Cyclic voltammetry studies on 4 V and 5 V plateaus of non-stoichiometric spinel Li_1+xMn_2-yO₄

Li_1+xMn_2-yO₄ spinels with various Li/Mn ratios were synthesized by a solid-state reaction. By X-ray diffraction analysis, Li₂MnO₃ was detected as a second phase with increasing the Li/Mn ratio; and the role of Li₂MnO₃ in Li_1+xMn_2-yO₄ spinel was discussed. A slow scanning cyclic voltammetry (CV) at the rate of 0.1 mV/s was adopted to characterize the evolutions of 4 V and 5 V plateaus of Li_1+xMn_2-yO₄ spinels. An additional Li⁺ insertion in 4 V region was observed in both Li-lack and Li-rich spinels at 3.95 V, which is different from the general Li⁺ insertion with weak Li-Li interaction and strong Li-Li interaction; and this plateau disappeared in the subsequent cycles. The 4.4 V/3.8 V plateaus correspondent to Li⁺ insertion and extraction of Li₂MnO₃ were discussed, and these plateaus have a high reversibility with cycling. The 5 V plateau was found only in the Li-rich samples, and this plateau has a tendency to emerge at higher voltage region with increasing Li/Mn ratio.