Reactivity of antimony oxides and MSb2O6 (M=Cu,Ni,Co), trirutile-type phases with metallic lithium

Isostrutural MSb2O6 (M=Cu,Ni,Co) trirutile-type phases were prepared by solid-state synthesis and their electrochemical activity towards lithium was investigated. NiSb2O6 and CoSb2O6 showed similar electrochemical behavior, with an uptake of about 18-19 Li per formula unit along the first reduction and only 6-7 Li reversibly removed upon subsequent cycling. This totally differs from the behavior of CuSb2O6 that was found to react with only about 7 Li during the first reduction, without any capacity recovered on subsequent charge. From X-ray diffraction data, both Cu and Co phases lead to an amorphous composite down to 0 V. From high-resolution transmission electron microscopy observations, reduced CoSb2O6 consists of 10-50 nm Co particles dispersed in a Li3Sb+Li2O matrix. For CuSb2O6, our results and observations enlightened a two-step reduction: First, the formation of Cu clusters through an electrochemically-driven exchange reaction leading to (Li2Sb2O6)-O-V, and then reduction of this matrix into 5 nm Sb domains dispersed in an insulating amorphous Li-rich Li-Sb-V-O matrix, preventing any further alloying reaction and any charge reaction. The complete charge irreversibility observed on Li/Sb2O5 half-cells confirmed this point, while the Li reduction of Sb2O3 emphasized a close similarity with the reactivity of the (Ni/Co)Sb2O6 phases, suggesting a first reduction step of Sb5+ into Sb3+, assuring conduction and subsequent cycling capacity. Despite the composite nature of the as-formed electrode and the very fine Sb particles, the reversibility of the alloying reaction was not found to be satisfactory for Li-ion cells.