Construction of Co3O4/ZnO Heterojunctions in Hollow N‐Doped Carbon Nanocages as Microreactors for Lithium–Sulfur Full Batteries

Abstract Lithium–sulfur (Li–S) batteries are promising alternatives of conventional Li‐ion batteries attributed to their remarkable energy densities and high sustainability. However, the practical applications of Li–S batteries are hindered by the shuttling effect of lithium polysulfides (LiPSs) on cathode and the Li dendrite formation on anode, which together leads to inferior rate capability and cycling stability. Here, an advanced N‐doped carbon microreactors embedded with abundant Co3O4/ZnO heterojunctions (CZO/HNC) are designed as dual‐functional hosts for synergistic optimization of both S cathode and Li metal anode. Electrochemical characterization and theoretical calculations confirm that CZO/HNC exhibits an optimized band structure that effectively facilitates ion diffusion and promotes bidirectional LiPSs conversion. In addition, the lithiophilic nitrogen dopants and Co3O4/ZnO sites together regulate dendrite‐free Li deposition. The S@CZO/HNC cathode exhibits excellent cycling stability at 2 C with only 0.039% capacity fading per cycle over 1400 cycles, and the symmetrical Li@CZO/HNC cell enables stable Li plating/striping behavior for 400 h. Remarkably, Li‐S full cell using CZO/HNC as both cathode and anode hosts shows an impressive cycle life of over 1000 cycles. This work provides an exemplification of designing high‐performance heterojunctions for simultaneous protection of two electrodes, and will inspire the applications of practical Li–S batteries.

Finally, the products were collected by centrifugation, washing, and drying process.
For the synthesis of Co MOFs and Zn MOFs, the process is the same as that of Co/Zn MOFs except for using 2 mmol of Co(NO 3 ) 2 ·6H 2 O or Zn(NO 3 ) 2 ·6H 2 O and stirred for 40 min or 6 h respectively.

Synthesis of CZO/HNC, Co 3 O 4 /HNC, and ZnO/HNC:
The as-prepared Co/Zn MOFs, Co MOFs and Zn MOFs were separately treated at 370 ℃ for 2 h in air with a heating rate of 0.5 ℃ min -1 . After cooling to room temperature, the obtained materials were labelled as CZO/HNC, Co 3 O 4 /HNC, and ZnO/HNC respectively.

Materials characterizations:
The structure and morphology of samples were characterized by the SEM (Hitachi S-4800), and AC TEM (Titan G2 60-300 cubed). XRD (Bruker-D8 ADVANCE) was used to investigate the crystal structures. The elemental status was obtained with XPS (Thermo Fisher Scientific). The nitrogen adsorption isotherm was collected with a SI-MP-10 (Quanatachrome). TGA (Pyris 1 DSC) was used to confirm the contents of sulfur in the samples under Ar flow with a heating rate of 10 ℃/min. Contact angel was measured on a CA-100C with the sessile drop technique.

Theoretical Calculations:
We have employed the first-principles to perform density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) formulation. We have chosen the projected augmented wave (PAW) potentials to describe the ionic cores and take valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 450 eV. Partial occupancies of the Kohn-Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV. The electronic energy was considered self-consistent when the energy change was smaller than 10 -5 eV. A geometry optimization was considered convergent when the energy change was smaller than 0.03 eV Å -1 . In our structure, the U correction is used for Co atoms. The vacuum spacing in a direction perpendicular to the plane of the structure is 20 Å for the Co 3 O 4 and ZnO surfaces. The Brillouin zone integration is performed using 3×3×1 Monkhorst-Pack k-point sampling for a structure. Finally, the adsorption energies 4 (E ads ) were calculated as E ads = E ad/sub -E ad -E sub , where E ad/sub , E ad , and E sub are the total energies of the optimized adsorbate/substrate system, the adsorbate in the structure, and the clean substrate, respectively. The free energy was calculated using the equation: where G, E ads , ZPE and TS are the free energy, total energy from DFT calculations, zero point energy and entropic contributions, respectively.

Electrochemical measurements:
Symmetric cells were assembled with two identical electrodes of CZO/HNC, The anode films were prepared by mixing with 90 wt% CZO/HNC and 10 wt% PVDF in NMP, followed with coating on the Cu foil and drying in a vacuum oven at 55 °C overnight. The Li capacity was controlled to be 10 mAh cm -2 by galvanostatic charging for 10 h at a current density of 1 mA cm -2 used for the subsequent tests (half cell and Li-S full cell). As for the symmetrical cell testing, two identical electrodes with pre-deposition capacity of 10 mAh cm -2 (Li@Cu or Li @CZO/HNC) were assembled and cycled. As for the CE measurements, bare Cu or CZO/HNC were used                       Table S1. Summary of the surface area and pore volume of the three products.