Confined Synthesis of Amorphous Al2O3 Framework Nanocomposites Based on the Oxygen‐Potential Diagram as Sulfur Hosts for Catalytic Conversion

Abstract Sulfur cathodes in Li–S batteries suffer significant volumetric expansion and lack of catalytic activity for polysulfide conversion. In this study, a confined self‐reduction synthetic route is developed for preparing nanocomposites using diverse metal ions (Mn2+, Co2+, Ni2+, and Zn2+)‐introduced Al‐MIL‐96 as precursors. The Ni2+‐introduced Al‐MIL‐96‐derived nanocomposite contains a “hardness unit”, amorphous aluminum oxide framework, to restrain the volumetric expansion, and a “softness unit”, Ni nanocrystals, to improve the catalytic activity. The oxygen‐potential diagram theoretically explains why Ni2+ is preferentially reduced. Postmortem microstructure characterization confirms the suppressive volume expansion. The in situ ultraviolet–visible measurements are performed to probe the catalytic activity of polysulfide conversion. This study provides a new perspective for designing nanocomposites with “hardness units” and “softness units” as sulfur or other catalyst hosts.


Synthesis of Al-MIL-96
Al-MIL-96 were prepared using co-solvent method.Typically, 5 mmol 1,3,5benzenetricarboxylic acid (BTC) and 5 mmol aluminum nitrate nonahydrate were successively dissolved in 60 mL mixture solution of 30 mL N,N-dimethylformamide (DMF)/30 mL deionized water.The resulting solution was transferred to a 100 mL Teflon high-pressure autoclave.After sealing, the autoclave was kept at 160 °C for 24 h, and then cool down to room temperature.The precipitate was collected by centrifugation, and washed several times with deionized water and methanol.Then the product was heated at 50 °C under vacuum in a drying oven for 24 h to remove the solvent in the MOF.

Synthesis of Al/M'-Mx (x = 1, 5, and 10) and Al/M'-M5-S
The bimetallic Al/M'-Mx samples were synthesized by the same method as for Al-MIL-96 except that a certain amount of nitrates of Mn 2+ , Co 2+ , Ni 2+ , and Zn 2+ was added together with aluminum nitrate nonahydrate into the solution.The molar content of BTC is the same as Al 3+ and second metal.Sublimed sulfur was introduced into MOFs through a melt-diffusion method at 155 °C.Taking Al-MIL-96 with 60 ω% sublimed sulfur-loading (Al-MIL-96-S) as an example, 100 mg of Al-MIL-96 powder was firstly grinded together with 150 mg of sublimed sulfur into a fine mixture that was transferred into a 25-mL Teflon-line sealed autoclave, and heated at 155 °C for 12 h to generate the Al-MIL-96-S sample.The other MOF crystals with 60 ω% sublimed sulfur-loading samples were obtained similarly.

Synthesis of Al/M'-M5-3h and the sulfur-loaded sample
The dried Al/M'-M5 were heated to 700 o C with a heating rate of 3 o C min -1 in an N2 atmosphere and keep 700 o C for 3 h, then directly cooled down to room temperature.The sulfur-loading process is the same as the Al/M'-M5-S except the sulfur content of 70%.

Synthesis of Al/Ni-M5-3h-H and Al/Ni-M5-3h-H-S
To remove the NiNCs in Al/Ni-M5-3h, 4 mol/L HCl were prepared, and then 60 mg Al/Ni-M5-3h was added for soaking 24 h.The precipitate was collected by centrifugation, and washed several times with deionized water and alcohol.Then the product was heated at 50 °C under vacuum in a drying oven for 24 h.The sulfur-loading process and sulfur content are the same as the Al/M'-M5-3h-S.

Characterization
The XRD patterns were performed by Rigaku MiniFlex 600 with Cu Kα radiation of 40 KV (λ = 1.5418Å).SEM images were obtained by Zeiss-Supra 55 microscope at an acceleration voltage of 5 KV.TEM and EDX elemental mapping scans were recorded using Tecnai G2 F30 S-TWIN at an acceleration voltage of 300 KV.The N2 adsorption-desorption isothermals were obtained by Autosorb-IQ3.Electron paramagnetic resonance (EPR) spectra were recorded on a Bruker A300 electron paramagnetic resonance spectrometer at room temperature.Raman spectra were obtained via INVIA REFLEX (Renishaw), in the range 150−4000 cm -1 .XPS analysis was carried out using Thermo Scientific ESCALAB 250Xi Xray photoelectron spectrometer with Al Kα radiation as the excitation source.The survey thickness is 2−3 nm.The accurate sulfur mass on each electrode was calculated according to TGA curves under N2 flow, 5 °C /min, and the elemental analysis data from Elementar, VarioELcube Co. (C, H, N, S mode).XAFS measurements of Al/M'-M5 were performed in Canadian Light Source, Saskatoon, Canada.XAFS measurements of Al/Ni-M5-3h and Al/Ni-M5-3h-S were performed in Shanghai Synchrotron Radiation Facility.
UV/Vis spectra of the above solutions (diluted 5 times before testing) were recorded by using a UV2550 instrument (Shimadzu, Japan).The concentration variations in these solutions was detected by the UV/Vis spectrocopy.

