Researchers at the University of Texas have discovered that introducing tellurium (Te) into a Li-Sulfur system as a cathode additive significantly improves the reversibility of Li plating/stripping by forming a tellurized and sulfide-rich solid-electrolyte interphase (SEI) layer on the Li surface.

The introduction of Te as a cathode additive in Li-S batteries greatly enhances the reversibility of Li plating and stripping. This is engendered by the formation of Li₂TeS₃ as a stabilizing interphasial component on Li-metal surface. Tellurium incorporation improves capacity retention significantly in anode-free full cells with no excess Li. Also, electrolyte decomposition on Li-metal surface is reduced, which helps improve the cyclability of lean-electrolyte pouch cells. Nanda et al.

In a paper in Joule, they report a “remarkable” improvement in cyclability in anode-free full cells with limited Li inventory and large-area Li-S pouch cells under lean electrolyte conditions.

Tellurium reacts with polysulfides to generate soluble polytellurosulfides that migrate to the anode side and form stabilizing lithium thiotellurate and lithium telluride in situ as SEI components. A significant reduction in electrolyte decomposition on the Li surface is also engendered. This work demonstrates Te inclusion as a viable strategy for stabilizing Li deposition and establishes a robust evaluation framework for preserving electro- chemical performance under limited Li and limited electrolyte conditio

While there has been substantial progress toward solving the numerous issues with S cathodes, a large excess of Li metal and liquid electrolyte is still required to enable long cycle life. A typical Li-S cell with a 4 mg cm^-2 S cathode and 0.75 mm thick Li-metal foil anode may have a li to S (Li/S) capacity ratio of 20 or higher. The electrolyte to S (E/S) ratio in such a cell might also exceed 20 µL mg^-1 of S. These unrealistic values, representative of literature, lead to overstated electrochemical performance and compromise system-level energy density. Reducing excess Li and electrolyte while maintaining reasonable capacities and cyclability is crucial to Li-S batteries achieving commercial viability.

—Nanda et al.

The team found that the introduction of tellurium (Te) as an additive in Li-S batteries engenders a unique tellurized and sulfide-rich solid-electrolyte interphase (SEI) comprising lithium thiotellurate (Li₂TeS₃) on the Li surface.

This type of SEI stabilizes Li deposition and ensures deposition of dense Li layers at the anode. This prevents electrolyte decomposition, curtails Li loss, and hence extends cycle life.

The anode-free full-cell configuration provides a reliable and robust framework for evaluating the dynamics of Li deposition in conjunction with various cathode systems.

Additionally, they found, a class of ternary sulfides similar to Li₂TeS₃ can be explored as artificial SEI layers to enable the stable operation of energy-dense, anode-free Li batteries.

Resources

• Nanda et al. (2020) “Anode-free, Lean-Electrolyte Lithium-Sulfur Batteries Enabled by Tellurium-Stabilized Lithium Deposition,” Joule doi: 10.1016/j.joule.2020.03.020