Metal S/Se Batteries

Elemental sulfur and selenium offer high gravimetric and volumetric capacity. Combining with alkali metal anodes, rechargeable metal-sulfur and metal-selenium batteries are one of the most important next-generation battery technologies. Our research focuses on fundamental studies on the reaction/degradation mechanisms and electrode/electrolyte designs to improve the energy density, efficiency and cycling stability of the metal-sulfur/selenium batteries.

  • Develop rotating-ring disk electrode technique for quantitative Li-S redox chemistry:

We develop quantitative rotating-ring disk electrode (RRDE) technique to probe the Li-S redox processes (Fig. 1).1 Our work demonstrated the strong influence of electrolyte solvent (e.g., dielectric constant and donor number (DN)) on the Li-S reaction pathways and rate capability of Li-S batteries.1 We show that the electrochemical steps of sulfur reduction exhibit fast reaction kinetics and only account for approximately one-quarter of the total capacity (i.e., ≈4 e /S8) within the short reaction time in RRDE experiments (seconds). The complete conversion of sulfur to Li2S can only be accomplished via chemical (i.e., potential-independent) polysulfide recombination/dissociation reactions that generate electrochemically reducible polysulfides with long reaction time (hours) in a closed battery cell.1 This work shows the common play ground between Li-O2 and Li-S electrochemistry and provides critical insights needed for designing efficient/stable electrolytes for rechargeable Li-S batteries.

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Fig. 1. Illustration of RRDE for sulfur electrochemistry.1 Characteristic Li-S RRDE voltammetry

  • Operando UV-Vis spectroscopic technique to reveal solvent-dictated Li-S reaction mechanism:

We develop operando UV-Vis spectroscopic technique coupled with RRDE to probe the roles of solvent and electrode additives on the Li-S redox and disproportionation behaviors (Fig. 2).2 We revealed solvent-dependent Li-S redox pathways that in high-DN solvents e.g. DMSO undergo series of electrochemical and chemical reactions involving S8   , S6   , S4    and S3    where S3    is the most stable and dominant reaction intermediates. In contrast, Li-S reactions in low-DN solvents e.g. DOL: DME mainly involve S4  . We investigate how the stability of the major polysulfide intermediates affect the Li-S cell reactions. We further show that solvents that facilitate the formation of redox-active polysulfides can improve the reaction rates of the Li-S cells. Our study reveals solvent-dependent Li-S reaction pathways and highlights the role of polysulfide stability in the efficiency of Li-S batteries.

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Fig. 2. Illustration of Operando UV-Vis for sulfur electrochemistry.2

  • A stable lithium–selenium interface via solid/liquid hybrid electrolytes

To address shuttle effect in lithium-selenium (Li−Se) batteries in glyme-based electrolyte, we develop hybrid electrolytes that permit fast solution-phase Li−Se redox reactions, eliminate polyselenides-shuttling and suppress the formation of lithium dendrite. The hybrid electrolytes consist of NASICON-type Li1.5Al0.5Ge1.5 (PO4)3 (LAGP) ceramics sandwiched by tetraethylene glycol dimethyl ether (TEGDME) electrolyte, enabling the use of low-cost and highly-scalable carbon, demonstrating full utilization of the Se cathode, superior cycling stability, and promising rate capabilities. Our work demonstrates that combining solid state electrolyte with liquid glyme electrolyte is key to achieve stable and high-performance Li−Se batteries.

Fig. 3. (Upper panel) Schematic of the Li–Se battery with hybrid electrolytes. Comparison of battery performance in hybrid electrolytes and liquid electrolyte. (Lower panel) Cycling test of Li-Se batteries with hybrid electrolyte. Test of polyselenides crossover through the LAGP solid state electrolyte. 3

Reference:

Lu Y.C.,* He Q., and Gasteiger H.A., “Probing the Lithium–Sulfur Redox Reactions: A Rotating-Ring Disk Electrode Study" Journal of Physical Chemistry C (2014) 118, 5733-5741 Link

Zou Q., and Lu Y.C.*, "Solvent-Dictated Lithium Sulfur Redox Reactions: An Operando UV–vis Spectroscopic Study" Journal of Physical Chemistry Letter, 2016, 7, pp 1518–1525 Link

Zhou Y., Li Z. and Lu Y.C.*, "A stable lithium–selenium interface via solid/liquid hybrid electrolytes: Blocking polyselenides and suppressing lithium dendrite" Nano Energy, 39, 2017, 554–561 Link