Magnetic-field-guided catalytic effect mitigates Li2S passivation of lithium–sulfur batteries
Lang Liao, Ruijin Meng, Chen Zhou, Shusheng Li, Wei Xu, Xiaoning Li, Kai Zhang, Lu Chen, Fangyi Shi, Duanzijing Liu, Hongying Hou, Chi Zhang, Jinhu Yang
TL;DR
Applying a magnetic field to a catalyst in lithium-sulfur batteries helps reduce performance issues by improving sulfur recycling and battery stability.
Contribution
Introducing magnetic-field-guided catalysis to simultaneously address polysulfide shuttle and Li2S passivation in Li–S batteries.
Findings
A 400-mT magnetic field enhances α-Fe2O3's catalytic activity for polysulfide conversion and Li2S deposition/dissociation.
Magnetic alignment and electron spin polarization improve the interaction between α-Fe2O3 and sulfur species.
The modified cathode shows excellent cycling stability and rate capability in Li–S batteries.
Abstract
Polysulfides shuttle and Li2S passivation are considered key problems of lithium–sulfur (Li–S) batteries, seriously hindering sulfur recycling for practical applications. However, the prevailing strategies for introducing electrocatalysts focus mainly on the polysulfides-shuttle effect, whereas Li2S passivation involving solid conversions with high energy barriers has been rarely studied and remains challenging. Herein, we propose that applying a magnetic field (MF) to weak ferromagnetism α-Fe2O3 shows an enhanced effect, not only suppressing the polysulfides shuttle, but also alleviating Li2S passivation by synchronously catalysing polysulfides conversion and Li2S deposition/dissociation. Experimental and theoretical studies reveal that the MF can promote the consistent alignment of magnetic-domain orientations in α-Fe2O3 and enhance the electron spin polarization of the Fe 3d orbital,…
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Figure 6- —National Natural Science Foundation of China10.13039/501100001809
- —Shanghai Municipal Education Commission10.13039/501100003395
- —Fundamental Research Funds for the Central Universities10.13039/501100012226
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TopicsAdvanced Battery Materials and Technologies · Extraction and Separation Processes · Advancements in Battery Materials
INTRODUCTION
Lithium–sulfur (Li–S) batteries have been regarded as one of the next-generation power systems because of their high energy density, low cost and environmental compatibility [1]. However, many thorny challenges limit their practical application, such as the inevitable ‘shuttle effect’ of soluble lithium polysulfides (LiPSs) intermediates and the sluggish reaction kinetics of sulfide conversion [2–6]. The weak interaction between the non-polar carbonaceous matrix [7–11] and polysulfides displays a limited effect on suppressing the shuttle effect and accelerating sulfide conversion over long-term cycling. Oxides, nitrides, sulfides, selenides, metal–organic frameworks, MXene and metal–N–C materials of group 3d transition metals are known to have strong adsorption and catalytic abilities for LiPSs [12–20]. In order to further improve the catalytic performance of catalysts, coordination modulation [21,22], defect control [18], heterojunctions [23], element doping [24,25] and applying external fields have been investigated.
It is noted that the whole electrochemical processes of Li–S batteries include conversion reactions from S_8_ to long-chain LiPSs (S_8_→Li_2_S_4_) and subsequently to short-chain Li_2_S_2_ and Li_2_S (Li_2_S_4_→Li_2_S_2_→Li_2_S) during the discharging process, followed by delithiation conversion reactions (Li_2_S→S_8_) during the charging process [1,3]. Relative to LiPS conversions (S_8_→Li_2_S_4_) at solid–liquid-phase interfaces, the short-chain sulfides conversions that undergo solid–solid phase reactions (Li_2_S_2_→Li_2_S) possess higher energy barriers [20], leading to slower conversion kinetics and depositing rates of Li_2_S,
which are prone to forming a Li_2_S passivation film on the electrodes. It is noteworthy that the dissociation of Li_2_S from the passivation film has a high energy barrier during the charging process [12], resulting in low sulfur utilization and thereby a poor cycling performance. However, unlike the polysulfides shuttle that can be mitigated through the intrinsic adsorption/catalytic effect of electrocatalysts, the issues for Li_2_S passivation films are much more challenging and have scarcely been investigated, as they require the regulation of Li_2_S-conversion dynamics and deposition/dissociation behaviors in the solid phase. Therefore, an effective strategy for alleviating Li_2_S passivation is highly desired for the development of Li–S batteries, especially with the simultaneous suppression of polysulfides shuttle through a simple and feasible method.
Recently, applying a magnetic field (MF) to transition-metal catalysts has emerged as a simple and effective approach in various electrochemical systems, including oxygen evolution reactions [26], oxygen reduction reactions [27], hydrogen evolution reactions [17], lithium-ion batteries, Li–O_2_ batteries and Li–S batteries. In lithium-ion batteries, research has focused on studying the inhibition of lithium dendritic growth on the Li anode. Li^+^ ions in the electrolyte are subjected to a Lorentz force that is perpendicular to the electric and magnetic fields. The Lorentz force will change the moving direction of Li^+^ ions, causing spiral motion of the charge and inducing convection in the electrolyte, which can effectively promote the transfer and uniform distribution of Li^+^ ions on the anode [28,29]. In addition, the MF promotes the directional alignment of magnetic material, creating a fast channel for Li^+^-ions migration [30]. In Li–O_2_ batteries, with the introduction of an external MF, the Lorentz force acts oppositely on the photogenerated electrons and holes, suppressing the recombination of charge carriers [31]. In Li–S batteries, an MF was used to investigate the catalyst-assisted trapping of LiPSs to suppress the shuttle effect and enhance the overall battery performance [32–35].
