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

Doping of lithium manganese spinels by zinc and cobalt of the type (Li_1xZn_x)(Mn_2–2x Co_2x)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 °C 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 occurred. XRD results prove the occupation of tetrahedral 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. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998