The redox-targeting reactions can disruptively boost the energy density but introduce additional free-energy loss. Here, the Nernstian-potential-driven redoxtargeting reaction considerably eliminates the voltage loss of the system while maintaining a high energy density. Driven by the Nernstian potential difference, the redox molecule, a ferrocene-grafted ionic liquid with standard potential identical to that of LiFePO 4 , reacts with the solid material both anodically and cathodically and exhibits near-unity solid material utilization, 95% voltage efficiency, and energy density.

Redox-targeting reactions of battery materials by redox molecules are extensively studied for energy storage since the first report in 2006. Implementation of the "redox-targeting" concept in redox flow batteries presents not only an innovative idea of battery design that considerably boosts the energy density of flow-battery system, but also an intriguing research platform applied to a wide variety of chemistries for different applications. Here, a critical overview of the recent progress in redox-targeting-based flow batteries is presented and the development of the technology in the various aspects from mechanistic understanding of the reaction kinetics to system optimization is highlighted. The limitations presently lying ahead for the widespread applications of "redox targeting" are also identified and recommendations for addressing the constraints are given. The adequate development of the redox-targeting concept should provide a credible solution for advanced large-scale energy storage in the near future.

Redox flow batteries (RFBs) have been extensively investigated because of their great operation flexibility and scalability for large-scale energy storage, yet they suffer from low energy density and relatively high cost when price per kWh is considered. Here, we report an aqueous redox flow lithium battery (RFLB) system based on the concept of Nernstian potential-driven redox targeting reactions of battery materials to address the above issues. With [Fe­(CN)6]4–/[Fe­(CN)6]3– and S2–/S2 2– as the redox mediators in the catholyte and anolyte, the cell reveals an anodic and cathodic volumetric capacity up to 305 and 207 Ah L–1 when LiFePO4 and LiTi2(PO4)3 are respectively loaded into the cathodic and anodic tank as energy storage materials. These are 4–6 times as high as that of the vanadium redox flow battery (VRB). In addition, with water-based electrolytes, the system presents notably enhanced Li+ conductivity in the membrane and consequently much improved power performance as compared to its nonaqueous counterpart. We anticipate that this work would be a paradigm and pave the way for the deployment of redox targeting-based flow battery technology for large-scale applications.

Heteroatom doping has been proved to effectively enhance the sloping capacity, nevertheless, the high sloping capacity almost encounters a conflict with the disappointing initial Coulombic efficiency (ICE). Herein, we propose a heteroatom configuration screening strategy by introducing a secondary carbonization process for the phosphate‐treated carbons to remove the irreversible heteroatom configurations but with the reversible ones and free radicals remaining, achieving a simultaneity between the high sloping capacity and ICE (≈250 mAh g−1 and 80 %). The Na storage mechanism was also studied based on this “slope‐dominated” carbon to reveal the reason for the absence of the plateau. This work could inspire to distinguish and filter the irreversible heteroatom configurations and facilitate the future design of practical “slope‐dominated” carbon anodes towards high‐power Na‐ion batteries.

The redox targeting reaction of Li-storage materials with redox mediators is the key process in redox flow lithium batteries, a promising technology for next-generation large-scale energy storage. The kinetics of the Li-coupled heterogeneous charge transfer between the energy storage material and redox mediator dictates the performance of the device, while as a new type of charge transfer process it has been rarely studied. Here, scanning electrochemical microscopy (SECM) was employed for the first time to determine the interfacial charge transfer kinetics of LiFePO/FePO upon delithiation and lithiation by a pair of redox shuttle molecules FcBr and Fc. The effective rate constant k was determined to be around 3.70-6.57 × 10 cm/s for the two-way pseudo-first-order reactions, which feature a linear dependence on the composition of LiFePO, validating the kinetic process of interfacial charge transfer rather than bulk solid diffusion. In addition, in conjunction with chronoamperometry measurement, the SECM study disproves the conventional "shrinking-core" model for the delithiation of LiFePO and presents an intriguing way of probing the phase boundary propagations induced by interfacial redox reactions. This study demonstrates a reliable method for the kinetics of redox targeting reactions, and the results provide useful guidance for the optimization of redox targeting systems for large-scale energy storage.