Materials preparation

Layered TiS₂ (99.8%, Strem Chemical Inc.) with an average particle size of 10 μm was used as purchased. A slurry of active material (70 wt.%), Super-P carbon (20 wt.%), and polyvinylidene fluoride (10 wt.%) dispersed in N-methyl-2-pyrrolidone was spread on a piece of stainless steel mesh (400 mesh, 0.8 cm²) and dried as the working electrode with active material mass loading of 0.5–1 mg cm^–2. To prepare samples for analysis, we prepared electrode by cold pressing 7 mg of TiS₂ powders onto stainless steel mesh at 10 MPa without using binder or conductive agent. Freshly polished magnesium foil (50 μm thick, 99.95%, GalliumSource, LLC) was used as both the counter and reference electrodes in 2- or 3-electrode cell tests. Standard APC electrolyte, a solution of 0.25 M [Mg₂Cl₃]⁺[AlPh₂Cl₂]^– in THF, was prepared following Aurbach et al. and was used as the Mg-ion electrolyte throughout this work¹⁹. PY14⁺ ion was added in the APC electrolyte by dissolving 1-butyl-1-methylpyrrolidinium chloride (PY14Cl, >98%, TCI America Co.) to the concentration of 0.2 M.