Synthesis, structure and electrochemical behaviour of new Ru-containing lithium-rich layered oxides

Abstract

Three new series of rocksalt related layered Li-rich oxides of formulas, Li[LixM0.5−x/2Ru0.5−x/2]O2 (I) [for M=Co, Ni; x=0.125, 0.25, 0.33], Li[LixM1−2xRux]O2 (II) [for M=Co, Ni; x=0.25] and Li[LixM1−3xMnxRux]O2 (III) [for M=Co, Ni; x=0.25], have been synthesized, structurally characterized and their electrochemical behaviour investigated. Oxides of series I for M=Co; x=0.125, 0.25 and 0.33 and for M=Ni; x=0.33 adopt the LiCoO2 (R-3m) structure. Oxides of series I for M=Ni; x=0.125 and 0.25 form in the Li2TiO3 (C2/c) structure. Series II oxides for M=Co; x=0.25 crystallize in the LiCoO2 (R-3m) structure; for M=Ni; x=0.25 form in the Li2RuO3 (C2/c) structure. Series III oxides for M=Co, Ni; x=0.25 adopt the Li2RuO3 (C2/c) structure. Electrochemical studies indicate that the Co-containing oxides exhibit a higher initial discharge capacity [for e.g. ~180mAhg−1 oxide of series I for M=Co, x=0.33 (Li4CoRuO6)] as well as a higher reversible discharge capacity [~130mAhg−1 for oxide of series I for M=Co, x=0.33 (Li4CoRuO6)] compared to the corresponding Ni-analogs. Participation of oxide ions (higher oxidation state of Ru) in the redox process could explain the higher discharge capacity during the first cycle. Reduced capacity (capacity fade) during the subsequent cycles could arise from the oxygen evolution due to the redox process (2O2−→2O−→O2), which is not reversible. The present work shows that ruthenium incorporation in rocksalt layered oxides along with Co/Ni appears to give a beneficial effect in producing a higher discharge capacity. In addition, the compounds crystallizing with the LiCoO2 related structure appears to give a better reversible capacity than the compounds crystallizing in the Li2RuO3 and Li2TiO3 structures.