Isovalent Co-Substitution of Iron and Titanium into Single-Crystal NMC622
Authors: Macintosh Macgregor , Obrovac Mark , Shakouri Mohsen 
Published: December 2024

Recently, all-dry (i.e., solvent-free) synthesis methods for Ni-rich nickel-manganese-cobalt (NMC) cathode active materials, Li(Ni
a
Mn
b
Co1-a-b
)O2 (NMC, a ≥ 0.5), have enabled the use of metal oxide feedstocks to be used as precursors.1 Such all-dry methods provide new opportunities for compositional modification in NMCs. In the search for more sustainable, high-performance Ni-rich alternatives to NMC, Fe and Ti have been identified as attractive substitutes for Co and Mn, respectively.2–4 Herein, using all-dry synthesis methods, both individual and isovalent co-substitution of Fe and Ti into single-crystal (SC) NMC622, according to Li(Ni0.6Mn0.2-y
Co0.2-x
Fe
x
Ti
y
)O2 (0 ≤ x, y ≤ 0.2), were conducted.

In these studies, it was found that Fe-substitution resulted in increased inter-site mixing, resulting in increased polarization and reduced capacity retention. However, Ti-substitution resulted in an increase in Ni3+ content, leading to improved capacity retention. These effects were found to be independent of each other, so that Ti additions could effectively offset some of the negative aspects of Fe-substitution. Such studies could point the way to enabling increased Fe-content in NMC. Additionally, layered Mn-free Li(Ni0.6Ti0.2Co0.2)O2 (NTC622) was produced as an endmember of this series with low levels of cation mixing. This is the first time to our knowledge that a fully ordered NTC622 composition has been reported. As a consequence of its ordered structure, the NTC622 made here was found to have far superior electrochemical performance in comparison to previous attempts reported in the literature.5,6 These results demonstrate the viability of Ti-rich cathode material compositions, such as NTC622, presenting an avenue for cathode materials synthesized by all-dry methods.7In this presentation the effects of Ti and Fe individual and co-substitution in single-crystal Li(Ni0.6Mn0.2-y
Co0.2-x
Fe
x
Ti
y
)O2 (0 ≤ x, y ≤ 0.2) made by all-dry methods will be discussed, including structural changes, changes in oxidation states as measured by X-ray absorption near-edge spectroscopy (XANES), and electrochemical performance; with special emphasis on the end-member composition of NTC622.

References

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Figure 1