Influence of Carbon Coating on Intercalation Kinetics and Transport Properties of LiFePO₄

Abstract

We report on a comparative study of Li + intercalation kinetics for LiFePO 4 materials with polydopamine-derived and glucosederived carbon coatings. We demonstrate that the uniform polydopamine-derived carbon coating affects the charge transfer rates and transport properties (electronic conductivity) of composite electrodes only slightly (compared to the inhomogeneous glucose-derived coatings), whereas much more pronounced effects are observed for the apparent diffusion coefficients and nucleation barriers. Thick and uniform carbon surface layers cause a sharp decrease in the ionic diffusion coefficients, which results in the loss of capacity at higher charge/discharge rates for LiFePO 4 composite electrodes. The new phase nucleation rates are also demonstrated to decrease significantly for the samples with thicker coatings inducing higher potential differences between charge and discharge curves. The analysis of the rate determining factors for the LiFePO 4 electrodes points to the rate limitations induced by the slow nucleation and slow ionic diffusion, whereas the formation of a polydopamine-derived uniform carbon layer tends to hinder the (de)intercalation reaction and does not provide additional benefits compared to glucose-derived carbon coatings comprising conductive yet inhomogeneous layers.[a] A. R. Iarchuk, Dr. V. A. Nikitina, Prof.

Electrochemical Analysis of the Mechanism of Potassium‐Ion Insertion into K‐rich Prussian Blue Materials

“…The peak‐to‐peak separations Δ E at the lowest scan rate (0.05 mV s −1 ) are 40 mV for the FeHCF_N and 55 mV for the FeHCF_M samples. The large Δ E value for the FeHCF_M electrode can be attributed to the polarization of the nucleation step, analogously to that observed for LiFePO 4 materials when the (de)insertion is controlled by the new phase nucleation and growth rates . Additionally, concentration polarization, which arises due to the large particle size of FeHCF_M, should also contribute to the observed difference in cathodic and anodic peak potentials.…”

“…This result strongly favors the two‐phase mechanism of K + (de)insertion into FeHCF_N, despite the undetected changes in the material crystal structure, as no other physical process in the PB materials, except for a first‐order phase transformation, can result in the observed linearity in ln( I ) vs. t dependences. Notably, despite the similar shapes of ln( I ) vs. t curves for PB samples and LiFePO 4 , the two types of ion insertion materials still differ significantly, as LiFePO 4 is characterized by extremely narrow single‐phase potential ranges, while those are much wider for PB (as evidenced by the symmetry of the CVs of FeHCF_N electrode in the 0.0–0.2 V potential interval). This result, however, does not interfere with our conclusion on the predominantly nucleation‐limited (de)insertion, which takes place in the two‐phase coexistence interval.…”

“…Examination of the electrochemical responses of PB electrodes implies a high probability of the two‐phase mechanism for both the FeHCF_N and FeHCF_M samples, which is suggested by the large peak‐to‐peak separations (>40 mV) in the Q vs. E dependencies, cathodic and anodic CV peaks asymmetry and the differences between the D values before and after the minimum. These features can only be explained if the thermodynamic origin of the hysteresis is assumed, i. e. the (de)insertion is controlled by the kinetics of the nucleation step …”

Metal‐Ion Coupled Electron Transfer Kinetics in Intercalation‐Based Transition Metal Oxides

“…[ 73,76 ] As for the carbon‐coated materials, much more subtle effects of the carbon coating on the intercalation pathways were recently reported, such as the decrease in the phase‐separation and new phase nucleation rates in LiFePO 4 materials. [ 77,78 ] Still, the degree of certainty in the conclusions regarding the effects of the artificial surface coatings on the intercalation kinetics is frequently compromised by the nonuniformity of the coatings and the cracking/morphology changes in the coating during continuous cycling, which are to be explored further.…”

“…[ 25,94–99 ] Despite the successful attempts [ 25,99 ] to apply phase field modeling approaches to reproduce the essential features in the galvanostatic charge/discharge profiles of LiFePO 4 and Li 4 Ti 5 O 12 phase‐transforming electrodes, voltammetry and chronoamperometry data for phase‐transforming intercalation systems has received rather limited attention. [ 77,100–105 ] As potentiodynamic and transient techniques in classical electrochemistry are most frequently used to study the kinetics of various processes, [ 9 ] in this Section we primarily focus on the application of similar approaches to the analysis of the kinetic data for phase‐transforming electrodes.…”