Manipulating Li2S Redox Kinetics and Lithium Dendrites by Core–Shell Catalysts under High Sulfur Loading and Lean‐Electrolyte Conditions

Abstract For practical lithium–sulfur batteries (LSBs), the high sulfur loading and lean‐electrolyte are necessary conditions to achieve the high energy density. However, such extreme conditions will cause serious battery performance fading, due to the uncontrolled deposition of Li2S and lithium dendrite growth. Herein, the tiny Co nanoparticles embedded N‐doped carbon@Co9S8 core–shell material (CoNC@Co9S8NC) is designed to address these challenges. The Co9S8NC‐shell effectively captures lithium polysulfides (LiPSs) and electrolyte, and suppresses the lithium dendrite growth. The CoNC‐core not only improves electronic conductivity, but also promotes Li+ diffusion as well as accelerates Li2S deposition/decomposition. Consequently, the cell with CoNC@Co9S8NC modified separator delivers a high specific capacity of 700 mAh g−1 with a low‐capacity decay rate of 0.035% per cycle at 1.0 C after 750 cycles under a sulfur loading of 3.2 mg cm−2 and a E/S ratio of 12 µL mg−1, and a high initial areal capacity of 9.6 mAh cm−2 under a high sulfur loading of 8.8 mg cm−2 and a low E/S ratio of 4.5 µL mg−1. Besides, the CoNC@Co9S8NC exhibits an ultralow overpotential fluctuation of 11 mV at a current density of 0.5 mA cm–2 after 1000 h during a continuous Li plating/striping process.

Micromeritics ASAP 2020 C to determine the specific surface area, pore volume and pore size distribution. X-ray photoelectron spectroscopy was performed by PHI 5000 Versa Probe III with a monochromatic Al Kα X-ray source, with a base pressure better than 5×10 -7 Pa for analysis. X-ray powder diffraction (XRD, PANalytical empyrean series 2, Netherlands) patterns were collected with CuKα radiation and Raman spectroscopy (LabRAM HR Evolution, France) were collected with 473 nm laser source. UV-vis spectroscopy of the solutions was collected by an ultraviolet and visible spectrophotometer (UV-3600). Elemental analysis was performed on the Thermo Fisher Scientific iCAP RQ inductively coupled plasma mass spectrometer (ICP-MS).

Nucleation and Dissolution of Li 2 S
0.5 M Li 2 S 8 solution was obtained by dissolving Li 2 S and S (molar ratio of 1:7) in tetraethylene glycol solvent under vigorous stirring at 60°C for 48 h. As-prepared samples, Super P and polyvinylidene fluoride (PVDF) (weight ratio of 8:1:1) were dispersed in NMP and coating on the Al foil with a diameter of 12 mm and dried at 60 °C for 12 h to be used as cathode. 25 μL LiTFSI electrolyte was added into the Li side and 25 μL Li 2 S 8 solution was added into the cathode side. S1 The cell was discharged galvanostatically to 2.06 V at 0.112 mA, and then discharged potentiostatically at 2.05 V until current decreased to 10 -5 A. To investigate the dissolution of Li 2 S, fresh cells were first discharged at a current of 0.10 mA to 1.80 V, and subsequently discharged at 0.01 mA to 1.80 V until full conversion of LiPSs into solid Li 2 S. Then, the cells were potentiostatically charged at 2.40 V for the dissolution of Li 2 S into LiPSs until charge current was below 10 -5 A. S2

Preparation of CoNC@Co 9 S 8 NC modified separator
The as-prepared CoNC@Co 9 S 8 NC and PVDF with a weight ratio of 9:1 was mixed in NMP to form a homogeneous slurry and then coated onto polypropylene (PP) separators (Celgard 2400). The obtained CoNC@Co 9 S 8 NC modified separator was dried under vacuum at 60 °C for 12 h and cut into 16 mm circular disks (mass loading ≈ 1.85 mg cm -2 ). CoNC and Co 9 S 8 NC modified separators were prepared through similar procedures.

Preparation of S cathode
CMK-3 and S with a weight ratio of 3:7 was added in 20 mL CS 2 solutions and then sonicated until the CS 2 solution evaporates completely. The mixture was heated 160 °C in an oven for 24 h under Ar atmosphere to get CMK-3/S. The composites, Super P and PVDF were mixed (with a weight ratio of 8:1:1) in NMP to form a slurry.
Then, the slurry was coated on carbon cloth and then dried in an oven at 60 °C for 12 h. The mass loading of S was 3.2 mg cm −2 ~ 8.9 mg cm −2 .

Assembly of Li 2 S 6 Symmetric Cells
Symmetric cells were assembled with two same electrodes of CoNC@Co 9 S 8 NC, CoNC and Co 9 S 8 NC. 0.2 M Li 2 S 6 solution (in DME/DOL) solution containing 1.0 M LiTFSI and 0.1 M LiNO 3 was used as the electrolyte. Cyclic voltammetry (CV) tests were carried out at scan rates of 0.5 mV s -1 and 20 mV s -1 between -1.0 V and 1.0 V on the CHI660E electrochemical workstation.

Electrochemical Tests
The electrochemical tests were carried out by using CR2032-type coin cells which were assembled in an Ar-filled glove box. The Li metal, as-prepared modified separator, and S cathode were used as anode, separator and cathode, respectively.
Electrolyte was the 1 M lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) and 0.1 M LiNO 3 in DOL/DME (1:1 by volume) solvents. The electrolyte/sulfur (E/S) was kept at ~12 μL mg -1 , or 4.5 μL mg -1 per cell. CV curves were tested at scan rates from 0.1 mV s -1 to 0.5 mV s -1 between 1.7 V and 2.8 V, and electrochemical impedance spectroscopic (EIS) were performed at a frequency range of 0.01 Hz-10 kHz on the CHI660E electrochemical workstation. The galvanostatic charge-discharge profiles were obtained using a Land battery tester with a voltage from 1.7 to 2.8 V vs Li + /Li.

Computational Methods
The present first principle DFT calculations are performed by Vienna Ab initio Simulation Package (VASP) S3 with the projector augmented wave (PAW) method. S4 The exchange-functional is treated using the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional. The energy cutoff for the plane wave basis expansion was set to 450 eV and the force on each atom less than 0.02 eV/Å was set for convergence criterion of geometry relaxation.15 Å vacuum was added along the z direction in order to avoid the interaction between periodic structures. The Brillouin zone integration are performed using 3×3×1 and 3×2×1 k-point sampling for Co (111) and Co 9 S 8 (440), respectively. The self-consistent calculations apply a convergence energy threshold of 10 -5 eV. The DFT-D3 method was employed to consider the van der Waals interaction. S5 Transition state searching were calculated using the climbing-image nudged elastic band (CI-NEB) method.
The adsorption energy of DME, DOL molecules and Li atom were calculated according to where E total is the total energy of the DME/DOL/Li adsorbed systems, E sub and E Li are the energies of the substrate and the isolated DME/DOL/Li, respectively. Figure S1. SEM images and XRD pattern of ZIF-67.