In Situ Probing Potassium-ion Intercalation-induced Amorphization in Crystalline Iron Phosphate Cathode Materials

Abstract

<p>Na-ion and K-ion batteries are promising alternatives for large-scale energy storage applications due to their abundancy and lower cost. However, designing an electrode structure to reversibly accommodate these large alkali-ions is the remaining challenge before their commercialization. Intercalation of these large ions could cause irreversible structural deformations and amorphization in the crystalline electrodes. The designing of new amorphous electrodes is another route to develop electrodes to store these ions reversibly. Lack of understanding of dynamic changes in the amorphous nanostructures during battery operation is the bottleneck for further developments. Here, we 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. 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.</p>

The impact of alkali‐ion intercalation on redox chemistry and mechanical deformations: Case study on intercalation of Li, Na, and K ions into FePO4 cathode

“…[ 26 ] A recent in situ XRD study also demonstrated the amorphization of the crystalline iron phosphate during the intercalation of K ions. [ 21 ]…”

“…XRD studies on K ion intercalation into iron phosphate demonstrated amorphization in the crystalline structure. [ 19,21 ] Therefore, progressive evolution of the strain rates in the electrodes is likely due to the occurrence of plastic deformation in the electrode structure. Constant strain rates during Li and Na intercalation can be interpreted as preserving crystalline structure while removing these ions from the host structure.…”

“…Composite electrodes were prepared by mixing pristine lithium iron phosphate (LiFePO 4 , LFP, Hanwha Chemical) with sodium carboxymethyl cellulose (binder, CMC, Aldrich) and conductive additive (carbon black, Alfa Aesar) in 8:1:1 mass ratio. Iron phosphate (FePO 4 , FP) composite electrode was formed by electrochemical displacement technique using a pristine LFP composite electrode [ 21–24 ] via galvanostatic cycle at a rate of C/10. FePO 4 electrodes were charged and discharged with Li, Na, or K ions by galvanostatic cycles at C/25 rate against Li, Na, or K counter electrodes, respectively.…”

“…Recently, we developed a new experimental approach to monitor dynamic physical and structural changes in the amorphous phase of the electrodes by combining in situ strain measurements via digital image correlation (DIC) and in‐operando XRD techniques. [ 21 ] The study detected the redox chemistry and the associated electrochemical strains in the amorphous phases of the iron phosphate electrode during K ion intercalation. [ 21 ]…”

“…[ 21 ] The study detected the redox chemistry and the associated electrochemical strains in the amorphous phases of the iron phosphate electrode during K ion intercalation. [ 21 ]…”