Synthesis and electrochemical properties of Li[Li(1-2x)/3NixMn(2-x)/3]O2 as a positive materials for rechargeable lithium batteries

Abstract

The presently commercialized lithium-ion batteries use layer structured LiCoO2 cathodes. Because of the high cost and toxicity of cobalt, an intensive search for new cathode materials has been underway in recent years. The research for layered LiMnO2 with the same structure as LiCoO2 is being pursued, but the materials tend to transform to the more stable spinel phase during the eletrochemical cycling due to Jahn-Teller ions of Mn3+ Recently, some research groups have studied to stabilize layered structure by using solid solution between Li2MnO3 and LiMO2 (M = Cr, Ni, Co) such as Li[Li(1-2x)/3NixMn(2-x)/3]O2 and Li[Li(1-x)/3Co(Cr)xMn(2-2x)/3]O2. Li2MnO3 has a layered structure similar to LiCoO2, LiNiO2, and LiCrO2. In Li2MnO3 and LiMO2 solid solution, M is the redox-active species, while tetravalent manganese in Li1/3Mn2/3 clusters is electrochemically inactive. It has been reported that the electrochemical capacity is resulted from oxidation of Cr3+ to Cr6+, Ni2+ to Ni4+, and Co3+ to Co4+ in Li[Li(1-x)/3CrxMn(2-2x)/3]O2, Li[Li(1-2x)/3NixMn(2-x)/3]O2, and Li[Li(1-x)/3CoxMn(2-2x)/3]O2, respectively. In this study, a sol-gel method was employed to prepare layered Li[Li(1-2x)/3NixMn(2-x)/3]O2 (x = 0.41, 0.35, 0.275 and 0.2) powders using glycolic acid as a chelating agent. The Li[Li(1-2x)/3NixMn(2-x)/3]O2 (x = 0.2) electrode delivers the discharge capacities of about 200 mAh g-1 at 30 ℃ and 240 mAh g-1 at 55 ℃ in the voltage 2.0 ∼ 4.6 V and 2.4 ∼ 4.5 V, respectively, with excellent cycling behavior.