Redox Nanocomposite Cathode Materials for Lithium Rechargeable Batteries

Abstract

Only a small number of Li-containing cathode materials groups are considered for practical use in Li-ion battery systems. The possible candidates of cathode material were limited to the crystals that contain both redox-active element and lithium ion in the open framework. This concept has been conventionally considered as a standard for searching the cathode materials. But, it could be constraints to restrict the choices of materials for cathode in Li-ion battery systems. To expand the sight for seeking new positive electrode material, we suggested a novel strategy to use various kinds of Li-free transition metal ionic compounds (MX, M = transition metal, X = anion or polyanion group) as a positive electrode material by blending with a Li ionic compound (LiY, Y = anion or polyanion group) in nanoscale.[1] MX and LiY provided a redox couple and lithium ion supply for an electrochemical reaction. This concept is unconventional with general system of electrode material (Li ions and transition metal ions are in the same crystal system). In this case, transition metal ion and Li ion in the nanocomposite do not exist in the same crystal system and spatially separated as a mixture of MX and LiY.

In this paper, we will introduce and discuss about the transition metal oxide system as a redox couple among the infinite possible combinations of lithium ionic compounds and metal ionic compounds. Transition metal oxides such as FeO, MnO were considered as promising anode materials due to the earth abundance and large capacity from conversion reaction.[2] Although they have redox potential below 1 V as anode material,[3][4] we will show that this transition metal oxides can be applied to the cathode materials (3 V-class) by making nanocomposite with lithium ionic compound. To better understand the mechanisms during electrochemical cycling we have performed XPS, XANES/EXAFS analysis, allowing us to study the local environment change of Fe or Mn during the charge and discharge reaction. FeO and MnO showed different aspects during the electrochemical reaction in terms of electrochemical activity and local environment change. We will discuss about the reaction mechanism in detail.