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Magnesium metal batteries (MMBs) currently face challenges suffering from severe Mg metal passivation and extremely high overpotential in conventional electrolytes. Herein, a strategy of using a low‐cost deep eutectic solution (DES) is proposed to modify Mg anode with the monolithic and compact coating of a MgCl2‐Al‐MgCl2 sandwich structure, enabling the stable and reversible Mg plating–stripping behavior. An organic/nanocrystal hybrid interphase is in‐situ built through a facile Mg‐Al displacement reaction between aluminum‐chloro clusters and Mg in AlCl3/Et3NHCl solution, and it can effectively minimize the adverse interfacial passivation reaction and surface diffusion barrier, affording the high ion‐conduction and electronic insulation. This DES‐assisted method guarantees a highly reversible cycling of Mg metal anode (over 5000 h at 0.1 mA cm−2 and 400 h at 2.0 mAh cm−2) in Mg(TFSI)2/DME electrolyte with the improved interfacial kinetics and low overpotential. Even at a much higher current density of 1 mA cm−2, the overpotential only undergoes a slight increase from 0.2 V (at 0.1 mA cm−2) to 0.23 V. The corresponding full cells with CuS and phenanthraquinone cathodes deliver satisfactory cyclic performance. The DES modification strategy provides a new solution to the design of robust and conductive solid electrolyte interphase for achieving high‐voltage and durable MMBs. [ABSTRACT FROM AUTHOR]

Conversion‐reaction metal fluoride cathodes have the advantages of low cost, environmental friendliness, and high energy density. However, their tailored synthesis and structure design under safe, facile, and scalable conditions have always been a puzzle, which limits the popularization of conversion‐type cathodes in lithium metal batteries. To address these challenges, here an iron fluoride heterostructure with FeF2 and FeF3 binary phases is proposed and prepared by deep eutectic solvent method under the assistance of cobalt cation as an oxidization promoter. The compact heterogeneous contact between tetragonal FeF2 and hexagonal tungsten bronze FeF3 phases enables the promotion and stabilization of interface charge transfer and topotactic conversion reaction. The narrower band gap of FeF2, higher theoretical capacity and thermodynamic potential of FeF3 synergistically contribute to the balance between cycling reversibility and capacity. The heterostructure cathode with an optimized molar ratio of FeF2/FeF3 can realize a highly reversible capacity of ≈520 mAh g−1 in the first tens of cycles and 305 mAh g−1 after 200 cycles. The typical two‐stage reaction plateaus are well preserved even under the high current density of 1000 mA g−1. The green synthesis strategy and heterostructure fluoride design open a new horizon for the development of high‐energy‐density metal fluoride batteries. [ABSTRACT FROM AUTHOR]

In Li metal batteries, the rational construction of artificial solid electrolyte interphase (ASEI) can homogenize the Li‐ion flowing and Li‐mass plating/stripping, and reinforce the mechanical and electrochemical stabilities of electrode–electrolyte interface. Here, an ordered‐mesoporous powder zirconium oxophosphate (ZrOP) is proposed to construct a Li‐ion conductive ASEI by brushing ZrOP on metallic Li. The P‐induced amorphization in ZrOP is expected to accelerate the spontaneous lithiation reaction and promote the generation of P‐contained Li‐ion conductors and LiF domains in ASEI during Li anode cycling. The electrochemical activation of ASEI with promoted ionic conductivity and interconnected porous network is favorable for the homogeneous nucleation and growth of Li metal grains (with ≈10 µm in size), and the suppression of Li dendrites and volume expansion during long‐term Li plating and stripping. The ZrOP‐modified Li symmetric cell enables a long lifespan of over 1600 h at 1 mA cm−2, and the corresponding LiNi0.8Co0.1Mn0.1O2 (NCM811) based full cell displays a stable cycling for at least 300 cycles with a capacity retention of 80% at 1 C. The modified thin Li foil (40 µm in thickness) anode also enables a multilayered pouch cell based on a high‐loading NCM811 cathode with stable cycling and reversible capacity over 1 Ah. [ABSTRACT FROM AUTHOR]

The application of solid‐state Li metal batteries based on NASICON‐type Li(1+x)AlxTi(2‑x)(PO4)3 (LATP) electrolytes has been hindered by the interface instability, originating from the spontaneous Ti4+ reduction reaction between LATP and Li. Here, a mesoporous fluorination interlayer based on high‐surface‐area AlF3 is proposed to improve the stability and compatibility of the LATP/Li interface and enable the reversible operation of NASICON‐type Li||FeF3 conversion batteries. The high Lewis acidity and porosity in the interlayer can buffer the volume evolution, increase the Li‐ion transference number and promote the formation of LiF domains. Time‐of‐flight secondary‐ion mass spectroscopy (ToF‐SIMS) proves the in situ construction of an F‐rich trap zone for Ti4+, which can pin the easily‐reduced Ti4+ in LATP and retard its conversion to Ti3+ during the electrochemical process. An electro‐fusion process at the LATP/Li interface occurs and successfully welds the solid electrolyte, AlF3 interlayer, and anode into a whole to mitigate the anode resistance, guaranteeing the fast interface migration of Li+. The corresponding Li||Li symmetric cells exhibit a stable cycling performance with small voltage hysteresis for at least 500 h. The ceramic‐type solid‐state Li‐Fe‐F cells modulated by the F‐rich transition layer can deliver a high capacity of 696.7 mAh g−1 and show a good cyclability of the conversion reaction. [ABSTRACT FROM AUTHOR]