Pre-constructing a mortice-tenon joint based-layer to achieve an enhanced SEI on Li metal anode†

For the efficient functioning of a lithium anode, it requires an ideal protective layer that has its own strength and strongly bonds with the substrate. There are many studies on the strength of such protective layers, but very few reports on their bond strength with substrates. Herein, a design strategy is proposedto pre-construct a based-layer, where a mortice-tenon joint will connect with the subsequent electrochemically active SEI that is set on a Li anode surface. Initially, a tightly bonded base layer was chemically formed via the reaction between 2-(fluorosulfonyl)difluoroacetate (DFSA) and lithium metal. Then, trimethylsilyl 2-(fluorosulphonyl)difluoroacetate (TSFSA), which has a similar molecular structure and same functional group as DFSA, was introduced to act as an SEI enhancer that can preferentially decompose over carbonate solvents under electrochemical conditions with the same components of the based-layer, which was thus strengthened to form an enhanced SEI (ESEI). The Li anode with ESEI achieved long cycling stability (≥ 2100 h) and a high average CE (99.2%) in carbonate electrolytes. Full cells with high cathode loading (20.5 mg cm⁻²) also achieved high cycling stability at low N/P ratios, demonstrating its great prospects for practical applications in high energy density Li-metal batteries.