Single-sulfur atom discrimination of polysulfides with a protein nanopore for improved batteries

Lithium-ion batteries are crucial for the energy transition, but improvements in lifetime and sustainability are needed. Smart batteries with integrated sensing and self-repairing capabilities are a promising area of research. Most research focuses on electrodes and electrolytes, but separator membranes also offer potential for incorporating chemical functionalities to capture unwanted species. This study investigates the use of biological nanopore sensors for high-resolution detection of polysulfides, which are detrimental to the performance of lithium-sulfur batteries due to the polysulfide shuttle effect. The shuttle effect involves the migration of soluble polysulfides (Li2Sn, 3 ≤ n ≤ 8) between the cathode and anode, leading to capacity fade. Current analytical techniques lack the single-molecule resolution needed to precisely monitor these species. This work explores the use of a protein nanopore (α-hemolysin) integrated within a lipid membrane, coupled with cyclodextrins (CDs), as a highly sensitive tool to address this limitation. Cyclodextrins are known for their host-guest interaction capabilities, which can be used to selectively trap polysulfides. The researchers hypothesize that this combination will allow for the precise identification and quantification of polysulfide species, ultimately leading to improved battery designs.

Previous studies have focused on improving lithium-sulfur batteries by modifying electrodes and electrolytes. Strategies for mitigating the polysulfide shuttle effect have included using mesoporous carbons or oxides for polysulfide confinement or adsorption, tuning electrolyte composition, and employing chemically-grafted separators. However, these approaches have limitations in selectivity. The use of cyclodextrins for polysulfide trapping has shown promise, with studies demonstrating supramolecular interactions between β-cyclodextrin and polysulfides. The nanopore technology has been successfully used for single-molecule detection of various biomolecules, including nucleic acids and proteins, demonstrating its high sensitivity and resolution. This study combines these two areas to develop a novel approach for single-molecule level detection and discrimination of polysulfide species.

The researchers synthesized sodium polysulfides (Na2Sn, n = 3, 4, 5) in aqueous solution and characterized them using UV-vis spectroscopy. The interaction between polysulfides and different cyclodextrins (α, β, γ) was investigated using 1H NMR spectroscopy, monitoring chemical shift changes of cyclodextrin protons upon polysulfide addition. A Job's plot method was used to determine the stoichiometry of the complexes, revealing a 1:1 stoichiometry. Association constants (Ka) were determined using NMR titration experiments and fitted using a 1:1 binding isotherm model. Molecular docking simulations were performed to support the experimental findings and provide insights into the binding modes. For the nanopore sensing, an α-hemolysin (α-HL) nanopore was embedded in a lipid bilayer, with the trans compartment containing β-cyclodextrin and Na2Sn/β-cyclodextrin complexes. Applying a potential difference across the membrane, the passage of molecules through the pore caused current blockades, which were analyzed to characterize the interaction of the complexes with the nanopore. The dwell times and blockade ratios were statistically analyzed to identify distinct signatures for each polysulfide species. Atomistic modeling, including molecular dynamics simulations of α-HL and the complexes, was used to estimate the nanopore hindrance and correlate the results with the experimental data. The thermal stability of the polysulfide/cyclodextrin complexes was also investigated via temperature-cycling NMR experiments to evaluate their suitability for temperature-responsive separator applications.

The NMR studies demonstrated a reversible equilibrium complexation between polysulfides and cyclodextrins, with β-cyclodextrin showing the highest affinity for Na2S5 (Ka = 181 ± 4 M-1). Job's plot analysis confirmed a 1:1 stoichiometry for the complexes. The association constants decreased with decreasing sulfur chain length. Molecular docking simulations supported the experimental results, showing polysulfides binding inside the cyclodextrin cavities. Nanopore experiments revealed a clear distinction between the current blockade signals generated by β-cyclodextrin and its complexes with different polysulfides. The blockade ratio increased with increasing sulfur chain length, allowing discrimination of polysulfides differing by a single sulfur atom. The calculated nanopore hindrance estimator, based on atomistic simulations, showed a good correlation with the experimental current blockade levels. Temperature-cycling NMR experiments indicated that the polysulfide/cyclodextrin complexes are thermally reversible, suggesting the possibility of creating temperature-responsive membranes for battery applications.

The study successfully demonstrates the use of a protein nanopore sensor coupled with cyclodextrins for single-sulfur-atom resolution in identifying polysulfides in aqueous solutions. The combination of experimental techniques and atomistic modeling provides strong evidence for the specific interaction between polysulfides and cyclodextrins, and the successful discrimination of these complexes in the nanopore. These findings pave the way for designing smart battery separator membranes capable of selectively trapping polysulfides, thereby mitigating the shuttle effect. The thermal reversibility of the complexes suggests a potential approach for on-demand release of polysulfides, enabling a regenerative separator functionality. The nanopore technique offers a valuable new tool for in-situ monitoring of battery electrolytes at the single-molecule level.

This multidisciplinary study demonstrates a novel approach for single-molecule detection and discrimination of polysulfides using a protein nanopore sensor combined with cyclodextrins. The high resolution achieved allows for distinguishing polysulfide species differing by only one sulfur atom. The findings have significant implications for designing smarter batteries with improved performance and lifetime. The thermally controlled reversibility of the complexes opens up exciting possibilities for developing stimuli-responsive battery separators. Future work should focus on adapting this methodology to organic solvents commonly used in Li-S batteries and exploring the use of alternative nanopore materials.

The study primarily focuses on aqueous solutions, while Li-S batteries typically utilize organic electrolytes. Adapting the method to organic solvents will require further investigation of polysulfide/cyclodextrin complexation equilibria in these media and potentially using alternative nanopore materials compatible with organic environments. The current study uses a simplified model system with β-cyclodextrin. Further studies are needed to explore the effectiveness of this approach using more complex membrane systems and real battery conditions.