Spinel LiNi_xMn_2-xO₄ as cathode material for aqueous rechargeable lithium batteries

Ni-doped spinel LiNi_xMn_2-xO₄ (x = 0, 0.05, 0.10) samples were prepared by a sol-gel method. Structure and morphology of the samples were characterized by X-ray diffraction, scanning electron microscopy, Brunnauer-Emmet-Teller method and inductively coupled plasma atomic absorption spectrometry. The electrochemical behavior as a cathode material (positive mass) for aqueous rechargeable lithium batteries (ARLBs) was investigated by cyclic voltammetry, electrochemical impedance spectroscopy, capacity measurements and cycling tests. The results show that the LiNi _0.1Mn_1.9O₄ electrode presents the best rate and cycling performance but low reversible capacity. In contrast, the LiNi _0.05Mn_1.95O₄ electrode shows a higher reversible capacity and relatively good cycling behavior. At a current density of 150 mA g^-1, LiNi_0.05Mn_1.95O₄ delivers a reversible capacity of 102 mA h g^-1. At the relative high current densities of 1500 and 3000 mA g^-1, the LiNi _0.05Mn_1.95O₄ electrode still delivers reversible capacities of 95.0 and 88.7 mA h g^-1, respectively. The Ni-doped samples show excellent cycling life in 0.5 mol L^-1 Li ₂SO₄ aqueous solution. The capacity retention ratios for LiNi_0.05Mn_1.95O₄ and LiNi_0.10Mn _1.90O₄ after 800 cycles at a current density of 1500 mA g^-1 are 79.4% and 91.1%, respectively, much higher than that for the undoped LiMn₂O₄ at only 37.8%.