Charge-discharge properties and reaction mechanism of cation-disordered rutile-type Li₁.₂MnFe₁.₂F₆.₈

Abstract

A carbon-coated Li₁.₂MnFe₁.₂F₆.₈ with a cation-disordered rutile structure is presented for the first time as a positive electrode material for lithium-ion batteries. For improved performance, the composite electrode is investigated at 90 °C using a thermally stable ionic liquid electrolyte. A combination of synchrotron X-ray diffraction, X-ray photoelectron spectroscopy and half-cell tests establish that the reversible extraction/insertion of lithium from/into Li₁.₂MnFe₁.₂F₆.₈ in the 2.5–4.5 V voltage range at 90 °C involves Fe(III)/Fe(II) and Mn(III)/Mn(II) redox reactions alongside partial conversion reactions between the rutile Li₁.₂MnFe₁.₂F₆.₈ and a mixture of LiF and rutile FeF₂. Deep discharge to 2.0 V involves a conversion reaction to LiF and metallic Fe and Mn accompanied by the partial preservation of the rutile structure.