However, the success of previous reports has been limited to liquid-phase- or liquid/solid-interface-involved systems, in which ions or LiPSs under an MF can diffuse freely with low energy barriers. It is noted that Li_2_S passivation is associated with complicated solid-phase-conversion reactions with sluggish kinetics and high energy barriers, and remains a great challenge. Given that MF can regulate the adsorption and catalytic behavior of catalysts and polysulfides, investigating the effects of the MF on the deposition state of Li_2_S on catalyst surfaces and the electronic interactions between catalysts and Li_2_S represents an innovative research direction. To date, the use of an MF to promote Li_2_S deposition/dissociation has not been thoroughly investigated.
In this work, an MF is utilized as a simple and effective regulation means, combined with magnetic α-Fe_2_O_3_ as a catalyst [36], to address the key challenging issues of Li_2_S passivation in Li–S batteries. The result demonstrates that MF-guided catalysis promotes not only polysulfides conversion, but also Li_2_S deposition/dissociation through the formation of electroactive loose films, synchronously realizing shuttle-effect suppression and Li_2_S anti-passivation for high-performance Li–S batteries. The mechanism stems from the fact that the MF can enhance the electron spin polarization of the Fe 3d electrons in α-Fe_2_O_3_, shifting the center of the Fe d-band towards the Fermi level and increasing the hybridization between Fe 3d and S 3p orbitals in α-Fe_2_O_3_–Li_2_S_6_ or α-Fe_2_O_3_–Li_2_S, which reinforces the adsorption and catalytic capability of α-Fe_2_O_3_ towards sulfur species. This work not only demonstrates a low-cost and effective strategy to promote the catalysis of Li_2_S deposition/dissociation and LiPSs, but also enriches the application of the MF effect in electrocatalysis and energy-storage fields. The additional mass burden and energy consumption introduced by MF-generating devices can be regarded as acceptable in remote and open operating environments, and in the context of large-scale clean-energy conversion and storage. Therefore, the findings of this study demonstrate significant potential for practical applications in such scenarios.
RESULTS AND DISCUSSION
Material characterizations and catalytic mechanism under an MF
Figure 1a shows a scanning electron microscopy (SEM) image of α-Fe_2_O_3_ in which the α-Fe_2_O_3_ particles exhibit an irregular polyhedral shape and have an average diameter of ∼100 nm. The corresponding X-ray diffraction (XRD) pattern in Fig. 1b shows the characteristic peaks of α-Fe_2_O_3_ particles with rhombohedral symmetry belonging to the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} R\bar{3}c\end{document} space group (PDF#72-0469). The magnetic property of α-Fe_2_O_3_ particles was characterized by using a vibrating sample magnetometer (VSM), as shown in Fig. 1c. The Morin transition of α-Fe_2_O_3_ particles occurs at ∼263 K (TM ≈ 263 K), above which the material becomes weakly ferromagnetic and below which it behaves as an antiferromagnet [37]. Therefore, at room temperature (298 K), α-Fe_2_O_3_ particles exhibit weak ferromagnetism with a saturation magnetization of 1.11 emu g^−1^ at 1500 mT, and a coercivity of 16.9 mT (Fig. 1c). When the applied MF is greater than the coercivity of the material, the alignment of the magnetic domain is oriented along the direction of the MF, as shown in Fig. 1d. The consistent alignment of the magnetic-domain orientations facilitates the migration of spin-selective electrons from the catalyst towards the reactants [38]. In this work, MF-guided catalysis using α-Fe_2_O_3_ catalyst not only promotes polysulfides conversion, but also facilitates Li_2_S deposition and dissociation, resulting in the formation of a highly electroactive, loosely structured Li_2_S film. This structure is also beneficial for fast and complete Li_2_S dissociation, as shown in Fig. 1e and f. This process simultaneously suppresses the shuttle effect and mitigates Li_2_S passivation, enabling high-performance Li–S batteries.
(a) Scanning electron microscopy image. Scale bar is 500 nm. (b) X-ray diffraction pattern and the crystal structure of the α-Fe2O3 particles. (c) Magnetic hysteresis loop of α-Fe2O3; the inset shows a magnified view. (d–f) Schematic diagrams of (d) the magnetic-domain orientations of α-Fe2O3, (e) the conversion of LiPSs and (f) Li2S deposition/dissociation with and without MFs.
