Electrochemical properties of LiMn_1-xM_xO₂ (M=Ni, Al, Mg) as cathode materials in lithium-ion cells

Abstract

Layered LiMn_1-xM_xO₂ (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_0.95Ni_0.05O₂ 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_0.95Al_0.05O₂ and LiMn_0.95Mg_0.05O₂ 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.