Fe³⁺/Fe²⁺ Redox Couple Approaching 4 V in Li_2–<i>x</i>(Fe_1–<i>y</i>Mn_<i>y</i>)P₂O₇ Pyrophosphate Cathodes

Li-metal pyrophosphates have been recently reported as novel polyanionic cathode materials with competent electrochemical properties. The current study presents a detailed analysis of inherent electrochemical properties of mixed-metal pyrophosphates, Li₂(Fe_1–<i>y</i>Mn_<i>y</i>)­P₂O₇, synthesized by an optimized solid-state route. They form a complete solid solution assuming a monoclinic framework with space group <i>P</i>2₁/<i>c</i>. The electrochemical analysis of these single-phase pyrophosphates shows absence of activity associated with Mn, where near-theoretical redox activity associated with Fe metal center was realized around 3.5 V. We noticed a closer look revealed the gradual substitution of Mn into parent Li₂FeP₂O₇ phase triggered a splitting of Fe³⁺/Fe²⁺ redox peak and partial upshifting in Fe³⁺/Fe²⁺ redox potentials nearing 4.0 V. Introduction of Mn into the pyrophosphate structure may stabilize the two distinct Fe³⁺/Fe²⁺ redox reactions by Fe ions in octahedral and trigonal-bipyramidal sites. Increase of the Gibb’s free energy at charged state by introducing Li⁺–Fe³⁺ and/or Li vacancy–Mn²⁺ pairs can be the root cause behind redox upshift. The underlying electrochemical behavior has been examined to assess these mixed-metal pyrophosphates for usage in Li-ion batteries

General Observation of Fe³⁺/Fe²⁺ Redox Couple Close to 4 V in Partially Substituted Li₂FeP₂O₇ Pyrophosphate Solid-Solution Cathodes

Exploring the newly unveiled Li₂<i>M</i>P₂O₇ pyrophosphate cathode materials for lithium-ion batteries, the current study reports the general observation of an unusually high Fe³⁺/Fe²⁺ redox potential close to 4.0 V vs Li/Li⁺ in mixed-metal Li₂<i>M</i>_<i>x</i>Fe_1–<i>x</i>P₂O₇ (<i>M</i> = Mn, Co, Mg) phases with a monoclinic structure (space group <i>P</i>2₁/<i>c</i>). Such a high voltage Fe³⁺/Fe²⁺ 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 SO₄^2– or F^–. 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 Fe³⁺/Fe²⁺ couple was stabilized by all dopants, either by larger Mn²⁺ or smaller Co²⁺ and Mg²⁺ ions, where Mg²⁺ 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 Fe³⁺/Fe²⁺ 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