Regulation of Li2S deposition/dissociation under an MF
Li_2_S deposition and dissociation experiments on α-Fe_2_O_3_/carbon paper (CP) were performed. The Li_2_S_4_→Li_2_S reaction contributes three-quarters of the theoretical capacity, while it is also the rate-determining step owing to sluggish kinetics. It thus governs the overall sulfur utilization in the electrode and determines the energy/power density of the Li–S battery. To investigate the effect of the MF on α-Fe_2_O_3_-catalysed Li_2_S conversion and deposition/dissociation, experiments were conducted under MFs of 0 and 400 mT, as shown in Fig. 2a and b. After a 400-mT MF was applied, the Li_2_S-deposition capacity catalysed by α-Fe_2_O_3_ exhibited an obvious increase from 488 to 532 mAh g^−1^. In addition, the deposition morphology of the Li_2_S on the α-Fe_2_O_3_/CP was observed by using SEM. A carbon nanofiber on the surface of the original CP shows a clean surface (Fig. S1). In contrast, the SEM images at points A and B and the energy-dispersive X-ray spectroscopy (EDS) elemental mappings at point B of the Li_2_S deposition with an MF of 0 mT show partially smooth passivation films, as shown by the dashed curves in Fig. 2a and Fig. S2. The passivation film is highly insulating, which reduces the amount and slows the rate of subsequent Li_2_S deposition [39], thereby resulting in a denser passivation layer. However, the Li_2_S product at point A' of the deposition curve under a 400-mT MF exhibits a loose and network-like morphology (Fig. 2b). The loosely stacked Li_2_S sheets serve as nucleation sites for subsequent Li_2_S deposition with a 400-mT MF, resulting in a thicker Li_2_S film with a fluffier and looser morphology (at the end point B'), which is different from the partly dense film at point B with a 0-mT MF, as illustrated in Fig. 2b and a, respectively.
Li2S-deposition and -dissociation curves of α-Fe2O3 with MFs of (a) 0 and (b) 400 mT. The insets show SEM images of the carbon nanofibers after Li2S deposition and dissociation. Scale bars are 2 μm. (c) XRD patterns of α-Fe2O3–Li2S 0 mT and α-Fe2O3–Li2S 400 mT, with commercial Li2S and α-Fe2O3 powders as references. The inset shows the (220) planes of α-Fe2O3–Li2S 0 mT and α-Fe2O3–Li2S 400 mT. (d) Projected density of states of Fe 3d and S 3p orbitals of α-Fe2O3–Li2S with and without MFs.
To further investigate the dynamic effects of the MF on Li_2_S nucleation and growth, we compared the Li_2_S-nucleation modes of α-Fe_2_O_3_ under an MF of 0 mT/400 mT with theoretical models from Scharifker–Hills. The nucleation rates of 2D progressive nucleation (2DP) and 2D instantaneous nucleation (2DI) are controlled by the crystal phase, whereas the nucleation rates of 3D progressive nucleation (3DP) and 3D instantaneous nucleation (3DI) are primarily determined by ion diffusion [40]. For α-Fe_2_O_3_/CP 400 mT, the nucleation of Li_2_S tends to follow the 3DI mode, as depicted in Fig. S3, which results in a faster deposition rate and an earlier attainment of the peak current (A', Tpeak = 901 s; A, Tpeak = 1051s), as shown in Fig. 2a and b. This suggests that a faster nucleation rate allows more achievable Li_2_S deposition (higher capacity) if 3D growth directions are allowed/activated [41]. To better understand the structure of Li_2_S nucleation, we characterized Li_2_S deposition on α-Fe_2_O_3_/CP with an MF of 0 mT/400 mT (α-Fe_2_O_3_–Li_2_S 0 mT and α-Fe_2_O_3_–Li_2_S 400 mT) by using XRD. The XRD measurements (Fig. 2c and Fig. S4) show that the full width at the half maximum (FWHM) of the Li_2_S (220) plane centered at 44.7 for α-Fe_2_O_3_–Li_2_S 400 mT is larger than that for α-Fe_2_O_3_–Li_2_S 0 mT. This suggests that the Li_2_S in α-Fe_2_O_3_–Li_2_S 400 mT possesses a reduced crystallinity with a looser morphology. The oxidation reaction of Li_2_S to soluble Li_2_Sn is also a crucial part of the electrochemical redox system, as it promotes the utilization of active materials. The Li_2_S dissociation curve of α-Fe_2_O_3_ 400 mT shows a higher peak current, earlier peak emergence and greater Li_2_S dissociation capacity (Fig. 2b), indicating stronger Li_2_S-oxidation kinetics and higher sulfur utilization compared with those of α-Fe_2_O_3_ 0 mT (Fig. 2a). In addition, a significant Li_2_S residue is observed on the α-Fe_2_O_3_/CP (0 mT, inset of Fig. 2a), while it is more fully dissolved on the α-Fe_2_O_3_/CP (400 mT, inset of Fig. 2b). The EDS elemental mappings (Figs S5 and S6) of α-Fe_2_O_3_/CP (400 mT) show a smaller amount of sulfur element (0.28 wt%) than that of α-Fe_2_O_3_/CP (0 mT) (2.08 wt%) after Li_2_S dissociation (Table S1). These results suggest that the MF effectively promotes the conversion of Li_2_S to Li_2_Sn (4 ≤ n ≤ 8). Furthermore, density functional theory (DFT) calculations were used to calculate the effect of the applied MF on the Fe d-band center of α-Fe_2_O_3_ (104)–Li_2_S, as shown in Fig. 2c and d. The projected density of states (PDOS) results demonstrate that the Fe d-band center value increases from −1.39 eV (without the MF) to −1.30 eV (with the MF) and the bandgap width decreases from 0.75 eV (without the MF) to 0.52 eV (with the MF). In addition, the hybridization degree (HD) between the Fe 3d and S 3p orbitals increases from 75.98% (without the MF) to 79.42% (with the MF). The results suggest that applying the MF can facilitate electron transfer and enhance the reaction kinetics of Li_2_S conversion.
