General
Observation of Fe<sup>3+</sup>/Fe<sup>2+</sup> Redox Couple Close
to 4 V in Partially Substituted Li<sub>2</sub>FeP<sub>2</sub>O<sub>7</sub> Pyrophosphate Solid-Solution Cathodes
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Abstract
Exploring the newly unveiled Li<sub>2</sub><i>M</i>P<sub>2</sub>O<sub>7</sub> pyrophosphate
cathode materials for lithium-ion
batteries, the current study reports the general observation of an
unusually high Fe<sup>3+</sup>/Fe<sup>2+</sup> redox potential close
to 4.0 V vs Li/Li<sup>+</sup> in mixed-metal Li<sub>2</sub><i>M</i><sub><i>x</i></sub>Fe<sub>1–<i>x</i></sub>P<sub>2</sub>O<sub>7</sub> (<i>M</i> = Mn, Co, Mg)
phases with a monoclinic structure (space group <i>P</i>2<sub>1</sub>/<i>c</i>). Such a high voltage Fe<sup>3+</sup>/Fe<sup>2+</sup> operation over 3.5 V has long been believed to be
possible only by the existence of much more electronegative but hygroscopic
anions such as SO<sub>4</sub><sup>2–</sup> or F<sup>–</sup>. Thereby, this is the first universal confirmation of >3.5 V
operation
by stable, simple phosphate material. High voltage (close to 4 V)
operation of the Fe<sup>3+</sup>/Fe<sup>2+</sup> couple was stabilized
by all dopants, either by larger Mn<sup>2+</sup> or smaller Co<sup>2+</sup> and Mg<sup>2+</sup> ions, where Mg<sup>2+</sup> is redox
inactive, revealing that the high voltage is induced neither by reduced
Fe–O bond covalency nor by contamination by the redox couple
of other transition metals. The cause of higher Fe<sup>3+</sup>/Fe<sup>2+</sup> redox potential is argued and rooted in the stabilized edge-sharing
local structural arrangement and the associated larger Gibbs free
energy in the charged state