Lu Zhang

Nonaqueous redox flow batteries are a promising technology that utilize redox-active species (i.e., redoxmers) in solution to store energy via electron-transfer (ET) reactions with electrodes. However, electron transfer (ET) phenomena at the interface of graphitic electrodes and nonaqueous media are poorly understood, with several non-idealities in the use of conventional models such as the Butler–Volmer model reported. Possibilities for these non-idealities include the adsorption of redox species at the electrode, fundamental ET limitations related to the density of states at the electrode, and the presence of chemical and spatial heterogeneities at the surface of the electrode. To this point, we present a computational and experimental approach to comparatively investigate the ET behavior of two redoxmers, ferrocene (Fc) and 2,3-dimethyl-1,4-dialkoxybenzene (C7) on single layer graphene (SLG), hydrogen-functionalized SLG (H-SLG), and pristine and hydrogen-functionalized graphite electrodes. Scanning electrochemical microscopy (SECM) experiments revealed enhanced ET kinetics for both redoxmers on H-SLG electrodes compared to pristine SLG electrodes, with the degree of functionalization playing a key role in this enhancement. Electrodes such as boron-doped diamond and hydrogenated graphite mirrored these enhancements. Density functional theory (DFT) calculations indicate only small differences in the binding strengths for Fc and C7 redoxmers on SLG and H-SLG surfaces, but Marcus–Hush–Chidsey (MHC) kinetic theory analysis suggests that the density of states (DOS) of the carbon electrode likely plays a crucial role in the observed ET enhancement. These findings refine our initial assumption of binding energy (BE) as a dominant factor for interfacial behavior in the case of Fc and C7 redoxmers. Our findings create new opportunities to explore systems with varying degrees of surface modification to understand and design better redox flow batteries.

The solubility of redox-active organic molecules (ROMs) in non-aqueous redox flow batteries (NRFBs) is a critical factor determining the energy density of the system. However, the scarcity of comprehensive solubility data has hindered electrolyte development. In this study, we systematically investigate the solubility behavior of ROMs in the presence of supporting salts to propose practical electrolyte formulations for NRFBs. Using automated high-throughput experimentation, we screen the solubility of 2,1,3-benzothiadiazole (BTZ) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in various organic solvents. Leveraging a Random Forest inference model, we identify a binary solvent mixture of mxylene and acetonitrile, which dissolves 3 M of both BTZ and LiTFSI—exceeding the previously reported 2 M limit in pure acetonitrile. This enhanced solubility is achieved by the inclusion of a LiTFSI-phobic yet BTZ-philic solvent, which counterbalances the solubility competition between BTZ and LiTFSI, with the latter favoring highly polar solvents. This work introduces a promising electrolyte design strategy for NRFBs and highlights the effectiveness of high-throughput screening combined with advanced data analysis for optimizing complex multi-component systems. Furthermore, it emphasizes the urgent need for more comprehensive solubility data to facilitate the development of practical NRFB electrolytes.

The study introduces novel asymmetric hydrofluoroether (HFE) designs, uncovers solvation-property relationships, and leverages molecular dynamics modeling for high-performance electrolytes in advanced lithium batteries.

Electrolyte design is critical for enabling next-generation batteries with higher energy densities. Hydrofluoroether (HFE) solvents have drawn a lot of attention as the electrolytes based on HFEs showed great promise to deliver highly desired properties, including high oxidative stability, ionic conductivity, as well as enhanced lithium metal compatibility. However, the structure-dynamics-properties relationships and design principles for high-performance HFE solvents are still poorly understood. Herein, we proposed four novel asymmetric HFE designs by systematically varying polyether and fluorocarbon structural building blocks. By leveraging molecular dynamics (MD) modeling to analyze the solvation structures and predict the properties of the corresponding 1 M lithium bis(fluorosulfonyl)imide (LiTFSI) solutions, we downselected the most promising candidate based on high conductivity, solvation species distribution, and oxidative stability for extensive electrochemical characterizations. The formulated electrolyte demonstrated properties consistent with the predictions from the simulations and showed much-improved capacity retention as well as Coulombic efficiency compared to the baseline electrolytes when cycled in lithium metal cells. This work exemplifies the construction of candidate electrolytes from building block functional moieties to engineer fundamental solvation structures for desired electrolyte properties and guide the discovery and rational design of new solvent materials.

Redox flow batteries (RFBs) hold promises for long-duration energy storage, with various chemistries demonstrated by academia and industry. However, their practical applications remain limited due to high active material cost, low energy density, narrow voltage window, and limited cyclability. Improving the performance of redox active materials (redoxmers) in RFBs is crucial for addressing these challenges. In this study, to better understand the impact of π-conjugation of aromatic redox cores on the redoxmers’ electrochemical behavior, we systematically investigated a series of dialkoxy benzene based redoxmers, including 1,4-dimethoxy benzene (DMB), 1,4-dimethoxynaphthalene (DMN), 2-ethyl-9,10-dimethoxyanthracene (EDMA). As the conjugation increases from DMB to DMN, we observed a lower redox potential, improved kinetic stability, and longer cycling life. However, further expanding the conjugation as in EDMA resulted in rapid O-dealkylation of the radical cation, possibly due to increased strain in methoxy groups. For DMN, the radical cation is observed to be prone to neutralization (likely via disproportionation) and can be recharged, suggesting a longer cycling life than the calendar life. Our study indicates that extending the π-system alters reactivity in multiple ways, and lowering the oxidation potential through π-conjugation to improve redoxmer stability should be cautiously pursued.

A novel simplicity-biased molecular structure generation method and Bayesian optimization based active learning have been used to discover a new class of heterocyclic molecules for organic non-aqueous redox flow batteries.

Redoxmers are organic active molecules storing electrochemical energy in nonaqueous redox flow batteries (NRFBs). Increasing the solubility of redoxmers is an important approach for increasing energy density of NRFBs as effective redoxmer concentration determines how much electricity can be stored in a given volume. Molecular engineering redoxmers towards liquid forms is regarded as one promising strategy as liquid redoxmers represent an extreme scenario where fluidity is maintained at maximum concentration using a minimum amount of supporting solvents. In this Perspective, recent examples of liquid redoxmers as well as their development strategy will be discussed.

Integrated computational and experimental studies of concentrated redoxmer/electrolyte solutions reveal that charging leads to softening of the solution.