Influence of oxidation system on chemical delithiation of LiFePO₄

Chemical delithiation is widely used in literature to study the structure-property-performance relationship of olivine LiFePO₄ before/after lithium extraction, but the influence of oxidation system on the structure, morphology and electrochemical performance of the resulted Li₁FePO₄ (0 ≤x<1) is seldom noticed. In this paper, large particle LiFePO₄ is synthesized via a modified solid-state reaction method and is chemically delithiated by NO₂BF₄ in ace- tonitrile or by K₂S₂O₈ in an aqueous solution. Solvents are found highly involved in chemical delithiation of LiFePO₄, and significant 0-H groups are identified in FePO₄ derived by K₂S₂O₈. Moreover, proton insertion into the crystal lat¬tice of Li_xFePO₄ (0 ≤x≤1) is demonstrated possible in such highly oxidative aqueous solutions. The synergism of dispersion, dissolution, delithiation, and more importantly protonation induces serious structure and morphology change to so-obtained FePO₄ and makes the particle size much smaller. Consequently, the specific discharge capaci¬ty is dramatically decreased with increased charge transfer impedance, and the charge/discharge curves are trans¬formed to slope-like profiles. Although these protons (including residual water and hydroxyls) could be removed by low temperature annealing, the battery performance can't be fully recovered due to the irreversible structure and morphology change. On the contrary, there is no obvious proton incorporation during chemical delithiation of LiFe- PO₄ by NO₂BF₄ in organic solutions, which leads to FePO₄ with almost invariant structure, morphology and perfor¬mance comparing to pristine LiFePO₄.