(Invited) Chimie Douce for Variety of Layered Materials

Abstract

This talk is dedicated to Prof. Claude Delmas and three of his magnificent study fields, namely, those on “Chimie Douce” (soft chemistry) [1], LiNiO2 [2] and Ni(OH)2 derivatives [3].Though LiNiO2 has been extensively studied as a powerful substitute for LiCoO2, its use as the source of Chimie Douce has been limited. This study shows possibilities to use LiNiO2 for obtaining metastable phases that are hardly obtained by high-temperature calcination. The NiO2 templet for Chimie Douce was obtained by the disproportionation reaction of LiNiO2 using proton as the catalyst [4,5]. Alkaline ion insertion to the NiO2 templet was employed using OH- as the reductant [6,7]. Low temperature heating is also effective for obtaining new polymorphs.The NiO2 templet has the nickel oxidation state of 3.8±0.1 and has the CdCl2 structure (O3) unless the Cdl2 structure (O1) is formed by using strong acids [8]. The Pourbaix diagram suggests that the NiO2 templet is stable only in the low pH regions and can be reduced in other regions. Treating NiO2 with LiOH leads to the formation of Li0.5NiO2 that is essentially the same as that obtained by electrochemical reactions. On the other hand, NaOH and KOH treatments typically produce M0.3NiO2•0.5H2O (P3), which corresponds to highly crystalline γ-NiOOH. The significant increase of the interlayer distance (Fig. 1) is caused by the water uptake on the Na+/K+ insertion. It is noteworthy that strongly solvated Li+ is inserted with desolvation while weakly solvated Na+/K+ are inserted as solvated.The P3 phases as rechargeable electrodes were inactive in non-aqueous lithium cells due to water decomposition on discharging, but were active in aqueous KOH electrolytes. Operando XRD analysis indicated that the cycle performance was better than that of the hydrated nickel hydroxide obtained by the conventional co-precipitation method, though the strong XRD peaks of the P3 phase weakened during discharging [9]. The longer interlayer distance of the discharged product suggests further water uptake on discharging. The use of Ni4+/ Ni3+/ Ni2+ redox couples is attractive for high energy aqueous rechargeable cells.In addition, we propose new proton insertion chemistry as a part of Chimie Douce application [10], which will be detailed in the presentation.