In situ AFM visualization of Li-O2 battery discharge products during redox cycling in an atmospherically controlled sample cell

In Situ Electrochemical Atomic Force Microscopy: From Interfaces to Interphases

The electrochemical interface formed between an electrode and an electrolyte significantly affects the rate and mechanism of the electrode reaction through its structure and properties, which vary across the interface. The scope of the interface has been expanded, along with the development of energy electrochemistry, where a solid-electrolyte interphase may form on the electrode and the active materials change properties near the surface region. Developing a comprehensive understanding of electrochemical interfaces and interphases necessitates three-dimensional spatial resolution characterization. Atomic force microscopy (AFM) offers advantages of imaging and long-range force measurements. Here we assess the capabilities of AFM by comparing the force curves of different regimes and various imaging modes for in situ characterizing of electrochemical interfaces and interphases. Selected examples of progress on work related to the structures and processes of electrode surfaces, electrical double layers, and lithium battery systems are subsequently illustrated. Finally, this review provides perspectives on the future development of electrochemical AFM.

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.

In-situ characterization of discharge products of lithium-oxygen battery using Flow Electrochemical Atomic Force Microscopy

The increasing interest in lithium-oxygen batteries (LOB), having the highest theoretical energy densities among the advanced lithium batteries, has triggered the search for in-situ characterization techniques, including Electrochemical Atomic Force Microscopy (EC-AFM). In this work we addressed the characterization of the formation and decomposition of lithium peroxide (Li₂O₂) on a carbon cathode using a modified AFM technique, called Flow Electrochemical Atomic Force Microscopy (FE-AFM), where an oxygen-saturated solution of the non-aqueous lithium electrolyte is circulated through a liquid AFM cell. This novel technique does not require keeping the AFM equipment inside a glove-box, and it allows performing a number of experiments using the same substrate with different electrolytes without disassembling the cell. We study the morphology of Li₂O₂ on graphite carbon using lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in dimethyl sulphoxide (DMSO) as electrolyte under different operational conditions, in order to compare our results with those reported using other electrolytes and in-situ and ex-situ EC-AFM.

Product formation during discharge: a combined modelling and experimental study for Li–O$$_2$$ cathodes in LiTFSI/DMSO and LiTFSI/TEGDME electrolytes

Li–air or Li–$$\text{O}_2$$ O 2 batteries are a promising energy storage technology due to the potentially high energy density. However, significant challenges related to reversible charge/discharge of these cells need to be solved. The discharge reaction is generally agreed to proceed via two main routes, which may occur simultaneously. These are the surface mechanism, leading to $$\text{Li}_2\text{O}_2$$ Li 2 O 2 product formation as surface films, or the solution mechanism, with solid particles formed in the pore structure of the cathode. A detailed understanding of the reaction mechanisms and the dynamic performance of the electrodes is key to further improvements. Here, we present a mathematical model for the discharge process, based on porous electrode theory, including effects of reactant transport and kinetic limitations, as well as the continuous change of properties due to the formation of reaction products via the solution mechanism and the surface mechanism. The model describes the dynamic change in the ratio of the surface and solution mechanism as a function of growth of film thickness, in line with recent findings. The model is able to predict the differences in experimentally obtained discharge curves between dimethyl sulfoxide and tetra ethylene glycol dimethyl ether solvents with 1M LiTFSI, with a minimum of free parameters. The model parameters are based on physical characterization of the materials and the electrodes, or determined by fitting to impedance spectra recorded during the discharge. The developed model and the methodology will provide a powerful tool for optimization of such electrodes.

Graphic abstract

Material balance in the O2 electrode of Li-O2 cells with a porous carbon electrode and TEGDME-based electrolytes

This work figures out the material balance of the reactions occurring in the O₂ electrode of a Li–O₂ cell, where a Ketjenblack-based porous carbon electrode comes into contact with a tetraethylene glycol dimethyl ether (TEGDME)-based electrolyte under more practical conditions of less electrolyte amount and high areal capacity. The ratio of electrolyte weight to cell capacity (E/C, g A h⁻¹) is a good parameter to correlate with cycle life. Only 5 cycles were obtained at an areal capacity of 4 mA h cm⁻² (E/C = 10) and a discharge/charge current density of 0.4 mA cm⁻², which corresponds to the energy density of 170 W h kg⁻¹ at a complete cell level. When the areal capacity was decreased to half (E/C = 20) by setting a current density at 0.2 mA cm⁻², the cycle life was extended to 18 cycles. However, the total electric charge consumed for parasitic reactions was 35 and 59% at the first and the third cycle, respectively. This surprisingly large amount of parasitic reactions was suppressed by half using redox mediators at 0.4 mA cm⁻² while keeping a similar cycle life. Based on by-product distribution, we will propose possible mechanisms of TEGDME decomposition and report a water breathing behavior, where H₂O is produced during charge and consumed during discharge.

The material balance in the O₂ electrode of a Li–O₂ cell with a Ketjenblack-based porous carbon electrode and a tetraethylene glycol dimethyl ether-based electrolyte under more practical conditions of less electrolyte amount and high areal capacity.