In many studies high capacity cathode materials are analyzed. One of the most promising materials is a Li-rich layered oxide. Other studies use high voltage cathode materials (e.g. LiMxMn2-xO4, with M = e.g. Ni, Co) in order to increase the operational voltage of the cell. Cells with high voltage cathodes suffer from unwanted side reactions with the electrolyte. This is because the operational voltage is higher than the stability window (ca. 1.0 – 4.3 V vs. Li+/Li0 in the presence of transition metal ions like Ni4+ and Mn4+) of current organic electrolytes. The reactions consume active lithium and a severe capacity fade occurs. On the other side, the increase of energy of high capacity cathode materials is limited due to the relatively low voltages of ca. 3.5 V vs. Li+/Li0 of these materials and the need for balancing of anode capacity.
In principle the high voltage spinel (LiNi0.5Mn1.5O4) offers the ability to increase the voltage and the capacity compared with other cathode materials. This can be reached when a metallic lithium anode is used. The spinel lattice can host a second lithium and therefore the theoretical capacity is increased from 147 to 294 mAh g-1. The increase in capacity during cycling between 2.0 and 5.0 V and the high operational voltage lead to a theoretical specific energy of more than 1000 Wh kg-1 based on the active materials. During intercalation of the second lithium, the spinel undergoes a phase transition from cubic to tetragonal because of a cooperative Jahn-Teller-distortion of Mn3+in the low voltage area. This is associated with material degradation and capacity loss.
Here we use different LiNi0.5Mn1.5O4-spinels doped with transition metals and treated with different temperature programs in order to increase the cycling stability. It is found that the stability and the initial capacities are directly dependent on the treatment and doping elements. At a rate of C/2 a capacity of 190 mAh g-1 is achieved with 92 % of the initial capacity retained after 100 cycles. The difference in the discharge behavior of the materials is analyzed based on electrochemical data, particle morphology and in-situ XRD analysis. A model, which can explain the different behaviors is introduced and will be discussed.