Although lithium metal has become a promising anode material for high-energy batteries owing to its high specific capacity and the lowest reduction potential, the continuous side reactions with electrolyte as well as the safety problem caused by Li dendrite growth restrict Li anode’s practical application. Herein, we demonstrate that N-fluoropyridinium (ArF⁺) bis(trifluoromethane)sulfonimide (TFSI⁻) as an electrolyte additive can protect the lithium metal by both solid electrolyte interphase (SEI) protection and electrostatic repulsion mechanisms. The ArF⁺ cations not only participate in forming F, N-containing SEI protective layer on Li surface, but also act as a cationic repellent during Li deposition to inhibit Li dendrite growth. As a result, the cycle performance of Li symmetric cells and Li||LiFePO₄ full cells were significantly improved by using ArFTFSI-added electrolyte. This study provides an electrolyte additive strategy for Li anode realizing SEI protection and electrostatic repulsion simultaneously.

Pseudocapacitive materials that store charges via reversible surface or near-surface faradaic reactions are capable of overcoming the capacity limitations of electrical double-layer capacitors. Revealing the structure–activity relationship between the microstructural features of pseudocapacitive materials and their electrochemical performance on the atomic scale is the key to build high-performance capacitor-type devices containing ideal pseudocapacitance effect. Currently, the high brightness (flux), and spectral and coherent nature of synchrotron X-ray analytical techniques make it a powerful tool for probing the structure–property relationship of pseudocapacitive materials. Herein, we report a comprehensive and systematic review of four typical characterization techniques (synchrotron X-ray diffraction, pair distribution function [PDF] analysis, soft X-ray absorption spectroscopy, and hard X-ray absorption spectroscopy) for the study of pseudocapacitance mechanisms. In addition, we offered significant insights for understanding and identifying pseudocapacitance mechanisms (surface redox pseudocapacitance, intercalation pseudocapacitance, and the extrinsic pseudocapacitance phenomenon in battery materials) by combining in situ hard XAS and electrochemical analyses. Finally, a perspective for further depth of understanding into the pseudocapacitance mechanism using synchrotron X-ray analytical techniques is proposed.

Similar to lithium-ion batteries (LIBs), during the first charge/discharge process of lithium-ion capacitors (LICs), lithium-intercalated anodes (e.g., silicon, graphite, and hard carbon) also exhibit irreversible lithium intercalation behaviors, such as the formation of a solid electrolyte interface (SEI), which will consume Li⁺ in the electrolyte and significantly reduce the electrochemical performance of the system. Therefore, pre-lithiation is an indispensable procedure for LICs. At present, commercial LICs mostly use lithium metal as the lithium source to compensate for the irreversible capacity loss, which has the demerits of operational complexity and danger. However, the pre-lithiation strategy based on cathode sacrificial lithium salts (CSLSs) has been proposed, which has the advantages of low cost, simple operation, environmental protection, and safety. Therefore, there is an urgent need for a timely and comprehensive summary of the application of CSLSs to LICs. In this review, the important roles of pre-lithiation in LICs are detailed, and different pre-lithiation methods are reviewed and compared systematically and comprehensively. After that, we systematically discuss the pre-lithiation strategies based on CSLSs and mainly introduce the lithium extraction mechanism of CSLSs and the influence of intrinsic characteristics and doping amount of CSLSs on LICs performance. In addition, a summary and outlook are conducted, aiming to provide the essential basic knowledge and guidance for developing a new pre-lithiation technology.