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), ~ 10(4) 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 (Cr(3+)/Cr(2+) 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.