Intrinsic instability of Fe4+ in electrochemically oxidized ramsdellite and orthorhombic Li1–xHxFeO2

Abstract

The cathodic materials r-Li _0.8 H _0.2 FeO ₂ (ramsdellite-type) and o-Li _0.7 H _0.3 FeO ₂ (orthorhombic form) were synthesized in an autoclave by reaction of iron oxohydroxides with lithium ethoxide in anhydrous ethanol. The compositions of the phases were determined both by means of elemental analyses and density measurements. The crystal structures of r-Li _0.8 H _0.2 FeO ₂ and o-Li _0.7 H _0.3 FeO ₂ were determined by the Rietveld method. The powders were made of homogeneously distributed submicronic needles. Electrochemical tests of the Li _1–x H _x FeO ₂ /Li systems were carried out along with X-ray diffraction and Mössbauer spectroscopy. The results from all the physical techniques employed taken together showed the instability of Fe ⁴⁺ formed during the recharge of the Li _1–x H _x FeO ₂ materials, forbidding cycling on the Fe ⁴⁺ /Fe ³⁺ electrochemical couple.