Driving Force behind the Amorphization in the Crystalline Cathode Structures during Alkali Metal Ion Intercalation for Electrochemical Energy Storage

Abstract

Development of cathode structures suitable for Na-ion and K-ion batteries is still one of the major challenges on the way to the design of next-generation alkali metal-ion batteries. Although Li, Na and K belong to the same alkali metal group with a single charge in their cation form, intercalation of Na+ and K+ ions in electrodes is difficult since ionic radii of Na+ (0.98 Å) and K+ (1.38 Å) are larger than that of Li+ (0.69Å). Intercalation of alkali metal ions results in volumetric expansions in the electrode structure. Even modest expansions in brittle cathodes can cause particle fracturing in a larger crystalline-size scale. Intercalation of larger ions can cause structural collapse and amorphization induced by continuous accumulation of strains and distortions. However, the lack of understanding behind the amorphization mechanisms in the crystalline electrodes upon ion intercalation materials hinders the development of electrode structures suitable for these large ions.In the first part of the talk, we will first report the utilization of in situ digital image correlation and in-operando X-ray diffraction (XRD) techniques to probe dynamic changes in the amorphous phase of iron phosphate during potassium intercalation1. A new experimental approach allows to monitor dynamic physical and structural changes in the amorphous phase of the electrodes. This method offers new insights to study mechanics of ion intercalation in the amorphous nanostructures.In the second part of the talk, we will discuss the electrochemical and mechanical response of the iron phosphate cathodes upon Li, Na and K ion intercalation. Strain evolution during Li and Na intercalation results in more linear dependence on the state of charge / discharge. However, strains generated in the electrode shows nonlinear behavior during insertion / extraction of K ions. Strain rate calculations showed that K ion intercalation results in a progressive increase in the strain rate, whereas Li and Na intercalation induce nearly constant strain rates2. Our results shows that strain rates are critical factor for the amorphization of the crystalline structure, rather than the absolute value of electrochemical strains. These observations provide a fundamental insight into the impact of alkali ions on the redox chemistry and associated chemomechanical deformations.Acknowledgement: This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (Award number DE-SC0021251).