Study Of5V Spinel LiNi_0.5Mn_1.5O₄and LiCoMnO₄for Lithium Ion Batteries

Operating voltage and the capacity of the cathode material are two major factors influencing the energy density of lithium-ion batteries. Spinel LiNi0.5Mn1.5O4is one of the most promising and attractive cathode materials due to various advantages, in particular its low cost, excellent rate capability, and relatively high working voltage (ca.4.7V). However, the theoretical capacity of LiNi0.5Mn1.5O4is only147mAh g-1when cycled between5.0V and3.0V. This relatively low capacity will hinder its future application in the high power field. However, theoretically, when cycled between5.0V and2.0V, two lithium ions can be inserted/extracted into/from the spinel structure, and its capacity can be as high as ca.294mAh g"1.In this work, Ti-substituted spinel LiNi0.5Mn1.5-xO4(0≤x≤0.6) samples were synthesized via a facial solid-state reaction method. The original LiNi0.5Mn1.5O4exhibited a high initial discharge capacity of239.1mAh g-1but over time showed severe capacity degradation, with a capacity retention of only44.5%after100cycles. Ti substitution decrease the initial discharge capacity, but an appropriate amount of Ti substitution clearly enhanced the cyclic stability of LiNi0.5Mn1.5-xTixO4in this wide voltage region. LiNi0.5Mn1.0Ti0.5O4sample showes an initial discharge capacity of206.5mAh g-1, with a relatively high capacity retention of57.6%after100cycles. When comparing the initial100-cycle discharge curves of LiNio.5Mn1.5O4and LiNi0.5Mn1.0Ti0.5O4, it can be seen that Ti substitution significantly improved the capacity retention at the2.7V voltage plateau which corresponds to the Mn4+/Mn3+redox reaction, while the decrease in capacity is mainly due to the shortening of the plateau at4.7V for Ni4+/Ni2+.XPS and XANES techniques were applied to examine the valence state of the transition metals in LiNi0.5Mn1.5O4and LiNi0.5Mn1.0Ti0.5O4samples. Both the XPS and XANES spectrums show that Ti substitution did not change the valence state of the other metals. The oxidation states of Ni and Ti were+2and+4respectively, while Mn showed a mixed oxidation state.Rietveld structural refinement results indicate that the transition metals (Ni, Mn and Ti) distributed randomly in the octahedral sites. Li+ions occupied the tetrahedral interstice, which is made up of MO6octahedrals. Since the ionic radius of Ti4+is much larger than that of Mn4+, Ti substitution enlarged the cell parameters and shrank the lithium ion diffusion channel, which may have resulted in a capacity decrease.Spinel LiCoxMn2-xO4(x=1,1.05,1.2) samples were prepared by a sol-gel method. The discharge capacity decreases with the increase of Co/Mn ratio, which may be due to a severe oxidation of the electrolyte. Among these compounds, LiCoMnO4delivered the highest capacity, but it suffered from severe capacity degradation during the charging/discharging process. A LiCoMnO4/Li4Ti5O12full-cells were fabricated and their electrochemical performance were investigated. Since the Li4Ti5O12anode presented an excellent cyclic stability, the design of the full-cell was based on the capacity limitations of Li4Ti5O12. In this work, the active material mass of the LiCoMnO4cathode was then selected to be roughly triple that of the Li4Ti5O12anode. The resulting full cell then showed a voltage plateau of ca.3.2V and a discharge capacity of115.9mAh g-1at a current density of170mA g-1, with only a decay of11.7%of the discharge capacity after100cycles.