Dynamics of Lithium Deposition in Li-S Batteries: An Elucidation with a Lithium Sulfide Cathode and a Bare Copper-Foil Current Collector without Any Free Lithium Metal
Authors: Nanda Sanjay , Gupta Abhay , Manthiram Arumugam 
Published: February 2020

The realization of high-energy density lithium-sulfur batteries requires the development of stable, reversible, and practical lithium-metal anodes. This necessitates a comprehensive study of the dynamics of lithium deposition in Li-S batteries. Most of the research in Li-S batteries has focused on half-cell studies, which include a thick metallic-lithium chip as the anode. With an infinite supply of lithium, this precludes understanding the impact of lithium degradation and loss on electrochemical performance. Hence, an investigation of Li-S cell performance in the absence of a large excess of lithium, i.e., under lithium-limited conditions, is acutely needed.
To this end, a novel full-cell configuration for Li-S batteries is presented here that comprises a lithium sulfide cathode paired with a bare copper or nickel foil as the hostless-anode substrate for lithium deposition. As the cell has only a limited and stoichiometric amount of lithium from the cathode without any free lithium metal on the anode, its electrochemical performance is limited by the efficiency of lithium plating and stripping. This allows for a more realistic assessment of Li-S cell performance. In addition, the role and dynamical behavior of the lithium anode can be effectively clarified by contrasting the electrochemical performance of the full cell to a comparable half cell.
Counter-intuitively, the Li2S full cell shows only a modest decline in electrochemical performance in comparison to the Li2S half cell. It demonstrates reasonable capacity retention with an average Coulombic efficiency of 96% over 100 cycles. In contrast, the capacity of a similar lithium-plating-based full cell employing LiFePO4 cathodes declines to near zero within 10 cycles. The Coulombic efficiency of plating and stripping a comparable amount of lithium on copper foil is vastly improved for Li2S counter electrode as opposed to Li-metal counter electrode. The exceptional robustness of the Li2S system points to an intrinsic stabilization of the lithium-deposition process in Li-S batteries. Detailed morphological and compositional characterization of the deposited lithium shows that the polysulfide intermediates formed help modulate the plating of lithium by forming deposits of Li2S and Li2S2, which facilitate the formation of a smooth planar morphology. A comparison of various electrolyte formulations in full-cell configuration reveals a sharp divergence in electrochemical performance, which is not observed in half-cell configuration. Emanating from the disparate lithium-deposition characteristics in different electrolytes, this helps provide insights into the critical solid-electrolyte interphase (SEI) components on the lithium surface that enable dynamic stabilization of lithium plating and stripping in Li-S batteries.
This configuration portrays a promising approach to a realistic and effective implementation of the lithium-metal anode in Li-S batteries. It is significant that a considerable enhancement in gravimetric and volumetric energy density can be achieved from eliminating excess lithium, while still maintaining reasonable electrochemical performance. The lithium-limited nature of this configuration makes it an ideal template for generating novel insights into the dynamics of lithium degradation and SEI formation. This work opens up a new frontier in research in Li-S batteries, bringing them one step closer to reality.

Figure: Schematic illustration of the deposited lithium on the copper foil from a Cu || Li2S full cell and the electrochemical performance of the Li || Li2S half cell compared with Cu || Li2S full cell.

Figure 1