Molecular skeleton programming of premediators in sulfur electrochemistry

a) Active materials design in organic flow redox batteries (OFRBs). b) Solvent molecule design in Li metal batteries. c) Redox mediator design in Li-air batteries. d) Organic relithiation reagent design for direct repairing of positive electrode materials in spent lithium-ion batteries (LIBs). TM represents transition metal. e) Functional group extended design for composite phase change materials (CPCMs) interface modification in thermal energy storage. Discussions: in OFRBs (a), the active materials in the positive and negative electrodes consist of redox-active organic molecules, such as anthraquinones, phenazines, fluorenones, viologens, and their derivatives. Modifying functional groups at positions beyond the redox-active center allows for the tuning of key properties including redox potential, electron transfer activity, and solubility. Therefore, applying molecular skeleton programming to a given class of active materials to identify configurations with optimal target performance can help expand the library of electrode materials for OFRBs. Another potential application is the design of solvent molecules in Li metal batteries (b). For instance, fluorination strategies are commonly used in the design of functional solvents. By selecting different parent solvent molecules and controlling the number and positions of fluorine substitutions, a series of fluorinated solvents with distinct properties can be created. Fluorination is often employed to weaken the binding energy between Li+ and solvents, promoting the formation of an anion-rich solvation structure and thereby enhancing the Li-metal interfacial stability. However, fluorination typically reduces the dielectric constant of the solvent, impairing its ability to dissociate Li salts and leading to decreased ionic conductivity. The molecular skeleton programming can be used to systematically investigate how fluorination sites and degrees influence the dielectric constant and Li+ binding energy, enabling the rational design of fluorinated solvents that balance the ion transport and Li+-solvent interaction. This contributes to Li metal batteries with both high ionic conductivity and stable interfaces. Third, for Li-air batteries (c), gaseous molecules (e.g., O 2 , CO 2 ) are reduced at the porous cathode to form insulating discharge products (such as Li 2 O 2 or Li 2 CO 3 ), which deposit on the electrode and often lead to surface passivation. A potential solution is to introduce redox mediators into the electrolyte, which bind gas molecules and shift the electrochemical reduction from the electrode surface to the solution phase. Quinone-based molecules (e.g., benzoquinones, anthraquinones) represent an important class of mediators. Modifying functional groups at different sites of the aromatic ring can modulate the mediator’s activation effect on gas molecules. Through rational molecular skeleton design, the influence of types and positions of the grafted group on mediation activity can be systematically studied. The fourth case involves the direct regeneration of spent LIBs positive electrode materials (d). Regarding the failure mechanism of positive electrodes, it is recognized that continuous phase transitions caused by compositional loss (e.g., Li deficiency) represent a common characteristic across various positive electrode materials. Therefore, to directly restore the structure of degraded positive electrodes, the primary step is to design suitable relithiation reagents for compositional replenishment. Taking spent nickel-cobalt-manganese layered oxide (NCM) as an example, the conventional regeneration approach involves physical mixing of inorganic Li salts (such as LiOH or Li 2 CO 3 ) with degraded NCM followed by high-temperature sintering. However, spent NCM particles are typically polycrystalline with abundant inter-particle voids, making it difficult for solid-phase Li sources to distribute uniformly via simple physical mixing. This results in inefficient contact between Li sources and NCM secondary particles and severely compromises the compositional homogeneity in the regenerated material. A potential strategy to improve the uniformity of Li source-degraded particles contact is to design organic intermediates that can react with Li metal or Li salts. The resulting Li-organic complexes can be dissolved in a suitable solvent to create a Li-rich liquid environment. By leveraging intermolecular interactions between the complexes and the degraded positive electrode material, uniform relithiation can be achieved in the liquid phase. In this process, the thermodynamic formation energy of the Li-organic complex can be tuned by designing the type and molecular skeleton of the reaction intermediates, ensuring the transformation of the Li source from a solid “inorganic phase” to a liquid “organic phase”. Moreover, the rich elemental composition and structural diversity of organic intermediates can be harnessed through side-chain functionalization to promote selective adsorption of the molecular complexes on the surface of the degraded positive electrode, thereby thermodynamically driving spontaneous contact between the Li source and positive electrode particles. The molecular skeleton programming protocol provides an operable platform for the above investigations. Based on this protocol, one can systematically explore how the elemental composition and chemical configuration of reaction intermediates influence the thermodynamic formation energy of Li-organic complexes and their adsorption energy on the crystal surfaces of degraded materials. This enables a deep understanding of the microscopic interaction mechanisms of lithiated molecules at the solid-liquid interface. Besides, it facilitates the identification of intermediates and the corresponding molecular skeletons that not only ensure rapid reactions with Li sources but also exhibit highly-efficient adsorption affinity. Such insights would offer a pathway toward homogeneous and complete repairing of spent positive electrode materials. Fifth, moving beyond the strategies in (a-d) that primarily focus on tailoring functional groups on a specific parent molecular skeleton, the programming paradigm can be further expanded by deconstructing the functional groups themselves (e). By resolving functional groups into independent dimensions of elemental composition and geometric structure, one can achieve “functional group skeleton programming” for more versatile surface modification protocols. A potential case for this expanded application is the investigation of complex interfacial behaviors in thermal energy storage, specifically for CPCMs. A critical bottleneck in CPCMs is the inherent trade-off between energy density (dependent on interfacial wettability) and power density (dependent on interfacial thermal conductivity). Since conventional surface functionalization strategy often entangles these two properties, we envision that dismantling surface functional group into its constituent geometric and elemental dimensions could open up the possibility to decouple these interfacial behaviors. By constructing a predictive functional group atlas that maps these independent dimensions to interfacial outcomes, this approach could provide a rational pathway for investigating universal interfacial issues in thermal energy storage systems.