Quantifying Interfacial Ion Transfer at Operating Potassium-Insertion Battery Electrodes within Highly Concentrated Aqueous Solutions.

Electrode/electrolyte interfacial ion transfer is a fundamental process occurring during insertion-type redox reactions at battery electrodes. The rate at which ions move into and out of the electrode, as well as at interphase structures, directly impacts the power performance of the battery. However, measuring and quantifying these ion transfer phenomena can be difficult, especially at high electrolyte concentrations as found in batteries. Herein, we report a scanning electrochemical microscope method using a common ferri/ferrocyanide (FeCN) redox mediator dissolved in an aqueous electrolyte to track changes in alkali ions at high electrolyte concentrations (up to 3 mol dm-3). Using voltammetry at a platinum microelectrode, we observed a reversible E1/2 shift of ∼60 mV per decade change in K+ concentrations. The response showed high stability in sequential measurements and similar behavior in other aqueous electrolytes. From there, we used the same FeCN mediator to position the microelectrode at the surface of a potassium-insertion electrode. We demonstrate tracking of local changes in the K+ concentration during insertion and deinsertion processes. Using a 2D axisymmetric, finite element model, we further estimate the effective insertion rates. These developments enable characterization of a key parameter for improving batteries, the interfacial ion transfer kinetics, and future work may show mediators appropriate for molar concentrations in nonaqueous electrolytes and beyond.

Impact of electrolyte decomposition products on the electrochemical performance of 4 V class K-ion batteries.

In the pursuit of long-life K-ion batteries (KIBs), half-cell measurements using highly reactive K metal counter electrodes are a standard practice. However, there is increasing evidence of electrolyte decomposition by K metal impacting electrode performance. Herein, we systematically explored the K metal-treated electrolytes KPF6, KN(SO2F)2 (KFSA), and their combination in ethylene carbonate/diethyl carbonate (EC/DEC), referred to as K-KPF6, K-KFSA, and K-KPF6:KFSA, respectively, after storage in contact with K metal. Through mass spectrometry analysis, we identified significant formation of carbonate ester-derived decomposition products such as oligocarbonates for K-KPF6, while K-KFSA predominantly generates anions combining FSA- with the solvent structures. Using three-electrode cells, we delineated the positive effects of the K-KFSA and K-KPF6:KFSA electrolytes on graphite negative electrode performance and the negative impact of oligocarbonates in K-KPF6 on K2Mn[Fe(CN)6] positive electrodes. The interactions between the decomposition products and the electrodes were further evaluated using density functional theory calculations. Full cell measurements using K-KPF6:KFSA showed an improved energy density and capacity retention of 78% after 500 cycles compared with an untreated electrolyte (72%). Hard X-ray photoelectron spectroscopy indicated the incorporation of the FSA-derived structures into the solid electrolyte interphase at graphite, which was not observed in K metal-free cells. Overall, this work indicates further complexities to consider in KIB measurements and suggests the potential application of decomposition products as electrolyte additives.

In situ Observation of Evolving H2 and Solid Electrolyte Interphase Development at Potassium Insertion Materials within Highly Concentrated Aqueous Electrolytes.

The solid-electrolyte interphase (SEI) is key to stable, high voltage lithium-ion batteries (LIBs) as a protective barrier that prevents electrolyte decomposition. The SEI is thought to play a similar role in highly concentrated water-in-salt electrolytes (WISEs) for emerging aqueous batteries, but its properties remain unknown. In this work, we utilized advanced scanning electrochemical microscopy (SECM) and operando electrochemical mass spectrometry (OEMS) techniques to gain deeper insight into the SEI that occurs within highly concentrated WISEs. As a model, we focus on a 55 mol/kg K(FSA)0.6 (OTf)0.4 electrolyte and a 3,4,9,10-perylenetetracarboxylic diimide negative electrode. For the first time, our work showed distinctly passivating structures with slow apparent electron transfer rates alike to the SEI found in LIBs. In situ analyses indicated stable passivating structures when PTCDI was stepped to low potentials (≈-1.3 V vs. Ag/AgCl). However, the observed SEI was discontinuous at the surface and H2 evolution occurred as the electrode reached more extreme potentials. OEMS measurements further confirmed a shift in the evolution of detectable H2 from -0.9 V to <-1.4 V vs. Ag/AgCl when changing from dilute to concentrated electrolytes. In all, our work shows a combined approach of traditional battery measurements with in situ analyses for improving characterization of other unknown SEI structures.

Active material and interphase structures governing performance in sodium and potassium ion batteries.

Development of energy storage systems is a topic of broad societal and economic relevance, and lithium ion batteries (LIBs) are currently the most advanced electrochemical energy storage systems. However, concerns on the scarcity of lithium sources and consequently the expected price increase have driven the development of alternative energy storage systems beyond LIBs. In the search for sustainable and cost-effective technologies, sodium ion batteries (SIBs) and potassium ion batteries (PIBs) have attracted considerable attention. Here, a comprehensive review of ongoing studies on electrode materials for SIBs and PIBs is provided in comparison to those for LIBs, which include layered oxides, polyanion compounds and Prussian blue analogues for positive electrode materials, and carbon-based and alloy materials for negative electrode materials. The importance of the crystal structure for electrode materials is discussed with an emphasis placed on intrinsic and dynamic structural properties and electrochemistry associated with alkali metal ions. The key challenges for electrode materials as well as the interface/interphase between the electrolyte and electrode materials, and the corresponding strategies are also examined. The discussion and insights presented in this review can serve as a guide regarding where future investigations of SIBs and PIBs will be directed.

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.

Probing the reversibility and kinetics of Li+ during SEI formation and (de)intercalation on edge plane graphite using ion-sensitive scanning electrochemical microscopy.

