Changes in the voltage profile of Li/${\mathrm{Li}}_{1+\mathit{x}}$${\mathrm{Mn}}_{2\mathrm{-}\mathit{x}}$${\mathrm{O}}_4$ cells as a function of x

${\mathrm{Li}}_{1+\mathit{x}}$ ${\mathrm{Mn}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{O}}_4$ is a very promising candidate as the cathode material in state-of-the-art Li-ion rechargeable batteries. The ability to retain the initial capacity of the electrochemical cell upon cycling depends on the amount of the excess Li, represented as x in ${\mathrm{Li}}_{1+\mathit{x}}$ ${\mathrm{Mn}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{O}}_4$. Thus it is important to measure the voltage profiles of Li/${\mathrm{Li}}_{1+\mathit{x}}$ ${\mathrm{Mn}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{O}}_4$ electrochemical cells carefully as a function of x. The twin peaks in the derivative curve, -dy/dV versus V, where y denotes the amount of intercalated Li, are found to be weakened with increasing x. With a simple lattice-gas model, we show that the two peaks in the derivative curve are consistent with order-disorder phase transitions of Li ions, and the weakening of the peaks with increasing x is due to the presence of intercalated Li atoms pinned to the excess Li atoms which are substituted for Mn in the host lattice. © 1996 The American Physical Society.