Homogeneous and mechanically stable solid–electrolyte interphase enabled by trioxane-modulated electrolytes for lithium metal batteries

The solid–electrolyte interphase (SEI) in lithium (Li) metal batteries is often heterogeneous, containing a diverse range of species and has poor mechanical stability. The SEI undergoes constant cracking and reconstruction during electrochemical cycling, which is accompanied by the exhaustion of active Li and electrolytes, hindering practical applications of the batteries. Here we propose an in situ structural design of SEI to promote its homogeneity and improve its mechanical stability. A bilayer structure of SEI is tailored through trioxane-modulated electrolytes: the inner layer is dominated by LiF to improve homogeneity while the outer layer contains Li polyoxymethylene to improve mechanical stability, synergistically leading to mitigated reconstruction of SEI and reversible Li plating/stripping. The coin cell consisting of an ultrathin Li metal anode (50 μm) and a high-loading cathode (3.0 mAh cm⁻²)—with the tailored bilayer SEI—achieves 430 cycles tested at 1.2 mA cm⁻², while the cell with an anion-derived SEI undergoes only 200 cycles under same conditions. A prototype 440 Wh kg⁻¹ pouch cell (5.3 Ah), with a low negative/positive capacity ratio of 1.8 and lean electrolytes of 2.1 g Ah⁻¹, achieves 130 cycles.