On the Origin and Implications of Li$_2$O$_2$ Toroid Formation in Nonaqueous Li-O$_2$ Batteries

TL;DR

This paper investigates how electrolyte additives like water influence Li$_2$O$_2$ toroid formation in nonaqueous Li-O$_2$ batteries, revealing a solution growth mechanism that enhances capacity and offers design insights for better electrolytes.

Contribution

It introduces a formalism to predict additive effects on Li$_2$O$_2$ growth, linking solution mechanisms to increased battery capacity and toroid morphology.

Findings

Trace additives trigger solution-based growth of Li$_2$O$_2$.

Solution growth mechanism leads to larger discharge capacities.

Li$_2$O$_2$ toroid formation is explained by the model.

Abstract

The lithium-air (Li-O$_2$) battery has received enormous attention as a possible alternative to current state-of-the-art rechargeable Li-ion batteries given their high theoretical specific energy. However, the maximum discharge capacity in nonaqueous Li-O$_2$ batteries is limited to a small fraction of its theoretical value due to the insulating nature of lithium peroxide, Li$_2$O$_2$, the battery$'$s primary discharge product. In this work, we show that the inclusion of trace amounts of electrolyte additives, such as H$_2$O, significantly improve the capacity of the Li-O$_2$ battery. These additives trigger a solution-based growth mechanism due to their solvating properties, thereby circumventing the Li$_2$O$_2$ conductivity limitation. Experimental observations and a growth model imply that this solution mechanism is responsible for Li$_2$ toroid formation. We present a general formalism describing an additive$'$s tendency to trigger the solution process, providing a rational design route for electrolytes that afford larger Li-air battery capacities.