Breaking Barriers to High‐Practical Li‐S Batteries with Isotropic Binary Sulfiphilic Electrocatalyst: Creating a Virtuous Cycle for Favorable Polysulfides Redox Environments

Abstract Investigations into lithium–sulfur batteries (LSBs) has focused primarily on the initial conversion of lithium polysulfides (LiPSs) to Li2S2. However, the subsequent solid–solid reaction from Li2S2 to Li2S and the Li2S decomposition process should be equally prioritized. Creating a virtuous cycle by balancing all three chemical reaction processes is crucial for realizing practical LSBs. Herein, amorphous Ni3B in synergy with carbon nanotubes (aNi3B@CNTs) is proposed to implement the consecutive catalysis of S8(solid) → LiPSs(liquid) → Li2S(solid) →LiPSs(liquid). Systematic theoretical simulations and experimental analyses reveal that aNi3B@CNTs with an isotropic structure and abundant active sites can ensure rapid LiPSs adsorption‐catalysis as well as uniform Li2S precipitation. The uniform Li2S deposition in synergy with catalysis of aNi3B enables instant/complete oxidation of Li2S to LiPSs. The produced LiPSs are again rapidly and uniformly adsorbed for the next sulfur evolution process, thus creating a virtuous cycle for sulfur species conversion. Accordingly, the aNi3B@CNTs‐based cell presents remarkable rate capability, long‐term cycle life, and superior cyclic stability, even under high sulfur loading and extreme temperature environments. This study proposes the significance of creating a virtuous cycle for sulfur species conversion to realize practical LSBs.

were added to the mixture, which was then homogenized for 30 minutes at room temperature using continuous sonication.In order to create pressure for the incorporation of S into the wrinkles of the aNi 3 B@CNTs, the suspension was then kept at 50 °C for 8 hours in a sealed container.Using a rotary evaporator, the carbon disulfide was removed from the homogeneous suspension.The resulting mixture powder was vacuum-dried at 45 °C for 12 hours, producing aNi 3 B@CNTs/S composites.To conduct a control experiment, cNi 3 B@CNTs/S composites were created using the same procedure.
Elemental mapping and energy-dispersive X-ray spectroscopy (EDS) were performed with the SEM microscope.X-ray diffraction (XRD; PANalytical X-Pert PRO, USA) was utilized to confirm the phase of the material.TGA (SDTQ600, USA) was carried out in N 2 with a 10 °C/min heating rate from room temperature to 600 °C.X-ray photoelectron spectroscopy (XPS) (K-Alpha+, USA) was employed to investigate the bonding characteristics and evaluate the adsorption mechanism.For determining the surface area and pore-size distribution, Brunauer-Emmett-Teller analysis was used.

LSBs assembly and measurements
First, the cathode was prepared by mixing the composite (aNi 3 B@CNTs, cNi 3 B@CNTs), carbon black, and PVDF powder in a 7:2:1 mass ratio, and dissolving them in NMP to form a homogeneous slurry.The cathode foil was created by coating the slurry onto carbon foil and vacuum-drying it there for an entire night at 60 °C.For LSBs with typical sulfur loading, the mass loading of the active material was controlled to be around 1.0 mg cm -2 .With the above electrode serving as the cathode, lithium foil serving as the anode, and Celgard 2300 PP serving as the separator, CR-2032 coin-type cells were assembled.The electrolyte used was 1.0 M LiTFSI in DOL and DME (V/V = 1:1) with 2% LiNO 3 additives.Galvanostatic charge/discharge test and cyclic voltammetry measurements were conducted within a voltage window of 1.7-3.1 V. On the Gamry Instrument Warminster (PA, USA), electrochemical impedance spectroscopy (EIS) was carried out in the frequency range of 0.01-10 5 Hz.

Polysulfide absorption test
The Li 2 S 6 solution was prepared by dissolving S and Li 2 S in a mixed solvent of DME (dimethoxyethane) and DOL (1,3-dioxolane) in a volume ratio of 1:1.The mixture was stirred vigorously at a temperature of 60 ℃ for 24 hours to promote dissolution.For the visual adsorption test, aNi 3 B@CNTs and cNi 3 B@CNTs powders were added individually to the Li 2 S 6 solution and soaked for 2 hours, resulting in aNi 3 B@CNTs-Li 2 S 6 and cNi 3 B@CNTs-Li 2 S 6 .After 2 hours, the absorption of LiPSs was visually observed.The optical properties of the upper and blank Li 2 S 6 solutions were analyzed using UV-vis spectroscopy to investigate changes before and after adsorption.Additionally, the precipitates were collected, dried, and subjected to XPS analysis to determine the chemical composition of the reaction products.

Symmetric-cell assembly and measurements
The electrode was prepared by mixing the host material (aNi 3 B@CNTs, cNi 3 B@CNTs) and PVDF in a weight ratio of 9:1 in NMP solvent.The resulting slurry was coated onto carbon foil.A 0.5 M Li 2 S 6 electrolyte was prepared by mixing S and Li 2 S powder with a molar ratio of 5:1 in a mixing solvent of DOL and DME (volume ratio 1:1) with 1 M LiTFSI salt at 60 ℃ for 12 hours in an Ar-filled glove box.Symmetric CR2032 coin cells were assembled with 0.5 M Li 2 S 6 electrolyte (25 μL) and two identical above electrodes (mass loading of about 1.0 mg cm-2) as both the working and counter electrode.CV measurement of the symmetric cell was performed over a voltage range of -1.5 to 1.5 V to evaluate polysulfide redox conversion kinetics.EIS was acquired in a frequency range of 0.01 to 10 5 Hz.

