  We have studied the electronic structure of
Li$_{1+x}$[Mn$_{0.5}$Ni$_{0.5}$]$_{1-x}$O$_2$ ($x$ = 0.00 and 0.05), one of the
promising cathode materials for Li ion battery, by means of x-ray photoemission
and absorption spectroscopy. The results show that the valences of Mn and Ni
are basically 4+ and 2+, respectively. However, the Mn$^{3+}$ component in the
$x$ = 0.00 sample gradually increases with the bulk sensitivity of the
experiment, indicating that the Jahn-Teller active Mn$^{3+}$ ions are
introduced in the bulk due to the site exchange between Li and Ni. The
Mn$^{3+}$ component gets negligibly small in the $x$ = 0.05 sample, which
indicates that the excess Li suppresses the site exchange and removes the
Jahn-Teller active Mn$^{3+}$.
