Valence band of ${\mathrm{LiNi}}_{\mathit{x}}$${\mathrm{Mn}}_{2\mathrm{-}\mathit{x}}$${\mathrm{O}}_4$ and its effects on the voltage profiles of ${\mathrm{LiNi}}_{\mathit{x}}$${\mathrm{Mn}}_{2\mathrm{-}\mathit{x}}$${\mathrm{O}}_4$/Li electrochemical cells

A high-voltage plateau at 4.7 V in ${\mathrm{LiNi}}_{\mathit{x}}$ ${\mathrm{Mn}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{O}}_4$/Li electrochemical cells appears with the introduction of Ni into the ${\mathrm{LiMn}}_2$ ${\mathrm{O}}_4$ spinel. The capacity of the 4.7-V plateau increases linearly at the expense of the 4.1-V plateau as x increases. Using ultraviolet photoelectron spectroscopy, we have studied the top of the valence band of ${\mathrm{LiNi}}_{\mathit{x}}$ ${\mathrm{Mn}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{O}}_4$ for a series of samples with 0.0⩽x⩽0.5. A component attributed to Ni 3d electrons was found at about 0.5-eV higher binding energy than the Mn 3d ${\mathit{e}}_{\mathit{g}}$ electrons. We propose that the increased voltage of the 4.7-V plateau is due to the increased energy required to remove electrons from Ni 3d levels compared to Mn 3d ${\mathit{e}}_{\mathit{g}}$ levels. © 1996 The American Physical Society.