The local structure and coordination environment of α-Fe_2_O_3_–Li_2_S (after Li_2_S deposition with 400 and 0 mT) were examined by using synchrotron radiation-based X-ray absorption near-edge structure (XANES) spectra and Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS). As shown in Fig. 3a, the XANES profiles of α-Fe_2_O_3_–Li_2_S 400 and 0 mT lie between those of Fe_3_O_4_ and α-Fe_2_O_3_, indicating that the average oxidation state of Fe is between those of the two reference samples. This can be attributed to the reduction of the iron valence induced by the sulfur-containing environment. As depicted in Fig. 3b, the FT-EXAFS fitting curves of α-Fe_2_O_3_–Li_2_S 400 mT and 0 mT show four characteristic peaks at about 1.5, 2.5, 3.2 and 5 Å, corresponding to the Fe–O first, Fe–Fe first, Fe–O second and Fe–S first coordinations, respectively. In this case, the sulfur atoms of Li_2_S do not replace oxygen atoms within the lattice of α-Fe_2_O_3_, and thus the Fe and S atoms in α-Fe_2_O_3_–Li_2_S form the long-range path Fe–S coordination, differing from that of the Fe–S chemical bond (2 Å) in the FeS_2_ powder (Figs S7 and S8). Instead, the sulfur atoms of Li_2_S are located farther away from the Fe atoms (5 Å). To obtain quantitative structural information, FT-EXAFS was fitted in the real-space (R-space), as shown in Fig. 3b and Table S2. Fe atoms in α-Fe_2_O_3_–Li_2_S 400 mT display a higher coordination number of 11.5 and a shorter Fe–S distance of 5.27 Å than those in α-Fe_2_O_3_–Li_2_S 0 mT (3.3 and 5.65 Å, respectively), due to the enhanced adsorption of Li_2_S on α-Fe_2_O_3_ under a 400-mT MF. The more powerful 3D and 2D wavelet transform EXAFS (WT-EXAFS) analysis of the Fe K-edge was conducted to provide high resolution in the k and R spaces, as shown in Fig. 3c–e and Figs S8–S10. As illustrated in Fig. 3c–e, all three samples exhibited maximum intensities at ∼5 and 8 Å^−1^ (k (Å^−1^) space), which are attributed to Fe–O and Fe–Fe, respectively. In contrast to the pristine α-Fe_2_O_3_ (Fig. 3e), the α-Fe_2_O_3_–Li_2_S 400 mT and 0 mT at 4.5–5.0 Å exhibited distinctly different coordination environments, with stronger WT oscillations and longer R-space coordination distances. This observed coordination region is attributed to Fe–S coordination (Fig. 3c and d), which is consistent with the R-space-fitting results (Fig. 3b). In summary, the coordination environment of α-Fe_2_O_3_ is significantly affected by the MF and Li_2_S deposition. Compared with a 0-mT MF, the application of a 400-mT MF results in a shorter Fe–S coordination distance, which facilitates the catalytic conversion of Li_2_S.
(a) Normalized Fe K-edge XANES spectra. (b) FT-EXAFS curves and fitting curves of α-Fe2O3–Li2S with 0/400-mT MF and pristine α-Fe2O3. (c–e) 3D WT-EXAFS contour maps of (c) α-Fe2O3–Li2S 400 mT, (d) α-Fe2O3–Li2S 0 mT and (e) pristine α-Fe2O3.
MF-guided adsorption and catalysis toward LiPSs
Strong adsorption of polysulfides by the catalyst could mitigate the shuttle effect of long-chain polysulfides (Li_2_Sn, 4 ≤ n ≤ 8) [42]. To study the effect of the MF on the adsorption of LiPSs by α-Fe_2_O_3_, the Li_2_S_6_-adsorption experiments were carried out with 0 and 400 mT of MF. After aging for 2 h, the Li_2_S_6_ solution containing α-Fe_2_O_3_ (α-Fe_2_O_3_–Li_2_S_6_) appeared more transparent, as shown in the insets of Fig. 4a. The two regions highlighted at 340 and 420 nm correspond to the absorption peaks of S_6_^2−^ and S_4_^2−^, respectively [43,44]. As shown in the magnified views in Fig. 4b, under a 400-mT MF, the peak intensities of S_6_^2−^ and S_4_^2−^ in the α-Fe_2_O_3_–Li_2_S_6_ solution are significantly reduced compared with those in the pure Li_2_S_6_ solution and the α-Fe_2_O_3_–Li_2_S_6_ 0 mT, indicating that the MF promotes the adsorption of LiPSs by α-Fe_2_O_3_.
