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

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

Pyrophosphate Chemistry toward Safe Rechargeable Batteries

We demonstrate that pyrophosphate anion can result in metal pyrophosphate cathode materials with high thermal stabilities. High temperature behaviors for the delithiated states of Li₂FeP₂O₇ and Li₂MnP₂O₇ in the <i>P</i>2₁/<i>c</i> symmetry are studied. Above 540 °C, the singly delithiated structure LiFeP₂O₇ undergoes an irreversible phase transformation to the ground state polymorph with a symmetry of <i>P</i>2₁. Intermediate delithiated compounds Li_2‑<i>x</i>FeP₂O₇ (0 < <i>x</i> < 1) convert to a mixture of LiFeP₂O₇ in the <i>P</i>2₁ symmetry and Li₂FeP₂O₇ in the <i>P</i>2₁/<i>c</i> symmetry. No decomposition is observed for both the singly and partially delithiated compounds until 600 °C showing the high thermal stabilities of the compounds. Analysis of phase stabilities reveals that LiFeP₂O₇ (<i>P</i>2₁/<i>c</i>) is intrinsically more stable than FePO₄ (olivine) against reduction (high temperature). Similar high thermal stability is also observed for Li_1.4MnP₂O₇. It decomposes to Li₂MnP₂O₇, Mn₂P₂O₇, LiPO₃, and O₂ at 450 °C, much higher than the olivine counterpart MnPO₄. The high stability of these metal pyrophosphates is rationalized by the stability of the P₂O₇^4‑ anion