X-ray absorption spectromicroscopy of Na0.67Fe0.25Mn0.75O2 and Na0.67Li0.2Fe0.2Mn0.6O2 primary particles for Na-ion batteries

Fe L3-edge, Mn L3-edge, Na K-edge, and O K-edge X-ray absorption spectroscopy (XAS) data were obtained from Na0.67Fe0.25Mn0.75O2 (FMO) and Na0.67Li0.2Fe0.2Mn0.6O2 (LFMO) primary particles in pristine and charged states with a spatial resolution of 30 nm using scanning transmission X-ray microscopy (STXM). The XAS data showed that the average oxidation number of Fe ions was +3 for all particles. For Mn ions, it was +3.9 (charged) and +3.6 (pristine) for the LFMO particles and +3.6 (charged) and +3.5 (pristine) for the FMO particles. Further, for Na ions, the value was +1 for both particles in the pristine state. The changes in the oxidation numbers implied the strong effect of Mn ions on charge exchange as the Na ions were intercalated. Compared to the FMO particles, the LFMO particles showed that (i) the Fe ions undergo fewer spectral feature changes during charging, (ii) Mn ions exchange charges dominantly toward the 4+ ion in the charged state, (iii) both the Mn and Fe ions possess more homogeneous spectral features in the charged state, (iv) the O ions undergo fewer spectral feature changes upon charging, and (v) Na ions possess more homogeneous spectral features in the pristine state. These spectral features support the enhancement of the structural stability of LFMO particles.