Fast Energy Storage of SnS2 Anode Nanoconfined in Hollow Porous Carbon Nanofibers for Lithium‐Ion Batteries

Abstract The development of conversion‐typed anodes with ultrafast charging and large energy storage is quite challenging due to the sluggish ions/electrons transfer kinetics in bulk materials and fracture of the active materials. Herein, the design of porous carbon nanofibers/SnS2 composite (SnS2@N‐HPCNFs) for high‐rate energy storage, where the ultrathin SnS2 nanosheets are nanoconfined in N‐doped carbon nanofibers with tunable void spaces, is reported. The highly interconnected carbon nanofibers in three‐dimensional (3D) architecture provide a fast electron transfer pathway and alleviate the volume expansion of SnS2, while their hierarchical porous structure facilitates rapid ion diffusion. Specifically, the anode delivers a remarkable specific capacity of 1935.50 mAh g−1 at 0.1 C and excellent rate capability up to 30 C with a specific capacity of 289.60 mAh g−1. Meanwhile, at a high rate of 20 C, the electrode displays a high capacity retention of 84% after 3000 cycles and a long cycle life of 10 000 cycles. This work provides a deep insight into the construction of electrodes with high ionic/electronic conductivity for fast‐charging energy storage devices.


Lithium ionic conductivity determined by galvanostatic intermittent titration technique (GITT).
1.2 Theoretical analysis of Li-ions adsorption behavior in the electrode.

Table S2.
Comparison of performance of Sn-based anode materials for Li-ion batteries.

Lithium ionic conductivity determined by galvanostatic intermittent titration technique (GITT)
To further study the lithium-ion diffusion properties of SnS2@N-HPCNFs electrode, the lithium-ion diffusion coefficient was calculated by GITT.The lithium-ion diffusivity ( Li D + ) could be calculated by the quotation as following [1] : Where t is the relaxation time,   is the molar volume,   is the mass loading of electrode, S is the contact area between electrode material and electrolyte, S E  is the voltage change caused by pulse, and t E  is the voltage change of constant current charge and discharge.In this GITT teat, the battery was discharged/charged between at 0.5 A g -1 for 900 s, and then relaxed under open circuit for 2 h (Figure S10).

Theoretical analysis of Li-ions adsorption behavior in the electrode.
To provide microscopic insight into the lithium storage performance of SnS2@N-HPCNFs, density functional theory (DFT) calculations were performed by the Vienna ab initio simulation package (VASP). [2-3]The ground state ion-electron wave-functions were described by the projected augmented wave (PAW) method. [4]Electron exchange-correlation was expressed by functional proposed by Perdew, Burke and Ernzerhof within the framework of generalized gradient approximation (GGA-PBE). [5]A cutoff energy of 550 eV was adopted for the planewave pseudopotential.The convergence thresholds during geometry optimization were set as 10 -4 eV and 0.01 eV/Å for total energy and residual force, respectively.The Grimme's dispersion correction with Becke-Johnson (BJ) damping functions (DFT-D3) [6] was adopted to include the weak van der Waals (vdW) interactions.
The original model of SnS2/carbon nanofibers heterostructures (SnS2@HPCNFs HSs) was composed of graphene 6×6 supercell and single-layer SnS2 4×4 supercell.The vacuum slab was set to be 30 Å to avoid the periodic interactions between neighboring layers.The SnS2/nitrogendoped carbon nanofibers heterostructures (SnS2@N-HPCNFs HSs) were then modeled by substituting one of the carbon atoms in graphene layer (see Figure S6).The N-doped carbon nanofiber (N-HPCNFs) was modeled by substituting one of the carbon atoms in bilayer graphene 6×6 supercell.In the meantime, bilayer SnS2 4×4 supercell and bilayer graphene 6×6 supercell were also modeled as references.The Brillouin zone was sampled by 3 × 3 × 1 kpoint meshes during structural relaxation.Denser 5 × 5 × 1 k-point meshes were applied for electronic property calculations.The climbing image nudged elastic band (CI-NEB) method [7-  8] was employed to calculate the diffusion pathway and diffusion barrier of Li ion in the interlayer of HSs.Discussion: Theoretical capacity of SnS2@N-HPCNFs.according to the calculation as following equation [9] :     C, 10 C, 20 C, 30 C, demonstrating a high reversible electrochemical reaction.However, the initial discharge/charge specific capacity of SnS2@N-HCNFs were 1170.3,1817.3 mAh g -1 at 0.1 C, with an ICE of 64.39%, which is lower than that of SnS2@N-HCNFs.Discussion: After extended cycling, the XRD spectra of the electrode approach an amorphous state, with broad and indistinct peaks, consistent with the results observed in electrodes with alloying/conversion reaction mechanisms. [10-11]The diffraction peak appearing at around 32.12°corresponded to the crystal plane of SnS2 (JCPDS NO. 23-0677), indicating that the SnS2 still existed after 10000 cycles.Furthermore, the SnS2@N-HPCNFs//LFP full cell displays a discharge capacity of 45.9 mAh g -1 at 1 C after 90 cycles with 0.7% capacity decay per cycle (Figure S19b), indicating superior rate capability and long cycling performance.

Figure S15 .
Figure S15.a) Rate capability, and b) long cycling performance of N-HPCNFs.

Figure
Figure S19.a) Rate capability, and b) long cycling performance of SnS2@N-HPCNFs//

Figure S7 .
Figure S7.The possible configurations of SnS2/nitrogen-doped carbon nanofibers heterostructures (SnS2@N-HPCNFs HSs), as well as their relative energies (ΔE) to total energy of Configuration 2.Discussion: To determine the most stable doping configurations of SnS2@N-HPCNFs, we have

Figure S11 .
Figure S11.The capacitive-controlled contribution at scan rate of 1.0 mV s -1 of SnS2@N-

Figure S15 .
Figure S15.a) Rate capability, and b) long cycling performance of N-HPCNFs.

Figure S20 .
Figure S20.The initial discharge and charge capacity of SnS2@N-HPCNFs||LFP were 139.45 and 168.86 mAh g -1 at 0.1 C, with a high initial Coulombic efficiency of 82.58%.

Table S2 .
Comparison of performance of Sn-based anode materials for Li-ion batteries.