Measurements of the Li 2 S nucleation/dissolution
A homogenous slurry was prepared by mixing 90 wt% aNi 3 B@CNTs and 10 wt% PVDF in NMP solvent under vigorous stirring.The slurry was coated on carbon foil, and then the foil was dried at 60 ℃ for 12 h.Lithium foil and PP were used as the anode and separator, respectively, while the cathode was constructed using aNi 3 B@CNTs material.On the cathode side, 20 mL of Li 2 S 8 was added, and on the Li anode side, blank electrolyte without Li 2 S 8 was added.The cell was discharged at 0.1 mA to 2.06 V galvanostatically to convert Li 2 S 8 to Li 2 S 6 , followed by potentiostatic discharging at 2.05 V to completely convert polysulfides to Li 2 S until the current decreased to 1.0 × 10 -5 mA.For the Li 2 S dissolution test, to fully transform S species into solid Li 2 S on catalytically reactive interfaces, the assembled cells cells were then potentiostatically charged at 2.40 V for the oxidation of Li 2 S into soluble LiPSs, with the charge ending when the charge current was below 10 -5 A. The cells were disassembled to observe the morphology of Li 2 S deposition/dissolution on both hosts.

Shuttle current measurements
LiNO 3 -free electrolyte was used for the shuttle current measurement to avoid passivating the lithium anode. [1,2]Prior to galvanostatic charging at 0.2 C to 2.8 V, cells underwent three cycles of discharge-charge.After that, the cells were discharged to 2.38 V and transferred to potentiostatic mode, which is when the shuttle current reached its maximum.When the potentiostatic current reached a steady state, it was referred to as the shuttle current.(Figure S6a-b).The Randles-Sevcik formula was employed to evaluate Li + diffusion efficiency, which is given by the equation: [3] Ip = (2.69 × 10 5 )  1.5    + 0.5   +  0.5 where Ip is the peak current, n is the electron charge number, A is the electrode area, D Li+ is the diffusion coefficient of Li + , C is the Li + concentration in the cathode, and v is the scan rate.Since n, A, C, and v are known, Ip and v 0.5 are correlated linearly, and the slope of the curve (Ip-v 0.5 ) is positively correlated with D Li + .The slope of aNi 3 B@CNTs/S was steeper compared to that for cNi 3 B@CNTs/S (as shown in Figure S6c).This indicates that aNi 3 B@CNTs exhibited a faster Li + diffusion rate, which facilitated the conversion of polysulfides throughout the entire charge/discharge process.Note: When the change in current density is the smallest, or when dI/dV=0, the baseline voltage and current density are determined as the values preceding the redox peaks.The onset current density and corresponding CV curves are 10 μA cm -2 beyond the corresponding baseline current density. [4,5]Specifically, for the cathodic peaks, the onset current density is 10 μA cm -2 more negative than the baseline current density, while for the anodic peak, it is 10 μA cm -2 more positive than the baseline current density.The inset shows the baseline voltages, with the colored region indicating the current density gap of 10 μA cm -2 .after cycles. [3]te:

Figure S7 .Figure S8 .
Figure S7.CV curves of symmetric cells with the aNi 3 B@CNTs electrode after the four three cycles.

Figure S11 .
Figure S11.Charge-discharge curves at various current densities of a) aNi 3 B@CNTs-based cell, and b) cNi 3 B@CNTs-based cell.

Figure S13 .
Figure S13.The fitted Randles EIS equivalent circuits of Li-S cells a) before cycling, and b)

Figure S15 .
Figure S15.Postmortem studies of the cycled lithium anodes.a-c) Digital pictures.d-f) SEM images.g-i) elemental mapping of sulfur.j-l) EDS analysis.

Figure S16 .
Figure S16.a) CV profiles comparison tested at 50 ℃ with a scan rate of 0.1 mV s -1 , and corresponding Tafel plots of b) peak B, and c) and peak C.

Figure S18 .
Figure S18.The cyclic performance of CNTs-based cell at 2C for 300 cycles.

Figure S20 .
Figure S20.Post HR-TEM image of a) aNi 3 B@CNTs after cyclic process, and b) cNi 3 B@CNTs after cyclic process.Several Fast Fourier transform (FFT)/inverse FFT pictures of the selected square regions are shown on the right; Post-XPS spectra of c) B1s, and d) Ni2p from aNi 3 B@CNTs; e) B1s, and f) Ni2p from cNi 3 B@CNTs electrocatalyst comparison before and after cyclic process.

Figure S21 .
Figure S21.Rate performance of aNi 3 B@CNTs-based cell at high sulfur loading.

Figure S23 .
Figure S23.Comparison of a).Cycle life, and b) Rate capability with previous reported materials.

Table S2 .
Corresponding comparison of cyclic performance at high sulfur loading with previous reported materials.

Table S3 .
Corresponding comparison of cyclic life with previous reported materials.

Table S4 .
Corresponding comparison of rate capability with previous reported materials.