Tellurium-Polysulfide Chemistry Enables Stable Operation of Lithium-Sulfur Batteries with Limited Lithium Inventory and Electrolyte Supply
Authors: Nanda Sanjay , Bhargav Amruth , Manthiram Arumugam 
Published: December 2020

The practical realization of lithium-sulfur (Li-S) batteries with high energy density is currently limited by the poor cyclability of the lithium-metal anode. Lithium plating and stripping in the liquid ether-based electrolyte is irreversible over long-term cycling and this leads to rapid capacity fade. Due to the reducing nature of lithium, parasitic side reactions with the electrolyte also occur with considerable severity during cell operation. This eliminates a significant portion of the available lithium inventory and electrolyte supply, and the cell fails prematurely. These problems can be sidestepped by employing surplus lithium and electrolyte in the cell, but this compromises the stack-level energy density. Stabilizing lithium deposition and reducing electrolyte decomposition is of critical importance to enabling commercially viable Li-S batteries with high energy density and long cycle life.
The chemistry of Li-S batteries is unique due to the generation of polysulfide intermediates at the cathode that are soluble in the electrolyte. The polysulfides migrate to the anode side, which fundamentally alters its interfacial chemistry by forming reduced sulfide species (Li2S/Li2S2) on the lithium surface. These species are known to have an intrinsically stabilizing effect on lithium deposition, although it is not sufficient to enable stable operation of Li-S batteries at reasonable current densities. Nevertheless, the modification of electrolyte-soluble polysulfide species offers a unique opportunity for tuning the interfacial chemistry of the lithium-metal anode towards stabilizing lithium deposition and reducing electrolyte decomposition.
Tellurium, which sits two position below sulfur in Group 16 of the periodic table, is an excellent candidate element for modifying the composition of polysulfide intermediates in Li-S batteries and forming potentially favorable components in the lithium SEI layer. It was found that elemental tellurium (Te0) could be facilely assimilated into polysulfide species to form soluble mixed polytellurosulfide species when left in an ethereal solution of Li2S6. This was confirmed by the characteristic isotopic signature of tellurium using mass spectrometry. The polytellurosulfide species were found to reduce on the surface of lithium metal to form red-colored lithium thiotellurate (Li2TeS3), which was confirmed by X-ray techniques. In contrast to Li2S, Li2TeS3 is expected to be a significantly better ionic conductor due to the reduced electron density around the sulfur atoms. This would help engender a uniform lithium-ion flux on the lithium surface, enabling homogenous lithium deposition.
Based on the results above, elemental tellurium was simply added in a 1 : 10 molar ratio to the sulfur/Li2S cathode to modify the chemistry of Li-S batteries. During cell operation, the generated polysulfides (Li2Sn) reacted with the Te0 additive in the cathode to form polytellurosulfide (Li2TexSy) species in-situ. The crossover of these tellurium-containing species to the anode side was confirmed by the detection of a significant amount of Li2TeS3 on the lithium surface. The favorable properties of Li2TeS3 as a lithium SEI component enabled the formation of dense and uniform lithium deposits and significantly improved lithium cycling efficiency. In the anode-free full cell configuration, which pairs a Li2S cathode with a bare anode current collector and contains zero excess lithium (N/P ratio = 1), the addition of tellurium improved cycle life by a factor of 7. The loss of the limited lithium inventory in the anode-free cell was reduced from 2% per cycle to 0.25% per cycle.
Time-of-flight secondary ion mass spectrometry (ToF-SIMS) revealed the drastically reduced electrolyte decomposition on lithium surface due to tellurium-stabilized lithium deposition. The reduced electrolyte decomposition could allow a limited electrolyte supply to be retained for a longer number of cycles, thus enabling extended cyclability under lean-electrolyte conditions. This was confirmed using large-area pouch cells employing a high-loading sulfur cathode (5.2 mg cm-2), a lithium-foil anode, and a low electrolyte/sulfur (E/S) ratio of 4.5 µl mg-1. Under such stringent cell design conditions, the pouch cells could be cycled stably for nearly 100 cycles with the addition of tellurium. In contrast, the control cell failed within 25 cycles.
This work demonstrates a novel, effective, and robust strategy of adding tellurium to the sulfur/Li2S cathode for stabilizing lithium deposition and reducing electrolyte decomposition in Li-S batteries. This improves cycle life in energy-dense cell configurations operating with a limited lithium inventory or electrolyte supply. Importantly, this approach is scalable as no expensive or complicated pre-treatments or coatings of the lithium-metal anode are necessary. The unique chemistry of tellurium in the polysulfide-rich environment of Li-S batteries opens a new frontier in research in Li-S batteries, bringing them one step closer to reality.

Figure 1