Ions at battery interfaces participate in both the solid-electrolyte interphase (SEI) formation and the subsequent energy storage mechanism. However, few in situ methods can directly track interfacial Li+ dynamics. Herein, we report on scanning electrochemical microscopy with Li+ sensitive probes for its in situ, localized tracking during SEI formation and intercalation. We followed the potential-dependent reactivity of edge plane graphite influenced by the interfacial consumption of Li+ by competing processes. Cycling in the SEI formation region revealed reversible ionic processes ascribed to surface redox, as well as irreversible SEI formation. Cycling at more negative potentials activated reversible (de)intercalation. Modeling the ion-sensitive probe response yielded Li+ intercalation rate constants between 10-4 to 10-5 cm s-1. Our studies allow decoupling of charge-transfer steps at complex battery interfaces and create opportunities for interrogating reactivity at individual sites.

High-Throughput Preparation of Metal Oxide Nanocrystals by Cathodic Corrosion and Their Use as Active Photocatalysts.

Nanoparticle metal oxide photocatalysts are attractive because of their increased reactivity and ease of processing into versatile electrode formats; however, their preparation is cumbersome. We report on the rapid bulk synthesis of photocatalytic nanoparticles with homogeneous shape and size via the cathodic corrosion method, a simple electrochemical approach applied for the first time to the versatile preparation of complex metal oxides. Nanoparticles consisting of tungsten oxide (H2WO4) nanoplates, titanium oxide (TiO2) nanowires, and symmetric star-shaped bismuth vanadate (BiVO4) were prepared conveniently using tungsten, titanium, and vanadium wires as a starting material. Each of the particles were extremely rapid to produce, taking only 2-3 min to etch 2.5 mm of metal wire into a colloidal dispersion of photoactive materials. All crystalline H2WO4 and BiVO4 particles and amorphous TiO2 were photoelectrochemically active toward the water oxidation reaction. Additionally, the BiVO4 particles showed enhanced photocurrent in the visible region toward the oxidation of a sacrificial sulfite reagent. This synthetic method provides an inexpensive alternative to conventional fabrication techniques and is potentially applicable to a wide variety of metal oxides, making the rapid fabrication of active photocatalysts with controlled crystallinity more efficient.

Interrogating Charge Storage on Redox Active Colloids via Combined Raman Spectroscopy and Scanning Electrochemical Microscopy.

Redox active colloids (RACs) are dispersible, cross-linked polymeric materials that incorporate a high concentration of redox-active motifs, making them attractive for next-generation size-exclusion redox flow batteries. In order to tap into their full potential for energy storage, it is essential to understand their internal charge mobility, capacity, and cyclability. Here we focus on using a combined suite of Raman spectroscopy and scanning electrochemical microscopy (SECM) tools for evaluating three important parameters that govern charge storage in viologen-RACs: their intraparticle redox active concentration, their reduction/oxidation mechanism, and their charge transfer rate. We addressed RACs using SECM imaging and single-particle experiments, from which the intraparticle diffusion and concentration parameters were elucidated. By using Raman spectroscopy coupled to surface interrogation SECM, we further evaluated their reversible redox properties within monolayer films of 80- and 135-nm-sized RACs. Most notably we have confirmed that the concentration and redox mechanisms are essentially unchanged when varying the RAC size. As expected, we see that larger particles inherently require longer times for electrolysis independent of the methodology used for their study. Our simulations further verify the internal concentration of RACs and suggest that their porosity enables solution redox active mediators to penetrate and titrate charge in their interior. The combined methodology presented here sets an important analytical precedent in decoupling the charge storage properties of new bulk materials for polymer batteries starting from probing low-dimensional assemblies and single particles using nano- and spectroelectrochemical approaches.

Soft Surfaces for Fast Characterization and Positioning of Scanning Electrochemical Microscopy Nanoelectrode Tips.

The testing of nanoelectrode tips for scanning electrochemical microscopy (SECM) is a slow and cumbersome task that often results in untimely electrode breakage due to crashing against a substrate. Here, we evaluated approach curves of nano- and microelectrodes to soft surfaces using SECM for a rapid and more convenient characterization and positioning protocol. Soft surfaces consisted of either a submerged argon bubble or a thin polydimethylsiloxane (PDMS) layer. While approach curves to Ar bubbles in the presence of a surfactant were promising for the characterization of microelectrode tips, their performance with nanoelectrodes was deficient. In contrast, approach curves to PDMS films allowed the rapid positioning of nanoelectrodes as small as 30 nm radius at speeds up to 5 µm/s without the risk of breakage. The nanoelectrodes were able to approach the polymer films multiple times without affecting their electrochemical performance. Furthermore, using a half-coated substrate with PDMS, nanoelectrodes could be retracted and positioned very close to the bare, hard substrate for characterization with traditional approach curves. We estimate time savings on tip characterization/positioning on the order of 10- to 100-fold. This simple procedure is easily implemented without the requirement of additional devices supplementing existing commercial SECM instruments.

Electrochemical Imaging of Photoanodic Water Oxidation Enhancements on TiO2 Thin Films Modified by Subsurface Aluminum Nanodimers.

Detecting metal plasmonic enhancements on the activity of semiconducting photoanodes for water oxidation is often obscured by the inherent electroactivity and instability of the metal in electrolyte. Here, we show that thin TiO2 photoanodes modified by subsurface Al nanodimers (AlNDs) display enhancements that are consistent with plasmon modes. We directly observed enhancements by mapping the oxygen evolution rates on TiO2/AlND patterns using scanning electrochemical microscopy (SECM) while exciting the surface plasmons of the nanodimers. This study highlights the importance of sample configuration for the in situ characterization of metal/photoanode interactions and suggests a route for Al-based plasmonics applied to photoelectrochemistry.