Promoting Polysulfide Redox Reactions through Electronic Spin Manipulation

Abstract

Catalytic additives able to accelerate the lithium-sulfur redox reaction are a key component of sulfur cathodes in lithium-sulfur batteries (LSBs). Their design focuses on optimizing the charge distribution within the energy spectra, which involves refinement of the distribution and occupancy of the electronic density of states. Herein, beyond charge distribution, we explore the role of the electronic spin configuration on the polysulfide adsorption properties and catalytic activity of the additive. We showcase the importance of this electronic parameter by generating spin polarization through a defect engineering approach based on the introduction of Co vacancies on the surface of CoSe nanosheets. We show vacancies change the electron spin state distribution, increasing the number of unpaired electrons with aligned spins. This local electronic rearrangement enhances the polysulfide adsorption, reducing the activation energy of the Li-S redox reactions. As a result, more uniform nucleation and growth of Li₂S and an accelerated liquid-solid conversion in LSB cathodes are obtained. These translate into LSB cathodes exhibiting capacities up to 1089 mA h g^-1 at 1 C with 0.017% average capacity loss after 1500 cycles, and up to 5.2 mA h cm^-2, with 0.16% decay per cycle after 200 cycles in high sulfur loading cells.

Keywords: cobalt selenide; lithium polysulfide; lithium−sulfur battery; spin polarization; vacancy.

Figure 1

Characterization of vacancies in CoSe, v1-CoSe, v2-CoSe and v3-CoSe samples. (a,b) XRD patterns. (c) Raman spectra. (d) Atomic resolution AC-HAADF-STEM image of CoSe and corresponding indexed power spectrum FFT. The CoSe interplanar distances were measured to be 0.181, 0.180, and 0.103 nm, at 60.0 and 30.0°, which correspond to the (11-20), (2-1-10) and (30-30) lattice planes as visualized along its [0001] zone axis. (e–h) Atomic resolution AC-HAADF-STEM images with the intensity profiles along the selected rectangular regions suggesting missing Co atoms in (e) CoSe, (f) v1-CoSe, (g) v2-CoSe, and (h) v3-CoSe, respectively. (i,j) Atomic model schemes of the vacancies for (i) CoSe and (j) v-CoSe samples. (k) STEM-EDS elemental maps of v2-CoSe. (l) Co and Se atomic fraction obtained from STEM-EDS maps. (m) High-resolution Co 2p XPS spectra. (n) Co L-edge EELS spectra.

Figure 2

Spin configuration characterization of CoSe and v-CoSe. (a,b) Co K-edge XANES spectra. (c) EXAFS oscillation extracted from K-edge spectra of the composites in k space. (d) EXAFS fit of the composites. (e,f) Co L_2,3 XAS (average of μ+ and μ−) of (e) CoSe and (f) v-CoSe at 50 K under an applied field B = 6 T. (g) XMCD (μ+ – μ−)/2 spectra. The inset shows the differential spin density between CoSe and v-CoSe. (h). EPR spectra. (i) Normalized magnetization curves at 2 K. The inset shows the differential spin density model between CoSe and v-CoSe. (j) Schematic diagram of electron spin–orbit configuration in Co²⁺. (k) Total density of states (TDOS) and partial density of states (PDOS).

Figure 3

Adsorption characterization of CoSe and v-CoSe. (a) 3D spin density model of CoSe in conjunction with Li₂S₄, (b) 2D spin density map of CoSe and (c) v-CoSe with Li₂S₄. (d) UV–visible spectrum showing Li₂S₆ adsorption experiments, with optical photos inserted. (e) Adsorption energy of the surface for different sulfur species. (f) Co 2p XPS spectra obtained for v-CoSe before and after Li₂S₆ adsorption. (g) Operando XRD patterns during charging and discharging of batteries based on CoSe/S and v-CoSe/S.

Figure 4

Catalytic characterization of CoSe and v-CoSe. (a) Fitted EIS spectra at 2.2 V and model circuit. (b,c) Arrhenius plot of R_ct calculated from CoSe (b) and v-CoSe (c). (d) Calculated activation energies at different voltages. (e) Gibbs free energy profiles and adsorption conformation of polysulfide species. (f) CV curves obtained from symmetric cells at a scan speed of 1 mV/s. (g,h) Potentiostatic discharge curves on CoSe (g) and v-CoSe (h) to study the Li₂S nucleation kinetics. (i) Energy level scheme for pristine polysulfide and after adsorption. (j) Energy level diagram showing orbital hybridization for Li₂S₄ according to DOS results. E_F represents the Fermi level of the substrate; τ_p indicates bonding states.

Figure 5

Electrochemical performance of v-CoSe/S and CoSe/S electrodes. (a) CV curves. (b) Tafel plots calculated from the reduction peaks I and II, respectively. (c) Peak current vs square root of scan rate in oxidation process. (d) GCD curves at 0.1 C. (e) Rate performance. (f) ΔE and Q₂/Q₁ values. (g) Long cycling performance at 1 C over 1500 cycles. (h) Comparison of the capacity obtained with the v-CoSe/S cathode with specific capacities of previously reported S catalysts electrodes.⁻

Figure 6

Electrochemical performance of v-CoSe/S at high sulfur loading. (a) Long-term cycling performance at 0.1 C. (b) GCD curves at 0.1 C from the 1st to the 200th cycles. (c) Rate performance from 0.1 to 4 C. (d) EIS before cycling. (f) Pouch cells powering a temperature and humidity monitoring clock.