DFT Computational methods and structures
The MIL-96-Al (002) surface was represented as a theoretical calculation model.All structure relaxations calculations within spin-polarized DFT were executed in the Vienna ab initio simulation package (VASP) [1] .The electron exchange−correlation interactions were described by the Perdew−Burke−Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) [2] .A cutoff energy of 350 eV was adopted for the plane-wave basis and the k-points were sampled using 1 × 1 × 1 Monkhorst-Pack mesh.To eliminate the possible interactions between neighboring slab layers, a vacuum layer with a thickness of 15.0 Å was set along the nonperiodic c-axis direction.The energy and force convergence thresholds for the iteration in the self-consistent field (SCF) were set to 0.01 eV and 0.01 eV Å -1 , respectively.The adsorption energy of Li2S4 and S8 (Eads) was calculated by: where E[substrate+Li2S4/S8], Esubstrate, and ELi2S4/S8 are the total energies of the adsorbed-substrate complex, the substrate, and the isolated Li2S4 or S8, respectively.

Li−S cell assembling and test
Electrode preparation: The slurry was mixed with sulfur-loaded sample, Super P, and

In situ UV/Vis measurement
The cathode slurry comprised of an active material powder Al/Ni-M5-S or Al/Ni-M5-3h-S, Super P, and PVDF with a mass ratio of 7:2:1.Then, the nickel foam was selected as the collector (1 × 0.6 cm 2 ).The sulfur mass loading on the electrode was ca.6 mg cm -2 , and the current density is 0.05 C. The in situ UV/Vis cells were assembled using Al/Ni-M5-S or Al/Ni-M5-3h-S electrode as the cathodes and Li metal as the anode, using a custom made in situ cuvette.The in situ cuvette cell was assembled in an Ar filled glove box and sealed in 3 mL of Li−S electrolyte.UV/Vis absorption spectra (UV/Vis, Shimadzu UVmini-1280 spectrophotometer) were used to detect the concentration and elemental chemical states of the LPS.

In situ XRD measurement
In situ XRD was performed on X-ray diffractometer (MiniFlex600-C) and a cell mould with Be.The mass ratio of slurry is the same as in situ UV/Vis test, and sulfur area mass loading on the electrode was 1.3 mg cm -2 , and the current density is 0.1 C. The current collector is Al foil.

In situ Raman measurement
In situ Raman was performed on Raman spectroscopy (NVIA) and a cell mould with glass, The mass ratio of slurry is the same as in situ UV/Vis test, and the sulfur area mass loading on the electrode was 1.2 mg cm -2 , and the current density is 0.1 C. The current collector is Cu mesh.Table S3.Elemental analysis data (mass fraction) of the sulfur-loaded samples.

Samples
polyvinylidene fluoride (PVDF) in a weight ratio of 7:2:1 in N-methyl-2-pyrrolidone as dispersant.The slurry was cast on the Al foil, and dried overnight at 50 °C under vacuum.The obtained working electrodes were cut to circular electrode with a diameter of 12 mm.The mass loading of active sulfur was ca.1.1 mg cm -2 .The CR2032-type coin cells were fabricated using the working electrode, lithium foil as the counter and anode electrode, Celgard 2400 as the separator.The electrolyte was used 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI Sigma-Aldrich (USA), 99.95%) in 1,3-dioxolane (DOL, Sigma-Aldrich (USA), 99.0%) and 1,2-dimethoxyethane (DME, Sigma-Aldrich (USA), 99.0% (volume ratio, 1:1) with 1 ω% LiNO3 in an argon-filled glove box (where both water and oxygen levels are below 0.1 ppm.The value of the electrolyte to S (E/S) ratio is ca.18 μL mg -1 (according to the weight of S).The GCD tests were estimated in the voltage window of 1.7−2.7 V.The rate capability was also tested by varying the current density from 0.1 C to 1 C (1 C = 1675 mA g -1 ) on a battery measurement system (CT3001A, Wuhan Land, China) at room temperature.CV and EIS curves were measured on an electrochemical workstation (CHI660E, Chenhua, Shanghai, China).

Figure S10 .
Figure S10.DFT calculations of the optimized geometric structures of adsorping surfaces.

Figure S12 .
Figure S12.Comparison of the UV/Vis absorption spectra of Al-MIL-96 and Al/M'-M5 supernatants at 3 h.

Figure S13 .
Figure S13.Comparison of the CV curves during third cycle of Al/M'-M5-S at 0.1 mV s -1 .

Table S4 .
Elemental analysis data (mass fraction) of the sulfur-loaded samples.