Lithium chromium pyrophosphate as an insertion material for Li-ion batteries

Lithium chromium pyrophosphate (LiCrP₂O₇) and carbon-coated LiCrP₂O₇ (LiCrP₂O₇/C) were synthesized by solid-state and sol–gel routes, respectively. The materials were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and conductivity measurements. LiCrP₂O₇ powder has a conductivity of ∼ 10⁻⁸ S cm⁻¹, ∼ 10⁴ times smaller than LiCrP₂O₇/C (∼ 10⁻⁴ S cm⁻¹). LiCrP₂O₇/C is electrochemically active, mainly between 1.8 and 2.2 V versus Li⁺/Li (Cr³⁺/Cr²⁺ redox couple), whereas LiCrP₂O₇ has limited electrochemical activity. LiCrP₂O₇/C delivers a reversible specific charge up to ∼ 105 mAh g⁻¹ after 100 cycles, close to the theoretical limit of 115 mAh g⁻¹. Operando XRD experiments show slight peak shifts between 2.2 and 4.8 V versus Li⁺/Li, and a reversible amorphization between 1.8 and 2.2 V versus Li⁺/Li, suggesting an insertion reaction mechanism.