Electrochemical Imaging of Interfaces in Energy Storage via Scanning Probe Methods: Techniques, Applications, and Prospects.

Developing a deeper understanding of dynamic chemical, electronic, and morphological changes at interfaces is key to solving practical issues in electrochemical energy storage systems (EESSs). To unravel this complexity, an assortment of tools with distinct capabilities and spatiotemporal resolutions have been used to creatively visualize interfacial processes as they occur. This review highlights how electrochemical scanning probe techniques (ESPTs) such as electrochemical atomic force microscopy, scanning electrochemical microscopy, scanning ion conductance microscopy, and scanning electrochemical cell microscopy are uniquely positioned to address these challenges in EESSs. We describe the operating principles of ESPTs, focusing on the inspection of interfacial structure and chemical processes involved in Li-ion batteries and beyond. We discuss current examples, performance limitations, and complementary ESPTs. Finally, we discuss prospects for imaging improvements and deep learning for automation. We foresee that ESPTs will play an enabling role in advancing EESSs as we transition to renewable energies.

An SECM-Based Spot Analysis for Redoxmer-Electrode Kinetics: Identifying Redox Asymmetries on Model Graphitic Carbon Interfaces.

The fundamental process in non-aqueous redox flow battery (NRFB) operation revolves around electron transfer (ET) between a current collector electrode and redox-active organic molecules (redoxmers) in solution. Here, we present an approach utilizing scanning electrochemical microscopy (SECM) to evaluate interfacial ET kinetics between redoxmers and various electrode materials of interest at desired locations. This spot-analysis method relies on the measurement of heterogeneous electron transfer rate constants (kf or kb ) as a function of applied potential (E-E0 '). As demonstrated by COMSOL simulations, this method enables the quantification of Butler-Volmer kinetic parameters, the standard heterogeneous rate constant, k0 , and the transfer coefficient, α. Our method enabled the identification of inherent asymmetries in the ET kinetics arising during the reduction of ferrocene-based redoxmers, compared to their oxidation which displayed faster rate constants. Similar behavior was observed on a wide variety of carbon electrodes such as multi-layer graphene, highly ordered pyrolytic graphite, glassy carbon, and chemical vapor deposition-grown graphite films. However, aqueous systems and Pt do not exhibit such kinetic effects. Our analysis suggests that differential adsorption of the redoxmers is insufficient to account for our observations. Displaying a greater versatility than conventional electroanalytical methods, we demonstrate the operation of our spot analysis at concentrations up to 100 mM of redoxmer over graphite films. Looking forward, our method can be used to assess non-idealities in a variety of redoxmer/electrode/solvent systems with quantitative evaluation of kinetics for applications in redox-flow battery research.

Pt/Polypyrrole Quasi-References Revisited: Robustness and Application in Electrochemical Energy Storage Research.

Choosing reference electrodes for nonaqueous electrochemical measurements, especially in energy storage research, is challenging due to lengthy experiments (>1 day), the lack of alternatives to the commonly used Ag/Ag+ reference electrode (RE), the introduction of junction potentials, and the possibility of sample contamination. Often, quasi-reference electrodes (QREs) such as Ag wires and Li metal strips are used. However, small changes in electrolyte composition can cause large potential drifts, and their surfaces may be reactive to the solution. Here, we propose an alternative QRE based on polypyrrole electrodeposited on Pt wire (PPyQRE) encased in a glass tube with the open end sealed with commercial frits. While freestanding PPyQRE wires have been reported in the literature, simple encasing of the PPyQRE overcomes the above-mentioned drawbacks of QREs while providing a reliable reference potential that is closer to the performance of an RE. During cyclic voltammetric and bulk electrolysis testing of a redox mediator in solution, the encased PPyQRE exhibited stable reference potentials over multiple charge/discharge cycles with minimal drift (∼5 mV) after ∼2.25 days of operation. We also tested the reliability of our reference during the testing of multilayer graphene Li-ion anodes, which often involve cycling samples at highly reducing potentials (<-3 V vs Fc/Fc+) over long durations (>1 day). In the same testing conditions, the Ag/Ag+ electrode led to observable Ag deposits on the graphene and large potential drifts (∼50 mV), while the PPyQRE exhibited no measurable drift and revealed changes in voltammetric features that were obscured by reference drift when using Ag/Ag+. Minor reference drifts of ∼30 mV over long usage of the PPyQRE (∼2 months) can be addressed by calibration with a ferrocene couple at the end of experiments. These results highlight the advantages of using an encased PPyQRE as a simple and practical reference electrode for electrochemical measurements in the field of nonaqueous energy storage research.