(a) Ultraviolet-visible (UV-vis) spectra for Li2S6 solution and that adsorbed by α-Fe2O3 after 2 h with MFs of 0 and 400 mT. The insets show the corresponding optical photos. (b) Magnified view of the absorbance curves in (a). (c) Fe 2p and (d) S 2p X-ray photoelectron spectroscopy spectra of α-Fe2O3 particles and α-Fe2O3–Li2S6 with MFs of 0 and 400 mT. (e) PDOS of Fe 3d and S 3p orbitals of α-Fe2O3–Li2S6 with and without MFs. (f) Schematic diagrams of the distribution center of the Fe d-band with and without MFs, and the orbitals interaction between the polysulfide and α-Fe2O3 with the MF.
To further investigate the MF-induced adsorption of the catalyst toward polysulfides, X-ray photoelectron spectroscopy (XPS) was employed to analyse the valence-state changes in Fe and S after the Li_2_S_6_-adsorption experiments, as shown in Fig. 4c and d. The initial peaks at 710.80 and 723.90 eV of the α-Fe_2_O_3_ particles are attributed to Fe 2p_3/2_ and Fe 2p_1/2_, respectively, which shift to lower binding energies of 710.69 and 723.59 eV after the adsorption of Li_2_S_6_ (α-Fe_2_O_3_–Li_2_S_6_, 0 mT), respectively. Moreover, the terminal sulfur (S_T_^−1^) and bridge sulfur (S_B_^0^) of the polysulfide in α-Fe_2_O_3_–Li_2_S_6_ 0 mT shift to higher binding energy compared with those of the Li_2_S_6_ powder. The decrease in the Fe valence and the increase in the S valence indicate the transfer of electrons from the LiPSs to α-Fe_2_O_3_ during the adsorption process. In contrast, under a 400-mT MF, the binding energy is increased by 0.24 and 0.08 eV for S_T_^−1^ and S_B_^0^, and decreased by 0.13 and 0.1 eV for Fe 2p_3/2_ and Fe 2p_1/2_, respectively, compared with that of α-Fe_2_O_3_–Li_2_S_6_ 0 mT, indicating that the MF enhances the chemical adsorption of Li_2_S_6_ by α-Fe_2_O_3_. XANES was utilized to characterize the S K-edge of α-Fe_2_O_3_–Li_2_S_6_ (with 0 and 400 mT of MF), as shown in Fig. S11. The results also show that the XANES energy of the S K-edge shifts towards higher energy after the MF is applied, implying electron transfer from Li_2_S_6_ to α-Fe_2_O_3_. These findings demonstrate that the MF enhances the chemical adsorption of Li_2_S_6_ by α-Fe_2_O_3_, consistent with the above XPS analysis.
DFT calculations were conducted to investigate the electron interactions between α-Fe_2_O_3_ and Li_2_S_6_. The optimized adsorption model of α-Fe_2_O_3_ (104)–Li_2_S_6_ is shown in Fig. S12. Figure 4e and Fig. S13 show the calculated density of states (DOS) and projected density of states (PDOS) for α-Fe_2_O_3_–Li_2_S_6_ with/without MF. In Fig. 4e, the bandgap width of α-Fe_2_O_3_–Li_2_S_6_ narrows from 1.20 to 0.69 eV after the MF is applied, indicating the improved conductivity of α-Fe_2_O_3_–Li_2_S_6_. In addition, the position of the metal d-band center relative to the Fermi energy level can be used to assess the adsorption and catalytic performances of the metal sites [45]. The Fe d-band center energy (Ed) of α-Fe_2_O_3_–Li_2_S_6_ increases from −2.72 to −1.40 eV after the MF is applied. This shift involves an increase in the orbital energy, suggesting that electrons are more likely to adopt a high-spin configuration [18,46]. Based on the results of the PDOS calculations, a schematic diagram illustrating the orbital interactions between Fe^3+^ with a high-spin 3d electronic structure and the polysulfide under the MSF is shown in Fig. 4f. According to the molecular orbital theory and ligand field theory, the dxz, dyz orbitals of the Fe^3+^ in α-Fe_2_O_3_ form two π bonds and two π* anti-bonding orbitals with the px, py orbitals of the sulfur atoms in the polysulfide. The \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} {{\mathrm{d}}}{{z}^2}\end{document} orbital of Fe^3+^ with the pz orbital of S form σ bond and σ* anti-bonding orbitals. Additionally, the dxy and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} {{\mathrm{d}}}{{x}^2 - {y}^2}\end{document} orbitals do not participate in bonding due to their inactivity compared with the other Fe^3+^ d states. Less electron filling in the anti-bonding orbitals (π* and σ*) facilitates the reduction of α-Fe_2_O_3_ and Li_2_S_6_ repulsion, enhancing the adsorption of polysulfides by α-Fe_2_O_3_ [22,23,47]. Specifically, the HD of the Fe 3d and S 3p orbitals were calculated over the range of −5 to 0 eV, as shown in Fig. 4e. The orbital HD can be used to measure electron interactions, where a high HD value indicates reduced electron repulsion with enhanced electron migration between the atoms and favors a catalytic effect [19,44,48]. The HD between the Fe 3d and S 3p orbitals is defined as \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \frac{{DO{S}{{\mathrm{overlapped}}}}}{{DO{S}{{\mathrm{S\ }}3{\mathrm{p\ states}}}}}\end{document} , where DOSoverlapped and DOSS 3p states are the integration of the overlapping Fe 3d with the S 3p states and the total S 3p states, respectively [49]. The HD of α-Fe_2_O_3_–Li_2_S_6_ is 98.95% with the MF and 77.49% without the MF, respectively. This indicates that the MF promotes the electron transfer between the Fe atoms of α-Fe_2_O_3_ and the S atoms of Li_2_S_6_, which can effectively enhance the conversion activity of the LiPS, reduce the LiPS shuttle effect and improve the utilization efficiency of the active material.
