LiCaFeF6: A zero-strain cathode material for use in Li-ion batteries.

A new zero-strain LiCaFeF 6 cathode material for reversible insertion and extraction of lithium ions is presented. LiCaFeF 6 is synthesized by a solid-state reaction and processed to a conductive electrode composite via high-energy ball-milling. In the first cycle, a discharge capacity of 112 mAh g −1 is achieved in the voltage range from 2.0 V to 4.5 V. The electrochemically active redox couple is Fe 3+ /Fe 2+ as confirmed by Mössbauer spectroscopy and X-ray absorption spectroscopy. The compound has a trigonal colquiriite-type crystal structure (space group P 3 ¯ 1 c ). By means of in situ and ex situ XRD as well as X-ray absorption fine structure spectroscopy a reversible response to Li uptake/release is found. For an uptake of 0.8 mol Li per formula unit only minimal changes occur in the lattice parameters causing a total change in unit cell volume of less than 0.5%. The spatial distribution of cations in the crystal structure as well as the linkage between their corresponding fluorine octahedra is responsible for this very small structural response. With its zero-strain behaviour this material is expected to exhibit only negligible mechanical degradation. It may be used as a cathode material in future lithium-ion batteries with strongly improved safety and cycle life. [ABSTRACT FROM AUTHOR]