Chemo-Mechanics of Alkali-Ion Intercalation into Iron Phosphate Composite Cathodes

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). Therefore, physical, and electrochemical behavior of the cathode materials in response to Na+ and K+ ion intercalation is expected to be fundamentally different than the response to Li+ ion. However, there is not much known about how electrochemical reactions and transport of ions take place in cathode materials with different alkali metal ions. There have been several studies focusing on electrochemical characterization and investigation of the structural changes in the electrode materials. Lack of insight into these reaction-transport mechanisms limits the design of novel cathode materials for Na-ion and K-ion batteries. Comparative studies between Li-ion, Na-ion and K-ion battery cathodes are critical to identify fundamental similarities and differences during intercalation. 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 strains and distortions. Although the amorphization in the structure can be easily identified by conventional diffraction or electron microscopy techniques, quantitative analysis of the physical changes in the structure during and after amorphization while cycling the battery electrode is critical.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 intercalation. A new experimental approach allows to monitor dynamic physical and structural changes in the amorphous phase of the electrodes. In-operando XRD demonstrates amorphization in the electrode’s nanostructure during the first charge / discharge cycle. In situ strain analysis detects the reversible deformations associated with redox reactions in the amorphous phases. This method offers new insights to study mechanics of ion intercalation in the amorphous nanostructures.In the second part of the talk, we will compare the electrochemical and mechanical response of the iron phosphate cathodes upon Li, Na and K ion intercalation by using electrochemical techniques and in situ digital image correlation. Iron phosphate model electrodes were prepared by electrochemical displacement technique in order to ensure identical morphology, structure and chemistry in the pristine iron phosphate electrodes. Strain evolution during Li and Na intercalation results in more linear dependence on the state of charge / discharge except the first discharge of NaFePO4. However, strains generated in the electrode shows nonlinear behavior during insertion / extraction of K ions. Interestingly, when the same amount of K and Na ions are intercalated, similar chemomechanical expansions were observed. However, 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 rates. Potential-dependent behaviors also demonstrate more sluggish redox reactions during K intercalation, compared to the Li and Na intercalation. These observations provide a fundamental insight into the impact of alkali ions on the redox chemistry and associated chemomechanical deformations.