Thermodynamically understanding Li2S deposition/dissociation and the polysulfide conversion
It is well known that, in Li–S batteries, the conversion reactions from Li_2_S_4_ to Li_2_S (Li_2_S_4_→Li_2_S_2_→Li_2_S) involve a liquid–solid transition (Li_2_S_4_→Li_2_S_2_) and a solid–solid transition (Li_2_S_2_→Li_2_S). The latter has a high energy barrier and serves as the rate-determining step that affects the discharge capacity of Li–S batteries. DFT Gibbs free energy calculations, as shown in Fig. 5a and Fig. S14, demonstrate the facilitating effect of the MF on the conversion of short-chain polysulfides from Li_2_S_4_ to Li_2_S, in which both the Li_2_S_4_→Li_2_S_2_ and the Li_2_S_2_→Li_2_S transformations under the MF have lower Gibbs free energies compared with those without the MF, leading to a faster reaction rate with the MF. In addition, the delithiation kinetics of Li_2_S during the charging process is also a factor that determines the reversibility of Li–S batteries. Thus, modeling of the decomposition of Li_2_S was performed to assess the reaction kinetics. The intrinsic kinetics barrier in the reaction can be deduced from the dissociation energy of Li_2_S into LiS and Li atoms. The optimized structures of the decomposition of Li_2_S on α-Fe_2_O_3_ catalysts with/without an MF and their energy barriers are shown in Fig. 5b and Fig. S15. It is obvious that the Li_2_S delithiation reaction under the MF has a lower energy barrier (0.45 eV) compared with that without the MF (0.78 eV). The decreased energy barrier is consistent with the experimentally observed more complete dissociation of Li_2_S on the α-Fe_2_O_3_/CP (400-mT MF) (Fig. 2b). Based on the above experimental and calculated results, we propose a mechanism for the conversion of short-chain polysulfides into Li_2_S and the dissociation of Li_2_S into long-chain polysulfides on α-Fe_2_O_3_ catalysts with/without an MF, as shown in Fig. 5c.
(a) Gibbs free energy for the discharging process from Li2S4 to Li2S on α-Fe2O3 with and without an MF. Optimized absorption configurations with the MF are shown in the insets. (b) Energy-barrier profiles of Li2S decomposition on α-Fe2O3 catalysts along with different reaction coordinates with and without MFs. Initial, transition and final states (IS, TS, FS) with MF are shown in the insets. (c) Schematic illustration of the catalytic mechanism of α-Fe2O3 catalysts involving the adsorption, nucleation, growth and dissociation of Li2S with and without MFs.
With an MF, α-Fe_2_O_3_ undergoes the consistent alignment of magnetic-domain orientations, which facilitates electron migration between the α-Fe_2_O_3_ catalyst and the LiPSs (Fig. 5c, Step I). Besides, the Fe d-band center moves closer to the Fermi level, along with the enhanced hybridization between the Fe 3d and S 3p orbitals, which strengthens the adsorption of polysulfides on α-Fe_2_O_3_ and accelerates catalytic reaction kinetics to form Li_2_S molecules that are deposited on the surface of the α-Fe_2_O_3_ on the electrodes (Step I). Subsequently, the deposited Li_2_S, as nucleation sites, capture LiPSs near the electrolyte–Li_2_S interface, followed by disproportionation to continuously generate Li_2_S (Step II) [49]. Due to the faster catalytic reaction kinetics of the MF-guided catalyst toward both LiPSs and solid Li_2_S (as shown in Fig. 2d), the 3D nucleation and growth mode were favored over the slower lateral diffusion mode (Fig. 2b) (Step II). Consequently, the rapid nucleation and growth of Li_2_S triggers the dynamically controlled deposition process, forming a loosely stacked Li_2_S layer (Step III). During the charging process, the dissociation of Li_2_S under the MF exhibits a lower energy barrier, facilitating its conversion into long-chain polysulfides, with a lower amount of Li_2_S remaining (Step IV). In contrast, without the applied MF, the magnetically disordered α-Fe_2_O_3_ catalyst shows weak adsorption and sluggish catalytic reaction kinetics towards the LiPSs and Li_2_S, rendering a 2D nucleation and growth mode (Fig. 5c, Step I and Step II). As a result, Li_2_S suffers from a slow and dense deposition during the conversion reaction, which is a thermodynamically dominated process, leading to the formation of an energy-stable passivation film (Step III). Accordingly, more Li_2_S residues are generated after the dissociation of the passivation film during the charging process without an MF (Step IV). The above results are confirmed by the SEM and EDS characterizations shown in Fig. 2a, b, Figs S5 and S6. Overall, the MF enhances the adsorption and conversion kinetics of polysulfides on α-Fe_2_O_3_, promotes the 3D nucleation and growth of Li_2_S, and reduces the energy barrier for Li_2_S dissociation.
