Preparation and Characterization of <em>x</em>Li<sub>2</sub>MnO<sub>3</sub>&middot;(1-<em>x</em>)Li[Ni<sub>1/3</sub>Mn<sub>1/3</sub>Co<sub>1/3</sub>]O<sub>2</sub> Cathode Materials for Lithium-Ion Batteries