Rate-Dependent Potential and Electrochemical Strain Hysteresis in Lithium Iron Phosphate Cathodes for Li-Ion Batteries
Authors: Ozdogru Bertan , Capraz Omer Ozgur Özgür 
Published: July 2021

The commercialization of Li-ion batteries (LIBs) at the beginning of the 1990s resulted in the portable consumer goods such as cell phones, laptops, and wearable technologies. Currently, LIBs can be found in electric cars, drones, and on-site energy storage sites. Recently, research on LIBs shifted to find a cheaper cathode material compared to lithium cobalt oxide to reduce the manufacturing costs for on-site application.1 Lithium iron phosphate (LFP) in this aspect found to be a good candidate, owing to its high thermal safety, high reversibility, and operating voltage (3.45 V vs Li/Li+).2 However, LFP still possess technical challenges that need to be understood for its large scale application. Rate dependent phase transition from lithium-rich LFP to lithium-poor LFP (FP, iron phosphate) and its effect on the LFP particle remains unclear. Depending on particle size, cycling rate, and cycling methods, different phase transition methods are proposed, such as core-shell3, domino-cascade4, and metastable solid solution5. On top of this complication, different lattice parameters of LFP and FP can cause mechanical and interfacial instabilities in the active material. Understanding the fundamental effect of cycling rate and charging/discharging method on the mechanical deformation of LFP active material is important for longer-lasting LIBs.

In this study, we aim to understand the rate dependency of the mechanical response in the LFP composite electrode during lithium intercalation via galvanostatic cycling at different rates. We combined digital image correlation (DIC) and galvanostatic intermittent titration technique (GITT) to understand this phenomenon. Similar to our previous study on LFP, we observed a nonlinear strain generation in the LFP electrode, resulting in a hysteresis between charging and discharging.6 More interestingly, in situ strain measurements showed that the nonuniformity of strain generation significantly decreased with slower scan rates. Our results suggest that even after a high current pulse, an increase in the relaxation time resulted in a remarkable decrease in the hysteresis, suggesting the formation of crystalline LFP and FP during relaxation. In this study, utilization of in situ strain measurement during GITT measurement helped to understand the phase transition pathway during different scan rates, as well as during the relaxation period.

Acknowledgment: The work was supported by the Department of Energy and we are thankful to Vijay Murugesan for fruitful discussions.

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