Electrochemical performance of α-Fe2O3/S cathode and system expansion
α-Fe_2_O_3_/S cathodes were assembled to investigate the effect of the MF on the electrochemical performance of Li–S batteries. The SEM image, EDS element mappings and XRD patterns of the α-Fe_2_O_3_/S cathode are shown in Figs S16 and S17, confirming that α-Fe_2_O_3_ and S are uniformly mixed in α-Fe_2_O_3_/S. Figure 6a presents the cyclic voltammetry (CV) curves of α-Fe_2_O_3_/S at 0.2 mV s^−1^ under 0 and 400 mT of MF. In this work, the MF was generated by using a permanent magnet (Fig. S18). Two reduction peaks appear at ∼2.3 V (Peak a) and ∼2.0 V (Peak b), which can be attributed to the formation of long-chain LiPSs and insoluble short-chain Li_2_Sn (n = 1, 2), respectively. Meanwhile, the oxidation peaks at ∼2.4 V correspond to the oxidation of Li_2_Sn. Peaks a, b and c of the CV curves were further analysed, as shown in Fig. 6b. Peaks a and b of the CV curves show higher peak voltages and onset voltages under the 400-mT MF, which suggests that the α-Fe_2_O_3_/S with a 400-mT MF has a higher discharge plateau and smaller voltage polarization during the discharge. Peak c at 400 mT has lower peak voltage and onset voltage than those of 0 mT, indicating lower charging voltages and smaller voltage polarization, which suggests that Li_2_S dissociation has faster conversion kinetics under the 400-mT MF. The voltage polarization of Peaks a, b and c is clearly presented by the Tafel plots (Fig. 6c, Fig. S19a and b), which show smaller Tafel slopes under the 400-mT MF. The results suggest that the MF-guided Fe_2_O_3_ catalytic system is conducive to promoting the conversion kinetics of polysulfides and Li_2_S. The magnetic hysteresis loops of the α-Fe_2_O_3_/S and sulfur powders were characterized by using a VSM, as shown in Fig. 6d. A significant decrease in the saturation magnetization of α-Fe_2_O_3_/S (0.21 emu g^−1^) is observed compared with that of α-Fe_2_O_3_ (1.11 emu g^−1^, Fig. 1c), which is due to the diamagnetic nature of sulfur. The coercivity slightly increases from 16.9 mT (α-Fe_2_O_3_) to 21.3 mT (α-Fe_2_O_3_/S), as shown in the inset of Fig. 6d. The first-cycle charge/discharge curves at 0.1 C (Fig. S20) also exhibit a smaller voltage polarization of 0.18 V under 400 mT compared with 0.21 V under 0 mT. Note that the α-Fe_2_O_3_/S 400 mT shows a larger current response, higher specific capacity and lower voltage polarization as compared with α-Fe_2_O_3_/S 0 mT, which suggests that the MF enhances the redox kinetics throughout the α-Fe_2_O_3_-catalysed conversion of polysulfides and Li_2_S. As shown in Fig. 6e, the equivalent circuit of electrochemical impedance spectroscopy (EIS) was used to simulate each circuit element. The charge-transfer resistance (Rct) of α-Fe_2_O_3_/S 400 mT (28.03 Ω) is lower than that of α-Fe_2_O_3_/S 400 mT (53.96 Ω), while the internal impedance (Rs) is decreased from 6.52 Ω (0 mT) to 4.11 Ω (400 mT), indicating that the MF can facilitate electron migration. The EIS results are consistent with the previous DFT calculations (Figs 4e and 2d), confirming that the MF reduces the bandgap widths of α-Fe_2_O_3_–Li_2_S_6_ and α-Fe_2_O_3_–Li_2_S, thereby promoting electron transfer between α-Fe_2_O_3_ and Li_2_S_6_/Li_2_S.
(a) CV curves and (b) peak voltages and onset potentials of Points a, b and c on CV curves. (c) Tafel plots of Peak b of α-Fe2O3/S with MFs of 0 and 400 mT. (d) Magnetic hysteresis loops of α-Fe2O3/S and sulfur. (e) EIS plots. Rs, internal resistance including the resistances of the electrolyte solution and electrodes; Rct, charge-transfer resistance; Rint, interphase resistance; W, Warburg resistance; CPE, capacitance contributed by the charge-transfer process. (f ) Rate performances and (g) long-cycle tests at 1 C of α-Fe2O3/S with MFs of 0 and 400 mT. (h) Cycling stability of α-Fe2O3/S pouch cell at 0.05 C under MF of 0 mT. (i) Digital photo of a light emitting diode (LED). (j) Cycling stability of α-Fe2O3/S cathodes with high sulfur loading of 3.6 mg cm−2 at 0.05 C.
