Beam damage in operando X-ray diffraction studies of Li-ion batteries.

Operando powder X-ray diffraction (PXRD) is a widely employed method for the investigation of structural evolution and phase transitions in electrodes for rechargeable batteries. Due to the advantages of high brilliance and high X-ray energies, the experiments are often carried out at synchrotron facilities. It is known that the X-ray exposure can cause beam damage in the battery cell, resulting in hindrance of the electrochemical reaction. This study investigates the extent of X-ray beam damage during operando PXRD synchrotron experiments on battery materials with varying X-ray energies, amount of X-ray exposure and battery cell chemistries. Battery cells were exposed to 15, 25 or 35 keV X-rays (with varying dose) during charge or discharge in a battery test cell specially designed for operando experiments. The observed beam damage was probed by µPXRD mapping of the electrodes recovered from the operando battery cell after charge/discharge. The investigation reveals that the beam damage depends strongly on both the X-ray energy and the amount of exposure, and that it also depends strongly on the cell chemistry, i.e. the chemical composition of the electrode.

Na-Ion storage in iron hydroxide phosphate hydrate through a reversible crystalline-to-amorphous phase transition.

Iron(iii) hydroxide phosphate hydrate Fe1.13(PO4)(OH)0.39(H2O)0.61 is investigated for the first time as a Na-ion battery cathode, which reveals that the material exhibits similar storage capacities for Na- and Li-ions at relatively low current rates (i.e. C/10). Interestingly, operando X-ray diffraction shows that insertion of Na-ions induces a solid solution transition in the crystalline Fe1.13(PO4)(OH)0.39(H2O)0.61 end-member simultaneously with a major amorphization. This result adds to the series of observations of phosphate-based materials undergoing order-disorder transitions during Na-ion storage. Fe1.13(PO4)(OH)0.39(H2O)0.61 is thus ideal for enhancing our knowledge on such phenomena. To this end, using total X-ray scattering with pair distribution function analysis, we show that the amorphous phase is Na-rich NaxFe1.13(PO4)(OH)0.39(H2O)0.61 with the local [FeO6]-[PO4] motif retained but with coherence lengths of only ca. 0.6 nm. Our investigation also reveals that the crystallinity of Fe1.13(PO4)(OH)0.39(H2O)0.61 is regained upon Na-extraction (battery recharge), i.e. the order-disorder transition is reversible.