications with respect to the validity of capacity fade models. There are structural chemical arguments for this cation distribution. These and other doped lithium manganese spinels were prepared by heat treatment (in the range between 500 and 800 ~ for 12 h) of solution precipitated precursors. Samples were characterised structurally and electrochemically by XRD and galvanostatic cycling. Extended cycling, storage in the charged state and storage in the discharged state were investigated.Pure phases of Zn-Co doped samples were obtained only for quenching the fired samples. Otherwise separation into a tetragonal spinel and a cubic spinel ococcurred. XRD results prove the occupation of telrahedral coordinated cation positions by zinc ions, in contradiction to the results of other authors. XRD profiles show an anisotropic line broadening, which is attributed to an anisotropic microstrain, maybe induced by a non-cooperative Jahn-Teller distortion. Capacity retention was better for extended cycling and worse for both kinds of storage, compared to a purely cobalt doped spinel. Therefore, thermodynamic stabilisation of the delithiated spinels could not be confirmed.

In Situ Formed Core–Shell LiZnxMn2–xO4@ZnMn2O4 as Cathode for Li-Ion Batteries

“…Considering the above-mentioned TM cation occupancy at Li tetrahedral sites at the shell (Figure b–d), the in situ formed structure at the surface of the Zn-doped samples should be spinel ZnMn 2 O 4 with some Zn atoms occupying Li sites and other Zn 2+ ions diffused into 16d Mn sites in bulk to form the LiZn x Mn 2– x O 4 phase, forming core–shell LiZn x Mn 2– x O 4 @ZnMn 2 O 4 . It is reported that the ionic radius of Zn 2+ in the tetrahedral coordination in the oxide spinel (0.60 Å) is quite similar to that of Li (0.59 Å), which further supports the feasibility of the Zn 2+ ion occupancy at tetrahedral Li sites to form the ZnMn 2 O 4 phase at the surface region.…”

“…In contrast, the gradients of O and Mn intensities increase from the surface to the center region. Considering the above-mentioned TM cation occupancy at Li tetrahedral sites at the shell (Figure It is reported that the ionic radius of Zn 2+ in the tetrahedral coordination in the oxide spinel (0.60 Å) is quite similar to that of Li (0.59 Å), 32 which further supports the feasibility of the Zn 2+ ion occupancy at tetrahedral Li sites to form the ZnMn 2 O 4 phase at the surface region.…”

“…Lee et al further reported, using 6 Li solid-state nuclear magnetic resonance (NMR), that Li ions even occupy the octahedral sites in the LiZn 0.5 Mn 1.5 O 4 material. However, most studies were based on diffraction techniques, , which exhibit poor spatial resolution. The detailed site occupations of Zn ions in the spinel at the atomic level remain poorly understood, although it is most important for exploring the fundamental interplay between the microstructural configuration and the electrochemical performance improvement of Zn-doped spinel LiMn 2 O 4 .…”