Meanwhile, Fig. 6f illustrates the rate performances of the α-Fe_2_O_3_/S cathode under 0/400 mT of MF, in which it can be observed that all the specific capacities of α-Fe_2_O_3_/S 400 mT exceed those of 0 mT from 0.1 to 2 C. Upon a return to 0.2 C, the α-Fe_2_O_3_/S 400 mT demonstrates an average reversible capacity of 807 mAh g^−1^ (388 mAh g^−1^, based on the mass of the entire cathode). α-Fe_2_O_3_/S 400 mT also shows a higher average capacity and more stable long cycling performance, as shown in Fig. 6g. In the initial cycles, α-Fe_2_O_3_/S 400 mT undergoes an activation state, resulting in a slight increase in the specific capacity. The capacity decay rate per cycle for α-Fe_2_O_3_/S 400 mT is only 0.018%, which is significantly lower than 0.076% observed for α-Fe_2_O_3_/S 0 mT. After 900 cycles at 1 C, α-Fe_2_O_3_/S 400 mT maintains a higher capacity of 431 mAh g^–1^ compared with that of 0 mT. High sulfur loading is a key parameter for achieving high energy density in Li–S batteries. In Fig. S21, under 400 mT, the battery with an α-Fe_2_O_3_/S cathode and a sulfur loading of 4.8 mg cm^−2^ (E/S = 12 μL mg^−1^) exhibits an average capacity of 690 mAh g^−1^ over 50 cycles at 0.05 C, which is significantly higher than 494 mAh g^−1^ under 0 mT. This electrochemical performance demonstrates that the MF favors the capacity enhancement and cycling stabilization of α-Fe_2_O_3_/S.
Cathodes with high sulfur content (60 wt%) and high areal sulfur loading (3.6 mg cm^−2^) were assembled into both coin cells and pouch cells. These cells demonstrated high specific capacity and excellent cycling stability even in the absence of an MF (Fig. 6h and i, Fig. S22 and Table S3).
As depicted in Fig. 6h, the initial specific capacity of the α-Fe_2_O_3_/S cathode in the pouch cell was 1150 mAh g^−1^ (693 mAh g^−1^, based on the mass of the entire cathode) at 0.05 C without an MF. This indicates that the pouch cell can deliver an energy density of 283 Wh kg^−1^. Subsequently, the coin cell was tested under a 400-mT MF. It delivered an initial specific capacity of 1095 mAh g^−1^ (660 mAh g^−1^, based on the mass of the entire cathode) and retained 791 mAh g^−1^ after 40 cycles (Fig. 6j). This performance is superior to that of the high-loading coin cell tested at 0 mT, showing enhanced specific capacity and cycling stability under the MF. Furthermore, KB/S (Ketjen black/sulfur, without an α-Fe_2_O_3_ catalyst) coin cells were assembled for rate performance testing. A minor performance improvement was observed under the MF; however, the enhancement was significantly more pronounced for the catalyst-containing cathode (α-Fe_2_O_3_/S), as clearly evidenced by the data shown in Figs S23, S24 and Table S4. Collectively, these results substantiate the practical application potential of the α-Fe_2_O_3_/S cathode under an external MF.
The MF-guided catalysis strategy has also been applied to a ferromagnetic nano-Fe_3_O_4_ catalyst-based system. The SEM image and magnetic hysteresis loop of Fe_3_O_4_ powder are shown in Fig. S25a and b. The adsorption of Li_2_S_6_ by the Fe_3_O_4_ powder was carried out as shown in Fig. S26a and b. These data indicate that MF promotes the adsorption of long-chain polysulfides by Fe_3_O_4_. The Li_2_S-deposition capacity shows a significant increase from 438 mAh g^−1^ (0 mT) to 519 mAh g^−1^ (400 mT), as shown in Fig. S27a and b. Additionally, as shown in the insets of Fig. S27a and b, Li_2_S exhibits a reticular structure at point A' of the deposition curve, while a 2D passivation film (dashed yellow curve, points A and B) is observed. At the endpoints (B and B') of the Li_2_S-deposition curves, an irregular multi-particle appearance is observed. However, Fe_3_O_4_/CP 400 mT shows looser and larger Li_2_S particles. In addition, Fe_3_O_4_/S was used as the cathode for the Li–S batteries. The CV curves, charge/discharge curves and rate performance of Fe_3_O_4_/S show that the MF reduced the voltage polarization, promoted the kinetics of the polysulfides and Li_2_S conversion, and enhanced the capacity and cycling stability (Figs S28a, b and S29).
CONCLUSION
In this work, we investigate the effect of an MF on the catalytic ability of magnetic Fe-based oxides in Li–S batteries. MF-guided catalysis by α-Fe_2_O_3_ promotes not only polysulfides conversion, but also Li_2_S deposition/dissociation, leading to the formation of a highly electroactive loose film, synchronously realizing shuttle-effect suppression and Li_2_S anti-passivation for high-performance Li–S batteries. The mechanism stems from the fact that the MF can enhance the electron spin polarization of the Fe 3d orbital in α-Fe_2_O_3_, shifting the center of the Fe d-band towards the Fermi level and increasing the hybridization between the Fe 3d and S 3p orbitals in α-Fe_2_O_3_–Li_2_S_6_ or α-Fe_2_O_3_–Li_2_S. As a result, under a 400-mT MF, the α-Fe_2_O_3_/S cathode shows outstanding cycling stability (≤900 cycles at 1 C with a 0.018% capacity decay rate per cycle) and excellent rate capability.
Supplementary Material
nwag039_Supplemental_File
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