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

Abstract

Batteries made of charge carriers from Earth‐crust abundant materials (e.g., Na, K, and Mg) have received extensive attention as an alternative to Li‐ion batteries for grid storage. However, a lack of understanding of the behavior of these larger ions in the electrode materials hinders the development of electrode structures suitable for these large ions. In this study, we investigate the impact of alkali ions (Li, Na, and K) on the redox chemistry and mechanical deformations of iron phosphate composite cathodes by using electrochemical techniques and in situ digital image correlation. Na‐ion and Li‐ion intercalation demonstrate a nearly linear correlation between electrochemical strains and the state of charge and discharge. The strain development shows nonlinear dependance on the state of charge and discharge for K ions. Strain rate calculations show that K ion intercalation results in a progressive increase in the strain rate for all cycles. Li and Na intercalation induce nearly constant strain rates with the exception of the first discharge cycle of Na intercalation. When the same amount of ions are inserted into the electrode, the electrode shows the lowest strain generation upon Li intercalation compared to larger alkali ions. Na and K ions induce similar volumetric changes in the electrode when the state of charge and discharge is around 30%. Although the electrode experiences larger absolute strain generation at the end of the discharge cycles upon Na intercalation, strain rates were found to be greater for K ions. Potential‐dependent behaviors also demonstrate more sluggish redox reactions during K intercalation, compared to Li and Na. Our quantitative analysis suggests that the strain rate, rather than the absolute value of strain, is the critical factor in amorphization of the crystalline electrode.

The Role of Transition Metals on Chemo‐Mechanical Instabilities in Prussian Blue Analogues For K‐Ion Batteries: The Case Study on KNHCF Versus KMHCF

“…[ 41 ] For example, strain rates in FePO 4 were ~0.3% and 2%‐SOD (strain rate per unit state‐of‐discharge) upon Li and Na ion intercalation, respectively. [ 42 ] Intercalation of K ions FePO 4 results in a progressive increase in strain rates up to 5‐%/SOD (strain per unit state‐of‐discharge), which leads to particle fracture and amorphization in the cathode. [ 39,42 ] In our study, if the mechanical deformations were the primary governing driving force behind the capacity fade, one should expect that both KMHCF and KNHCF should decay at a similar rate since they experience similar reversible and irreversible electrochemical strains.…”

Probing the Formation of Cathode-Electrolyte Interface on Lithium Iron Phosphate Cathodes via In Operando Mechanical Measurements

“…For strain measurements, once dried, the composite electrode was peeled off from aluminum foil and cut into a rectangular shape that was appropriate for the custom strain cells. [47][48][49] Electrochemical Investigation: The fabricated free standing rectangular-shaped LiFePO4 composite electrodes were characterized using galvanostatic and cyclic voltammetry tests. Galvanostatic cycles were performed at C/10 discharge rate.…”

Probing the Formation of Cathode-Electrolyte Interface on Lithium Iron Phosphate Cathodes via In Operando Mechanical Measurements

“…A detailed description of the custom cell was provided in the previous works. [47][48][49] Optical investigation of the LFP electrodes occurred using the digital image correlation (DIC) technique with a Grasshopper3…”