Fig. 1: Crystal structure and lithium-ion diffusion properties.
a, The XRD patterns of the Li3N samples processed by different ball-milling speeds (the ball-milling time was 8 h for all samples). The commercially available Li3N is usually a mixture of α- and β-phases. α-Li3N (space group P6/mmm) transforms into β-Li3N (space group P63/mmc) at an increased pressure. The ball-milling method (speed 400 rpm) is chosen to create high pressure for obtaining pure β-Li3N from the commercial mixed-phased Li3N. b, Arrhenius plots of β-Li3N as a function of the ball-milling time (the ball-milling speed is constant of 400 rpm) and commercial Li3N. The lithium-ion conductivity of Li3N is evaluated by the alternating current (a.c.) impedance method using pressed pellets. The plots compare the Arrhenius behaviour of commercial Li3N with β-Li3N samples milled at 400 rpm for varying durations (8 h, 16 h and 24 h). c, Lithium-ion conductivity at 25 °C and activation energy of β-Li3N plotted as functions of ball-milling duration, conducted at a consistent speed of 400 rpm. For comparison, commercial Li3N (without ball milling) is also presented. This figure summarizes the room-temperature ionic conductivities and activation energies as a function of ball-milling times. d, SXRD patterns and TOF neutron diffraction data (bank 3) with corresponding Rietveld refinement results for β-Li3N-400rpm-16h. The corresponding crystal structures with a focus on lithium and nitrogen vacancies are refined using the Rietveld method. e, Crystal structure of vacancy-rich β-Li3N and the calculated formation energy of single neutral lithium vacancy at 2b and 4f sites (2b site, 1.43 eV; 4f site, 0.81 eV), respectively. f, Schematic illustration of vacancy-mediated superionic diffusion mechanism of vacancy-rich β-Li3N. EHOP, lithium-ion hopping energy; Ea, activation energy for lithium-ion conduction; n, concentration of mobile lithium ions; σ, ionic conductivity.
