Electrochemical Properties of $LiMn_{1-x}M_{x}O_{2}$ (M=Ni, Al, Mg) as Cathode Materials in Lithium-Ion Cells

Layered $LiMn_{1-x}M_{x}O_{2}$ (M=Ni, Al, Mg) has been synthesized over a wide range of composition by ion-exchange from their sodium precursors and characterized. Both the lattice parameters (a and c) of the hexagonal unit cell increase with x for Ni and Mg substitutions but decrease for Al substitution. As reflected by the broad powder X-ray diffraction peaks, scanning electron micrographs show agglomeration of submicrometer-size particles. The discharge capacity of these cathode materials varies with the nature and concentration of the substituents. $LiMn_{0.95}Ni_{0.05}O_{2}$ exhibits a specific capacity of about 220 mAh $g^{-1}$ during the initial cycling, which decays gradually to about 150 mAh $g^{-1}$ after 30 charge-discharge cycles. $LiMn_{0.95}Al_{0.05}O_{2}$ and $LiMn_{0.95}Mg_{0.05}O_{2}$ yield capacity values of 180 and 150 mAh $g^{-1}$, respectively. Cyclic voltammetry, galvanostatic intermittent titration, and ac impedance data corroborate the observed discharge capacity behavior of these cathode materials