After our initial work on Li2MnO3, we discovered that there was a debate in the literature for a related compound. The more promising Li1.2Mn0.54Ni0.13Co0.13O2. It is the Li and Mn rich analogue to the Li[NiMnCo]O2 oxides used in commercial batteries.
What was this debate? It was whether the material existed as a solid solution or if it was an intergrowth or mix of Li2MnO3 and Li[NiMnCo]O2. Ie, if the composition crystallised as one or two phases. Here is an excerpt highlighting the debate pubs.acs.org/doi/10.1021/cm…
And here is what the two proposed models are. In the multi-phase model the hexagonal ordering occurs exclusively for Li and Mn whereas in the single phase Li in the transition metal layer can also have Ni and Co as nearest neighbours.
Why does this matter? It has been argued that the additional capacity, specifically the anionic redox capacity, originated from the Li2MnO3 phase. Thus, its presence is necessary for that additional capacity.
So why is this so difficult? Surely a diffraction experiment can tell you whether multiple phases are present. Lets play a game of spot the difference🙂. Below are diffraction patterns (X-rays and neutrons) from a confirmed "single" and "multi" phase samples
Aside from intensity differences in the neutron diffraction pattern, they look pretty similar. Figuring out this puzzle is further complicated since the compound has disorder over several length scales. Short to long: Atomic site mixing, stacking faults and phase segregation
Disorder over multiple length scales means to answer this question multiple characterisation tools are needed. This is what we did, looking at both local and long range information. The methods were diffraction, raman spectroscopy, magnetic susceptibility, and STEM measurements
What did we learn? Is the material one or two phases?? The answer is: both and it depends on how you synthesise the material. Similar to the first paper, solid state gives an inhomogeneous distribution of elements whereas the sol gel method is more homogeneous
It is not too surprising that this has led to conflicting results over the years. It does raise some interesting ideas about tailoring phase intergrowth via different synthetic pathways however.
The full story can be found here: pubs.acs.org/doi/10.1021/ac…
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