Leo W. Gordon

Ionic liquids (ILs) are low-temperature molten salts, where ion transport is primarily governed by ion–ion interactions. Yet, the impact of organic IL cations on critical electrolyte properties such as ion dissociation and overall transport behavior in lithium-salt-doped ILs remains poorly understood. Moreover, despite their critical role in designing IL-based electrolytes for energy storage applications, ion–ion interactions and ion-specific transport under an applied electrical potential are seldom quantified, largely due to the unique experimental and computational challenges involved. Herein, we compare transport properties obtained using 1H, 7Li, and 19F pulsed-field gradient nuclear magnetic resonance (NMR) and electrophoretic NMR (eNMR) with those measured by electrochemical impedance spectroscopy. Non-equilibrium molecular dynamics (MD) simulations and eNMR confirm the presence of negatively charged [Li(TFSI) n ](1−n) aggregates that migrate towards the positive electrode, resulting in negative lithium transference numbers. Equilibrium MD simulations reveal a vehicular Li ion transport mechanism facilitated by long-lived aggregates with Li+ cations strongly bound to multiple TFSI− anions. Finally, we observe an inverse relationship between the apparent charge of the TFSI− anion in the neat IL, which is dictated by the IL cation, and Li+ transport in the salt-doped systems. This highlights the opportunity to tune electrolyte performance by tailoring cation chemistry.

A quantitative electrochemical technique is presented for the identification of charge storage mechanisms in energy storage materials using cyclic voltammetry at various scan rates. This approach enables the distinction between Faradaic diffusion-limited, capacitive, and Faradaic non-diffusion-limited (pseudocapacitive) processes based on their characteristic dependencies on time of mass and charge transfer. Unlike traditional qualitative techniques that rely on the shape of voltammograms, the proposed tool determines the relative contributions of each mechanism with high accuracy across a range of electrochemical conditions. Representative Faradaic, capacitive and pseudocapacitive electrode and electrolyte materials were selected to exemplify each mechanism and validate the technique. To facilitate data analysis, a MATLAB graphical user interface was developed in addition to a Python-based Jupyter notebook. The tools automate the extraction of mechanism-specific contributions and visualize their variation with electrode potential, enabling efficient interpretation of large-scale datasets. This tutorial provides a standardized protocol for quantifying charge storage mechanisms in diverse electrode-electrolyte systems. The ability to differentiate and quantify charge storage mechanisms supports the rational design of energy storage materials with tailored energy and power characteristics. The method is broadly applicable to batteries, supercapacitors, and hybrid systems, and offers a foundation for advancing energy storage technologies that require both high performance and mechanistic clarity.

Selenium (Se) is an attractive positive electrode material for rechargeable aluminum (Al) batteries due to its high theoretical capacity of 2037 mA h g^–1 and its higher electronic conductivity compared to sulfur. Selenium can undergo a series of electrochemical reactions between Se(−II) and Se(IV), resulting in a six-electron capacity per Se atom. However, existing Al−Se battery literature is inconsistent regarding the different electrochemical reactions possible, while the conditions enabling the electrochemical reduction of Se to Al₂Se₃ are not well understood. Here, we demonstrate that this electrochemical reduction is achievable using amorphous selenium but is suppressed for crystalline selenium. We further show that the electrochemical oxidation of Se to SeCl₄, which occurs at higher potentials, reduces the long-range order of crystalline Se and enables its discharge to Al₂Se₃. Solid-state ⁷⁷Se nuclear magnetic resonance (NMR) measurements further establish that the local Se helical structures are maintained upon the loss of crystallinity.

Rechargeable aluminum-organic batteries are composed of earth-abundant, sustainable electrode materials while the molecular structures of the organic materials can be controlled to tune their electrochemical properties. Aluminum metal batteries typically use electrolytes based on chloroaluminate ionic liquids or deep eutectic solvents that are comprised of polyatomic aluminum-containing ions. Quinone-based organic electrodes store charge when chloroaluminous cations (AlCl₂⁺) charge compensate their electrochemically reduced carbonyl groups, even when such cations are not natively present in the electrolyte. However, how ion speciation in the electrolyte affects the ion charge storage mechanism, and resultant battery performance, is not well understood. Here, we couple solid-state NMR spectroscopy with electrochemical and computational methods to show for the first time that electrolyte-dependent ion speciation significantly alters the molecular-level environments of the charge-compensating cations, which in turn influences battery properties. Using 1,5-dichloroanthraquinone (DCQ) for the first time as an organic electrode material, we utilize dipolar-mediated solid-state NMR experiments to elucidate distinct aluminum coordination environments upon discharge that depend significantly on electrolyte speciation. We relate DFT-calculated NMR parameters to experimentally determined quantities, revealing insights into their origins. The results establish that electrolyte ion speciation impacts the local environments of charge-compensating chloroaluminous cations and is a crucial design parameter for rechargeable aluminum-organic batteries.

Today’s electrochemical energy storage technologies aim to combine high specific energy and power, as well as long cycle life, into one system to meet increasing demands in performance. These properties, however, are often characteristic of either batteries (high specific energy) or capacitors (high specific power and cyclability). To merge battery- and capacitor-like properties in a hybrid energy storage system, researchers must understand and control the co-existence of multiple charge storage mechanisms. Charge storage mechanisms can be classified as faradaic, capacitive, or pseudocapacitive, where their relative contributions determine the operating principles and electrochemical performance of the system. Hybrid electrochemical energy storage systems can be better understood and analyzed if the primary charge storage mechanism is identified correctly. This tutorial review first defines faradaic and capacitive charge storage mechanisms and then clarifies the definition of pseudocapacitance using a physically intuitive framework. Then, we discuss strategies that enable these charge storage mechanisms to be quantitatively disentangled using common electrochemical techniques. Finally, we outline representative hybrid energy storage systems that combine the electrochemical characteristics of batteries, capacitors and pseudocapacitors. Modern examples are analyzed while step-by-step guides are provided for all mentioned experimental methods in the Supplementary Information.