Electrocatalysis in Lithium Sulfur Batteries under Lean Electrolyte Conditions

Abstract

The presence of electrocatalysis in lithium-sulfur batteries has been proposed but not yet sufficiently verified. In this study,m olybdenum phosphide (MoP) nanoparticles are shown to play ad efinitive electrocatalytic role for the sulfur cathode working under lean electrolyte conditions featuring alow electrolyte/active material ratio:the overpotentials for the charging and discharging reactions are greatly decreased. As aresult, sulfur electrodes containing MoP nanoparticles show faster kinetics and more reversible conversion of sulfur species, leading to improvements in charging/discharging voltage profiles,c apacity,r ate performance,a nd cycling stability. Taking advantage of the electrocatalytic properties of MoP, high-performance sulfur electrodes were successfully realized that are steadily cyclable at ah igh areal capacity of 5.0 mAh cm À2 with ac hallenging electrolyte/sulfur (E/S) ratio of 4 mL E mg À1 S .Lithium sulfur batteries (LSBs) have as uperior theoretical specific energy of 2600 Wh kg À1 ,b ut an unsatisfactory cycle life. [1] Thecathode material of LSBs,namely sulfur,islimited in cycling stability,d espite its high theoretical specific capacity of 1675 mAh g À1 . [2] Thec apacity decay is tightly associated with the dissolution and diffusion of lithium polysulfide (LPS) intermediates in the electrolyte,w hich on the one hand facilitates the kinetics of electrochemical conversion [3] but on the other hand causes loss of active material and side reactions. [4] Using polar material surfaces, for example,f unctionalized carbons, [5] metal oxides, [6] sulfides, [7] phosphides, [8] nitrides, [9] and metal-organic frameworks, [10] to confine LPS via chemical interactions has been found to be one of the most effective ways for improving the cycling stability of sulfur electrodes.B esides immobilizing LPS on the cathode,i ti sa lso crucial to ensure their rapid electrochemical conversion and thus capacity contribution. Fori nstance,w eh ave recently reported on molybdenum phosphide (MoP) nanoparticles serving an electrocatalystlike function for stabilizing the sulfur cathode. [8b] While the extension of cycle life was significant, electrocatalysis was only preliminarily implied by the larger cyclic voltammetry current on the MoP-modified electrode in aL PS electrolyte. Herein, we report anew observation of akey evidence for the electrocatalytic role played by MoP nanoparticles in LSBs:reduction of overpotential for the charging/discharging reactions of the sulfur cathode,w hich is only obvious under lean electrolyte conditions (that is,w ith al ow electrolyte/ active material ratio). Fora no rdinary sulfur electrode,t he average charging/discharging voltage hysteresis at ac urrent density of 0.8 mA cm À2 increases greatly to 0.43 Vw hen the electrolyte/sulfur (E/S; mL E mg À1 S )ratio is lowered to 6. With MoP nanoparticles incorporated in the electrode structure, the voltage hysteresis is reduced to 0.18 V. MoP effectively catalyzes the electrochemical conversion reactions of sulfur, which helps the...

Metal Organic Framework Derivative Improving Lithium Metal Anode Cycling

“…The cell exhibited the typical two‐plateau charging–discharging voltage profiles (Figure 4b), indicating that the modified separator does not interfere with the electrochemical conversion of sulfur. Employing a simple mixture of carbon nanotubes and sulfur powder as the cathode material, the cell with a sulfur mass loading of 2 mg cm −2 showed a significant improvement in capacity retention upon cycling than the control cell with a plain separator (Figure 4c), verifying that the a‐TiO 2 ‐BDC material is effective in mitigating the active material loss caused by LPS shuttling and accumulation of the nonconductive Li 2 S …”

Multifunctional Transition Metal‐Based Phosphides in Energy‐Related Electrocatalysis

“…Besides, lowing the amount of electrolyte is beneficial for achieving high energy density at the cell level but in lean electrolyte the kinetics of the sulfur redox reactions are limited, resulting in the formation of “dead” sulfur on the electrode surface upon cycling. More recently, Wang and co‐workers arrived at a new level by uncovering the electrocatalytic effects of MoP on sulfur redox reactions in the lean electrolyte . Figure c presented charging/discharging voltage profiles of the CNT‐S electrode and MoP‐CNT‐S electrode under a lean electrolyte/sulfur (E/S) ratio of6 mL E mg −1 S .…”

“…c) Representative charging/discharging voltage profiles of the MoP–CNT‐5‐S and CNT‐S electrodes at under lean electrolyte conditions (E/S = 6; sulfur mass loading: ≈4.0 mg cm −2 ). Reproduced with permission . Copyright 2018, John Wiley and Sons.…”

“…More recently, Wang and co-workers arrived at a new level by uncovering the electrocatalytic effects of MoP on sulfur redox reactions in the lean electrolyte. [183] Figure 15c presented charging/ discharging voltage profiles of the CNT-S electrode and MoP-CNT-S electrode under a lean electrolyte/sulfur (E/S) ratio of 6 mL E mg −1 S . Obviously, the improved specific capacity of 863 mAh g −1 at 0.8 mA cm −2 was achieved on MoP-CNT-S electrode because the MoP nanoparticles catalyzed the transformation of dissolved LPS into solid Li 2 S/Li 2 S 2, and more LPS tended to be electrochemically active on the MoP surface.…”

Sandwich‐Like Ultrathin TiS2 Nanosheets Confined within N, S Codoped Porous Carbon as an Effective Polysulfide Promoter in Lithium‐Sulfur Batteries

“…[39] As such, the sulfur utilization of both plateaus deteriorates over time. [41] SU H (Figure 3c), SU L (Figure 3d), and the corresponding capacity (Capacity H , Capacity L ) are plotted as a function of the cycles. [41] SU H (Figure 3c), SU L (Figure 3d), and the corresponding capacity (Capacity H , Capacity L ) are plotted as a function of the cycles.…”