Cathode/Electrolyte Interface-Dependent Changes in Stress and Strain in Lithium Iron Phosphate Composite Cathodes

Abstract

Dynamic interactions between electrode and electrolyte species during electrochemical cycling have an impact on the interfacial kinetics and mechanical stability of the electrode materials. In this study, we employ in-situ stress and strain measurements to investigate potential-dependent mechanical changes in lithium iron phosphate (LiFePO 4 ) cathodes during cyclic voltammetry in LiPF 6 and LiClO 4 -containing electrolytes. Analysis of the stress and strain derivatives in LiClO 4 -containing electrolytes exhibits single peaks during both lithiation and delithiation that coincide with LiFePO 4 phase transformations. An additional feature in the stress and strain derivatives is observed in LiPF 6 -containing electrolytes at the onset of the delithiation process. The current peak splitting in LiPF 6 is larger than in LiClO 4 , and electrochemical impedance spectroscopy measurements show higher impedances in LiPF 6 versus LiClO 4 -containing electrolytes in lithiated LiFePO 4 . The larger current peak splitting and higher impedance in LiPF 6 electrolytes suggest the potential-dependent growth of a thick and resistive cathode/electrolyte interface (CEI) layer on LiFePO 4 cathodes. We hypothesize that kinetic limitations in Li + transport through the CEI leads to additional stress and strain development at the electrode surface.

In Situ Probing Potassium-Ion Intercalation-Induced Amorphization in Crystalline Iron Phosphate Cathode Materials

“…To better understand nanoscale structural changes in the electrode during charge and discharge cycles, capacity and strain derivatives with respect to voltage were calculated to investigate the charge behavior and physical response of the FePO 4 electrode during K intercalation. Previous studies on graphite, 18,20 lithium manganese oxide (LMO), 19,28,29 lithium iron phosphate (LFP), 30 and sodium iron phosphate (NFP) 23 electrodes showed that the evolution of the strain derivatives with potential closely matches with the phase transformations in the electrode structure. In these studies, the location of the strain derivative peaks was in good agreement with the evolution of the electrochemical response of the materials associated with the nanoscale changes in their structure.…”

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

“…Broader current peak indicates the greater surface resistance towards the electrochemical reactions at the electrode / electrolyte interface. 49 The associated mechanical behavior of the electrode during cyclic voltammetry is shown in Fig 2 B and D, respectively. Strain values were set to zero at the beginning of each lithiation and delithiation cycle in order to compare mechanical deformations in each charge / discharge process.…”

“…Previous studies showed CEI/SEI layer formation on the electrode surface which mostly causes additional resistance in batteries. 49,51,52 Electrochemical impedance spectroscopy (EIS) analyses were performed at different voltage values from 3.5 to 4.4 V during the first charge. Fig.…”

“…The first semicircles in our results were attributed to the sum of charge transfer resistance, LFP/Al interlayer, and CEI layer formation which is represented as surface resistance (Rsurface). 49 Fittings applied only on the semicircles due to the Warburg element is corresponding to Li + diffusion into solid-phase electrode. 56 Decrease in first semicircle sizes of both LiClO4 and LiTFSI salts containing systems is visible when voltage values increase (Fig.…”

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

“…To better understand nanoscale structural changes in the electrode during charge and discharge cycles, capacity and strain derivatives with respect to voltage were calculated to investigate the charge behavior and physical response of the FePO4 electrode during K intercalation. Previous studies on graphite 15,24 , lithium manganese oxide (LMO) [25][26][27] , lithium iron phosphate (LFP) 28 and sodium iron phosphate (NFP) 21 electrodes showed that the evolution of the strain derivatives with potential closely matches with the phase transformations in the electrode structure. In the studies, the location of the strain derivative peaks was in good agreement with evolution of the electrochemical response of the materials associated with the nanoscale changes in their structure.…”