Synthesis and characterization of Sn4+ substituted Li-rich/Mn-rich NMC cathode materials for Li-ion batteries

Abstract

The rocksalt-type layered Li-rich/Mn-rich lithium nickel manganese cobalt oxide (NMC, xLi2MnO3-(1-x)LiMO2 (M = Mn, Ni, Co)) is considered as a promising alternative for the commercialised LiCoO2 cathode material for Li-ion batteries 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. Characteristic for Li-rich/Mn-rich NMC is the migration of manganese accompanied by oxygen loss. The migration of the octahedrally coordinated TM to other octahedral sites in neighbouring layers is believed to occur via intermediate tetrahedral (Th) sites. As Sn4+ does not tend to adopt Th coordination, the substitution of Sn4+ for Mn4+ has been investigated 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 report the synthesis of Sn-substituted Li-rich/Mn-rich NMC with average composition Li1.2Ni0.13Mn0.54-ySnyCo0.13O2 (y~0.03-0.11, y=0.54) via a facile aqueous carbonate co-precipitation route followed by an anneal at two different temperatures. The XRD pattern of unsubstituted Li1.2Ni0.13Mn0.54Co0.13O2 contain peaks related to an α-NaFeO2 type structure and a Li2MnO3 type structure. The XRD patterns of Li1.2Ni0.13Mn0.54-ySnyCo0.13O2 (y~0.03-0.11) contain next to peaks ascribed to the α-NaFeO2 and Li2MnO3 type structure peaks related to a Sn-rich phase. The composition of the Sn-rich phase is determined by means of STEM-EDX. Furthermore, STEM-EDX indicates a homogeneous distribution of the transition elements and tin. For Li1.2Ni0.13Sn0.54Co0.13O2 a different phase is formed as compared to the materials with zero or partial substitution of Sn4+ for Mn4+ as indicated by the absence of the majority of peaks related to the α-NaFeO2 type structure.

Galvanostatic cycling is performed at different C rates. For Li1.2Ni0.13Mn0.54-ySnyCo0.13O2 (y~0.11) a slightly shorter potential plateau at ~ 4.5V is obtained, indicating less oxygen release as compared to the unsubstituted NMC.2,3 The first discharge capacity of the substituted sample is slightly lower than the unsubstituted Li1.2Ni0.13Mn0.54Co0.13O2. When replacing all Mn4+ by Sn4+ a drastically lower first discharge capacity of has been obtained next to a much shorter potential plateau. The charged and pristine materials are further analysed by advanced characterization techniques to correlate the electrochemical behaviour with crystal structure changes upon charging.