Electrochemical delithiation of the binary LiAl, Li3Al2, Li9Al4 and boron–doped phases

Abstract

The process of electrochemical delithiation of LiAl (structure type (ST) NaTl, space group (SG) Fd m ), Li 3 Al 2 (own ST, SG R m ), Li 9 Al 4 (own ST, SG C 2/ m ) intermetallic compounds and doped by boron LiAl 1- y B y , Li 3 Al 2- y B y , Li 9 Al 4- y B y phases was investigated by X-ray powder diffraction and scanning electron microscopy. The samples were synthesized by arc melting of pressed pellets of pure components under an argon atmosphere and further annealing of the alloys at 200 ºC for 1 month in sealed evacuated silica tubes without quenching. Electrochemical delithiation of the phases, which were used as anode materials, was carried out in the two-electrode prototype of the battery “Swagelok-cell”. LiCoO 2 (ST NaFeO 2 ) was used as cathode material. The anode and cathode materials were separated by pressed cellulose to avoid contact between them. An electrolyte for batteries consisted of 1 M Li[PF 6 ] solution and a mixture of aprotic solvents ethylene carbonate and dimethyl carbonate (1:1 vol. ratio). X-ray phase analysis was carried out on powder data obtained on automatic diffractometer DRON-2.0M (Fe K α -radiation). Investigated alloys contained the expected phases and small amount of other phases. Solid solutions of the substitution had homogeneity ranges about 5 at. % of boron and were characterized by reduced unit cell parameters. All observed binary and ternary phases demonstrated electrochemical ability. The unit cell parameters of the binary and ternary phases reduced after the delithiation. The lithium mobility of the electrodes on the basis of the LiAl, Li 3 Al 2 , and Li 9 Al 4 alloys was 0.16 Li/f.u., 0.52 Li/f.u., and 1.6 Li/f.u., respectively. The better results were obtained for the electrodes based on the B-doped alloys: 0.18 Li/f.u. for the LiAl 1- y B y , 0.61 Li/f.u. for the Li 3 Al 2- y B y , and 1.8 Li/f.u. for the Li 9 Al 4- y B y phase. In all cases the surface morphology of the electrodes after 50 cycles of delithiation was changed and became more porous. The small particles (80‒500 nm) aggregation increased as a result of material amorphization and interaction of the electrode surface with electrolyte. Keywords: synthesis, solid solution of the substitution, electrochemical delithiation, Li-ion batteries.