Isovalent Co-Substitution of Iron and Titanium into Single-Crystal NMC622

Abstract

Substitutional Li[Ni0.6Mn0.2Co0.2]O2 oxides (known as NMC622) were made by all-dry synthesis with Fe and Ti substituting Co and Mn, respectively. The substitutions were performed in three series, Fe substitution for Co, Ti substitution for Mn, and Fe and Ti co-substitution for Co and Mn, according to the formula Li(Ni0.6Mn0.2-yCo0.2-xFexTiy)O2. The resulting oxides were evaluated as cathode materials for Li-ion batteries. Fe-substitution for Co resulted in increased intersite mixing, resulting in increased polarization and capacity fade. Ti-substitution for Mn also resulted in increased intersite mixing, but the mixing was due to Ti3+ in the Li-layer. As a result, Ti-substituted NMCs had improved capacity retention and reduced polarization. These effects were independent of each other, so that Ti could partially offset the negative aspects of Fe-substitution. Additionally, layered Mn-free Li(Ni0.6Ti0.2Co0.2)O2 (NTC622) was produced as an endmember of this series for the first time with low intersite mixing and superior electrochemical performance in comparison to previous reports. These results demonstrate benefits of all-dry Ti-substitution in NMC and the all-dry synthesis method as an avenue towards new cathode composition discovery.