Coordinated mapping of Li+ flux and electron transfer reactivity during solid-electrolyte interphase formation at a graphene electrode.

Interphases formed at battery electrodes are key to enabling energy dense charge storage by acting as protection layers and gatekeeping ion flux into and out of the electrodes. However, our current understanding of these structures and how to control their properties is still limited due to their heterogenous structure, dynamic nature, and lack of analytical techniques to probe their electronic and ionic properties in situ. In this study, we used a multi-functional scanning electrochemical microscopy (SECM) technique based on an amperometric ion-selective mercury disc-well (HgDW) probe for spatially-resolving changes in interfacial Li+ during solid electrolyte interphase (SEI) formation and for tracking its relationship to the electronic passivation of the interphase. We focused on multi-layer graphene (MLG) as a model graphitic system and developed a method for ion-flux mapping based on pulsing the substrate at multiple potentials with distinct behavior (e.g. insertion-deinsertion). By using a pulsed protocol, we captured the localized uptake of Li+ at the forming SEI and during intercalation, creating activity maps along the edge of the MLG electrode. On the other hand, a redox probe showed passivation by the interphase at the same locations, thus enabling correlations between ion and electron transfer. Our analytical method provided direct insight into the interphase formation process and could be used for evaluating dynamic interfacial phenomena and improving future energy storage technologies.

Impact of Surface Modification on the Lithium, Sodium, and Potassium Intercalation Efficiency and Capacity of Few-Layer Graphene Electrodes.

In a conventional lithium-ion battery (LIB), graphite forms the negative electrode or anode. Although Na is considered one of the most attractive alternatives to Li, achieving reversible Na intercalation within graphitic materials under ambient conditions remains a challenge. More efficient carbonaceous anode materials are desired for developing advanced LIBs and beyond Li-ion battery technologies. We hypothesized that two-dimensional materials with distinct surface electronic properties create conditions for ion insertion into few-layer graphene (FLG) anodes. This is because modification of the electrode/electrolyte interface potentially modifies the energetics and mechanisms of ion intercalation in the thin bulk of FLG. Through first-principles calculations; we show that the electronic, structural, and thermodynamic properties of FLG anodes can be fine-tuned by a covalent heteroatom substitution at the uppermost layer of the FLG electrode, or by interfacing FLG with a single-side fluorinated graphene or a Janus-type hydrofluorographene monolayer. When suitably interfaced with the 2D surface modifier, FLG exhibits favorable thermodynamics for the Li+, Na+, and K+ intercalation. Remarkably, the reversible binding of Na within carbon layers becomes thermodynamically allowed, and a large storage capacity can be achieved for the Na intercalated modified FLG anodes. The origin of charge-transfer promoted electronic tunability of modified FLGs is rationalized by various theoretical methods.

Nernstian Li+ intercalation into few-layer graphene and its use for the determination of K+ co-intercalation processes.

Alkali ion intercalation is fundamental to battery technologies for a wide spectrum of potential applications that permeate our modern lifestyle, including portable electronics, electric vehicles, and the electric grid. In spite of its importance, the Nernstian nature of the charge transfer process describing lithiation of carbon has not been described previously. Here we use the ultrathin few-layer graphene (FLG) with micron-sized grains as a powerful platform for exploring intercalation and co-intercalation mechanisms of alkali ions with high versatility. Using voltammetric and chronoamperometric methods and bolstered by density functional theory (DFT) calculations, we show the kinetically facile co-intercalation of Li+ and K+ within an ultrathin FLG electrode. While changes in the solution concentration of Li+ lead to a displacement of the staging voltammetric signature with characteristic slopes ca. 54-58 mV per decade, modification of the K+/Li+ ratio in the electrolyte leads to distinct shifts in the voltammetric peaks for (de)intercalation, with a changing slope as low as ca. 30 mV per decade. Bulk ion diffusion coefficients in the carbon host, as measured using the potentiometric intermittent titration technique (PITT) were similarly sensitive to solution composition. DFT results showed that co-intercalation of Li+ and K+ within the same layer in FLG can form thermodynamically favorable systems. Calculated binding energies for co-intercalation systems increased with respect to the area of Li+-only domains and decreased with respect to the concentration of -K-Li- phases. While previous studies of co-intercalation on a graphitic anode typically focus on co-intercalation of solvents and one particular alkali ion, this is to the best of our knowledge the first study elucidating the intercalation behavior of two monovalent alkali ions. This study establishes ultrathin graphitic electrodes as an enabling electroanalytical platform to uncover thermodynamic and kinetic processes of ion intercalation with high versatility.