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curves of the spin standard TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxyl) with concentrations varying from 0.01 mM to 100 mM. Lithium cation diffusion coefficient (DLi) of the range of samples was determined by 7Li pulsed field gradient (PFG) NMR measurement at Larmor frequency of 2π×232.98 rad MHz at 20 °C using a 14.1 T (600 MHz 1H) NMR spectrometer (Agilent, USA) equipped with a 5 mm z- gradient probe (Doty Scientific, USA), which can generate a maximum gradient strength of ~31 T/m. The echo height, S(g), recorded as a function of gradient strength, g, was fitted with the Stejskal-Tanner equation, 45 S(g)=S(0)e¿¿ (1) where S(g) and S(0) are the echo heights at the gradient strengths of g and 0, respectively, D is the diffusion coefficient, γ is 7Li gyromagnetic ratio, Δ is the time interval between the two gradient pulses, i.e. diffusion delay, and δ is the gradient length. The PFG-echo profiles were obtained using the stimulated echo sequence employing the bipolar gradient pulses (Dbppste, a vender supplied sequence, Agilent, USA) as a function of gradient strength varied with 16 equal steps. The maximum gradient strength was chosen according to the echo height at the maximum gradient strength. The 90° pulse length, Δ and δ were 8 μs, 30 and 2 ms, respectively. Micro-Raman measurements were performed using a 633 nm laser source, which was attenuated using a variable neutral density filter wheel (to 25 μW/μm2), reflected off a dichroic beam splitter, and focused onto the sample using a 10X air objective. Our commercial Raman microscope is based on an inverted optical microscope (Nikon Ti-E) coupled to a Raman spectrometer (Horiba LabRAM HR). The backscattered light is collected through the same objective, transmitted through the beam splitter cube, and dispersed through a 600 l/mm grating. The effective resolution of the instrument in 3 cm-1 using this configuration. Operando X-ray absorption spectroscopy measurements Li-S battery cells of LiǀZ_0.5 M Li2S4ǀECF_Li2S8 were assembled in specially prepared CR2032 coin-cell cases. The cathode case was punched with a rectangular hole of 2 mm (horizontal) × 1 mm (vertical) at the center and tightly patched using 7 μm-thick Kapton tape, which has X-ray transmission of ~81% at 2470 eV). After assembly, no electrolyte leakage was observed. Li-S cells were tightly sealed in an aluminum foil envelope and then shipped to the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory for the XAS measurements. The operando XAS experiments were performed at Beamline 10.3.2 and 5.3.1 at the ALS. The X-ray beam size was 8 μm (horizontal) × 5 μm (vertical). The XAS spectra were collected in partial fluorescence-yield mode and calibrated using elemental sulfur spectra. All the S K-edge XAS spectra were collected under constant helium gas flow in a small chamber in front of the cell, and were acquired continuously during the discharging process. Electrochemical measurements The electrochemical characteristics of Li-S cells were investigated by PCGA, which is a common technique for characterization of electrochemical processes. In our case, PCGA was carried out by setting stepwise potential scans of 5 mV with a minimum current limit of 50 μA at Page 13 of 24PDF Image | A lithium-sulfur battery with a solution-mediated pathway
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