Electrochemical lithiation of the Y5-xDyxGe3 and Y5-xSmxGe3 phases

The phases Y 5- x Dy x Ge 3 (Y 2.4 Dy 2.6 Ge 3 ) and Y 5- x Sm x Ge 3 (Y 3.6 Sm 1.4 Ge 3 ) with Mn 5 Si 3 -type of structure were investigated by X-ray powder diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy as anode materials for Li-ion battery prototype. The ingot samples (Y 30 Dy 32.5 Ge 37.5 and Y 40 Sm 22.5 Ge 37.5 ) of alloys were synthesized by arc-melting of pure components under purified argon atmosphere with the further annealing of the alloys at 600 ºC for 720 h in evacuated silica tubes. Electrochemical lithiation of the phases were carried out in the two-electrode “Swagelok-cell” prototype of the batteries. As cathode (during of discharging) we used powder of rhombohedral LiCoO 2 (structure type α-NaFeO 2 ). An electrolyte for battery consisted of 1 MLi[PF 6 ] solution in themixture of aprotic solvents. The process of lithiation consists of two stages: inclusion of Li-atoms in the vacant octahedral voids (2 b ) of structure and further partial substitution of Sn-atoms by lithium. After electrochemical insertion of Li we obtained the solid solutions Li x + y Y 2.4 Dy 2.6 Ge 3- y and Li x + y Y 3.6 Sm 1.4 Ge 3- y with increased unit cell parameters: a = 8.4430(3)→8.4322(3) Å, c = 6.3418(3)→6.3450(3) Å, V = 391.50→391.71 Å 3 (for Dy-containing electrode) and a = 8.4586(1)→8.4690(6) Å, c = 6.3405(2)→6.351(1) Å, V = 392.99→395 Å 3 (for Sm-containing electrode). During of the Li-insertion in the octahedral voids, the Mn 5 Si 3 -type structure is transformed into the superstructure of Hf 5 CuSn 3 -type. Electrodes based on the Y 2.4 Dy 2.6 Ge 3 and Y 3.6 Sm 1.4 Ge 3 during discharge showed 0.24 and 0.22 Li per formula unit, accordingly. The value of electrochemically active lithium content correlates well with the results of electrochemical measurements of Sn-containing phases with Mn 5 Si 3 -type structure. Еlectrochemical processes lead to the expansion of the profile of diffraction peaks on X-ray powder patterns, grinding and amorphization of grains and the change the morphology of the surface. The composition of the grains after lithiation practically did not change, with the exception of a slight decrease in the germanium content up to 1.9–2.6 at. %, that explains the processes of partial replacement of germanium atoms by lithium. Keywords: intermetallic compound, solid solutions, electrochemical lithiation, Li-ion batteries.