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

Lithium chromium pyrophosphate (LiCrP 2 O 7 ) and carbon‐coated LiCrP 2 O 7 (LiCrP 2 O 7 /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. LiCrP 2 O 7 powder has a conductivity of ∼ 10 −8  S cm −1 , ∼ 10 4 times smaller than LiCrP 2 O 7 /C (∼ 10 −4  S cm −1 ). LiCrP 2 O 7 /C is electrochemically active, mainly between 1.8 and 2.2 V versus Li + /Li (Cr 3+ /Cr 2+ redox couple), whereas LiCrP 2 O 7 has limited electrochemical activity. LiCrP 2 O 7 /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.