Magnesium Anode Protection by an Organic Artificial Solid Electrolyte Interphase for Magnesium-Sulfur Batteries

Abstract

In the search for post-lithium battery systems, magnesium-sulfur batteries have attracted research attention in recent years due to their high potential energy density, raw material abundance, and low cost. Despite significant progress, the system still lacks cycling stability mainly associated with the ongoing parasitic reduction of sulfur at the anode surface, resulting in the loss of active materials and passivating surface layer formation on the anode. In addition to sulfur retention approaches on the cathode side, the protection of the reductive anode surface by an artificial solid electrolyte interphase (SEI) represents a promising approach, which contrarily does not impede the sulfur cathode kinetics. In this study, an organic coating approach based on ionomers and polymers is pursued to combine the desired properties of mechanical flexibility and high ionic conductivity while enabling a facile and energy-efficient preparation. Despite exhibiting higher polarization overpotentials in Mg-Mg cells, the charge overpotential in Mg-S cells was decreased by the coated anodes with the initial Coulombic efficiency being significantly increased. Consequently, the discharge capacity after 300 cycles applying an Aquivion/PVDF-coated Mg anode was twice that of a pristine Mg anode, indicating effective polysulfide repulsion from the Mg surface by the artificial SEI. This was backed by operando imaging during long-term OCV revealing a non-colored separator, i.e. mitigated self-discharge. While SEM, AFM, IR and XPS were applied to gain further insights into the surface morphology and composition, scalable coating techniques were investigated in addition to ensure practical relevance. Remarkably therein, the Mg anode preparation and all surface coatings were prepared under ambient conditions, which facilitates future electrode and cell assembly. Overall, this study highlights the important role of Mg anode coatings to improve the electrochemical performance of magnesium-sulfur batteries.

Keywords: artificial solid electrolyte interphase; coating techniques; electrochemical impedance spectroscopy; ionomers; magnesium anode; magnesium−sulfur battery.

Figure 1

Chemical structure of Aquivion (red), SPEEK (green), and c-PAN (blue). Both ionomers comprise one sulfonyl functional group per repetition unit but exhibit different ion exchange capacity values: IEC (Aquivion) = 1.3 mequiv/g, IEC (SPEEK) = 2.7 mequiv/g, cf. IEC (Nafion) = 0.9 mequiv/g.

Figure 2

Surface morphology of (a) scraped Mg, (b) Mg-Aquivion/PVDF, (c) Mg-SPEEK/PVDF, and (d) Mg-PAN. (e) Poor edge coverage and (f) thickness approximation in the case of the PAN-coating (∼ 1 μm) on the Mg substrate (∼100 μm).

Figure 3

AFM images of Mg-Aquivion/PVDF (spin-coated) revealing the porous macro- (left) and microstructures (right) with the white frames indicating the zoomed areas. (a–d) Height sensor and (e–h) peak force error.

Figure 4

Potential profiles of symmetrical Mg–Mg cells with pristine and coated Mg electrodes (vs Mg RE). (a) Polarization at 0.1, 0.2, 0.5, 1.0, and final 0.1 mA cm^–2 with an initial 50 and 10 h intermediate rest at OCV. Potential trend during (b) 50 h OCV and (c–g) polarization cycles at different current densities with evolving voltage spike and overpotential asymmetry during stripping and plating.

Figure 5

Schematic interface of a-SEI-coated Mg anodes: (a) initially, (b) during extended OCV in contact with electrolyte, (c) after polarization with a closed a-SEI layer and an in situ SEI formed underneath via polymer decomposition, and (d) coating failure due to morphology changes and in situ SEI formation via B(hfip)₄^– decomposition beneath the a-SEI. Bottom: scheme of the Mg surface covered with an Aquivion/PVDF layer. In the case of an intact a-SEI, polysulfides and B(hfip)₄^– anions are repelled. Nevertheless, an in situ SEI composed of MgO, MgF₂, and MgS is likely via solvent and polymer decomposition.

Figure 6

(a) Stripping and (b) plating potential during the final cycle of each polarization interval. (c) Current-dependent asymmetry in the plating and stripping potential for all investigated Mg anodes.

Figure 7

Nyquist plots of potentiostatic impedance spectra of pristine and coated Mg anodes during the initial 50 h OCV (cf. Figure 4b) applying a 5 mV amplitude in a frequency range of 300 kHz to 0.1 Hz.

Figure 8

Bode plots of galvanostatic impedance spectra of pristine and coated Mg anodes during stripping at 0.1 mA cm^–2 (polarization in Figure 4c,g) applying a 5 mV amplitude in a frequency range of 300 kHz to 0.1 Hz.

Figure 9

(a) 48 h OCV with subsequent discharge at C/20 and (b) corresponding optical cell images collected during operation. Different Mg anodes in an otherwise identical optical cell setup were applied (50:40:10 wt % S/KB/CMC-SBR cathode, GF/C separator, 0.2 M Mg[B(hfip)₄]₂/G1 electrolyte). In the case of the Mg pellet, an additional separator was used.

Figure 10

Comparison of pristine and coated Mg anodes in Mg–S cells cycled at C/10.

Figure 11

Fourier transform infrared (FTIR) spectra of a Mg-Aquivion/PVDF anode (Mg–S cell, post mortem) in comparison to PVDF and Aquivion powder. Partial in situ cation exchange and consequent cross-linking of the SO₃^– groups are observed.

Figure 12

XPS depth profile of a Mg-Aquivion/PVDF anode cycled at C/10 for 150 cycles in a Mg–S cell with a 0.2 M Mg[B(hfip)₄]₂/G1 electrolyte.

Figure 13

Discharge capacity and Coulombic efficiency of Mg–S cells cycled at C/10. Influence of (a) Mg foil oxidation and PVDF binder, (b) coating thickness and homogeneity, and (c) different coating techniques.