Electrochemical Redox Mechanism in 3.5 V Li_2‑<i>x</i>FeP₂O₇ (0 ≤ <i>x</i> ≤ 1) Pyrophosphate Cathode

Abstract

Li₂FeP₂O₇ pyrophosphate is the latest phosphate-based polyanionic cathode material operating at 3.5 V (vs Li+/Li). Capable of two-dimensional Li⁺-ion diffusion, the pyrophosphate has a complex three-dimensional crystal structure, rich in Li–Fe antisite defects. The electrochemical (de)­lithiation of pristine Li₂FeP₂O₇ involves permanent structural rearrangement, as reflected by the voltage drop between the first and subsequent charging segments. The current article presents the structural analysis of the electrochemical redox mechanism of Li₂FeP₂O₇ cathode coupling <i>in situ</i> and <i>ex-situ</i> structural characterization. Contrary to previous reports, it involves a single-phase redox reaction during (de)­lithiation cycles involving a minimal <2% volume expansion. Further, it forms a rare example of cathode showing positive expansion upon delithiation similar to LiCoO₂. The mechanism of single-phase (de)­lithiation and related (ir)­reversible structural arrangement is elucidated