Electrochemical characterization of monoclinic and orthorhombic Li3CrF6 as positive electrodes in lithium-ion batteries synthesized by a sol-gel process with environmentally benign chemicals

Lithium transition metal fluorides (Li3MF6; M = Fe, V) with cryolite structure are investigated as positive electrode materials for lithium-ion batteries. A novel sol-gel process with trifluoroacetic acid as fluorine source was used to synthesize monoclinic and orthorhombic Li3CrF6. A ball milling process with Li3CrF6, binder, and conductive agent was applied to form a Li3CrF6 composite, which was electrochemically characterized against lithium metal for the first time. The electrochemical properties of two different modifications are quite similar, with a reversible specific capacity of 111 mAhg−1 for monoclinic Li3CrF6 and 106 mAhg−1 for orthorhombic Li3CrF6 (1 eq. Li ≙ 143 mAhg−1). The electrochemically active redox couple CrIII/CrII was confirmed by X-ray photoelectron spectroscopy.