Study on the thermal stabilities and their electrochemical properties of $LiNi_{1-x-y}Co_xTi_yO_2$

Abstract

The lithium nickelate is the possible candidate, which can replace the commercially available lithium cobaltate, and have a greater discharging capacity with the problems like the lithium deficiency during synthesis and thermal instability at charged state. These problems hinder the commercialization of lithium nickelate as the positive electrode for the lithium ion battery. To solve these matters, many researches have been done in the fields like oxygen flowing environment during the synthesis and the substitution in lithium or nickel site. However, more importantly, the strong nickel reducibility, which is originated from the electron affinity of nickel, is the key role in both perspectives. During the synthesis of lithium nickelate, the lithium deficiency is induced by the nickel reduction, and for the charging stage, the reduction of nickel lead to the thermal decomposition to rock salt structure at above ambient temperature. Therefore, in this study, to limit the nickel reducibility, there are two ways of approaches. The one is involving with the $Ni^{3+}$ precursors. Generally, the oxidation state of the available nickel precursor for the many different synthesis routes is $2^+$. There’s no commercially available $Ni^{3+}$ precursor because of instability and hard to be synthesized. If the lithium nickelate is prepared by using $Ni^{2+}$ precursor with heavy oxidation scheme, the nickel reducibility to $2^+$ state cannot be sustained. Then, the more stable $Ni^{3+}$ precursor is synthesized in this study, and used in several synthesis routes. For the proper $Ni^{3+}$ precursor, β- NiOOH is chosen, because that the structure is same as lithium nickelate, $α-NaFeO_2$, and great tendency to interchange ion between hydrogen and alkali earth metals, i.e. lithium. The β-NiOOH is prepared from $Ni(OH)_2$, which is one of the $Ni^{2+}$ precursor, at 125℃ for 24 hours with 300ml/min of flowing oxygen. The pre-oxidized powder is determined as desirable phase, NiOOH...