The development of solid state batteries is constrained by imperative factors like the high reactivity of lithium with solid electrolytes, high grain boundary resistance of the solid electrolyte, and interfacial resistance between the solid electrolyte and electrodes. In the present work we exploited Li1.5Al0.5Ge1.5(PO4)3 (LAGP) as a solid electrolyte and noticed a spontaneous reactivity of lithium with LAGP. Further, we observed that  introduction of a wetted polypropylene (PP) layer between LAGP and lithium successfully prevented the undesirable reaction between lithium and the surface of the solid electrolyte. Aside from that, it also considerably decreased the interfacial resistance and polarization. This strategy has been applied to both Li-ion and Li–S battery systems, and we observed considerable improvement in the electrochemical performance of these solid state devices. The lithium ion battery retained 100% capacity and Coulombic efficiency after 50 cycles with an observed capacity of 190 mAhg–1. After 200 cycles with varying C-rates, 98.5% capacity retention is observed. Similarly, in the Li–S battery, LAGP effectively restricted the polysulfide shuttle and maintained 78% of the initial capacity after 200 cycles. Such high capacity retention is the outcome of the PP layer which functions as an interlayer that protects the LAGP surface against reacting with the lithium metal anode as inferred by post-mortem analysis.

Magnesium–Sulfur batteries are one of most appealing options among the post‐lithium battery systems due to its potentially high energy density, safe and sustainable electrode materials. The major practical challenges are originated from the soluble magnesium polysulfide intermediates and their shuttling between the electrodes, which cause high overpotentials, low sulfur utilization, and poor Coulombic efficiency. Herein, a functional Mo6S8 modified separator is designed to effectively address these issues. Both the experimental results and density functional theory calculations show that the electrochemically active Mo6S8 layer has a superior adsorption capability of polysulfides and simultaneously acts as a mediator to accelerate the polysulfide conversion kinetics. Remarkably, the magnesium–sulfur cell assembled with the functional separator delivers a high specific energy density (942.9 mA h g−1 in the 1st cycle) and can be cycled at 0.2 C for 200 cycles with a Coulombic efficiency of 96%. This work demonstrates a new design concept toward high‐performance metal–sulfur batteries.

In non-aqueous Mg batteries, inactive adsorbed species and the passivation layer formed from the reactive Mg with impurities in the electrolyte seriously affect the Mg metal/electrolyte interface. These adlayers can impede the passage of Mg 2+ ions, leading to high Mg plating/stripping overpotential. Herein, we report the properties of a new additive, bismuth triflate (Bi(OTf) 3 ), for chlorine-free Mg electrolyte to enhance Mg plating/stripping from initial cycles. The beneficial effect of Bi(OTf) 3 can be ascribed to the Bi/Mg 3 Bi 2 formed in-situ on the Mg metal surface，which increases the charge transfer during the on-off transition by reducing the adsorption of inactive species on the Mg surface and enhancing the resistance of the reactive surface to passivation. This simple method provides a new avenue to improve the compatibility between Cl-free Mg electrolyte and Mg metal anode.

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