Electrochemical Strain Evolution in Iron Phosphate Cathodes during Li and Na Intercalation
Authors: Çapraz Ömer Özgür , Özdogru Bertan 
Published: May 2020

The ever-growing energy demand associated with the increasing human population, environmental concerns and technological developments puts pressure on society to harvest electricity from renewable sources. Due to their intermittent nature, the adoption of renewable energy depends on further developments in energy storage technology. Although Li-ion batteries dominate the current energy-storage landscape, their application in large-scale energy storage is constrained by continuously growing lithium prices as well as limited Li resources. Sodium-ion batteries have attracted attention in the search for cost-effective batteries with a minimum sacrifice on the performance. Already-existing LIB facilities can be adopted for manufacturing NIBs without requiring expensive upgrades1. Similar to Li-ion batteries, performance of sodium-ion batteries also depends on mechanical integrity of the electrodes and interfacial stability associated with solid-electrolyte interface formation. Physical and chemical properties of Na ions are intrinsically different than Li ions. Lack of insight into the influence of different alkali ions on the interfacial dynamics and mechanical degradations of electrodes limits the design of novel materials. Understanding how electrochemical reactions with different alkali ions affect reaction processes and mechanics of electrodes is required to develop advanced Na-ion batteries. In this work, we investigated in-situ strain generation in iron phosphate cathode during Li and Na intercalation.
Digital image correlation technique is used to measure in-situ strain evolution in the composite iron phosphate electrodes2. Iron phosphate (FePO4) was chosen as a model cathode system since both Li and Na can be reversibly intercalated into the FePO4 structure. The composite cathode was prepared by mixing active materials with CMC binder and conductive carbon in 8:1:1 mass ratio, respectively. The electrodes are cycled between 2.6-4.4 V during Li intercalation and 2.0-4.0 V during Na intercalation. Sodium intercalation leads to almost three-times larger strain evolution in the electrode compared to Li intercalation when the electrodes cycled at 50 uV/s. Strain derivative with respect to voltage was calculated using the finite difference method. Local minima in strain derivatives closely matched the current peaks during both Li and Na insertion into the FePO4 structure. Phase transformation in the LFP and NFP, measured by ex-situ XRD corresponds well with the current peaks observed during the cycling3,4. Our results revealed that Na intercalation results in distinct phase transitions and unprecedentedly large electrochemical strains in iron phosphate cathode electrodes compared to Li intercalation .

References

1. Kubota, K., Dahbi, M., Hosaka, T., Kumakura, S. & Komaba, S. Towards K-Ion and Na-Ion Batteries as “Beyond Li-Ion”. Chem. Rec.
18, 459–479 (2018).
2. Çapraz, Ö. Ö. et al. Controlling Expansion in Lithium Manganese Oxide Composite Electrodes via Surface Modification. J. Electrochem. Soc.
166, A2357–A2362 (2019).
3. Ramana, C. V., Mauger, A., Gendron, F., Julien, C. M. & Zaghib, K. Study of the Li-insertion/extraction process in LiFePO4/FePO4. J. Power Sources (2009). doi:10.1016/j.jpowsour.2008.11.042
4. Moreau, P., Guyomard, D., Gaubicher, J. & Boucher, F. Structure and stability of sodium intercalated phases in olivine FePO 4. Chem. Mater. (2010). doi:10.1021/cm101377h