Synthesis of Sn-substituted Li-rich/Mn-rich NMC via an aqueous carbonate co-precipitation method

Abstract

The rocksalt-type layered lithium-rich/Mn-rich lithium nickel manganese cobalt oxide (NMC) is considered as a promising alternative for the commercialised cathode material for Li – ion batteries LiCoO2 due to its higher capacities in the range of 250–300 mAh/g,1 its lower cost and higher thermal stability. However, Li-rich/Mn-rich NMC suffers from large voltage decays, most probably related to structural changes ascribed to transition metal (TM) migration during cycling.

Migration of the octahedrally coordinated TM to other octahedral sites is believed to occur via intermediate tetrahedral (Th) sites. As Sn4+ does not tend to adopt Th coordination, the substitution of Sn4+ for Mn4+ is under investigation as a way to decrease the voltage fade. Co-precipitation by using carbonate–based precipitating agents is a well-established route to prepare NMC. Here, we evaluate the synthesis of Sn-substituted Li-rich/Mn-rich NMC with composition Li1.2Ni0.13Mn0.54Co0.13O2 via aqueous carbonate co-precipitation followed by an anneal at elevated temperatures. Tin substitution degrees were varied from 5 % till 100 % of Mn4+ replaced by Sn4+. It is shown by XRD that the spherical micrometer-sized particles obtained after co-precipitation and before anneal contain for the whole investigated composition range a tin oxohydroxo phase. The role of the pH and the metal-to-precipitating agent ratio applied during co-precipitation synthesis on the formation of this tin non-carbonate phase has been investigated. The XRD patterns recorded after anneal show additional peaks not indexed to NMC. The structure and composition of the end product has been further investigated by STEM with emphasis on the correlation between the tin oxohydroxo phase and the end product.