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

Abstract

Intensive research in the field of lithium ion intercalating systems over the last several decades resulted in the design of hundreds of active material and electrolyte systems for practical battery applications. [1][2][3] Given the high priority of achieving maximum capacity, energy density, and rate capability characteristics of the Li-ion batteries, as well as emerging Na-ion and K-ion batteries, the focus of the majority of studies on the Electrochemical metal-ion intercalation systems are acknowledged to be a critical energy storage technology. The kinetics of the intercalation processes in transition-metal based oxides determine the practical characteristics of metal-ion batteries, such as the energy density, power, and cyclability. With the emergence of post lithium-ion batteries, such as sodium-ion and potassium-ion batteries, which function predominately in nonaqueous electrolytes of special formulation and exhibit quite varied material stability with regard to their surface chemistries and reactivity with electrolytes, the practical routes for the optimization of metal-ion battery performance become essential. Electrochemical methods offer a variety of means to quantitatively study the diffusional, charge transfer, and phase transformation rates in complex systems, which are, however, rather rarely fully adopted by the metal-ion battery community, which slows down the progress in rationalizing the ratecontrolling factors in complex intercalation systems. Herein, several practical approaches for diagnosing the origin of the rate limitations in intercalation materials based on phenomenological models are summarized, focusing on the specifics of charge transfer, diffusion, and nucleation phenomena in redox-active solid electrodes. It is demonstrated that information regarding rate-determining factors can be deduced from relatively simple analysis of experimental methods including cyclic voltammetry, chronoamperometry, and impedance spectroscopy.

Evidence of Bulk Proton Insertion in Nanostructured Anatase and Amorphous TiO2 Electrodes

“…This well-defined horizontal charging plateau is again indicative of a two-phase active materials transformation. 42 What is interesting to point out here is that the marked contrast we observed here between the anatase and amorphous TiO2 is readily comparable to literature reports of Li + insertion from aprotic organic electrolytes. 44,[48][49][50][51][52][53][54] Concerning the discharge curves, if we disregard the interference of HER at the end of the charging processes, the curves are almost symmetrical (with respect to the charging curves) regardless of the TiO2 phase.…”

“…The non-zero value of Ep observed at equilibrium in CV was proposed as a diagnostic criterion for a phase transformation of the active electrode material during an ion insertion, and its magnitude reflects the energetic barrier associated with nucleus formation. 42 In the present case of reversible H + insertion, a significantly lower Ep value is observed, thus suggesting that the associated phase transformation occurs through a less energetic nucleation barrier than with Li + . This observation is fully consistent with the minor orthorhombic distortion predicted between anatase and the H0.5TiO2 crystalline structures.…”

“…These divergences can be phenomenologically taken into account using the well-established concept of the Frumkin intercalation isotherm, which considers energetic interactions between insertion sites. 41,42 In that context, the equation governing the equilibrium potential of TiO2 as a 6 function of its redox state through reaction 2 is given by the following modified Nernst equation:…”

“…In the Frumkin framework, such a phase transformation is thus associated with g < gcrit. 42 On contrario, the insertion of Li + in amorphous TiO2 was characterized by a pair of symmetric but very broad faradaic waves in CV (meaning thus a g value > 0), 44,45,[48][49][50][51][52][53] leading some groups to conclude that in amorphous TiO2, the reversible Li + insertion proceeded through a solid solution reaction over a distribution of insertion potentials (reflecting the highly disordered structure of TiO2). 44,54 To better understand the dynamics of the proton insertioncoupled electron transfer reaction, CVs were recorded at different scan rates ranging from 1 mV•s -1 to 1 V•s -1 ( Figure S3).…”

Evidence of Bulk Proton Insertion in Nanostructured Anatase and Amorphous TiO2 Electrodes

“…These divergences can be phenomenologically taken into account using the well-established concept of the Frumkin intercalation isotherm, which considers energetic interactions between insertion sites. 41,42 In that context, the equation governing the equilibrium potential of TiO 2 as a function of its redox state through reaction 2 is given by the following modified Nernst equation:…”