Good evening,
I am glad to see our spectrum collected at 6K (not published) looks quite like yours. However, we simply couldn't measure any splitting for the other positions (we measured them all at 6K later on).
The limited capacity we observed seemed to fit with the limited capacity observed by other folks in Tours who worked on a Na/Fe-Sb system you may have heard of.
By curiosity, did your spectra collected at RT look like ours at all?
Moreover, did you have the chance to measure other spectra at 5 K ?
Anyways, congrats on your nice work !
Article Performance and mechanism of FeSb2 as negative electrode for Na-ion