Depletion of Lithium Inventory and Devolution of Lithium-Electrolyte Interface in Lithium-Sulfur Batteries
Authors: Nanda Sanjay , Manthiram Arumugam 
Published: December 2020

The lithium-sulfur (Li-S) chemistry holds tremendous potential for realizing the next generation of rechargeable batteries with high energy density and low cost. Despite its promise, the practical development of Li-S batteries is hindered by its limited cyclability, which is mainly caused by the poor plating and stripping efficiency of the lithium-metal anode. In liquid electrolytes, lithium deposits adopt a mossy dendritic filamentous morphology with high surface area. Due to the reactive nature of lithium, the electrolyte irreversibly decomposes on the lithium surface, which consumes both the lithium and electrolyte supply in the cell. However, significant questions remain about the mechanisms that underlie lithium inventory loss and the role of the lithium-electrolyte interface. Further commercial adoption of Li-S batteries is conditional on developing an effective strategy for improving the reversibility of lithium deposition, which is in turn dependent on developing a clear mechanistic understanding of lithium degradation with cycling.

In this study, we use the anode-free full cell configuration, which pairs a Li2S cathode with a bare nickel foil as the anode current collector. Since there is no excess lithium in the system (N/P ratio = 1), the cyclability of anode-free full cells depends on the electrochemically accessible lithium inventory in the system. By identifying the sulfur-limited and lithium-limited regimes in the cycling of anode-free full cells, the lithium inventory loss rate per cycle (LILR) can be quantitatively estimated. This parameter provides an effective, robust, and quantifiable description of lithium degradation with cycling in cells with a limited lithium inventory. Lithium degradation displays a strong dependence on the applied current density, with the LILR increasing from 0.6% per cycle to 1.8% per cycle when the current rate is increased from C/10 to C/5.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was applied to analyze the chemical composition of the deposited lithium in the anode-free full cells with different cycle numbers. The integrated intensity for Li2
-, which is indicative of metallic lithium, was found to show negligible variation with cycle number. In contrast, the electrochemically accessible lithium inventory is known to show a clear decrease with cycling. Thus, there is poor correlation between the lithium inventory available with cycle number and the amount of metallic lithium detected with ToF-SIMS. This indicates that lithium inventory depletion in Li-S batteries is not just due to the loss of lithium in parasitic side reactions with the electrolyte to form lithium SEI compounds (Li+), but largely due to the formation of electrochemically inaccessible “dead” metallic lithium. Further analysis with ToF-SIMS revealed the growth of a thick layer of electrolyte decomposition products on the surface of the deposited lithium. Since the decomposition products are expected to be ionically insulating, lithium-ion access to and from the metallic lithium underneath is blocked, which renders it electrochemically inaccessible or “dead”.

The bulk of the deposited lithium is found to be composed of fully reduced decomposition products, such as Li2O, Li2S, LiH, and LiOH. In particular, the hydrogen-containing decomposition products such as LiH and LiOH are found to show a sharp decrease in concentration with cycling. This is attributed to the substantial gas evolution that occurs at the lithium anode during cell operation. These gases include H2, CH4, and C2H4, and their generation is correlated with the loss of lithium inventory with cycling. This could play an additional role in limiting electrochemical accessibility to the trapped metallic lithium by blocking off ionic and electronic conduction pathways, especially if the gases themselves are trapped in void spaces of the porous lithium deposit.

This work illuminates the mechanisms that underlie the loss of lithium inventory in Li-S batteries by a careful application of the anode-free full cell configuration and analytical tools, such as ToF-SIMS. The role of the devolving lithium-electrolyte interface in engendering lithium degradation with cycling is clarified. The framework demonstrated in this work and the conclusions drawn are applicable not just to Li-S batteries, but to lithium-metal-based batteries in general. It is hoped that the insights generated in this work can motivate efforts towards enabling stable operation of lithium-metal anodes and long cycle life for energy-dense Li-S batteries.

Figure 1